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The injection moulding process. Introduction

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It consist in introducing a molten thermoplastic into a mold at high temperature and pressure. The mold gives the final shape, cools off and ejects the part.

Injection molding process

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Injection of large dimensions components

[www.dpg.com]

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Microinjection

[www.sms-k.com]

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Size

Lenght

Thickness

Complexity

Injection moldingparameters

Geometry

Mold

Injection moldingmachine

Process

Rheology PVT curves Cristallynity degree

Material

Tools

Process-structure-properties

relationships

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Clampingunit Mold

Injection unit

Control system

Heating

Homogenizing

Inject

The injection molding machine

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Parting line

Temperingunit

Feedingsystem

Cavity

Ejectingsystem

Injectionpoint (gate)

The mold (basics)

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a) Mold closes

b) Injectionc) Holding

d) Melting material e) Cooling

f) Mold opening, ejection

The injection cycle

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The injection cycle

[www.dmeuniversity.com]

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e) Cooling

b) Injection/filling

c) Holding pressure

d) Plasticating

f) Ejection

StartEnd

a) Mold closes

Time

The injection cycle

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The injection process: Main parameters

Temperature: melt temp. mould temp Pressures: injection max, holding Time: injection, holding pressure, mould

opening, cycle Shot size: screw travel Speed: Injection speed

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The injection process: Melt and Mould Temperature

160-180175~210190~21060~90POM

190~250250~280250~28050~80PA66

PA6

200~210210~230210~23050~80PA type

140~170190~220190~24030~70PMMA

230~270280~340300~35070~115PC

130~140140~200170~20020~60Soft PVC

Hard PVC

140~150170~200170~20020~60PVC type

140~180220~290220~32510~80PP

130~200220~300230~31020~60HDPE

LDPE

130~200220~300230~31020~60LDPE types

150~160200~250200~27010~80SAN

150~160190~260200~27010~80ABS

150~160190~260200~27010~75HIPS

PS

150~160200~260200~28010~75PS types

RearMiddleNozzle

plastic injection molding temperature of barrelMouldtemperature

Plastic type

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The injection process: Meld and Mould Temperature

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Sprue

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Runner

Gate

Part

Advancing

Flow Front

Flow Length

Flow is driven by pressure Overcomes the melt's resistance to flowPlastics flow from high to low pressure areas Pressure decreases along the flow length

The injection cycle: Pressure

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Cooling time

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Mold closed

Cycle time

The injection cycle: Typical Pressure-time cycle evolution

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Time (s)0 5 15 33 35

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Cooling

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Inner of thecavityGate

The injection cycle: Temperature-time evolution

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1

1 Mold closes. Channels are filled

Inner of cavity

Gate

2 Part filling

2 3

3 Holding pressure. Gate is frozen

4 Ejection

4 5

The injection cycle: Pressure and temperature evolution

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= FSShear stress

vy v

y =. Strain rate

= .Viscosisty

Newtonian

Pseudoplastic

The injection cycle: Materials viscosity

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= .Viscosity

Newtonian

Pseudoplastic

Tp

.

The injection cycle: Materials viscosity

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n: Cross exponent . P= pressureB:Arrhenius constant T=temperatureTb:Reference temperature* =Shear stress=Shear rate0=viscosity extrapolated at zero shear rater

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vF

h

hv

AF

F/A

v/h

1

Newtonian Viscosity

hv

Generalization: )(

Injection molding

rateshear :Typical shear rate for Polymer processes (sec)-1

Extrusion 102~103Calendering 10~102Injection molding 103~104Comp. Molding 1~10

Shear Thinning

~ 1 sec-1 for PE

The injection cycle: Materials viscosity

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Material viscosity

Injection speed.

Profile of velocitiesMould and partgeometry

Runner/channel geometry. Cold/hot runners Gate shape.Part shape: thickness, length, weight.

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Time (s)0 5 10 15

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PA66 at 290C

PA66 at 270C

PA66 30% glass fiberat 290C

The injection cycle: Factors influencing injection pressure

Material. Viscosity. Fillers, etc... Melt temperature Mould temperature

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Unfilled polyamide at290C

Unfilled polyamide at270C

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Glass fiber filledpolyamide at 290C

http://islnotes.cps.msu.edu/trp/inj

The filling stage is clearly non isothermal. There is a growing frozenlayer of polymer near the mould wall. The cross section or the meltpath changes (diminish) with time. There is a local heating due toviscous flow and shear effects and a global cooling from the mouldwalls. The type of the flow is called Fountain flow.

The injection cycle: Factors influencing injection pressure

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Frozen layer

Mould wall

Mould wall

Streching ofmaterial.

orientation

The prediction of the exactflow shape during theinjection is difficult becausethe material cools down andsolidifies. The profile is notfully parabolic. The plastic atthe mould wall is solid andforms a skin while the innerpart (core) is still melted.

P

Advancingmelt front

The injection cycle: Fountain flow

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Shear rate - min max

Molecular orientation is caused by shear flowThe highest amount of shear is inside the frozen layer

Produces the highest orientation

The injection cycle: Shear

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The fountain flow induces a particular orientation in the injection mouldingof particulate-filled polymers. Theorientation is higher near the moldwalls.

The shear stress aligned the fibers in the flow direction and in the strechingdirection.

[http://islnotes.cps.msu.edu/trp/inj]

The injection cycle: Fiber orientation

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An important issue is the effect of the different cooling rate betweenplastic at the mold wall and core. The former cools very quicly (highfreezing rate) while the latter cools very slowly (low freezing rate). Therefore, the outer polymer layer may have lower crystallinity degreethan the inner layer due to uneven cooling.

Skin-core structure

Melt flow

Skin

Gate

[PhD Thesis Hans Ziudema. Univ. Tech. Eindhoven. 2000]

The injection cycle: Skin-core and frozen layer

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v

T

Amorphous

p

Tg

v

T

Semicrystalline

Tg Tf

p

PVT curves

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p1

p2

p3

v

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2

1-2 Holding

TEjection

3

4

2-4 Cooling

0

1

0-1 Injeccin

5

4-5 Cooling outsidemould

Troom

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The process curve

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Material type v

TTEXPTAMB

C

Amorphous Semicrystalline Filled with particles

Amorphous

Semicrystalline

Fillers and additives

The process curve

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p1

p2

p3

v

T

2

TEJECTION

3

4

0

1

5

1-2 Holding

2-4 Cooling

0-1 Injection

4-5 Cooling out ofmould

TROOM

Material type Amorphous

Semicrystalline

Fillers and particles

Process Pressure

3

The process curve

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Material type Amorphous

Semicrystalline

Fillers and particles

Process PressureMelt and mouldtemperatures

Tv

Fast coolSlow cool

Part geometry Thickness, shape,

The process curve

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Material type Amorphous

Semicrystalline

Fillers

Process Pressure Melt and mouldtemepratures

Part geometry Thickness, shape

Flow lenght

Core-shell structure

The process curve

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Pressure

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Thermaldegradation

Unfilledparts

Flash

Melting and plastificationproblems

The process curve: process window.