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Need more Heat Man! Heated Bed issues.

Posted by bdcannon 
Need more Heat Man! Heated Bed issues.
January 18, 2012 09:31PM
Hey Everyone!

So I have the Mk1 Heated bed, RAMPS 1.2, and the firmware I'm running is Marlin 1.0.

The issue I have is that, when I'm trying to print ABS, I can't get the heated bed hot enough. The bed is usually reported around 96 C. I see most people run their beds around 110 C. I have two 18 amp, power supplies running in parallel so its not that I am current limited. I thought perhaps, it was the PID settings in Marlin, but I turned them off and I have the same problem.

I was wondering if anybody else ever has the same problems or any words of wisdom.

Stay Calm Out There

Re: Need more Heat Man! Heated Bed issues.
January 18, 2012 10:38PM
I know this isn't your direct question... but i have almost identical setup but print in pla and only need to get to 55c on the heated bed. it's an option... though a pain to clear out the abs from the extruder well enough to start pla.
Re: Need more Heat Man! Heated Bed issues.
January 18, 2012 10:50PM
Hey Anthong,

I used to print only in PLA for this exact reason. I wanted to be able to print in ABS as though.

Re: Need more Heat Man! Heated Bed issues.
January 18, 2012 10:54PM
I'm suffering through the same problem.What I figured out was that my heat bed did not have power to get up to temp but, had the power to maintain the heat. I placed aluminum foil over the heat bed until the bed gets to temp then slide it off and whalla! I figure out how to fix this later.
Re: Need more Heat Man! Heated Bed issues.
January 19, 2012 12:00AM
yea its a pain that's why I use a heater pad with aluminum plate... back to your problem... if you have the means test the pcb separate with a higher voltage.. Alot of people with your problem separate (if you have the means) the bed voltage and crank it to 16 or so... I have a pcb and have toyed with it with glass but find the aluminum plate better....only diff is the plate has more mass.. that means slower top end without backlash... BUT i can tell you.. i can do 150mm+ without backlash which is good for a thick plate..

Re: Need more Heat Man! Heated Bed issues.
January 19, 2012 04:08AM
Exactly why I had to get rid of my PCB bed. Couldn't get it hot enough. I run a power resistor bed now and it gets up to 130C no problem.

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Re: Need more Heat Man! Heated Bed issues.
January 19, 2012 12:59PM
Lets get oriented first.

The top of the MK2 bed is the solid copper side.

The bottom has the traces, and solder points for optional LEDs.

The thermistor is mounted to the bottom, center, and held there with kapton tape.

Now, if the MK2 is open to the air on top and bottom, it will never get hot enough, because the heat that it generates will create updrafts that will continuously cool it. You need two insulating layers. The top layer should be more condutive than the bottom.

To remedy this, place a 1/8" glass pane on top, affix to the corners with Kapton. My preference is Borosilicate Glass, which is also known as Pyrex. It's not cheap. An 8"x8" will set you back $35. This takes care of the top.

For the bottom, use at least three sheets of corrugated card board. The important thing is to heat the air under the MK2, but not let it escape, or radiate. Avoid using any aluminum tape, as this will not insulate, but radiate.

Re: Need more Heat Man! Heated Bed issues.
January 19, 2012 04:56PM
Hey Wildseyed,

I have the MK1 bed still and I am using a layer of glass, not Pyrex but I thought I'd give it a chance since I already had it. It actually seems to be working out just fine.

I have the bottom of the MK1 covered in 3 layers of ceramic insulating tape, I kept adding to try and reduce the loss of radiating ambient heat, but to no avail.

I actually don't have my thermistor in the center of my plate though. I decided to place it off center closer to the edge of the bed. After a few checks with my thermocoulple during prints, I found that since I often place the prints in the center of the bed, right over the thermistor, that the thermistor is reporting the temperature of the hot spot where the plastic is and not the average temperature of the bed. I guess there is two ways to look at this, if we place the thermistor directly under where we print, then we report the temperature of where the plastic meets the surface. Is that desirable or is it the other way around? Do we want to detect the average temperature of the bed by placing it off center? Or is my logic flawed here?

I think I might have to just go back to the shop and build myself a new build plate using power resistors. I think it will perform better anyways.

Thanks again for all the input guys.

~Stay Calm Out There

Re: Need more Heat Man! Heated Bed issues.
January 20, 2012 02:25PM
My thought on the thermistor placement is that I want to know what the temperature is where plastic meets the HBP. My reasoning is that I know that too much heat is just as bad as too little, so measuring average temperature would not do much good.

I have looked at my HBP using FLIR cameras at various times, and the temperature profile across the surface is almost linear until you get withing 20mm of the edges. If the print bed is not moving, it will get warmer in the center, but only by a few degrees.

As for the ceramic tape that you are using on the bottom, I think a less dense, thicker material works best (card board, in other words)
Re: Need more Heat Man! Heated Bed issues.
January 21, 2012 01:04PM
I found a good way to print ABS on a heated glass surface, I sprayed some varnish on the glass, heated it to 100°C for the first layer then lower the temp to 60°C, the printed part stuck verry well on varnished glass an pops out by itself when the bed cools to ambient temperature.

The extruder temperature is set to 240°C for the first layer then 210°C, the temperatures may be inaccurate since the thermocouples are close to the cold end on my extruder :
Of course you will have to apply a new varnish layer for each print or just make several varnished glass bed in advance.
Re: Need more Heat Man! Heated Bed issues.
January 22, 2012 03:44PM

I'd try to connect your hotbed *directly* to the 12V output of one of your power supplies while you monitor the bed temperature with the thermistor (or an external thermometer you may have laying around). Don't leave this unattended and disconnect your power supply once you have reached the temperature you want (ie. 110 degrees).

If after a few minutes the temperature estabilizes below the desired temperature you know that testbed is not going to work no matter you disable PID or not. Conversely if the target temperature is reached, you know the bed is ok.

It is always important to keep all the bed-related wiring thick enough.
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