Heating Box
The heating box is essentially a big plywood box lined with tin foil. It traps the radiant heat given off by the heaters and funnels it up into the plastic sheet you're trying to soften. There really isn't much to say about it, except that the box has to be large enough to accommodate the heat source, and lined with a reflective material.
Radiant heat is wonderful in that unlike convection (using an oven element, for example), it heats up whatever it shines on without really heating the air in between. When the box is lined with a thin reflective material (in my case tinfoil), the heat bounces off the foil without heating up the wood underneath. I've kept the thing on for 10 minutes once (cooking some very thick white plastic), and the wooden box remained cool to the touch!
Interestingly enough, after keeping it on for 10 minutes, I turned the heaters off, removed the cooked plastic, stuck my hand into the space between the heaters and the floor, and discovered that the air here was only a few degrees above ambient temperature. Why? Because heat rises. Brilliant!
Really, using a radiant heater is by far the best solution for home/garage vacuum forming in my humble opinion. Way better than throwing plastic into the same oven you cook lasagna in. Just be smart, don't turn it on and leave it unattended.
Anyway, here is the box lined with tin foil, which is wrapped around the edges and held in place using electrical tape.
Also, it's worth noting that after this photo was taken, I have elevated the heaters by about 8" (by placing them on top of empty 1 gal paint cans). I've also angled them away from each other so that they are not pointing straight up as seen here. I found that if facing straight up, the plastic heats up quicker in the middle where the arcs of reflection intersect. Angling the heaters a few degrees away from each other heats the plastic more uniformly, which is the desired result.
To see the entire vacuum forming setup in action, check out the video on this page.
* * * * * * *
- Heating Box