The flow rate of the melt varies over the wall thickness during filling.
Shear forces arise between the layers of different velocities.
This way a thought volume element is deformed (diamond in the picture).
The time dependent change of the angle (gamma) is called shear rate.
High shear rates lead to shear heating, molecular chains deform and might break.
The quality of the product is reduced.

Shear Rate in Plastic Industry

Injection Molding Process: The slicing view shows high shear rates around the gate.

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This glossary of plastic industry is provided by PLEXPERT Canada Inc.