A dimensionally unstable part continues to move after normal cooling. It may change during storage, assembly, transport, or moderate reheating. The immediate forming result can appear acceptable while the long-term shape is not.
Residual stress from forming near the lower end of the usable temperature range is a common cause. Uneven cooling, high sheet orientation, moisture conditioning, crystallization, and service temperatures near the material’s softening range can also change dimensions.
Evaluation
Parts should be measured after a defined conditioning period, not only at the machine. Thermal cycling or service simulation may be required for critical applications. Directional movement should be compared with the sheet extrusion direction.
The process should reduce stress through suitable forming temperature, complete tool contact, and balanced cooling. Material selection must match the service environment. A cooling fixture can improve repeatability, but it cannot make an unsuitable polymer stable at a temperature beyond its intended use.