Friday, 30 April 2010

Flash bang bed

As my MK3 heated bed on HydraRaptor has been working well I decided to scale it up for Mendel.

Buying aluminium that is flat seemed to be a hit and miss affair until a friend told me that what I need is tooling plate and put me in touch with a company that sells it. They recommended C250 cast machined tooling plate. It wasn't cheap (I got 5 pieces 200 × 200mm for ~ £140) but they are all flat.

I can't find a geometric definition of flatness. It is given as +/- 0.4mm for a 6mm sheet of C250 (I would have preferred 5mm to reduce the mass a bit but that is +/- 0.8mm). I take it to mean that all the points on the surface of a metre square plate will lie in a volume 0.8mm high. For a 200mm piece I expect the deviation to be about 1/5 of that, i.e. 0.16mm assuming it is a single curve rather than wavy. Since the bed can be levelled at the corners the deviation in the middle should be about half that again, 0.08mm, just about acceptable for raft-less printing.

When I tried levelling the bed I ran into a problem though. With my Dibond bed I could level each corner because it can flex a bit. With the rigid aluminium bed I can only level three out of the four corners at a time. When I move the nozzle to each corner in turn it behaves as if two diagonally opposite corners are lower than the other two. That would imply the plate is not flat, but I know it is when I put a straight edge across it. I think this means that the two y-axis bars are not quite level with each other at both ends, causing the bed to twist about the y-axis as it traverses it. I expect it could be corrected by adjusting the frame but I haven't got my head around what to adjust and in what direction yet.

Given that I am using 188W on a 150mm bed on HydraRaptor, to get a similar warm up time I would need 335W. That seems a lot to get from a PSU, so I decided to make it mains driven. I found that I could get 47Ω TO220 resistors cheaper than other values. Five in series across the mains gives about 250W, so I used two strings of five to give 500W. That gives a warm up time of about three minutes.

Equally spacing four or nine resistors on a square is easy but placing ten is an interesting problem. I used the solution to packing ten circles in a square that I found here. This is my layout with 16 magnets as well.

And here it is wired up: -

I used wire with PTFE insulation rated to 300°C. I have an earth connection of course. It would be a good idea to have a second earth in case the first one breaks due to the constant bed movement. I also fitted a 150°C thermal cut out that came out of a microwave oven. With 500W it would get very hot indeed if the control circuit failed.

I intended to mount the magnets the way I did before, by drilling holes not quite through, leaving a rim to retain them. I didn't tighten my drill stop enough and went all the way through so I decided to glue them in with JB-Weld.

I placed the bed onto a sheet of glass with some cling film on it. I then dropped in the magnets and glued them. When I turned it over the next day I found the magnets were sticking up from the surface. The glue must expand as it sets pushing the magnets down and lifting the plate!

I tapped them down with a punch but, unsurprisingly, they fell out the first time the bed was heated. In the end I jammed them in with PET tape. Drilling part way through is a much better solution.

I mounted the bed on top of the Dibond bed with nylon stand-offs.

Not an ideal solution as a lot of z-travel is lost, but the thermal cut-out is quite deep.

I used chocolate block connectors to wire up the mains. To make them safe and provide strain relief for the cables I RepRapped some plastic covers.

The lids just clip on with some tabs that fit into small slots. They didn't fit very tightly, I need to make the tabs bigger and a tighter fit. A boss and a screw hole would have been better I think.

For safety all the wires should be inside the cover as everything accessible should be double insulated. I will make it wider at my next attempt.

The bed worked well for the first few objects I made. Simple bang-bang control gave about 10°C overshoot initially but settles down before the object build starts so does not really matter. One thing I have realised is that the nylon pillars expand about 0.1mm when they warm up so I give them some time to do that otherwise the first layer has varying height.

I got some new ABS from that turned out to be white, I was expecting natural as that is easier to work with. It seems to need higher temperatures to get it to stick to itself and the bed. I am extruding at 240°C with the bed at 140°C for the first layer and 110°C after that. I built one object like that and then disaster struck. The bed heated to 140°C and levelled off. While the extruder was heating I heard a few pops and crackles. When I looked at the temperature graph I saw the bed temperature soaring. Before I had time to think what was happening there was a loud bang and flash from underneath the bed and the 5A fuse in the plug blew.

What happened was one of the resistors developed a short between its tab and one of the connections. That caused a path to earth which increased the power on the remaining four in the chain. Several of those went short circuit as well in a chain reaction which ended up shorting the mains.What I couldn't explain at first was why the firmware did not turn it off and why the thermal cut-out did not cut the power. It turns out that I had swapped the live and neutral connections in the IEC connector, which meant that the solid state relay and the cut-out were in the neutral connection. As soon as the first resistor shorted it had bypassed all the control, not good!

I had originally chosen the resistors when I was making a bed for PLA at 60°C. Looking at the datasheet they have a maximum operating temperature of 155°C but they are de-rated to zero wattage at that temperature, so by putting 50W into them at 140°C I am grossly over loading them. I have abused AL clad and vitreous enamel resistors in this way and not had any problems but the TO220 seem far less robust. I don't know what they use for the tab insulation but I wouldn't be surprised if it was epoxy. The high voltage may also have been a factor as the ones on HydraRaptor have survived a similar overload so far. They have the same de-rating curve, but are made by a different company.

I rebuilt the bed and changed my firmware to stay inside the power curve by reducing the PWM ratio as the temperature increases. Unfortunately , I found I could only get to 130°C so I had to change the zero power point to 200°C to get to 140°C in a reasonable time. Even then it takes 400 seconds instead of 175.

So far it is holding up, but it is nowhere near as fast as I wanted. A shame because I had bought 50 of the 47Ω resistors, but I think I will have to scrap them and go back to AL clad. The smallest ones that I have used before are not rated for mains voltage so I will need some bigger ones. PCB or stick on silicone heaters are starting to look more attractive!


  1. why not just use nichrome wire instead of resistors?

  2. Not got an RCD on your bench? I don't like playing with mains stuff without one.. bitten too many times.

  3. If I use nichrome I have to find a way of attaching it with a good thermal connection and good electrical insulation. I also have to work out the correct length and somehow coil it up. When I buy resistors I am paying a company to do all these things for me.

    I have about 50 sockets in use in my room. Some are on an RCD but that one wasn't. A 2A fuse for the heater would have been good as well except it would have also been in the neutral, so useless.

  4. I've been using the Silicone 115V heaters from Omega on my RepStraps, and they work really well. A bit pricey though. See

    The only problem I've had with the silicone heaters is that if you use the pressure sensitive adhesive, the adhesive is only rated to 149C, so you need make sure you don't overtemp them on startup. My thermocouple is located on the outer rim of the build surface, so the temperature at the center of the heater can have quite a bit higher temp than the outside until things stabilize.

  5. I too have recently purchased a silicone Omega heater so I'll be curious to see how it works out with the pressure sensitive adhesive. My build platform is 8" x 8" x .25" thick Aluminum and the heater measures 6" x 6" and I also plan to mount the thermocouple to the outer rim so the info is good to know.

    With regard to cast Aluminum tooling plate I was recently at a friend's machine shop and he gave me (for free!) a 24" x 42" x 3/8" thick piece of Mic6 Aluminum tooling plate that had been ground to within .001" over the entire surface. It was originally used for vacuum tooling so it has micro holes drilled all the way through it over a large percentage of its surface. I'm sure I'll find a use for it!

  6. You should design in conformance with EN 60950 (security of products) ;-)
    It will be easier with your fire insurance...^^

  7. I know, I know: Two defects at the same time.

  8. For flatness definition:

    Flatness checking:

  9. Thanks Anonymous, or is it Anonymouses?

  10. They are "The Anonymous" ;)

    Anyway, following on from our previous discussion... I want to build a heated bed, but you no longer recommend the TO220, for reasons mentinoed above, and I am still wary of 240v...even more so after hearing about "Flashbangs"! so I have been looking for a suitable alu-clad resistor for a 12v heated bed powered by a 400W PSU. This is what I was thinking of going with so far:

  11. How many are you planning to use?

  12. I think the resistors I chose will run at 53W when 12v applied so, I don't think I can use any more than 7 with a 400W PSU, I was thinking of buying a PSU just for the purpose of running the bed though so if you think I need more power prehaps I should look at a higher wattage PSU. Also I was thinking of heating a 232x230x6mm bed so I could well need more than 7. I am not really too sure exactly, which is why I asked for your words of experiance ;)

  13. ...Also something which I forgot to consider is that although the power supply is rated at 400W it can actually only supply 20Amps over the +12v line. which means I actually only have 240W to play with.

  14. With 500W I got a warm up time of ~ 180 seconds so you would get 180 * 232 * 230 / (200 * 200) * 400 / 500 = 192 seconds.

    400W at 12V is 33A though so you need a good MOSFET and thick wires. Nine 3.3R are easier to distribute.

  15. Hmm, crossed in the post and I got it wrong anyway.

    240W will be 180 * 232 * 230 / (200 * 200) * 500 / 240 = 500 seconds.

  16. So this rough calculation is your warm up time multiplied by the ratio of your bed size to mine, multiplied by the ratio of your power rating to mine....?

    Anyway so it will take my bed over 8 minutes to warm up. Its not unreasonable, but I am going to have to look at hefty mosfets, and buy a new PSU. The more I think about it using 240v is much better (like you have already decided yourself). I am just worried about safety.

  17. I think I will try a silicone flexible heater with adhesive next together with a thermal cut-out, an inline 2A fuse and two independent earth straps. That should be safe and everything in spec. They don't cost much more than nine or ten resistors anyway.

  18. Maybe I will wait and see what you come up with then. My main reason for wanting a heated bed is that I am having trouble getting the first layer to stick, but I am in very early days, maybe there is more things I can try to remedy the problem, such as adjusting the z offset, different surface coverings (such as the PET tape I have bought) and even different materials (like PLA). I haven't figured out how to print a raft in reprap host software, prehaps I will have to give skienforge a try aswell.

  19. If you are using ABS and a cold bed it won't stick to PET or Kapton at all. It needs to be acrylic or Foamex or double sided sticky tape (apparently the papery type used to hold carpets down works well).

  20. I intend to make more machines, although what usually happens when I buy things is that my ideas move on before using the others, or sometimes before materials have arrived!

  21. Why not use a soldering iron heater? Thats designed for mains voltage and you can get really high wattage ones too. Just stick a few in a block of AL and wire them together.

    And for the nichrome wire idea, why not just use thermally conductive epoxy. Thats what Makergear is doing with their heated build platforms:
    If you use a huge amount of nichrome then it would be suitable for mains usage. Also for the electrically conductive problem, just use insulated nichrome. It has a fiberglass insulation that conducts plenty of heat.

  22. Soldering iron elements are expensive and usually 50W or less so it would take 10 to get 500W.

    I can't find a source of thermally conductive epoxy in the UK apart from JB-Weld, but that is completely crap in all applications I have tried to use it.

    Also I can't get insulated nichrome in the UK and it would need a lot for 500W, so difficult to manage.

    I have just ordered a 490W 7" x 7" silicone heater with pressure sensitive adhesive.

  23. Looking forward to seeing your design based on a 7"x7" silicone heater. Will it still need a Solid State Relay for Power Control? Would a mechanical relay be suitable or do you think it will not switch fast enough?

  24. I will use an SSR. A mechanical relay could be made to work but by default my firmware would switch it too quickly and wear it out. When the bed is at temperature the heater flickers on and off. I could sample it less often and have a bit more temperature ripple.

  25. Hi nophead, sorry to ask such a noobie question, but could you give me some pointers on how to wire up the mains powered heated bed please? My current design is running at 120W from 19v, but it takes ages (20mins) to reach 120C even with a balsa blanket.
    I see you are using an SSR with on/off control, but I can't see from your pics which wires go where.

  26. The two AC terminals of the SSR go in series with the live connection of the heater. It's a good idea to include a fuse rated for just a bit more than the heater current and essential to have an earth lead to the bed that will not break with the movement.

    The + pin goes to 5V and the - pin goes to an output pin on the micro which is set to logic zero to turn the bed on.

    If the bed can get too hot when powered on 100% include a thermal fuse to ensure you don't get meltdown if your software crashes or the SSR goes short circuit.

  27. How did the flexible heaters work out? They were ? 115V?

  28. Yes I got 115V volt by mistake so I have never used them. I can't find 200mm x 200mm @ 240V.

  29. I see this one that's about 1/4 the size (area) but looks hot enough and runs on 220v.