Friday, 1 January 2010

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 ...

23 comments:

  1. This looks like a very neat solution a couple of questions tho.

    Will the foam in the foam board melt?

    Will you get hot spots where the heat is radiating out from the resistors making much hot areas at the center point of each of the power resistors?

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  2. The foamboard had a small gap between it and the resistors but it seems to have buckled on its excursion to 230C. I have some ceramic wool which is a much better solution but I don't like the fibres it gives off. I would have to seal it in well. I may just go with an air gap for simplicity or perhaps a sheet of balsa.

    I think that the thermal conductivity of aluminium 6mm thick is so low that temperature will be pretty even, but I have no evidence to back that up.

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  3. The aluminium looks very sexy.

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  4. Balsa will also burn but you could get a sheet of peek as an insulator.

    or

    To cut up into strips to make a frame with two central dividers between the resistors to restrict any air movement within the frame.

    Or you could make a fiberglass cover. I believe that fiberglass epoxy resin is OK up to 150C I think.

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  5. Hmm I'm not sure if fiberglass gives off any nasty gasses tho.. I have seen it used for heat insulation thou

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  6. Thanks for your great heated bed article.

    Another insulation option is wool. From the "Wool Burns" page at:
    http://www.strobel.com/wool_burns.htm

    wool has an ignition temperature of 600 C, higher than other common textile insulators like cotton at 255 C or rayon at 420 C.

    Besides buying wool at a clothing store, you can also get wool insulation batts, like Heatkeeper in the UK:
    http://www.heatkeeper.co.uk/insulation.html

    or Sheep Wool Insulation in Ireland:
    http://www.sheepwoolinsulation.ie/about/

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  7. Wow, I know how much time and dedication can go into such thorough blog posts! I always look forward to them :)

    But back on topic. I also have plans for a heated bed for a while now. Maybe you can comment on its viability. I hope to use two 2mm sheets of aluminium with kapton taped (and insulated) nichrome between them. Just need to find time to cut the sheets. Wanted to put two 6mm sheets of acrylic around that, to balance out the warping force from the temperature gradient. Together the sheets will probably resist a substantial amount of force from printed object, so much that there will be no cumlative bending.

    I may also try a 90'deg air feed. Already built it, just need a proper way to affix it to the X carriage of my darwin and still be able to direct the airflow properly.

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  8. Hi Erik,
    Yes I think nichrome and Kapton on AL will work but I think you need to pick the length and gauge carefully. You can work out the total power from 1Kw/m^2 plus some margin. But then you need to distribute that amongst enough nichrome so that it does not get hot enough to burn the Kapton. Might need some trial and error as it depends on the thermal resistance between the nichrome and the AL.

    If you don't have the correct guage then you might need to use parallel loops.

    That is why I went for resistors, more expensive and more work to mount them, but somebody elae has worked out the nichrome length and guage and how to electrically insulate it but thermally couple it.

    TO220 style resistors are probably a better choice in this low temp application, IIRC they are good up to 150C. Slimmer and easier to mount.

    I have also been thinking about a jet of hot air around the nozzle. Same temp as the bed, fast airflow to cool the filament to below Tg quickly, but concentrated so as not to waste too much energy.

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  9. @Enrique,
    Yes wool looks ideal, thanks for that. I had no idea it could stand high temperatures.

    @BodgeIt,
    PEEK is not that good an insulator compared to air. I think most plastics are ~ 0.17 compared to air ~0.02 and wool ~0.04.

    I doubt balsa would burn at 100C, especially as it would have about 1mm air gap between it and the resistors. It may warp though and not as good an insulator as wool.

    Even with a bit of warped foamboard it seems to work, i.e. my XY table stays cool when the bed is 100C. I should really have covered the top side of the foam board with AL foil to reflect any radiation.

    I didn't try it with just the 15mm air gap. It may well have been fine like that.

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  10. An alternative to power resistors for the heating element is a kapton heater - they are basically a thin heating element sandwiched between two pieces of kapton film. They provide very even heating, are available in many sizes, shapes and wattage ratings. They even have models that can be run off mains voltage. The other percieved advantage I see is because of the low profile of a kapton heater you would have more room to install an insulator.

    (www.omega.com/pptst/KHR_KHLW_KH.html)

    I don't know how available they are outside the US but I can testify that they work very well!

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  11. Bad Link in my above post - sorry! Here's the correct link:

    http://www.omega.com/pptst/KHR_KHLV_KH.html

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  12. Hi Shane,
    Yes they are quite a nice solution, and we can get them in the UK, but they are still 115V only for the bigger ones. I would have to get two 3" x 6" and wire them in series. It then gets expensive ~ £70. The economics are a lot better in the US as I could use one 6" x 6" and they are a similar price in dollars as they are in pounds here, so considerably cheaper.

    Also they are only rated at 200C. That is fine for plastic extrusion, but my original intention was to also solder SMT which is why I chose resistors that I have previously used at 240C. If I was aiming for plastic extrusion only then a PCB with a zig-zag track would be my first thing to try.

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  13. Actually the silicone ones look a better bet. They seem cheaper, are available for 230V and just about reach soldering temperatures.

    http://www.omega.co.uk/ppt/pptsc.asp?ref=SRFR_SRFG

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  14. Does the aluminium plate warp when heated to 230degrees?

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  15. I didn't notice it warp. I expect that since it is such a good conductor both sides will be very similar temperatures, so both expand together.

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  16. For lowering thermal capacity, you could go with a thinner sheet of aluminium.

    Needs to work out the fixation though. Like soldering aluminium rods to the sheet, and cutting thread on it. Soldering aluminium is tricky...

    Laszlo

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  17. Another possibility is use what reptile pet owners use on their cages for heating up them up. They come in different sizes for various sizes of "tanks" and have a sticky side that you can peel and stick to the underside of the plate.

    Just an example,
    http://www.tinyurl.com/ycud9qb

    Drake

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  18. @Laslo,
    6mm was about the thinnest that I could blind tap to leave a flat surface. Some of the back of the plate could be milled away in between the resistors leaving some ribbing for strength perhaps.

    @Drake,
    They are limited to ambient plus 20C, so only about 40C which is not hot enough.

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  19. @nophead
    If you use an oversized one it actually gets hotter then using a "correct size one" for a given tank.

    Plus thats just one example.. Forgotten where but they do make a variable heat one.

    drake

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  20. I have made some silicon that I showed to NASA (they were unimpressed) where I took an oxy acetylene flame, for 30 seconds to a 1/4" 4"x4" piece I cast for them. It glowed red and chuffed off white powder (probably quartz), at a 1/4" radius where the flame point was the center. That was read on a thermal imager to be 4500°F at the center radiating out about 1" diameter to 800°F and at the edges at room temperature. I wanted to cast a giant sock for them at the underside of the space shuttle. It remains flexible and does not crack. When the flame was shut off it quickly went to 800°F all around the piece and slowly cooled.

    It is not an inexpensive solution but you could mold the entire undercarriage with this. It will eventually come up to temp but with your balsa wood idea there should be enough insulation to limit the heat soak. I can make some up for anyone interested or simply go to Home Depot and they have a fire stop from Dow Corning that is similar but very expensive.

    Another thing to consider as far as heat capacity of your metal. Yes Aluminum comes up to temp quick and evenly but it also like copper will heat soak in areas creating cold spots in others where the cold is more prevalent. One problem seen on the stove top, hot in the center cool on the sides. Trying cast iron, once it is up to temperature and it stays there with little power needed thus no cold pots. Just look a fajita pan for proof. The whole pan comes up to temperature(350°F) and keeps hot cooking the food off the burner for a long time. Metals all come up to temp with everything else being equal the same. It is how long they stay hot after the power cycles off that I think you are looking for. Aluminum and Copper cool off too quickly. I think what is happening here is your efficiency through the aluminum plate. The bottom is very hot, heat rising up through it, and constantly hitting cool air; and what does aluminum like to do? Act like a heat sink and it does just that. So you have to dump a lot of power all the time. With cast iron, you heat it up, it is hot all over. Much more efficient, because cast iron makes a terrible heat sink. Problem is you have to make a sand cast and cast your own bed. It can be surfaced but does not hold a screw well. Sorry, I don’t have a very good answer on that one for you.

    BTW, Be very careful with your PTFE. At about 350° F it out-gasses hydrogen fluoride gases. Extremely toxic. Honestly I would cast up a nozzle for you out of my silicon before using PTFE. And after all wouldn't you like the ability to run higher temp materials like casting aluminum?

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  21. Many thanks for the detailed calculations :))) Now its possible for me to build my own, based on a PC-Powersupply and NiChrome Wire .... I know its not the best solution, but it will produce some heat :)

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  22. Simone Iannucci5 December 2011 17:50

    Does it have to be 6mm or can it be lower?

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  23. I can be thinner but thinner AL plate isn't guaranteed to be as flat.

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