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Strange heatbed heating problem

Posted by barakori 
Strange heatbed heating problem
February 28, 2015 02:47AM
I've been printing exclusively with PLA over the last 3 months, and decided to try switching back to ABS.

I sliced something for 225C/105C and preheated the headbed. Heating was much slower than I remember from a few months back,
Actually, although the temp was set to 105C, it stopped at 97C. After 10 minutes of not increasing by a single degree, I decided to try and print anyway. I re-sliced with 96C for the heatbed (so that the print would start) and started the print.

Once the print started, the heatbed started heating again, and actually reached 105C two minutes into the print.

As I print now, I'm hearing popping sounds, and as far as I can see, printing is going well (so far).

In the past, It used to take ~10 minutes to heat the heatbed to 110C.

Any tips on what's going on?

Re: Strange heatbed heating problem
February 28, 2015 03:27AM
Check the connections at both ends of the cables supplying power to the heat bed. There may be a poor joint.

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Strange heatbed heating problem
February 28, 2015 01:24PM
There is probably a 100c limit set in the firmware. It can be over written to 110c while printing. Have you updated your firmware or host software recently? This might account for a change.

The popping sound you are hearing is moisture in your filament. Search for methods of drying your filament and pick the one you feel most comfortable with.
Re: Strange heatbed heating problem
March 12, 2015 02:36AM
Thanks for the tips. All seem right on the spot.

I tightened the screws on the RAMPS side (the connections to the heatbed itself came pre-connected).
Indeed, I updated the firmware recently, but I checked, and couldn't see anything that limits the temp to 100C.
The popping sound just solved itself - don't know how.

The heating problem exists. It doesn't even reach 100C. I can only heat up to ~90C, and when I actually print,
it gets a little higher (~95C). Any other thing I should check?

I see the following in my configuration:

#define ABS_PREHEAT_FAN_SPEED 255 // Insert Value between 0 and 255

Anything else I should check in my config (attached the full config)?

open | download - Configuration.h (33 KB)
Re: Strange heatbed heating problem
March 12, 2015 03:07AM
It's possible the heatbed PTC fuse in the RAMPS could be acting strange. You can check the voltage effectively reaching the heatbed while 1. heating and while 2. printing, if there is indeed a big difference then it's the fuse. Replace with a fuse holder and a fast automobile fuse rated 15A.
Re: Strange heatbed heating problem
March 12, 2015 06:12AM
If the ntc is at the center of the heated bed and you're printing your object at the center it could be that the heat of the object is transfered to the ntc through the glass plate, hence the warm up, but this is a very local temperature increase.

I had the same idea that it took very little time to heat to 120 degrees, like 5 min. on 12V but I forgot it was during testing without the glass plate and with a few sheets of kitchen paper towel on it for insulation. It could be that the heat characteristics have changed like cooling fans, ambient temp., print plate, insulation, draft from a window, winter vs summer etc.

Since I've switched to 24V for the heated bed only, the 110 degrees temp is reached within 5 min. From 21 degrees and I'm using PID. Once the temperature is reached, it's very constant with deviations of 0.1 degree. The mk2 heated bed is 12V only, but by using pwm at 127 (50%) it draws about 10A at 24V, just within the fuses limit. I've been running it for weeks without any problems.
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