Intro: Make a Good, Cheap, Upgradeable Sheet Plastic Vacuum Former
Or you can make nice and surprisingly sturdy plastic stuff in amazing shapes, amaze your friends with your technical prowess, and be the life of the party.
Vacuum forming is a technique for shaping sheet plastics into 3D shapes, which you can do at home, easily and cheaply. And it's fun. It's the easiest way to make an infinite variety of shapes in plastic, or to make molds for casting shapes in other materials, such as concrete.
The basic technique is to
0. clamp a sheet of plastic to a frame (such as a windowscreen-type aluminum frame)
1. heat it in an oven (such as your kitchen oven) until it's soft and rubbery
2. stretch it over a convex mold of an interesting shape (such as a life cast of your sweetie's face), and
3. suck the plastic inward onto that mold with a vacuum system (such as your household vacuum cleaner)
Once the plastic cools, you pull it off the mold and trim off the excess plastic, leaving a copy of whatever shape you sucked the plastic onto.
In this instructable, I'll show you how to make a cheap but good vacuum former, using mostly things you have around the house, or can buy very cheaply. The whole thing shouldn't cost more than about $30 to $50, maybe less depending on what shortcuts or substitutions you choose, and what materials and tools you have lying around. It also shouldn't or take more than an hour or two to make. (Plus a shopping trip to a home improvement store and an office supply store, and letting some silicone cure overnight; you can use epoxy if you're in a big hurry and want to do it all in an evening.)
Here's a movie of the vacuum former in action:
Relatively few people know about vacuum forming, or how easy it is. They're mostly radio control model builders---who use it for making thin plastic parts for airplanes, or bodies for cars or helicopters, or hulls for boats---or they're Star Wars fans who use it for making their own costume armor.
It's unfortunate that vacuum forming know-how is mostly limited to these little niches, because vacuum forming can be used for many purposes, artistic and practical. If you like making stuff in general, and especially if you like non-rectilinear stuff that doesn't look "homemade," you should know how to vacuum form.
You can use vacuum forming to make:
1. intermediate molds for modifying and combining sculptural shapes (this allows you to sculpt in whatever medium is easiest, and transfer the shapes to plastic, making one copy or many)
2. sturdy custom parts out of thick plastic to protect delicate machinery. (Using cheap homemade equipment, I've vacuum formed shells from 1/4" thick plastic that are sturdy enough to stand on.)
3. three-dimensional, internally-illuminated signs from scintillating textured plastic
4. flexible, cushiony custom liners from thermoformable foam
5. relief sculptures of various kinds
6. molds for casting chocolates, soaps, candles, or concrete relief sculptures
7. decorative architectural reliefs, or decorative shells that can be reinforced for structural purposes
8. stage props and costume parts in hard plastic or soft foam,
9. zillions of things you'll probably think of.
Industrially, vacuum forming is used for making all kinds of things, from disposable plastic cups and lids to sinks and hot tubs and McDonald's golden arches to full-sized boat hulls. (If you've never seen a 30-foot sheet of plastic sucked into a boat shape in a few seconds, trust me, it's pretty cool.)
For vacuum forming at home, the main limitation is usually space for the equipment---the size of your vacuum former is proportional to the size of plastic sheet you need to form. The $30-50 vacuum former described here doesn't take up much storage space at all, and can handle thin plastic sheets as big as will fit in your oven.
For larger stuff, you need a custom oven---not very difficult or expensive to make, but a little bigger all around than the plastic it will heat.
For thick plastics (more than about 3/32" or 1/8" thick, depending on several variables) you often need a stronger vacuum than a vacuum cleaner will provide, and again the cost and size of the equipment are roughly proportional to the size of plastic sheet you will be forming. The cost can be under $50 for a high-vacuum system for thick plastic sheets up to about 12" x 18", using a converted bike pump, or an electric air pump of some sort from a thrift store. (Such as a kitchen vacuum sealer, a tire inflator air compressor, or a "nebulizer" air pump.)
The vacuum former described here will work very well with an inexpensive high vacuum system, getting professional quality results for thick plastic, for under $100. If you want a standalone vacuum oven, so that you can use it somewhere besides your kitchen, you can make a medium-sized one (12 x 20 inches) for $30.
For now, let's make a good fast cheap vacuum former that you can do a lot with, using your kitchen oven and vacuum cleaner; it's mainly a board with a hole in it, which you can store on a shelf. You can soup it up later, if you want.
Step 1: Understand the Design
Our vacuum former consists mostly of
(1) a board (called a "platen") with a hole in the middle of it,
(2) a vacuum cleaner that sucks air through that hole,
(3) a pair of frames we can clamp together around the edge of a sheet of plastic, and
(4) a kitchen oven.
To use it, we'll do three basic things:
(1) heat the plastic in the oven until it's soft and rubbery and stretchable
(2) stretch it over the shape we want to copy, and
(3) suck it down around that thing, and let it cool in that shape
In more detail, we'll
0.a. Support the board on something near the oven. The support(s) can be pretty much anything, or any convenient pair of things that is reasonably sturdy, allows us to route the hose to the vacuum cleaner without kinking it, and can be put very near the oven we're using.
0.b Put some things in the oven which we can support the plastic-holding frames on. (Glasses made of actual glass, for instance.)
0.c. Preheat the oven. This usually gives us more even heat.
0.d Position some object that we want to shape plastic over on the board, over the hole, but with some spacers under it, so that air can flow from around the the object, under it, and to the hole in the board.
1.a Clamp a sheet plastic between the pair of frames and support it on three or four things in the oven (such as glasses made of actual glass)
1.b Wait a few minutes for the plastic to get hot and rubbery and stretchable. For most plastics, we can tell how stretchable it is by how much it sags under its own weight. When it sags about the right amount, we know it's ready.
2.a. (Turn on the vacuum cleaner, open the oven, and) QUICKLY but carefully remove the plastic from the oven with gloved hands...
2.b. ...stretch the plastic down over the shape we're copying, until the frame meets the board, creating a kind of "tent" of hot rubbery plastic over our form and stretching down to the board, and...
2.c. ...let the vacuum cleaner suck air out from under the "tent," by sucking air from under the form, and in turn from around it. This will suck the stretched, rubbery plastic inward into the desired shape, in about one second, and the plastic will cool enough to solidify in the new shape in about 10 to 20 seconds.
To make this work well, and flexibly, we'll add a few basic enhancements:
1) We'll put a foam rubber gasket on the board, the size and shape of our plastic-clamping frames. That way, when we stretch the plastic over our mold, we can press the frame against the gasket to make a seal. When the vacuum cleaner sucks air from around the mold, it will do a better job because it's not sucking air through any little gaps between the frame and the board.
2) We'll make the gasket removable, so that we can use different-sized gaskets (and plastic-clamping frames) for different-sized sheets of plastic. The obvious benefit of this is you can waste less plastic if you make different-sized things. A less obvious benefit is that it helps you use odd-sized scraps that you get from cutting the larger size out of a sheet of plastic. A much less obvious benefit is that you often get better results for certain difficult-to-form shapes, by using plastic that is somewhat bigger than the thing you're making, but not a whole lot bigger.
(I won't explain that here, but if you're interested you can check out this thread on www.rcuniverse.com, about "webbing" problems and ways to avoid them: http://www.rcuniverse.com/forum/m_5086453/anchors_5086453/mpage_1/key_/anchor/tm.htm#5086453 One of the best ways is to use plastic sheets about the right size for your project.)
Making the gasket removable is easy. Instead of sticking the self-stick foam rubber directly to the platen (board), well stick it to a slightly oversized sheet of something flexible---such as thin plastic, and tape that down to the board.
3) We'll make our frames out of pieces of aluminum windowscreen frame material, with internal aluminum corner braces. That will let us mix and match a few side lengths to make frames of different sizes and proportions for different projects.
4) We'll use a 3/4" galvanized floor flange (plumbing fitting) under the hole in the platen, as part of our connection to the vacuum cleaner hose. This will let us replace the vacuum cleaner with a more powerful---but surprisingly cheap---vacuum system later, if we want. A more powerful vacuum system lets you form thicker plastic and still get good detail. (If you know you'll never need to do that this, you could just make the platen hole the size of your vacuum cleaner hose, or some attachment that fits it, and glue the hose or the attachment permanently to the hole. That would be cheap and easy, but you would lose flexibility for later upgrades.)
If you've seen other homemade vacuum formers, you've likely seen "vacuum boxes" several inches thick covered with pegboard. Don't make one of those. You don't need a bunch of holes in your platen; One big one works at least as well if you're only forming one object at a time, and if you want to distribute the vacuum across several smaller molds, there are other ways of doing it. (Many industrial vacuum formers use one-big-hole platens.)
Thick "vacuum forming boxes" are likely to collapse if you ever add a powerful vacuum system---and to reduce the vacuum system's effectiveness, because the air inside the box has to be pulled out.
(If you decide later that you really want a many-hole platen, you should make a thin "sandwich construction" platen; you can use your one-hole platen as the bottom layer of the "sandwich," so starting with a one-hole platen is a good way to go.)
Step 2: Gather Materials and Tools
A kitchen oven or some other way of heating plastic. (Don't use a heat gun; it's hard to heat a non-tiny sheet of plastic evenly with a spot-heating device like a heat gun.) If you want to build a small standalone vacuum forming oven cheaply, check out my $30 oven design at www.rcgroups.com: http://www.rcgroups.com/forums/showthread.php?t=621858
A good vacuum cleaner, preferably a powerful (1000 watts or more) canister vacuum. If you don't already have one, you can get one for $5 at the Goodwill Outlet Store (a.k.a. Blue Hanger Store). Don't buy a shop vac just for vacuum forming; they don't suck any harder than a high-wattage household vac, and they're bulky. (If you already have a shop vac, though, you can go ahead and use it.) Don't buy a new canister vacuum, either; the centrifugal pumps in vacuum cleaners don't have seals that wear out, so a used one that works is fine; it'll suck as hard as a brand new one costing 20 times as much. (If the filter or bag is dirty, just take it out; you don't need a filter. If a few particles do get sucked through it, they won't hurt it; centrifugal pumps are nice that way.)
A 3/4" MDF (medium-density fiberboard) board at least 2 inches bigger than the inside dimensions of your oven, cut down to 2 inches bigger each way. If you have a big enough scrap board around---maybe a piece of plywood or thick particle board---it will probably do. It needs to be pretty smooth on one side (the top). (MDF will cost about 5 dollars for a 2 x 2 foot sheet, or about 9 dollars for a 2 x 4 foot sheet at Home Depot.) You can substitute a scrap board you have lying around (such as 1/2" or thicker plywood), but it should be smooth on the top side. (MDF is nice and smooth.)
A 3/4" galvanized floor flange (plumbing fitting). (Less than $3.00.)
A 3/4" x 2" threaded pipe nipple (or "riser"); PVC plastic or galvanized is fine. (That's just a short piece of pipe threaded on both ends, about 50 cents.)
A little PTFE tape, a.k.a. "Teflon tape," used for sealing plumbing joints. (About $1.)
Four 3/4" long wood screws, fairly large diameter but small enough to fit through the holes in the floor flange. (About $1.)
8 aluminum windowscreen frame corner braces, for 7/16" or 3/8" thick frame material. (5/16" will do for small frames and thin plastic, but thicker is better for larger or thicker plastic; small differences in thickness have a significant effect on stiffness). You want aluminum corner braces, not just plastic ones that fit aluminum frames. I use 3/8" corners from a local True Value (25 cents each) with 7/16" frame material from Home Depot, and that works fine.
2 or 3 sticks, 6 or 7 or 8 feet long, of aluminum 3/8" or 7/16" windowscreen frame material that goes with the aluminum frame corners. You'll need enough for four frame sides in each of the two dimensions of the plastic you'll be using, plus a couple of inches extra per stick. (About $4-5 per stick.)
1 box of a dozen binder clips, large size, from an office supply store. (Three or four dollars.)
A sheet of thin plastic 2" bigger each way than your chosen plastic sheet size, or just 1" bigger in a pinch, maybe a flimsy GARAGE SALE sign or a piece of the thin plastic you intend to vacuum form. (See step 7.) Unfortunately, Home Depot's big (14 x 19) signs are not flimsy or cheap, but Lowe's has 15 x 19 signs for $3. (Wal-Mart has them for $2, and smaller ones for $1 or so.)
A little silicone caulk or silicone sealant, or maybe epoxy, or rubber cement. (Any kind of gap-filling glue will work, if it doesn't set extremely quickly like hot glue. Tacky putty will do temporarily, in a pinch.)
A 10-foot roll of foam rubber weatherstrip, at least 1/4" thick and 1/2" wide, preferably 3/8" thick and 3/4" wide. You want the kind that's just foam rubber self-stick tape with a rectangular cross-section. (Three or four dollars at Home Depot.)
Some washers or coins to use as spacers.
Masking tape or (preferably) blue painter's tape.
Duct tape may come in handy, as it often does. (See Step 8.)
Some aluminum window screen material is also nice to have, but optional. Screening from a junked window screen is fine.
The main thing on this list that you likely won't find on a trip to an office supply store and a Home Depot is the aluminum window screen corner braces. (Home Depot and Lowe's only seem to sell plastic ones these days.) If you're not in a hurry, or can't find them locally, you can buy the frame corners online, and wait a few days for them to be delivered. http://www.Builderdepot.com sells a tub of twenty 3/8" frame corners for $6. (Having extra corners is nice, so that you can have some different-sized frames without having to take them apart and reassemble them when you switch sizes.)
Another option is to make your frames out of wood. That's not my favorite way, but it does work---you can use wooden frames for a vacuum former, and many people do. (They'll eventually char and/or warp, but they won't catch fire in the oven; they won't be in a hot enough oven, or not for long enough.) If you're in a hurry and especially if you have the scrap wood around, that may be the way to go, at least to get started. I won't discuss wooden frames any further, because they don't get you the mix-and-match size advantage, and tend reduce the maximum size that will fit in a kitchen oven, but if you want to pursue that option, see this thread on www.tk560.com: http://www.tk560.com/phpBB2/viewtopic.php?t=466&highlight=draft+++simple+former )
My Lowes doesn't sell small sheets of MDF, or thick weatherstrip 3/4" wide, so if you want to only hit one home improvement store, try Home Depot first.
You should probably take your vacuum cleaner hose to the store, and look for a plumbing fitting that adapts 3/4" pipe threads to roughly the size of your hose (inside or outside). It doesn't have to fit well, but anything that gets you closer is good. (See Step 8.) If you already have a shop vacuum with a large hose, and will be using that, you'll want an adapter from the large hose size to the small hose size. (About four dollars.)
While you're at the store, you may want to pick up a 2 x 4 sheet of textured styrene, sold as a fluorescent light diffuser panel for suspended ceilings. It's fun stuff to vacuum form. (Acrylic panels are nice, too, but a little trickier to heat and form.)
You'll also need some basic tools:
(1) a drill and a reasonably large bit (such as 1/4"), plus a bit that's somewhat smaller than your screws
(2) a screwdriver that fits your screws
(3) an electric saw such as a portable jigsaw or circular saw, unless you have the board cut to size at the store. (That's usually free; see Step 3.)
(4) a hacksaw
(5) a miter box you can use the hacksaw with
A portable jigsaw is good to have, but not strictly required. A hole saw (drill attachment) that can cut a 1 1/2" diameter hole is also nice, but not necessary.
If you make your frames of wood, you won't need the hacksaw, and maybe not the miter box.
Step 3: Make a Platen
The heart of your vacuum former is the platen, which is just a piece of MDF (medium-density fiberboard) a little bigger than your kitchen oven---about an inch bigger all the way around. The extra inch will be useful for taping down a sheet holding the frame gasket.
Measure the inside dimensions of your oven, side to side and front to back, and add two inches to each measurement. That's how big your platen board should be. (A little bigger isn't a problem, unless it makes it hard to fit your board and yourself near your oven in your kitchen.)
The easy way to make a board that size is to buy the next larger size sheet of MDF at your local home improvement store, and have them cut it to size for you. They usually make the first two cuts for free, if you buy the board, so it shouldn't cost anything and will save you a little hassle. (They don't do "precision cuts," but we don't need precision for this.)
Now make a hole 1 1/2" in diameter in the middle of the board. I used a hole saw attachment on my drill to make a neat hole, but it doesn't actually matter. If you have a drill and a portable jigsaw, you can drill a starter hole with the drill to get the jigsaw blade through the board, and cut out the 1 1/2" hole with the jigsaw.
(Don't make the common mistake of making the hole the same size as the diameter of your pipe. That creates an air flow bottleneck right around the hole, where the air must squeeze under the mold to get into the hole. You want a hole with a larger circumference than your pipe.)
Rather than cutting out a 1 1/2" hole, you could just drill a bunch of 1/4" holes in the middle of the board. (If you don't have a jigsaw or a hole saw, this is the way to go.) Start with a circle 1 1/4" in diameter, and drill 1/4" holes spaced about 1/4" or so apart, i.e., with centers about every half-inch around the circle. (Again, make pilot holes so your holes don't wander into each other.) Then drill a few more holes scattered in the middle of that. (You need 10 or 12 quarter-inch holes to avoid creating a bottleneck, with 7 or 8 in the outer circle; for a platen bigger than kitchen oven-sized, you'd need more.)
The platen shown is actually 1/2" MDF, because that's what I had around. 3/4" MDF is nicer; it's more than three times as rigid.
Step 4: Attach the Floor Flange to the Platen Board
Now you need to attach the floor flange to the bottom of the board, over the hole. Center the flange over the hole, and mark the spots where the screw holes are with a pencil.
Drill pilot holes into the board at the centers of those circles. (The pilot holes should be about the diameter of your screws' shafts, not including the threads, or a shade smaller.) Drill most of the way through the board, but not all the way.
Clean the surface of the flange where it meets the board, using soapy water and then non-soapy water. Galvanized fittings often have a thin protective layer of oil on them, which can keep sealants and glues from bonding well to them.
While you're at it, dampen the area around the hole just a little, maybe pressing a damp paper towel to it. (This will make silicone set up faster. You don't want it really wet, which may make the MDF swell; you just want a touch of moisture.)
Put a bead of silicone caulk or silicone sealant around where the flange meets the board, going on both sides of each screw hole.
Press the flange to the board, and screw it down. Wipe around the edge of the flange with a paper towel or something to remove excess silicone.
(You can use epoxy instead of silicone, if you're in a hurry, or if that's what you have handy. In that case, don't dampen the MDF, but do make sure the flange isn't oily.)
Step 5: Make 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.)
Step 6: Clamping Plastic
This step isn't really part of making the vacuum former, but you'll need to know how to clamp the plastic, so here goes:
Orient both frames so that the useless groove is on top, so that the side with the wider flat area is on the bottom. You may want to mark each frame on the top, so you don't forget. (Maybe "BOTTOM FRAME, THIS SIDE UP" and "TOP FRAME, THIS SIDE UP.")
Take a sheet cut to your frame size, put it between your two frames, and clamp the frames together around it with binder clips.
For most thin plastics, and sheets up to about 12 x 18 inches, 6 binder clips are sufficient---one at the middle of each shorter ends, and two spaced about 1/3 and 2/3 of the way along each of the the long sides. (For thick plastics, larger sheets, or difficult-to-form plastics like acrylic or polycarbonate, you may need more clamps. A clamp every few inches is generally enough.)
Because our clamps go on the outside of the frames, they can interfere with the seal between the bottom frame and the gasket if we're not careful. If they don't sit flat against the bottom, there can be a gap that the gasket doesn't seal, and if they're too far out, the rolled edge of the binder clip can hit the gasket, which may cause an air leak. (And if you do it a lot, you'll tear up your gasket.)
Look at the bottom of each clamp. Make sure the rolled edge sticks inward around the plastic about 1/4" past the frames, and the bottom is flat against the bottom of the bottom frame.
(Unless you plastic is just the right thickness, the clamps won't be quite flat on both top and the bottom; they'll usually taper in a bit, or for thick plastic they'll flare out. Just make sure they're flat against the bottom, and let the top do whatever it wants.)
Now remove the bent-wire handles from the binder clips, at least on the bottom, so that the handles won't get in the way of making a seal. (Just squeeze the handle so that the ends come inward away from the rolled edge of the binder clip at the ends, and pull it away.) Put those aside in a pile.
If you don't have a tight fit in your oven, you can leave the top handles on, flipped back and outward. If you do have a tight fit, remove them too, so they won't stick outward past the frames and be a problem.
Step 7: Make a Removable Weatherstrip Gasket
Now make the gasket for your chosen frame size. You want it easily removable, so that you can use different-sized plastic and frames for different projects, so don't just stick the weatherstrip right down on the platen.
Instead, use a thin sheet of flexible material, such as 1/32" plastic, maybe the same stuff that you'll be vacuum forming. (You might think it would stick, but it won't.) One of those cheap flexible "garage sale" signs from a hardware store will do fine, too.
You'll put the gasket on this "tape-down sheet," and tape the sheet to the platen with masking tape or painter's tape over the edges. (You might think this would make a lousy seal, but it works fine; vacuum sucks the tape in so that it seals better. Positive pressure would blow it right off.)
Cut the plastic 1/2" or 1" bigger all around than your frame size---only a half inch if it's almost the size of your board (or the plastic sheet you're cutting it from), but an inch otherwise. An inch is nice, but a half inch will do fine. You don't want it way oversized, because that just makes a bigger edge you have to seal with tape.
Now cut a hole in the middle so that your tape-down sheet won't block the vacuum hole in the middle of the platen.
(If you're applying this technique to a many-hole platen, which you can also do, the tape-down sheet should be big enough to cover all the holes, and the hole in the middle should be big enough to expose most of the holes inside the gasket.)
Mark the rectangle where your frame will meet your gasket, both the inside and the outside edges. That's where you want the gasket.
I like to make mitered corners in my gaskets, cutting a 90-degree vee out of the material at the corners, but not quite all the way through---leave about 1/8" of foam at the outside edge, rather than cutting all the way through it. Other people cut theirs square.
The weatherstrip is flexible and stretchable, which can make it difficult to apply neatly, evenly, and in a straight line, if you peel the backing tape off of it too soon. So don't. Peel the backing paper off a few inches at a time, and carefully smooth it down without stretching it. (There should be some slack.)
That's especially important at the corners. Don't cut your pieces too short, or cut your vee-notches too soon. Lay the stuff down almost to the corner, and then cut it a shade too long, maybe 1/16", rather than a little too short---lay it down slightly scrunched at the corners, so that the foam presses against the joint and holds it closed, rather than being stretched and having the joint gape open.
(Don't obsess about this, either, though---if you get it wrong, you can fill the gap with silicone, or re-do it, and it will be fine.)
If you're in a hurry, don't bother to seal the corner joints. You can go back and silicone them later, when you won't be using the thing overnight. (Or use rubber cement, which sets up quickly.)
You probably don't want to actually tape the sheet to the platen at this point; wait until you've adapted your vacuum hose. If you do tape it down, be careful not to damage the gasket when you're fiddling around with the bottom side of the board.
Step 8: Adapt Your Vacuum Cleaner Hose to the Nipple
This is the only part I can't give you precise instructions for, because vacuum cleaner hose diameters vary somewhat by brand.
You should probably take your vacuum cleaner hose to the hardware store, and find some plumbing fitting that adapts 3/4" pipe threads to something that fits your vacuum hose (inside or outside) reasonably closely. Then you'll have less shimming or packing or taping to do.
(I got lucky. My Shark vacuum's hose fits perfectly and snugly inside a fitting meant for 1" unthreaded pipe. So for 94 cents, I got a perfect adapter---a PVC pipe elbow with a 3/4" threads on one end and a 1" socket on the other. It took a few minutes to find it in the plumbing department at Lowe's, and about 30 seconds to install.)
If you're using a shop vac with a standard large (2 1/2") diameter hose and an adapter to the standard small (1 1/4") diameter, the adapter will likely fit right over 3/4" pipe nipple, with the pipe threads going inside the (unthreaded) adapter. (That's not how it's designed to work, but it works.) You can just screw the adapter over the nipple, with epoxy on the threads and around the outside where they meet, to permanently connect them and seal the joint. (If you don't want to commit to permanently joining them, you can pack the threads with tacky putty, screw them together, smear a little tacky putty around the outside of the joint, and wrap duct tape around the whole mess.)
For most household vacuum cleaner hoses, the hose will fit loosely over the nipple, and you need to shim it out a little so that it fits snugly. A few wraps of duct tape may fill the gap. If the gap is very large, you may want to wrap some craft foam or leftover weatherstrip around it first.
If you have an extra vacuum hose attachment you don't mind sacrificing, you may want to cut it down and epoxy the base of it to the pipe nipple, to make a nice special-purpose adapter. If the gap is small, say 1/16" you can just fill it with epoxy. If it's larger, you may want to make a shim out of some appropriate-thickness piece of flexible plastic. (Or a piece of similar-diameter tubing, slit and wrapped around the nipple, epoxied in place and smoothed over at the seam.)
In figuring out how to kludge this together, keep in mind that
(1) if you're only using a vacuum cleaner for suction, you don't need an absolutely perfect seal---a little seepage is not a problem for a vacuum cleaner to keep up with
(2) if you want a very good seal, you can use something soft like tacky putty or even modeling clay, applied to the OUTSIDE of a joint that seeps a little, and it will make a great seal. A vacuum leak sucks the squishy stuff inward, sealing the joint better, rather than blowing it off. (Tacky putty is nicer than modeling clay because it's less greasy and easier to get off, so it's less of a problem if you decide to do something better later. Either will make a great seal. Rubber cement can be a good sealant too, and it's not as hard to peel off as silicone, much less epoxy, if you want to re-do the joint later.)
(3) Silicone seals very well, but thick layers may not cure in reasonable time. (Moisture in the air is what makes it cure, and moisture takes a long time to diffuse through thick silicone.) Don't fill large gaps with it unless you want to wait days for it to set, and don't expect even thin silicone sandwiched between impermeable materials to set up quickly.
(4) Rubber cement doesn't fill gaps as well as silicone, because it shrinks as it dries; it's fine for a sealing layer around the outside of a joint, though.
(5) Duct tape alone often works fine, temporarily. It works best if the two surfaces at the joint are flush, so that you don't get any wrinkles across the joint. You may want to shim the smaller-diameter surface on the outside, to bring it up to the same diameter as the larger one. (A bunch of smooth wraps of duct tape on the narrower piece may work, followed by a smooth wrap of duct tape around the joint. Then you can detach the parts by unwrapping that final wrap, and put another final wrap across the joint you reattach the pieces.)
(6) If you're making a permanent adapter, and the threads on both ends of the nipple are not helping, you can use a longer nipple and cut the threaded part off one end with a hacksaw. (PVC cuts easily with a hacksaw.) Then you won't have to pack the threads to keep them from creating a leak.
Step 9: Tape the Gasket to the Platen
Once your silicone or glue has set up, center the tape-down sheet holding the gasket on your board, and carefully tape it down with masking tape or painter's tape. 3/4" masking tape will do fine, but 1" wide painter's tape is nicer.
Apply the tape carefully to each edge, unrolling it and smoothing it on halfway on the tape-down sheet, and halfway off (on the platen). If you get a wrinkle, lift the tape up back to the wrinkle, pull it taut, and smooth it down again.
Step 10: Screw the Nipple to the Platen Flange
Wrap several layers of PTFE tape (teflon tape) around the threads of the pipe nipple, where it will screw into the floor flange. You need several layers, because the tape is very thin, and you want to build up enough thickness to fill any mis-fits in the threads and make a good seal.
Now screw the pipe nipple into the flange, hand tight.
Step 11: Tips on Using Your Vacuum Former
I won't give detailed instructions for vacuum forming here---that's for another instructable I'm working on.
For more information on vacuum formers and vacuum forming, check out my web site, http://www.vacuumformerplans.com (It has links to several other good sources of information.)
I will give a few tips that are somewhat specific to this design, though:
(1) Pick a size of plastic that's appropriate for what you're making. You need some "extra" plastic between your mold and the inside edge of the gasket. A good general rule of thumb is that if your mold has steep sides, the extra area should be as wide as the mold is tall. If the sides are more rounded or gently sloping, you don't need as much. If you have too much extra plastic around the mold, you may get webbing. (Wrinkles caused by the plastic stretching too much and not being able to suck inward onto the mold without folding over on itself.) If you have about the right size sheet of plastic, and you're still getting webbing, there are other fixes; go to my site and click the link for the webbing article.
(2) Be careful about the binder clips; remember to make them flat against the bottom of the bottom frame, with the rolled edge a bit inward from the frame so that you don't tear up your gasket.
You will sometimes bring the frame down a little out of alignment, and dent the inner part of gasket with the rolled edges of binder clips. That's no big deal.
You can add guide rails to prevent this, and bring the plastic down straight every time. All you need is three strips of something reasonably stiff, sticking up just outside the gasket. If you put two along one edge and one along an adjacent edge, that defines a corner that you can nestle the frame into just before bringing it down, and press it lightly into the "corner" as you lower it. L-shaped guide rails can be clamped to the platen anywhere you want them, for different sizes of plastic.
There are designs out there for systems with their own ovens, which have a frame that pivots to raise the plastic off the oven and bring it down onto the platen. For a small former, that's not necessary. L-shaped clamp-on guides work fine, are adjustable, and let you bring the plastic straight down rather than swinging it in an arc.
(3) Put your mold up on spacers such as washers or coins, about the thickness of pennies, to ensure that there's room for air to flow under the mold and to the platen hole.
You can also use a piece of aluminum window screen under the mold, to keep the mold from sitting quite flat against the platen. For plastic up to about 12 x 16 inches, I often use a piece of window screen folded once each way, to make four layers, and no other spacers. This makes one thick, porous "spacer" under the whole mold, which air can flow right through. In effect, it makes thousands of platen holes, including hundreds right around the edge of the mold, where they count most.
(Even if you have a many-hole platen, one layer of window screen is a good idea. It keeps the mold from blocking the holes it's sitting on, and keep the plastic from sucking quite flat to the platen and blocking the holes right around the mold.)
For larger molds, you may need taller spacers, about the thickness of nickels. Tall molds trap more air under the "tent" of plastic that you create when you stretch the plastic under the mold, and taller spacers allow more air can to flow under the mold in the crucial first second. If the gap between your mold and the platen is too big, though, the plastic may suck in around the edge of your mold and need to be cut off. A combination of windowscreen and penny-sized spacers usually works well.
Butane Flame made it!