Propellant and Composite Post Cure Oven


This describes some recent history and work on my Propellant and Composite Post Cure Oven.

I originally built this to cure PBAN Composite Propellants as they require a 140F cure temperature to cure in several days (vs months at room termperature). As I got into making epoxy composite structures I started using it also for both accelerated cures as well as post cures of these structures. It's very handy to have an epoxy lamination, made with 24 hr slow cure epoxy, ready to handle if not fly in several hours. I've also built a Airframe curing and post cure oven.

Current Construction

Propellant and Composite Post Cure Oven

This photo shows the current construction of the oven. All walls and the door are about 2.75" thick - essentially available wood originally skinning 2" thick Corning pink styrene foam insulation board.

The inside dimensions are about 12..5" wide by about 28" deep by about 24" tall. The width and depth is designed to accomodate Jelly Roll baking pans which make very handy trays. The extra depth to accomodate the heat source in the back, and the height as tall as possible to fit under a workbench.

The window is triple pane and I find worth the effort to install (whether or not triple pane, it's triple as I had the material handy).

Most folks seem to make curing ovens by taping the foam board together and working out an arrangement of lights. I went the extra two miles as I'm largely comitted to PBAN for my composite propellants. Ok, I just can't help myself. One nice advantage is that this oven will quickly get to temperature with as little as a 150-175W bulb even in a cold garage. That means faster start up times and also faster recovery times for when the oven is accessed. It also provides a very gradual ramp down, which can be useful for proper post cure ramp downs. For instance, taking several hours to drop from 140F down to 100F.

Inside view with propellant curing rack and heater

Here's an inside view. You can see my propellant curing rack (for 54mm and 38mm grains), and an interim heat source.

This is Version II with the new lining. I had a couple of temperature excursions. They caused the pink foam to overexpand and pop the original luan plywood liner and distort the walls. I wish I had pictures of that. It also warped the inside window pane as you can see. I used it this way until I had a break to rebuild. I have replaced the inner window panel with polycarbonate which is heat tolerant in this range. It used to be plexiglass as  the middle and outer panes remain.

I gutted the box to replace the insulation. It turned out to be a push vs starting from scratch as I'd used lots of construction adhesive and it was difficult to gut without destroying it. But I perservered.

To avoid overexpansion of the foam I am ensuring I don't have over temp excursions, and I also used R-15 fiberglas insulation, which will not expand. I then also re-lined it with Hardi-Backer cement board instead of plywood. This provides for much greater tolerance for proximity to heat sources. The black spots are a fireplace pointing compound I used to spackle the recessed screw holes.

I must note that Hardi-Backer board is a Royal PIA to work with. Cutting it is an inhalation hazard and it will quickly make any blade ignorant. Scoring and snapping is best, but if you guess wrong and need to file to fit, well, you'll have some biceps when you're done. Next time? I dunno.

It IS very heat resistant. With plywood I had an elaborate arrangement of aluminum heat shields around my light bulb heat sources. Hardi Backer  does also seemto be hygroscopic as I haven't had a run when it's not quite steamy when opened. I may try to seal it with some sort of high temp epoxy or furnace sealer. Next time? I dunno. Maybe back to luan with a waterglass fire retardant coating?

Thermostat detail inside oven

In version one I had a fancy electronic temperature controller. After a couple temperature excursions I am re-designing. Right now I have mounted a water heater thermostat into the upper corner. Depending on the model they have a working range in the 100F - 190F range. This one 90F to 175F. That's a decent range for most work. PBAN cures at 140F usually. AeroPoxy post cures at 170-180F.

The only disadvantage of these is the hysteresis. It's about 18F. So if it's set to 150F, it won't reset and come back on until it's dropped to 142F. It's workable, but I prefer the tighter range of electronic controls, where I usually ran within a couple degrees and could have had it pegged within 1F if I didn't want to be nice to my bulb.

No matter what controls you use, I very strongly recommend a mechanical high current sturdy high temp limit switch even if using electronic controls for your fine temp control. Right now, the water heater thermostat suffices for both, but when I add the electronics back in I will either leave this one in, or put in a CanTherm snap action thermostat at perhaps 190F as an absolute over temp backup.

Do note that I very highly recommend (1) your overtemp limit be positioned at the top of the oven, and (2) that your temperature source be positioned near the bottom of the oven. Even in this small oven I found the temperature difference as high as 40F between the top and bottom, and in my overtemp occurances the foam at the top had over expanded and nearly melted back while that at the bottom was undamaged. So having your measurement at the top ensures no part of the oven reaches over temp, and having your temperature source at the bottom encourages at least some convection circulation.


I think I rattled off most of the history of this thing already. But I'll trim the content elsewhere and move the history more down here.

Insulation & Lining

Pink Foam

R-Matte Plus 3

Fiberglass Insulation R-15. (peel off the flamable tar paper layer)

Hardi-Backer board

Temperature Regulation,  Heat Sources, and Fans.

water heater thermostats

These are water heater thermostats I salvaged from my 6 year old heater that ruptured it's tank. That problem was the Sacrificial Anode and if you don't know what that is you should learn and check yours and save yourself a lot of money on unnecessary repairs.

The left unit will allow either itself or the right unit to be operational, but not both. We don't care about that mode. They're otherwise identical. The left unit also has a 190F overtemp safety limit on top, shown as the red button. That limit is a handy temp limit, but note that overtemp limit doesn't automatically reset. You may or may not care. If it does overtemp, it may well be best to have it shut down and stay that way.

If you can't scavenge some you can pick them up new very inexpentsively.

I will elaborate more on electronic controls in a bit.

1500 watt space heater

Here's a 1500 watt space heater. I really only need 175-200 watts to run this oven, but 1500W sure gets it toasty quick, and does seem to help if I want to push 175-180F for epoxy post cures. The Hardi-Backer board seems to be able to take proximity to this heater.

Usually, however, I run either a single 200W or 2x 200W bulbs. I will illustrate that. I also recommend a fan to stir the air - otherwise you will  have uneven temperature distribution. This 1500 w heater has a built in fan. I use an  external fan with the light bulbs.

For Fans, you have to be aware that most weren't designed for these temperatures. Actually, most heaters weren't either.

You will also notice in the top picture the ground fault interrupter. In the first excursion it tripped my GFCI in the garage. I decided I didn't want to miss out on that bit of safety no matter what it was plugged into so I've included one in the cabinet itself for an extra measure of safety.

Aside from electronic temperature controllers, you may also consider more manual regulation. There are some easy choices. A light dimmer works great for lights and lower powered heaters. Most will actually handle upwards of 600W which is a lot of heat, though not enough for most heaters proper. For higher wattage a router speed controller is an excellent choice and a 1500W resistive load is a cake walk for them.

router speed controller

Here's my router speed controller. It also works well for controlling the heat in a KNDX/KNSB Candy Propellant kettle. Do note that the fans in some heaters will not like the speed controllers and they usually won't spin. My ceramic heater shown above has a computer style fan inside that won't work with this controller, much to my dismay.