An Introduction to Vacuum Forming

by Flint Mitchell

Last updated 03/14/2019

Please note: always wear insulated gloves when handling the heated plastic frame or the plastic shelf.

Items needed that you can purchase:

Items you need to make:

Please note: All of the illustrations are numbered in the below paragraphs.

vacuum forming is a versatile plastic molding method that has been used to make items as varied as toys to candy molds to robot parts, and beyond.  vacuum forming is quick, easy, and inexpensive, especially when compared to other molding methods, such as resin casting.

Briefly, vacuum forming consists of putting a softened sheet of plastic over a vacuum form mold. A vacuum is turned on. Air is sucked out of the area between the plastic and the mold. When cool (this takes only a second or two), you have a copy of your original.

This article will cover improvised vacuum forming with materials that are readily available. You can also buy vacuum forming machines in various sizes. With the vacuum form machines, a lot of the guesswork is taken out of the process. At the same time, these machines can be quite expensive--and the results are no better than with improvised equipment.

Please note that you have limits as to the size something can stick out above the main mold. In other words, you can't vacuum form a sailboat top with an entire mast. The plastic will stretch way too thin.

A) There are two types of vacuum forming modelmakers can use: male mold and female mold. With male mold vacuum forming, you put a positive mold of the item you want to copy on a vacuum forming table, and copy that. All of the detail is on the inside of your copy (when I was hired to make specialty candy molds, I used this method).

With female mold vacuum forming, you make a negative mold of the item to be copied, and then drill holes in the mold. This mold is put over a vacuum form box. With this method, all of the fine details are on the outside of the copy. You do, however, have to deal with slight outward bumps where each of the vacuum holes were drilled (I chose this method when making model kits).

For both of the above methods, you need a vacuum form box and frame. Usually, what I do on a project is make a box that is slightly larger than the item I want to mold. This saves on plastic. You can also make a vacuum form table and frame in a generic size (say one foot by one foot) and adapt your projects to fit on that.

There is an upper limit as to the size of your copy. The main factor in determining how big you can make your copies is how much vacuum you can produce, and how well sealed your box is.

B) A vacuum form box is a fairly simple thing to make: picture a hollow box with a hole on the side that will tightly accept a small vacuum cleaner hose. On the top of the box are holes for the vacuum to go through. You can either drill your own holes in a piece of wood, use pegboard, or attach a flat mesh screen to the top of the box.

Around the edges of the box should be something to seal the vacuum. I have used strips of plywood. Other people prefer rubber window insulation. Whatever you use, it should be continuous, with no gaps to let air through.

C) Be sure to seal the inside of your vacuum form box so there are no leaks. If you are making a box custom for a project, be sure that there is at least 1" space all around on the forming space so that a vacuum seal can develop.

D) And next, the frame. You need something to hold the plastic while it is being heated. You can construct a frame, either from angle aluminum or wood. Make sure that the frame just fits over the box. If you wish, install stops around the box so that all you need to do is place the frame on the box, rather than having to hold it at a specific distance.

E) You need a method of clamping the two halves of the frame together. On small projects, I have used spring clamps. On a larger project, I attached window latches to the sides of the frame. These were standard cast (not stamped) window latches that were mounted slightly off center so that as you turned the latch, it tightened more and more.

F) To make the plastic hold tightly onto the frame, glue coarse sandpaper onto each half of the frame that touches the plastic. Rubber cement works great for this purpose.

G) If you are using an oven, you need some way of standing the plastic frame inside of the oven. The plastic will droop, and you do not want it to stick to the oven rack. What I have usually done is make a ring out of sheet metal which the plastic holding frame can sit on.

OK, now you have your vacuum form box, frame, sheet styrene, heat source and plastic stand. All you need is a male mold.

H) About the only warning here is that something with undercuts won't work. The plastic will either not form all the way, or will stick to your mold.

Set up your plastic stand, vacuum form box, plastic frame, vacuum cleaner and so on. Set your oven to 350 degrees to start (you might want to adjust the temperature up or down as you go along). Put the plastic shelf in the oven. When the oven is hot enough, put the plastic frame on the shelf. And then watch carefully. The plastic does not need to melt. It just needs to get soft enough to form. Look for it to droop. Individual projects will require more or less droop, depending on their size.

When you think the droop is enough, pull the frame out of the oven and place on the vacuum form box. Turn on the vacuum cleaner, and be prepared to be awed. The plastic forms over the mold almost instantaneously. Picture a Hollywood style morphing effect and you get a general idea of what it looks like.

Please note that you have to watch the plastic droop for each piece of plastic you mold. In other words, you can't time the amount of heat the plastic gets. Heating it for a minute for one piece might be perfect for that piece, or too little or too much for the next piece. The above is a lot easier if you have someone working with you, especially if one handles the frame and the other merely turns the vacuum cleaner on.

The plastic will form instantly, but it doesn't hurt to keep the vacuum on a little longer: sometimes freshly formed plastic can warp. It's always better to err on the side of caution in this case.

I) Look over your plastic copy. Is it what you wanted? Are some details in the middle unclear? If they are, consider drilling small holes where the unclear details are, so that more vacuum will be applied to them. You will have small inward dots, but they can be filled with Bondo during the sanding and painting process.

What is great about male mold vacuum forming is the smoothing effect it has. Since the greatest detail in on the inside, some roughness will not come through to the outside.

If you want, you could use a mix of wood, plastic, modeling clay and so on to make your master, and if the master is reasonably uniform, your copy will look good.

Female mold vacuum forming is a slightly different approach. Generally, what I have done is make a vacuum form box for a female mold project, and duct tape the female mold to it.

Basically, you start out with a master to make a mold from. You can use various materials to copy your master. I have heard of concrete being used. I use plaster. You need to build a box around your master to hold the plaster. I glue the master down onto a smooth backing. The master must be flush with the backing: no undercuts.

You will want to paint your master and box with some form of mold release. I put a bar of soap into some water and let it soak until I have a thin, milky liquid. I use that to brush the master and master box. This is allowed to dry.

Mix your plaster: 2 parts plaster to 1 part water. On one vacuum forming project, I had to use a great deal of plaster. No utensils were big or sturdy enough to mix it. So I had to use my arms and hands. And I quickly learned that in cases like that, it's best to use warm water.

Pour the mixed plaster over the master. Level off the top of the plaster, so it will fit on the vacuum box easily.

Why not simply pour some plaster into a box and push the master down into the plaster? Because bubbles rise in plaster. By casting upside down, if there are any air bubbles they will rise away from the mold. In fact, you can borrow a method used by cake makers: if you are confident that your master and master box are secured, you can bounce it up and down with the plaster, to help bring any bubbles to the top. I have heard of a simple vibrating device that will also do the job.

Wait a day or two for the plaster to dry (this is actually longer than it needs, but it's better to be safe than sorry). Unmold the mold from the master.

Look over your female mold: are all of the details there? Did the mold flake off at any fine detailing? A friend of mine had a simple philosophy in the case of mold damage: never redo anything, no matter how much damage. He would always find ways to repair the plaster molds he made. He used such things as styrene, bits of wire, Bondo and so on to repair his molds. And he always got them to work.

Never be afraid to do some cutting, sanding, or even minor replastering to improve your molds. When your mold is satisfactory, you need to drill holes in so the vacuum can pull the plastic through the mold. If you are making a simple bubble shape., one hole in the bottom of the bubble will suffice. Generally, the less holes, the better. Areas of detail would be improved by drilling a hole by them. I recommend a 1/16" carbide reinforced bit.

One time I had to drill holes that were deeper than the bit I had. I just got some wire, and hammered it into the holes from the top. This sometimes made the holes larger on the bottom, but since that has no contact with the plastic, that was not a concern.

J) You will need to make a raised ridge on your plaster mold, to seal it. Generally, I would make a square mold, and put sheet styrene, cut in strips, around it. As in male molding, the strips should be continuous. Inside of the strips, all around the mold, drill holes so the vacuum can make the seal.

Put your mold on top of your vacuum form box. Seal it with duct tape or whatever, so air does not escape.

At this point, the instructions are the same as for male mold vacuum forming.

Some clever variations:

You can use the above to make windows, or clear RC auto bodies, as well. Just ask for clear vinyl in the .030" size. Clear vinyl is a true joy to work with, especially in the smaller sizes: when heated, it turns cloudy-- but when it is hot enough to mold, it turns clear again.

If you have an area that does not produce the detail in the mold, all is not lost. Keep the plastic in the frame and on the mold. Apply a heat gun to the difficult area, and either turn on your vacuum, or rub over it with your gloved hand--or both.

A heat gun is excellent for heating small vacuum form projects. I made 1" domes using a small mold and a heat gun as the heat source. The heat gun was applied to the plastic in a circular pattern until the proper droop was attained. Then the vacuum cleaner was turned on, making an excellent copy.

Books on the Subject:

Do It Yourself Vacuum Forming for the Hobbyist (Vacuum Form Press).

Click here to order this book

A winner! More than you will ever need to know about vacuum forming. I had been vacuum forming for several years, and I learned a lot of new tricks form this book. Covers mainly male mold vacuum forming. My favorite vacuum forming trick from this book: making a flip top vacuum form with an electric frying pan.


The Prop Builder's Molding and Casting Handbook (Betterway Press).

Click here to order this book

This book is great! It not only covers vacuum forming (and how to make your own two foot vacuum form machine), it covers molding and casting methods with the usual materials (resin, RTV rubber), as well as obscure casting/molding materials such as silicon caulk, hot glue, water putty, and so on. A must!

Plastic and Plastic Supplies:

Any city of any size will have several plastic suppliers. In the St. Louis Yellow pages, for example, the list of plastics suppliers goes on for several pages.