1366638848_rma_rotomolding_guide_20110701

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Ravago Manufacturing Americas13001 Almeda Road

Houston, Texas 77045

713-433-5604 Fax 713-433-9783

The technical information, suggested uses and applications presented are made without charge and are believed to be reliable; however Ravago Manufacturing Americas disclaims responsibility

for results of use of this information. Ravago Manufacturing Americas makes no warranties, either expressed or implied, concerning our materials, including any warranties of merchantability or

fitness for a particular use. All users should rely upon their own tests in determining suitability.

13001 Almeda Road, Houston, TX 77045 Tel: (713)433-5604

TECHNICAL NOTE

Rotational Molding Guide

Introduction:Rotational molding is a process where plastic materi-

als are formed into useful articles by rotating a mold

containing the plastic material in an oven sufficiently

hot to melt the plastic. The mold is rotated in two

directions to cause the plastic to coat all surfaces of

the mold. There is little pressure inside the mold dur-

ing the forming process. After the plastic is thorough-

ly melted to coat the mold surface the mold and plas-tic is cooled, the part is removed from the mold.

The rotational molding process is ideally suited to

fabrication of hollow parts. It also lends itself to pro-

duction of rather large parts. The process can ac-

commodate articles with complex geometry.

Because the process is carried out at low pressure, the

molds can be fabricated from relatively inexpensive

materials. The relatively low cost of tooling allows

rotational molding to be used to produce parts whose

production volume would not amortize the high cost

of expensive injection or blow molds.

At first examination, the rotational molding process

seems simple. Those who have had experience pro-

ducing parts by rotational molding understand that the

rotomolding is a complex manufacturing process,

with many variables that can affect the quality articles

produced.

Polyethylene is a particularly good material for rota-

tional molding. When used as a fine mesh powder,

polyethylenes melting characteristics easily form a

solid part. Polyethylene has adequate heat stability to

withstand the relatively high temperature and long

heating cycles of rotational molding. Parts formed

from polyethylene have an excellent combination of

strength, toughness and other mechanical properties.

In order to optimize the properties, it is necessary to

use proper molding conditions to form articles.

Muehlstein rotational molding products are designed

for optimized processability and properties in rota-

tional molding. These products are compounded from

prime quality materials by a process designed to yield

consistent material, box-to-box and shipment-to-

shipment.

This bulletin is designed to help the customer find the

optimum molding conditions for their application.

Because of the diversity of article made by the rota-

tional molding process and the different types of rota-tional molding machines and molds used in the roto-

mold process, it is impossible to establish universal

optimum molding condition that are optimum for all

circumstances. We can offer recommendations for

starting conditions and guidelines for adjustments to

the process conditions to help optimize the process

for the particular part being molded.

Process Control:In order to achieve consistent quality rotationally

molded articles, it is important that proper molding

conditions are used and, as with any manufacturingprocess, that all variable in the manufacturing process

be controlled as closely as possible. Each rotational

molding cycle must be preformed under the same

conditions as the previous cycle, as nearly as possi-

ble.

Because different parts are often placed on the same

arm or different arms of a multi-arm machine, often

the molding cycle is a compromise to achieve ac-

ceptable quality for all parts being molded.

Mold Design:Proper mold design is critical for production of quali-ty parts. While the design of molds is outside the


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Page 2 Ravago Manufacturing Americas 20110701

scope of this guide, there are a couple of areas of

mold design that should be highlighted.

Mold Vent ing:

Mold venting is important to good part quality. Prop-

er venting will allow the part to be heated and cooled

at about atmospheric pressure. This will eliminateblowholes at the parting line. Proper venting will pro-

long the contact of the cooling part with the inner

surface of the mold, resulting in a shorter cooling

cycle and post-mold warpage. Lower pressure differ-

ential also results in longer mold life.

In general, mold vents of sufficient size and number

to allow equalization of the inner mold pressure. Vent

tubes should extend to the center of the mold. The

inside end should be loosely packed with fiberglass to

prevent the unfused powder from exiting the mold

early in the heating cycle.

In order to be efficient vent tubes must be kept clean.If the fiberglass becomes clogged with plastic powder

the vent tub will no longer function to equilibrate

pressure.

Drying:

Polyethylene does not absorb moisture easily. There-

fore, under normal conditions, pre-drying of the pul-

verized powder is not required. Under high humidity

conditions, or when specialty grades, such as flame

retardant, are used drying may be necessary.

Measurement of Charge

Production of consistent parts requires that each step

of the rotational molding process be repeated precise-

ly every cycle. This is especially important that the

same weight of polymer be added to the mold for

each part. The bulk density of pulverized polyeth-

ylene for rotational mold has some variability. There-

fore, material should be measured on a weight rather

than a volume basis.

Oven Temperature:

Proper cure of the rotationally molded part requires

sufficient heat to melt the polyethylene for long

enough time duration to completely fuse the polyeth-

ylene. The oven temperature must not e so high that

the polyethylene is oxidized. On a multi arm machine,

the heating and cooling cycles must be of equal time.

On one hand, the molder would like the oven temper-

ature to be as high as possible to shorten the heating

time to a minimum. On the other hand, keeping the

oven temperature as low as possible gives optimum

part appearance, part toughness, reduces the cooling

required and reduces thermal stress on the mold. The

rotational molder must decide what combinations of

these variable results in acceptable part quality and

productivity.

Starting Conditions:

Our experience has shown that the following condi-

tions are good starting points. The molders experi-

ence with existing molds and specific molding ma-

chines may suggest slightly different conditions. In

general, Muehlstein polyethylene rotational molding

compounds can be processed at 50 to 75F lower

oven temperatures than competitive polyethylenes,

especially if a Muehlstein precolored polyethylene is

replacing a dry-blended color.

Oven temperature: 500 to 600F

(260 to 315C)

Oven time: 10 to 25 minutes

You can reduce the trial and error involved in

time/temperature parameters by measuring the PIAT

(Peak Internal Air Temperature). The following table

is a general guide for PIAT for RMA standard resins.

These are suggested for a wall thickness of around

0.125 in and adjustments will need to be made for

thinner or thicker walls:

Table 1.

GRADE PIAT C

HMP-304 210

HMP-305 200

HMP-307 190

HMP-301 215

HMP-315 204

HMP-325 202

AQUATUF HMP-334 205

AQUATUF HMP-337 207

Cool ingLike heating cycles, there is no universal best cooling

cycle. For a multi-arm molding machine, the cooling

time can be no longer than the heating time. On the

other hand, high cooling rates increase the potential

for warped parts.

It is usually best to start the cooling with several

minutes of air-cooling. This is followed by atomized

water, air/water or water spray for the bulk of the

cooling time. It is beneficial to finish the cooling cy-


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cle with several minutes of air cooling, allowing the

outside surface of the mold to dry.

Polyethylene is semi-crystalline polymer. The degree

of crystallinity is partly controlled by the cooling rate

of the part. Rapid cooling results in lower crystallini-

ty. Unfortunately, rapid cooling produces parts with

low crystallinity at the mold side of the part and high

crystallinity at the inner surface of the part. This dis-parity manifests itself as warpage.

It has been found that warpage is minimized with

slow cooling. This must be accomplished with the

constraints of the cooling time available. Slow cool-

ing may adversely affect other physical properties,

such as impact strength.

AQUATUFHigher density, high performance res-

ins in the 0.942 to 0.952 range tend to be more crys-

talline and have a higher shrink rate. These may need

to be cooled more slowly to prevent warpage, and

every attempt made to hold the part against the mold

surface until the release point [This is best determinedby a Rotolog type trace on the cooling cycle].

Common problems:

Part Surface:

Several factors should be considered when at-

tempting to improve the quality of the parts sur-

face.

The surface of the part can be no better than

the surface of the mold.

Mold design is also of importance in theproduction of part especially in parts with

recesses, high definition or inserts.

Selection of the appropriate melt index poly-

ethylene is another factor.

The dry flow and particle size distribution of

the molding power will also affect surface

quality.

The selection of the proper combination of

heating and cooling conditions is also im-

portant factors.

Rotation speed and rotation ratio effect partsurface quality

Consistent application and minimum use ofmold release.

References:

1. Introduction to Rotational Molding Seminar,

Dr. Glenn Beall, Association of Rotational

Molders, Chicago IL.

2. Rotational Molding Troubleshooting Manual,

(ARM-102-1089), Association of Rotational

Molders, 1989, Chicago, IL


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Typical Rotation Ratios Typical Speed

(RPM)

Ratio Shapes

Major

axis

Minor

axis

8 to 1 Oblongs (Horizontal)Straight tube (Horizontal)

8 9

5 to 1 Some defroster ducts 5 6

4.5 to 1 Balls & gloves 8 9.75

3.3. to 1 Any shape having overlap-

ping lines of rotation at 4

to1

10

12

12.25

14.5

4 to 1 Cubes, balls or odd shapes

Rectangular boxes, horses

with bent legs

8

1010

12.5

2 to 1 Rings, tires, balls

Any rectangle that shows

two or more thin sides

when run at 4 to1

Picture frames, Round flat

shapes

Horses with straight legs,

Auto crash pads (vertical)

6

8

10

12

9

12

15

18

1 to 2 Parts that should run 2 to 1

but show thin side walls.

5

7

15

21

1 to 3 Flat rectangles e.g. gas

tanks, suitcases, tote bin

covers, etc.

4

6

9.5

15

22.5

361 to 4 Tires, curved air ducts, Pipe

angles, flat rectangles, Balls

whose sides are thin at 4 to

1.

4

5

6

20

25

30

1 to 5 Cylinders, vertical 4 24

Vertical - Mounted parallel to major axis

Horizontal - Mounted perpendicular to major axis.


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Rotational Molding Trouble-shooting Guide


Problem Probable Cause Possible Solution

Warped

parts

Inadequate venting Provide adequate

venting - 3/8-1/2

diameter vent per ft3

of molded volume is

suggested for thin

walled parts.

Non-uniform coolingof part caused by resin

pulling away from

mold.

Rotate mold duringcooling cycle. Provide

adequate venting and

make sure vents are

not clogged. Use less

mold release. Check

for too effective mold

release agent. Avoid

large flat panels in

part design if possible.

Reduce cooling rate

during initial part

cooling cycle. Increase

the cooling medium

temperature, air cool,

and then water cool.Apply air pressure

through spine during

cooling

Non-uniform cooling

caused by uneven wall

thickness in the part.

See suggested reme-

dies under problem

heading Uneven wall

thickness of molded

parts.

Non-uniform cooling

caused by sections of

the mold being

shielded from heat

and cooling medium.

Mount mold to elimi-

nate shielding prob-

lems, add baffles to

direct heat and cool-

ing into recessed orshielded areas.

Uneven cooling

caused by clogged

water nozzles

Check and clean noz-

zles on a periodic

schedule.

Over-cured part. Deg-

radation of the resin

due to high tempera-

ture and/or excessive-

ly long heating cycle.

Decrease oven tem-

perature or heating

time.

Highly under-fused

part. Some degree of

under-fusion is advis-

able especially in the

case of low melt-index

resins to prevent deg-

radation; however,

highly underfused

parts can cause signif-

icant loss of impact

strength.

Increase oven temper-

ature or total heating

time. Increase heat-

transfer coefficient,

e.g. steel or aluminum.

Improper coloring Select pigment and

pigment loading that

does not affect resin.


Use precolored, com-

pounded resin.

Resin type Use proper resin hav-

ing adequate melt

index and molecular

weight distribution for

application.

Moisture on resin or

pigment.

Only use dry powder

and/or pigment.

Poor

impact

strength

Resin selection not

correct

Use lower density or

lower melt index resin

Density increase dur-

ing slow cooling

Increase cooling rate

to maintain a lower

density.

Part design not ap-

propriate

Review and alter mold

design if necessary,

eliminate sharp cor-

ners and narrow pas-sages.

Insufficient fusion of

resin

Increase oven time

and/or temperature.

Improper coloring Select pigment and

pigment level that

does not effect im-

pact. Use pre-colored,

compounded resin.

Over-curing of resin.

Degradation of resin

due to long-term high

temperatures.

Decrease oven tem-

perature or heating

cycle.

Parts

sticks in

the mold

Insufficient amount of

mold release agent or

the release agent has

deteriorated with use.

Reapply or use more

release agent. Old

release may have to

be removed and a

new one applied.

Ineffective release

agent or mold release

does not withstand

elevated tempera-

tures.

Use suitable mold

release agent that is

effective for resin and

temperature used.

Apply according to

suppliers instructions.

Mechanical interfer-

ence during part re-moval

Locate mold parting

line at undercut ortaper side walls of

mold.

Roughness and poros-

ity of mold surface

provide areas where

resin may adhere.

Refinish damaged

mold surfaces, plug,

weld and sand

smooth.

Presence of resin at

parting line due to

internal mold pressure

forcing semi-molten

Provide adequate

venting, 3/8 to

diameter vent per

cubic foot of mold


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resin through parting

line.

volume is suggested

for thin walled parts.

Build-up of degraded

resin in the mold may

be caused by burning

of thin walled sections.

Clean mold periodical-

ly. Reduce oven tem-

perature.

Shrinking onto large

deep inserted areas.

Provide adequate

taper to mold walls.

Use effective mold

release on insert are-

as. Remove part while

warm. Provide ade-

quate means for ap-

plying force to sepa-

rate mold halves.

Undercuts in mold Design mold to place

undercuts at parting

linen so that mold has

draft angle for part

removal.

Low shrinkage value

for resin.

Use higher density

polyethylene grade.

Blow

holes

through

the part

or ring-

worm

effect

under

thin wall

surface

other

than at

Porosity in the cast

aluminum mold

Obtain better quality

castings. Drill through

void and drive pin or

weld from inside.

Relieve from outside

by drilling into void.

Remove parts from

molds while warm to

touch. This helps drive

moisture out of pores.

the part-

ing line

Pores or holes in

welds.

Use proper welding

rod and procedure.

Weld inside surface

first to get good

penetration.

Excessive

flashing

at mold

parting

line.

Internal mold pressure

during heating cycle

tends to force semi-

molten resin out

through the parting

line.

Provide adequate

venting and make sure

vents are not clogged.

Remate mold parting

line and adjust mold

clamp pressure evenly.

Clean mold flange to

prevent gapping and

apply new mold re-

lease on flange. Re-

duce internal air pres-

sure if used. Use lower

melt index pressure.

Bubbles

on the

mold

parting

line.

During the first stages

of cooling, there will

be a rush of air into

the part to fill the

resultant partial vacu-

um. If there is inade-

Vent the mold to at-

mosphere pressure.

Relocate vent to mid-

dle of mold. Use glass

wool in vent. Use

Teflon as vent tube.


quate venting, air will

penetrate the molten

resin, at parting line,

becoming trapped as

the part wall solidifies.

Make sure tube is

adequate size.

Poor mold parting

line.

Remate molding part-

ing line and adjust

mold clamp pressureevenly. Clean mold

flange to prevent

gapping and apply

new mold release on

flange.

Discol-

oration of

interior

surface of

part.

Degradation of resin

due to high tempera-

ture and/or excessive-

ly long heating cycle.

..

Decrease oven tem-

perature or heating

cycle, or purge part

with inert gas (nitro-

gen). Use resin with

the proper amount

and type of antioxi-

dant. Check pigment

for heat stability.Powder

bridging

or not

filling

narrow

passages

of mold.

Mold design incorrect. Modify mold by in-

creasing width to

depth ratios across the

mold opening. Design

corners of mold with

more generous radii.

Avoid ribs with less

than 4x wall thickness.

Poor pourability (dry

flow) of powder.

Make sure powder has

acceptable pourability

and bulk density.

Powder does not melt

or flow properly.

Use finer mesh pow-

der or resin with a

higher melt index.

Cold spots on mold. Avoid any shielding

mold areas. Check for

mold wall thickness

uniformity.

Improper mold rota-

tion.

Use correct ratio and

rotation speed.

Poor part

stiffness

Part wall too thin Add more powder to

initial charge.

Resin selection not

correct.

Use rein of higher

density.

Part design not ap-

propriate.

Review and alter mold

design if necessary.

Under fused parts. Increase oven temper-


cycle. Increase heat-

transfer rate by using

thinner mold walls, or

make the mold from

materials of greater

heat-transfer coeffi-

cient, e.g. steel or

aluminum. Try filling

molds while hotter.

Light-

ning

effect in

Moisture in pigment

or resin.

If dry-blending, dry

pigment or use pig-

ment from unopened


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colored

parts.

container. Use pre-

compounded color

resin powder. Dry

resin completely or

replace.

Static build-up Add small amount of

mineral oil to resin or

commercially availableanti-stat. Make certain

that all mixing and

molding equipment is

adequately grounded

with high surface cop-

per cable.

Pigment not ground

properly.

Use 100-mesh pig-

ment or pulverize

pigment prior to mix-

ing. Use pre-

compounded color

resin powder.

Blow

holesthrough

part

around

insert

Poor fit on inserts

allowing moisture orvapors to be trapped

around insert and

expand, blowing a

hole in the part.

Refit insets and relieve

to allow trappinggases to escape to the

outside of the mold.

Drill a small hole

through the insert bolt

to relieve gas pres-

sure.

Bridging of resin be-

cause of close dimen-

sions.

Change insert dimen-

sions or location to

allow powder to flow

without bridging.

Speckled

colors

and

lumps of

pigment

in dry

blended

colors

Insufficient blending Break up agglomer-

ates of pigment be-

fore blending. Use

high intensity mixer. If

unable to achieve a

desirable color bal-

ance, use a com-

pounded color resin

powder.

Long

oven

cycles

Heat-transfer rate not

adequate to melt all

resin, excessively thick

mold.

Increase heat-transfer

rate by using thinner

mold walls, or make

the mold from materi-

als with greater heat-



Heating not efficient. Increase air velocity

around mold during

heating cycle. Checkoven for air leaks.

Low oven tempera-

ture.


ature. Recalibrate

instruments on regular

schedule.

Resin powder too

coarse

Use finer mesh pow-

der

Poor melt flow Use higher melt index

resin.

Extended cooling Reduce air-water cool-

ing ratio


Long-

term part

failure

Part over-cured during

molding

Decrease oven tem-

perature or heating

cycle

Photo-degradation of

part caused by ultra-

violet light from sun

or internal lighting

(florescent)

Use UV stabilized resin

in application. Add

suitable UV stable

pigment.

Stress-cracking due to

multi-axial stresses in

part. Cracking may

have been accelerated

by chemical environ-

ment and/or tempera-

ture

Use polyethylene

grade with good envi-

ronmental stress crack

resistance (ESCR).

Modify design around

the areas containing

inserts. Examine parts

in field use to deter-

mine adequacy of

design around stress

concentration points.

Inadequate resin addi-

tive system.

Antioxidant type and

level of concentration

may be inadequate.Reduce level of inter-

nal mold release if

used.

Color change due to

oxidation. Light col-

ored parts may look

yellow or pink.

Reduce oven tem-

perature.

Improper colorants or

blending.

Use colorants that

disperse well in base

resin. Use pre-

compounded color

resin powder for op-

timum dispersion of

color and stabilizers.

Uneven

wall

thickness

of mold-

ed parts.

Improper mold rota-

tion

Vary ratio and speed

of rotation of mold to

obtain even coverage

and adequate number

of powder trackings

Mold shielding Mount mold to elimi-

nate shielding.

Uneven mold wall

thickness

Use care in designing

molds to prevent

excessive variations in

mold wall thickness

(thin spots attract

more resin)

Inadequate powder

properties. Low bulk

density, no powder

pourability, large

amount of fluff, parti-

cles have many tails

that entangle into

clumps during mold-

ing.

Obtain an acceptable

quality powder. F

using bulk powder

storage, empty stage

silos before refilling to

prevent accumulation

of fine particles in

storage silo.

Buffeting or air flow in

deep dished areas.

Avoid deep dished

areas whenever possi-

ble. Reduce thickness

of mold in dished


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areas. Open handles

so air can flow

through kiss-offs in

mold.

Highly

under-

fused

parts,with

Oven temperature not

high enough to drive

air bubbles out of part

walls.



cycle.

many

small

bubbles

in wall or

rough

powdery

inside

Heat transfer rate not

adequate to melt

resin.

Increase heat transfer

rate by using thinner

mold walls or make

mold from materials

with greater heat-



surface. Resin powder too

coarse.

Use finer mesh pow-

der.

Moisture in mold Reduce moisture in

mold by running with

warm molds and dry

mold before chargingpowder.

Poor

flow-out

into mold

recesses

Poor mold design Design shallow re-

cesses with generous

radii on edges. Pre-

heat recessed areas

with torch for 30-

seconds before charg-

ing. Add heat deflec-

tors or thermal pins.

Improper mold rota-

tion

Change ratio and/or

speed of rotation.

Melt index of resin too

low

Increase melt index of

resin.

Documents

1366638848_rma_rotomolding_guide_20110701