1. Injection molding
Residual Stresses in Injection Molding
Four stages
1.
2.
3.
4.
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•
•
•
Injection Molding
Simple stress model
Residual Stress measurements
Conclusions
Fill
Hold
Cool
Pressure [MPa]
Basic understanding and measurement
Filling
Holding
Cooling
Ejection
Eject
Pressure profile
time [s]
K.M.B. (Kaspar) Jansen
k.m.b.jansen@tudelft.nl
1
2
Associate professor
Faculty of Industrial Design Engineering
Product Engineering Section
Schematic of thermoplastic Injection molding machine
1. Injection molding
2. Simple stress model
Residual stresses in finished products
•
•
•
•
•
•
•
•
•
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Due to crystallization, thermal shrinkage and pressure
Invisible
Affects product performance
More sensitive to stress cracking
Warpage, tolerance problems
Undesired optical effects (birefringence)
Shrinkage is prevented in mold (ribs, etc.)
Layer-wise solidification
Each layer freezes under different pressure
Pressure profile freezes-in
Pressure [MPa]
2. Simple stress model
40 -
P(t)
20 -
-10 -
• Stress after ejection:
-10 compressive
compressive
September 24, 2012
Validation studies
z = thickness coordinate
1
1
• Residual stresses
2
2
1 2
1
Jansen, Polym. Eng. Sci. 38, p.2030 (1999)
Jansen, Intern. Polym. Process. 9, p.82 (1994)
α
1 2
1
• Shrinkage after molding:
α
Note:
Thermal stresses vanish after ejection and cooling down
Reason: Prevention of in-mold shrinkage
September 24, 2012
0-
-20 -
2. Simple stress model
With equations
• Solidification pressure:
• Expansion upon ejection:
tensile
Ejection
time [s]
3
+10 -
0-
60 -
• Hydrostatic strain:
MPa
MPa
1
2
∞
Jansen, Intern. Polym. Process. 13, p.99 (1998)
Jansen, Polym. Eng. Sci. 38, p.838 (1998)
5
September 24, 2012
6
 Works amazingly well!
1
2. Simple stress model
2. Simple stress model
Warpage due to uneven cooling
Warpage due to uneven cooling
• Injection Molded plate
- Low hold pressures warp to hot side; high to cold side
curvature
12 1
2
• Injection molded corner product
tensile
Hot
Cold
•
/
Intern. Polym. Process. 13, p.417 (1998)
/
compressive
- Model predicts correct trends, but
- underpredicts warpage by factor 2
Intern. Polym. Process. 13, p.417 (1998)
 Fine tuning of angle with Ph and ΔTw possible!
September 24, 2012
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September 24, 2012
2. Conclusions about stress model
• Model is simple and gives a clear understanding
• For simple geometries model works as good as
simulation tools
• Frozen-in cavity pressure profile determines stresses
and shrinkage (not the thermal stresses!)
• Surface layer and core are in tension; sub-surface
layer in compression
3. Residual Stress Measurements
Overview of methods
•
MPa
•
+10 -
tensile
0-
• Shrinkage and warpage follow from same model
• Shrinkage and warpage can be tuned with holding
pressure and mold temperatures
8
•
•
-10 compressive
September 24, 2012
9
birefringence
• Sensitive to both orientation and stress (PC, PMMA more for stress,
PS more for orientation)
• Stresses relieve near cutting surface  not useful
Layer Removal
• Elaborative
• Stress relaxation due to milling heat
Hole drilling method
• Only “average” stress level possible  not useful
Environmental Stress Cracking test
• Only “average” stress level possible  not useful
September 24, 2012
10
3. Residual Stress Measurements
3. Residual Stress Measurements
Outline of Layer Removal test
Example of Layer Removal method
1. Mill top layer (50-200 µm)
2. Residual stresses are no longer
balanced
zr
3. Release from vacuum rig
h(x)
4. Measure curvature κ
 Back-Calculate stress distribution
6 1
4
2
Treuting and Read, J Appl Phys 22, p.130 (1951)
September 24, 2012
11
deflection profiles
curvature profile
September 24, 2012
stress distribution
12
Hastenberg et al., Pol Eng Sci 32, p.506 (1992)
Zoetelief et al.,
Pol Eng Sci 36, p.1886 (1996)
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3. Residual Stress Measurements
3. Residual Stress Measurements
Layer Removal method: Problems and solutions
• Stress relaxation after production
Why Excimer laser ablation
•
KrF has high energy photons (3.5-8 eV)
 Store in freezer before use
• Melting and stress release during milling
0
C-O
3.6
C-C bond strength
6.2
10 eV
 Use sharp tool and speed < 1500 rpm
• Creep effects after milling
KrF excimer laser
 Due to flattening in test rig! Wait 10 min (96% recovery)
Nd:YAG laser
CO2 laser
• Resolution: milling <0.1 mm is difficult
 Will give problems near surface
curvatures
data
• Data analyis of κ(zr) curve: differentiation error
Polyfit 4
•
direct bond breaking; no melting!
•
Pulsed operation: shockwaves remove debris
•
Ultra thin layers possible (< 1 µm)
Polyfit 6
 Do not fit a polynomial over all data points
but use a sliding window fit
September 24, 2012
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1
September 24, 2012
0.5
Jansen, Polym. Eng. Sci. 38, p.2030 (1999)
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Note: only works well with Polycarbonate
0
3. Residual Stress Measurements
3. Residual Stress Measurements
Comparison with model predictions
Comparison between standard milling and laser ablation
• Stress levels of 5 MPa tensile to -10 MPa compression
• Typical error margin: ± 0.5 MPa
• Good consistency; differences only near surface (200 µm)
• 500-1000 rpm milling similar to laser ablation
• 2000 rpm not OK: surface stresses changed
High viscosity; high Ph
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small surface stress
compressive sub-surface layer
good match with model
High viscosity; low Ph
-
large compressive surface stress
almost quenching like stress profile
reasonable match with model
Low viscosity; high Ph
September 24, 2012
- large tensile surface stress
- “inversion” of stress profile
2012
-September
low24,stresses
in core
- good match with model
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4. Conclusions
MPa
• Stresses in injection molding are mainly due to frozenin pressure variations
• Warpage is due to asymmetric stress distribution
• Stress profile, warpage and shrinkage can be
estimated is a relatively simple way
tensile
+10 -
0-
-10 compressive
• Layer Removal method is suitable to measure stress
profiles
 but carefulness is required
• Depending on viscosity and holding pressure the
frozen-in stress profiles can be tuned from
compressive to tensile!
September 24, 2012
‐1
‐0.5
0
0.5
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