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[drcrash]

Mattax,

I don't think you need the heating elements very close to the plastic, if the inside of the oven is metal like Harley Guy's.

Infrared crosses space very efficiently, so the trick is to make sure it gets to the plastic before being absorbed by something else. If your oven walls were perfectly reflective, you could make the oven as deep as you wanted and it would make no efficiency difference---all of the IR would bounce around until it eventually hit the plastic.

Aluminum is extremely reflective to IR---over 90%, and over 95% at most frequencies. If you have aluminum walls, the IR could bounce several times off walls before hitting the plastic, and it'd still be very efficient.

(Of course, if you have metal walls you want to make sure the nichrome coils never touch them, or you'll short across segments of nichrome.)

One advantage of a reflective interior is that the oven doesn't need to get as hot, or need as much insulation. If more of the IR bounces around until it hits plastic, less of it heats the oven itself.

The issues are somewhat different for flip-flop designs than for over-and-unders. It seems to me that flip-flops benefit from convection more than over-and-unders; the hot air heated by hot oven walls rises to heat the plastic. Hot air rising is not so good when the plastic is at the bottom of the oven, so it's more advantageous to reflect the IR to heat the plastic directly, rather than having the walls absorb it, convert it to heat, and conduct it to the air. (Unless you're going to circulate that air like a convection oven; natural convection may not be your friend.)

I'd consider using thin metal over mineral wool insulation. Aluminum flashing is pretty cheap, and probably fine if the coils are not very close to it.

I'd also consider having the plastic rise into the oven, like Harley Guy does. (Maybe just use an aluminum flashing skirt if your oven isn't deep enough.) That should reduce convective cooling of the bottom of the plastic, especially around the edges.

(If you wanna go nuts, I consider having a pair of doors under the plastic, with some heating elements in them, too, at least around the edges.)

[drcrash]

Here's a machine with a lift linkage similar to the Proto Form's, but easier to see how it works. (Especially given the clear side view.)

http://www.hobbymolding.com/hm_forum/phpBB2/viewtopic.php?t=319

I think the Proto Form's geometry is a little different, and probably better.

(The bottom pivot on the Proto Form is not lined up directly under the top one; it's a little further back. I'm not sure how that affects the throw and leverage issues. And I think the protoform allows the top pivot to come down past the bottom one, mashing the frame down past the platen like Mattax was talking about.)

[drcrash]

Oops. That last post should have been in the "An Idea for a Drop down Frame..." thread:

http://www.tk560.com/phpBB2/viewtopic.php?t=356&start=0&postdays=0&postorder=asc&highlight=

[jegner]