Meaning
Structural phase distribution within heterogeneous elastomers defines rubber domain morphology, governing impact resistance and melt rheology during injection cycles. Elastomeric phase separation occurs during thermal cooling in the mould cavity, setting the final impact modifier dispersion state before ejection. Poor domain sizing results in brittle part failures and high scrap rates under load.
Material specifications dictate nominal particle dimensions, whereas part specifications demand actual cross sectional dispersion uniformity across thin walls. Virgin polymer batches maintain predictable domain sizes, while excessive regrind blending alters viscosity ratios and leads to coarse domain coalescence. Datasheet values assume optimal thermal profiles, but moulders holding tight cycle times frequently encounter shear induced domain elongation that degrades low temperature impact strength.
Phase Separation
Cooling rates dictate how elastomer particles segregate from the thermoplastic continuous matrix during the solidification stage. Thermal gradients across thick wall sections promote uneven domain growth, creating localized stress concentration zones. Excessive barrel temperatures erase previous thermal history, dissolving dispersed domains into a uniform melt that fails to precipitate correct microstructures upon cooling.
Mold temperature control units stabilise this separation window by removing heat uniformly through the cavity walls. Faster tool cycles compress the residence time available for phase development, leaving insufficient duration for complete domain separation.
Shear Rate
Screw rotation speeds during plastication dictate initial elastomer droplet breakup before the melt enters the runner system. High screw speeds generate excessive mechanical work, tearing elastomer particles into sub optimal dimensions that reduce overall impact efficiency. Gate geometry introduces secondary shear fields, elongating spherical domains into fibrous strands that cause anisotropic mechanical performance in the finished article.
Viscosity mismatches between the continuous phase and the dispersed rubber phase amplify shear sensitivity during high speed injection phases. Optimising injection velocity profiles prevents excessive domain deformation while maintaining complete cavity fill without surface defects.
Dimensional Stability
Shrinkage rates vary directly with domain dispersion quality, affecting final part tolerances and assembly fit. Coarse domain structures promote uneven volumetric contraction, causing warp and sink marks on flat surfaces. Regrind incorporation shifts the melt flow index, altering the shrinkage profile away from virgin material baselines established in tool design.
Moulders compensate for domain variation by adjusting packing pressure, though over packing induces internal stresses that counteract the benefits of a well dispersed rubber phase. Final part performance depends on maintaining consistent domain morphology across every production shift.
