Spring-in

Stress locked into a material or assembly in the absence of external load, arising from cure shrinkage, thermal mismatch during cooling, welding or forming. Residual stresses cause distortion, such as spring-in of composite parts, and can add to service stresses to promote cracking.

The programmed schedule of temperature, pressure and vacuum against time under which a thermosetting laminate or adhesive is cured, including heating ramps, dwells and cool-down. The cycle is defined in the material's processing data and qualified for the specific part and process.

The moulds, mandrels, jigs and fixtures on and in which parts are laminated, formed and cured. Tooling material selection, from GRP and epoxy tooling board through to nickel alloy and steel, is driven by cure temperature, thermal expansion match, accuracy requirements and production volume.

A laminate whose ply orientations and materials mirror each other about the mid-plane. Symmetry prevents coupling between in-plane loads and bending, so the laminate remains flat after cure instead of warping.