Meaning
Metrological inspection protocols establish whether physical features on a manufactured component align with design drawing limits. In technical moulding, dimensional tolerance verification compares actual part geometries against engineering drawings after volumetric shrinkage and post-mould cooling reach equilibrium. The protocol measures critical features like hole diameters, wall thicknesses, and flat surfaces using coordinate measuring machines, optical gauges, or calibrated hard fixtures.
Verification stops at physical geometry, leaving mechanical strength and resin composition to distinct destructive testing routines.
Shrinkage Differential
Moulding tools are cut oversize to anticipate resin contraction, yet differential cooling rates across thick and thin sections cause anisotropic shrink patterns. Semi-crystalline resins like polyoxymethylene contract non-uniformly between the flow direction and transverse direction, creating out-of-round bores or bowed walls. Verifying dimensions identifies tool steel corrections before high-volume production launches, isolating whether an out-of-spec feature stems from steel errors or process instability.
Process engineers vary pack pressure, holding time, and mould temperatures to pull out-of-tolerance features back within specification before requesting tool modifications.
Cavity Qualification
Multi-cavity production tooling requires discrete dimensional verification for every individual impression across several consecutive moulding cycles. Viscosity shifts between virgin resin lots and recycled regrind blends alter flow balance, forcing outer cavities to pack differently than central cavities. Quality departments apply statistical capability metrics to each cavity, rejecting runs where process capability indices fall below required thresholds.
Compliance Criterion
Part release protocols demand clear boundaries between part compliance and raw material certification. A resin lot may meet every datasheet specification for melt flow index and density while still failing dimensional tolerance verification due to inadequate mould cooling cycles.
