As of yesterday, the "foam oven of doom" has had it's first successful trial run!
It ran for a total of 3 hours without catching fire or exploding, or something going horribly horribly wrong.
Overall, I'm quite pleased with its performance.
Now for the tech stuff -- after a lot of testing, I found that with an 1,100 watt hot plate, I needed to set it to 'high' rather than the low or medium settings that I originally intended. Reason being -- the interval in which the heat cycles from on, then off, and on again, is too far apart (approx. 12min on low, 8 on medium) to maintain a constant temp. 'High' on the other hand, has a cycle of about 5 minutes, give or take.
With the plate going at such furious temperatures, I realized that the oven would need to be vented so that the heat would stay at an appropriate 160˚ – 190˚F range.
I ended up cutting a 2 inch hole in the top for heat to escape (fig. 1). And repositioned the fan so that it sucked air in from the outside of the oven (fig. 2).
This arrangement not only facilitated convection, but also kept the oven temperature cycling steadily from a high of 190˚F to a low of 160˚F, and up again.
This means the moulds were averaging 175˚F.
I monitored all this with a cheap digital probe thermometer -- the probe itself was poked through the top of the oven close to the center for a general reading.
Finally, I finished it off with an old oven rack sourced from a junk shop (fig. 3). It didn't fit perfectly at first, so I had my uncle cut it down with a grinder. It was installed with 8 1.75in screws -- 4 on each side -- the rack can now support 20+ pounds with ease.
As you can see, a foam oven wasn't the 'easy weekend project' that I thought it would be. It's a ton of work, and problem solving, and testing.
But if this test is any indication, it could turn out to be quite useful.
Just as an aside: I continually monitored this thing while it was running. And I noticed that the longer it ran, the "gap" in the temperature cycle started to close -- originally 160˚ – 190˚F eventually turned to 170˚ – 192˚F… after 3 hours mind you.
So, on a long enough timeline, I imagine the oven would get hotter than the 'foam-zone' -- obviously, not by a lot, but enough that I might have to make a minor adjustment to the vent system.
Overall, though, it seems to be shockingly consistent for a homemade hotbox.