Meaning
Cross-sectional structural stratification develops across injection molded walls due to rapid thermal quenching and high velocity shear gradients during filling. Evaluating skin core morphology identifies how flow-induced molecular orientation at the mold wall transitions into unoriented spherulitic structures within the part interior. Material specifications often report isotropic bulk properties that fail to capture localized property variations across part thickness.
High mold temperatures and slower injection speeds reduce skin layer thickness while broadening the transition zone.
Morphological Formation
Molten polymer coming into contact with cold cavity surfaces freezes instantaneously, capturing high molecular orientation in a thin outer skin layer. Sub-surface layers experience peak shear stresses during packing, forming highly oriented intermediate shear zones. Core regions solidify slowly under hydrostatic pressure, allowing polymer chains to relax and form isotropic spherulitic structures.
Part wall thickness dictates the relative volume fraction occupied by core versus skin layers.
Structural Stress
Differential cooling rates and shear histories generate residual stress gradients across the wall thickness. Highly oriented skin layers exhibit high tensile strength along the flow direction but reduced transverse strength. Unbalanced shrinkage between highly crystalline core regions and quenched amorphous skin layers drives part warpage upon mold ejection.
Regrind addition disrupts core crystallization patterns, altering cross-sectional stress balances.
Property Impact
Layered structural morphology dictates impact resistance, flexural performance, and anisotropic thermal expansion in molded parts. Optimizing injection speed profiles controls skin thickness to minimize warpage. Balance across cross-sectional layers ensures dimensional stability under mechanical stress.
