as I read the comments of mcgiver found the missing clue for my observations. I was evaluating the impact of different extrusion speeds and got a clogged nozzle due to the blockage of the filament in the PTFE tube.
In this picture you can see on the right side the original filament and on the left the widened part as long as the isolator guide.
I have worked out an explanation which I would like to share and discuss.
So lets start with a stupid comment about what we are doing:
We are pushing a solid filament into a heated nozzle in order to get material out of the nozzle tip.
But how is it working in detail? Once thing is clear: we have to liquefy the filament in order to press it through the nozzle. With the proper temperature the material becomes as liquid as water. Now we have only to apply pressure to the liquid(!) to push it through the nozzle. But only pushing the filament is not doing the job. Like water the molten filament will flow to the minimum resistance, which in this case is the PTFE tube. The area by the gap between the inner PTFE diameter (1.8mm) and the filament (1.75mm) is larger than the nozzle cross section, so the majority of the liquid will flow trough the PTFE tube in the isolator.
Now two main mechanisms will take place in combination:
- Since the PTFE tube is not heated, the liquid will cool down since, after some distance, it will solidify again. Hereby it will fill the whole cross section of the PTFE tube with solid filament, forming a seal for the liquid. Having this seal any movement of the filament towards the nozzle will build up a pressure in the liquid and start the extrusion through the nozzle.
- this sealing takes place at temperatures around 150°C. At these temperatures nearly all filaments are no longer solid but ductile. This means, putting a pressure on the material will widen the diameter until the full diameter of the PTFE tube is filled. This will increase the length of the seal further.
So it is necessary to build up such a sealing, but with increasing seal length the contact area between filament-seal and PTFE tube is increasing too an we need more force to move the filament within the guide. Since the force of the extruder motor is limited, there is a condition when the seal length is too long for the extruder to move the filament: a clogged nozzle.
From this described mechanism we can derive two precautions for preventing nozzle clogging:
- provide good cooling to the PTFE tube, i.e. do turn on the fan as standard (low to medium values are sufficient)
- print at low to moderate extrusion rates. With increasing extrusion rate you have to increase the pressure inside the system, which let it move the seal point more far into the PTFE guide and which will increase the seal length due to ductile deformation.
And the clogging due to retraction? Just remember to pull a chewing gum: it will become thinner in the middle.
The same will happen to the Filament. The ductile material at the end of the seal will be pulled and becoming thinner. The more you pull the more material of the seal will be removed, loosing the contact to the tube sidewall and finally leave a gap. Since the pressure inside the system was released by the retraction too, nothing bad will happen now.
But after retraction the filament is pushed forward a large amount in short time, giving the liquid a lot of possibilities to flow trough the opened gap towards the PTFE tube before creating a new seal with elongated length.
By a repeated procedure this will end up in a seal length which cannot be handled any longer by the extruder motor.
So the issue at retraction is to maintain the initially generated seal. This implies, that the retraction length should be minimized, the always given retraction length of 3mm is a good starting point, however, I would try to reduce it.
Finally, there may arise the question which is the proper extrusion speed. Just try a simple setup:
Put your print head in a similar condition like during filament load/unload (G1 X100 Y100 Z100), switch on the heater to the working extrusion temperature for the filament, switch on the fan (M106 S100), and set the printer in to relative mode (M83).
After reaching the nominal temperature do a manual extrusion of a larger filament part (5cm: G1 Tx E50 F100) with x the selected nozzle (0: default or 1).
If you hear a clicking at the extruder motor your pushing too fast, the motor cannot handle the necessary pressure. Adjust the F number to a value where the extrusion is running smoothly (without clicking).
According to my observations the final result is in contrary to high speed prints. Any feedback on this experiment will be appreciated.