A small kiln with big expectations: Fusing PMC, Solder PCBs, Harden/Temper/Anneal/Carburize Steel, Pottery, Glass, Melting Precious Metals
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Required Max Temperatures:
Max ramp rate required is 1C/s under 260C. Hitting 3C/s is a nice to have.
In-field firmware reprogramming.
Safe to run for 8-10 hours.
With all the virus frenzy behind us now, finally got back into making some progress.
A lot has happened since the last update.
1. Lined the inside of the kiln with 1400C KAO wool and learned a couple of things:
- It is not easy to mount a hot element on KAO wool.
- You have to glue KAO wool to the fire bricks using mortar and the density of high temp wool makes it not want to stick to the sides and top. Eventually used some pins made from Nichrome to pretty much staple it to the bricks.
- Used 25mm wool, reducing the inside dimensions by 50mm on all axis.
- Reached 900 C in about 10 minutes and shortly thereafter reached room temperature as the element burned right where it enters the kiln. Suspect the mounting/"hanging" of the element put too much strain on the single strand entry into the hot face and gravity pretty much did the rest.
- Even though the KAO wool product I used only had a non-carcinogenic irritation risk during handling, after it was heated to over 700 C, apparently it forms structures that are pretty much back to carcinogenic when inhaled again. This prompted me to seal the hotface to prevent those small needles from circulating and venting. Used some water glass; it worked okay but its like glue and sticks to everything. For some reason a yellow deposit formed just behind where the element touched the wool/glass sealed surface after firing. Hoping it was sulfur and not chromium.
- To avoid stuff sticking to the water glass and thus sealing the door shut once fired, had to use some kiln wash on the door to create a barrier.
All up, the KAO wool lining made the internal go hotter faster using less electricity, but the lack of structural integrity on the KAO wool walls caused a element failure and the super fast temperature climb and fall caused the sealed glass structure to crack and decompose even after the second firing. This made the kiln a pretty unsafe tool as it was and decided to abandon the KAO wool hot surface idea.
2. Having removed the KAO wool, the overall structure was back to the original design. Difference now was that the bricks all had mortar between them, sealing the inside more than it was. Cut deeper groves to take the element a bit better and replaced it with some off the shelve 1200W elements (5mm x 480mm).
- Testing revealed similar performance to the original design, suggesting that the sealing did not make that big a difference and convection was not that big an energy drain.
- Cheap hacksaw blades work like a charm to cut the grooves and a lot less dust than power tools. They work for cutting the bricks in half as well but I used 5 blades in total for 4 brick splits and grooves on 3 bricks. Probably because the blades are just carbon steel and the stuff the bricks are made of is the same stuff they put on emery paper which in turn tends to be used to sharpen metal blades (doing the blunting role in this case).
At this point, the only way to get to the ramp rates I was after at higher temperatures was to either go to a 15A power point or start looking at 3 phase/multiple feed points to power the coils. High voltage is scary and adding so much complexity is not something I'd like to troubleshoot and maintain. So major turning point for the kiln design was in order.
3. Pretty much ditched the original brick layout and reduced the overall design to use 7 bricks. Cut all but the base 2 bricks to 50mm thickness and created a 127x127x280 internal size oven. Wrapped the lot in some of that leftover KAO wool from point 1 above and only fitted one coil. Some notes and current performance:
- Front door is a stainless box wrapping some of that 25mm KAO wool.
- Brick structure is now snugly wrapped in a KAO wool blanked, 25mm thick. As a result, the biggest heat leak is the 25mm thick door.
- With less brick mass to heat up and less surface area to conduct heat to the outside, the kiln now goes from room temperature (20 C) to 1000 C in under an hour and up to 700 C in about 20 minutes. Sounds slow, but to...
Read more »After the last test, decided to add some fiber padding to the kiln inside. Faster ramp times and better sealing keeping more heat in (is the theory). The blanket is 25mm thick and I'm trying to work out how to mount the element now as it used to be stapled to the brick walls. The fiber is not rigid enough to support the staple approach and it will be coated with water glass which in theory will be molten at the element operating temperatures..
As a result I created some water glass as a side project to seal the fibers and keep them out of lungs.
Currently the frame is pickling in vinegar to get rid of the mill scale. That stuff is evil :)
Once the pickling is done, I'll finish up some final welding needed to get some handles and feet of the oven attached.
Painting then comes next. I managed to get 1093C resistant paint from the auto store. It needs to bake at 200C before it is ready and I don't have anything large enough to bake it in. Thinking I'll just blast it with a flame for a bit and hope the oven at full speed will do the rest on first launch.
Tweaked PID controller to stabilize target swings. Still needs further tweaking, maybe even adding Auto Tune as the oven behaves differently at different temperatures. Increased the PWM from 25% max to 100% max to do a full load test.
Wrapped most of the oven in foil, shiny side inward. Made a big difference to perceived heat emission and it completely blinded the laser temp sensor (was reading room temp all over the surface)
Ran up to 800C to check the thermal stability of everything hooked up. Some findings:
Overall the construction seems sound enough and electronics/electrics can handle the design well.
Think there is a massive loss to convection caused by the gaps between bricks. Also noticed the max power the coils are willing to draw peaked at about 1650W suggesting a 3rd coil might be needed to reduce the resistance. Trying to keep the max current consumption under 10A to avoid needing a special wall socket.
Some other random notes:
Next potential steps:
Great news! First integration test went okay. Hooked one set of coils up to the assembled controller box and fired it up to 108 deg C. Held it for about 1H.
Only two things got hot: The inside of the oven and the 9V PSU in the controller box. Didn't have the 80mm fan on so hoping it will keep the PSU temperature in check. It was hovering around 50 degrees with ambient at 18.
Had some teething issues with noise in the I2C bus and the GPU controller ran out of RAM causing random things to show on the display. Don't think it is all fixed but getting close.
Before running the test I limited the PWM to 25% of its max capacity. This translates to the coil only ever getting power 25% of the time max.
Some measurements while cruising at 108C (18C ambient):
- Coil at 65 Ohms
- Max Temp 110.1C/Min Temp 106.7C
- Max Power 500W / Min Power 270W
- Power meter power factor of 50
- Power meter 4A max
- Multi meter AC Voltage over coil Min 40V Max 80V
- Clamp meter Amps at 0.3A - 0.45A
To the untrained eye these numbers don't add up. The trained eye doesn't care to do the math to recon why not :)
Not sure what's next. Really want to check the controller box can handle full on state for 8 hours. That needs a couple of small problems solved before it can be executed (like the stuck terminal block screw for the second coil, lugs and insulation for the remaining coil wires and more schedules to ramp the temperature to various intermediate levels).
The PID needs some serious tuning but 2.5 degrees swing around the target is not bad for an initial guess.
Wonder how one can calculate the efficiency of the insulation and oven design using the results from above?
Thought I'd add some of the journey towards the final product.
Where is it up to?
- Main kiln body manufactured and fired up to 700C.
- PID Controller Software Developed (Alpha)
- LCD Touch Screen Controller Developed (Alpha)
- Master Controller Software Developed (Alpha)
What's still left?
- Controller Box Manufacturing
- Electronics mounting
- Final Component Integration
- Tweaking body design
Kiln body design in links. Will add code to GitHub as soon as initial integration testing is successful.
Decide on kiln size. Some factors to consider:
These will drive the max wattage needed, internal size and the type of materials that can be used as the max temperature limits the range.
K26 bricks matched my max temperature requirements and angle iron was most economical to purchase for the frame build. The min. ramp rate suggested the oven should have some fiber lining to reach temperature quicker. Kanthal A1 wire could handle the temperature and weight was not much of a problem for me.
With a 10A/240V socket available, I opted for around 2000W as I could get an SSR, fuse holder/fuse, high temperature silicon wire and some terminal blocks that could handle the amperage fairly easily.
Calculate what size wire and how much is needed, used https://kruegerpottery.com/products/element_240v_16amp
The shape of the coil, width between windings and internal layout of coil are impacted by:
Used Fusion 360 to model the K26 bricks, layout the easiest manufacturing pattern and then play with coil layout on the model until it fit with minimum installation interference.
Simples, PID does the heavy lifting. Have a K-type (1200deg) probe hooked up to a Max31855 amplifier hooked up to a Arduino Nano which in turn is controlling an SSR using PWM on a custom Kanthal coil. The Arduino Nano receives it's target temperature/ramp rate from a master controller that lives on a another MCU which is responsible for schedules and translating user input into target temperatures.
mybura
Matt Moses
twl
extremerockets
Seems like a huge range of temperatures for one machine. How do you plan to hit your target temperatures on both ends of the scale?