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In this instructable, a user creates a heated bed from a wooden plank, some nails, insulated copper wire, and a DC switch. He does place an aluminum bed on top of it, but for our heated chamber, we won't use any aluminum on top of these wires. We aren't trying to heat an aluminum bed, but an entire chamber.

enter image description here Assume I place a wire loom like this underneath my 3D printer. (I might place some aluminum foil between the wood and the copper wire, to reflect heat upwards more? Is that safe?)

Then assume I have a cardboard box that has aluminum foil lining the walls on its inside. Assume the total size of my printer is 500mm x 500mm x 500mm. When I place the cardboard box over my printer, do you think it would take less than 1 hour for the top of the cardboard box to reach 60 degrees Celsius?

Do you think there are any other hazards here I am not considering?

Could I, theoretically, do this entirely with cardboard, and no wood?

Could I, theoretically, line the inside of my cardboard box like this? Or might the cardboard ignite?

I'd like to use cardboard because it is both lightweight and insulative. I can remove it easier to check all areas of my printer, unlike an enclosure.

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K Mmmm
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    To be honest this sounds like a fire waiting to happen. Even with the aluminum foil you will be conducting heat down into the structure of the heater. Using a cardboard box as the enclosure is commonly done but also not recommended and requires supervision. The difference being it has no direct (hopefully) contact with the heating element. – Perplexed Dipole Sep 04 '19 at 15:17
  • Ok. I will try this with an Infrared Heat Lamp instead of the extra large heated bed. This is a bad question so I may delete it soon. – K Mmmm Sep 05 '19 at 01:45
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    Using the heated bed to heat up the chamber is not a bad idea, in fact it is the easiest way to do so. What is dangerous about your idea is making a DIY heater out of cardboard and wire. – Perplexed Dipole Sep 05 '19 at 04:28
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    Most certainly mount it in an enclosed box of nonflammable material. Compare, e.g., with the slicone heating mats used in reptile cages. The risks you're taking with DIY wiring is that the wire itself can overheat and burn unless you have really good knowledge of thermistors, control circuitry, etc. – Carl Witthoft Sep 05 '19 at 15:03
  • I was thinking recently that using a hair dryer or two might be a better way to get the air temperature in the chamber up to whatever level you need. – CrossRoads Sep 06 '19 at 04:00
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    Yea, hair dryers use a lot of amps though. They use around 15A at 120V! My fuse is rated at less than 15A... – K Mmmm Sep 12 '19 at 02:40

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Cardboard is fundamentally paper. Paper ignites - just as Ray Bradbury claims - at around 233 °C or 451 °F.

A thin heater wire usually glows red hot under operation. Not dark red, not blood cherry, not dark cherry, it's usually medium cherry to dark orange in operation, as one can easily tell by looking into a trusty toaster oven, which also shows us how powerful of a heating element this is.

In a typical toaster, heater wires are mounted in a metal frame and backed by some non-flammable material that looks like paper but is not. In a 2004 patent for such a method, they use a sheet of mica. But how hot is our wire? Well, under mains power, the wire of a toaster reaches 700-900 °C within seconds, heating the inner chamber of the toaster despite the slots to about 200-250 °C so quick that the bread roasts deliciously... until the heat controlling bimetal strip releases it up.

We operate these wires for at best a few minutes. In an electric forge, pretty much the same wires with a little more thickness to them are run at lemon glow for hours... and they are embedded in firebrick material to prevent them from igniting stuff touching them by accident!

How we know the temperature? Incandescence charts of course: enter image description here

Conclusion

Paper or cardboard backing on heater wires is a bad idea. Mounting the heater wire to a metal frame that is as nonconductive as possible and without a chance that something accidentally touches it is needed. Fiberglass or rock material would serve as a good carrier, an aluminium block with radiators would give a good heating-to-surface mix, especially is precautions are taken to make the wire surface nonconductive.

Even using wood close to such a heater is a gamble I would not want to take. It's also best to mount the heating element into an enclosure that can't touch the wooden outer chamber and that stays put if the enclosure is opened or removed.

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