Cheap low-temperature oven

[ezalys]

I'd like to make an oven for curing adhesives that require an elevated temperature for a proper cure. I'd like for the temperature to be settable from 60 degrees C to 75 C plus or minus 5 degrees over time, and over the volume of the container. I'd like for the volume to be about 6" x 6" x 6" or 15 cm x 15 cm x 15 cm.

Primary objectives are to keep the cost down and to make the oven out of easily available materials from home depot or mcmaster carr or similar. Ideally would prefer to repurpose some home appliance. The ideal for me might be just repurposing a toaster and adding a temperature controller to it, but I worry about a toaster having consistent temperature through its volume. This feeeels like something that someone must have bodged together at some point, tried, experimented with, and therefore has experience with. I'd love to hear from them.

I think the weird part here is the low temperature. Tons of people have done SMD reflow ovens. I'm wondering about how a toaster will function with such a low setpoint. I do, fortunately, have a fan that's good up to 90 celsius, so I could stick that inside to agitate the air inside somewhat. I think my hesitancy to try this mostly is due to having experience with dedicated low temperature ovens that feature a boatload of insulation (take for example https://www.globalindustrial.com/p/medical-lab/laboratory-equipment/le-laboratory-shakers/steel-door-incubator-10-180-270w?infoParam.campaignId=T9F&gclid=Cj0KCQiAg_HhBRDNARIsAGHLV520X3p0496UjQs_pm782ODjZq3ah3Ju1ZMHENPwxVry78agLIucDPUaAgpnEALw_wcB ). That insulation must be there for a reason. I just wonder what, quantitatively, I'm giving up by not having it.

[DaJMasta]

I wouldn't expect a toaster to do that low, but try an egg incubator.  Something for bird/reptile eggs may be able to go that high.


Conversely, you could try hacking a toaster oven's thermal controls to do it, maybe disabling a heating coil or just reducing the current through one for a slower response and finer PID control.

[jmelson]

A toaster will do FINE with an added-on thermostat.  You might also need to provide some kind of power limiting, so the heating elements never get over XX% of full power.  Some thermocouple temperature controllers might be able to handle that.  Now, a big volume of adhesive in a roughly cubic pot is not going to heat evenly.
But, putting a thermocouple on the side of the pot should give good control of the temperature of the OUTSIDE of the pot.

Jon

[ezalys]

I would absolutely add a PID controller to the thing. Sorry, by use a toaster oven, I meant use a toaster oven with some custom controls.

[ConKbot]

Definitely a convection toaster oven for even temperature, plus a PID controller (from Omega engineering if you want it to just work reliably, or ebay if you want it cheaper)  and a SSR for switching the elements. Since you're in 120V land, rewire the heater elements in series so they would be in a nominal 240V configuration, so you'd drop the power to 1/4 of original or just disconnect some if you only need 1/2 power, etc.

If you get a PID controller with an auto tune feature, let it tune itself with a medium-small size chunk of metal inside to let it have some thermal mass to tune to, but not an excessive amount.

Make sure to keep thermal fusing and normal fusing in place.

I use an omega temp controller, +SSR to control a heater for a snake terrarium, and with the output on a 1s period (No need for slower, as there are no mechanical contacts to preserve) it stays within 1degree F all the time. Just had to let it auto-tune for an hour or two before adding stuff to the inside of the tank.

[ejeffrey]

I think 5 degree variation across the volume is probably unrealistic with a toaster oven without at least additional insulation and air circulation.  I also don't think it is really necessary for curing most epoxies.  At least the ones I have worked with have cure time vs. temperature plots on the datasheet and a very broad acceptable temperature range for an ideal cure.  So curing at 50C takes a lot longer than at 80C but produces the same result.  Just cure for the longer time and it should be fine -- unless your workpiece is sensitive to the longer cure time nothing bad should happen.  So make sure you really need this.  Your workpiece also probably has much higher thermal conductivity than air, so even if there is some variation in the oven, if you put a thermocouple on your workpiece and stabilize that the temperature of the epoxy will probably show less variation than the ambient temperature of the oven.

If you do actually need or want 5 degree uniformity I would look at an egg incubator .  The built in temperature controller might not let you set it that high but if you provide your own regulator (and possibly your own heater) it should work fine.  If you want to go complete DIY, just get an appropriately sized cooler from home depot and add a reptile heater mat and a PID controller.  You can get much better than 5 degree uniformity and stability this way.

[rhb]

Add a fan with a long motor shaft and some insulation to a toaster oven.

Use an MCU to provide a PWM input to a suitable solid state relay/MOSFET/SCR/Triac based on a few thermistors.

Or make an insulated box, and use a thrift store hair dryer with PID-PWM control of the heating element.  Ideal arrangement would be an insulated metal bucket with tangential feed from the hair dryer at the base and a vent hole at the top and a metal grill in the middle to hold the part to be cured.

[Poe]

Our high-tech setup uses a wooden box (24" x 10" x 12") with a 60W incandescent light bulb inside.  It's basically an ezbake oven. 

We adjust the temp by un/covering vent holes.  After leaving it warm up for a half hour we put the PCBAs inside.  After another ten minutes, it's pretty consistent across the entire inside area and over an entire day.  Well within 5 degrees Canadian.

The bulb light and air are baffled to prevent hot/cold spots.

[ezalys]

Poe: What temperature does this box reach?

[metrologist]

Bought an Oster convection toaster oven for the kitchen. Its temperature starts at 250F, but that is about 1/3 turn on the dial. Lower is a Warm setting where the marking is a band that gets narrower to a point at 0. Just slightly up from 0, the thermostat clicks. This oven seems to be able to regulate temperature from ambient to 400F. I am not sure how stable and repeatable it is. Maybe just putting a variac on it will move the temp control up to a better range on the dial.

Maybe I will stick a thermocouple in there tonight and see what sort of low temp control I can achieve. I bet if you fiddle with that knob long enough, you could get any temp you wanted right out of the box...

[Poe]

Poe: What temperature does this box reach?

We've never had it hotter than 105°C, but that was with two 40W bulbs.  I'd guess the single 60W could get to 90°C. 

We've used it to bake boards, cure conformal coatings, baking LEDs, and pre-heating boards with large thermal sinks.  Best 30minute investment our company ever made.

The inside is lined with some ~0.25" thick foil backed insulation, aluminum foil and metallic duct tape for good measure.  The insulation I found at the local hardware store for <$13, but I can't recall the original purpose.... weatherproof pipes maybe.

[strawberry]

connet incadescent bulbs in series to last long and get more IR