Hot bed

Making a heated bed to combat warping has been on my "to do" list for a long time. In fact I ordered the materials more than a year ago. My plan was to use an aluminium plate with many small power resistors screwed on the back.

The plate is 8" square to match my table and 6mm thick. A friend with a CNC machine shop kindly machined it for me. It saved me a lot of hard work with a hacksaw and file and looks a lot better as well.



I estimated that it would need about 50W to raise the temperature to 100°C, so I aimed for 100W to give a reasonable margin for control. I used 9 10W 12Ω resistors wired in parallel. Driven from 12V this would take 9A giving a power of 108W.

The holes in the resistors are only big enough for M2 screws. I drilled blind holes and tapped them with a plug tap, actually a broken tapered tap that I ground to a flat end.



Tapping small holes in aluminium is tricky, that was how the tap came to be broken in the first place. The correct size hole for M2 is 1.6mm but I drilled it 1.7mm to make it easier to tap. In fact aluminium is so ductile that the peaks of the thread are still the correct diameter. I.e. the 1.7mm drill would not fit the hole after tapping and the thread was a good tight fit on the bolts. I used paraffin for lubrication.

Soldering the resistors was fun.



I used stout wire to handle 9A and high temperature solder because I fancied using it as a hot plate for soldering. My 50W iron did not have enough power to melt the solder when the resistors were mounted with two thick copper wires leading from them. To get round that I placed it on a silicone matt and powered it up to raise the temperature to 100°C and then soldered it while hot and live, not something I would recommend. As the iron bit is grounded I had to solder all the 0V connections first and then swap the polarity.

The original plan was to power it from a 12V PC power supply and switch it with a big MOSFET. Initial tests with a bench power supply showed it took about 15 minutes to warm up to 80°C. When calculating the power I had forgotten take into account the specific heat capacity of the thick sheet of aluminium. I didn't want to add 15 minutes to the build time, so I decided to double the power. I have abused these resistors before and got away with it. I changed the wiring slightly to make a series parallel combination with a total resistance of 12Ω and fed it from 48V AC giving 192W.

I used a big 350W transformer and controlled the mains to it with a solid state relay. Since the temperature is controlled there is no real point in using a regulated DC supply. It is much more efficient to use AC and avoid the losses associated with rectification and smoothing. It also allows me to use the same control hardware and firmware that I used for the SMT oven.

I made some PEEK insulating stand-offs to mount it on my XY table with a gap of about 6mm below the resistors: -



I wrapped the feed points around two of these to make the transition to a lower temperature with PTFE sleeving before using normal flex to handle the movement of the table.



I also added some foam board to insulate the top of my X-Y table.



This just fills some of the air gap under the plate to prevent air circulating and convecting heat downwards.

I made some PTFE washers to go under the nuts that hold it down by slicing up a failed extruder insulator: -



These deformed considerably when I heated the table to 230°C, highlighting why PTFE insulators fail when used in an extruder.

Here is the final result mounted on the machine: -



I added Kapton tape around the edge as I thought it would stop hot air escaping from underneath, but it didn't seem to make a lot of difference.

Here is the open loop response at full power: -



Although it can reach the required temperature, it is much too slow for SMT soldering. It needs to be able to rise at about 1°C / second for that. So I will stick to using the oven for soldering for now. I was hoping to be able to paste boards, place components and then solder with the board still on the table, but it obviously needs a lot more power.

Here is the response using bang-bang control from the host at one second intervals.



Some analysis: the initial rise rate is about 20°C in 75 seconds. The specific heat capacity of aluminium is 0.9 J /gK and the total weight of the bed plus resistors is 700g. So with 192W the time taken to rise 20°C should be 0.9 × 700 × 20 / 192 = 66 seconds, reasonable agreement as we ignored any heat loss.

The initial fall rate is 5°C in 85 seconds while at a temperature of ~80°C above ambient. So the rate of heat loss is 0.9 * 700 * 5 / 85 = 37W. Looking at the steady state the power is on for about 1 in 6, which would be 32W, so again reasonable agreement.

The plate is ~ 200mm square so its area is 0.04m² so it looks like we need about 1kw / m² to reach the sort of temperatures needed for HDPE and probably twice that to have reasonable warm-up time and control. Mendel's build area is also 200mm square, so would require a similar power.

You might have noticed the thermocouple is covered with a piece of ceramic cloth in the photo above. This is what happens if it is just stuck down with Kapton tape:-



You can see that as the temperature rises you get increasing thermal noise. Even with the ceramic cover in place you can see similar noise on the open loop test when the temperature was much higher. I think the reason for this is the convective air currents causing chaotic air turbulence. If you think about it you have hot air rising but, away from the edges, the only way cold air can replace it is by falling through the rising air.

A better place to put the thermocouple would be under the bed to avoid the convection currents, but I wanted to try controlling the surface temperature when it was covered by a bed material. Here is what happens with the thermocouple on top of a 3mm thick sheet of smoked acrylic: -



The set point is 95°C in this case. Clearly a case where bang-bang does not work too well, with 5°C overshoot and 3°C undershoot.

The acrylic loses about 15°C between the bottom and the top surface. That makes it curl upwards, so it would need a frame around the edge to hold it down. Fortunately I have one made from HDPE laminated with aluminium so it should stand the heat. It also adds a significant time lag.

Another problem is that acrylic has a glass transition at about 114°C. When the control was of the top surface temperature, the bottom surface exceeded that during the overshoots and went soft.

So I will need to implement PID for top surface control, but I had a suspicion that a transformer was not going to like PWM into its primary much. Anyhow I put the thermistor back onto the plate and moved the bang-bang control from the PC to the firmware in preparation for building something. Bang-bang was an apt name for what happened next. When the temperature crossed the set point it started dithering the mains on and off. The transformer sounded like it wanted to jump off the desk and then blew its 3A anti-surge mains fuse.

The solid state relay turns the power on at the zero point crossing of the mains, and off when the current is zero. Current builds up slowly through an inductor so what could possibly be wrong? I had noticed big transformers thump when you connect them to the mains, but I had always assumed it was because the secondary usually has a big smoothing capacitor to charge up. However, this was a purely resistive load, and even with no load attached the transformer thumps on start-up, so some reading up on transformer theory was required!

It turns out that transformers take a big surge current and turning on at the zero crossing point is actually the worst point to turn them on. The reason is that when a transformer is running, being an inductor, the current lags behind the voltage by 90°. So normally when the voltage is crossing zero, the current is at its maximum reverse polarity and over the next half cycle of voltage it goes though zero and then to its maximum positive value: -

If the current starts at zero then over the first half cycle it will rise to twice its normal value: -

That would not be too bad except for the fact that transformers usually run with their core close to magnetic saturation for efficiency reasons. That means the core saturates during start-up. The inductance disappears and then the only thing limiting the current is the DC resistance of the primary, about 3.3Ω in my case, so the current can be enormous. Counter intuitively, the best time to turn a transformer on is when the mains is at its peak voltage.

So I learned something I didn't know about transformers. The fix was simple, I added a solid state relay to the secondary circuit and plugged the transformer into the mains. Bang-bang control then was able to pulse it very quickly due to the dithering caused by noise, which ends up with some proportional control, that reduces the temperature swings to a fraction of a degree.



So I can add PID control firmware and it can be shared with my oven control but I have an extra solid state relay and a big transformer. A better solution would be to pick the resistor values to give the correct wattage when wired in series across the mains. Of course mains on a moving table is not the safest design. I would use a heavy three core flex with an earth lead to the plate and a second independent earth strap for safety.

It turns out the first use for my heated bed was not to combat warping, but actually something more essential, details coming soon ...