Meaning
Laminar flow velocity gradients across melt channels induce distinct molecular orientation and phase separation layers in processed polymers. Experiencing shear stratification during high-speed injection molding alters mechanical properties and optical clarity across the part wall thickness. High shear rates near channel walls align polymer chains parallel to flow, creating a highly oriented skin layer over a less oriented core.
Structural layer formation governs anisotropic mechanical behavior but does not dictate overall volumetric shrinkage.
Skin Layering
Flow geometry and injection velocity determine the thickness of high-shear surface zones. In cases of severe shear stratification, multi-phase polymer blends or filled compounds separate into distinct phase bands, causing surface delamination and weld line weakness. Processors lower injection speeds or raise melt temperatures to reduce shear rate differentials across the flow front.
Filler Distribution
Fiber reinforcement orientation varies significantly between high-shear skin and low-shear core regions. Glass fibers subjected to shear stratification align tightly along outer part walls while remaining randomly oriented near the center. This non-uniform fiber distribution creates anisotropic thermal expansion, leading to post-molding part warpage and dimensional distortion.
Defect Mitigation
Processing adjustments reduce localized velocity gradients within complex runner systems and narrow gates. Controlling shear stratification requires optimizing gate thickness and melt channel transitions to maintain uniform shear rates across the flow profile. Balanced shear distribution improves part toughness and prevents localized cosmetic defects in semi-crystalline resins.
