1. Using Moldflow to evaluate internal stresses and shrinkage
B. Buffel
W. Six

2. Content
Overview of Moldflow shrinkage models
Case study

3. Shrinkage models Midplane and Dual Domain uncorrected residual stress
Corrected residual in mold stress (CRIMS)
Residual strain
3D mesh
Based on volumetric shrinkage and PVT and CTE data
(Note: high element quality is required
to avoid shear locking in thin walled product)
Uncorrected residual stress
This option will be selected when there is no shrinkage data available for the material. In this case, the Fill+Pack analysis predicts the residual stress values within in the part, based on the flow and thermal history during the molding cycle.
Corrected residual in-mold stress (CRIMS)
This option is the default when shrinkage testing has been performed on the material. This model is the most accurate because it is obtained by correlating actual tested shrinkage values with Fill+Pack analysis predictions.
Note: You can view the CRIMS model coefficients for the dataset(s) that are available for the selected material. The analysis always uses the dataset that is appropriate for the selected material, analysis sequence, and process settings.
Residual strain
This option is selected for those materials where the CRIMS model was found to not adequately describe the shrinkage behavior of the material.
Note: You can view the residual strain model coefficients for the dataset(s) that are available for the selected material. The analysis always uses the dataset that is appropriate for the selected material, analysis sequence, and process settings.

4. residual stress model + corrections for “industrial processing”
Shrinkage models
CRIMS model
Simulation software does not capture alle processing effects on shrinkage and warpage based on standard laboratory data.
CRIMS =
residual stress model + corrections for “industrial processing”
One of the biggest problems though is, properly capturing how the material used in the molding of a part will react as all materials behave differently under different circumstances. This extends to Simulation Moldflow based on how this data is presented to the software for simulation. As Simulation Moldflow uses laborartory material data for a large percentage of material classifications (whether tested internally or provided by a material supplier) the solvers don't always capture the effects of processing parameters on the material behavior. Due to this, theoretical calculations of part shrinkage may often exhibit unacceptably large errors.
CRIMS (Corrected Residual In Mold Stress) provides a unique method that combines the theoretical model for residual stress and a model for morphology development. This provides a correction of errors due to the use of material data that are obtained under lab conditions rather than those experienced by the material during actual injection molding.

5. CRIMS model 28 injection molding conditions are
tested to evaluate influence of:
Part thickness
Melt temperature
Mold temperature
Injection time and profile
Packing time and profile
Cooling time
Source:

6. CRIMS model
Source:

7. Illustration Stiffened plate 100x100x2mm Rib 5mm high PP
3 different shrinkage models

8. Stress tensor (warp) result
Illustration
Uncorrected
residual stress
CRIMS
Residual strain
the Stress tensor result shows the stress in the selected direction throughout the part (after the ejected part has cooled to room temperature) in the midplane and in the 3D solutions.
-1,1 … 8,7MPa
-2 … 9,1MPa
-3,1 … 6,6MPa
Stress tensor (warp) result

9. Illustration Uncorrected residual stress CRIMS Residual strain
-0,18 … 0,07mm
-0,42 … 0,16mm
-0,24 … 0,08mm
Z-deflection

10. Illustration: influence of fiber orientation
Using CRIMS
PP 30% GF
Injecting parallel and perpendicular to rib stiffener
Parallel flow front

11. Illustration: influence of fiber orientation CRIMS model
Fiber orientation tensor (normalized thickness 0,88)
-0,1…0,02mm
-1,3 … 0,8mm
Z-deflection

12. Fiber orientation tensor (normalized thickness 0,88)
Illustration: influence of fiber orientation uncorrected residual stress model
Fiber orientation tensor (normalized thickness 0,88)
-0,1…0,02mm
-0,5 … 0,2mm
Z-deflection

13. Case study: - PPS housing

14. Content
PPS Housing
Situation
Simulation results
Conclusions

15. PPS housing 2 used meshtypes 3D mesh Midplane mesh
Flow  a lot of cleanup!
Weldsurfaces  stress results
Moldtemperatures  cooling results
Pathlines

16. PPS housing
Situation: Housing for a valve, product shows cracks in after limited use

17. PPS housing  handbuild mesh  3D model  SCHULADUR A3 GF30
Pressure load
 handbuild mesh
 3D model
 SCHULADUR A3 GF30

18. PPS housing Proces settings. Injection temperature 260°C
Mold temperature 70°C
3 Injection points
Ejection temperature 178°C
Packing 80%
Flow problems and resulting internal stresses due to warp

19. Possible strenght reduction of 75% ! (1997, R.Seldén et al)
PPS housing
Filling and weldsurfaces
Meeting angle ~ 0°! End of flow
Far from inj. point

20. PPS housing
Fiber orientation

21. high E-mod, high strength
PPS housing
Low E-mod, low strength
Fiber orientation and Volumetric shrinkage
high E-mod, high strength

22. Large local variations = internal stress
PPS housing
Volumetric shrinkage, fiber orientation dependent
Large local variations = internal stress

23. PPS housing
warp

24. PPS housing
Internal stress due to warp

25. PPS housing
Internal stress due to warp

26. PPS housing Conclusion
Filling the product in 3 points leads to various weldsurfaces
Disturbed fiber orientation
Less strength
Crack initiation
Film gate – higher mold surface temperature (variotherm)
Rib structure avoids shrinkage of the bottom.
Leads to large (50MPa) internal stresses in the product.
Fiber orientation leads to large variations in stiffness
Take into account in during the design process

27. Thank you for your attention
Contact: