Tolerance Grades a Toolroom Signs under DIN 16742
DIN 16742 defines nine tolerance grades based on resin shrinkage variance, requiring toolmakers to split tool-bound and parting-line tolerances during sign-off.
Polyoxymethylene Contraction
Meaning
High volumetric reduction in acetal homopolymer or copolymer melts occurs during solidification due to rapid crystallization of highly ordered molecular structures. Polyoxymethylene contraction drives significant post-molding dimensional change as cooled parts transition from melt phase into dense crystalline solids. The property governs steel mold cavity sizing, stopping once post-mold cooling and secondary crystallization reach thermal equilibrium.
Crystalline Shrinkage
Dense packing of linear polymer chains produces high volumetric contraction during cooling. Experiencing polyoxymethylene contraction causes linear shrinkage rates between one and three percent depending on wall thickness and molding conditions. Thick part sections shrink significantly more than thin sections, creating sink marks and internal void formation if packing pressure is inadequate.
Mold designers enlarge cavity dimensions to account for high contraction rates in precision acetal gears.
Processing Controls
Holding pressure, melt temperature, and mold temperature govern the final density of molded polyoxymethylene components. Controlling polyoxymethylene contraction requires extended hold times to force additional melt into the cooling cavity until gates freeze. Low mold temperatures freeze the surface rapidly, suppressing crystallization and resulting in lower immediate shrinkage but higher post-mold dimensional drift.
High mold wall temperatures yield higher initial shrinkage, superior dimensional stability, and optimum surface hardness.
Tolerance Boundary
Post-molding dimensional changes continue for forty-eight hours after ejection as secondary crystallization completes. Unmanaged polyoxymethylene contraction causes part warpage, out-of-round gear profiles, and premature mechanical assembly failure. Engineering designs for tight-tolerance acetal parts specify precise conditioning periods before dimensional inspection against drawing requirements.