The Physics of Shrinkage

When a heated plastic sheet is vacuum-formed over a mold, it cools rapidly. As plastic cools, it shrinks. If you form plastic over a perfectly vertical 90-degree block, the plastic will shrink tight against the sides of the block, essentially locking itself onto the mold. You will never get it off without destroying the part.

What is a Draft Angle?

To prevent parts from locking onto the tool, engineers must design draft angles into the mold. A draft is a slight taper (angle) applied to all vertical walls of the mold.

• Male Molds: When the plastic drapes over the tool (shrinking onto it), we require a minimum draft angle of 3 to 5 degrees on all vertical surfaces.
• Female Molds: When the plastic is sucked into a cavity (shrinking away from the walls), a smaller draft angle of 1 to 2 degrees is often sufficient.

Texture and Draft

If your mold has a textured surface (like a faux-leather grain), you must increase the draft angle. A general rule of thumb is to add 1 degree of draft for every 0.001 inches of texture depth. If you don't, the texture itself will act like thousands of tiny hooks, locking the part to the mold.