1

PLE ndash 305 Extrusion Theory and Application

Medical Extrusion Research Assignment

Focus Multilumen Tubing Extrusion Die Design

Dr Wickman

October 26 2010

Jason McNulty Jesse Pischlar

2

Contents The Extrusion Process as Related to Medical Manufacturing 4

Overview of the Medical Extrusion Market and its Associated Technologies 4

Overview of the Medical Tubing Extrusion Process 5

The Extruder5

Drive and Motor 5

Gearbox 5

Feed Section 5

Screw and Barrel 5

Temperature Control 6

Base 6

Melt Pumps Internal Air Control Systems and Crossheads 6

Tube Extrusion Process Control and Evaluation 7

Laser Gages 7

Gamma Backscatter Probes 8

Ultrasonic Reflection8

Pressure Transducers 8

Multilumen Tubing Die Design for Extrusion 10

Rheological and Mechanical Design of Extrusion Dies 10

Extrusion Die Design Guidelines and Recommendations 10

In General 10

Establishing the Flow Channel Configuration of the Die 11

Die Guidelines and Recommendations Regarding the Processing of Specific Materials 12

Materials for Extrusion Dies 13

Draw Down Ratios 14

Diameter Draw Ratio (DDR) 15

Wall Draw Ratio (WDR) 15

Area Draw Ratio (ADR) 15

Draw Ratio Balance (DRB) 16

Sizing Ratio (SR) 17

Land Length 17

Taper Angles 18

Simulation Software and the Die Design Process 18

3

Figures and Graphics 20

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas 22

Works Cited 25

4

The Extrusion Process as Related to Medical Manufacturing

Overview of the Medical Extrusion Market and its Associated Technologies

Extruded medical tubing dates back to the 1930rsquos (Sparacino 1999) Since then the

overarching goal and push for medical device manufacturers has been to create products that

are less invasive Extruded tubing products such as catheters are constantly shrinking in

diameter while being expected to offer more features and greater performance ldquoWhat used to

work as a 6 French catheter is 5 French today and people want it to be a 3 French tomorrowrdquo

(Conley 2006) Moreover new medical extrusion applications are consistently requiring tighter

tolerances additional lumens more sophisticated co-extrusions complex braiding styles and

multi-durometer sections Regarding multilumen tubing new specifications may call for

anywhere up to eight lumens some with wire inside some with braids and some with different

lining methods (Conley 2006) In 2006 die manufacturer Guill Tool amp Engineering indicated

ldquoAs a percentage of the business one of our customers says that about 50 of the requests are

multilumen compared with 25 a year agordquo (Conley 2006)

With the smaller physical size of todayrsquos medical tubing it becomes difficult to retain all of the

desired properties using a single material Increasingly the solution has been co-extrusion

There is also a growing demand for tubing with stiffness that varies with length making tooling

more complex (Conley 2006)

PVC remains the leading polymer for medical tubing with polyurethane polyolefins and blends

or alloys such as thermoplastic elastomers also commonly used For more demanding

applications engineering plastics such as polyamideimide polyester polycarbonate or various

fluoropolymers can be selected Many resins can be compounded with optical or x-ray

opacifiers such as titanium dioxide barium sulfate or bismuth subcarbonate further increasing

the number of potential materials and grades (Colbert 1996)

Within a single tube there could potentially be two or three unique durometers Some original

equipment manufacturers (OEM) are manufacturing braided catheters that require changing the

braid as well as the hardness of the polymer material along the braid (Conley 2006) More

complex yet some catheters utilize alternating durometers braid styles and contain multiple

lumens for various purposes For these requirements the extruder is essentially being

oscillated ndash going from one durometer to the other and back (Conley 2006)

5

Overview of the Medical Tubing Extrusion Process

See the ldquoFigures and Graphics Sectionrdquo for photographs of the various systems described

below

The Extruder

The extruder is considered the main component in a tubing extrusion system There are six

major components that contribute to maintaining a consistent process during daily production

Drive and Motor

This system supplies the power to the screw it must be sized properly to ensure constant speed

and required torque to process the particular polymer Currently most machines come equipped

with AC Vector drives (Sparacino 1999) To accurately maintain diameter and wall thickness of

intricate polymer tubes a uniform flow rate of melt from the extruder must be guaranteed All

extruders producing extremely tight tolerances will exhibit surging as a result of electrical drive

control fluctuations screw design and the normal rheological variation in the polymer To

overcome this a precision rotary gear pump is used to provide steady pressure and accurate

metering of the polymer to the die head in controlled surge-free manner (Colbert 1996)

Gearbox

The gearbox transmits the power from the drivemotor system to the screw Service factor

efficiency lubrication and gear design are of most importance Double reduction gearboxes with

carburized or hardened gears and internal lubrication systems are most desirable (Sparacino

1999)

Feed Section

Two main functions are to isolate the polymer from the barrel heat until it is fed to the screw

and allow the polymer to free flow to the screw A separate machined casting that is cored to

allow cooling with a feed opening more than one diameter long will accomplish both

(Sparacino 1999)

Screw and Barrel

This combination must take the polymer which for tubing is normally in pellet form and convey

melt mix and meter it into the die A barrel made from a steel alloy with a liner cast into it of the

proper composition that will withstand the pressure corrosiveness and abrasiveness of the

melting process is required A barrier screw that is properly designed to separate the solid from

the melted polymer and mix it properly functions best (Sparacino 1999) The matching of

extruder screw design to the melting and rheological characteristics of the plastic to be

processed is fundamental to extruder performance Thus there is no such thing as a ldquogeneral-

purposerdquo screw The key extrusion criteria of output plastication solids conveying and power

consumption are influenced by screw design variables such as channel depth number of flights

helix angle compression ratio flight clearance and flight geometry (Colbert 1996)

6

Temperature Control

Although 80 of the required heat should be put into the polymer by proper screw design the

temperature control system must maintain the level accurately Discrete temperature controllers

with PID settings to control induction heating of the barrel through finned castings with built in

rod heaters are commonly used Microprocessor and Programmable logic controllers are also

available allowing recipe storage and other advantages that will be discussed later In most

medical tubing applications cooling is coordinated with air blowers for each zone (Sparacino

1999)

Base

The base is the foundation for all of the individual components of the extruder Without a sturdy

base and a well-designed barrel support vibration is transferred to the polymer and product

surface finish is compromised (Sparacino 1999)

Melt Pumps Internal Air Control Systems and Crossheads

Melt pumps are necessary in some applications however they introduce to the system another

component that must be controlled and maintained They are necessary when part tolerance is

closer than the +- 1 that a screw alone can produce or when batch to batch material

consistency cannot be guaranteed As stated earlier the die gives the extruded product its initial

shape The transition from the extruder to the die set must be streamlined with no obstacles or

ldquohang-uprdquo points for material to stagnate on The inventory of material in the transition must also

be held to a minimum Calculation for the die size the pin size and the land length must be

made in conjunction with the material being extruded the sizing method to be used and the

final part dimensions (Sparacino 1999)

Internal air control becomes necessary when used as a sizing method for multi-lumen or bump

tube products The die must have built into it a free passage for air to control inner diameters of

the tube and lumens The passage is either through the spider legs in an in line die or through

the core tube in a crosshead die High pressure plan air must be decreased to useable pressure

(usually 5 psi or less) and maintained accurately For bump tube products vacuum sizing does

not aid in the sizing of the tube In a modern day applications the entire system both water and

air are a part of the vacuum system This makes the cooling water weightless as compared to

the tubing passing through (Sparacino 1999)

After the material is prepared it is fed usually by gravity to the rotating screw of an extruder

The polymer is heated fed compressed and metered The homogeneous melt is fed either to a

melt metering pump or directly into an extrusion die in line or crosshead which will give the

thermoplastic mass its initial shape (Sparacino 1999)

The final size of the extruded product can be accomplished in one of three ways

1 Free extrusion where the pressure on the inside of the tube is held at atmospheric level

and the speed of the puller as compared to the speed of the extruder sizes the part

2 Applying a controlled source of compressed air to the inside of the tube as it is cooled

7

3 Vacuum sizing by controlling the pressure outside the tube as compared to the

atmospheric pressure on the inside of the product as it is being cooled (Sparacino

1999)

The production of precision multilumen tubes or the insertion of forming wires or guidewires

requires cross-head die extrusion In this process the polymer melt enters the die at right angles

to the outlet which allows lumen characteristics to be controlled by individual pressurized air

supplies fed from the back of a cross-head die and into the tube via precision-bore injector

needles To ensure that a manufacturerrsquos multilumen tubing will sustain precise flow levels the

extrusion process must include some means of maintaining the consistency of all tube

dimensions A typical tolerance range is plus or minus 1 which for a tube with an internal

diameter of 16mm translates to accuracies of plus or minus 10 micrometers (Colbert 1996)

In a multilumen tube each lumen has a defined end use There are two principal methods used

for controlling the shape of each lumen In the first bore-forming mandrel wires can be inserted

temporarily into the tube as the polymer overlays the mandrels accurate lumens can be formed

by removing the precision-gauge wires after cooling Alternatively separate air-pressure control

for each lumen can be achieved by using air injection needles It is possible to accurately adjust

and maintain pressure differentials at low pressures For example a pressure of 0017 bar can

be maintained to within plus or minus 0002 bar The relative flow rate required to maintain

lumen size at a given die speed can be computed but care must be taken to ensure that the air

supply used for pressure regulation is subject to the same influences as the ambient air

surrounding the extrusion line (Colbert 1996)

Tube Extrusion Process Control and Evaluation

In order to ensure patient satisfaction every tube shipped from the manufacturing facility must

be within specification In order to achieve this requirement the extrusion system can utilize a

number of post processing product evaluation techniques For example the tube can be

measured in line using ultra-sonic sensors to measure the wall thickness or a dual scanner laser

can be used positioned before the puller (Sparacino 1999) However current trends are

towards on-line real-time monitoring of key parameters so as to achieve ldquoprocessed-in qualityrdquo

Automatic control of processing parameters is therefore used to keep the product within

specified quality limits (Colbert 1996) Common real-time process control techniques include

the following

Laser Gages

Laser gages offer accurate and rapid measurement of outside diameter by measuring a shadow

created when the tube obscures a fine beam of rapidly scanning lights Dual-plane laser gages

measure OD in tow planes providing both average OD and ovality with a resolution of 1

micrometer (Sparacino 1999)

8

Gamma Backscatter Probes

These devices use gamma-radiation backscatter to determine wall thickness down to 005 mm

for tubes with diameters as small as 1mm Probes measure wall thickness at a single point

around the tube a number of probes can be used if measurements of multiple points around the

diameter are required (Sparacino 1999)

Ultrasonic Reflection

This technique involves aligning the product in an ultrasonic gage placed in a water bath and

arranging transducers around it Each transducer sends out a transmission pulse that is partially

reflected off the outer wall of the tube While the partial reflection returns to the transducer the

remainder of the initial transmission pulse continues through the product wall The difference in

density between the two pulses allows wall thickness to be calculated Enhancing the signal

with digital processing can allow measurements to an accuracy of plus or minus 5 micrometers

of tubes as small as 10mm OD with wall thicknesses of 013mm or less

Pressure Transducers

As it relates to process control techniques food and medical applications cannot use the most

common type of transducer because it introduces mercury to the process Additionally these

standard oil-filled sensors can fail to operate within specifications after as little as 5 months at

570 degrees Fahrenheit (Naitove) Dynisco LLC a company in Franklin Massachusetts has

developed multiple new pressure transducers that can be utilized in a medical device

manufacturing environment Their new PT528 series has been documented to operate within

specification for almost a year Its maximum operating temperature is 617 degrees Fahrenheit

though the recommended limit for optimum life span is 527 degrees Fahrenheit Its accuracy is

rated at plus or minus 5 with repeatability of 02 at a price of $690 to $1400 (Naitove)

Conventional push-rod transducers have the same accuracy and repeatability specifications as

oil-filled sensors with comparable cost and can be used at temperatures up to 750 degrees

Fahrenheit Though these types of transducers may be used in medical manufacturing

environments they experience more zero-shift error in response to changes in either ambient or

process temperature than other types of transducers Due to their thicker diaphragms push-rod

transducers are less accurate at low pressures (250 to 500 psi) (Naitove)

For the medical manufacturing market Dynisco has developed a new line of pressure

transducers that encompass all of the features of the previously discussed components in a

FDA acceptable design The newly dubbed ldquoNon-toxic transducersrdquo including a sodium-

potassium liquid fill are up to twice as accurate and repeatable as push-rod and oil-filled types

They are also the most sensitive at low pressures and can handle the highest temperatures (up

to 1000 degrees Fahrenheit) Their fill solution does not degrade over time and they are the

least sensitive to temperature induced zero-shift error The main advantage of NaK sensors is

cost (around $1000 to $1200) and their lack of an explosion-proof rating (Naitove)

Statistical process control can be achieved by using measuring instruments such as those

discussed previously Data can be gathered (typically at 100 scanssec) and rapidly converted

9

by a process controller to provide waveform readings or live trend charges When the data are

viewed statistically deviation trends can be seen allowing process adjustments to be made by

a control feedback loop The most advanced current technology allows two independent loops

to be used Typically one loop controls haul-off or screw speed and the other controls air

pressure or vacuum (Colbert 1996)

Although the use of plastics in medical applications represents less than 2 of total

consumption the high ldquoadded valuerdquo of the final products is of considerable commercial interest

to material suppliers and end processors (Colbert 1996) Increasing product accuracy will lead

to significant reductions in material usage and resultant cost savings For example reducing the

tolerance from +- 008 mm down to +- 001 mm on a 100 mm inner diameter tube with a 0225

mm wall thickness yields material savings of 125 Assuming material costs of approximately

$10000 per ton this would represent cost saving of $1250hr at a 10 kghr production rate

(Colbert 1996)

10

Multilumen Tubing Die Design for Extrusion

Rheological and Mechanical Design of Extrusion Dies

In plastics processing the primary objective of the rheological design of an extrusion die or

distributor is a uniform velocity distribution in the polymer melt at the end of the flow channel

This requirement arises from the desire for a product that changes its dimensions only slightly

due to superimposed local velocity profiles after exiting from the die It is possible to meet this

requirement with an appropriate design for the flow channel (Michaeli Industrial Practice for the

Design of Extrusion Dies 2004)

The mechanical design of extrusion dies the calculation of the forces and deformations arising

during the operation of the die is important for two reasons

1 To assure that the die will not be damaged during operation

2 To assure that the distribution channel retains the geometry established by the

rheological design also during its operation (Michaeli Mechanical Design of Extrusion

Dies 2004)

The important applications of the mechanical design are

Design of screw joints and sealing surfaces with respect to the internal pressure

Design of walls for a permissible deformation by the internal pressure

Design of systems for the adjustment of the geometry of the die in the exit region

(Michaeli Mechanical Design of Extrusion Dies 2004)

The mechanical design is always closely linked to the rheological design First of all in the

rheological design the geometry of the flow channel is defined Then the pressure distribution

must be estimated conservatively by computation of the flow for the die within the expected

operating range with the material of the highest viscosity at the lowest mass temperature and

the highest mass throughput With that the isotropic pressures and shear stresses at the wall

existing in the die are known The forces acting in the die can be calculated from the stresses

and the areas of the flow channel walls The weight of the die also has to be considered in the

design of medium size and large dies (Michaeli Mechanical Design of Extrusion Dies 2004)

Extrusion Die Design Guidelines and Recommendations

In General

A die should consist of as few individual parts as possible in order to minimize the time

needed for assembly and cleaning Care must be taken to center the parts of the die

accuratey relative to each other and the die should be manufactured with close fits

(Michaeli Industrial Practice for the Design of Extrusion Dies 2004)

11

A small number of individual parts contributes to fewer joints in the die body and the flow

channel Fewer joints minimizes the possibility of leaks and therefore the potential for

material to get caught and degrade within them Also joints should be placed in

advantageous cross sections to simplify the cleaning of the die (Michaeli Mechanical

Design of Extrusion Dies 2004)

The sealing surfaces since they cannot be avoided should be as flat and small as

possible in order to assure a uniform distribution of sealing forces over the entire sealing

surface (Michaeli Mechanical Design of Extrusion Dies 2004)

The gap between stationary and moving parts of the die can be sealed by inserting a

gasket cord or an oversized packing strip in a groove in the stationary part of the die

(Michaeli Mechanical Design of Extrusion Dies 2004)

The die should be held together by few large heat resistant bolts (as opposed to many

small ones) since the service life of larger diameter bolts is longer The bolts should be

easily accessible without having to dismantle the strip heaters (Michaeli Mechanical

Design of Extrusion Dies 2004)

For frequently used threaded connections Helicoil inserts should be used to prevent

premature wear and unanticipated thread failures Fasteners should be designed to

withstand a safety factor of at least 200 when the head is exposed to maximum

pressure (Hendess 2002)

The die assembly should be designed so that it can be handled when hot

Component supports should be planned for the disassembly process

Jacking screws allow for simple disassembly of precision components especially

when the head is full of molten polymer (Hendess 2002)

Establishing the Flow Channel Configuration of the Die

When possible the melt should be supplied to the die centrally (Michaeli Mechanical

Design of Extrusion Dies 2004)

There must not be any dead spots or corners in the flow channels (sites of melt

stagnation) Sharp sudden transitions in cross section or changes in direction must be

avoided (Michaeli Mechanical Design of Extrusion Dies 2004)

Flow lines always lead to lower quality extruded products Their formation should be

avoided or diminished and their number reduced by a proper design of the flow channel

(Michaeli Mechanical Design of Extrusion Dies 2004)

Surface finishes for sealing surfaces should be 32 microinch or better Melt flow surfaces

should be 16 microinch or better with 4 to 8 microinch preferred (Hendess 2002)

12

Melt flow channels from component to component should have sharp edges to prevent

melt stagnation areas when assembled

Sharp edges are defined as being less than a 002rdquo break

Gaps between head components greater than 0003rdquo will generally allow polymer

leakage (Hendess 2002)

Die Guidelines and Recommendations Regarding the Processing of Specific Materials

Rigid polyvinylchloride (PVC-U)

Gentle streamlining of melt flow channels

Tight temperature control of the flow channel surfaces

Non-invasive melt temperature measurement

Good corrosion and wear resistance of melt contact surfaces

Use of restrictor bushing not breaker plates for screen pack support (Hendess 2002)

Chlorinated polyvinylchloride (CPVC)

Similar recommendations to PVC-U with exceptions to the following

Extreme streamlining of melt flow channels

High corrosion resistance (Hendess 2002)

Polyolefins

Incorporate strainer basket heads or spiral distributor flow channels to accommodate

high weld line sensitivity (Hendess 2002)

Thermoplastic urethanes (TPU)

Good flow channel streamlining

Temperature control of internal die components

Non-invasive melt temperature measurement (Hendess 2002)

Fluoropolymers

Good flow channel streamlining

Good temperature control of internal die components

Non-invasive melt temperature measurement

Extreme corrosion resistance for melt contact surfaces to withstand hydrogen fluoride

exposure

Internal heating to accommodate extreme melt fracture sensitivity (Hendess 2002)

Polyether-etherkeytones (PEEK) Polysulfones (PS)

Good flow channel streamlining

Tight temperature control of internal die components

Non-invasive melt temperature measurement

Corrosion and wear resistance of melt contact surfaces not of much concern (Hendess

2002)

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

2

Contents The Extrusion Process as Related to Medical Manufacturing 4

Overview of the Medical Extrusion Market and its Associated Technologies 4

Overview of the Medical Tubing Extrusion Process 5

The Extruder5

Drive and Motor 5

Gearbox 5

Feed Section 5

Screw and Barrel 5

Temperature Control 6

Base 6

Melt Pumps Internal Air Control Systems and Crossheads 6

Tube Extrusion Process Control and Evaluation 7

Laser Gages 7

Gamma Backscatter Probes 8

Ultrasonic Reflection8

Pressure Transducers 8

Multilumen Tubing Die Design for Extrusion 10

Rheological and Mechanical Design of Extrusion Dies 10

Extrusion Die Design Guidelines and Recommendations 10

In General 10

Establishing the Flow Channel Configuration of the Die 11

Die Guidelines and Recommendations Regarding the Processing of Specific Materials 12

Materials for Extrusion Dies 13

Draw Down Ratios 14

Diameter Draw Ratio (DDR) 15

Wall Draw Ratio (WDR) 15

Area Draw Ratio (ADR) 15

Draw Ratio Balance (DRB) 16

Sizing Ratio (SR) 17

Land Length 17

Taper Angles 18

Simulation Software and the Die Design Process 18

3

Figures and Graphics 20

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas 22

Works Cited 25

4

The Extrusion Process as Related to Medical Manufacturing

Overview of the Medical Extrusion Market and its Associated Technologies

Extruded medical tubing dates back to the 1930rsquos (Sparacino 1999) Since then the

overarching goal and push for medical device manufacturers has been to create products that

are less invasive Extruded tubing products such as catheters are constantly shrinking in

diameter while being expected to offer more features and greater performance ldquoWhat used to

work as a 6 French catheter is 5 French today and people want it to be a 3 French tomorrowrdquo

(Conley 2006) Moreover new medical extrusion applications are consistently requiring tighter

tolerances additional lumens more sophisticated co-extrusions complex braiding styles and

multi-durometer sections Regarding multilumen tubing new specifications may call for

anywhere up to eight lumens some with wire inside some with braids and some with different

lining methods (Conley 2006) In 2006 die manufacturer Guill Tool amp Engineering indicated

ldquoAs a percentage of the business one of our customers says that about 50 of the requests are

multilumen compared with 25 a year agordquo (Conley 2006)

With the smaller physical size of todayrsquos medical tubing it becomes difficult to retain all of the

desired properties using a single material Increasingly the solution has been co-extrusion

There is also a growing demand for tubing with stiffness that varies with length making tooling

more complex (Conley 2006)

PVC remains the leading polymer for medical tubing with polyurethane polyolefins and blends

or alloys such as thermoplastic elastomers also commonly used For more demanding

applications engineering plastics such as polyamideimide polyester polycarbonate or various

fluoropolymers can be selected Many resins can be compounded with optical or x-ray

opacifiers such as titanium dioxide barium sulfate or bismuth subcarbonate further increasing

the number of potential materials and grades (Colbert 1996)

Within a single tube there could potentially be two or three unique durometers Some original

equipment manufacturers (OEM) are manufacturing braided catheters that require changing the

braid as well as the hardness of the polymer material along the braid (Conley 2006) More

complex yet some catheters utilize alternating durometers braid styles and contain multiple

lumens for various purposes For these requirements the extruder is essentially being

oscillated ndash going from one durometer to the other and back (Conley 2006)

5

Overview of the Medical Tubing Extrusion Process

See the ldquoFigures and Graphics Sectionrdquo for photographs of the various systems described

below

The Extruder

The extruder is considered the main component in a tubing extrusion system There are six

major components that contribute to maintaining a consistent process during daily production

Drive and Motor

This system supplies the power to the screw it must be sized properly to ensure constant speed

and required torque to process the particular polymer Currently most machines come equipped

with AC Vector drives (Sparacino 1999) To accurately maintain diameter and wall thickness of

intricate polymer tubes a uniform flow rate of melt from the extruder must be guaranteed All

extruders producing extremely tight tolerances will exhibit surging as a result of electrical drive

control fluctuations screw design and the normal rheological variation in the polymer To

overcome this a precision rotary gear pump is used to provide steady pressure and accurate

metering of the polymer to the die head in controlled surge-free manner (Colbert 1996)

Gearbox

The gearbox transmits the power from the drivemotor system to the screw Service factor

efficiency lubrication and gear design are of most importance Double reduction gearboxes with

carburized or hardened gears and internal lubrication systems are most desirable (Sparacino

1999)

Feed Section

Two main functions are to isolate the polymer from the barrel heat until it is fed to the screw

and allow the polymer to free flow to the screw A separate machined casting that is cored to

allow cooling with a feed opening more than one diameter long will accomplish both

(Sparacino 1999)

Screw and Barrel

This combination must take the polymer which for tubing is normally in pellet form and convey

melt mix and meter it into the die A barrel made from a steel alloy with a liner cast into it of the

proper composition that will withstand the pressure corrosiveness and abrasiveness of the

melting process is required A barrier screw that is properly designed to separate the solid from

the melted polymer and mix it properly functions best (Sparacino 1999) The matching of

extruder screw design to the melting and rheological characteristics of the plastic to be

processed is fundamental to extruder performance Thus there is no such thing as a ldquogeneral-

purposerdquo screw The key extrusion criteria of output plastication solids conveying and power

consumption are influenced by screw design variables such as channel depth number of flights

helix angle compression ratio flight clearance and flight geometry (Colbert 1996)

6

Temperature Control

Although 80 of the required heat should be put into the polymer by proper screw design the

temperature control system must maintain the level accurately Discrete temperature controllers

with PID settings to control induction heating of the barrel through finned castings with built in

rod heaters are commonly used Microprocessor and Programmable logic controllers are also

available allowing recipe storage and other advantages that will be discussed later In most

medical tubing applications cooling is coordinated with air blowers for each zone (Sparacino

1999)

Base

The base is the foundation for all of the individual components of the extruder Without a sturdy

base and a well-designed barrel support vibration is transferred to the polymer and product

surface finish is compromised (Sparacino 1999)

Melt Pumps Internal Air Control Systems and Crossheads

Melt pumps are necessary in some applications however they introduce to the system another

component that must be controlled and maintained They are necessary when part tolerance is

closer than the +- 1 that a screw alone can produce or when batch to batch material

consistency cannot be guaranteed As stated earlier the die gives the extruded product its initial

shape The transition from the extruder to the die set must be streamlined with no obstacles or

ldquohang-uprdquo points for material to stagnate on The inventory of material in the transition must also

be held to a minimum Calculation for the die size the pin size and the land length must be

made in conjunction with the material being extruded the sizing method to be used and the

final part dimensions (Sparacino 1999)

Internal air control becomes necessary when used as a sizing method for multi-lumen or bump

tube products The die must have built into it a free passage for air to control inner diameters of

the tube and lumens The passage is either through the spider legs in an in line die or through

the core tube in a crosshead die High pressure plan air must be decreased to useable pressure

(usually 5 psi or less) and maintained accurately For bump tube products vacuum sizing does

not aid in the sizing of the tube In a modern day applications the entire system both water and

air are a part of the vacuum system This makes the cooling water weightless as compared to

the tubing passing through (Sparacino 1999)

After the material is prepared it is fed usually by gravity to the rotating screw of an extruder

The polymer is heated fed compressed and metered The homogeneous melt is fed either to a

melt metering pump or directly into an extrusion die in line or crosshead which will give the

thermoplastic mass its initial shape (Sparacino 1999)

The final size of the extruded product can be accomplished in one of three ways

1 Free extrusion where the pressure on the inside of the tube is held at atmospheric level

and the speed of the puller as compared to the speed of the extruder sizes the part

2 Applying a controlled source of compressed air to the inside of the tube as it is cooled

7

3 Vacuum sizing by controlling the pressure outside the tube as compared to the

atmospheric pressure on the inside of the product as it is being cooled (Sparacino

1999)

The production of precision multilumen tubes or the insertion of forming wires or guidewires

requires cross-head die extrusion In this process the polymer melt enters the die at right angles

to the outlet which allows lumen characteristics to be controlled by individual pressurized air

supplies fed from the back of a cross-head die and into the tube via precision-bore injector

needles To ensure that a manufacturerrsquos multilumen tubing will sustain precise flow levels the

extrusion process must include some means of maintaining the consistency of all tube

dimensions A typical tolerance range is plus or minus 1 which for a tube with an internal

diameter of 16mm translates to accuracies of plus or minus 10 micrometers (Colbert 1996)

In a multilumen tube each lumen has a defined end use There are two principal methods used

for controlling the shape of each lumen In the first bore-forming mandrel wires can be inserted

temporarily into the tube as the polymer overlays the mandrels accurate lumens can be formed

by removing the precision-gauge wires after cooling Alternatively separate air-pressure control

for each lumen can be achieved by using air injection needles It is possible to accurately adjust

and maintain pressure differentials at low pressures For example a pressure of 0017 bar can

be maintained to within plus or minus 0002 bar The relative flow rate required to maintain

lumen size at a given die speed can be computed but care must be taken to ensure that the air

supply used for pressure regulation is subject to the same influences as the ambient air

surrounding the extrusion line (Colbert 1996)

Tube Extrusion Process Control and Evaluation

In order to ensure patient satisfaction every tube shipped from the manufacturing facility must

be within specification In order to achieve this requirement the extrusion system can utilize a

number of post processing product evaluation techniques For example the tube can be

measured in line using ultra-sonic sensors to measure the wall thickness or a dual scanner laser

can be used positioned before the puller (Sparacino 1999) However current trends are

towards on-line real-time monitoring of key parameters so as to achieve ldquoprocessed-in qualityrdquo

Automatic control of processing parameters is therefore used to keep the product within

specified quality limits (Colbert 1996) Common real-time process control techniques include

the following

Laser Gages

Laser gages offer accurate and rapid measurement of outside diameter by measuring a shadow

created when the tube obscures a fine beam of rapidly scanning lights Dual-plane laser gages

measure OD in tow planes providing both average OD and ovality with a resolution of 1

micrometer (Sparacino 1999)

8

Gamma Backscatter Probes

These devices use gamma-radiation backscatter to determine wall thickness down to 005 mm

for tubes with diameters as small as 1mm Probes measure wall thickness at a single point

around the tube a number of probes can be used if measurements of multiple points around the

diameter are required (Sparacino 1999)

Ultrasonic Reflection

This technique involves aligning the product in an ultrasonic gage placed in a water bath and

arranging transducers around it Each transducer sends out a transmission pulse that is partially

reflected off the outer wall of the tube While the partial reflection returns to the transducer the

remainder of the initial transmission pulse continues through the product wall The difference in

density between the two pulses allows wall thickness to be calculated Enhancing the signal

with digital processing can allow measurements to an accuracy of plus or minus 5 micrometers

of tubes as small as 10mm OD with wall thicknesses of 013mm or less

Pressure Transducers

As it relates to process control techniques food and medical applications cannot use the most

common type of transducer because it introduces mercury to the process Additionally these

standard oil-filled sensors can fail to operate within specifications after as little as 5 months at

570 degrees Fahrenheit (Naitove) Dynisco LLC a company in Franklin Massachusetts has

developed multiple new pressure transducers that can be utilized in a medical device

manufacturing environment Their new PT528 series has been documented to operate within

specification for almost a year Its maximum operating temperature is 617 degrees Fahrenheit

though the recommended limit for optimum life span is 527 degrees Fahrenheit Its accuracy is

rated at plus or minus 5 with repeatability of 02 at a price of $690 to $1400 (Naitove)

Conventional push-rod transducers have the same accuracy and repeatability specifications as

oil-filled sensors with comparable cost and can be used at temperatures up to 750 degrees

Fahrenheit Though these types of transducers may be used in medical manufacturing

environments they experience more zero-shift error in response to changes in either ambient or

process temperature than other types of transducers Due to their thicker diaphragms push-rod

transducers are less accurate at low pressures (250 to 500 psi) (Naitove)

For the medical manufacturing market Dynisco has developed a new line of pressure

transducers that encompass all of the features of the previously discussed components in a

FDA acceptable design The newly dubbed ldquoNon-toxic transducersrdquo including a sodium-

potassium liquid fill are up to twice as accurate and repeatable as push-rod and oil-filled types

They are also the most sensitive at low pressures and can handle the highest temperatures (up

to 1000 degrees Fahrenheit) Their fill solution does not degrade over time and they are the

least sensitive to temperature induced zero-shift error The main advantage of NaK sensors is

cost (around $1000 to $1200) and their lack of an explosion-proof rating (Naitove)

Statistical process control can be achieved by using measuring instruments such as those

discussed previously Data can be gathered (typically at 100 scanssec) and rapidly converted

9

by a process controller to provide waveform readings or live trend charges When the data are

viewed statistically deviation trends can be seen allowing process adjustments to be made by

a control feedback loop The most advanced current technology allows two independent loops

to be used Typically one loop controls haul-off or screw speed and the other controls air

pressure or vacuum (Colbert 1996)

Although the use of plastics in medical applications represents less than 2 of total

consumption the high ldquoadded valuerdquo of the final products is of considerable commercial interest

to material suppliers and end processors (Colbert 1996) Increasing product accuracy will lead

to significant reductions in material usage and resultant cost savings For example reducing the

tolerance from +- 008 mm down to +- 001 mm on a 100 mm inner diameter tube with a 0225

mm wall thickness yields material savings of 125 Assuming material costs of approximately

$10000 per ton this would represent cost saving of $1250hr at a 10 kghr production rate

(Colbert 1996)

10

Multilumen Tubing Die Design for Extrusion

Rheological and Mechanical Design of Extrusion Dies

In plastics processing the primary objective of the rheological design of an extrusion die or

distributor is a uniform velocity distribution in the polymer melt at the end of the flow channel

This requirement arises from the desire for a product that changes its dimensions only slightly

due to superimposed local velocity profiles after exiting from the die It is possible to meet this

requirement with an appropriate design for the flow channel (Michaeli Industrial Practice for the

Design of Extrusion Dies 2004)

The mechanical design of extrusion dies the calculation of the forces and deformations arising

during the operation of the die is important for two reasons

1 To assure that the die will not be damaged during operation

2 To assure that the distribution channel retains the geometry established by the

rheological design also during its operation (Michaeli Mechanical Design of Extrusion

Dies 2004)

The important applications of the mechanical design are

Design of screw joints and sealing surfaces with respect to the internal pressure

Design of walls for a permissible deformation by the internal pressure

Design of systems for the adjustment of the geometry of the die in the exit region

(Michaeli Mechanical Design of Extrusion Dies 2004)

The mechanical design is always closely linked to the rheological design First of all in the

rheological design the geometry of the flow channel is defined Then the pressure distribution

must be estimated conservatively by computation of the flow for the die within the expected

operating range with the material of the highest viscosity at the lowest mass temperature and

the highest mass throughput With that the isotropic pressures and shear stresses at the wall

existing in the die are known The forces acting in the die can be calculated from the stresses

and the areas of the flow channel walls The weight of the die also has to be considered in the

design of medium size and large dies (Michaeli Mechanical Design of Extrusion Dies 2004)

Extrusion Die Design Guidelines and Recommendations

In General

A die should consist of as few individual parts as possible in order to minimize the time

needed for assembly and cleaning Care must be taken to center the parts of the die

accuratey relative to each other and the die should be manufactured with close fits

(Michaeli Industrial Practice for the Design of Extrusion Dies 2004)

11

A small number of individual parts contributes to fewer joints in the die body and the flow

channel Fewer joints minimizes the possibility of leaks and therefore the potential for

material to get caught and degrade within them Also joints should be placed in

advantageous cross sections to simplify the cleaning of the die (Michaeli Mechanical

Design of Extrusion Dies 2004)

The sealing surfaces since they cannot be avoided should be as flat and small as

possible in order to assure a uniform distribution of sealing forces over the entire sealing

surface (Michaeli Mechanical Design of Extrusion Dies 2004)

The gap between stationary and moving parts of the die can be sealed by inserting a

gasket cord or an oversized packing strip in a groove in the stationary part of the die

(Michaeli Mechanical Design of Extrusion Dies 2004)

The die should be held together by few large heat resistant bolts (as opposed to many

small ones) since the service life of larger diameter bolts is longer The bolts should be

easily accessible without having to dismantle the strip heaters (Michaeli Mechanical

Design of Extrusion Dies 2004)

For frequently used threaded connections Helicoil inserts should be used to prevent

premature wear and unanticipated thread failures Fasteners should be designed to

withstand a safety factor of at least 200 when the head is exposed to maximum

pressure (Hendess 2002)

The die assembly should be designed so that it can be handled when hot

Component supports should be planned for the disassembly process

Jacking screws allow for simple disassembly of precision components especially

when the head is full of molten polymer (Hendess 2002)

Establishing the Flow Channel Configuration of the Die

When possible the melt should be supplied to the die centrally (Michaeli Mechanical

Design of Extrusion Dies 2004)

There must not be any dead spots or corners in the flow channels (sites of melt

stagnation) Sharp sudden transitions in cross section or changes in direction must be

avoided (Michaeli Mechanical Design of Extrusion Dies 2004)

Flow lines always lead to lower quality extruded products Their formation should be

avoided or diminished and their number reduced by a proper design of the flow channel

(Michaeli Mechanical Design of Extrusion Dies 2004)

Surface finishes for sealing surfaces should be 32 microinch or better Melt flow surfaces

should be 16 microinch or better with 4 to 8 microinch preferred (Hendess 2002)

12

Melt flow channels from component to component should have sharp edges to prevent

melt stagnation areas when assembled

Sharp edges are defined as being less than a 002rdquo break

Gaps between head components greater than 0003rdquo will generally allow polymer

leakage (Hendess 2002)

Die Guidelines and Recommendations Regarding the Processing of Specific Materials

Rigid polyvinylchloride (PVC-U)

Gentle streamlining of melt flow channels

Tight temperature control of the flow channel surfaces

Non-invasive melt temperature measurement

Good corrosion and wear resistance of melt contact surfaces

Use of restrictor bushing not breaker plates for screen pack support (Hendess 2002)

Chlorinated polyvinylchloride (CPVC)

Similar recommendations to PVC-U with exceptions to the following

Extreme streamlining of melt flow channels

High corrosion resistance (Hendess 2002)

Polyolefins

Incorporate strainer basket heads or spiral distributor flow channels to accommodate

high weld line sensitivity (Hendess 2002)

Thermoplastic urethanes (TPU)

Good flow channel streamlining

Temperature control of internal die components

Non-invasive melt temperature measurement (Hendess 2002)

Fluoropolymers

Good flow channel streamlining

Good temperature control of internal die components

Non-invasive melt temperature measurement

Extreme corrosion resistance for melt contact surfaces to withstand hydrogen fluoride

exposure

Internal heating to accommodate extreme melt fracture sensitivity (Hendess 2002)

Polyether-etherkeytones (PEEK) Polysulfones (PS)

Good flow channel streamlining

Tight temperature control of internal die components

Non-invasive melt temperature measurement

Corrosion and wear resistance of melt contact surfaces not of much concern (Hendess

2002)

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

3

Figures and Graphics 20

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas 22

Works Cited 25

4

The Extrusion Process as Related to Medical Manufacturing

Overview of the Medical Extrusion Market and its Associated Technologies

Extruded medical tubing dates back to the 1930rsquos (Sparacino 1999) Since then the

overarching goal and push for medical device manufacturers has been to create products that

are less invasive Extruded tubing products such as catheters are constantly shrinking in

diameter while being expected to offer more features and greater performance ldquoWhat used to

work as a 6 French catheter is 5 French today and people want it to be a 3 French tomorrowrdquo

(Conley 2006) Moreover new medical extrusion applications are consistently requiring tighter

tolerances additional lumens more sophisticated co-extrusions complex braiding styles and

multi-durometer sections Regarding multilumen tubing new specifications may call for

anywhere up to eight lumens some with wire inside some with braids and some with different

lining methods (Conley 2006) In 2006 die manufacturer Guill Tool amp Engineering indicated

ldquoAs a percentage of the business one of our customers says that about 50 of the requests are

multilumen compared with 25 a year agordquo (Conley 2006)

With the smaller physical size of todayrsquos medical tubing it becomes difficult to retain all of the

desired properties using a single material Increasingly the solution has been co-extrusion

There is also a growing demand for tubing with stiffness that varies with length making tooling

more complex (Conley 2006)

PVC remains the leading polymer for medical tubing with polyurethane polyolefins and blends

or alloys such as thermoplastic elastomers also commonly used For more demanding

applications engineering plastics such as polyamideimide polyester polycarbonate or various

fluoropolymers can be selected Many resins can be compounded with optical or x-ray

opacifiers such as titanium dioxide barium sulfate or bismuth subcarbonate further increasing

the number of potential materials and grades (Colbert 1996)

Within a single tube there could potentially be two or three unique durometers Some original

equipment manufacturers (OEM) are manufacturing braided catheters that require changing the

braid as well as the hardness of the polymer material along the braid (Conley 2006) More

complex yet some catheters utilize alternating durometers braid styles and contain multiple

lumens for various purposes For these requirements the extruder is essentially being

oscillated ndash going from one durometer to the other and back (Conley 2006)

5

Overview of the Medical Tubing Extrusion Process

See the ldquoFigures and Graphics Sectionrdquo for photographs of the various systems described

below

The Extruder

The extruder is considered the main component in a tubing extrusion system There are six

major components that contribute to maintaining a consistent process during daily production

Drive and Motor

This system supplies the power to the screw it must be sized properly to ensure constant speed

and required torque to process the particular polymer Currently most machines come equipped

with AC Vector drives (Sparacino 1999) To accurately maintain diameter and wall thickness of

intricate polymer tubes a uniform flow rate of melt from the extruder must be guaranteed All

extruders producing extremely tight tolerances will exhibit surging as a result of electrical drive

control fluctuations screw design and the normal rheological variation in the polymer To

overcome this a precision rotary gear pump is used to provide steady pressure and accurate

metering of the polymer to the die head in controlled surge-free manner (Colbert 1996)

Gearbox

The gearbox transmits the power from the drivemotor system to the screw Service factor

efficiency lubrication and gear design are of most importance Double reduction gearboxes with

carburized or hardened gears and internal lubrication systems are most desirable (Sparacino

1999)

Feed Section

Two main functions are to isolate the polymer from the barrel heat until it is fed to the screw

and allow the polymer to free flow to the screw A separate machined casting that is cored to

allow cooling with a feed opening more than one diameter long will accomplish both

(Sparacino 1999)

Screw and Barrel

This combination must take the polymer which for tubing is normally in pellet form and convey

melt mix and meter it into the die A barrel made from a steel alloy with a liner cast into it of the

proper composition that will withstand the pressure corrosiveness and abrasiveness of the

melting process is required A barrier screw that is properly designed to separate the solid from

the melted polymer and mix it properly functions best (Sparacino 1999) The matching of

extruder screw design to the melting and rheological characteristics of the plastic to be

processed is fundamental to extruder performance Thus there is no such thing as a ldquogeneral-

purposerdquo screw The key extrusion criteria of output plastication solids conveying and power

consumption are influenced by screw design variables such as channel depth number of flights

helix angle compression ratio flight clearance and flight geometry (Colbert 1996)

6

Temperature Control

Although 80 of the required heat should be put into the polymer by proper screw design the

temperature control system must maintain the level accurately Discrete temperature controllers

with PID settings to control induction heating of the barrel through finned castings with built in

rod heaters are commonly used Microprocessor and Programmable logic controllers are also

available allowing recipe storage and other advantages that will be discussed later In most

medical tubing applications cooling is coordinated with air blowers for each zone (Sparacino

1999)

Base

The base is the foundation for all of the individual components of the extruder Without a sturdy

base and a well-designed barrel support vibration is transferred to the polymer and product

surface finish is compromised (Sparacino 1999)

Melt Pumps Internal Air Control Systems and Crossheads

Melt pumps are necessary in some applications however they introduce to the system another

component that must be controlled and maintained They are necessary when part tolerance is

closer than the +- 1 that a screw alone can produce or when batch to batch material

consistency cannot be guaranteed As stated earlier the die gives the extruded product its initial

shape The transition from the extruder to the die set must be streamlined with no obstacles or

ldquohang-uprdquo points for material to stagnate on The inventory of material in the transition must also

be held to a minimum Calculation for the die size the pin size and the land length must be

made in conjunction with the material being extruded the sizing method to be used and the

final part dimensions (Sparacino 1999)

Internal air control becomes necessary when used as a sizing method for multi-lumen or bump

tube products The die must have built into it a free passage for air to control inner diameters of

the tube and lumens The passage is either through the spider legs in an in line die or through

the core tube in a crosshead die High pressure plan air must be decreased to useable pressure

(usually 5 psi or less) and maintained accurately For bump tube products vacuum sizing does

not aid in the sizing of the tube In a modern day applications the entire system both water and

air are a part of the vacuum system This makes the cooling water weightless as compared to

the tubing passing through (Sparacino 1999)

After the material is prepared it is fed usually by gravity to the rotating screw of an extruder

The polymer is heated fed compressed and metered The homogeneous melt is fed either to a

melt metering pump or directly into an extrusion die in line or crosshead which will give the

thermoplastic mass its initial shape (Sparacino 1999)

The final size of the extruded product can be accomplished in one of three ways

1 Free extrusion where the pressure on the inside of the tube is held at atmospheric level

and the speed of the puller as compared to the speed of the extruder sizes the part

2 Applying a controlled source of compressed air to the inside of the tube as it is cooled

7

3 Vacuum sizing by controlling the pressure outside the tube as compared to the

atmospheric pressure on the inside of the product as it is being cooled (Sparacino

1999)

The production of precision multilumen tubes or the insertion of forming wires or guidewires

requires cross-head die extrusion In this process the polymer melt enters the die at right angles

to the outlet which allows lumen characteristics to be controlled by individual pressurized air

supplies fed from the back of a cross-head die and into the tube via precision-bore injector

needles To ensure that a manufacturerrsquos multilumen tubing will sustain precise flow levels the

extrusion process must include some means of maintaining the consistency of all tube

dimensions A typical tolerance range is plus or minus 1 which for a tube with an internal

diameter of 16mm translates to accuracies of plus or minus 10 micrometers (Colbert 1996)

In a multilumen tube each lumen has a defined end use There are two principal methods used

for controlling the shape of each lumen In the first bore-forming mandrel wires can be inserted

temporarily into the tube as the polymer overlays the mandrels accurate lumens can be formed

by removing the precision-gauge wires after cooling Alternatively separate air-pressure control

for each lumen can be achieved by using air injection needles It is possible to accurately adjust

and maintain pressure differentials at low pressures For example a pressure of 0017 bar can

be maintained to within plus or minus 0002 bar The relative flow rate required to maintain

lumen size at a given die speed can be computed but care must be taken to ensure that the air

supply used for pressure regulation is subject to the same influences as the ambient air

surrounding the extrusion line (Colbert 1996)

Tube Extrusion Process Control and Evaluation

In order to ensure patient satisfaction every tube shipped from the manufacturing facility must

be within specification In order to achieve this requirement the extrusion system can utilize a

number of post processing product evaluation techniques For example the tube can be

measured in line using ultra-sonic sensors to measure the wall thickness or a dual scanner laser

can be used positioned before the puller (Sparacino 1999) However current trends are

towards on-line real-time monitoring of key parameters so as to achieve ldquoprocessed-in qualityrdquo

Automatic control of processing parameters is therefore used to keep the product within

specified quality limits (Colbert 1996) Common real-time process control techniques include

the following

Laser Gages

Laser gages offer accurate and rapid measurement of outside diameter by measuring a shadow

created when the tube obscures a fine beam of rapidly scanning lights Dual-plane laser gages

measure OD in tow planes providing both average OD and ovality with a resolution of 1

micrometer (Sparacino 1999)

8

Gamma Backscatter Probes

These devices use gamma-radiation backscatter to determine wall thickness down to 005 mm

for tubes with diameters as small as 1mm Probes measure wall thickness at a single point

around the tube a number of probes can be used if measurements of multiple points around the

diameter are required (Sparacino 1999)

Ultrasonic Reflection

This technique involves aligning the product in an ultrasonic gage placed in a water bath and

arranging transducers around it Each transducer sends out a transmission pulse that is partially

reflected off the outer wall of the tube While the partial reflection returns to the transducer the

remainder of the initial transmission pulse continues through the product wall The difference in

density between the two pulses allows wall thickness to be calculated Enhancing the signal

with digital processing can allow measurements to an accuracy of plus or minus 5 micrometers

of tubes as small as 10mm OD with wall thicknesses of 013mm or less

Pressure Transducers

As it relates to process control techniques food and medical applications cannot use the most

common type of transducer because it introduces mercury to the process Additionally these

standard oil-filled sensors can fail to operate within specifications after as little as 5 months at

570 degrees Fahrenheit (Naitove) Dynisco LLC a company in Franklin Massachusetts has

developed multiple new pressure transducers that can be utilized in a medical device

manufacturing environment Their new PT528 series has been documented to operate within

specification for almost a year Its maximum operating temperature is 617 degrees Fahrenheit

though the recommended limit for optimum life span is 527 degrees Fahrenheit Its accuracy is

rated at plus or minus 5 with repeatability of 02 at a price of $690 to $1400 (Naitove)

Conventional push-rod transducers have the same accuracy and repeatability specifications as

oil-filled sensors with comparable cost and can be used at temperatures up to 750 degrees

Fahrenheit Though these types of transducers may be used in medical manufacturing

environments they experience more zero-shift error in response to changes in either ambient or

process temperature than other types of transducers Due to their thicker diaphragms push-rod

transducers are less accurate at low pressures (250 to 500 psi) (Naitove)

For the medical manufacturing market Dynisco has developed a new line of pressure

transducers that encompass all of the features of the previously discussed components in a

FDA acceptable design The newly dubbed ldquoNon-toxic transducersrdquo including a sodium-

potassium liquid fill are up to twice as accurate and repeatable as push-rod and oil-filled types

They are also the most sensitive at low pressures and can handle the highest temperatures (up

to 1000 degrees Fahrenheit) Their fill solution does not degrade over time and they are the

least sensitive to temperature induced zero-shift error The main advantage of NaK sensors is

cost (around $1000 to $1200) and their lack of an explosion-proof rating (Naitove)

Statistical process control can be achieved by using measuring instruments such as those

discussed previously Data can be gathered (typically at 100 scanssec) and rapidly converted

9

by a process controller to provide waveform readings or live trend charges When the data are

viewed statistically deviation trends can be seen allowing process adjustments to be made by

a control feedback loop The most advanced current technology allows two independent loops

to be used Typically one loop controls haul-off or screw speed and the other controls air

pressure or vacuum (Colbert 1996)

Although the use of plastics in medical applications represents less than 2 of total

consumption the high ldquoadded valuerdquo of the final products is of considerable commercial interest

to material suppliers and end processors (Colbert 1996) Increasing product accuracy will lead

to significant reductions in material usage and resultant cost savings For example reducing the

tolerance from +- 008 mm down to +- 001 mm on a 100 mm inner diameter tube with a 0225

mm wall thickness yields material savings of 125 Assuming material costs of approximately

$10000 per ton this would represent cost saving of $1250hr at a 10 kghr production rate

(Colbert 1996)

10

Multilumen Tubing Die Design for Extrusion

Rheological and Mechanical Design of Extrusion Dies

In plastics processing the primary objective of the rheological design of an extrusion die or

distributor is a uniform velocity distribution in the polymer melt at the end of the flow channel

This requirement arises from the desire for a product that changes its dimensions only slightly

due to superimposed local velocity profiles after exiting from the die It is possible to meet this

requirement with an appropriate design for the flow channel (Michaeli Industrial Practice for the

Design of Extrusion Dies 2004)

The mechanical design of extrusion dies the calculation of the forces and deformations arising

during the operation of the die is important for two reasons

1 To assure that the die will not be damaged during operation

2 To assure that the distribution channel retains the geometry established by the

rheological design also during its operation (Michaeli Mechanical Design of Extrusion

Dies 2004)

The important applications of the mechanical design are

Design of screw joints and sealing surfaces with respect to the internal pressure

Design of walls for a permissible deformation by the internal pressure

Design of systems for the adjustment of the geometry of the die in the exit region

(Michaeli Mechanical Design of Extrusion Dies 2004)

The mechanical design is always closely linked to the rheological design First of all in the

rheological design the geometry of the flow channel is defined Then the pressure distribution

must be estimated conservatively by computation of the flow for the die within the expected

operating range with the material of the highest viscosity at the lowest mass temperature and

the highest mass throughput With that the isotropic pressures and shear stresses at the wall

existing in the die are known The forces acting in the die can be calculated from the stresses

and the areas of the flow channel walls The weight of the die also has to be considered in the

design of medium size and large dies (Michaeli Mechanical Design of Extrusion Dies 2004)

Extrusion Die Design Guidelines and Recommendations

In General

A die should consist of as few individual parts as possible in order to minimize the time

needed for assembly and cleaning Care must be taken to center the parts of the die

accuratey relative to each other and the die should be manufactured with close fits

(Michaeli Industrial Practice for the Design of Extrusion Dies 2004)

11

A small number of individual parts contributes to fewer joints in the die body and the flow

channel Fewer joints minimizes the possibility of leaks and therefore the potential for

material to get caught and degrade within them Also joints should be placed in

advantageous cross sections to simplify the cleaning of the die (Michaeli Mechanical

Design of Extrusion Dies 2004)

The sealing surfaces since they cannot be avoided should be as flat and small as

possible in order to assure a uniform distribution of sealing forces over the entire sealing

surface (Michaeli Mechanical Design of Extrusion Dies 2004)

The gap between stationary and moving parts of the die can be sealed by inserting a

gasket cord or an oversized packing strip in a groove in the stationary part of the die

(Michaeli Mechanical Design of Extrusion Dies 2004)

The die should be held together by few large heat resistant bolts (as opposed to many

small ones) since the service life of larger diameter bolts is longer The bolts should be

easily accessible without having to dismantle the strip heaters (Michaeli Mechanical

Design of Extrusion Dies 2004)

For frequently used threaded connections Helicoil inserts should be used to prevent

premature wear and unanticipated thread failures Fasteners should be designed to

withstand a safety factor of at least 200 when the head is exposed to maximum

pressure (Hendess 2002)

The die assembly should be designed so that it can be handled when hot

Component supports should be planned for the disassembly process

Jacking screws allow for simple disassembly of precision components especially

when the head is full of molten polymer (Hendess 2002)

Establishing the Flow Channel Configuration of the Die

When possible the melt should be supplied to the die centrally (Michaeli Mechanical

Design of Extrusion Dies 2004)

There must not be any dead spots or corners in the flow channels (sites of melt

stagnation) Sharp sudden transitions in cross section or changes in direction must be

avoided (Michaeli Mechanical Design of Extrusion Dies 2004)

Flow lines always lead to lower quality extruded products Their formation should be

avoided or diminished and their number reduced by a proper design of the flow channel

(Michaeli Mechanical Design of Extrusion Dies 2004)

Surface finishes for sealing surfaces should be 32 microinch or better Melt flow surfaces

should be 16 microinch or better with 4 to 8 microinch preferred (Hendess 2002)

12

Melt flow channels from component to component should have sharp edges to prevent

melt stagnation areas when assembled

Sharp edges are defined as being less than a 002rdquo break

Gaps between head components greater than 0003rdquo will generally allow polymer

leakage (Hendess 2002)

Die Guidelines and Recommendations Regarding the Processing of Specific Materials

Rigid polyvinylchloride (PVC-U)

Gentle streamlining of melt flow channels

Tight temperature control of the flow channel surfaces

Non-invasive melt temperature measurement

Good corrosion and wear resistance of melt contact surfaces

Use of restrictor bushing not breaker plates for screen pack support (Hendess 2002)

Chlorinated polyvinylchloride (CPVC)

Similar recommendations to PVC-U with exceptions to the following

Extreme streamlining of melt flow channels

High corrosion resistance (Hendess 2002)

Polyolefins

Incorporate strainer basket heads or spiral distributor flow channels to accommodate

high weld line sensitivity (Hendess 2002)

Thermoplastic urethanes (TPU)

Good flow channel streamlining

Temperature control of internal die components

Non-invasive melt temperature measurement (Hendess 2002)

Fluoropolymers

Good flow channel streamlining

Good temperature control of internal die components

Non-invasive melt temperature measurement

Extreme corrosion resistance for melt contact surfaces to withstand hydrogen fluoride

exposure

Internal heating to accommodate extreme melt fracture sensitivity (Hendess 2002)

Polyether-etherkeytones (PEEK) Polysulfones (PS)

Good flow channel streamlining

Tight temperature control of internal die components

Non-invasive melt temperature measurement

Corrosion and wear resistance of melt contact surfaces not of much concern (Hendess

2002)

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

4

The Extrusion Process as Related to Medical Manufacturing

Overview of the Medical Extrusion Market and its Associated Technologies

Extruded medical tubing dates back to the 1930rsquos (Sparacino 1999) Since then the

overarching goal and push for medical device manufacturers has been to create products that

are less invasive Extruded tubing products such as catheters are constantly shrinking in

diameter while being expected to offer more features and greater performance ldquoWhat used to

work as a 6 French catheter is 5 French today and people want it to be a 3 French tomorrowrdquo

(Conley 2006) Moreover new medical extrusion applications are consistently requiring tighter

tolerances additional lumens more sophisticated co-extrusions complex braiding styles and

multi-durometer sections Regarding multilumen tubing new specifications may call for

anywhere up to eight lumens some with wire inside some with braids and some with different

lining methods (Conley 2006) In 2006 die manufacturer Guill Tool amp Engineering indicated

ldquoAs a percentage of the business one of our customers says that about 50 of the requests are

multilumen compared with 25 a year agordquo (Conley 2006)

With the smaller physical size of todayrsquos medical tubing it becomes difficult to retain all of the

desired properties using a single material Increasingly the solution has been co-extrusion

There is also a growing demand for tubing with stiffness that varies with length making tooling

more complex (Conley 2006)

PVC remains the leading polymer for medical tubing with polyurethane polyolefins and blends

or alloys such as thermoplastic elastomers also commonly used For more demanding

applications engineering plastics such as polyamideimide polyester polycarbonate or various

fluoropolymers can be selected Many resins can be compounded with optical or x-ray

opacifiers such as titanium dioxide barium sulfate or bismuth subcarbonate further increasing

the number of potential materials and grades (Colbert 1996)

Within a single tube there could potentially be two or three unique durometers Some original

equipment manufacturers (OEM) are manufacturing braided catheters that require changing the

braid as well as the hardness of the polymer material along the braid (Conley 2006) More

complex yet some catheters utilize alternating durometers braid styles and contain multiple

lumens for various purposes For these requirements the extruder is essentially being

oscillated ndash going from one durometer to the other and back (Conley 2006)

5

Overview of the Medical Tubing Extrusion Process

See the ldquoFigures and Graphics Sectionrdquo for photographs of the various systems described

below

The Extruder

The extruder is considered the main component in a tubing extrusion system There are six

major components that contribute to maintaining a consistent process during daily production

Drive and Motor

This system supplies the power to the screw it must be sized properly to ensure constant speed

and required torque to process the particular polymer Currently most machines come equipped

with AC Vector drives (Sparacino 1999) To accurately maintain diameter and wall thickness of

intricate polymer tubes a uniform flow rate of melt from the extruder must be guaranteed All

extruders producing extremely tight tolerances will exhibit surging as a result of electrical drive

control fluctuations screw design and the normal rheological variation in the polymer To

overcome this a precision rotary gear pump is used to provide steady pressure and accurate

metering of the polymer to the die head in controlled surge-free manner (Colbert 1996)

Gearbox

The gearbox transmits the power from the drivemotor system to the screw Service factor

efficiency lubrication and gear design are of most importance Double reduction gearboxes with

carburized or hardened gears and internal lubrication systems are most desirable (Sparacino

1999)

Feed Section

Two main functions are to isolate the polymer from the barrel heat until it is fed to the screw

and allow the polymer to free flow to the screw A separate machined casting that is cored to

allow cooling with a feed opening more than one diameter long will accomplish both

(Sparacino 1999)

Screw and Barrel

This combination must take the polymer which for tubing is normally in pellet form and convey

melt mix and meter it into the die A barrel made from a steel alloy with a liner cast into it of the

proper composition that will withstand the pressure corrosiveness and abrasiveness of the

melting process is required A barrier screw that is properly designed to separate the solid from

the melted polymer and mix it properly functions best (Sparacino 1999) The matching of

extruder screw design to the melting and rheological characteristics of the plastic to be

processed is fundamental to extruder performance Thus there is no such thing as a ldquogeneral-

purposerdquo screw The key extrusion criteria of output plastication solids conveying and power

consumption are influenced by screw design variables such as channel depth number of flights

helix angle compression ratio flight clearance and flight geometry (Colbert 1996)

6

Temperature Control

Although 80 of the required heat should be put into the polymer by proper screw design the

temperature control system must maintain the level accurately Discrete temperature controllers

with PID settings to control induction heating of the barrel through finned castings with built in

rod heaters are commonly used Microprocessor and Programmable logic controllers are also

available allowing recipe storage and other advantages that will be discussed later In most

medical tubing applications cooling is coordinated with air blowers for each zone (Sparacino

1999)

Base

The base is the foundation for all of the individual components of the extruder Without a sturdy

base and a well-designed barrel support vibration is transferred to the polymer and product

surface finish is compromised (Sparacino 1999)

Melt Pumps Internal Air Control Systems and Crossheads

Melt pumps are necessary in some applications however they introduce to the system another

component that must be controlled and maintained They are necessary when part tolerance is

closer than the +- 1 that a screw alone can produce or when batch to batch material

consistency cannot be guaranteed As stated earlier the die gives the extruded product its initial

shape The transition from the extruder to the die set must be streamlined with no obstacles or

ldquohang-uprdquo points for material to stagnate on The inventory of material in the transition must also

be held to a minimum Calculation for the die size the pin size and the land length must be

made in conjunction with the material being extruded the sizing method to be used and the

final part dimensions (Sparacino 1999)

Internal air control becomes necessary when used as a sizing method for multi-lumen or bump

tube products The die must have built into it a free passage for air to control inner diameters of

the tube and lumens The passage is either through the spider legs in an in line die or through

the core tube in a crosshead die High pressure plan air must be decreased to useable pressure

(usually 5 psi or less) and maintained accurately For bump tube products vacuum sizing does

not aid in the sizing of the tube In a modern day applications the entire system both water and

air are a part of the vacuum system This makes the cooling water weightless as compared to

the tubing passing through (Sparacino 1999)

After the material is prepared it is fed usually by gravity to the rotating screw of an extruder

The polymer is heated fed compressed and metered The homogeneous melt is fed either to a

melt metering pump or directly into an extrusion die in line or crosshead which will give the

thermoplastic mass its initial shape (Sparacino 1999)

The final size of the extruded product can be accomplished in one of three ways

1 Free extrusion where the pressure on the inside of the tube is held at atmospheric level

and the speed of the puller as compared to the speed of the extruder sizes the part

2 Applying a controlled source of compressed air to the inside of the tube as it is cooled

7

3 Vacuum sizing by controlling the pressure outside the tube as compared to the

atmospheric pressure on the inside of the product as it is being cooled (Sparacino

1999)

The production of precision multilumen tubes or the insertion of forming wires or guidewires

requires cross-head die extrusion In this process the polymer melt enters the die at right angles

to the outlet which allows lumen characteristics to be controlled by individual pressurized air

supplies fed from the back of a cross-head die and into the tube via precision-bore injector

needles To ensure that a manufacturerrsquos multilumen tubing will sustain precise flow levels the

extrusion process must include some means of maintaining the consistency of all tube

dimensions A typical tolerance range is plus or minus 1 which for a tube with an internal

diameter of 16mm translates to accuracies of plus or minus 10 micrometers (Colbert 1996)

In a multilumen tube each lumen has a defined end use There are two principal methods used

for controlling the shape of each lumen In the first bore-forming mandrel wires can be inserted

temporarily into the tube as the polymer overlays the mandrels accurate lumens can be formed

by removing the precision-gauge wires after cooling Alternatively separate air-pressure control

for each lumen can be achieved by using air injection needles It is possible to accurately adjust

and maintain pressure differentials at low pressures For example a pressure of 0017 bar can

be maintained to within plus or minus 0002 bar The relative flow rate required to maintain

lumen size at a given die speed can be computed but care must be taken to ensure that the air

supply used for pressure regulation is subject to the same influences as the ambient air

surrounding the extrusion line (Colbert 1996)

Tube Extrusion Process Control and Evaluation

In order to ensure patient satisfaction every tube shipped from the manufacturing facility must

be within specification In order to achieve this requirement the extrusion system can utilize a

number of post processing product evaluation techniques For example the tube can be

measured in line using ultra-sonic sensors to measure the wall thickness or a dual scanner laser

can be used positioned before the puller (Sparacino 1999) However current trends are

towards on-line real-time monitoring of key parameters so as to achieve ldquoprocessed-in qualityrdquo

Automatic control of processing parameters is therefore used to keep the product within

specified quality limits (Colbert 1996) Common real-time process control techniques include

the following

Laser Gages

Laser gages offer accurate and rapid measurement of outside diameter by measuring a shadow

created when the tube obscures a fine beam of rapidly scanning lights Dual-plane laser gages

measure OD in tow planes providing both average OD and ovality with a resolution of 1

micrometer (Sparacino 1999)

8

Gamma Backscatter Probes

These devices use gamma-radiation backscatter to determine wall thickness down to 005 mm

for tubes with diameters as small as 1mm Probes measure wall thickness at a single point

around the tube a number of probes can be used if measurements of multiple points around the

diameter are required (Sparacino 1999)

Ultrasonic Reflection

This technique involves aligning the product in an ultrasonic gage placed in a water bath and

arranging transducers around it Each transducer sends out a transmission pulse that is partially

reflected off the outer wall of the tube While the partial reflection returns to the transducer the

remainder of the initial transmission pulse continues through the product wall The difference in

density between the two pulses allows wall thickness to be calculated Enhancing the signal

with digital processing can allow measurements to an accuracy of plus or minus 5 micrometers

of tubes as small as 10mm OD with wall thicknesses of 013mm or less

Pressure Transducers

As it relates to process control techniques food and medical applications cannot use the most

common type of transducer because it introduces mercury to the process Additionally these

standard oil-filled sensors can fail to operate within specifications after as little as 5 months at

570 degrees Fahrenheit (Naitove) Dynisco LLC a company in Franklin Massachusetts has

developed multiple new pressure transducers that can be utilized in a medical device

manufacturing environment Their new PT528 series has been documented to operate within

specification for almost a year Its maximum operating temperature is 617 degrees Fahrenheit

though the recommended limit for optimum life span is 527 degrees Fahrenheit Its accuracy is

rated at plus or minus 5 with repeatability of 02 at a price of $690 to $1400 (Naitove)

Conventional push-rod transducers have the same accuracy and repeatability specifications as

oil-filled sensors with comparable cost and can be used at temperatures up to 750 degrees

Fahrenheit Though these types of transducers may be used in medical manufacturing

environments they experience more zero-shift error in response to changes in either ambient or

process temperature than other types of transducers Due to their thicker diaphragms push-rod

transducers are less accurate at low pressures (250 to 500 psi) (Naitove)

For the medical manufacturing market Dynisco has developed a new line of pressure

transducers that encompass all of the features of the previously discussed components in a

FDA acceptable design The newly dubbed ldquoNon-toxic transducersrdquo including a sodium-

potassium liquid fill are up to twice as accurate and repeatable as push-rod and oil-filled types

They are also the most sensitive at low pressures and can handle the highest temperatures (up

to 1000 degrees Fahrenheit) Their fill solution does not degrade over time and they are the

least sensitive to temperature induced zero-shift error The main advantage of NaK sensors is

cost (around $1000 to $1200) and their lack of an explosion-proof rating (Naitove)

Statistical process control can be achieved by using measuring instruments such as those

discussed previously Data can be gathered (typically at 100 scanssec) and rapidly converted

9

by a process controller to provide waveform readings or live trend charges When the data are

viewed statistically deviation trends can be seen allowing process adjustments to be made by

a control feedback loop The most advanced current technology allows two independent loops

to be used Typically one loop controls haul-off or screw speed and the other controls air

pressure or vacuum (Colbert 1996)

Although the use of plastics in medical applications represents less than 2 of total

consumption the high ldquoadded valuerdquo of the final products is of considerable commercial interest

to material suppliers and end processors (Colbert 1996) Increasing product accuracy will lead

to significant reductions in material usage and resultant cost savings For example reducing the

tolerance from +- 008 mm down to +- 001 mm on a 100 mm inner diameter tube with a 0225

mm wall thickness yields material savings of 125 Assuming material costs of approximately

$10000 per ton this would represent cost saving of $1250hr at a 10 kghr production rate

(Colbert 1996)

10

Multilumen Tubing Die Design for Extrusion

Rheological and Mechanical Design of Extrusion Dies

In plastics processing the primary objective of the rheological design of an extrusion die or

distributor is a uniform velocity distribution in the polymer melt at the end of the flow channel

This requirement arises from the desire for a product that changes its dimensions only slightly

due to superimposed local velocity profiles after exiting from the die It is possible to meet this

requirement with an appropriate design for the flow channel (Michaeli Industrial Practice for the

Design of Extrusion Dies 2004)

The mechanical design of extrusion dies the calculation of the forces and deformations arising

during the operation of the die is important for two reasons

1 To assure that the die will not be damaged during operation

2 To assure that the distribution channel retains the geometry established by the

rheological design also during its operation (Michaeli Mechanical Design of Extrusion

Dies 2004)

The important applications of the mechanical design are

Design of screw joints and sealing surfaces with respect to the internal pressure

Design of walls for a permissible deformation by the internal pressure

Design of systems for the adjustment of the geometry of the die in the exit region

(Michaeli Mechanical Design of Extrusion Dies 2004)

The mechanical design is always closely linked to the rheological design First of all in the

rheological design the geometry of the flow channel is defined Then the pressure distribution

must be estimated conservatively by computation of the flow for the die within the expected

operating range with the material of the highest viscosity at the lowest mass temperature and

the highest mass throughput With that the isotropic pressures and shear stresses at the wall

existing in the die are known The forces acting in the die can be calculated from the stresses

and the areas of the flow channel walls The weight of the die also has to be considered in the

design of medium size and large dies (Michaeli Mechanical Design of Extrusion Dies 2004)

Extrusion Die Design Guidelines and Recommendations

In General

A die should consist of as few individual parts as possible in order to minimize the time

needed for assembly and cleaning Care must be taken to center the parts of the die

accuratey relative to each other and the die should be manufactured with close fits

(Michaeli Industrial Practice for the Design of Extrusion Dies 2004)

11

A small number of individual parts contributes to fewer joints in the die body and the flow

channel Fewer joints minimizes the possibility of leaks and therefore the potential for

material to get caught and degrade within them Also joints should be placed in

advantageous cross sections to simplify the cleaning of the die (Michaeli Mechanical

Design of Extrusion Dies 2004)

The sealing surfaces since they cannot be avoided should be as flat and small as

possible in order to assure a uniform distribution of sealing forces over the entire sealing

surface (Michaeli Mechanical Design of Extrusion Dies 2004)

The gap between stationary and moving parts of the die can be sealed by inserting a

gasket cord or an oversized packing strip in a groove in the stationary part of the die

(Michaeli Mechanical Design of Extrusion Dies 2004)

The die should be held together by few large heat resistant bolts (as opposed to many

small ones) since the service life of larger diameter bolts is longer The bolts should be

easily accessible without having to dismantle the strip heaters (Michaeli Mechanical

Design of Extrusion Dies 2004)

For frequently used threaded connections Helicoil inserts should be used to prevent

premature wear and unanticipated thread failures Fasteners should be designed to

withstand a safety factor of at least 200 when the head is exposed to maximum

pressure (Hendess 2002)

The die assembly should be designed so that it can be handled when hot

Component supports should be planned for the disassembly process

Jacking screws allow for simple disassembly of precision components especially

when the head is full of molten polymer (Hendess 2002)

Establishing the Flow Channel Configuration of the Die

When possible the melt should be supplied to the die centrally (Michaeli Mechanical

Design of Extrusion Dies 2004)

There must not be any dead spots or corners in the flow channels (sites of melt

stagnation) Sharp sudden transitions in cross section or changes in direction must be

avoided (Michaeli Mechanical Design of Extrusion Dies 2004)

Flow lines always lead to lower quality extruded products Their formation should be

avoided or diminished and their number reduced by a proper design of the flow channel

(Michaeli Mechanical Design of Extrusion Dies 2004)

Surface finishes for sealing surfaces should be 32 microinch or better Melt flow surfaces

should be 16 microinch or better with 4 to 8 microinch preferred (Hendess 2002)

12

Melt flow channels from component to component should have sharp edges to prevent

melt stagnation areas when assembled

Sharp edges are defined as being less than a 002rdquo break

Gaps between head components greater than 0003rdquo will generally allow polymer

leakage (Hendess 2002)

Die Guidelines and Recommendations Regarding the Processing of Specific Materials

Rigid polyvinylchloride (PVC-U)

Gentle streamlining of melt flow channels

Tight temperature control of the flow channel surfaces

Non-invasive melt temperature measurement

Good corrosion and wear resistance of melt contact surfaces

Use of restrictor bushing not breaker plates for screen pack support (Hendess 2002)

Chlorinated polyvinylchloride (CPVC)

Similar recommendations to PVC-U with exceptions to the following

Extreme streamlining of melt flow channels

High corrosion resistance (Hendess 2002)

Polyolefins

Incorporate strainer basket heads or spiral distributor flow channels to accommodate

high weld line sensitivity (Hendess 2002)

Thermoplastic urethanes (TPU)

Good flow channel streamlining

Temperature control of internal die components

Non-invasive melt temperature measurement (Hendess 2002)

Fluoropolymers

Good flow channel streamlining

Good temperature control of internal die components

Non-invasive melt temperature measurement

Extreme corrosion resistance for melt contact surfaces to withstand hydrogen fluoride

exposure

Internal heating to accommodate extreme melt fracture sensitivity (Hendess 2002)

Polyether-etherkeytones (PEEK) Polysulfones (PS)

Good flow channel streamlining

Tight temperature control of internal die components

Non-invasive melt temperature measurement

Corrosion and wear resistance of melt contact surfaces not of much concern (Hendess

2002)

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

5

Overview of the Medical Tubing Extrusion Process

See the ldquoFigures and Graphics Sectionrdquo for photographs of the various systems described

below

The Extruder

The extruder is considered the main component in a tubing extrusion system There are six

major components that contribute to maintaining a consistent process during daily production

Drive and Motor

This system supplies the power to the screw it must be sized properly to ensure constant speed

and required torque to process the particular polymer Currently most machines come equipped

with AC Vector drives (Sparacino 1999) To accurately maintain diameter and wall thickness of

intricate polymer tubes a uniform flow rate of melt from the extruder must be guaranteed All

extruders producing extremely tight tolerances will exhibit surging as a result of electrical drive

control fluctuations screw design and the normal rheological variation in the polymer To

overcome this a precision rotary gear pump is used to provide steady pressure and accurate

metering of the polymer to the die head in controlled surge-free manner (Colbert 1996)

Gearbox

The gearbox transmits the power from the drivemotor system to the screw Service factor

efficiency lubrication and gear design are of most importance Double reduction gearboxes with

carburized or hardened gears and internal lubrication systems are most desirable (Sparacino

1999)

Feed Section

Two main functions are to isolate the polymer from the barrel heat until it is fed to the screw

and allow the polymer to free flow to the screw A separate machined casting that is cored to

allow cooling with a feed opening more than one diameter long will accomplish both

(Sparacino 1999)

Screw and Barrel

This combination must take the polymer which for tubing is normally in pellet form and convey

melt mix and meter it into the die A barrel made from a steel alloy with a liner cast into it of the

proper composition that will withstand the pressure corrosiveness and abrasiveness of the

melting process is required A barrier screw that is properly designed to separate the solid from

the melted polymer and mix it properly functions best (Sparacino 1999) The matching of

extruder screw design to the melting and rheological characteristics of the plastic to be

processed is fundamental to extruder performance Thus there is no such thing as a ldquogeneral-

purposerdquo screw The key extrusion criteria of output plastication solids conveying and power

consumption are influenced by screw design variables such as channel depth number of flights

helix angle compression ratio flight clearance and flight geometry (Colbert 1996)

6

Temperature Control

Although 80 of the required heat should be put into the polymer by proper screw design the

temperature control system must maintain the level accurately Discrete temperature controllers

with PID settings to control induction heating of the barrel through finned castings with built in

rod heaters are commonly used Microprocessor and Programmable logic controllers are also

available allowing recipe storage and other advantages that will be discussed later In most

medical tubing applications cooling is coordinated with air blowers for each zone (Sparacino

1999)

Base

The base is the foundation for all of the individual components of the extruder Without a sturdy

base and a well-designed barrel support vibration is transferred to the polymer and product

surface finish is compromised (Sparacino 1999)

Melt Pumps Internal Air Control Systems and Crossheads

Melt pumps are necessary in some applications however they introduce to the system another

component that must be controlled and maintained They are necessary when part tolerance is

closer than the +- 1 that a screw alone can produce or when batch to batch material

consistency cannot be guaranteed As stated earlier the die gives the extruded product its initial

shape The transition from the extruder to the die set must be streamlined with no obstacles or

ldquohang-uprdquo points for material to stagnate on The inventory of material in the transition must also

be held to a minimum Calculation for the die size the pin size and the land length must be

made in conjunction with the material being extruded the sizing method to be used and the

final part dimensions (Sparacino 1999)

Internal air control becomes necessary when used as a sizing method for multi-lumen or bump

tube products The die must have built into it a free passage for air to control inner diameters of

the tube and lumens The passage is either through the spider legs in an in line die or through

the core tube in a crosshead die High pressure plan air must be decreased to useable pressure

(usually 5 psi or less) and maintained accurately For bump tube products vacuum sizing does

not aid in the sizing of the tube In a modern day applications the entire system both water and

air are a part of the vacuum system This makes the cooling water weightless as compared to

the tubing passing through (Sparacino 1999)

After the material is prepared it is fed usually by gravity to the rotating screw of an extruder

The polymer is heated fed compressed and metered The homogeneous melt is fed either to a

melt metering pump or directly into an extrusion die in line or crosshead which will give the

thermoplastic mass its initial shape (Sparacino 1999)

The final size of the extruded product can be accomplished in one of three ways

1 Free extrusion where the pressure on the inside of the tube is held at atmospheric level

and the speed of the puller as compared to the speed of the extruder sizes the part

2 Applying a controlled source of compressed air to the inside of the tube as it is cooled

7

3 Vacuum sizing by controlling the pressure outside the tube as compared to the

atmospheric pressure on the inside of the product as it is being cooled (Sparacino

1999)

The production of precision multilumen tubes or the insertion of forming wires or guidewires

requires cross-head die extrusion In this process the polymer melt enters the die at right angles

to the outlet which allows lumen characteristics to be controlled by individual pressurized air

supplies fed from the back of a cross-head die and into the tube via precision-bore injector

needles To ensure that a manufacturerrsquos multilumen tubing will sustain precise flow levels the

extrusion process must include some means of maintaining the consistency of all tube

dimensions A typical tolerance range is plus or minus 1 which for a tube with an internal

diameter of 16mm translates to accuracies of plus or minus 10 micrometers (Colbert 1996)

In a multilumen tube each lumen has a defined end use There are two principal methods used

for controlling the shape of each lumen In the first bore-forming mandrel wires can be inserted

temporarily into the tube as the polymer overlays the mandrels accurate lumens can be formed

by removing the precision-gauge wires after cooling Alternatively separate air-pressure control

for each lumen can be achieved by using air injection needles It is possible to accurately adjust

and maintain pressure differentials at low pressures For example a pressure of 0017 bar can

be maintained to within plus or minus 0002 bar The relative flow rate required to maintain

lumen size at a given die speed can be computed but care must be taken to ensure that the air

supply used for pressure regulation is subject to the same influences as the ambient air

surrounding the extrusion line (Colbert 1996)

Tube Extrusion Process Control and Evaluation

In order to ensure patient satisfaction every tube shipped from the manufacturing facility must

be within specification In order to achieve this requirement the extrusion system can utilize a

number of post processing product evaluation techniques For example the tube can be

measured in line using ultra-sonic sensors to measure the wall thickness or a dual scanner laser

can be used positioned before the puller (Sparacino 1999) However current trends are

towards on-line real-time monitoring of key parameters so as to achieve ldquoprocessed-in qualityrdquo

Automatic control of processing parameters is therefore used to keep the product within

specified quality limits (Colbert 1996) Common real-time process control techniques include

the following

Laser Gages

Laser gages offer accurate and rapid measurement of outside diameter by measuring a shadow

created when the tube obscures a fine beam of rapidly scanning lights Dual-plane laser gages

measure OD in tow planes providing both average OD and ovality with a resolution of 1

micrometer (Sparacino 1999)

8

Gamma Backscatter Probes

These devices use gamma-radiation backscatter to determine wall thickness down to 005 mm

for tubes with diameters as small as 1mm Probes measure wall thickness at a single point

around the tube a number of probes can be used if measurements of multiple points around the

diameter are required (Sparacino 1999)

Ultrasonic Reflection

This technique involves aligning the product in an ultrasonic gage placed in a water bath and

arranging transducers around it Each transducer sends out a transmission pulse that is partially

reflected off the outer wall of the tube While the partial reflection returns to the transducer the

remainder of the initial transmission pulse continues through the product wall The difference in

density between the two pulses allows wall thickness to be calculated Enhancing the signal

with digital processing can allow measurements to an accuracy of plus or minus 5 micrometers

of tubes as small as 10mm OD with wall thicknesses of 013mm or less

Pressure Transducers

As it relates to process control techniques food and medical applications cannot use the most

common type of transducer because it introduces mercury to the process Additionally these

standard oil-filled sensors can fail to operate within specifications after as little as 5 months at

570 degrees Fahrenheit (Naitove) Dynisco LLC a company in Franklin Massachusetts has

developed multiple new pressure transducers that can be utilized in a medical device

manufacturing environment Their new PT528 series has been documented to operate within

specification for almost a year Its maximum operating temperature is 617 degrees Fahrenheit

though the recommended limit for optimum life span is 527 degrees Fahrenheit Its accuracy is

rated at plus or minus 5 with repeatability of 02 at a price of $690 to $1400 (Naitove)

Conventional push-rod transducers have the same accuracy and repeatability specifications as

oil-filled sensors with comparable cost and can be used at temperatures up to 750 degrees

Fahrenheit Though these types of transducers may be used in medical manufacturing

environments they experience more zero-shift error in response to changes in either ambient or

process temperature than other types of transducers Due to their thicker diaphragms push-rod

transducers are less accurate at low pressures (250 to 500 psi) (Naitove)

For the medical manufacturing market Dynisco has developed a new line of pressure

transducers that encompass all of the features of the previously discussed components in a

FDA acceptable design The newly dubbed ldquoNon-toxic transducersrdquo including a sodium-

potassium liquid fill are up to twice as accurate and repeatable as push-rod and oil-filled types

They are also the most sensitive at low pressures and can handle the highest temperatures (up

to 1000 degrees Fahrenheit) Their fill solution does not degrade over time and they are the

least sensitive to temperature induced zero-shift error The main advantage of NaK sensors is

cost (around $1000 to $1200) and their lack of an explosion-proof rating (Naitove)

Statistical process control can be achieved by using measuring instruments such as those

discussed previously Data can be gathered (typically at 100 scanssec) and rapidly converted

9

by a process controller to provide waveform readings or live trend charges When the data are

viewed statistically deviation trends can be seen allowing process adjustments to be made by

a control feedback loop The most advanced current technology allows two independent loops

to be used Typically one loop controls haul-off or screw speed and the other controls air

pressure or vacuum (Colbert 1996)

Although the use of plastics in medical applications represents less than 2 of total

consumption the high ldquoadded valuerdquo of the final products is of considerable commercial interest

to material suppliers and end processors (Colbert 1996) Increasing product accuracy will lead

to significant reductions in material usage and resultant cost savings For example reducing the

tolerance from +- 008 mm down to +- 001 mm on a 100 mm inner diameter tube with a 0225

mm wall thickness yields material savings of 125 Assuming material costs of approximately

$10000 per ton this would represent cost saving of $1250hr at a 10 kghr production rate

(Colbert 1996)

10

Multilumen Tubing Die Design for Extrusion

Rheological and Mechanical Design of Extrusion Dies

In plastics processing the primary objective of the rheological design of an extrusion die or

distributor is a uniform velocity distribution in the polymer melt at the end of the flow channel

This requirement arises from the desire for a product that changes its dimensions only slightly

due to superimposed local velocity profiles after exiting from the die It is possible to meet this

requirement with an appropriate design for the flow channel (Michaeli Industrial Practice for the

Design of Extrusion Dies 2004)

The mechanical design of extrusion dies the calculation of the forces and deformations arising

during the operation of the die is important for two reasons

1 To assure that the die will not be damaged during operation

2 To assure that the distribution channel retains the geometry established by the

rheological design also during its operation (Michaeli Mechanical Design of Extrusion

Dies 2004)

The important applications of the mechanical design are

Design of screw joints and sealing surfaces with respect to the internal pressure

Design of walls for a permissible deformation by the internal pressure

Design of systems for the adjustment of the geometry of the die in the exit region

(Michaeli Mechanical Design of Extrusion Dies 2004)

The mechanical design is always closely linked to the rheological design First of all in the

rheological design the geometry of the flow channel is defined Then the pressure distribution

must be estimated conservatively by computation of the flow for the die within the expected

operating range with the material of the highest viscosity at the lowest mass temperature and

the highest mass throughput With that the isotropic pressures and shear stresses at the wall

existing in the die are known The forces acting in the die can be calculated from the stresses

and the areas of the flow channel walls The weight of the die also has to be considered in the

design of medium size and large dies (Michaeli Mechanical Design of Extrusion Dies 2004)

Extrusion Die Design Guidelines and Recommendations

In General

A die should consist of as few individual parts as possible in order to minimize the time

needed for assembly and cleaning Care must be taken to center the parts of the die

accuratey relative to each other and the die should be manufactured with close fits

(Michaeli Industrial Practice for the Design of Extrusion Dies 2004)

11

A small number of individual parts contributes to fewer joints in the die body and the flow

channel Fewer joints minimizes the possibility of leaks and therefore the potential for

material to get caught and degrade within them Also joints should be placed in

advantageous cross sections to simplify the cleaning of the die (Michaeli Mechanical

Design of Extrusion Dies 2004)

The sealing surfaces since they cannot be avoided should be as flat and small as

possible in order to assure a uniform distribution of sealing forces over the entire sealing

surface (Michaeli Mechanical Design of Extrusion Dies 2004)

The gap between stationary and moving parts of the die can be sealed by inserting a

gasket cord or an oversized packing strip in a groove in the stationary part of the die

(Michaeli Mechanical Design of Extrusion Dies 2004)

The die should be held together by few large heat resistant bolts (as opposed to many

small ones) since the service life of larger diameter bolts is longer The bolts should be

easily accessible without having to dismantle the strip heaters (Michaeli Mechanical

Design of Extrusion Dies 2004)

For frequently used threaded connections Helicoil inserts should be used to prevent

premature wear and unanticipated thread failures Fasteners should be designed to

withstand a safety factor of at least 200 when the head is exposed to maximum

pressure (Hendess 2002)

The die assembly should be designed so that it can be handled when hot

Component supports should be planned for the disassembly process

Jacking screws allow for simple disassembly of precision components especially

when the head is full of molten polymer (Hendess 2002)

Establishing the Flow Channel Configuration of the Die

When possible the melt should be supplied to the die centrally (Michaeli Mechanical

Design of Extrusion Dies 2004)

There must not be any dead spots or corners in the flow channels (sites of melt

stagnation) Sharp sudden transitions in cross section or changes in direction must be

avoided (Michaeli Mechanical Design of Extrusion Dies 2004)

Flow lines always lead to lower quality extruded products Their formation should be

avoided or diminished and their number reduced by a proper design of the flow channel

(Michaeli Mechanical Design of Extrusion Dies 2004)

Surface finishes for sealing surfaces should be 32 microinch or better Melt flow surfaces

should be 16 microinch or better with 4 to 8 microinch preferred (Hendess 2002)

12

Melt flow channels from component to component should have sharp edges to prevent

melt stagnation areas when assembled

Sharp edges are defined as being less than a 002rdquo break

Gaps between head components greater than 0003rdquo will generally allow polymer

leakage (Hendess 2002)

Die Guidelines and Recommendations Regarding the Processing of Specific Materials

Rigid polyvinylchloride (PVC-U)

Gentle streamlining of melt flow channels

Tight temperature control of the flow channel surfaces

Non-invasive melt temperature measurement

Good corrosion and wear resistance of melt contact surfaces

Use of restrictor bushing not breaker plates for screen pack support (Hendess 2002)

Chlorinated polyvinylchloride (CPVC)

Similar recommendations to PVC-U with exceptions to the following

Extreme streamlining of melt flow channels

High corrosion resistance (Hendess 2002)

Polyolefins

Incorporate strainer basket heads or spiral distributor flow channels to accommodate

high weld line sensitivity (Hendess 2002)

Thermoplastic urethanes (TPU)

Good flow channel streamlining

Temperature control of internal die components

Non-invasive melt temperature measurement (Hendess 2002)

Fluoropolymers

Good flow channel streamlining

Good temperature control of internal die components

Non-invasive melt temperature measurement

Extreme corrosion resistance for melt contact surfaces to withstand hydrogen fluoride

exposure

Internal heating to accommodate extreme melt fracture sensitivity (Hendess 2002)

Polyether-etherkeytones (PEEK) Polysulfones (PS)

Good flow channel streamlining

Tight temperature control of internal die components

Non-invasive melt temperature measurement

Corrosion and wear resistance of melt contact surfaces not of much concern (Hendess

2002)

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

6

Temperature Control

Although 80 of the required heat should be put into the polymer by proper screw design the

temperature control system must maintain the level accurately Discrete temperature controllers

with PID settings to control induction heating of the barrel through finned castings with built in

rod heaters are commonly used Microprocessor and Programmable logic controllers are also

available allowing recipe storage and other advantages that will be discussed later In most

medical tubing applications cooling is coordinated with air blowers for each zone (Sparacino

1999)

Base

The base is the foundation for all of the individual components of the extruder Without a sturdy

base and a well-designed barrel support vibration is transferred to the polymer and product

surface finish is compromised (Sparacino 1999)

Melt Pumps Internal Air Control Systems and Crossheads

Melt pumps are necessary in some applications however they introduce to the system another

component that must be controlled and maintained They are necessary when part tolerance is

closer than the +- 1 that a screw alone can produce or when batch to batch material

consistency cannot be guaranteed As stated earlier the die gives the extruded product its initial

shape The transition from the extruder to the die set must be streamlined with no obstacles or

ldquohang-uprdquo points for material to stagnate on The inventory of material in the transition must also

be held to a minimum Calculation for the die size the pin size and the land length must be

made in conjunction with the material being extruded the sizing method to be used and the

final part dimensions (Sparacino 1999)

Internal air control becomes necessary when used as a sizing method for multi-lumen or bump

tube products The die must have built into it a free passage for air to control inner diameters of

the tube and lumens The passage is either through the spider legs in an in line die or through

the core tube in a crosshead die High pressure plan air must be decreased to useable pressure

(usually 5 psi or less) and maintained accurately For bump tube products vacuum sizing does

not aid in the sizing of the tube In a modern day applications the entire system both water and

air are a part of the vacuum system This makes the cooling water weightless as compared to

the tubing passing through (Sparacino 1999)

After the material is prepared it is fed usually by gravity to the rotating screw of an extruder

The polymer is heated fed compressed and metered The homogeneous melt is fed either to a

melt metering pump or directly into an extrusion die in line or crosshead which will give the

thermoplastic mass its initial shape (Sparacino 1999)

The final size of the extruded product can be accomplished in one of three ways

1 Free extrusion where the pressure on the inside of the tube is held at atmospheric level

and the speed of the puller as compared to the speed of the extruder sizes the part

2 Applying a controlled source of compressed air to the inside of the tube as it is cooled

7

3 Vacuum sizing by controlling the pressure outside the tube as compared to the

atmospheric pressure on the inside of the product as it is being cooled (Sparacino

1999)

The production of precision multilumen tubes or the insertion of forming wires or guidewires

requires cross-head die extrusion In this process the polymer melt enters the die at right angles

to the outlet which allows lumen characteristics to be controlled by individual pressurized air

supplies fed from the back of a cross-head die and into the tube via precision-bore injector

needles To ensure that a manufacturerrsquos multilumen tubing will sustain precise flow levels the

extrusion process must include some means of maintaining the consistency of all tube

dimensions A typical tolerance range is plus or minus 1 which for a tube with an internal

diameter of 16mm translates to accuracies of plus or minus 10 micrometers (Colbert 1996)

In a multilumen tube each lumen has a defined end use There are two principal methods used

for controlling the shape of each lumen In the first bore-forming mandrel wires can be inserted

temporarily into the tube as the polymer overlays the mandrels accurate lumens can be formed

by removing the precision-gauge wires after cooling Alternatively separate air-pressure control

for each lumen can be achieved by using air injection needles It is possible to accurately adjust

and maintain pressure differentials at low pressures For example a pressure of 0017 bar can

be maintained to within plus or minus 0002 bar The relative flow rate required to maintain

lumen size at a given die speed can be computed but care must be taken to ensure that the air

supply used for pressure regulation is subject to the same influences as the ambient air

surrounding the extrusion line (Colbert 1996)

Tube Extrusion Process Control and Evaluation

In order to ensure patient satisfaction every tube shipped from the manufacturing facility must

be within specification In order to achieve this requirement the extrusion system can utilize a

number of post processing product evaluation techniques For example the tube can be

measured in line using ultra-sonic sensors to measure the wall thickness or a dual scanner laser

can be used positioned before the puller (Sparacino 1999) However current trends are

towards on-line real-time monitoring of key parameters so as to achieve ldquoprocessed-in qualityrdquo

Automatic control of processing parameters is therefore used to keep the product within

specified quality limits (Colbert 1996) Common real-time process control techniques include

the following

Laser Gages

Laser gages offer accurate and rapid measurement of outside diameter by measuring a shadow

created when the tube obscures a fine beam of rapidly scanning lights Dual-plane laser gages

measure OD in tow planes providing both average OD and ovality with a resolution of 1

micrometer (Sparacino 1999)

8

Gamma Backscatter Probes

These devices use gamma-radiation backscatter to determine wall thickness down to 005 mm

for tubes with diameters as small as 1mm Probes measure wall thickness at a single point

around the tube a number of probes can be used if measurements of multiple points around the

diameter are required (Sparacino 1999)

Ultrasonic Reflection

This technique involves aligning the product in an ultrasonic gage placed in a water bath and

arranging transducers around it Each transducer sends out a transmission pulse that is partially

reflected off the outer wall of the tube While the partial reflection returns to the transducer the

remainder of the initial transmission pulse continues through the product wall The difference in

density between the two pulses allows wall thickness to be calculated Enhancing the signal

with digital processing can allow measurements to an accuracy of plus or minus 5 micrometers

of tubes as small as 10mm OD with wall thicknesses of 013mm or less

Pressure Transducers

As it relates to process control techniques food and medical applications cannot use the most

common type of transducer because it introduces mercury to the process Additionally these

standard oil-filled sensors can fail to operate within specifications after as little as 5 months at

570 degrees Fahrenheit (Naitove) Dynisco LLC a company in Franklin Massachusetts has

developed multiple new pressure transducers that can be utilized in a medical device

manufacturing environment Their new PT528 series has been documented to operate within

specification for almost a year Its maximum operating temperature is 617 degrees Fahrenheit

though the recommended limit for optimum life span is 527 degrees Fahrenheit Its accuracy is

rated at plus or minus 5 with repeatability of 02 at a price of $690 to $1400 (Naitove)

Conventional push-rod transducers have the same accuracy and repeatability specifications as

oil-filled sensors with comparable cost and can be used at temperatures up to 750 degrees

Fahrenheit Though these types of transducers may be used in medical manufacturing

environments they experience more zero-shift error in response to changes in either ambient or

process temperature than other types of transducers Due to their thicker diaphragms push-rod

transducers are less accurate at low pressures (250 to 500 psi) (Naitove)

For the medical manufacturing market Dynisco has developed a new line of pressure

transducers that encompass all of the features of the previously discussed components in a

FDA acceptable design The newly dubbed ldquoNon-toxic transducersrdquo including a sodium-

potassium liquid fill are up to twice as accurate and repeatable as push-rod and oil-filled types

They are also the most sensitive at low pressures and can handle the highest temperatures (up

to 1000 degrees Fahrenheit) Their fill solution does not degrade over time and they are the

least sensitive to temperature induced zero-shift error The main advantage of NaK sensors is

cost (around $1000 to $1200) and their lack of an explosion-proof rating (Naitove)

Statistical process control can be achieved by using measuring instruments such as those

discussed previously Data can be gathered (typically at 100 scanssec) and rapidly converted

9

by a process controller to provide waveform readings or live trend charges When the data are

viewed statistically deviation trends can be seen allowing process adjustments to be made by

a control feedback loop The most advanced current technology allows two independent loops

to be used Typically one loop controls haul-off or screw speed and the other controls air

pressure or vacuum (Colbert 1996)

Although the use of plastics in medical applications represents less than 2 of total

consumption the high ldquoadded valuerdquo of the final products is of considerable commercial interest

to material suppliers and end processors (Colbert 1996) Increasing product accuracy will lead

to significant reductions in material usage and resultant cost savings For example reducing the

tolerance from +- 008 mm down to +- 001 mm on a 100 mm inner diameter tube with a 0225

mm wall thickness yields material savings of 125 Assuming material costs of approximately

$10000 per ton this would represent cost saving of $1250hr at a 10 kghr production rate

(Colbert 1996)

10

Multilumen Tubing Die Design for Extrusion

Rheological and Mechanical Design of Extrusion Dies

In plastics processing the primary objective of the rheological design of an extrusion die or

distributor is a uniform velocity distribution in the polymer melt at the end of the flow channel

This requirement arises from the desire for a product that changes its dimensions only slightly

due to superimposed local velocity profiles after exiting from the die It is possible to meet this

requirement with an appropriate design for the flow channel (Michaeli Industrial Practice for the

Design of Extrusion Dies 2004)

The mechanical design of extrusion dies the calculation of the forces and deformations arising

during the operation of the die is important for two reasons

1 To assure that the die will not be damaged during operation

2 To assure that the distribution channel retains the geometry established by the

rheological design also during its operation (Michaeli Mechanical Design of Extrusion

Dies 2004)

The important applications of the mechanical design are

Design of screw joints and sealing surfaces with respect to the internal pressure

Design of walls for a permissible deformation by the internal pressure

Design of systems for the adjustment of the geometry of the die in the exit region

(Michaeli Mechanical Design of Extrusion Dies 2004)

The mechanical design is always closely linked to the rheological design First of all in the

rheological design the geometry of the flow channel is defined Then the pressure distribution

must be estimated conservatively by computation of the flow for the die within the expected

operating range with the material of the highest viscosity at the lowest mass temperature and

the highest mass throughput With that the isotropic pressures and shear stresses at the wall

existing in the die are known The forces acting in the die can be calculated from the stresses

and the areas of the flow channel walls The weight of the die also has to be considered in the

design of medium size and large dies (Michaeli Mechanical Design of Extrusion Dies 2004)

Extrusion Die Design Guidelines and Recommendations

In General

A die should consist of as few individual parts as possible in order to minimize the time

needed for assembly and cleaning Care must be taken to center the parts of the die

accuratey relative to each other and the die should be manufactured with close fits

(Michaeli Industrial Practice for the Design of Extrusion Dies 2004)

11

A small number of individual parts contributes to fewer joints in the die body and the flow

channel Fewer joints minimizes the possibility of leaks and therefore the potential for

material to get caught and degrade within them Also joints should be placed in

advantageous cross sections to simplify the cleaning of the die (Michaeli Mechanical

Design of Extrusion Dies 2004)

The sealing surfaces since they cannot be avoided should be as flat and small as

possible in order to assure a uniform distribution of sealing forces over the entire sealing

surface (Michaeli Mechanical Design of Extrusion Dies 2004)

The gap between stationary and moving parts of the die can be sealed by inserting a

gasket cord or an oversized packing strip in a groove in the stationary part of the die

(Michaeli Mechanical Design of Extrusion Dies 2004)

The die should be held together by few large heat resistant bolts (as opposed to many

small ones) since the service life of larger diameter bolts is longer The bolts should be

easily accessible without having to dismantle the strip heaters (Michaeli Mechanical

Design of Extrusion Dies 2004)

For frequently used threaded connections Helicoil inserts should be used to prevent

premature wear and unanticipated thread failures Fasteners should be designed to

withstand a safety factor of at least 200 when the head is exposed to maximum

pressure (Hendess 2002)

The die assembly should be designed so that it can be handled when hot

Component supports should be planned for the disassembly process

Jacking screws allow for simple disassembly of precision components especially

when the head is full of molten polymer (Hendess 2002)

Establishing the Flow Channel Configuration of the Die

When possible the melt should be supplied to the die centrally (Michaeli Mechanical

Design of Extrusion Dies 2004)

There must not be any dead spots or corners in the flow channels (sites of melt

stagnation) Sharp sudden transitions in cross section or changes in direction must be

avoided (Michaeli Mechanical Design of Extrusion Dies 2004)

Flow lines always lead to lower quality extruded products Their formation should be

avoided or diminished and their number reduced by a proper design of the flow channel

(Michaeli Mechanical Design of Extrusion Dies 2004)

Surface finishes for sealing surfaces should be 32 microinch or better Melt flow surfaces

should be 16 microinch or better with 4 to 8 microinch preferred (Hendess 2002)

12

Melt flow channels from component to component should have sharp edges to prevent

melt stagnation areas when assembled

Sharp edges are defined as being less than a 002rdquo break

Gaps between head components greater than 0003rdquo will generally allow polymer

leakage (Hendess 2002)

Die Guidelines and Recommendations Regarding the Processing of Specific Materials

Rigid polyvinylchloride (PVC-U)

Gentle streamlining of melt flow channels

Tight temperature control of the flow channel surfaces

Non-invasive melt temperature measurement

Good corrosion and wear resistance of melt contact surfaces

Use of restrictor bushing not breaker plates for screen pack support (Hendess 2002)

Chlorinated polyvinylchloride (CPVC)

Similar recommendations to PVC-U with exceptions to the following

Extreme streamlining of melt flow channels

High corrosion resistance (Hendess 2002)

Polyolefins

Incorporate strainer basket heads or spiral distributor flow channels to accommodate

high weld line sensitivity (Hendess 2002)

Thermoplastic urethanes (TPU)

Good flow channel streamlining

Temperature control of internal die components

Non-invasive melt temperature measurement (Hendess 2002)

Fluoropolymers

Good flow channel streamlining

Good temperature control of internal die components

Non-invasive melt temperature measurement

Extreme corrosion resistance for melt contact surfaces to withstand hydrogen fluoride

exposure

Internal heating to accommodate extreme melt fracture sensitivity (Hendess 2002)

Polyether-etherkeytones (PEEK) Polysulfones (PS)

Good flow channel streamlining

Tight temperature control of internal die components

Non-invasive melt temperature measurement

Corrosion and wear resistance of melt contact surfaces not of much concern (Hendess

2002)

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

7

3 Vacuum sizing by controlling the pressure outside the tube as compared to the

atmospheric pressure on the inside of the product as it is being cooled (Sparacino

1999)

The production of precision multilumen tubes or the insertion of forming wires or guidewires

requires cross-head die extrusion In this process the polymer melt enters the die at right angles

to the outlet which allows lumen characteristics to be controlled by individual pressurized air

supplies fed from the back of a cross-head die and into the tube via precision-bore injector

needles To ensure that a manufacturerrsquos multilumen tubing will sustain precise flow levels the

extrusion process must include some means of maintaining the consistency of all tube

dimensions A typical tolerance range is plus or minus 1 which for a tube with an internal

diameter of 16mm translates to accuracies of plus or minus 10 micrometers (Colbert 1996)

In a multilumen tube each lumen has a defined end use There are two principal methods used

for controlling the shape of each lumen In the first bore-forming mandrel wires can be inserted

temporarily into the tube as the polymer overlays the mandrels accurate lumens can be formed

by removing the precision-gauge wires after cooling Alternatively separate air-pressure control

for each lumen can be achieved by using air injection needles It is possible to accurately adjust

and maintain pressure differentials at low pressures For example a pressure of 0017 bar can

be maintained to within plus or minus 0002 bar The relative flow rate required to maintain

lumen size at a given die speed can be computed but care must be taken to ensure that the air

supply used for pressure regulation is subject to the same influences as the ambient air

surrounding the extrusion line (Colbert 1996)

Tube Extrusion Process Control and Evaluation

In order to ensure patient satisfaction every tube shipped from the manufacturing facility must

be within specification In order to achieve this requirement the extrusion system can utilize a

number of post processing product evaluation techniques For example the tube can be

measured in line using ultra-sonic sensors to measure the wall thickness or a dual scanner laser

can be used positioned before the puller (Sparacino 1999) However current trends are

towards on-line real-time monitoring of key parameters so as to achieve ldquoprocessed-in qualityrdquo

Automatic control of processing parameters is therefore used to keep the product within

specified quality limits (Colbert 1996) Common real-time process control techniques include

the following

Laser Gages

Laser gages offer accurate and rapid measurement of outside diameter by measuring a shadow

created when the tube obscures a fine beam of rapidly scanning lights Dual-plane laser gages

measure OD in tow planes providing both average OD and ovality with a resolution of 1

micrometer (Sparacino 1999)

8

Gamma Backscatter Probes

These devices use gamma-radiation backscatter to determine wall thickness down to 005 mm

for tubes with diameters as small as 1mm Probes measure wall thickness at a single point

around the tube a number of probes can be used if measurements of multiple points around the

diameter are required (Sparacino 1999)

Ultrasonic Reflection

This technique involves aligning the product in an ultrasonic gage placed in a water bath and

arranging transducers around it Each transducer sends out a transmission pulse that is partially

reflected off the outer wall of the tube While the partial reflection returns to the transducer the

remainder of the initial transmission pulse continues through the product wall The difference in

density between the two pulses allows wall thickness to be calculated Enhancing the signal

with digital processing can allow measurements to an accuracy of plus or minus 5 micrometers

of tubes as small as 10mm OD with wall thicknesses of 013mm or less

Pressure Transducers

As it relates to process control techniques food and medical applications cannot use the most

common type of transducer because it introduces mercury to the process Additionally these

standard oil-filled sensors can fail to operate within specifications after as little as 5 months at

570 degrees Fahrenheit (Naitove) Dynisco LLC a company in Franklin Massachusetts has

developed multiple new pressure transducers that can be utilized in a medical device

manufacturing environment Their new PT528 series has been documented to operate within

specification for almost a year Its maximum operating temperature is 617 degrees Fahrenheit

though the recommended limit for optimum life span is 527 degrees Fahrenheit Its accuracy is

rated at plus or minus 5 with repeatability of 02 at a price of $690 to $1400 (Naitove)

Conventional push-rod transducers have the same accuracy and repeatability specifications as

oil-filled sensors with comparable cost and can be used at temperatures up to 750 degrees

Fahrenheit Though these types of transducers may be used in medical manufacturing

environments they experience more zero-shift error in response to changes in either ambient or

process temperature than other types of transducers Due to their thicker diaphragms push-rod

transducers are less accurate at low pressures (250 to 500 psi) (Naitove)

For the medical manufacturing market Dynisco has developed a new line of pressure

transducers that encompass all of the features of the previously discussed components in a

FDA acceptable design The newly dubbed ldquoNon-toxic transducersrdquo including a sodium-

potassium liquid fill are up to twice as accurate and repeatable as push-rod and oil-filled types

They are also the most sensitive at low pressures and can handle the highest temperatures (up

to 1000 degrees Fahrenheit) Their fill solution does not degrade over time and they are the

least sensitive to temperature induced zero-shift error The main advantage of NaK sensors is

cost (around $1000 to $1200) and their lack of an explosion-proof rating (Naitove)

Statistical process control can be achieved by using measuring instruments such as those

discussed previously Data can be gathered (typically at 100 scanssec) and rapidly converted

9

by a process controller to provide waveform readings or live trend charges When the data are

viewed statistically deviation trends can be seen allowing process adjustments to be made by

a control feedback loop The most advanced current technology allows two independent loops

to be used Typically one loop controls haul-off or screw speed and the other controls air

pressure or vacuum (Colbert 1996)

Although the use of plastics in medical applications represents less than 2 of total

consumption the high ldquoadded valuerdquo of the final products is of considerable commercial interest

to material suppliers and end processors (Colbert 1996) Increasing product accuracy will lead

to significant reductions in material usage and resultant cost savings For example reducing the

tolerance from +- 008 mm down to +- 001 mm on a 100 mm inner diameter tube with a 0225

mm wall thickness yields material savings of 125 Assuming material costs of approximately

$10000 per ton this would represent cost saving of $1250hr at a 10 kghr production rate

(Colbert 1996)

10

Multilumen Tubing Die Design for Extrusion

Rheological and Mechanical Design of Extrusion Dies

In plastics processing the primary objective of the rheological design of an extrusion die or

distributor is a uniform velocity distribution in the polymer melt at the end of the flow channel

This requirement arises from the desire for a product that changes its dimensions only slightly

due to superimposed local velocity profiles after exiting from the die It is possible to meet this

requirement with an appropriate design for the flow channel (Michaeli Industrial Practice for the

Design of Extrusion Dies 2004)

The mechanical design of extrusion dies the calculation of the forces and deformations arising

during the operation of the die is important for two reasons

1 To assure that the die will not be damaged during operation

2 To assure that the distribution channel retains the geometry established by the

rheological design also during its operation (Michaeli Mechanical Design of Extrusion

Dies 2004)

The important applications of the mechanical design are

Design of screw joints and sealing surfaces with respect to the internal pressure

Design of walls for a permissible deformation by the internal pressure

Design of systems for the adjustment of the geometry of the die in the exit region

(Michaeli Mechanical Design of Extrusion Dies 2004)

The mechanical design is always closely linked to the rheological design First of all in the

rheological design the geometry of the flow channel is defined Then the pressure distribution

must be estimated conservatively by computation of the flow for the die within the expected

operating range with the material of the highest viscosity at the lowest mass temperature and

the highest mass throughput With that the isotropic pressures and shear stresses at the wall

existing in the die are known The forces acting in the die can be calculated from the stresses

and the areas of the flow channel walls The weight of the die also has to be considered in the

design of medium size and large dies (Michaeli Mechanical Design of Extrusion Dies 2004)

Extrusion Die Design Guidelines and Recommendations

In General

A die should consist of as few individual parts as possible in order to minimize the time

needed for assembly and cleaning Care must be taken to center the parts of the die

accuratey relative to each other and the die should be manufactured with close fits

(Michaeli Industrial Practice for the Design of Extrusion Dies 2004)

11

A small number of individual parts contributes to fewer joints in the die body and the flow

channel Fewer joints minimizes the possibility of leaks and therefore the potential for

material to get caught and degrade within them Also joints should be placed in

advantageous cross sections to simplify the cleaning of the die (Michaeli Mechanical

Design of Extrusion Dies 2004)

The sealing surfaces since they cannot be avoided should be as flat and small as

possible in order to assure a uniform distribution of sealing forces over the entire sealing

surface (Michaeli Mechanical Design of Extrusion Dies 2004)

The gap between stationary and moving parts of the die can be sealed by inserting a

gasket cord or an oversized packing strip in a groove in the stationary part of the die

(Michaeli Mechanical Design of Extrusion Dies 2004)

The die should be held together by few large heat resistant bolts (as opposed to many

small ones) since the service life of larger diameter bolts is longer The bolts should be

easily accessible without having to dismantle the strip heaters (Michaeli Mechanical

Design of Extrusion Dies 2004)

For frequently used threaded connections Helicoil inserts should be used to prevent

premature wear and unanticipated thread failures Fasteners should be designed to

withstand a safety factor of at least 200 when the head is exposed to maximum

pressure (Hendess 2002)

The die assembly should be designed so that it can be handled when hot

Component supports should be planned for the disassembly process

Jacking screws allow for simple disassembly of precision components especially

when the head is full of molten polymer (Hendess 2002)

Establishing the Flow Channel Configuration of the Die

When possible the melt should be supplied to the die centrally (Michaeli Mechanical

Design of Extrusion Dies 2004)

There must not be any dead spots or corners in the flow channels (sites of melt

stagnation) Sharp sudden transitions in cross section or changes in direction must be

avoided (Michaeli Mechanical Design of Extrusion Dies 2004)

Flow lines always lead to lower quality extruded products Their formation should be

avoided or diminished and their number reduced by a proper design of the flow channel

(Michaeli Mechanical Design of Extrusion Dies 2004)

Surface finishes for sealing surfaces should be 32 microinch or better Melt flow surfaces

should be 16 microinch or better with 4 to 8 microinch preferred (Hendess 2002)

12

Melt flow channels from component to component should have sharp edges to prevent

melt stagnation areas when assembled

Sharp edges are defined as being less than a 002rdquo break

Gaps between head components greater than 0003rdquo will generally allow polymer

leakage (Hendess 2002)

Die Guidelines and Recommendations Regarding the Processing of Specific Materials

Rigid polyvinylchloride (PVC-U)

Gentle streamlining of melt flow channels

Tight temperature control of the flow channel surfaces

Non-invasive melt temperature measurement

Good corrosion and wear resistance of melt contact surfaces

Use of restrictor bushing not breaker plates for screen pack support (Hendess 2002)

Chlorinated polyvinylchloride (CPVC)

Similar recommendations to PVC-U with exceptions to the following

Extreme streamlining of melt flow channels

High corrosion resistance (Hendess 2002)

Polyolefins

Incorporate strainer basket heads or spiral distributor flow channels to accommodate

high weld line sensitivity (Hendess 2002)

Thermoplastic urethanes (TPU)

Good flow channel streamlining

Temperature control of internal die components

Non-invasive melt temperature measurement (Hendess 2002)

Fluoropolymers

Good flow channel streamlining

Good temperature control of internal die components

Non-invasive melt temperature measurement

Extreme corrosion resistance for melt contact surfaces to withstand hydrogen fluoride

exposure

Internal heating to accommodate extreme melt fracture sensitivity (Hendess 2002)

Polyether-etherkeytones (PEEK) Polysulfones (PS)

Good flow channel streamlining

Tight temperature control of internal die components

Non-invasive melt temperature measurement

Corrosion and wear resistance of melt contact surfaces not of much concern (Hendess

2002)

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

8

Gamma Backscatter Probes

These devices use gamma-radiation backscatter to determine wall thickness down to 005 mm

for tubes with diameters as small as 1mm Probes measure wall thickness at a single point

around the tube a number of probes can be used if measurements of multiple points around the

diameter are required (Sparacino 1999)

Ultrasonic Reflection

This technique involves aligning the product in an ultrasonic gage placed in a water bath and

arranging transducers around it Each transducer sends out a transmission pulse that is partially

reflected off the outer wall of the tube While the partial reflection returns to the transducer the

remainder of the initial transmission pulse continues through the product wall The difference in

density between the two pulses allows wall thickness to be calculated Enhancing the signal

with digital processing can allow measurements to an accuracy of plus or minus 5 micrometers

of tubes as small as 10mm OD with wall thicknesses of 013mm or less

Pressure Transducers

As it relates to process control techniques food and medical applications cannot use the most

common type of transducer because it introduces mercury to the process Additionally these

standard oil-filled sensors can fail to operate within specifications after as little as 5 months at

570 degrees Fahrenheit (Naitove) Dynisco LLC a company in Franklin Massachusetts has

developed multiple new pressure transducers that can be utilized in a medical device

manufacturing environment Their new PT528 series has been documented to operate within

specification for almost a year Its maximum operating temperature is 617 degrees Fahrenheit

though the recommended limit for optimum life span is 527 degrees Fahrenheit Its accuracy is

rated at plus or minus 5 with repeatability of 02 at a price of $690 to $1400 (Naitove)

Conventional push-rod transducers have the same accuracy and repeatability specifications as

oil-filled sensors with comparable cost and can be used at temperatures up to 750 degrees

Fahrenheit Though these types of transducers may be used in medical manufacturing

environments they experience more zero-shift error in response to changes in either ambient or

process temperature than other types of transducers Due to their thicker diaphragms push-rod

transducers are less accurate at low pressures (250 to 500 psi) (Naitove)

For the medical manufacturing market Dynisco has developed a new line of pressure

transducers that encompass all of the features of the previously discussed components in a

FDA acceptable design The newly dubbed ldquoNon-toxic transducersrdquo including a sodium-

potassium liquid fill are up to twice as accurate and repeatable as push-rod and oil-filled types

They are also the most sensitive at low pressures and can handle the highest temperatures (up

to 1000 degrees Fahrenheit) Their fill solution does not degrade over time and they are the

least sensitive to temperature induced zero-shift error The main advantage of NaK sensors is

cost (around $1000 to $1200) and their lack of an explosion-proof rating (Naitove)

Statistical process control can be achieved by using measuring instruments such as those

discussed previously Data can be gathered (typically at 100 scanssec) and rapidly converted

9

by a process controller to provide waveform readings or live trend charges When the data are

viewed statistically deviation trends can be seen allowing process adjustments to be made by

a control feedback loop The most advanced current technology allows two independent loops

to be used Typically one loop controls haul-off or screw speed and the other controls air

pressure or vacuum (Colbert 1996)

Although the use of plastics in medical applications represents less than 2 of total

consumption the high ldquoadded valuerdquo of the final products is of considerable commercial interest

to material suppliers and end processors (Colbert 1996) Increasing product accuracy will lead

to significant reductions in material usage and resultant cost savings For example reducing the

tolerance from +- 008 mm down to +- 001 mm on a 100 mm inner diameter tube with a 0225

mm wall thickness yields material savings of 125 Assuming material costs of approximately

$10000 per ton this would represent cost saving of $1250hr at a 10 kghr production rate

(Colbert 1996)

10

Multilumen Tubing Die Design for Extrusion

Rheological and Mechanical Design of Extrusion Dies

In plastics processing the primary objective of the rheological design of an extrusion die or

distributor is a uniform velocity distribution in the polymer melt at the end of the flow channel

This requirement arises from the desire for a product that changes its dimensions only slightly

due to superimposed local velocity profiles after exiting from the die It is possible to meet this

requirement with an appropriate design for the flow channel (Michaeli Industrial Practice for the

Design of Extrusion Dies 2004)

The mechanical design of extrusion dies the calculation of the forces and deformations arising

during the operation of the die is important for two reasons

1 To assure that the die will not be damaged during operation

2 To assure that the distribution channel retains the geometry established by the

rheological design also during its operation (Michaeli Mechanical Design of Extrusion

Dies 2004)

The important applications of the mechanical design are

Design of screw joints and sealing surfaces with respect to the internal pressure

Design of walls for a permissible deformation by the internal pressure

Design of systems for the adjustment of the geometry of the die in the exit region

(Michaeli Mechanical Design of Extrusion Dies 2004)

The mechanical design is always closely linked to the rheological design First of all in the

rheological design the geometry of the flow channel is defined Then the pressure distribution

must be estimated conservatively by computation of the flow for the die within the expected

operating range with the material of the highest viscosity at the lowest mass temperature and

the highest mass throughput With that the isotropic pressures and shear stresses at the wall

existing in the die are known The forces acting in the die can be calculated from the stresses

and the areas of the flow channel walls The weight of the die also has to be considered in the

design of medium size and large dies (Michaeli Mechanical Design of Extrusion Dies 2004)

Extrusion Die Design Guidelines and Recommendations

In General

A die should consist of as few individual parts as possible in order to minimize the time

needed for assembly and cleaning Care must be taken to center the parts of the die

accuratey relative to each other and the die should be manufactured with close fits

(Michaeli Industrial Practice for the Design of Extrusion Dies 2004)

11

A small number of individual parts contributes to fewer joints in the die body and the flow

channel Fewer joints minimizes the possibility of leaks and therefore the potential for

material to get caught and degrade within them Also joints should be placed in

advantageous cross sections to simplify the cleaning of the die (Michaeli Mechanical

Design of Extrusion Dies 2004)

The sealing surfaces since they cannot be avoided should be as flat and small as

possible in order to assure a uniform distribution of sealing forces over the entire sealing

surface (Michaeli Mechanical Design of Extrusion Dies 2004)

The gap between stationary and moving parts of the die can be sealed by inserting a

gasket cord or an oversized packing strip in a groove in the stationary part of the die

(Michaeli Mechanical Design of Extrusion Dies 2004)

The die should be held together by few large heat resistant bolts (as opposed to many

small ones) since the service life of larger diameter bolts is longer The bolts should be

easily accessible without having to dismantle the strip heaters (Michaeli Mechanical

Design of Extrusion Dies 2004)

For frequently used threaded connections Helicoil inserts should be used to prevent

premature wear and unanticipated thread failures Fasteners should be designed to

withstand a safety factor of at least 200 when the head is exposed to maximum

pressure (Hendess 2002)

The die assembly should be designed so that it can be handled when hot

Component supports should be planned for the disassembly process

Jacking screws allow for simple disassembly of precision components especially

when the head is full of molten polymer (Hendess 2002)

Establishing the Flow Channel Configuration of the Die

When possible the melt should be supplied to the die centrally (Michaeli Mechanical

Design of Extrusion Dies 2004)

There must not be any dead spots or corners in the flow channels (sites of melt

stagnation) Sharp sudden transitions in cross section or changes in direction must be

avoided (Michaeli Mechanical Design of Extrusion Dies 2004)

Flow lines always lead to lower quality extruded products Their formation should be

avoided or diminished and their number reduced by a proper design of the flow channel

(Michaeli Mechanical Design of Extrusion Dies 2004)

Surface finishes for sealing surfaces should be 32 microinch or better Melt flow surfaces

should be 16 microinch or better with 4 to 8 microinch preferred (Hendess 2002)

12

Melt flow channels from component to component should have sharp edges to prevent

melt stagnation areas when assembled

Sharp edges are defined as being less than a 002rdquo break

Gaps between head components greater than 0003rdquo will generally allow polymer

leakage (Hendess 2002)

Die Guidelines and Recommendations Regarding the Processing of Specific Materials

Rigid polyvinylchloride (PVC-U)

Gentle streamlining of melt flow channels

Tight temperature control of the flow channel surfaces

Non-invasive melt temperature measurement

Good corrosion and wear resistance of melt contact surfaces

Use of restrictor bushing not breaker plates for screen pack support (Hendess 2002)

Chlorinated polyvinylchloride (CPVC)

Similar recommendations to PVC-U with exceptions to the following

Extreme streamlining of melt flow channels

High corrosion resistance (Hendess 2002)

Polyolefins

Incorporate strainer basket heads or spiral distributor flow channels to accommodate

high weld line sensitivity (Hendess 2002)

Thermoplastic urethanes (TPU)

Good flow channel streamlining

Temperature control of internal die components

Non-invasive melt temperature measurement (Hendess 2002)

Fluoropolymers

Good flow channel streamlining

Good temperature control of internal die components

Non-invasive melt temperature measurement

Extreme corrosion resistance for melt contact surfaces to withstand hydrogen fluoride

exposure

Internal heating to accommodate extreme melt fracture sensitivity (Hendess 2002)

Polyether-etherkeytones (PEEK) Polysulfones (PS)

Good flow channel streamlining

Tight temperature control of internal die components

Non-invasive melt temperature measurement

Corrosion and wear resistance of melt contact surfaces not of much concern (Hendess

2002)

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

9

by a process controller to provide waveform readings or live trend charges When the data are

viewed statistically deviation trends can be seen allowing process adjustments to be made by

a control feedback loop The most advanced current technology allows two independent loops

to be used Typically one loop controls haul-off or screw speed and the other controls air

pressure or vacuum (Colbert 1996)

Although the use of plastics in medical applications represents less than 2 of total

consumption the high ldquoadded valuerdquo of the final products is of considerable commercial interest

to material suppliers and end processors (Colbert 1996) Increasing product accuracy will lead

to significant reductions in material usage and resultant cost savings For example reducing the

tolerance from +- 008 mm down to +- 001 mm on a 100 mm inner diameter tube with a 0225

mm wall thickness yields material savings of 125 Assuming material costs of approximately

$10000 per ton this would represent cost saving of $1250hr at a 10 kghr production rate

(Colbert 1996)

10

Multilumen Tubing Die Design for Extrusion

Rheological and Mechanical Design of Extrusion Dies

In plastics processing the primary objective of the rheological design of an extrusion die or

distributor is a uniform velocity distribution in the polymer melt at the end of the flow channel

This requirement arises from the desire for a product that changes its dimensions only slightly

due to superimposed local velocity profiles after exiting from the die It is possible to meet this

requirement with an appropriate design for the flow channel (Michaeli Industrial Practice for the

Design of Extrusion Dies 2004)

The mechanical design of extrusion dies the calculation of the forces and deformations arising

during the operation of the die is important for two reasons

1 To assure that the die will not be damaged during operation

2 To assure that the distribution channel retains the geometry established by the

rheological design also during its operation (Michaeli Mechanical Design of Extrusion

Dies 2004)

The important applications of the mechanical design are

Design of screw joints and sealing surfaces with respect to the internal pressure

Design of walls for a permissible deformation by the internal pressure

Design of systems for the adjustment of the geometry of the die in the exit region

(Michaeli Mechanical Design of Extrusion Dies 2004)

The mechanical design is always closely linked to the rheological design First of all in the

rheological design the geometry of the flow channel is defined Then the pressure distribution

must be estimated conservatively by computation of the flow for the die within the expected

operating range with the material of the highest viscosity at the lowest mass temperature and

the highest mass throughput With that the isotropic pressures and shear stresses at the wall

existing in the die are known The forces acting in the die can be calculated from the stresses

and the areas of the flow channel walls The weight of the die also has to be considered in the

design of medium size and large dies (Michaeli Mechanical Design of Extrusion Dies 2004)

Extrusion Die Design Guidelines and Recommendations

In General

A die should consist of as few individual parts as possible in order to minimize the time

needed for assembly and cleaning Care must be taken to center the parts of the die

accuratey relative to each other and the die should be manufactured with close fits

(Michaeli Industrial Practice for the Design of Extrusion Dies 2004)

11

A small number of individual parts contributes to fewer joints in the die body and the flow

channel Fewer joints minimizes the possibility of leaks and therefore the potential for

material to get caught and degrade within them Also joints should be placed in

advantageous cross sections to simplify the cleaning of the die (Michaeli Mechanical

Design of Extrusion Dies 2004)

The sealing surfaces since they cannot be avoided should be as flat and small as

possible in order to assure a uniform distribution of sealing forces over the entire sealing

surface (Michaeli Mechanical Design of Extrusion Dies 2004)

The gap between stationary and moving parts of the die can be sealed by inserting a

gasket cord or an oversized packing strip in a groove in the stationary part of the die

(Michaeli Mechanical Design of Extrusion Dies 2004)

The die should be held together by few large heat resistant bolts (as opposed to many

small ones) since the service life of larger diameter bolts is longer The bolts should be

easily accessible without having to dismantle the strip heaters (Michaeli Mechanical

Design of Extrusion Dies 2004)

For frequently used threaded connections Helicoil inserts should be used to prevent

premature wear and unanticipated thread failures Fasteners should be designed to

withstand a safety factor of at least 200 when the head is exposed to maximum

pressure (Hendess 2002)

The die assembly should be designed so that it can be handled when hot

Component supports should be planned for the disassembly process

Jacking screws allow for simple disassembly of precision components especially

when the head is full of molten polymer (Hendess 2002)

Establishing the Flow Channel Configuration of the Die

When possible the melt should be supplied to the die centrally (Michaeli Mechanical

Design of Extrusion Dies 2004)

There must not be any dead spots or corners in the flow channels (sites of melt

stagnation) Sharp sudden transitions in cross section or changes in direction must be

avoided (Michaeli Mechanical Design of Extrusion Dies 2004)

Flow lines always lead to lower quality extruded products Their formation should be

avoided or diminished and their number reduced by a proper design of the flow channel

(Michaeli Mechanical Design of Extrusion Dies 2004)

Surface finishes for sealing surfaces should be 32 microinch or better Melt flow surfaces

should be 16 microinch or better with 4 to 8 microinch preferred (Hendess 2002)

12

Melt flow channels from component to component should have sharp edges to prevent

melt stagnation areas when assembled

Sharp edges are defined as being less than a 002rdquo break

Gaps between head components greater than 0003rdquo will generally allow polymer

leakage (Hendess 2002)

Die Guidelines and Recommendations Regarding the Processing of Specific Materials

Rigid polyvinylchloride (PVC-U)

Gentle streamlining of melt flow channels

Tight temperature control of the flow channel surfaces

Non-invasive melt temperature measurement

Good corrosion and wear resistance of melt contact surfaces

Use of restrictor bushing not breaker plates for screen pack support (Hendess 2002)

Chlorinated polyvinylchloride (CPVC)

Similar recommendations to PVC-U with exceptions to the following

Extreme streamlining of melt flow channels

High corrosion resistance (Hendess 2002)

Polyolefins

Incorporate strainer basket heads or spiral distributor flow channels to accommodate

high weld line sensitivity (Hendess 2002)

Thermoplastic urethanes (TPU)

Good flow channel streamlining

Temperature control of internal die components

Non-invasive melt temperature measurement (Hendess 2002)

Fluoropolymers

Good flow channel streamlining

Good temperature control of internal die components

Non-invasive melt temperature measurement

Extreme corrosion resistance for melt contact surfaces to withstand hydrogen fluoride

exposure

Internal heating to accommodate extreme melt fracture sensitivity (Hendess 2002)

Polyether-etherkeytones (PEEK) Polysulfones (PS)

Good flow channel streamlining

Tight temperature control of internal die components

Non-invasive melt temperature measurement

Corrosion and wear resistance of melt contact surfaces not of much concern (Hendess

2002)

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

10

Multilumen Tubing Die Design for Extrusion

Rheological and Mechanical Design of Extrusion Dies

In plastics processing the primary objective of the rheological design of an extrusion die or

distributor is a uniform velocity distribution in the polymer melt at the end of the flow channel

This requirement arises from the desire for a product that changes its dimensions only slightly

due to superimposed local velocity profiles after exiting from the die It is possible to meet this

requirement with an appropriate design for the flow channel (Michaeli Industrial Practice for the

Design of Extrusion Dies 2004)

The mechanical design of extrusion dies the calculation of the forces and deformations arising

during the operation of the die is important for two reasons

1 To assure that the die will not be damaged during operation

2 To assure that the distribution channel retains the geometry established by the

rheological design also during its operation (Michaeli Mechanical Design of Extrusion

Dies 2004)

The important applications of the mechanical design are

Design of screw joints and sealing surfaces with respect to the internal pressure

Design of walls for a permissible deformation by the internal pressure

Design of systems for the adjustment of the geometry of the die in the exit region

(Michaeli Mechanical Design of Extrusion Dies 2004)

The mechanical design is always closely linked to the rheological design First of all in the

rheological design the geometry of the flow channel is defined Then the pressure distribution

must be estimated conservatively by computation of the flow for the die within the expected

operating range with the material of the highest viscosity at the lowest mass temperature and

the highest mass throughput With that the isotropic pressures and shear stresses at the wall

existing in the die are known The forces acting in the die can be calculated from the stresses

and the areas of the flow channel walls The weight of the die also has to be considered in the

design of medium size and large dies (Michaeli Mechanical Design of Extrusion Dies 2004)

Extrusion Die Design Guidelines and Recommendations

In General

A die should consist of as few individual parts as possible in order to minimize the time

needed for assembly and cleaning Care must be taken to center the parts of the die

accuratey relative to each other and the die should be manufactured with close fits

(Michaeli Industrial Practice for the Design of Extrusion Dies 2004)

11

A small number of individual parts contributes to fewer joints in the die body and the flow

channel Fewer joints minimizes the possibility of leaks and therefore the potential for

material to get caught and degrade within them Also joints should be placed in

advantageous cross sections to simplify the cleaning of the die (Michaeli Mechanical

Design of Extrusion Dies 2004)

The sealing surfaces since they cannot be avoided should be as flat and small as

possible in order to assure a uniform distribution of sealing forces over the entire sealing

surface (Michaeli Mechanical Design of Extrusion Dies 2004)

The gap between stationary and moving parts of the die can be sealed by inserting a

gasket cord or an oversized packing strip in a groove in the stationary part of the die

(Michaeli Mechanical Design of Extrusion Dies 2004)

The die should be held together by few large heat resistant bolts (as opposed to many

small ones) since the service life of larger diameter bolts is longer The bolts should be

easily accessible without having to dismantle the strip heaters (Michaeli Mechanical

Design of Extrusion Dies 2004)

For frequently used threaded connections Helicoil inserts should be used to prevent

premature wear and unanticipated thread failures Fasteners should be designed to

withstand a safety factor of at least 200 when the head is exposed to maximum

pressure (Hendess 2002)

The die assembly should be designed so that it can be handled when hot

Component supports should be planned for the disassembly process

Jacking screws allow for simple disassembly of precision components especially

when the head is full of molten polymer (Hendess 2002)

Establishing the Flow Channel Configuration of the Die

When possible the melt should be supplied to the die centrally (Michaeli Mechanical

Design of Extrusion Dies 2004)

There must not be any dead spots or corners in the flow channels (sites of melt

stagnation) Sharp sudden transitions in cross section or changes in direction must be

avoided (Michaeli Mechanical Design of Extrusion Dies 2004)

Flow lines always lead to lower quality extruded products Their formation should be

avoided or diminished and their number reduced by a proper design of the flow channel

(Michaeli Mechanical Design of Extrusion Dies 2004)

Surface finishes for sealing surfaces should be 32 microinch or better Melt flow surfaces

should be 16 microinch or better with 4 to 8 microinch preferred (Hendess 2002)

12

Melt flow channels from component to component should have sharp edges to prevent

melt stagnation areas when assembled

Sharp edges are defined as being less than a 002rdquo break

Gaps between head components greater than 0003rdquo will generally allow polymer

leakage (Hendess 2002)

Die Guidelines and Recommendations Regarding the Processing of Specific Materials

Rigid polyvinylchloride (PVC-U)

Gentle streamlining of melt flow channels

Tight temperature control of the flow channel surfaces

Non-invasive melt temperature measurement

Good corrosion and wear resistance of melt contact surfaces

Use of restrictor bushing not breaker plates for screen pack support (Hendess 2002)

Chlorinated polyvinylchloride (CPVC)

Similar recommendations to PVC-U with exceptions to the following

Extreme streamlining of melt flow channels

High corrosion resistance (Hendess 2002)

Polyolefins

Incorporate strainer basket heads or spiral distributor flow channels to accommodate

high weld line sensitivity (Hendess 2002)

Thermoplastic urethanes (TPU)

Good flow channel streamlining

Temperature control of internal die components

Non-invasive melt temperature measurement (Hendess 2002)

Fluoropolymers

Good flow channel streamlining

Good temperature control of internal die components

Non-invasive melt temperature measurement

Extreme corrosion resistance for melt contact surfaces to withstand hydrogen fluoride

exposure

Internal heating to accommodate extreme melt fracture sensitivity (Hendess 2002)

Polyether-etherkeytones (PEEK) Polysulfones (PS)

Good flow channel streamlining

Tight temperature control of internal die components

Non-invasive melt temperature measurement

Corrosion and wear resistance of melt contact surfaces not of much concern (Hendess

2002)

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

11

A small number of individual parts contributes to fewer joints in the die body and the flow

channel Fewer joints minimizes the possibility of leaks and therefore the potential for

material to get caught and degrade within them Also joints should be placed in

advantageous cross sections to simplify the cleaning of the die (Michaeli Mechanical

Design of Extrusion Dies 2004)

The sealing surfaces since they cannot be avoided should be as flat and small as

possible in order to assure a uniform distribution of sealing forces over the entire sealing

surface (Michaeli Mechanical Design of Extrusion Dies 2004)

The gap between stationary and moving parts of the die can be sealed by inserting a

gasket cord or an oversized packing strip in a groove in the stationary part of the die

(Michaeli Mechanical Design of Extrusion Dies 2004)

The die should be held together by few large heat resistant bolts (as opposed to many

small ones) since the service life of larger diameter bolts is longer The bolts should be

easily accessible without having to dismantle the strip heaters (Michaeli Mechanical

Design of Extrusion Dies 2004)

For frequently used threaded connections Helicoil inserts should be used to prevent

premature wear and unanticipated thread failures Fasteners should be designed to

withstand a safety factor of at least 200 when the head is exposed to maximum

pressure (Hendess 2002)

The die assembly should be designed so that it can be handled when hot

Component supports should be planned for the disassembly process

Jacking screws allow for simple disassembly of precision components especially

when the head is full of molten polymer (Hendess 2002)

Establishing the Flow Channel Configuration of the Die

When possible the melt should be supplied to the die centrally (Michaeli Mechanical

Design of Extrusion Dies 2004)

There must not be any dead spots or corners in the flow channels (sites of melt

stagnation) Sharp sudden transitions in cross section or changes in direction must be

avoided (Michaeli Mechanical Design of Extrusion Dies 2004)

Flow lines always lead to lower quality extruded products Their formation should be

avoided or diminished and their number reduced by a proper design of the flow channel

(Michaeli Mechanical Design of Extrusion Dies 2004)

Surface finishes for sealing surfaces should be 32 microinch or better Melt flow surfaces

should be 16 microinch or better with 4 to 8 microinch preferred (Hendess 2002)

12

Melt flow channels from component to component should have sharp edges to prevent

melt stagnation areas when assembled

Sharp edges are defined as being less than a 002rdquo break

Gaps between head components greater than 0003rdquo will generally allow polymer

leakage (Hendess 2002)

Die Guidelines and Recommendations Regarding the Processing of Specific Materials

Rigid polyvinylchloride (PVC-U)

Gentle streamlining of melt flow channels

Tight temperature control of the flow channel surfaces

Non-invasive melt temperature measurement

Good corrosion and wear resistance of melt contact surfaces

Use of restrictor bushing not breaker plates for screen pack support (Hendess 2002)

Chlorinated polyvinylchloride (CPVC)

Similar recommendations to PVC-U with exceptions to the following

Extreme streamlining of melt flow channels

High corrosion resistance (Hendess 2002)

Polyolefins

Incorporate strainer basket heads or spiral distributor flow channels to accommodate

high weld line sensitivity (Hendess 2002)

Thermoplastic urethanes (TPU)

Good flow channel streamlining

Temperature control of internal die components

Non-invasive melt temperature measurement (Hendess 2002)

Fluoropolymers

Good flow channel streamlining

Good temperature control of internal die components

Non-invasive melt temperature measurement

Extreme corrosion resistance for melt contact surfaces to withstand hydrogen fluoride

exposure

Internal heating to accommodate extreme melt fracture sensitivity (Hendess 2002)

Polyether-etherkeytones (PEEK) Polysulfones (PS)

Good flow channel streamlining

Tight temperature control of internal die components

Non-invasive melt temperature measurement

Corrosion and wear resistance of melt contact surfaces not of much concern (Hendess

2002)

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

12

Melt flow channels from component to component should have sharp edges to prevent

melt stagnation areas when assembled

Sharp edges are defined as being less than a 002rdquo break

Gaps between head components greater than 0003rdquo will generally allow polymer

leakage (Hendess 2002)

Die Guidelines and Recommendations Regarding the Processing of Specific Materials

Rigid polyvinylchloride (PVC-U)

Gentle streamlining of melt flow channels

Tight temperature control of the flow channel surfaces

Non-invasive melt temperature measurement

Good corrosion and wear resistance of melt contact surfaces

Use of restrictor bushing not breaker plates for screen pack support (Hendess 2002)

Chlorinated polyvinylchloride (CPVC)

Similar recommendations to PVC-U with exceptions to the following

Extreme streamlining of melt flow channels

High corrosion resistance (Hendess 2002)

Polyolefins

Incorporate strainer basket heads or spiral distributor flow channels to accommodate

high weld line sensitivity (Hendess 2002)

Thermoplastic urethanes (TPU)

Good flow channel streamlining

Temperature control of internal die components

Non-invasive melt temperature measurement (Hendess 2002)

Fluoropolymers

Good flow channel streamlining

Good temperature control of internal die components

Non-invasive melt temperature measurement

Extreme corrosion resistance for melt contact surfaces to withstand hydrogen fluoride

exposure

Internal heating to accommodate extreme melt fracture sensitivity (Hendess 2002)

Polyether-etherkeytones (PEEK) Polysulfones (PS)

Good flow channel streamlining

Tight temperature control of internal die components

Non-invasive melt temperature measurement

Corrosion and wear resistance of melt contact surfaces not of much concern (Hendess

2002)

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

13

Materials for Extrusion Dies

In general the materials used for extrusion dies should meet the following requirements

Can be readily machined

Are resistant to pressure temperature and wear

Have sufficient strength and toughness

Have sufficient surface hardness

Can be readily polished to a satisfactory surface (without porosity)

Respond adequately to heat treatment

Have minimum tendency to distortion and change in dimensions during the heat

treatment

Are resistant to (corrosive) chemical attack

Offer possibilities for surface treatment (eg chromium plating nitriding)

Have a good thermal conductivity

Are free of internal or residual stresses (Michaeli Mechanical Design of Extrusion Dies

2004)

Additionally the following questions should be asked and answered when selecting the material

for an extrusion die

What type of compound will be processed Details that should be known include

o Processing temperature range

o Corrosion potential

o Anticipated wear by additives

What is the nature and the magnitude of the mechanical stresses The bending stresses

present are of significant and crucial importance for the selection of the material

By what process will the die be manufactured Machining methods are possible for

materials with strength up to approximately 1500 Nmm2 however the most

advantageous machining conditions exist at strengths of 600-800 Nmm

What heat treatment is required and does it tend to cause distortions or dimensional

changes (Michaeli Mechanical Design of Extrusion Dies 2004)

For medical extrusion applications dies are normally constructed of stainless steel which must

be hardenable and capable of achieving a good polish (Colbert 1996) The three general

stainless steel categories include austenitic martensitic and precipitation hardening (PH)

grades The austenitic stainless steels have good corrosion resistance but have low strength

and hardness and cannot be heat treated The martensitic materials are hardenable but at the

cost of reduced corrosion resistance The best stainless steels for plastic extrusion tooling are

the precipitation hardening grades The 17-4 and 15-9 grades have good corrosion resistance

high strength and can be hardened to 50Rc with a procedure that does not distort the final

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

14

product Passivation of the stainless steel prior to use is a must for ensuring corrosion

resistance (Hendess 2002)

Furthermore the lower thermal conductivity of stainless steels compared to non-stainless steels

is an asset to the plastic extrusion process Dies made of stainless steels take longer to heat up

and thus take longer to change temperature As a result they are more thermally stable when

the desired processing temperature is reached (Hendess 2002)

Draw Down Ratios

The requirements for medical tubing with respect to dimensional tolerances and overall quality are stricter than almost any other application This coupled with the small tubing sizes typically produced presents challenges to the producers of medical tubing The requirements are often so exact that strict adherence to fundamental extrusion design principles are essential for companies seeking to improve their extrusion performance (Rauwendaal 2009) Important issues in the design of tubing tooling are the various draw ratios that define the tooling and the extrusion process The dimensions of the tip (mandrel) and die are determined by the draw down in the extrusion process There are various draw ratios in tubing extrusion that describe how the tubing is drawn down at the exit of the die including diameter draw ratio wall draw ratio area draw ratio draw ratio balance and sizing ratio (Rauwendaal 2009) For all equations below refer to Figure 1 for clarification regarding dimensions

Figure 1

Dt = tip diameter Dd = die diameter Do = tubing outer diameter Di = tubing inner diameter

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

15

Diameter Draw Ratio (DDR)

The average diameter of the tip and die divided by the average diameter of the tubing

(Rauwendaal 2009) Wall Draw Ratio (WDR)

The gap between the tip and die divided by the wall thickness of the tubing

(Rauwendaal 2009)

Area Draw Ratio (ADR)

Commonly referred to as Draw Down Ratio (DDR) or simply Draw Ratio

The cross sectional area between the tip and die divided by the tubing cross sectional area

A high ADR increases orientation and the chance of pinholes and breakaways

A low ADR reduces orientation and increases the chance of melt fracture

Figure 2

(Rauwendaal 2009)

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

16

Draw Ratio Balance (DRB)

The diameter ratio of the die and tip divided by the diameter ratio of the tubing In other words the balance between the rate the outside of the cone draws down and the rate the inside of the cone draws down (BampH Tool Company 2010)

o Most products made by drawing a plastic melt are smaller in cross sectional area than the tooling gap (BampH Tool Company 2010)

When the DRB equals 1 the annular shape of the tubing is the same as the annular shape of the tooling

When the DRB is greater than 1 the inner diameter (ID) of the tubing relative to the outer diameter (OD) will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter)

For a stable tubing extrusion process the DRB should be equal to or greater than one

Figure 3

(Rauwendaal 2009) The need to understand the relationship between the dimensions of the tooling and those of the final product is met by understanding both Draw Down Ratio and Draw Ratio Balance It is required that both the tooling designer and extruded product manufacturer understand and are able to communicate these relationships to each other (BampH Tool Company 2010)

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

17

Given the small dimensions of many intricate multilumen tubes high drawdown ratios are often employed to allow die dimensions to be large enough to be practical By contrast high drawdown ratios can be used with polyamide and many fluoropolymers which are therefore more commonly specified for precision microbore or multilumen tubes (Colbert 1996) Sizing Ratio (SR)

The wall draw ratio divided by the diameter draw ratio

A balanced draw occurs when the sizing ratio ranges from 10 to 13

When the SR is larger than 13 there is a danger of getting tear holes in the tubing

Low SR values can cause instabilities in the sizing of the tubing

Rubbers and high molecular weight polymers can be run with low SR values

Low viscosity polymers should be run with high SR values

High SR values will increase orientation and the chance of breakaways and require higher internal andor lower external air pressure to obtain tubing size (Rauwendaal 2009)

Land Length In addition to the tip and die diameter the land length and the cone angle are important design parameters In many situations a long land length is desired because a long land tends to

Reduce tip and die drool

Increase orientation

Reduce the chance of pinholes

Reduce the swelling of the extrudate (die swell)

Improve shape definition (Rauwendaal 2009) The main drawback of a long land length is increased diehead pressure Since the land region usually has the highest restriction to flow a longer land can increase pressure substantially Another drawback of a long land length is that a long tip is more susceptible to mechanical deformation the tip can bend more easily This is a particular concern in small diameter tubing Typical rules for the land length are

Land length divided by gap between tip and die (LH) from 101 to 201

Land length divided by the diameter of the tip (LDt) from 101 to 251 (Rauwendaal 2009)

The gap between the tip and the die H is half the die diameter minus half the tip diameter or H = 05Dd- 05Dt The land length values that follow from these rules often result in excessive pressures with dealing with high viscosity materials In many cases therefore the pressure drop will determine what land length is practical (Rauwendaal 2009)

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

18

Taper Angles

The taper angle used in self-centering tooling typically ranges from 30-40 degrees in adjustable tooling from 8-15 degrees Research has found that the entry angle affect melt fracture in certain polymers such as LDPE When the entry angle is as large as 120 degrees melt fracture occurs in LDPE At smaller entry angles melt fracture does not occur In other polymers such as HDPE the entry angle has no effect on the extrudate distortion (Rauwendaal 2009)

Figure 4 A self-centered crosshead die

(Rauwendaal 2009)

Simulation Software and the Die Design Process

Modern computational tools exist that provide information in a practical and cost-effective way to improve the die design process Finite element techniques are coupled with advanced free surface calculations and several rheological models to allow for the realistic simulation of the three dimensional flow of plastics or rubber through complex dies and the anticipated deformationswell of the material(s) outside the die The combination of the designersrsquo knowledge of the extrusion process with the insight provided by numerical simulation should result in savings in the number of trial dies therefore reducing cost time-to market and scrap material In addition virtual experimentation introduces a more reproducible engineering practice in the design of extrusion dies (Marchal Burton Franceschetti De Rijcke Chauvin amp Metwally 2007) Commercial packages such as Diecalc and Polyflow are currently being used to assist in die design These packages simulate the flow inside the die providing information such as temperature and velocity fields pressure drops and distribution residence times and stresses This information can then be utilized by a designer to develop a new design or evaluate the performance of an existing die A designer is offered flexibility in that the die geometry processing conditions and the material can be easily changed This minimizes the time and

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

19

effort required for the changes and a few design alternatives can then be selected for prototyping (Munot Mead Orroth amp Stacer 1999) Sometimes simulation is the only way for a quick solution to problems such as excessive pressure losses existence of stagnation zones unbalanced flow and excessive shear heating Andrejewski in his work on die design has observed a 167 increase in the production rates and an 18 fold reduction in set up times using Polyflow software for die design (Munot Mead Orroth amp Stacer 1999)

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

20

Figures and Graphics

Figure 5 An extrusion system for the production of multilumen catheter tubing

(Extrudex Kunststoffmaschinen 2008)

Extruder

Laser Measuring Head

Vacuum Calibration Bath

Controller

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

21

Figure 6 A crosshead for multilumen tube extrusion

(Extrudex Kunststoffmaschinen 2008)

Figure 7 Air module for a four-lumen tube

(Extrudex Kunststoffmaschinen 2008)

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

22

Correspondence With Chuck Keyes Medical Extrusion Engineer at Biotech Manufacturing Center of Texas From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Monday October 11 2010 1046 PM To keyes-extrusionearthlinknet Subject Information Regarding Multi-Lumen Tubing Die Design

Chuck Please allow me to introduce myself My name is Jesse Pischlar and I am a senior at the University of Wisconsin-Stout pursuing bachelorrsquos degrees in both Plastics Engineering and Manufacturing Engineering This summer I was an intern in Medtronicrsquos Cardiac Rhythm Disease Management division and worked extensively with single and multi-lumen tubing while performing laser welding experiments on cardiac leads I discovered your website during my research endeavors for an assignment in a class Extrusion Theory and Application The assignment focuses on exploring die design for multi-lumen tubing a topic in which you have extensive experience and expertise I was wondering if you would be willing to suggest any authors articles websites databases etchellip to peruse to gain further knowledge regarding this topic Your help would be greatly appreciated Thank you for taking the time to read this and I look forward to hearing from you Sincerely Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar

From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 628 AM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design Hi Jesse I believe yoursquore going to be surprised to learn that medical extrusion especially multi-lumen extrusion die design involves just as much art as it does science Itrsquos like painting a picture using scientific brushes and paint Die design for multi-lumens will vary greatly from one extrusion house to another

23

I designed all the dies to extrude the multi-lumen catheters shown within the picture above My multi-lumen dies adapt to a special crosshead I designed which allows me to independently adjust the stainless steel hypodermic tubing (needles) that used to form the inner dimensions (ID) of the multi-lumen catheters The die tip of the hypodermic needles can be shaped to yield inner dimensions that are not round The ability to independently adjust the hypo needles during the set-up allows me to quickly obtain the shape and dimensions I require to conform to the print Irsquom sorry to say that I cannot recommend a book pertaining to multi-lumen die design What books that are on the market show multi-lumen die designs that are totally useless The reason for this is because of secrecy Medical extrusion houses that have their own extrusion engineer who can design heads and dies will do whatever it takes to maintain all the designs as top secret Competition The secret is the art Without the art the science is pretty much useless Itrsquos like owning a car without an engine The best way to learn the art is hands-on training although many people do not have the artistic abilities Not everybody can learn how to paint like Leonardo Davinci If you wishhellip I can send you PDF prints of one of my male and female multi-lumen die designs however it will not include the print to my special multi-lumen crosshead The male and female multi-lumen die prints should be scientifically helpful and they may offer an understanding of the art involved Thankshellip The Chuck

Chuck Keyes Web Site httpwwwchuck-keyescom Medical Extrusion Engineer and Device Designer Home Address 6359 CR 4511 Athens Texas 75752 Tel 903-675-7432 Home E-Mail charleskeyesearthlinknet Medical Extrusion Engineer Web Page httphomeearthlinknet~keyes-extrusionid12html

24

From Pischlar Jesse [mailtopischlarjmyuwstoutedu] Sent Wednesday October 13 2010 1028 AM To keyes-extrusionearthlinknet Subject RE Information Regarding Multi-Lumen Tubing Die Design

Chuck Thank you taking the time to read my email and for the information As I had presumed and discussed with my professor finding information regarding this topic will certainly be a challenge due to the secrecy involved in the design of multi-lumen dies I certainly recognize and appreciate the level of competition involved in die design and the fact that sharing critical information could negatively impact business The prints for one of your male and female multi-lumen die designs would be greatly appreciated Irsquom confident that I would learn a lot in perusing them Again thank you for your help It is greatly appreciated Jesse J Pischlar Manufacturing Engineering amp Plastics Engineering University of Wisconsin-Stout Wisconsins Polytechnic University Menomonie Wisconsin 54751 507 301 8923 --- httpwwwjessepischlarcom httpwwwlinkedincominjessepischlar From keyes-extrusion [mailtokeyes-extrusionearthlinknet] Sent Wednesday October 13 2010 129 PM To Pischlar Jesse Subject RE Information Regarding Multi-Lumen Tubing Die Design

Hi Jesse I have attached four (pdf) prints showing the die design and assembly for a triple lumen 10 French 40D PEBAX (Nylon) catheter This triple lumen catheter is shown more than once in the picture I sent you This heart catheter contains 20 Barium sulfate and 2 Ti02 thus allowing the surgeon to view it under X-Ray for positioning within the human heart The catheter can monitor pressures feed medication and work with guide wires Believe me I doubt if anybody in the world has a better design than this one The extrusion set-up for this job is about a 12 hour and all three lumens plus the outer diameter of the tube (OD) are round within 001 Please let me know what you think of my 3x lumen die design prints Take the time to study them and imagine the dies joined with a crosshead and mounted on a 15rdquo extruder Sometimes I think about writing a book about medical extrusions Irsquom getting up there in age and I havenrsquot figured out how to connect my brain to a computer so I can download all my extrusion knowledge before I take in my last breath of oxygen The Chuck

25

Works Cited

BampH Tool Company (2010) Understanding and Communicating Draw Down Ratio Balance

Retrieved October 20 2010 from BampH Tool Company httpwwwbhtoolcomadvisor-article3htm

Colbert J (1996 March) Achieving Precision Tube Extrusion for Medical Applications Medical Plastics and Biomaterials

Conley B (2006) The extrusion process is challenged by advancing medical technology requirements Rubber World

Extrudex Kunststoffmaschinen (2008 September 30) Extrusion of single- and multi-lumen catheter tubes with thermoplastic materials Retrieved October 20 2010 from

httpdownloadsgerman-pavilioncomdownloadspdfexhibitor_19892pdf

Hendess P M (2002) Plastic Extrusion Forming Heads For Pipe and Tube Designs and Materials

Marchal T Burton T Franceschetti G De Rijcke J Chauvin C amp Metwally H M (2007) Numerical Balancing of Coextrusion Dies A Validation Study With a TPV-Based Hose ANTEC Michaeli W (2004) Extrusion Dies for Plastics and Rubber - Design and Engineering Computations (3rd Edition) In W Michaeli Extrusion Dies for Plastics and Rubber - Design and Engineering Computations (3rd Edition) (p 116) Hanser Publishers Michaeli W (2004) Extrusion Dies for Plastics and Rubber - Design and Engineering Computations (3rd Edition) In W Michaeli Extrusion Dies for Plastics and Rubber - Design and Engineering Computations (3rd Edition) (pp 305 320 - 323) Hanser Publishers

Munot A Mead J L Orroth S A amp Stacer R G (1999) Use of Stereolithography for Extrusion Dies ANTEC

Naitove M H (nd) New Pressure Transducer For Food amp Medical Extrusion Plastics Technology Rauwendaal C (2009 August 28) Tooling Design for Tubing Sparacino C (1999) Manufacturing Close Tolerance Medical Tubing ANTEC