Meaning
Polymer chains align under high velocity gradients near cavity walls during the injection molding cycle, forming a shear thin layer along the flow path. Low viscosity characterizes this peripheral zone because chain stretching reduces flow resistance adjacent to the cold steel. Orientation persists as the melt freezes against the mold surface, establishing mechanical anisotropy between the core and the skin.
Wall shear stress dictates the thickness of this boundary region, which grows when injection velocity increases or melt temperature drops. Regrind batches alter this profile because degraded molecular weight distributions change pseudo plastic behavior under identical processing conditions.
Thermal Gradient
Cooling rates dominate solidification kinetics across the cross section immediately after cavity fill concludes. Mold temperature differentials control how quickly the oriented molecules relax before freezing locks the morphology in place. Thick wall parts retain heat longer, allowing molecular chains to recoil and thereby reducing the net birefringence of the final molded article.
Excessively cold tooling accelerates skin formation, creating brittle zones that crack under impact loading during end use. Process engineers adjust barrel heaters and coolant flow rates to balance crystallization kinetics against orientation retention.
Viscosity Shift
Apparent viscosity drops sharply inside the high shear zone because pseudo plastic fluids thin under mechanical stress. Pressure losses across the runner system depend directly on this rheological response, governing the clamp tonnage required to prevent flash. Material specifications list melt flow rates measured under standard loads, but shop floor conditions rarely match capillary rheometer environments.
Virgin resin maintains predictable flow curves, whereas recycled polymer batches exhibit broader molecular weight distributions that shift the onset of shear thinning. Moulders must monitor cavity pressure traces to detect viscosity variations before short shots or flash events occur on the production line.
Dimensional Variance
Molecular orientation gradients across the part thickness generate internal stresses that drive post molding warpage. Differential shrinkage occurs because transverse chain orientation contracts differently than longitudinal orientation during cooling down phases. Part specifications tolerate narrow flatness windows, requiring precise control over packing pressure and holding duration to mitigate distortion.
Excessive shear thinning during injection leads to uneven density distributions, causing finished housings to warp out of tolerance upon ejection from the mold. Measuring residual stress via polarized light microscopy reveals whether process drift originated from thermal imbalance or velocity fluctuations.
