Introduction: How to Vacuum Form Foam (Reusing the Same Master, Many Times)

Styrene + Foam + Heat = huge mess of melted plastics and burnt foam.

This is how I prepare my foam to keep it from sticking to the styrene, and minimize the amount it shrinks with each casting.

Step 1: Prep Materials

  • DAP Plaster of Paris (any plaster will probably work)
  • Brush
  • Water
  • Container
  • Your master (I found some nicely milled Iron-Man heads laying around a shop)
  • Sand paper (not pictured)

Step 2: Get It Plastered

The idea is to try to cover up little holes in the foam, while giving it a nice, hard, temperature-resistant cover.

The trick is to work with very small amounts of plaster at a time. I usually mix no more than 4oz of water with the recommended amount of plaster (my plaster called for a 2:1 ratio of plaster to water).

I usually swirl it around for a couple of minutes, until I feel like it’s starting to set, and isn’t just pouring down the brush anymore, being careful not too wait too long, or it starts to clump.

This is why I usually only work with 2oz or 4oz at a time. If the mixture is too watery when applied to the foam, it tends to just drip off, leaving very thin amounts of plaster. And, if you mix up a whole bunch, and it starts to set before you apply all of it, you get a bunch of clumps everywhere. No good either.

The plaster should feel a little like watery glue when applying it.

Cover the whole model with it, wait 30 minutes for it to dry, and repeat. I usually put 3 to 4 coats of plaster. How many coats really depends on the type of plaster, the type of foam, how thick I’m applying it, and how much of it is sticking to the foam.

I then wait at least 24 hours for the plaster to fully dry before sanding it a little bit.

Step 3: Game Day Materials

  • Plastered master
  • Towels
  • Petroleum jelly
  • Bowl with ice-cold water
  • Styrene (not pictured)
  • Vacuum forming setup (not pictured, but some more info here)

Step 4: It's Getting Hot in Here...

I set up my heaters, turn them on, and while the plastic is heating up, I throw the towels into the ice-cold water and apply a nice thick coat of petroleum jelly to the plastered masters. In addition to the plaster, this helps keep the masters from melting and losing their shape.

Step 5: Do It !!

Once the plastic is nice and melty, I carefully place it over the masters on the vacuum table.

When all the suckable air has been sucked, I cover the plastic with cold, wet towels. This also keeps the plastic from melting the foam.

Step 6: ​Wait a Little Bit, But Not Too Long

Once the plastic is cool enough to touch, I lift the frame, tap on top of the plastic, and if everything went ok the masters will fall right off. Sometimes a hard tap with the heel of the hand is enough to dislodge the foam. Otherwise, compressed air can be blown into the gap between the foam and the plastic.

Jiggle, jiggle, release!

Step 7: ​No Foam Was Accidentally Melted in the Making of This Instructable

But the Iron Man head is too small for me.

I guess I'm more of a Tony Starks anyway. . . .

Step 8: Addendum: Vacuum Form Table

I'm using a setup that a friend built a while ago, and unfortunately I don't have pictures for a step-by-step.

The heating chamber is just a 4-sided box, lined with foil for heat retention, with 2 heaters inside.

The actual vacuum table is made up of 3 layers of 1/2" MDF, glued together:

  • The bottom layer has a hole for the shop vac.
  • The middle layer is just a frame, it provides a hollow chamber inside the table.
  • And the top layer has a bunch of holes.