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Mold materiel that is safe for the oven and will not stick to lexan/PETG? Answered

I'm making a helmet, and I hit a wall: I need to make the visor. I have the helmet made, and I need to make a negative mold of the visor, which I would put in the oven, with the PETG/lexan to "melt" the plastic to form over the negative mold in the home oven.

But, again, issue is what mold materiel can I use which will be safe up to 300 degree Fahrenheit and not stick to a semi-melted plastic?



Best Answer 5 years ago

Your plan isn't going to work. I tried something similar some years ago. You can't get the plastic to flow into the contours of the mold properly unless you heat it to the melting point. And if it melts, you can't control the thickness, the high points will be too thin and the low points too thick. And the Lexan will distort the light passing through it and make it difficult to see through.
To do the job properly, it should be vacuum formed or if there are no complex curves or angles in the mold you may be able to bend the heated and softened Lexan around the mold.

It's not really a mold, persay. I'm basically making a mold of the inside of a motorcycle helmet, and then taking that mold, and placing lexan over that. I'm not actually making anything terribly intrastate, just some way to flex the Lexan for a visor.

In that case, you might be able to warm the Lexan, heat it in an oven and bend it by hand over a plaster of paris mold. I've done that with a moderate amount of success when I was doing a lot of train modeling.
Cast the plaster of paris to the inside of the existing visor, let it cure and fill all the pin holes and lightly sand it smooth. When you're bending the Lexan, the more hands you have to press the warmed Lexan onto the mold, the better. It may help if you warmed the plaster in the oven for a few minutes before attempting, it will give you a little more time to work with the Lexan.

Oh! That's brilliant!
By chance, would a heat gun do the job as well? I have this feeling that the lexan will be a bit hot.

The problem I have with a heat gun is that it is hard to heat the Lexan evenly. You'll get hot spots and maybe even blisters. I would use the oven to heat the Lexan for sure. Pick up a couple of pairs of cotton gloves for yourself and whoever you recruit to help you or keep a couple of tea towels handy. And remember you want to heat the Lexan gently, pliable enough to bend but not so soft that you can thin it out under pressure.
It helps if you have a few scraps to practice on to get a feel for what works best.

Makes sense, and sounds fairly easy to do. Thanks for the answer!


has lots of high temperature options and instructional videos.


*you can mold molten lead with many of these silicones which are good for 400-700F.

Fantastic! That'll do beautifully!

However, one finally question: any ideas on a cheaper silicone? Not to sound cheap, but I would probably one use the mold once.

unfortunately you get what you pay for - and smooth-on is one of the few suppliers of good molding materials

If your making a pattern (a positive) then vacuum forming is the way to go.

You can make a good pattern from MDF.

A red neck Vacuum former isn't hard to make.

As here