Basic Technical Criteria for High Flow Polypropylene Homopolymer Selection
Select high flow polypropylene homopolymers by balancing melt flow rate against impact loss, verifying narrow molecular weight distribution and peroxide residues.
Gate Shear
Meaning
Localized friction generated when a polymer melt passes through a restricted gate area represents a fundamental heating mechanism in injection moulding. High gate shear reduces the viscosity of the flowing resin and facilitates the filling of thin-walled cavities. This phenomenon must be controlled to avoid thermal degradation and structural defects in the finished part.
Thermal Effect
High shear rates generated during the filling phase can increase the local melt temperature beyond the thermal stability limit of the polymer. When gate shear becomes excessive, the polymer chain molecules are broken down, resulting in a reduction of molecular weight and a decline in the mechanical performance of the moulded part. The viscosity of pseudoplastic resins drops dramatically under these high shear rates, which can be useful if managed within the process limits.
If the process is not optimized, the material properties will deviate from those of the virgin resin.
Component Defect
Cosmetic flaws and structural failures often result from unbalanced flow and excess shear heating at the point of cavity entry. Too much gate shear produces jetting or splay marks, which makes the part unacceptable for consumer-facing applications. In glass-filled polymers, the intense mechanical action can fracture the fibers, which decreases the structural strength of the finished part.
This degradation reduces the maximum load the part can withstand during service.
Tooling Optimization
Tooling design directly dictates the shear rate experienced by the polymer during the injection cycle. Adjusting the gate dimensions and land length is the primary method used to control gate shear and optimize the fill profile. If the gate is too small, the high velocity increases the shear rate, whereas an oversized gate makes automatic de-gating difficult.
Engineers balance these variables to maintain process stability and part quality.