Thermoforming Part Design

Thermoforming generally makes simple parts without undercuts. Minor undercuts can sometimes be made if the undercut is small enough so the material can pull off of the mold without permanent distortion. For large undercuts, special sliding mold pieces are needed.

Ribs, gussets, and bosses are generally not possible in thermoforming. The part is formed from a flat plastic sheet, so applications are limited.

Figure 1 shows a method to stiffen and strengthen flat sections. Using this corrugated design will increase the wall’s strength and rigidity, while reducing potential warpage.

As with any plastic part design, sharp angles should be avoided wherever possible. Break all sharp corners with radii.

Extremely smooth surfaces should be avoided because of the potential of trapping air between the plastic sheet and the mold. Smooth surfaces are also difficult to remove from the mold because of the vacuum.

The thickness of the sheet stalk used to thermoform the part can be any thickness that can be effectively heated and formed. For most materials, the thickness limit is ½ inch. The most common sheet thickness ranges from 0.08” to 0.16”, although sheets as thin as 0.020” can be thermoformed.