Optimizing Mechanical Restraint Forces in Non-Rigid Polymer Metrology under ISO 5459 Guidelines
Balancing restraint forces under ISO 5459 ensures accurate datum seating without distorting non-rigid polymer part features during coordinate metrology.
Contact Pad Geometry
Meaning
Physical design specifications for the load-bearing surface of clamping fixtures distribute holding forces across moulded thermoplastic components. Contact pad geometry determines the localized pressure applied to the part during post-mould extraction and subsequent machining operations. If the surface area is too small, localized stress concentration exceeds the yield strength of the polymer at eject temperature, leading to permanent cosmetic marks or dimples.
A broader pad redistributes these mechanical forces evenly across the part profile, which is particularly important when dealing with soft, high-flow resins that exhibit slow cooling rates or lower heat deflection temperatures.
Force Distribution
Mechanical analysis of the contact area determines the maximum force a freshly ejected polymer part can withstand. When contact pad geometry matches the local contour of the injection moulded part, the compressive stress remains below the material’s proportional limit. This prevents localized bending and holds the part in true alignment during degating.
A misaligned or flat pad on a curved surface reduces the contact to a single point, which multiplies the localized stress and yields the polymer.
Defect Prevention
Part aesthetics depend directly on avoiding localized surface indentation and gloss changes during handling. Poor contact pad geometry often causes micro-cracking or stress whitening in amorphous resins like polycarbonate. For semi-crystalline materials, the pressure from an incorrect pad can accelerate localized crystallization, leaving a permanent visual blemish.
Designing the contact pad with radiused edges instead of sharp corners mitigates these localized stress risers.
Operational Limit
Continuous production cycles demand that the selected pad shape accommodates normal volumetric shrinkage variations. Because part shrinkage fluctuates with regrind ratio and melt temperature, contact pad geometry must incorporate enough compliance to prevent binding. Silicone or elastomeric pads offer a passive mechanical buffer that absorbs these dimensional differences without transferring excessive stress to the cooling part.
This compliance maintains consistent holding action without manual fixture adjustments.