Meaning
Velocity gradients developing at the interface between molten polymer and frozen cavity walls generate localized frictional heat during high speed injection moulding. Internal fluid friction from boundary layer shear reduces local melt viscosity, accelerating flow into thin mold sections. Viscous dissipation within this narrow zone dictates immediate wall surface finish and orientation of filler fibers.
Shearing ceases when mold filling halts and hydrostatic packing pressure equalizes across the cavity volume.
Frictional Heating
High shear rates along cold cavity surfaces create thermal energy that maintains resin fluidity despite low tool temperatures. Automated cavity pressure sensors record fill velocity changes driven by boundary layer shear during the boost phase of injection. Rapid injection speeds maximize frictional heat generation while lowering overall clamping force requirements.
Molecular Orientation
Polymer chains aligning parallel to flow vectors under high shear stresses develop anisotropic mechanical properties across the part cross section. Wall zones exhibit high tensile strength along flow paths but diminished impact resistance perpendicular to flow lines. Fiber filled resins experience orientation changes that alter local part shrinkage rates.
Differential shrinkage across boundary layer shear zones causes post moulding warpage in semi crystalline polymers.
Rheological Limit
Extreme shear rates exceed polymer chain stability, causing mechanical degradation and localized discoloration in clear resins. Material datasheets report shear viscosity curves measured on capillary rheometers, which differ from actual tool boundary conditions. Mold surface roughness alters local wall friction during filling operations.
Excess boundary layer shear degrades flame retardant additives near gate locations.
