Shear stress is stress applied to the melt as it flows through the mold. This stress is caused by excessive friction between layers of plastic flowing in the cavity.
This type of stress can occur when the material viscosity is high and/or the material flow rate is high. Shear stress often manifests around the gate due to high injection speeds, and at the end of flow if a constant flow rate is used. High shear stress can cause the plastic to degrade and fail due to stress cracks. The red line in the following diagram represents the area of maximum shear stress. This is between the dark blue frozen layer and the light blue melt.
What to do
To reduce internal shear stress:
- Slow down the flow rate.
- Increase wall thickness.
- Avoid differential orientation and differential shrinkage.