Meaning
Dimensional reduction occurs as a thermoplastic resin cools from its melt temperature to its solid state after being injected into a mould. This polyoxymethylene shrinkage is relatively high due to the semi-crystalline nature of the material, often ranging between 1.5 and 2.5 percent. The phenomenon is driven by the tight packing of polymer chains as they form crystalline structures during solidification.
Crystallization Effect
Rapid cooling in the tool can trap the material in a less organized state, leading to further dimensional changes over time. Post-moulding polyoxymethylene shrinkage can continue for several days as the part reaches thermal and structural equilibrium. Moulders often use heated tools to promote a more complete crystallization and stabilize the part dimensions sooner.
Process Variables
Injection pressure and hold time directly influence the final size of the part by packing more resin into the cavity to compensate for the contraction. If the hold pressure is too low, the polyoxymethylene shrinkage will be more pronounced, potentially leading to sink marks or voids in thick sections. Tool designers must account for these factors when sizing the cavities.
Dimensional Control
Uniform wall thicknesses help maintain a consistent rate of contraction across the entire geometry. This prevents the development of internal stresses that lead to warping or structural weakness. Correct gate positioning ensures that the part packs out evenly before the resin freezes.
