An undercut mechanically prevents a rigid formed part from moving in the tool-release direction. Shrinkage can tighten the lock around positive features or pull material into negative recesses.
Flexible materials and shallow undercuts may release by controlled deformation, but the allowable strain must be proven at the actual demolding temperature and wall thickness. A part that releases during setup can still crack or distort after the process stabilizes and the tool reaches production temperature.
Tooling solutions
Split molds, retractable cores, collapsible sections, removable inserts, or a different parting direction are the reliable options for significant undercuts. Air eject and release agents reduce friction but cannot remove a true mechanical interference.
Demolding earlier may temporarily increase flexibility, yet it can also create warpage, stress whitening, or permanent distortion. The undercut and release method should be designed together rather than corrected through cooling changes after the tool is built.