testing 1 2 313 Aug 2007 I finished the test-oven a couple days ago, using a single halogen linear tube to heat the plastic.
The test oven is a lot smaller than the real oven: 33 by 33cm. The plastic needed to be cut to size. Jim Egner (TK560) of http://www.tk560.com/ demonstrates how to cut plastic in one of his movies. Of course, it's not because he can do it that it works for me.
The idea is to break the plastic in pieces and to use a cutting knife to cut grooves where the plastic should break.
The first one was no problem, but the second time I broke the plastic this happened:
Now I know I should be more careful.
The second problem arose when I tried to clamp the plastic in the wooden frame. The wooden frame is too big and the clamps can barely get around it. So I decided to break out my new drill stand and drill a couple not-too-deep holes where the clamps fit in.
Heating the plastic went pretty fast. The plastic was very close to the lamp and the plastic melted more rapidly in the middle than the outsides.
I have nothing decent to use as a mold, so the mold I used is just some kind of ashtray that was lying around in the basement.
The result: [G2:11891]
I wasn't really happy with this result because the heat didn't spread out well enough to heat all the plastic.
But hey, that's why it's a test right ?
For the second attempt, I quickly cut some 10cm high spacers where the frame could rest on.
For this test, I also reused the plastic from test 1. Because polystyrene is a thermoplast, that should be possible, and it was :)
Again, although the heat was spread out more evenly, I didn't like the result.
I've decided to go with more lamps, and less power per lamp. The final oven will have 27 lamps of 150W each. Each 33x33 square (9 of those in the final oven) will have 3 lamps positioned in such a way that the heat spreads out nicely.
I'm playing with the idea of attaching relays to every lamp so I can switch them on/off separately. When using a smaller sheet of plastic, there is no reason to turn on all the lamps anyway.
Right now, the vacuform oven tests are on hold. My dad called me yesterday and said he has a couple meters of nichrome wire that I can use. If it's enough, I can surely do that.
But, I'm more in favor of the halogen spots setup, because I can easily replace those linear tubes with new ones if they burn out. An extra advantage is that the linear tubes don't expose their wire to plain air, so there is no danger of electrocution. (Of course, for safety's sake, I should put big red flashing "HIGH VOLTAGE" signs around the oven)