PP 2007 Lecture 6 Injection Moldingweb.engr.oregonstate.edu/~atre/PP 2007 Lecture 6 Injection...
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Polymer Processing Lecture 6 Injection Molding
Summer 2007
S.V. Atre 1
Injection Molding
Lecture 6 Chapter 6
Key Issues to Address
� Process
� Process Analysis
� Problem Solving
� Research Issues
Polymer Processing Lecture 6 Injection Molding
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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
Polymer Processing Lecture 6 Injection Molding
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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
Polymer Processing Lecture 6 Injection Molding
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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
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