Electricians! Craftspeople! Help me build a box that gets hot!

March 12, 2009 8:53 AM

I'm looking for any suggestions as I'm trying to make a new, improved version of a box my professor built by hand 30 years ago, which has two stainless steel surfaces which get hot. His box is bigger than I need and not as spiffy as I'd like.

I want to make a box. The box should be approximately 14 x 20 inches; the depth is as small as is safely possible (presumably no less than 4 inches, but the smaller the better). The box is made with a wooden frame, and stainless steel sheeting on the two large sides, attached presumably by screws. The box will stand upright on its smallest surface. Inside the box is (are) something to heat up the air inside of the box, and the stainless steel surfaces, so the outside surface can reach approximately 50 degrees Celsius/120 degrees Fahrenheit (less than 40/100 would make the box next to useless, and more than 60/140 is too hot). The surface should reach this temperature within 30 minutes of being plugged in/turned on, more preferably 10 or 15. The stainless steel surface should be as evenly heated as possible, with no/limited hot or cool spots. On the side of the box is a dial/dimmer which allows the temperature of the surfaces to be controlled. The box plugs into a regular outlet. The box does not explode or burn or hurt the people who are using it or near it. The box can be made by someone with very basic but not special electrical knowledge from parts available in-store or online. The cost of building the box (not including the stainless steel) does not exceed $100, or at a stretch, $200.
- What are the things (thing) inside the box that produce the heat? Are they light bulbs? Heating elements? Ceramic "bulbs"? Aquarium heaters?
- How is the temperature controlled? I'm gong to have to put a thermometer in there, right?
- What keeps the heat even across the surface of the box? Insulation? the nature of the thing producing the heat? some other material placed on the inside of the stainless steel sheets?
-What keeps the box from blowing up or burning?
In case it matters, the box will be used for drying sheets of handmade paper.
Any thoughts/suggestions will be very much appreciated!!

posted by segatakai to sports, hobbies, & recreation (17 comments total)

To be clear - the box is closed; you cannot see what's in it?

It has two large surfaces that radiate heat in opposite directions.

Essentially, if I hold my hand up, palm out to you as it to indicate STOP, my palm and the back of my hand would be the two sides of this box that put out heat?

Is there some reason the faces need to be stainless steel? Is there a reason this needs to be one device that puts out heat on two sides? Would two devices work if they were thin enough?
posted by phearlez at 8:58 AM on March 12

Assuming all of the above it accurate, here's what I'd use as my basis:

http://www.amazon.com/Presto-07039-Professional-22-Inch-Electric/dp/B0000Z6JMS

I don't believe the handles and things have any essential electronics, so they can probably be cut off. That leaves you a surface that's probably under an inch wide.

Put two back to back (well, bottom to bottom) and you could adhere them with some heat-resistant epoxy or even just a couple of small c-clamps.

From there your only problem is that they get way hotter than you want, so you need some sort of gadget to cycle them on and off to keep them at the temp you want. There's probably off-the-shelf solutions for this, but if I (as a software weenie) had to make something like this tonight I'd use an Arduino, temp sensor, and a solid-state relay.

A smarter person could probably create something more finely tuneable that actually regulates the input voltage so the tempperature shifts are more subtle, but I am not that person.
posted by phearlez at 9:07 AM on March 12

I have recently built a couple of similar boxes for doing encaustic monoprints. I use 4 100w light lightbulbs and a dimmer switch to control them. You can buy an inexpensive thermometer from R&F Handmade paints for $11. Stainless steel is not a good heat conductor - you'd be better off with either galvanized steel (which is what I used) or anodized aluminum. Aluminum is a better conductor of heat but costs about twice what stainless does. I used scrap wood to build my boxes and insulated the wooden sides, using metal duct tape (this is not the stuff everyone makes stuff out of - it's an adhesive backed aluminum foil) to protect wiring from getting hot. My boxes are 18 x 24" and I run them at around 180 degrees - using lower powered light bulbs or setting the dimmer lower would work fine.
posted by leslies at 9:27 AM on March 12

Check out this $33 stainless steel warming tray:

• 300-watt warming-tray server for use at any special event
• 3 variable heat controls ranging from 130 to 185 degrees F
• 2 cool-touch handles offer safe transport; 4 non-stick feet
• Stainless-steel base can be used as stand-alone warming tray
• Measures 26 by 14-2/5 by 2-2/3 inches; 90-day warranty

Also available at Amazon for $45.
posted by jamjam at 9:40 AM on March 12

I don't really understand why you apparently don't know how your professor's box works (worked?) or how it's built. It seems like your specs are pretty specific and restrictive - is there some flexibility? Knowing what it's for...

You know those newfangled ranges that are basically an electric burner under glass? How about two cheap hot plates (or a double), covered with a window pane? I think the glass would transfer the heat more evenly than stainless steel, and the hot plates would have dials for heat adjustment.

Alternately I might try suspending some screen (even just an old window screen) above the hot plates - maybe with the glass in place as well.
posted by attercoppe at 9:40 AM on March 12

to follow up...
yes the box is closed and has 2 surfaces that radiate heat in opposite directions. In professional situations, where the box is a welded container, heated internally by steam, I believe stainless steel is standard because it doesn't transmit (much) trace metal to the paper being dried on it. I know that steel is a problem, but I have also seen brass used. My understanding of metals is insufficient to say whether aluminum would be appropriate.
The griddle is an interesting solution. Ultimately it doesn't need to be one unit, or necessarily enclosed. The problem with the griddle is the surface. First, there is a raised lip around the edge which would get in the way (though, this is perhaps negligible). More importantly, the surface of the griddle is presumably teflon coated, which would allow for poor adhesion of the paper during drying. I also think the 'pebbly-ness' of the surface would be a problem in terms of adhesion -- it needs to be quite smooth, like a stainless steel countertop.
Also, I just noticed I edited out that the original box has five 250 watt heat lamps inside. leslies, how long does it take for your box to get to 180 degrees with your 100w bulbs? That certainly sounds like a simple solution.
Thanks for your thoughts so far; please keep them coming!
posted by segatakai at 9:55 AM on March 12

Throw us a bone here... what is it used for?
posted by Liver at 10:04 AM on March 12

ugh. ok, to follow up again. My professor's box is heatlamps in a box. the main problem with my doing exactly the same thing is that I need/want something more portable/smaller/slimmer. The size of the heat lamps necessitates the depth of the box being about 10 inches. Any slimmer and they cause significant hotspots on the surface (actually they do even at that size). I was thinking that maybe there was something that had a slimmer profile that would give similar heat.
attercoppe, glass is actually TOO smooth, and I would worry about it breaking. Also, the unit needs to stand upright.
jamjam's first link looks VERY promising! thanks!!
Liver, if you look at the second last line in the OQ, you will see that the box is used for drying sheets of handmade paper.
Thank you!!
posted by segatakai at 10:11 AM on March 12

I would use a couple* of light bulbs and a thermostat. All electrical and structural components to make it are thus available off-the-shelf at your local hardware store, probably everything for under $30.

*or lots of small ones if you want to make it thinner

If you use bulbs instead of heating elements, paint the inside of the steel black, and paste some tin-foil on the inside of the wood frame, so that more of the heat goes to the steel, which, being thermally conductive, will distribute the heat evenly.

The disadvantage of bulbs is that while they make the project simple, they'll eventually burn out. Heating elements (or making you own with resistance wire if you feel technically comfortable doing so) are a more robust solution.
posted by -harlequin- at 10:18 AM on March 12

What about using flexible silicone rubber heaters such as those shown here? A little bit pricey perhaps, but you wouldn't have to worry about making the box thin nor about evenness of heat. Perhaps controlled by a thermostat with an external sensor attached to the underside of the heated surface near the edge?
posted by FishBike at 10:33 AM on March 12

It takes my box 10-15 minutes to heat up enough. I don't actually attach the metal to the frame - I use it lying down with one surface but I attached a wooden lip to the box and just clamp the lid on with binder clips so it's easy to change the bulbs. That should work on a vertical box as well. In terms of metal I find that the galvanized steel doesn't react with my pigments/monoprints so it should be ok with paper as well and will transmit heat better than stainless.
posted by leslies at 10:53 AM on March 12

Putting a slow fan inside will keep the surface temperatures more even than using several small heat sources, like light bulbs, with no fans. A hairdryer with a cobbled-up thermostat sounds about right.
posted by TruncatedTiller at 10:54 AM on March 12

I hadn't realized that things were coming in physical contact with the metal sheets.

Normal glass would be too slick but what about frosted or etched glass? Use something suitably thick and you will be no more likely to break it than you are to seriously dent your existing setup. It also distributes heat well, reducing your hotspots.

The best thing about it is that you could make it by sandwiching a a back and forth pattern with an uninsulated copper wire between two pieces of glass. Run a current through the wire to heat it up.

Either use a glass bit to put holes in the four corners and clamp them together that way (which you can use also to put in some sort of hanging material) or clamp the edges with C-shaped materials. Or google "high temperature adhesives" - should be no trouble finding stuff that will hold up to under 150F temps.
posted by phearlez at 10:59 AM on March 12

You can also sandblast glass to add texture, and can use finer or coarser blasting material to alter the texture.
posted by phearlez at 12:45 PM on March 12

My father made a box like this using scrap wood, a thermostat, a fan and several 100 watt lightbulbs; the box ensured that his bread dough could rise at optimal temperatures. He used flashing for the sides and a pizza stone for the base. Would ceramic tile work for you? Could you hang the paper sheets from wires or string that criss-crossed the top... that way they wouldn't touch the metal? If you used a pair of magnets, with a paperclip "hook" superglued to one of them then there wouldn't be any imprint on the paper.
posted by carmicha at 1:40 PM on March 12

Note that the griddle objections (lip, teflon coating) you raised could be overcome with a five minute date with a milling machine. If you want even heating you could in theory make a big spiral of thin nichrome (or stainless) wire and use it as a heating element right up against the surface of the stainless sheets (you'd need a thin, heat-safe electric insulator like Kapton between the wire and the sheet, of course).

Also, precise temperature control on a device with a 15-30 minute settling time is going to be a pain in the butt; to do it really well you'd probably have to gather experimental data to be able to predict the behavior of the actual as-built box.
posted by range at 5:58 PM on March 12

Thanks for all your suggestions! It never occurred to me that there would be a ready-made object available for less than $50 that would do what I want and be slimmer than any box I could build, AND be already made of the material I desire. I'm going to try jamjam's link (I've ordered it and will test it), but if that doesn't work as well as I think it will, I'll go back to the box, using suggestions made here. Thanks again!
posted by segatakai at 7:45 AM on March 19