MK2 Heated Bed Does Not Reach 110 Degrees

Hi, I am having trouble getting my school's RepRapPro Mendel heated bed to heat up enough for ABS printing. It is an MK2 PCB heated bed and it should be drawing around 10-12amps max from all the sources I have looked at, which should let it reach 110 degrees with the help of insulation. The stack on our heated bed is standard for the RepRapPro machines: MK2 heated bed under an aluminum plate under a glass build surface.

Without insulation, we can reach about 68 degrees C at maximum. With corrugated cardboard covering the sides and most of the bottom, we can bump that up to the upper 70s (the actual temperature of the aluminum plate, measured with an IR thermometer, is about 3-5 degrees higher than what the thermistor reads). I read in a forum thread that it might be possible that the heated bed could sustain a higher temperature, but not reach it on its own. With this in mind, I tried insulating the top of the bed with some styrofoam during the heating process. Being totally insulated, It was able to reach 110degrees in about one hour, but it quickly dropped and settled at 83 degrees when I took the insulation off the top.

Having exhausted my ideas, I tried to cover the entire printer in a trash bag to see if I could replicate a solution found on the forums. Needless to say, this was a bad idea. The bed got up to 90 degrees on its own, but the hot end fan had no cool air to suck in, which caused part of the X carriage to melt.

Normally the bed takes a little less than 20 minutes to heat up from 25 degrees C to 50 degrees C, which is what we use for PLA printing. From what I've read on MK2s, this is a long time. Could the power supply be an issue? Its output is 20A. I was also wondering if stronger heating element like this might be a good investment?: http://store.qu-bd.com/product.php?id_product=61
It might require separate power, but one minute to 120 degrees is very fast compared to what we are currently experiencing.

Any help with this problem would be very much appreciated!

Often times the PCB heated beds end up at a higher resistance than they should, which means they draw less current and don't heat as fast. Can you measure the actual resistance of the heated bed? Also, can you measure the voltage at your power supply when the heated bed is on? That would indicate whether the power supply is the problem.

Cameron provides some good tips, and the answers will help lead you in the right direction.

I had several issues getting my bed to 110, but it does now, and within 15 minutes or so. Each of these provided some improvement:

* Resoldered the connections into the heated bed.
* Ensured that I had insulation (teflon cooking sheet and cardboard) between the PCB and my aluminium sheet.
* Replaced the wire between my power supply and RAMPS. As it turned out, it wasn't rated for the amps.

All my voltages seemed good the whole way, so discovering and fixing to all these issues took several months.

Good luck.

Cameron,

I was not able to test the voltage of the power supply with the bed on because the school computer with all the software on it was having trouble connecting. The voltage of the power supply with only the hot end fan running is 11.6 volts, rather than the advertised 12 volts. The resistance of the bed was about 1.3-1.4 ohms measured from the screw connectors and the solder joints on the bed (Rats, I just realized that I forgot to disconnect the bed from the board for this measurement. I will have the correct measurement the next time I can get my hands on the printer.)

Karmavore,

I think the solder connections to the bed might be a problem, if nothing else is faulty. They look like some burnt flux got stuck in the joint. The wires from the power supply to the controller board (Melzi) are very thick. I think they are rated for the full amperage value of the power supply, but I will have to look into that.

We had a temporary setup earlier where we were routing the extruder motor to a second Sanguinololu board due to a faulty stepper controller on the Melzi, but we fixed that. Now we have a two long and rather thick wires (that were being used to power the Sanguinololu as a slave controller to the Melzi) hooked up to the XLR socket on the printer. Could these wires be doing anything to the power coming into the Melzi board?


Thanks for all the help so far!

mrShrimp Wrote:
-------------------------------------------------------
> Cameron,
>
> I was not able to test the voltage of the power
> supply with the bed on because the school computer
> with all the software on it was having trouble
> connecting. The voltage of the power supply with
> only the hot end fan running is 11.6 volts, rather
> than the advertised 12 volts. The resistance of
> the bed was about 1.3-1.4 ohms measured from the
> screw connectors and the solder joints on the bed
> (Rats, I just realized that I forgot to disconnect
> the bed from the board for this measurement. I
> will have the correct measurement the next time I
> can get my hands on the printer.)

1.3 Ohms is a bit high on the bed, giving you just over 100 Watts of power. My heated bed is twice the area but uses 360 Watts. If your power supply voltage is dropping significantly with the heated bed on, you will also lose a lot of power to the heated bed.