Why the corners are always thinnest. A vacuum-formed part is made from a fixed quantity of plastic: whatever was in the sheet. Nothing is added during forming, so every square millimetre of extra surface area comes from thinning somewhere. The plastic that reaches the deepest point has travelled furthest and stretched most. This is quantified as the draw ratio — depth against width — and above roughly 1:1 on a simple box shape you should expect real thinning problems. It is also why a tall narrow part is much harder than a wide shallow one, regardless of how good your vacuum is.

Thermoplastic versus thermoset. This process only exists because HIPS, PETG and ABS are thermoplastics: heating them breaks no chemical bonds, so they soften, take a new shape and can in principle be reheated and reshaped again. Bakelite and the other thermosets cannot do this — their polymer chains are cross-linked during curing, and reheating degrades rather than softens them. That single distinction determines which plastics can be formed this way and which must be cast or machined.

Male and female tooling. Forming over a raised buck is drape forming, and it gives good detail on the inside surface and best thickness on the top face. Forming down into a cavity gives good outside detail but thins the base more severely. Neither is correct in general — choose by which surface has to look right and which dimension has to be accurate.

Where it came from. Thermoplastic sheet forming grew out of wartime work on acrylic aircraft canopies, and vacuum forming became a mainstream industrial process during the 1950s alongside the arrival of cheap sheet polystyrene. It is now the backbone of disposable packaging — the blister pack, the yoghurt pot, the interior door card of a car. It stayed in the workshop for the same reason it succeeded in industry: the tooling is nearly free.

Vacuum Molding

Blueprints (1)

Vacuum Forming