Heated Bed Turing off once extruder fan come on!

Okay so printer was working fine in PLA. Now moving to ABS i had to add cork to the bottom of the printbed to get to 110*c (it can get upto around 120 now!), but it keeps cutting out after a few layers and seems to coincie with the extruder fan coming on. Once it drops to 100*c the part knocks off the print bed and game over! I thought it may be the current draw so added a second psu to drive the heatbed independantly but still the same. Any ideas as to where to go next?

Thanks
DemonsRun

Check the heated bed mosfet and see how hot its running. It may be neccesary to replace it with a better one. The irlr7843 would be a good choice for example.

Edited 1 time(s). Last edit at 07/26/2015 03:48PM by imqqmi.

Hi

Thanks for the quick reply. I have held the mosfet and while it is warm it is not red hot (i can pinch it whithout buring!). And i have a fan trained on it (and my stepper drivers). Also i have just disconnected the extruder fan and it works now... would the fan be drawing that much current? or does it point to somthing else? I will look how much a new mosfet is and swap if theres nothing else to try just in case.

Thanks

Hi

I have done a little more investigation tonight and it is quite wired! If you run a print and then send M106 S255 (to turn the fan on) the the printbed heat looses heat. Send M106 S0 and the fan goes off and the bed starts to heat up again.

At first i thought it may be a power issue and @imqqmi suggested that the mosfet mat not be up to it. But am i right in thinking that the heated bed is on 11amp rail and d9 is on the 5 amp rail? so not sure why that would happen??

Is it still worth me changing the mosfet?

Any ideas because I have none!

Thanks

Perhaps the fan is blowing cool air over the heat bed and cooling it down? Or does the heated bed LED go off too? When printing ABS you shouldn't use a print cooling fan anyway, just a hot end fan, which should be wired to 12V permanently.

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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].

Hi. Thanks for the reply.

Tried that by having the printhead way up and the yellow pages over the printbed and it still lost heat. The LED stays on too.

I have a perm on fan on the hot end and a pwm one on the extruder. Whats tge reason for not using it with ABS, would you not need it for bridging ect? Or am i missing somthing...

Thanks again for the reply

Can you measure the voltage across the heat bed with and without the print cooling fan on? What sort of fan is it? Most 12V fans take only a few hundred mA, but maybe yours takes a lot more and the power supply voltage is dropping.

The reason for not using a print cooling fan with ABS is that because of the higher extrusion temperature, the extruded ABS cools faster anyway. If you cool it more, it will not adhere well to the previous layer, resulting in delamination. I've not done any long bridges in ABS, I guess a fan on a low setting turned on just for bridges might help.

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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].

Hi

Thanks again.

As requested the voltage drops:
During Innitial Heating: 11.26V
Once Printing with no fan on: 11.23-11.26V
Once Printing with fan on: 11.21-11.24V

Which, to me, doesnt seem that great a difference.

I didnt realise tho that you did not require the fan for ABS, which is all im printing in ATM so will prob just grin and bear it.
Im also looking into a dual extruder so the fan pin will get moved to D4 then anyways.

Whilst im on it are there any other major differences between printing PLA vs ABS apart from temps and the fan?

Loving how much easier ABS is to get to stick vs PLA on masking tape!

Thanks everyone

I agree, that's not much of a difference in voltage. If your heated bed is using PID control, you could try changing it to bang-bang in case there is an interaction between different PID channels on your electronics and firmware.

PLA is easier to print than ABS. It is less prone to warping and doesn't produce anything like the same level of noxious fumes and particulates. Using a heated bed, it can be printed on plain glass, as well as on Kapton tape or glass coated with hairspray. But ABS plastic is tougher and has more give than PLA.

There is also PETG which is claimed to have some of the advantages of both PLA and ABS, but I haven't tried it yet.

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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].

I'm not sure what your power supply voltage is, but I'd be more concerned that you are losing that much voltage tomthe bed, period. That, or the power supply is weak - you might try the same test, but meter both the board input terminals, the board output terminals, and the power supply. Supply OK and board low? -> too small on the wiring supply to board. Supply OK and board OK? -> crappy MOSFETS. Low supply? -> Power supply not up to the task. Board out OK and bed low? -> too small on the wiring board to bed. The interaction with the fan, though, makes me think power supply, wiring supply to board, or board itself. A couple of quick calcs show that on a 100w heatbed, dropping to 11.25v, it will only output 85 . . .

- Tim