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

Content Overview of Moldflow shrinkage models Case study

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.

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.

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: http://autodesk.typepad.com

CRIMS model Source: http://autodesk.typepad.com

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

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

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

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

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

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

Case study: - PPS housing

Content PPS Housing Situation Simulation results Conclusions

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

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

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

PPS housing Proces settings. Injection temperature 260°C Mold temperature 70°C 3 Injection points Ejection temperature 178°C Packing 10s @ 80% Flow problems and resulting internal stresses due to warp

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

PPS housing Fiber orientation

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

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

PPS housing warp

PPS housing Internal stress due to warp

PPS housing Internal stress due to warp

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

Thank you for your attention Contact: bart.buffel@kuleuven.be wim.six@kuleuven.be