Extruder temp drops while printing

I am running Marlin 2.0.7.2 on a basically custom cube printer. Printer was working great for about 5 or 6 prints, then it started acting oddly. The extruder heater is set to 230 at beginning of print. It has no problem reaching that temp and holding it, until it starts printing. Once it starts printing the extruder temp begins to drop slowly, and by slowly I mean 1 degree every minute or so. It will drop to min temp eventually at which time print fails with the min temp error.

I have changed the wiring from MOSFET right up to the the heater connection on the hotend. I am getting 30v from MOSFET, and same voltage all the way up to the connector where the heater element connects. I can't check voltage past that point so I stuck a new heater in it. No change. I am using the heater connection on motherboard as a trigger for the MOSFET, and the blue led on the MOSFET is on steady once heater drops more than 2 degrees below setting. Currently the print job has been running for 13 minutes, hotend temp is set at 230, but is actually reading 221. I don't think it can be the thermistor since it is such a slow drop in temp, and the thermistor is accurately measuring room temp prior to first print.

Any ideas? I have attached my config files.

Do you have a part cooling fan that is perhaps blowing on the nozzle or heater block?

What happens if you pause the print? Does the temperature rise again?

Another thought (although you seem to have already covered it) is that there's a bad connection somewhere in the heater wiring, so there is intermittent power supply to the heater as the hotend moves around. Can you put an LED in parallel with the heater element so you can see when it is actually getting power?

Quote
frankvdh
Do you have a part cooling fan that is perhaps blowing on the nozzle or heater block?

What happens if you pause the print? Does the temperature rise again?

Another thought (although you seem to have already covered it) is that there's a bad connection somewhere in the heater wiring, so there is intermittent power supply to the heater as the hotend moves around. Can you put an LED in parallel with the heater element so you can see when it is actually getting power?

Thanks for the reply.

I did the splice into the heater wires after the last connection already to check for power. I know that is not really a proper test, I can get a 30v reading but it may not be high enough current to run anything. Didn't have anything that was 24v to see if there was sufficient current.

Funny you mention the cooling fans blowing on the nozzle and bed. There are two of them blowing on the nozzle and bed(one each side) and a third one blowing on the "radiator" portion of the hotend. They were all running for all the builds since I got the printer running again the other day. Did not have any issues except for a slight curling on the corners of the item being printed. I was running a bed temp of 50 degrees, and the extruder temp of 230 for first layer, then dropping 10 degrees every 10 layers until 200 then staying there. After seeing the curling and looking for a solution, the first thing I tried was dropping the bed temp. No bed heater led to filament not wanting to stick, so I set it at 35 and set extruder to start at 220. Filament stuck just fine, but then I started having the issue of the temp dropping. After doing all the other things and posting here that it dawned on me that this started after dropping bed temp, and the only thing that I could figure that connected them were those two fans. Pulled the plug on those fans and problem solved. This mainboard (MKS SGEN_L v2.0) has two fan outputs, so I will have to see if I can get those two outputs to do dedicated fan control. Slicer only has control for one fan, but I could use that to control speed of those two fans, and just run the third fan at 100% to cool the radiator, or put an external speed control on it.

Yes, the radiator cooling fan can just be wired direct to the power supply. There's no benefit (apart from noise) in turning that off.

Ideally you want fine (about the same size as the extruder nozzle) jets of air aimed at the tip of the nozzle to cool the filament to solid as soon as it is extruded. Ideally, you want those air jets to be able to flow away freely to take the heat away..

• You might try adding a small outlet to limit the flow from your part fans to the point of the heater nozzle. But you may need quite high pressure (but low flow) to get it working well, and you get into all sorts of aerodynamics.
• Two fans opposite each other is bad (I don't know how you actually have them set up)... that will just create turbulence and make the air bounce around the place.
• I think that one fan is probably better than two, unless you have four at right angles and can change which fan is running depending on head direction.
• Probably quite low fan speeds will be better than high speeds.
• The ideal isn't achievable; those air jets will inevitably bounce off the part being built, so flows will change depending on what's already been printed, the direction the head is moving, and other stuff.

i have attached a pic from the site I got it from. It comes with 3 30mm fans, and I printed up an adaptor to put a 40mm in the hole where the one fan only blows through radiator fins. The other two are directed down at the base of the hotend and bed. I currently have it set up on two controls now, the radiator only fan is on all the time, and is also connected to the fan in the box over the motherboard and drivers. The two others are controlled by what the slicer put in the gcode file that is put on sdcard and put into the TFT35 to print. It was working quite well when the bed temp was at 50 and all three fans running 100%. Since I have been printing items that have 4 walls and a floor, once the floor is done the hotend is only printing about 1/2 inch in width so the air from the two fans is blowing mostly into free air space. I think that is why I did not see this issue until I lowered the bed temp.

Marlin looks like it will support turning fans on and off based on motor status. If the motors are enabled, it will run the fan, and once it is done printing, it will run the fans for however long you want once the motors are disabled. I think I will use that connection for the radiator and case fans, and just use the port that is used by slicer to control the others fans, based on layer level and desired speed at that level. I am using Simplify3d for slicing. Simplify3d by default does not turn on any motors until it is ready to print, so that will keep the single fan off since no motors are enabled until bed/extruder are up to temp, and the two other fans will not turn on until it meets what I specified in the slicer.

The extruder with the three fans and complete hotend was about $30CAN when I ordered. Only just set it up less than a week ago.

Thanks for the tips, I was thinking of upsizing the other two fans, but it looks like it will put the fans too low and end up hitting the bed long before the tip does. Unless I can find a curved adaptor. That may help in redirecting the air as well. Maybe I can fill in the current slot it is using with epoxy and drill holes at a steeper angle with a small drill bit, something like a 0.125 or a size or 2 smaller. Maybe just block off the top of the existing port.

Those axial fans are better at sucking than blowing. For blowing, you want a radial fan.