Meaning
Layered structural organization formed across the cross-section of injection molded parts arises from differential shear rates and cooling velocities during melt solidification. Characterizing skin core shell morphology reveals distinct structural zones comprising a highly oriented rapidly frozen outer skin, a sheared intermediate shell and a slowly cooled unoriented core. This structural arrangement forms during melt processing of semi-crystalline and filled polymers, applying across solid wall cross-sections until wall thickness becomes too thin to sustain distinct thermal gradients.
Layer Formation
Melt contact with cold mold cavity walls causes rapid thermal quenching, trapping polymer chains in high orientation within the surface skin. Underneath this skin, skin core shell morphology features an intermediate shell layer where high shear forces align molecular chains and reinforcement fibers along the flow direction. At the center, slow cooling and low shear allow polymer chains to form isotropic spherulitic structures in semi-crystalline resins like polypropylene.
Mold temperature and injection speed directly dictate the relative thickness of these individual structural layers.
Fiber Orientation
Discontinuous glass fibers respond dynamically to local velocity gradients across the wall thickness during cavity filling. In fiber-reinforced components, skin core shell morphology creates a sandwich structure where surface fibers align along flow, shell fibers experience shear alignment, and core fibers reorient transversely. This anisotropic distribution leads to directional mechanical properties, causing warpage when thermal expansion differs between skin and core layers.
Moulders must balance wall thickness and gate location to control structural asymmetry across parts.
Defect Sensitivity
Variations in processing conditions alter layer proportions, impacting impact strength and surface appearance. When processing regrind resin, inconsistent molecular weight distributions disrupt skin core shell morphology, altering shell thickness and promoting sink marks or core void formation. Datasheet values based on uniform test bars fail to reflect the non-uniform stress distribution across multi-layered part walls under flexural loading.
Optimizing packing pressure helps compress core voids while maintaining stable surface skin integrity.
