Prevent stopping and heating in the middle of the print

Question

In my latest 3D model, the Prusa i3 MK3 stops, heats and then continues the print. However, there's no noticeable drop in temperature. The display shows 210 °C/90 °C (nozzle/bed) as expected. I have printed the same part 3 times now and the behavior is similar. I've seen a maximum of 211 °C/90 °C and a minimum of 209 °C/89 °C.

Unfortunatly, it seems that stopping the head seems no good idea. The nozzle is very close to the object and it seems as if either there's a bit of filament leaking, causing too much filament, or the existing filament is melting.

The irregularities are always in the same XYZ positions. The print is still usable for my needs, but I'd like to understand this behavior better and find a fix, if possible. My meterial is HDGlass (PETG) and I'm printing with default settings, 20 % infill and first layer 200 °C, following layers 210 °C.

I've looked up the print quality troubleshooting guide. It looks a bit like blobs and zits, just much less and much more subtle than in the example there. Also, the solution is not applicable, because Slic3r does not show a planned retraction in those positions.

My model is available online and the issue appears near the large bottom layers and the large top layers.

My questions are:

• what is the problem called / what problem category do I have here?
• what is a potential fix?
• since the filament also extrudes at 200 °C, can I prevent the printer from stopping the movements unless the nozzle temperature drops below e.g. 207 °C, although the nominal temperature should be 210 °C?

Answer 1

Your allowable temperature tolerance might be too low.

You have seen what I call emergency-halt the print on getting too far away from the set print-temperature. As described, the allowable tolerance seems to be 1 degree, which means pretty much any variation in the printer head's temperature will trigger the stop before the software even can counteract it.

To counteract this, you might try to increase your allowable range slightly, for example to about 2.5 of more degrees, either way, giving the printer time to compensate without halting the print.

How the tolerance area works

• Let's assume the printer has 210 degrees with a stable heat input from the cartridge and monitored by the thermosensor.
• conditions are met to start printing.
• Filament is pushed into the nozzle, picking up heat energy from the heater block.
• The temperature of the heater block drops, as the thermal energy (=heat) input from the cartridge did not yet include the new thermal energy drain in the shape of the filament.
• The Thermosensor registers the thermal drop a little bit later and notifies the chip
• The chip increases the voltage on the heater cartridge with a little time difference
  • With a very low tolerance, it is possible to fall below the threshold before the cartridge could heat up to compensate
• the cartridge pumps in way more heat than needed, raising the temperature rapidly back into the print-temperature tolerance area...
• ...and overshoots the aimed for 210 degrees.
• The thermosensor only registers the aimed temperature with a short delay
• the chip cuts the voltage to the heater cartridge lower
• but there is still a lot of extra thermal energy banked in the heater cartridge, heating the block over the aimed 210 degrees
  • which again can trigger the stop for the temperature rose over the allowable tolerance.