Heated Bed can't reach 100 degrees

Hi There,

I've just started (trying) to print with ABS. However I've found that the heated bed on my tri-colour mendel is unable to reach the temperatures I need.

I've been struggling with this one for a couple of days now, I'm running out of ideas (ones that aren't drastic anyway.) The voltage measured at the pads of the PCB heater is 11.6V while it's trying to heat up so there doesn't seem to be any drastic voltage drop anywhere

So far I've added 18mm of corrugated cardboard between the heater PCB and the MDF base. This has improved things a fair bit, but it still can't get above 105 degrees and this takes about 40 minutes. People have written on forums of heated beds getting up to 130 degrees in 15-20 minutes so something must be seriously wrong. This was just heating the bed! I tried a print when the bed temperatures was at 105, the code first heated up the extruder then heated up the bed. Except the bed didn't heat up it started to very slowly cool from 104.5 down to 98 degrees!

I've tried using PID auto tuning M303 gcode, for some reason it will only auto tune my first extruder. If I send M303 E-1 S100 C5 it simply heats up and tests extruder 0. The firmware I'm running is RepRapPro's Marlin 1.0.4

Can I use a solid state relay and hook up my MK2 heated bed to the mains? Will it go pop if I do this?

My Mendel uses two Melzi boards powered at 12V. Can I put a higher voltage into these to give a few more amps to the heater without cooking anything?

I'd really appreciate any ideas people have about this one. I'd be really happy if I had one of the beds that'll heat up to 125 degrees in a few minutes. Even if I do need to buy a new heating element and power it off the mains!

Thanks,

Pete

Have you measured the voltage at the board to make sure it's 12v? If not then of course turn up the power. If it is then you might be using too small wire going to the bed and dropping the voltage. There should not be a problem cranking it up a bit, I have mine set at 12.6v and it's been running for 1.5 years now with at least 8 hours a week use.

what gauge wire are you using to connect the bed, can you double up the wiring. a quarter ohm less resistance will provide a lot more power to the heater bed.

as far as i know heated bed pid tuning is experimental. use full power bang bang mode. the mass that heats up is large enough that it will take several seconds to change a degree in C so you should be fine.

Edited 1 time(s). Last edit at 08/06/2013 05:13AM by jamesdanielv.

Or you could ask me, Pete!

Our ABS printing advice, including notes on PSU voltage: [reprappro.com]
tmorris9 is right, you can turn up the PSU. However, check your voltage isn't dropping while the heated bed is on. The Tricolour does run the PSU near the 20A limit, you may want to consider getting a 12V 30A supply.

Most people who talk about getting up to 130C in 20 minutes don't have the aluminium heat spreader the RRP machines do. The extra thermal mass takes time to heat up, but means the bed is a consistent temperature across it's surface.

The Melzi can run at up to 24V - the Huxley kits are shipped with a 19V power supply. However, I wouldn't advise going higher than 14V with the standard Mendel heatbed; assuming a resistance of 1.3 ohms, it will draw around 14 / 1.3 = 10.8A, or 14 x 10.8 = 150 watts of power. Don't wire the heatbed up to the mains! It has far too low resistance for that.

1.0.7 is the current version of the master Melzi firmware. Update to that, but make sure you follow the instructions (ie use Arduino IDE 1.0.5) [reprappro.com] . We haven't played around with the PID autotune on the hot ends as they tend to heat up nice and quick and are stable, so I'd say that isn't your problem, and to leave it as standard. The bed control is bang/bang, ie not PID, and PID control is not in our version of the firmware.

Ian
RepRapPro tech support

Edited 1 time(s). Last edit at 08/10/2013 10:14AM by droftarts.

yes perhaps directing you towards the vendor is a best solution.

This is a pretty common problem I think. The way I've dealt with it is thermal paste and aluminum foil. Go to radio shack and pick up some heat sink compound, the kind you use for computer processors. Then, cut a piece of aluminum foil about the size of your build surface. Apply the surface of the aluminum foil with a light coat of thermal paste. Then apply it to the "bottom" side of your heated bed.

Then it depends on what you use as a printing substrate (I use glass with PET tape, aka window tint). Use some more paste on the bottom side of your printing substrate and place it atop of the foil. Viola. It's all about thermal conductivity.

@ohioplastics: I think you're talking about regular Prusa-style heated beds, that are just heatbed PCB and glass. The RepRapPro heated beds are a MDF, PCB, aluminium and glass sandwich. See: [reprappro.com]

However, what you suggest is feasible. A customer recently did this, putting thick silver foil between the MDF and back of the PCB, which he reckoned improved the heat up time. Make sure if you do this, you do not short the soldered-on power wires or thermistor!

We could make the aluminium heat spreader thinner, it's 3mm at the moment. 1mm would be ideal, but then fixing it is an issue, as the heads of the countersunk screws stick up. Making it thinner would also improve the heat up time.

I think a aluminium foil head spreader is very useful for Prusa-style heated beds, so the glass is more evenly heated. Many people now use mirrors for printing on, which effectively does the same thing.

Ian
RepRapPro tech support

Also placing a folded towel or other insulation on top of the bed while heating will speed up the heating time quite a bit. Just remember to remove it before printing.
Turning the voltage up a bit will solve the temperature limit problem. P= I^2 *R so going from 11.6 to 12.6 (8% increase) will increase the power by 18% which should substantially increase the top temperature you can reach.