Help me design an oven for curing foam latex!
December 13, 2016 5:17 PM   Subscribe

I'd like to design a box-style oven that is of dimensions suitable for curing (“baking”) prosthetic appliances made of foam latex, for special effects makeups. I have some tutorials to base my plans on, but if you know about how ovens work, I have some questions about how I can refine the design.

The primary (and most detailed) source I plan to follow is Todd Debreceni’s Foam Oven Tutorial on YouTube. It feels like his is a revision on the tutorial in the December 2009 issue of Stop Motion Magazine, but they both feel a little rough around the edges. I've found a few other builds online, but they're maddeningly vague. There is one book that was written about the subject back in 1994, but it seems to be long out of print, and I have little hope of borrowing it through inter-library loan (though I did submit the request, just in case).

Anyway, if you look at the Debreceni build, it's mostly 2x2" framing with foil-covered foam insulation on the inside, and plywood cladding on the outside. The heat source is a 3x24" strip heater, mounted in the bottom, and there is what seems like a massively overpowered blower and duct system for forcing convection from the top of the box back down to the bottom.

I plan to improve on monitoring and automation by using an Arduino with a temperature probe and relay to control power to the heater. Beyond that, I have these questions about efficiency and safety:
  • How can I calculate what size/wattage heater I need? Is that 24" model overkill?
  • Same with the blower: I read in an article about convection ovens for powder coat curing that the fan needs to be able to turn over the air in the space 3X per minute. In a 4'x3'x2' enclosure (24 cubic feet), that means the fan only needs to be capable of 72 CFM. Is this accurate? If so, would a 100 CFM duct fan do the trick?
  • Do I actually need this sort of external duct system to move the air, or would a fan inside the box do just as well (as in a standard home convection oven)? If so, where should the fan best be located inside the enclosure?
  • The powder coat article also made me think the oven needs an exhaust/vent to release VOCs and ammonia that is being baked out of the latex. How can I calculate how big of a vent I need in the top? Do I need a matching intake opening in the bottom? How does this affect convection?
  • Does the foil-covered foam insulation protect the box enough for the temperatures needed (≤185ºF)?
  • Do I need specially-rated high-heat wiring for the power to the heater? What certification should I be looking for?
Any additional pointers on safety and/or oven design are most welcome — thanks!
posted by mboszko to Media & Arts (7 answers total)
Arduino might be overkill.
posted by Leon at 5:22 PM on December 13, 2016

Response by poster: Leon, the Arduino might be overkill, but I've already got one handy, and I plan to program it to let me do multi-temp runs with timed segments, and notify me when it's done, so I'm okay with that. ;-)
posted by mboszko at 5:30 PM on December 13, 2016

Best answer: How hot do you need the oven to get? Duct fans are only designed for temperatures generated by heating systems (probably 135F max, maybe less) so if are circulating 200F air you risk burning up the motor windings and or bearings. You are probably better off having the motor of your fan unit isolated from your heated space. This is how convection oven fans are setup. Speaking of which if it was me I'd repurpose one or two of those (probably used to save money). A fan and blade from a self cleaning convection oven can handle over a 1000F.

Generally only if the wire is inside the heated area in which case you need wire (actually the insulation) with a temperature rating higher than the air temperature it will be exposed to plus a margin for current induced heating. There are a bunch of calculation to figure this exactly but for a one off DIY project I'd just go big and use something like the wire used for waffle irons. Connections should be made with a crimp on ring connector iif you use that heater.

You need to have enough watts to a) heat the contents to the desired temperature in the desired time and heat any make up air. With enough insulation and zero makeup air even a 60W lightbulb will eventually heat your box to a couple hundred degrees. So you either calculate the amount of heat you need to heat your makeup air (mass of makeup air over time X heat needed to raise that mass from ambient to cure temperature factoring in relative humidity) or as long as the fumes aren't poisonous or flammable I'd handle it the way stoves do with a simple vent pipe in the top to equalize pressure. Reading the GM Foam site it doesn't sound like the offgassing will impede cure so if you can set up your oven either outside or in a very well ventilated location this approach may still work.

If you need make up air you can put another vent in the bottom (possibly with a fan) and just keep adding heaters until your temperatures stabilize. To give you an idea of the upper end something like pipe bending oven with a 4" opening at each end uses 2300 watts to fairly rapidly heat a 4" PVC pipe to too hot to touch.

Having said all that a search around revealed this tutorial from Smooth-On which uses a construction heater to neatly solve heat, ventilation and circulation problems all in one UL listed form using an electric construction heater. You could have you controller regulate a shutter to control temperatures. Or you could hack the heater and vary the power output of the heating coil via PWM. The built in thermostat on say this DeWalt unit wouldn't be an impediment because it measures the temperature coming in not out.
posted by Mitheral at 7:50 PM on December 13, 2016 [1 favorite]

Response by poster: Mitheral, thanks — that's very helpful! Some clarifications and additional questions:
How hot do you need the oven to get?
For the latex, ≤185ºF, but to give myself more flexibility, let’s say up to 212ºF (100ºC) so I can also use it to heat accelerate/cure some resins.
You need to have enough watts to a) heat the contents to the desired temperature in the desired time and heat any make up air.
Is there a better formula I can use to calculate this? I found some back-of-the-envelope calculations on Quora which looks to work out in my ideal, perfectly-insulated, no-vent box, using that strip heater, it would only take about six minutes to heat the box from 15.6ºC to 100ºC:
  • specific heat capacity of air: 1.006 kJ/kgC
  • density of air: 0.036 kg/ft3
  • volume of box: 4'×3'×2' = 72 ft3
  • mass = density × volume = 0.036 × 72 = 2.592 kg
  • temperature differential: 100ºC − 15.6ºC (cool basement temp) = 84.4ºC
  • energy needed = 1.006 × 2.592 × 84.4 = 222.077 kJ
  • energy output: 600W = 600J/s (assuming perfect operation of that strip heater, though I couldn't find any efficiency details on the McMaster-Carr website)
  • heating time: 220,077J ÷ 600J/s = 366 seconds = 6.1 minutes
Of course, my design will not be perfectly efficient, and if I add a vent, it will be even less so. Any way to calculate how much air is expected to vent? I suspect it may be based on a number of factors I don't really know (including fan size, speed, placement), but figured it's worth asking. I guess it only matters in that the heater is able to keep up with offset from any venting?

Is there any conventional wisdom as to the placement of the fan? I could put it in the back wall, as in a household convection oven, but maybe it would do more good at the top, pushing the hot air back down? I don't know anything about air circulation physics, so I'm just guessing here.
Reading the GM Foam site it doesn't sound like the offgassing will impede cure so if you can set up your oven either outside or in a very well ventilated location this approach may still work.
I do plan to put the oven outside while curing. My workshop is not well ventilated at all.
Having said all that a search around revealed this tutorial from Smooth-On[… ]
Is this the one you mean? Thanks! Somehow I missed that one. That seems rather clever, though I wonder if having that open hole in the side of the box will lead to uneven heating areas inside the box, especially when it's cold outside. But maybe that won't matter as much as I think it might.
posted by mboszko at 1:13 PM on December 14, 2016

That's it; must have forgot the link. I'd think because the heat source is pressurizing the oven you wouldn't have to worry much about cold air coming in. If you do just cover up some of the vents until you get a positive pressure inside.

It's been years since I've done any make up air calculations and then for commercial kitchens so I'll have to refresh my memory. Unfortunately I've got a crazy busy few days coming up; I'll try to get back to it next week.

Keep in mind though your object will undoubtedly have a higher mass then the air. If you put anything in the oven it'll take quite a bit longer than 6 minutes to hit 100C. Also the linked foam material might perform better if the temperature rises more slowly so as to minimize the temperature differential fromt he surface to the middle.

mboszko: "Is there any conventional wisdom as to the placement of the fan?"

I don't know but considering how easy it would be to move a fan around I wouldn't sweat it much. Try a few different positions and see what works best would be my approach. Keep in mind if you are doing any hollow object you might want air to be directed inside the hollow.

mboszko: "energy output: 600W = 600J/s (assuming perfect operation of that strip heater, though I couldn't find any efficiency details on the McMaster-Carr website)"

Electric heaters are always 100% efficient.
posted by Mitheral at 10:19 PM on December 14, 2016 [1 favorite]

Response by poster: Amazingly enough, interlibrary loan came through on delivering Foam Latex Ovens : Design, Materials and Construction, so I'm devouring it now. There are loads of calculation formulas for load based on air volume and number of molds, good suggestions for heaters (specialized oven heaters, or finned strip heaters), and where the fan should be (basically — he recommends a high temp, high-CFM blower, and putting the heater right in the output path, which seems like overkill to me, but… eh.).

Also, there's a ton of info on safety and trying to meet regulations for commercial ovens (assuming you'll be using this to make product for sale), which means while the wood-framed boxes I see elsewhere on the internet might work fine (the ignition point for wood being way above 200º), its certainly not going to pass muster if a fire marshal comes to inspect things. A bolted or welded steel frame made of L-bar is recommended, and duct board is much recommended over foil-faced polyurethane foam, as the urethane foams are not rated for the temperatures required without potential off-gassing.

In all, it probably means I can't afford to build the oven I want right now, but I now know what I need to do to plan for the oven I will build in a year or so.
posted by mboszko at 5:23 PM on January 10, 2017

Best answer: You might find it more cost effective to build a sheet metal box within a box insulated by Fiberglas. If you make the door one side you only need four threaded rods at the other end to support the inner liner. Fiberglas insulation is cheap. The whole thing is non combustible; easy to clean, easy to modify and you can attach items directly to the sheet metal (possibly with the aid of threaded insters) anywhere you want.
posted by Mitheral at 12:02 PM on January 11, 2017

« Older Is that a monkey on your back or are you happy to...   |   How do I hire a US lawyer from abroad? Newer »
This thread is closed to new comments.