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K.M.B. (Kaspar) Jansenk.m.b.jansen@tudelft.nl
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Residual Stresses in Injection Molding
Basic understanding and measurement
1. Injection Molding 2. Simple stress model
3. Residual Stress measurements 4. Conclusions
Associate professor Faculty of Industrial Design Engineering
Product Engineering Section Schematic of thermoplastic Injection molding machine
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1. Injection molding
Four stages • Filling • Holding • Cooling • EjectionFill Hold Cool Eject
time [s] Pressure [MP a] Pressure profile 3
1. Injection molding
Residual stresses in finished products• Due to crystallization, thermal shrinkage and pressure • Invisible
• Affects product performance • More sensitive to stress cracking
• Warpage, tolerance problems
• Undesired optical effects (birefringence)
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2. Simple stress model
• Shrinkage is prevented in mold (ribs, etc.) • Layer-wise solidification• Each layer freezes under different pressure • Pressure profile freezes-in
time [s] Pressure [MP a] 20 40 60 10 0 +10 -MPa tensile compressive P(t) 20 10 0 -MPa compressive Ejection September 24, 2012 5
2. Simple stress model
With equations• Solidification pressure: • Hydrostatic strain: • Expansion upon ejection: • Stress after ejection:
Note: Thermal stresses vanish after ejection and cooling down Reason: Prevention of in-mold shrinkage
1 2 1 2 1 1 2 α z= thickness coordinate September 24, 2012 6
2. Simple stress model
Validation studies• Residual stresses
Jansen, Polym. Eng. Sci. 38, p.2030 (1999) Jansen, Intern. Polym. Process. 9, p.82 (1994)
• Shrinkage after molding:
Jansen, Intern. Polym. Process. 13, p.99 (1998) Jansen, Polym. Eng. Sci. 38, p.838 (1998)
α ∞
1 2
1 2
1
2
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2. Simple stress model
Warpage due to uneven cooling • Injection Molded plate- Low hold pressures warp to hot side; high to cold side
- Model predicts correct trends, but - underpredicts warpage by factor 2
Intern. Polym. Process. 13, p.417 (1998)
curvature 12 1 2 / / Hot Cold tensile compressive September 24, 2012 8
2. Simple stress model
Warpage due to uneven cooling • Injection molded corner product• Intern. Polym. Process. 13, p.417 (1998)
Fine tuning of angle with Phand ΔTwpossible!
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2. Conclusions about stress model
• Model is simple and gives a clear understanding • For simple geometries model works as good assimulation 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
• Shrinkage and warpage follow from same model
• Shrinkage and warpage can be tuned with holding
pressure and mold temperatures 10
0 +10 -MPa tensile compressive September 24, 2012 10
3. Residual Stress Measurements
Overview of methods• 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
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3. Residual Stress Measurements
Outline of Layer Removal test1. Mill top layer (50-200 µm) 2. Residual stresses are no longer
balanced
3. Release from vacuum rig 4. Measure curvature κ
Back-Calculate stress distribution
h(x)
4 2
6 1 4 2
zr
Treuting and Read, J Appl Phys 22, p.130 (1951)
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3. Residual Stress Measurements
Example of Layer Removal methodHastenberg 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
Layer Removal method: Problems and solutions • Stress relaxation after production Store in freezer before use
• Melting and stress release during milling
Use sharp tool and speed < 1500 rpm
• Creep effects after milling
Due to flattening in test rig! Wait 10 min (96% recovery)
• Resolution: milling <0.1 mm is difficult
Will give problems near surface
• Data analyis of κ(zr)curve: differentiation error Do not fit a polynomial over all data points
but use a sliding window fit
0 0.5 1 curvatures data Polyfit 4 Polyfit 6 September 24, 2012 14
3. Residual Stress Measurements
Why Excimer laser ablation• KrF has high energy photons (3.5-8 eV)
• direct bond breaking; no melting! • Pulsed operation: shockwaves remove debris • Ultra thin layers possible (< 1 µm)
Jansen, Polym. Eng. Sci. 38, p.2030 (1999) Note: only works well with Polycarbonate
0 10 eV C-O 3.6 6.2 C-C bond strength CO2laser Nd:YAG laser KrF excimer laser September 24, 2012 15
3. Residual Stress Measurements
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; low Ph
- large compressive surface stress
- almost quenching like stress profile
- reasonable match with model
High viscosity; high Ph
- small surface stress
- compressive sub-surface layer
- good match with model
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3. Residual Stress Measurements
Comparison with model predictionsLow viscosity; high Ph
- large tensile surface stress
- “inversion” of stress profile
- low stresses in core
- good match with model
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• Stresses in injection molding are mainly due to frozen-in pressure variations
• Warpage is due to asymmetric stress distribution • Stress profile, warpage and shrinkage can be
estimated is a relatively simple way
• 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!
