Thick sections cool slower than thin sections. The thin section first solidifies, and the thick section is still not fully solidified. As the thick section cools, it shrinks and the material for the shrinkage comes only from the unsolidified areas, which are connected, to the already solidified thin section.
This builds stresses near the boundary of the thin section to thick section. Since the thin section does not yield because it is solid, the thick section (which is still liquid) must yield. Often this leads to warping or twisting. If this is severe enough, the part could even crack.
Warping can also be caused due to non-uniform mold temperatures or cooling rates.
- Non-uniform packing or pressure in the mold.
- Alignment of polymer molecules and fiber reinforcing strands during the mold fill results in preferential properties in the part.
- Molding process conditions--too high a injection pressure or temperature or improper temperature and cooling of the mold cavity. Generally, it is best to follow the resin manufacturer's guidelines on process conditions and only vary conditions within the limits of the guidelines.
It is not good practice to go beyond the pressure and temperature recommendations to compensate for other defects in the mold. If runners need to be sized differently to allow for a proper fill, or gate sizes that need to be changed, then those changes need to happen.
Otherwise the finished parts will have too much built in stresses, could crack in service or warp-leading to more severe problems such as customer returns or field service issues.