PP 2007 Lecture 6 Injection Moldingweb.engr.oregonstate.edu/~atre/PP 2007 Lecture 6 Injection Molding... · Polymer Processing Lecture 6 Injection Molding Summer 2007 S.V. Atre 2

Polymer Processing Lecture 6 Injection Molding

Summer 2007

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Injection Molding

Lecture 6 Chapter 6

Key Issues to Address

� Process

� Process Analysis

� Problem Solving

� Research Issues


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Injection Molding

Polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity where it solidifies; molded part is then removed from cavity

� Produces discrete components almost always to net shape

� Typical cycle time ∼∼∼∼10 to 30 sec., but cycles > 1 minute not uncommon

� Mold may contain multiple cavities

Injection Molded Parts ( Moldings)

� Complex and intricate shapes � Shape limitations:

– Capability to fabricate a mold whose cavity is the same geometry as part

– Shape must allow for part removal from mold

� Part size from ∼ ∼ ∼ ∼ 50 g up to ∼∼∼∼ 25 kg� Injection molding is economical only

for large production quantities


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Part Cost

Polymers for Injection Molding

� Injection molding is the most widely used molding process for thermoplastics

� Some thermosets and elastomersare injection molded– Modifications in equipment and operating

parameters must be made to avoid premature cross-linking of these materials


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Foam Injection Molding

Molding of thermoplastic parts that possess dense outer skin surrounding lightweight foam center

� Part has high stiffness-to-weight ratio� Produced either by introducing a gas into molten

plastic or by mixing an ingredient with resin � Foam in contact with cold mold surface collapses to

form dense skin, while core retains cellular structure

Injection Molding of Thermosets

� Equipment and operating procedure must be modified to avoid premature cross-linking– Reciprocating-screw injection unit with shorter

barrel length

� Barrel temperatures are relatively low � Cross-linking occurs in a heated mold� Curing is the most time-consuming step


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Reaction Injection Molding

Two highly reactive liquid ingredients are mixed and immediately injected into a mold cavity where chemical reactions leading to solidification occur

� RIM was developed with polyurethane to produce large automotive parts such as bumpers and fenders

� Other materials: epoxy, urea-formaldehyde

Powder Injection Molding (PIM)

Polymers are mixed with metal or ceramic powders and injected into a mold

� The polymers are removed from the molded part by several methods and the particles are fused together thermally (sintering ) to form a net-shaped part

� Combines material performance with shape complexity and high volume manufacturing


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PIM: Overall Approach

debind

shape

flow

sinter

powder

design

Injection Molding Machine

� Two principal components:– Injection unit – melts and delivers polymer melt, operates

much like an extruder – Clamping unit – opens and closes mold each injection cycle


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Injection Unit of Molding Machine

� Consists of barrel fed from one end by a hopper containing plastic pellets

� Inside the barrel is a screw which:1. Rotates for mixing and heating the polymer2. Acts as a ram to inject molten plastic into mold

� Non-return valve near tip of screw prevents melt flowing backward

� Later in molding cycle ram retracts to its former position

Clamping Unit of Molding Machine

� Functions : 1. Holds two halves of mold in proper alignment

with each other2. Keeps mold closed during injection by applying

a clamping force to resist injection force3. Opens and closes the mold at the appropriate

times in molding cycle


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Machine Cost

(1) mold is closed

Molding Cycle: 1


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(2) melt is injected into cavity

Molding Cycle: 2

(3) screw is retracted

Molding Cycle: 3


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(4) mold opens and part is ejected

Molding Cycle: 4

Injection Molding Cycle

1

2

34

5

1. Mold Close2. Filling3. Pack and Hold4. Cooling5. Mold opening

and ejection

* Fractions of total cycle time


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Cycle Time

Cycle Time


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Process Analysis: Cooling

� Cooling Time (Plate)

� Cooling Time (Cylinder)

α – thermal diffusivity, TM – melt temperatureTW – mold temperature, TD – average part temperature during ejection

The Mold

� Various types of mold for injection molding:– Two-plate mold– Three-plate mold– Hot-runner mold


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(a) closed (b) open

Two-Plate Mold

Two-Plate Mold Features

� Cavity – has geometry of part but slightly oversized to allow for shrinkage– Created by machining of the mating surfaces of

two mold halves

� Distribution channel through which polymer melt flows into mold cavity

� Sprue - leads from nozzle into mold� Runners - lead from sprue to cavity (or

cavities)� Gates - constrict flow of plastic into cavity


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More Two-Plate Mold Features

� Ejection system – function is to eject molded part from cavity at end of cycle – Ejector pins built into moving half of mold usually

accomplish this function

� Cooling system - water is circulated through passageways in mold to remove heat from hot plastic

� Air vents – to permit evacuation of air from cavity as polymer melt rushes in

Three-Plate Mold

Uses 3 plates to separate parts from sprue and runner when mold opens

� Advantages over two-plate mold:– Allows automatic operation of molding machine

� As mold opens, runner and parts disconnect & drop by gravity into two containers under mold

– Flow of molten plastic is through a gate at the base of part rather than side, allowing more even distribution of plastic melt into sides of cup


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Hot-Runner Mold

� Eliminates solidification of sprue and runner by locating heaters around the corresponding runner channels

� While plastic in mold cavity solidifies, material in sprue and runner channels remains molten, ready to be injected into cavity in next cycle– This saves material that otherwise would be scrap

Mold Cost


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Cost Model

Shrinkage

� Polymers have high thermal expansion coefficients, so significant shrinkage occurs during cooling in mold

� Typical shrinkage values:

Plastic Shrinkage, mm/mmNylon-6,6 0.020Polyethylene 0.025Polystyrene 0.004PVC 0.005


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Compensation for Shrinkage

� Dimensions of mold cavity must be larger than specified part dimensions:

Dc = Dp + DpS + DpS2

where Dc = dimension of cavity; Dp = molded part dimension, and S = shrinkage value

Shrinkage Compensation Factors

� Fillers in the plastic tend to reduce shrinkage

� Injection pressure – as pressure is increased, it forces more material into the mold cavity, and shrinkage is reduced

� Compaction time - similar effect - forces more material into cavity during shrinkage

� Molding temperature - higher temperature lowers the polymer melt viscosity, allowing more material to be packed into mold


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Injection Molding Stage

Mold Close → Fill → Pack→ Cool →Mold Open → Part Eject

Repeat cycle

Molding Defects

Air trapWeld-line

Short shot

Flashing

Filler-polymer separation

Jetting


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Molding a Square Plate

Plate thickness: 3 mm, 2 mm and 1 mm

Roshan Urval, OSU

Maximum Injection Pressure

0

10

20

30

40

50

0 1 2 3 4 5 6 7 8 9 10

Cases

Pi (MPa)

3 mm

2 mm

1 mm

0.4 MPa

0.8 MPa

4.2 MPa

Too High: Jetting

Too Low: Short shots and weld-lines


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Clamping Force

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7 8 9 10

Cases

f c (ton)

3 mm

2 mm

1 mm

Too High:Reduces mold life

Too Low:Flashing

Cooling Time

Cooling Time

0

1

2

3

4

5

0 1 2 3 4 5 6 7 8 9 10

Cases

t c (s)

3 mm

2 mm

1 mm

Too long:Long cycle times

Too short:Short shot, ejector pin marks and weld-lines


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Temperature Distribution

1 mm 2 mm 3 mm

Melt Front Temperature Difference (ΔΔΔΔMFT)

140 °°°°C

130 °°°°C

140 °°°°C

110 °°°°C

140 °°°°C

64 °°°°C

∆MFT = 76 °°°°C ∆MFT = 20 °°°°C ∆MFT = 10 °°°°C

Process Conditions: Tm = 140 °°°°C, Tw = 45 °°°°C, tf = 1.0 s

• ΔΔΔΔMFT is important for uniform material properties.

• Residual stresses lead to warpage and cracking.

Large ΔΔΔΔMFT mainly due to fast cooling of molten feedstock.

• Large ΔΔΔΔMFT leads to residual stress.

Temperature Distribution


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Residual Stress at Part Surface

α – thermal expansion coefficient, E – modulusTg – glass transition temperature, TF – final part temperature

Short Shot Formation

(Tm = 140 °°°°C, Tw = 45 °°°°C, tf = 1.0 s)

1 mm 2 mm 3 mm

10.46% filled 52.40% filled 95.90% filled

Increased surface area causes the melt to solidify prematurely


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0

2

4

6

8

10

12

14

25 30 35 40 45 50 55 60

Mold Temperature (oC)

Fill

Tim

e (s

)Filled 1mm

Filled 2mm

Filled 3mm

1mm Short shot

2 mm Short shot

3 mm Short shot

Process Window

3 mm

2 mm

1 mm

Process Window for Plastics


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Injection Molding Experiments

Predicted vs. Experimental Mold Filling Pattern.

Microscale Powder Injection Molding (µPIM)


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µ-PIM Applications

MiniaturizationMicro heat exchangerMicro reactorMicro mixerInkjet nozzle

Micro Pattern / FeatureMicro channelMicro lensMicro texture matrix

µ-PIM: Revenue vs. Part Size

IMM, 2002

Microchannel Arrays

Carl Wu


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Material Homogeneity

Channel Image & X-section, 100x

Short filling

Micrograph Morphology, 1000x

Heterogeneity

Molded MCAs

Feedstock Material

Alumina (BASF Catamold AO–F)Composition:

Powder: Alumina (0.4 µm), Binder: PolyacetalPhysical Properties:

Density, Modulus, PVT

Thermal Properties:

Specific Heat, Thermal Conductivity, Transition Temperature

Rheological Properties:

Viscosity, Shear Rate, Temperature


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Results: Optical Microscopy

short shots, distortion, cracks, polymer skin

There is more polymer on the surface of ribs?

Results: Profilometry

Dimensions of 50 µm green & sintered MCAsInhomogeneities get magnified after sintering

Green

Sintered


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Results: XPS

Al/C Distribution Gradient

0.08

0.13

0.18

0.23

0.28

50 RibTop

50 topdow n

50 Mid 50Bottom

50 TopTrench

50 lowtrench

50 Body

Locations

Al/C ratio represents concentration of powder/polymerThere is more polymer on the surface of ribs

Results: XPS

Al/C Ratio Comparion in Rib and Bulk Areas

0

0.05

0.1

0.15

0.2

0.25

0.3

500a 50Control 50HP100 50VFR12

Sample Groups

Al/C

C Ribs

Bulks

Al/C ratio represents concentration of powder/polymerVaries as a function of process parameters


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Results: Nanoindentation

Indentation: The modulus of 50 µm Green Samples

0

2

4

6

8

10

50µm Ribs Center Base

Sampling Locations

GP

a )

Modulus Avg

Powder concentration in ribs is greater than then bulk

50 μm

21-35

36-50

1-3

…16-18

T1 T2 T6 T8T7

4-6 19

20

Body

Ribs

Results: SEM

Sinteredsamples


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Analysis: Mold Filling Behavior

PIMSolver

Developed for PIM

Moldflow Simulation

Wax binder

Polyacetal

0-30 MPa

0-3 MPa

Green Microfabrication

Flow-Independent Processing with Laser / Ion Beams

2-5 µµµµm20-100 µµµµm

• particle size limited?

Channel dimensions: 1 – 500 µµµµm


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Process Analysis: Molding Cycle

HW 4

� Problem 6.1


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You should have learnt …

� Injection Molding Process� Process Variants� Injection Molds� Research Issues in Microfabrication� Process Analysis

Next Class

� Nanotechnology & Microfabrication

Documents

PP 2007 Lecture 6 Injection Moldingweb.engr.oregonstate.edu/~atre/PP 2007 Lecture 6 Injection Molding... · Polymer Processing Lecture 6 Injection Molding Summer 2007 S.V. Atre 2