Troubleshooting cracking problems

Cracking can cause part failure, a short part life and be visually unacceptable.

Cracks may not be obvious until several days or weeks after production. Hence, it is better to recognize and remove the potential problem of cracking before production.

Causes

• High residual stresses. Cracks may occur in regions where internal shear stresses are frozen into the part.
• Weld line weaknesses. When two or more flow paths meet during a filling process, structural problems and/or visibly unacceptable results will occur.
• Differential shrinkage. Differential orientation, packing and cooling cause differential shrinkage resulting in high internal stress levels being frozen in.

Remedies

• Minimize residual stress. Program the ram speed or increase wall thickness to reduce flow induced stresses. Check for the recommended maximum shear stress value for the material (recorded in the Autodesk Moldflow materials database)
• Minimize differential shrinkage.

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.