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Step 5Make a matched pair of clamping frames
Now you need to decide what size frames you want to make and use first. You can make different-sized frames later, any size from a few inches across to whatever size will fit in your oven. (That's probably about 16 x 22 inches.)
To start with, you probably want to do things smaller than that. That will save on plastic while you get the hang of vacuum forming.
If you make different-sized frames, but with one dimension the same, you can take your old frames apart and use those frame sides for the new frames. So, for example, if you eventually want 16 x 20 inch frames, you can make 12 x 16 inch frames now, and reuse the 16-inch sides when you make the 20-inch sides.
For many reasonably small items, 12 x 16 inches is a good size for frames. You can divide sheets whose dimensions are 2 x 4 or 4 x 8 feet into 12 x 16 in sizes with no waste, and 12 x 16 inches will accommodate most RC plane canopies, most full-size masks, many enclosures for small electrical and mechanical projects, etc. (You can also cut 12 x 18 inch sheets of craft foam down by a couple of inches, and not waste much foam.)
All other things being equal, non-square rectangular shapes are better than squares. Most things are not square, and if something won't fit the short way, it may fit the longer way.
Have a look at your windowscreen frame material and the aluminum corner braces. See how the ends of the corner pieces fit INSIDE the frame material, with a funny groove going along the inside edge of the frame on one side. (That groove is for a rubber strip that holds windowscreen in, and it's useless to us, but you should know where it goes.)
The groove has to be along the inside edge of the screen frame, or the corner things won't go in right. (They have a little alignment tab on them that fits in a small slot in the frame material.) When making your miter cuts, make sure that the non-groove edge is the long edge, and the grooved edge is the short one.
If your aluminum frame corners are like mine, you need to miter cut the frame material, at 45 degrees, so that the two pieces can slide over the internal corner brace and completely cover it, meeting at a 45 degree join.
Each piece is shorter on the inside edge (where the groove is) than on the outside. It's the longer outer edges that should have the same dimensions as your plastic. Cut four pieces the size of the shorter dimension, and four pieces the size of the longer one. (This will require a separate miter cut for each end of each piece---16 cuts---because the frame material is asymmetrical and the remaining piece is always mitered the wrong way.)
Use a miter box and a hacksaw for these cuts, because you want the pieces to meet pretty closely. Be sure to clamp the material you're cutting. That helps make a reasonably precise cut.
Now put the two frames together, sliding the cut pieces on over the corners.
Look at both of them, paying particular attention to the corners. Pick the one whose fit is best, with the least gap at the joins on both the top and bottom, to use as your bottom frame---that's the critical one for making a seal. (The plastic needs to be flat against the top of the bottom frame, and the bottom of that frame is what will meet the gasket.) If they're both about the same, but with a sloppy fit on some corners and a good fit on others, mix and match the parts to make one neatish frame and one sloppyish one.
Mark the better frame BOTTOM FRAME with a permanent marker or something.
(Don't obsess about making the corners meet perfectly, though. If you're using a vacuum cleaner as your vacuum source, tiny leaks don't matter a lot; a vacuum cleaner can suck a whole lot of air in a hurry, and keep ahead of very small leaks. If you upgrade to a high vacuum system later, you can neaten up your frames then.)
To start with, you probably want to do things smaller than that. That will save on plastic while you get the hang of vacuum forming.
If you make different-sized frames, but with one dimension the same, you can take your old frames apart and use those frame sides for the new frames. So, for example, if you eventually want 16 x 20 inch frames, you can make 12 x 16 inch frames now, and reuse the 16-inch sides when you make the 20-inch sides.
For many reasonably small items, 12 x 16 inches is a good size for frames. You can divide sheets whose dimensions are 2 x 4 or 4 x 8 feet into 12 x 16 in sizes with no waste, and 12 x 16 inches will accommodate most RC plane canopies, most full-size masks, many enclosures for small electrical and mechanical projects, etc. (You can also cut 12 x 18 inch sheets of craft foam down by a couple of inches, and not waste much foam.)
All other things being equal, non-square rectangular shapes are better than squares. Most things are not square, and if something won't fit the short way, it may fit the longer way.
Have a look at your windowscreen frame material and the aluminum corner braces. See how the ends of the corner pieces fit INSIDE the frame material, with a funny groove going along the inside edge of the frame on one side. (That groove is for a rubber strip that holds windowscreen in, and it's useless to us, but you should know where it goes.)
The groove has to be along the inside edge of the screen frame, or the corner things won't go in right. (They have a little alignment tab on them that fits in a small slot in the frame material.) When making your miter cuts, make sure that the non-groove edge is the long edge, and the grooved edge is the short one.
If your aluminum frame corners are like mine, you need to miter cut the frame material, at 45 degrees, so that the two pieces can slide over the internal corner brace and completely cover it, meeting at a 45 degree join.
Each piece is shorter on the inside edge (where the groove is) than on the outside. It's the longer outer edges that should have the same dimensions as your plastic. Cut four pieces the size of the shorter dimension, and four pieces the size of the longer one. (This will require a separate miter cut for each end of each piece---16 cuts---because the frame material is asymmetrical and the remaining piece is always mitered the wrong way.)
Use a miter box and a hacksaw for these cuts, because you want the pieces to meet pretty closely. Be sure to clamp the material you're cutting. That helps make a reasonably precise cut.
Now put the two frames together, sliding the cut pieces on over the corners.
Look at both of them, paying particular attention to the corners. Pick the one whose fit is best, with the least gap at the joins on both the top and bottom, to use as your bottom frame---that's the critical one for making a seal. (The plastic needs to be flat against the top of the bottom frame, and the bottom of that frame is what will meet the gasket.) If they're both about the same, but with a sloppy fit on some corners and a good fit on others, mix and match the parts to make one neatish frame and one sloppyish one.
Mark the better frame BOTTOM FRAME with a permanent marker or something.
(Don't obsess about making the corners meet perfectly, though. If you're using a vacuum cleaner as your vacuum source, tiny leaks don't matter a lot; a vacuum cleaner can suck a whole lot of air in a hurry, and keep ahead of very small leaks. If you upgrade to a high vacuum system later, you can neaten up your frames then.)
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I made a quickie vacuum former awhile back out of parts laying around the house, I had an expanded metal shelf laying around that worked for a base.
I took a different tact with connecting the vacuum that seemed to work well. I used a flexible sink tailpipe that happens to fit my shop-vac perfectly. For mounting it to the MDF, I just used expanding insulation. The few times I used it, it worked well, how well it would hold up over extended, heavy use, I'm not sure. ( Vacuum connection )
The only problem I had was with the frame. I kept meaning to get back to it, but never did , well until now that is ;)
Thanks again.