Meaning
Volumetric change that occurs in a polymer when it transitions between its molten, amorphous state and its solid, crystalline state. In semi-crystalline materials like polyethylene, a density transition occurs during cooling as the random molecular chains organize into dense, ordered crystalline regions. This physical shift causes a significant volumetric contraction, which is much greater than the simple thermal contraction of the amorphous regions alone.
Mould designers must account for this shrinkage to ensure the finished parts meet the specified dimensional tolerances.
Structural Impact
Crystallinity levels govern the magnitude of the volumetric contraction during the solidification phase. Higher crystallinity results in a more pronounced contraction because the ordered regions pack molecules closer together than the disordered amorphous regions. This differences in regional shrinkage across a part can generate high internal stresses, which frequently lead to warpage or sink marks in thick sections.
Regrind materials can exhibit altered crystallization kinetics due to the presence of nucleating impurities, modifying this volumetric behavior compared to virgin resin.
Process Adaptation
Mould pressure must be maintained at a high level during the packing stage to feed additional melt into the cavity as the volume decreases. This compensation is critical because the density transition occurs rapidly as the resin cools below its crystallization temperature. If the gate freezes prematurely, no additional material can enter the cavity, which results in internal voids or surface depression in the thickest regions of the molded part.
Process engineers use cavity pressure sensors to monitor this transition and optimize the duration of the packing and holding phases. This real-time feedback allows for adjustments in holding pressure to counteract the rapid volumetric change, preventing dimensional errors and reducing the reject rate of the molding cycle.
Physical Outcome
The final density of the molded part dictates both its mechanical and barrier properties. High-density regions possess superior chemical resistance and stiffness because the tightly packed chains restrict the diffusion of solvents and the slip of polymer layers. However, this high density also leads to a reduction in optical clarity and impact toughness.
Achieving the correct balance of density across the run requires precise control of the mould temperature and the cooling rate.
