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.
Thermoplastic Deformation
Meaning
The permanent, heat-induced distortion of a polymeric component subjected to external mechanical stresses during or immediately after moulding represents a major source of dimensional inaccuracy. Thermoplastic deformation occurs when a part is ejected or handled before its temperature has dropped below its heat deflection or glass transition point, where the material behaves as a viscoelastic fluid rather than a rigid solid. Under these conditions, the polymer chains can slide past one another under relatively low clamping or ejection forces, locking in a warped or distorted shape.
It dictates the minimum cooling time required in the cycle to ensure that parts can be handled without losing their dimensional tolerances.
Viscoelastic Behavior
Thermoplastic materials display both elastic and viscous characteristics when heated above their softening point. During thermoplastic deformation, the elastic recovery is insufficient to overcome the viscous flow induced by the external load. This means that once the force is removed, the part does not return to its original moulded shape.
This behaviour is highly dependent on both the temperature of the melt and the magnitude of the applied force.
Defect Mechanism
Improper handling of warm parts often leads to surface dimples, bending, or overall part warpage. In many cases, this occurs when ejector pins push against a part that has not cooled sufficiently. The pin localized force pushes through the soft plastic, creating an indentation or a hole.
This defect requires the operator to extend the cooling phase of the moulding cycle, which increases overall production costs.
Process Control
Optimizing the mold temperature and cooling time is the primary method to prevent these distortions. By ensuring that the part is fully solidified before ejection, moulders can eliminate post-mould warping. This is often achieved by using conformal cooling channels in the mold tool.