All About Plastic Injection Molding

8/9/2019 All About Plastic Injection Molding

1/33

Plastic Injection Molding

www.engmatl.com


2/33

Injection Molding

3 major functional units; injection, mold, clamping


3/33


is a manufacturing technique for making parts fromthermoplastic and thermoset materials

In contrast to the extrusion (which makes continuousparts of constant cross section), injection molding make

discrete parts (with complex and variable cross section) Molten plastic is injected at high pressure into a mold,

which is the inverse of the desired shape.

The mold is made from metal, usually eithersteel oraluminium

widely used for manufacturing a variety of parts, from thesmallest component to entire body panels ofcars


4/33

The key to success in injection molding are to have;

Proper machine for good melting and injecting of theresin

The proper resin to appropriate part performance A good mold for part definition and removal

Proper operation for efficient molding cycle (mold cycledepends on the design of the mold and manufacturingparameters)



5/33

The most commonly used thermoplastic

materials are: Polystyrene (low-cost, lacking the strength andlongevity of other materials)

ABS oracrylonitrile butadiene styrene (a co-polymeror mixture of compounds used for everything fromLego parts to electronics housings)

Nylon (chemically resistant, heat-resistant, tough andflexible - used for combs)

Polypropylene PP (tough and flexible - used forcontainers

PVC (more common in extrusions as used for pipes,window frames, or as the insulation on wiring where itis rendered flexible by the inclusion of a highproportion ofplasticiser).


6/33

Injection Unit

Purpose: to liquefy the plasticmaterials and then inject theliquid into mold

Resin is introduced throughhopper

Some machines can haveseveral hoppers (to fed filler,colorants, other additives)-Injection molding act as mixer

However, due to limited sizeof barrel, mixing capability ispoor


7/33

From hopper hole (feed throat)

Barrel made of heavy steel cylinder to

withstand the pressure and temperature

involved in melting the resin

2 types of system used in injection

molding;

Reciprocating screw- similar to extruder screwbut with unique reciprocating action

Ram injector

Injection Unit


8/33

Design of screw- similar to an extrusion screw 3 sections;

Feed section- to advance the resin

Compression section- to melt the resin

Metering section- to homogenize the resin and pump it forward

The screw of injection molding machine is shorter thanextruder, L/D ratios are 12:1 and 20:1

Low L/D ratios suggest the mixing is less efficient in theinjection molding machine

The compression ratio (diameter of root at feed zone to

the diameter of root at metering zone) often in the rangeof 2:1 and 5:1

Low compression ratio means less mechanical action isadded during melting process

Injection Unit


9/33

Important measure of the size of an

injection molding is weight of resin that

can be injected, called shot size

Typical shot size range from 20g to 20 kg

Since shot size depends on the density of

the plastic, PS has been chosed as the

standard for rating the machine

Injection Unit


10/33

Reciprocating Screw Injection

Molding Machine Resin is melt by mechanical shear and thermalenergy from heaters

The molten resin is conveyed to a space at the

end of the screw- collects in a pool Here, the mold is closed


11/33

The entire screw move forward and pushes themolten resin out through the end of barrel

To ensure the resin does not flow backward, a

check valve ornonreturn valve is attached to theend of screw

Normally the screw will stay in the forwardposition, until resin began to harden in the mold


Molding Machine


12/33

Retraction of the screw, create space at the end of thescrew

Cooling of the part in the mold, until it can be removed

While the part is cooling, the screw turns and meltsadditional resin


Molding Machine


13/33

Advantages

More uniform melting

Improved mixing or additives and dispersionthroughout the resin

Lower injection pressure

Fewer stresses in the part

Faster total cycle


Molding Machine


14/33

In this type of injection molding, the resin is fed from a

hopper into the barrel, and heated through thermalenergy from the heaters

The molten resin is collect in a pool in a barrel celledinjection chamber

The molten resin is then push forward by the action of

plunger (ram or piston) To five better mixing, the molten resin is pushed past a

torpedo/spreader, impart shear to the melt

Ram Injection- Injection Molding Machine


15/33

Moulds

Designing and making mold for injectionmolding is more complicated than making

extrusion die

Mold Parts mold is placed in between

stationary plate and the moveable plate


16/33

The connection from the injection unit to themold is through the nozzle

The channel that run through the stationary plate

of the mold is called the sprue channel (materialthat is in the channel is called the sprue)

The solid sprue is removed from the finishedpart assembly after the part is ejected from the

mold Resin flow from the sprue through the runner

(connecting channel) to the mold cavities

Moulds


17/33

Mold Bases

Assembly of variousmold parts

Mold bases can be

purchased as entireunits, then the

cavities are cut from

A & B plates


18/33

Runners

Distribution system for the resin from the sprueto the cavities

Flow characteristics (viscosity), temperature and

other factors are important in determining therunner diameter and length

If the diameter of the runner is too small or thelength is too long,the resin can freeze in the

runner before the mold is completely full If the runner system is too large, excess material

would be ejected and too much regrind created


19/33

If the resins have a high viscosity, larger runnersare needed compared to low viscosity resin

The optimum flow of the resin through therunner system depends on the shape anddiameter of the channel

Round channel give the best flow characteristicsbut difficult to machine

Machining cost can be reduce by machining one

side of the mold plates Better shape where the depth of the channel is

at least two-thirds the size of the width and thesides are tapered between 2 to 5.

Runners


20/33

Runner Channel Shapes


21/33

Secondary Runners

Secondary runnerchannel are used formulticavity molds

The flow into thesecondary channelshould bestreamlined (angle in

flow direction) The streamlined

minimizes shear onthe resin


22/33

Gates

The end of runner and the entry path into

the cavity

The gate shape can also affect the fillingof the cavity, dimension and properties of

the parts

Gate is the most restricted point in

injection molding system, i.e. for

reinforcement and filler + polymer systems


23/33

Gate Design

Small rectangular opening atthe end of the runner channel,connect to the edge of cavity

Edge gate can be below theparting line if the channel andpart are also below the partingline

Or it can be symetricaly aboutthe parting line, if the runnerchannel and part are at bothside of parting line

Edge gate


24/33

Submarine Gate

Starts from the edge of therunner, and goes into the cavityedge at an angle

It narrows to a point as it movesfrom the runner to the cavity

The advantage; separation ofthe parts and the runner isautomatic

Disadvantage; gate cannot beused for some resins becauseof high shear

Submarine Gate


25/33

Tab Gate

By connecting the runner directly

into the cavity with no reduction

in runner cross-section

Used for very large parts where a

reduction in flow would disturb

the resins flow pattern and might

result inadequate flow into the

cavity


26/33

Fan Gate

Made by reducing thethickness and not thediameter of the runner

channel as it goes into thecavity

Used for intermediate size,and when reinforcement inthe resin cannot flow throughthe edge gate


27/33

Ring Gate

Used to make hollow

cylinder parts

The ring gate covers theentire top of the cylinder

part so that the resin flow

is downward into the wall

of the part


28/33

Cavities

Are actual molding locations

Resin enter the cavities through gate, fills

the cavities, and cools to form the solid.The parts are ejected and finished

Cavities are the heart of the molding

process, and must be precisely prepared

The shape of the cavities determines the

shape of the part


29/33

Materials & Product Consideration

Almost all thermoplastic can be injectionmolded

Resin with low melt viscosity is required;

so that the flow through runner, gate,cavity easily done with minimuminjection pressure

Resin with injection molding grades havelow molecular weight and narrowmolecular weight distribution


30/33

Part Shapes

Hollow parts can be created by allowing

the moveable plate to protrude into the

cavity of the stationary plate


31/33

Threads can be placed on the inside of a

part by using a core pin that is inserted

into the cavity where the threads are

desired

Part Shapes


32/33

A hollow part with a hole on the side is evenmore complicated (the core pin is used)

The core pin slide into position after the mold isclosed

The core pin seals against the surface of themoveable plate, prevent flow of resin into thearea

Part Shapes


33/33

Authors request: Please add more useful

information & re-upload the file.

THANKS

Documents

All About Plastic Injection Molding