PLA Suddenly Jamming Mid-Print

[Alibert]

In a makerbot-type printer (such as the Qidi Tech you have) the cooling block (the large alu bar which is used to fix the extruder with) will get hotter than, for example a E3D type cooling block. This is normal and a by-product of the design. However, you should be able to touch it without problems.

There are two things that determine the temperature of this block: 1) the amount of heat put into it (from the hotend) and 2) the amount of heat being taken out of it (via the cooling fins and fan).

As to 1): Do you still have the glass-fiber insulation around the heating block? If not, buy some from aliexpress and re-do it.

As to 2): Is the fan running? Is the cooling fin part with the flat side to the cooling block and the fins to the fan? You can improve the heat transfer by putting some thermal paste (as for CPU's in PC's) between the fins and the block (makes it a bit messy when dismantling though).

Finally, the makerbot design was intended to have ptfe lined throats (the part between the cooling block and the hotend) as the cooling block can not make a sharp temperature transition in the throat. If you want to print PLA, you will need this ptfe liner there.

As a beside: I have just installed a micro-swiss all-metal upgrade in one of my Qidi printers to be able to print in the 250-300 C range (polycarbonate and PEI). This one has a two-part throat, an alu outer part (good thermal conductance) and a steel inner/low part (poor thermal conductance) to the heating block. With extra thermal paste between the alu outer part and the cooling block it is claimed to be able to achieve the sharp temperature transition needed. Just tested it at 265 C (PC-MAX polycarbonate) and it worked just fine, no jamming. Will be doing PEI later this week when the sampel comes in (at 280 C).

One last final thought:
I have had a heck of a time in repairing the printer of the local primary school. It would also give all kinds of grief as to jamming, extruder grinding and patchy extrusion. It turned out that the thermistor leads would make contact and short at random times by vibration/head movement (the leads were jammend under a fixing screw keeping the fan in place). This would make the mainboard 'think' that the temperature was suddenly 300 C and it would stop heating. As the heating block cooled down through lack of heating the plastic would solidify and jam. I found that out by using a monitor function to monitor the temperature, heat the hot end to 150 C, and then wiggle the leads of the thermistor. When I did that I saw the monitored temperature jumping up and down. Took me a long time to find that one.

[printbus]

Not sure any of this helps, but...

a) Stepper motors are designed to run hot. I've never seen the temperature on spec sheets, but multiple sites mention stepper motors capable of operating at temperatures of 85 to 100 degrees C. That's hot. They are intended for heat to pulled out of the mounting surface of the motor, something 3D printers rarely do a good job of.

b) Based on the data sheet, those 42HB34F08AB 06 are lower current/higher coil resistance than what at least the reprap wiki recommends for 3d printer use. This could explain why these motors run hotter than others for a given current setting on the stepper driver.

c) As Bob mentioned, you'd typically have issues with the stepper driver before you'd have problems with the motor itself. The stepper driver chips are designed to simply stop driving the motor if the chip gets too hot. Motors don't have any kind of protection built into them.

d) Be aware that once enabled, stepper motors ALWAYS have current flowing through them, and thus are ALWAYS consuming power and generating heat. The only time this isn't true is when the printer firmware has the stepper motors disabled. Once the stepper drivers are enabled, coils are energized to act as a brake and provide the motor with holding torque. This could explain why motors are cool when the printer idles with the hot end hot but then heat up in use.