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Molding Plexiglas

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Jan 6, 2010
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Midland Ontario Canada
Molding Plexiglas

There has been some discussion about molding Plexiglas. In an effort to save people some frustration and money, I am sharing a bit that I have learned over the years. I have done a lot of research and experimenting, and scrapped a lot of valuable plexiglas in the process. I have also built a number of ovens of varying complexity. For whatever reason, experts either don't want to share info, or just don't know the details of why they do it the way they do, "we have always done it this way".

First some background. Plexiglas is a trade name for Acrylic. Lexan is a trade name for Polycarbonate. They are very different. Plexi is harder and more resistant to scratching, but cracks easier. Lexan is softer and tougher, can be bent cold in a metal brake, and won't crack when drilled. Lexan is more easily damaged by chemicals although Acrylic also can suffer from chemical attack. Both can be molded hot. Lexan often needs to "cook" for a while to drive the moisture out of it before raising to molding temperature. For aircraft windshields Acrylic is usually the better choice.

There are 2 basic types of Acrylic, Cast and Extruded. Extruded is more common and costs a bit less. Cast costs about 25% more. As with anything, both expand and contract with temperature changes, but Cast expands/contracts equally in both directions where extruded does not. This isn't a big problem for homebuilts because we aren't working to extreme precision. The big advantage to cast is that it has more of an elastic memory. If you screw up molding a part, like not getting it into shape before it cools, you can reheat it and it will return to the flat original shape and you can try the mold again. This memory is so pronounced that in experiments with Cast, I squeezed hot, 1/4" thick plexi, with vice grips down to 1/8" thick, leaving the vise grip imprint in the plexi when it cooled. I then reheated the samples and let cool. There was no evidence of the vise grip imprints! That's powerful memory.

The normal way to mold a windshield is flat wrap drape forming. You can make a mold from a sheet of steel or aluminum bent around supports, or sometimes use an old windshield as the mold. In this there will be no compound curves as you would have in a blown bubble. Another sheet of some type (metal) is used to support the plexi while heating in the oven. Soft cotton flannel or "Billiard Table cloth" is placed on the sheet under the plexi to prevent it from getting scratched. The mold is not in the oven, and flannel is also covering the mold surface. When the plexi is hot enough, and it takes some time for the heat to soak into the center of the sheet (about 20 minutes for 1/8" thick) remove it from the oven with gloves, and drape it over the mold. It will be as flexable as a wet noodle. Be quick and accurate, you only have about 20 seconds to get it into shape on the mold. If it cools too much before in shape, it will look like it molded ok, but there will be internal stress that will cause crazing and cracks in a year or two. If you plan ahead, you could leave "tabs" sticking out from edges of the plexi and corresponding locators on the mold. Just don't locate them so that the plexi can't shrink a bit as it cools or it will cause stress. You can make the mold, and the plexi, a bit bigger than needed, that way placement is less critical and you can trim to finished shape after it cools. You want the plexi to cool slowly on the mold to reduce stress in the plastic. If using a steel mold, you could put a small heater under it or a light bulb, to heat the mold a bit so it doesn't "chill" the plexi. You should also cover the part with soft flannel and blankets after it's on the mold, and leave it to cool slowly. The reason putting the mold in the oven is normally not a good idea, is that the plexi is soft enough to imprint even the soft flannel texture. Think about the cool "before soft" sheet of plexi resting on the mold. It will have point loading where it touches the mold making chances of imprinting worse, and those "points" will move around the mold as the plexi softens and slowly bends down. So why doesn't the mold still imprint the plexi a little when it's out of the oven? Even many molders don't know the reason. It seems that when you remove the plexi from the oven it starts to cool, and just like it took time for the heat to soak in, it takes time for it to leave. The outside surfaces cool faster than the inside. This means that in the short time it took to get the plexi from the oven to the mold, the surface has cooled and "hardened" just enough to resist imprinting, even though the center is still soft and the sheet is still very flexible.

The oven doesn't need to be fancy, but temperature control is critical. You can buy a very capable PID controller, with a thermocouple and solid state relay, for well under $100. It will not only give you extremely precise control, it will ensure the oven doesn't overshoot the desired temp as it heats up. The other point about ovens that I have found to be critical, is they should be much bigger than needed to fit the sheet. At least 3 times the volume you think you need to allow air movement. You also need a fan in the oven to circulate the air and get even temperature.

There are a lot of other points, and I will add more, but I hope this info helps.

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