The need for uniform walls
- 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.
Uniform wall thicknesses reduce/eliminate this problem.
- Uniform walled parts are easier to fill in the mold cavity, since the molten plastic does not face varying restrictions as it fills.
What if you cannot have uniform walls, (due to design limitations) ?
- When uniform walls are not possible, then the change in section should be as gradual as possible.
- Coring can help in making the wall sections uniform, and eliminate the problems associated with non-uniform walls.
- Warping problems can be reduced by building supporting features such as gussets.