Anisotropic Shrinkage Variance in Glass-Filled Polyamide Secondary Tooling Nests
Secondary tooling nests for glass-filled polyamide accommodate anisotropic shrinkage through floating datum pins aligned with transverse contraction vectors.
Melt Front Shear Stress
Meaning
Physical property describes the frictional force exerted by a molten polymer as it moves across the internal surfaces of a mould. Managing melt front shear stress prevents the degradation of long chain molecules near the part skin. Excessive pressure at the interface leads to visible surface defects.
Molecular Orientation
Polymer chains align in the direction of flow when the velocity gradient is steep. If melt front shear stress exceeds the material threshold, the resulting internal tension creates weak points in the finished part. High orientation levels often result in non-uniform shrinkage and subsequent warping.
Parts with high shear history may exhibit reduced impact strength in specific directions.
Surface Quality
Visual imperfections like gate blush or dull spots appear when the flow velocity is not optimized for the cavity geometry. Low melt front shear stress maintains a consistent finish across the entire surface of the moulding. Rapid injection speeds increase the stress level at the wall.
This effect is most pronounced in thin walled sections where the gap between the frozen layer and the core flow is narrow. Temperature control of the melt and the tool helps to keep these forces within safe limits for the specific resin grade.
Moulding Physics
Rheological data from the resin manufacturer provides the limits for shear rate and stress. Using melt front shear stress as a guide, process engineers adjust the fill profile to maintain a constant speed. This control stabilizes the production run.