Another way to get the same effect is to have different-sized frames and gaskets for one platen. The platen I usually use is just a board with a hole in it. (For making one thing at a time, that works as well as anything else.) To use different-sized frames, I put down a sheet of flexible plastic with the right gasket stuck to it, and tape it down around the edges.
ORIGINAL: Chad Veich
Probably the best thing I have done to help prevent webbing is to build different size vacuum boxes so that I can choose the one that best fits the part being molded. Lots of excess plastic flopping around is the recipe for webbing. The added benefit is less wasted plastic as well.
By the way, you don't really need pairs of frames; just one rigid frame will do. Then you can clip strips of something to the top to hold the plastic down along each edge---that's how lots of industrial ther****rmers do it. The strips don't even have to fit---only one does. If you want to uses a bunch of different sizes of plastic, that can save time and money on frames. At most you need one good rigid frame per size.
For thin plastic, aluminum windowscreen edging is good enough, if you use aluminum corners. You can mix and match the side parts, so you only need one pair in each length to make the frame you need. (Unless you want to make squares... then you need four of each size. So I don't do squares. :-) For the upper strips, you can use anything that's at least long enough---say, the two next-longer sizes you've got lying around. You just leave the extra hanging off the corners, like this:
...and clip them on with binder clips. If you're using a gasket sealing system and use binder clips, they create a potential leak, because they go under the bottom frame as well as over the top strips. I've found that they work fine anyway for a vacuum-cleaner based system, if you're careful to make sure that they're flat against the bottom frame, and the rolled edge is inward of where you'll hit the gasket. The leakage is small enough that a high-volume low-vacuum system just doesn't care. (Likewise for the very small seams at the corners when you plug the bottom frame together using the aluminum corners.)
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Right now I'm making a heavier-duty set of adjustable frames for a no-gasket high-vacuum system, so that I can do all sizes and shapes in thick plastic. That's easy enough for a system where the plastic seals directly to the edge of the platen, because the bottom frame doesn't have to be airtight. Unfortunately it means that the hard problem is making all the different raised platen edges you want---they do have to be airtight. I'm thinking of trying to make a set of telescoping platens with silicone edging. (I'm thinking silicone edges should work as well as metal, and sometimes better, and should be able to take the heat.) Or I might just make a telescoping mold and cast a bunch of platen-tops out of solid silicone caulk... not sure yet.
Yes. Female molds can be great. They can be harder to make, though, especially if they're non-small.
One other idea not discussed here which I have tried is pulling the plastic into a female mold rather than over a male plug. [...] [T]here is not really any chance of webs forming when you are sucking the heated plasitc down into a cavity.
For bubble canopies, if the exact shape isn't critical, an easier way to do it is by gravity forming and/or "free blowing." Instead of making an actual female mold, you just make a teardrop-shaped cutout, put it on top of a box, and suck air out of the box, pulling the plastic down into it to make an actual bubble. (When they started calling them "bubble" canopies, they weren't kidding.) People making full-size vehicles do this, so I'm wondering why more modelers don't.
For full-size stuff, at least, you can actually get a fair bit of control of the shape by varying how much gravity-sag you use and how much vacuum, and by tilting the box. If you just use gravity, you'll get a catenary-like curve that never reaches 180 degrees. If you use vacuum, you'll get something more bubbly, that's rounder and may flare outward and come back in a bit. You can also use gravity to change the front-to-back profile; if you put the nose-end down, the plastic will sag toward that end and enlarge the front, making a more exaggerated teardrop shape.
Industrial ther****rming people do this sort of thing for actual production, without ever making a mold. They work out how hot to get the plastic, how far to let it sag under gravity, how long to apply how much vacuum, etc.... and then do it many times with surprisingly consistent results, all without ever making an actual mold.
I'm not sure how well that works at model scales, because the force of gravity isn't much on a little thin piece of plastic. I'm thinking it may work okay with thick PETG for fairly large model canopies, and it's on my list of things to try. You may also need fairly high-quality plastic. (Most full-size bubbles are done in cast plate plastic, not the usual extruded stuff. It's got a more consistent thickness, and doesn't have the residual stresses from the extrusion process.)
Once you get one canopy you like, you can use it as a mold, and vacuum form more the normal way. All with no sanding.
(I just hate sanding things to shape. I'm bad at it, and if I work that hard on something, I don't want to risk breaking it by flying it or something crazy like that. :-) I'm also not the best pilot. Bad combination. That's one reason I took up vacuum forming---so I could make things easily enough that I wouldn't be too afraid of breaking them. Lately I've been hung up on designing vacuum formers, but some day I'll get back to smashing planes into things. :-) )
BTW, the "cutout" doesn't have to be a two-dimensional hole in a flat plate. It can be the three-dimensional shape of where the canopy is supposed to meet the fuselage. (Or if you want to get fancy, you can intentionally make it somewhat different and oversized, to control the shape of the bubble, and then cut the resulting bubble it down to fit the fuselage. If you're willing to work that hard, you could probably make some pretty-close-to-scale canopies.)
And if what you're making is going to be used as a mold for the actual canopies, you can use any material that's easy to form the way you want. (Such as really thick plastic, so that you can use gravity better, and then form thin plastic over it.)
One thing I really want to try is making custom fuselages that way. I'm pretty sure some full-sized ultralight stuff has been made that way, either free-blowing the body halves and reinforcing them with foam and carbon spars, or free-blowing a shape and using it as a mold for composite layup. One problem there is that if you don't have very consistent technique and materials, it's not going to be symmetrical enough. One thing I want to try is making one half, casting over it in something thin and rubbery, and then using that as a mold, turning it inside out to make the other half. That's tricky because it has to be very flexible but not distort too much when you cast in it. I think that's doable if you're clever, and you could take advantage of the flexibility to "fix up" the shape a little.