Meaning
Polymer migration describes the physical movement of low molecular weight additives, unreacted oligomers or plasticizer fractions from the bulk interior of a plastic component toward its exterior boundary under thermal and mechanical gradients. This internal transport mechanism dictates surface energy properties, adhesion performance and chemical resistance in finished moulded goods over time. The phenomenon ceases to govern material behavior once temperature drops well beneath the glass transition point or crystalline melting threshold, halting molecular diffusion entirely.
Resin Kinetics
Additive bleed out during injection moulding begins within the plasticising screw as localized shear heating lowers viscosity for specific chain fractions. Material selection dictates the initial concentration gradient driving this internal mass transfer. Virgin polyolefins exhibit predictable additive retention rates based on molecular weight distribution width, whereas high regrind ratios introduce degraded fractions with accelerated diffusion coefficients.
Thermal Gradient
Barrel temperature set points and mould face temperatures establish the thermodynamic force that pushes unbound species across the boundary layer. Processing operators must balance cooling channel efficiency against cycle time targets to freeze mobile fractions before surface accumulation occurs. Uncontrolled thermal persistence inside heavy section parts causes blooming defects, leaving a powdery residue that invalidates subsequent painting and bonding operations.
Spec Limit
Laboratory data sheets quote resin migration resistance under static test conditions, but production floors experience dynamic mechanical stress that alters real world performance. Moulders hold tighter tolerances on additive loading than raw material suppliers guarantee in standard technical documentation. Economic viability collapses when excessive surface exudation forces secondary solvent cleaning steps to salvage out of specification mouldings.
