Meaning
Mechanical degradation processes break polymer chains through high shear forces, reducing the molecular weight of the resin. When shear scission occurs, the long polymer chains are broken into shorter segments by the high stresses generated in the melt. This change causes a decrease in the melt viscosity and lowers the strength of the final moulded part.
Moulders must optimize processing parameters to prevent this degradation.
Viscosity Change
The breakdown of long polymer chains alters the flow behavior of the resin. Through shear scission, the melt viscosity drops as it passes through narrow restrictions.
Gate Design
Injection mould gates must be sized correctly to prevent excessive shear stress as the melt enters the cavity. When the gate is too small, shear scission occurs as the polymer is forced through the narrow opening at high speed. This degradation leads to cosmetic defects like splay and blush around the gate area.
It can also cause the part to crack easily near the gate during ejection or in service. Enlarging the gate reduces the shear rate and prevents this material breakdown.
Mechanical Weakness
Shorter polymer chains are less effective at forming the physical entanglements that give a plastic part its tough behavior. When shear scission is not controlled, the finished parts exhibit low impact resistance and are prone to brittle failure. This loss of performance is especially problematic in structural or safety-critical components.
It cannot be recovered by adding virgin resin to the mixture because the molecular weight has been permanently reduced.
