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.
Hygroscopic Swelling
Meaning
Volumetric expansion in polar polymers occurs when ambient moisture molecules diffuse into the amorphous matrix and force polymer chains apart. Hygroscopic swelling increases component dimensions in engineering plastics like polyamides and polycarbonates following post-molding environmental exposure. The behavior governs post-mold dimensional stability and mechanical modulus in humid conditions, stopping when equilibrium moisture content is reached.
Dimensional Distortions
Absorbed water molecules act as plasticizers, lowering glass transition temperatures while expanding part geometries. Experiencing hygroscopic swelling alters critical interference fits and pin alignments in precision automotive connectors. Dimension changes occur unevenly across parts with varying wall thicknesses, inducing internal stress and post-mold warpage.
Molders account for environmental moisture uptake when cutting steel tool cavities for hygroscopic materials.
Moisture Absorption
Diffusion rates depend on relative humidity and ambient temperature. Managing hygroscopic swelling requires conditioning molded parts in controlled humidity chambers to reach equilibrium before final inspection. Saturated nylon components can expand by several percent relative to their dry-as-molded dimensions.
Storage conditions for raw resin pellets require sealed desiccant packaging to prevent moisture uptake prior to melt processing.
Tolerance Threshold
Material datasheets report dry-as-molded dimensions alongside conditioned properties to clarify environmental effects. Ignoring hygroscopic swelling leads to assembly binding or structural failure in outdoor applications. Precision components designed for tight-tolerance optical or mechanical assemblies require non-hygroscopic alternatives like polyphenylene sulfide when moisture-induced movement is unacceptable.