Meaning
Molecular breakdown occurring as a polymer melt passes through a restricted flow area can compromise the structural integrity of a finished part. In injection moulding, gate shear degradation refers to the damage inflicted on polymer chains when they are subjected to excessive shear stress as they flow through the gate. This high-velocity passage breaks the long polymer chains into shorter segments, reducing the molecular weight of the resin.
Consequently, the mechanical properties of the moulded product are severely diminished.
Surface Flaw
Visual defects often appear near the injection point when the material suffers from localized damage. The broken polymer chains from gate shear degradation create cosmetic issues such as splay, discoloration, or halo marks around the gate. These surface imperfections lead to high reject rates in cosmetic consumer products.
Gate Geometry
Sizing the entry point correctly is the primary method of preventing material breakdown. An increase in gate size reduces the shear rate and minimizes gate shear degradation during the filling phase. This adjustment lowers the melt velocity as the resin enters the cavity, protecting the structural properties of the polymer.
Material Susceptibility
Resins with high molecular weight are particularly sensitive to high shear forces. Material choices like polycarbonate require larger gates to prevent gate shear degradation compared to easier-flowing resins like polyethylene. Selecting the appropriate gate configuration based on material behavior protects the long-term performance of the component.
