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Step 11Tips 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.
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.
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http://www.widgetworksunlimited.com/Styrene_Thermoform_Plastic_s/51.htm
but they seem a little on the pricey side. I'm still searching for a wholesaler. Anybody got any leads?
(can you give us the oven temp and time in the oven as well)
The most common plastics for beginners, and also common for non-beginners are High Impact Polystyrene (a.k.a. HIPS, or High Impact Styrene, or HIS) and PETG. HIPS is usually opaque, and PETG is crystal clear. They're both pretty cheap and pretty good, and easy to form.
I'd recommend starting with plastic between 1/32 and 1/16 inch thick. (Or .030" to .060" or so.) Thinner plastic cools so quickly you may have trouble getting it to the platen and formed before it cools. Thicker plastic is harder to form without high vacuum, and is more expensive.
The most economical place to buy it is usually a local sheet plastic supplier. You can buy a 4 x 8 foot sheet of thin plastic for what a few small sheets would cost at a hobby shop or craft store. You can probably roll it up to get it into your car. (While it's still flat, thin plastic is flexible. Forming it into a 3D shape makes it much more rigid.) Look in the phone book and call around, to find a supplier that has no minimum order, or a low one like $25. Some places don't want to talk to you if you're not buying at least $100 worth of stuff. Others are happy to sell you a sheet or two of thin plastic. Unfortunately you generally have to call around to find out which is which.
Online, you can get sheet plastics from places like TAP Plastics and Professional Plastics.
I often vacuum form textured plastic light diffuser ceiling panels from Home Depot or Lowe's, which come in 2 x 4 foot sheets. The styrene ones are easier to form than the acrylic ones, but harder to find, and more fragile.
I also vacuum form craft foam from craft stores. (Hobby Lobby has good prices on craft foam---a 12-pack of 12 x 18 inch sheets 2mm thick is $4.00. ) That's a little different from most plastics, though, because it doesn't sag when it gets hot enough to form.
You can find links about vacuum forming diffuser panels and craft foam on my web site. ( http://www.vacuumformerplans.com )
Acrylics can be a problem because they absorb moisture from the air, and if you heat them, you get little bubbles. In my experience with diffuser panels, the bubbles have been small and don't show much. (They don't show at all for the translucent white stuff, just for the clear.) To avoid the bubbles, you may need to "pre-dry" acrylics, baking them at 200 degrees for a half hour to a couple of hours, depending on the thickness. That drives off the water.
Two other commonly vacuum formed plastics are ABS and polycarbonates (like Lexan (TM)). They both can absorb moisture from the air and require pre-drying, too.
I'd stick with HIPS or PETG, starting out, to avoid having to deal with that. (Craft foam doesn't have a problem with moisture.)
You can get a piece of PETG about 6" x 13" (IIRC) by splitting cutting the ends off a 3-liter soda bottle, splitting it, and unrolling it. A lot of RC plane "bubble canopies" are made of pop bottle PETG.
I'm working on another Instructable, with detailed instructions for vacuum forming and plastics sources, but that probably won't be done for a few weeks.
The thread is about moldmaking for vacuum forming, including casting several vent holes into the buck (male vacuum forming mold) to let air out of concavities.
The air does flow under the mold to the hole. That is why you need spacers, or several layers of windowscreen.
In the mold-making thread, you'll see a picture of the mold sitting on a one-hole platen, on top of folded-over windowscreen, as described in this instructable. It's not as nice a platen as the one in the instructable, but it's the same general setup.