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.

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

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.



    • Indoor Lighting Contest

      Indoor Lighting Contest
    • Metal Contest

      Metal Contest
    • Make It Fly Challenge

      Make It Fly Challenge

    19 Discussions


    3 years ago

    I can't remember seeing it mentioned on any of the Instructables that I've seen, but you don't have to use a base plate with holes in it for the forming- what they did in the shop of a former employer was to put something breathable (we used carpet padding) between the form and the single air hole. That meant the base could be solid and the form didn't have to have any holes in it.

    You could probably use a sponge or other similar items as well.

    Involved Observer

    3 years ago

    Nice! I've been thinking about doing some vacuforming with foam masters, but have been worried about exactly this issue (melting the buck/master) - thanks for sharing this, you've saved me a ton of work!

    1 reply

    Hi ! It definitely helped, but it was still hard to remove the plug that some of the plaster got chipped and would benefit from being replied between castings. But definitely worth a try. Good luck.


    5 years ago on Introduction

    I think I've seen that vacuformer before....Say hi to the guys for me!


    5 years ago on Introduction

    I have an idea that make your vacuum machine all-in-one and easy to use.

    If you put legs to yout vacuum table, and a hinge in the side of the frame that holds the plastic sheet, you can rotate 180º to have 2 positions in it, one for heating and the other for vacuum.

    This way, plastic will be heated from one side and formed by the other.

    I'll add a picture.

    I hope I explained it propperly.

    1 reply

    5 years ago on Step 7

    I don't quite understand the point of this other than to make vacuum forms from a styrofoam master. (I think, properly speaking, the piece of styrofoam you are using to make a vacuum form from would be called the master, not the mold.) What is the function of the reproduced shape in vacuum formed styrene? I suppose one could use the vacuum formed styrene as a mold for casting something so long as the item you were casting in the mold didn't heat up to a point where the styrene would lose its shape.

    2 replies

    Reply 5 years ago on Introduction

    Yeah! I meant master! Thanks. I'll change the title and text to make it more clear.

    The point is exactly that: to make many plastic objects from the same foam master. Prepping one master with plaster and vaseline is cheaper and faster than milling multiple masters.

    Sometimes I vacuum form toys. Other times I vacuum form masks. Recently I've been trying to vacuum form 6 pieces for a sculpture/installation I'm working on. In all cases, I like to get many plastic objects out of the same master, so I don't have to keep milling the same thing over and over and over...

    The Iron Man head/helmet/mask here was really just a test piece.


    Reply 5 years ago on Introduction

    Cool. Back when I worked in a jewelry shop I did a little lost wax casting. I had thought about using either a clay or a styrofoam master to make a vacu-formed mold from which wax masters could be cast to use in the lost wax casting. I never tried it. Not sure if the vacu-formed plastic could take the heat from the melted wax without losing its shape.


    5 years ago on Introduction

    Maybe its just me, but I'm not understanding this. Perhaps a video would better illustrate the steps.

    1 reply

    Reply 5 years ago on Introduction

    How to vacuum form? or how to build a table? : )

    I didn't really write this as generic as it could be. It's specific to vacuum forming foam, without burning it or getting it stuck on your plastic...

    Let me know what isn't clear so I can fix it.


    5 years ago on Step 7

    Very cool tutorial! I can't wait to try this either. And I am down with legamin, can you share links or ideas where to look to build a rig like your Vacuum Table. Thanks!

    1 reply


    I'm using a setup that a friend built a while ago, and unfortunately I don't have pictures for a step-by-step, but ... from what I can tell, the setup is very similar to this:

    The heating chamber is 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.

    Hope that helps!


    5 years ago on Step 7

    I can't wait to begin experimenting! I'm very new to this so I was hoping for more info on materials, building the heating chamber, building the vacuum table!molds etc. very cool though!

    1 reply