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.
Mechanical Restraint Forces
Meaning
The structural loads applied by tooling to resist the internal pressure and thermal contraction of a polymer part during its formation define a major process variable. Mechanical restraint forces prevent demoulded components from warping as they undergo asymmetric cooling and volumetric shrinkage. These forces must be maintained until the polymer temperature falls below its glass transition or heat deflection point, where the material gains sufficient stiffness to resist self-deformation.
If the holding forces are released too early, the unbalanced internal stresses warp the part and distort its critical dimensions.
Stress Generation
Internal stress accumulates as the outer skin of the part solidifies while the inner core remains molten and hot. When mechanical restraint forces are active, they oppose this differential shrinkage by holding the part in a fixed geometric state. This action forces the polymer to yield microscopically, which relaxes the molded-in stresses and stabilizes the final shape.
If the restraint is unevenly distributed, it introduces new, localized stress concentrations that can crack brittle resins like polystyrene.
Deformation Prevention
Maintaining dimensional flatness in thin-walled or asymmetrical parts requires a carefully calculated pressure profile. Mechanical restraint forces must be high enough to resist the high-shrinkage directions of semi-crystalline polymers like polypropylene. However, exceeding the compressive yield strength of the warm plastic creates permanent indentation or shear marks.
Moulders use force-limiting clamp designs to balance these two competing issues.
Tooling Retention
Secure positioning during secondary processes like ultrasonic welding requires secure holding action. If mechanical restraint forces are insufficient, the part slips under assembly vibrations. This slippage ruins the weld quality and scuffs the cosmetic surface.
Properly calibrated clamping cylinders ensure that the part remains immobile throughout the entire automation sequence.