Standard Uniaxial Testing Deficiencies in Amorphous Polymer Characterization
Uniaxial tensile tests misrepresent multi-axial yield, physical aging, and strain-rate sensitivity in amorphous polymers, causing structural failure in real parts.
Injection Molding Residual Stress
Meaning
Locked-in internal tension and compression profiles develop inside molded thermoplastic parts due to non-uniform cooling rates and flow-induced molecular orientation. Accumulating injection molding residual stress causes part warpage, post-mold dimensional drift, environmental stress cracking, and unexpected premature failure during service. The phenomenon affects all melt-processed polymers, but becomes negligible in thin, fully annealed parts cooled at equilibrium rates.
Thermal Gradient
Rapid chilling at cold mold surfaces creates skin layers frozen under tension while hot core resin cools slowly and contracts under constraint. Differential volumetric contraction forces the rigid outer skin into compression while pulling the cooling core into tensile stress. High packing pressure forces extra melt into the cavity, offsetting thermal shrinkage but introducing intense flow stresses near the gate.
Gate location dictates the direction and magnitude of these internal stress fields.
Warping Mechanism
Unbalanced stress distribution across wall sections forces part geometry to distort upon mold ejection. Parts warp as internal tensile and compressive forces find mechanical equilibrium outside the restrictive metal mold cavity. Amorphous resins retain high residual stress because frozen chain orientations cannot relax below the glass transition temperature.
Adding un-dried regrind resin alters melt viscosity, causing inconsistent packing and erratic stress profiles across mold cavities.
Stress Mitigation
Thermal annealing relieves locked-in stresses by allowing localized polymer chain relaxation.