What Is Wall Thickness?

What Is Wall Thickness in Injection Molding?

Designers start with a nominal wall, the base thickness of the shell, and then add secondary features such as ribs, bosses and holes to it. Most molding problems trace back to places where that nominal wall changes suddenly.

The reason is cooling. Plastic shrinks as it cools, and a thick section keeps cooling and shrinking after the thinner walls around it have set. That uneven shrinkage pulls the surface inward (a sink mark), leaves a hollow inside the part (a void), or bends the part out of shape (warpage).

Thickness also sets the cycle. Cooling time generally dominates the injection molding cycle, and a thicker wall takes longer to cool before the part can be ejected. MIT's injection molding course notes give three reasons to keep walls thin and uniform: shorter cycle time, better dimensional stability and fewer surface defects. They suggest holding the nominal wall within ±10%.

Why Not Just Make the Wall Thicker?

A thicker wall is stiffer, but it is rarely the best way to add stiffness. The same course notes list three options: increase the thickness, switch to a stiffer grade (glass-filled polypropylene, for example), or add ribs. Ribs usually win, because they add stiffness without creating a thick section.

Ribs need care too. Where a rib meets the wall, the material is locally thicker. One US patent on molded thermoplastic articles notes that a sink mark forms opposite a rib once it exceeds about 25% of the wall thickness in thermoplastic olefins, and about 40% in many other plastics. That is why ribs are designed noticeably thinner than the wall they support.

How to Design Wall Thickness

• Pick one nominal wall early and design everything else relative to it.
• Where thickness must change, taper it gradually instead of stepping it.
• Core out thick areas such as solid bosses and heavy corners instead of leaving them solid.
• Add stiffness with ribs and gussets, kept thinner than the wall they support.
• Draft both faces of a wall in the same direction so the thickness stays constant.

Wall thickness is one of the first things checked in the DFM review that opens every plastic injection molding project.