Building Molds


I'm interested in doing some vacuforming, and there's tons of stuff on that. But there's not much in the way of the molds ro use. Can you make a mold from anything and then form the ABS on it? Or is it like 'glass/resin where you need a release agent? Does the ABS tend to stick onto the mold?

I would like to put together something using my PVC Board, then use that as a mold for an ABS version. Will the two plastics stick to each other, or would the heat of the ABS warp/bend the sintra?

I know it's a lot of questions, but I'm just curious. You know 'measure twice, cut once' train of thought.
I know the sintra has a pretty low melting point. I'm guessing even lower than than the ABS. It might work once or twice bit in the end I think wood, metal or even plaster as a mold material will hold up better. Just my inexperienced guess. I've never vac'd anythng myself.
I've successfully Vacuum-formed over a mold made from sintra without any problems. Don't see any problems using PVC, but if your a little gun shy, just wax up your mold for extra security.
You can vac over just about anything that will stand up to the heat of the hot plastic your using, and the pressure of the suction from the vac'ing process..

A lot of people seem to be using MDF/fiber board, to make molds or "bucks" as they can be called.

If you don't have something perfect the first time, then a good cheat is to vacuform a thin piece of plastic over your product( like rigid foam, plaster, wood, pvc, resin, etc..) then take the vacuformed plastic piece and fill it with something hard and durable like a hard resin or ultracal... etc.. then you have a good mold to vac several copies from.

Just some ideas for ya...

The plastic never actually melts, and so won't stick to your pattern in the usual sense. However, if your pattern has paint on it anywhere the paint may melt and stick to your plastic. If you made the patter by glueing together layers of MDF and then carving, be wary- any glue that is exposed by carving may melt and stick when it contacts the hot plastic.

And while the plastic won't stick to things, the vacuum forming process does create a vacuum seal between the part and the pattern. Normal air pressure is roughly 15 psi. If your vacuum formed part has a surface area of 36 square-inches (or 6" by 6") the pressure holding that plastic to the pattern is around 540 pounds. When releasing the plastic, keep in mind that you don't want so much to yank the pieces apart as break that seal. if the part is round and smooth, this won't be terribly difficult. If it has flat surfaces or surfaces nearly perpindicular to the surface on which you do your forming, this becomes harder. More curves and folds will make the part less flexible. You want to "wiggle" the part to release it- that is, break the vacuum seal by flexing the part as much as you can. Another good trick is to use a compressor with one of those needle tips, like you would use to blow up a basketball. try and wiggle the tip in between the plastic and the part, and then blow air in. The force of the air will often break that seal, and let the part release easily. Tapping the part with a rubber mallet can wor, as will tapping in thin wooden shims between the part and the pattern. The technique which works best will vary from part to part, so try them all until something works.
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