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DIY curing oven


John Kimball

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I’ve always wanted to have real composite curing oven in my work shop. I only work in small objects, so an economical small curing oven has been out of the question. I decided to buy a the smallest toaster oven I could find on the Brazilian rain forest site and settled on a cheap ($54) oven with a dial thermostat.

I was pretty pleased with it, but it had some issues that kept me from trusting it.

1) it had 4 burner units. This can cause extreme heat on the surface of the part which can flash cure prepreg and even melt the vacuum bag.

2) The heat was wither On or Off, no ramp rate or easing to the temperature that prepreg really needs.

3) It required you to use a 60 minute timer to operate it. This is annoying because most cures are over 2 hours.

4) I also want to use it for a warming oven to cure paint and speed cure adhesive. This oven could only go as low as 130ºF, so it was still a bit high for my liking.

5) It got pretty hot on the outer shell when heating and caused the room to heat up and was kind of dangerous in my opinion.

With all that in mind and after some research, I decided to “modify” this cheap oven into something a little more usable for my needs.

I started by replacing the dial thermostat with a simple PID temperature controller with solid state relay (SSR) and a toggle power switch to bypass the silly timer. There was plenty of room inside for some simple electronics. I also added ceramic insulation to the areas that would allow. I also eliminated the 2 top burners to slow down the heating process. The remaining 2 burners are more than enough to heat the tiny space.

It was at this point that things started spiraling out of control. I realized that I wanted to control the power to the heaters even if the controller was powered on, so I added another switch. The SSR was mounted to a heat sink, so I figured it probably got pretty hot, so I mounted a small cooling fan to the back of the oven to provide some circulation while it’s running. It was running great, but it still got pretty hot on the outside, especially on the bottom. I found some heat deflection material that insulates with a very thin layer and put that on the bottom and the back of the unit. The PID allowed me to set a temperature and it would climb to that temp and then hold there. I thought I would be satisfied with that, but alas, I wasn’t. I found another PID controller that was programmable for ramp and soak for up to 20 different programs. A gold mine. and it was affordable. I also needed a way to monitor the temps of the part, so I added some internal thermocouple receptacles to the interior with connections on the front panel for easy readings.

This is where almost done turned into start over and, oh yeah, I’ll need vacuum in there. More research. I found a small vacuum pump about 4” x 3” x 1.5”. Perfect. It only pulls 20 in/Hg, but that will do for my projects just fine. so I bought a small vacuum gauge, some high temp vacuum hose and a few fittings and got back to building the oven properly. I added another switch to control the vacuum pump and mounted the vacuum gauge. I also added some more heat deflection material to the interior to keep it more efficient. I covered the glass with the insulation, so I also covered the glass with a carbon fiber plate to make it look a bit more finished.

Next I needed some small vacuum ports for my tiny hoses and parts, so I fabbed a couple of them from some brass fittings and washers.

The controller is fantastic. It can be tuned to the oven or even a mold or part in the oven for optimal heat control. It also has Bluetooth, so I can use my phone or iPad to program, control, and monitor cure cycles.

All said, I spent about $250 to build the final version. Considering that a small lab oven could cost much more than that, I’m pleased.

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11 hours ago, John Kimball said:

IMG_7443.thumb.jpeg.60de6de884e00db4285c669a64299901.jpeg

I’ve always wanted to have real composite curing oven in my work shop. I only work in small objects, so an economical small curing oven has been out of the question. I decided to buy a the smallest toaster oven I could find on the Brazilian rain forest site and settled on a cheap ($54) oven with a dial thermostat.

I was pretty pleased with it, but it had some issues that kept me from trusting it.

1) it had 4 burner units. This can cause extreme heat on the surface of the part which can flash cure prepreg and even melt the vacuum bag.

2) The heat was wither On or Off, no ramp rate or easing to the temperature that prepreg really needs.

3) It required you to use a 60 minute timer to operate it. This is annoying because most cures are over 2 hours.

4) I also want to use it for a warming oven to cure paint and speed cure adhesive. This oven could only go as low as 130ºF, so it was still a bit high for my liking.

5) It got pretty hot on the outer shell when heating and caused the room to heat up and was kind of dangerous in my opinion.

With all that in mind and after some research, I decided to “modify” this cheap oven into something a little more usable for my needs.

I started by replacing the dial thermostat with a simple PID temperature controller with solid state relay (SSR) and a toggle power switch to bypass the silly timer. There was plenty of room inside for some simple electronics. I also added ceramic insulation to the areas that would allow. I also eliminated the 2 top burners to slow down the heating process. The remaining 2 burners are more than enough to heat the tiny space.

It was at this point that things started spiraling out of control. I realized that I wanted to control the power to the heaters even if the controller was powered on, so I added another switch. The SSR was mounted to a heat sink, so I figured it probably got pretty hot, so I mounted a small cooling fan to the back of the oven to provide some circulation while it’s running. It was running great, but it still got pretty hot on the outside, especially on the bottom. I found some heat deflection material that insulates with a very thin layer and put that on the bottom and the back of the unit. The PID allowed me to set a temperature and it would climb to that temp and then hold there. I thought I would be satisfied with that, but alas, I wasn’t. I found another PID controller that was programmable for ramp and soak for up to 20 different programs. A gold mine. and it was affordable. I also needed a way to monitor the temps of the part, so I added some internal thermocouple receptacles to the interior with connections on the front panel for easy readings.

This is where almost done turned into start over and, oh yeah, I’ll need vacuum in there. More research. I found a small vacuum pump about 4” x 3” x 1.5”. Perfect. It only pulls 20 in/Hg, but that will do for my projects just fine. so I bought a small vacuum gauge, some high temp vacuum hose and a few fittings and got back to building the oven properly. I added another switch to control the vacuum pump and mounted the vacuum gauge. I also added some more heat deflection material to the interior to keep it more efficient. I covered the glass with the insulation, so I also covered the glass with a carbon fiber plate to make it look a bit more finished.

Next I needed some small vacuum ports for my tiny hoses and parts, so I fabbed a couple of them from some brass fittings and washers.

The controller is fantastic. It can be tuned to the oven or even a mold or part in the oven for optimal heat control. It also has Bluetooth, so I can use my phone or iPad to program, control, and monitor cure cycles.

All said, I spent about $250 to build the final version. Considering that a small lab oven could cost much more than that, I’m pleased.

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This is awesome! What a cool project!

Is the heat in the oven distributed fairly evenly after removing the other heating elements?

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17 minutes ago, lo_0l said:


This is awesome! What a cool project!

Is the heat in the oven distributed fairly evenly after removing the other heating elements?

The oven is small enough that it heats well. My main goal was to keep too many elements from scorching the part. And with the PID controller, it dies a great job of managing heat. 

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This is truly incredible John!  The final product is quite stylish as well!  

You have me thinking now about how to implement an oven in my home work shop. I have been thinking about trying to figure something out for quite some time now.  I want one that is larger though, perhaps the size of a typical kitchen oven. 

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  • 3 weeks later...
On 10/2/2023 at 2:12 PM, John Kimball said:

Shouldn’t be a problem. It’ll take me a minute to collect it though. 
the best part is that you could do this same thing with an old kitchen oven if you need larger space. 

Hi John - Mainly looking for the PID temp controller with Bluetooth capability. I'm not fully interested in vacuum just yet but am sure others would be interested in seeing what parts are needed for that.

I'm also looking for a good temp recorder that doesn't cost an arm and a leg like the Graphtec bricks! Or can the PID/ SSR/ Phone app record all that?

Thanks!

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19 hours ago, Anthony Fairhurst said:

Hi John - Mainly looking for the PID temp controller with Bluetooth capability. I'm not fully interested in vacuum just yet but am sure others would be interested in seeing what parts are needed for that.

I'm also looking for a good temp recorder that doesn't cost an arm and a leg like the Graphtec bricks! Or can the PID/ SSR/ Phone app record all that?

Thanks!

I’m still working on the BOM, but should be able to get something soon. I really like my PID with Bluetooth. It doesn’t necessarily record, but you can monitor it with a nice graph. However, if you walk away and lose Bluetooth, you lose the record. I use my iPad to record, so I can walk away. The screenshot below is an example of what it can do. Just keep in mind that it can only show 1 thermocouple. The 3 values are: Set point (red), TC value (blue) and power input (green)(% of full power). 

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