First off, you need the right extruder design. Specifically, the filament path between the drive gear or hob and hot end must be extremely well-constrained. 1.75mm TPE filaments (thermoplastic elastomers) will buckle in an instant if given the chance. That means they will try to squeeze out of any little opening in the filament drive path rather than being forced through the nozzle.
- Is there a gap >1mm anywhere between your extruder drive gear and hot end? Including the groove in the drive gear itself. If so, either change extruders or print something to fill the gap. Many popular extruders already have Ninjaflex conversion parts uploaded on Thingiverse or Youmagine.
- Are you using a 1.75mm bowden drive? You probably shouldn't bother with the softer TPEs like Ninaflex. Harder TPEs like Semiflex may be ok, but it's still difficult. 3mm bowden drives may perform ok. Direct drive extruders are highly recommend.
- TPUs (thermoplastic urethanes) in particular can be sticky in the filament feed path between the spool and extruder. Try to minimize corners and turns (even inside PTFE tubes) between the extruder and spool. As a general rule of thumb, don't exceed 180 degrees of cumulative tube curvature in the entire path from the spool to the nozzle.
Once your filament feed path is fully enclosed, the filament will be constrained and unable to squeeze out the side or wrap around your drive gear. That's the most important step.
The next problem is simply loading the filament. Purging out PLA or ABS with a soft TPE can be challenging because of the force required to purge the old material. Many default loading routines are actually too fast for TPEs and will cause the TPE filament to bunch up rather than purging the previous material. It tends to extrude better when there is nothing else in the way. Some tips:
- Do some cold-pulls to clear out the hot end as much as possible before loading the TPE. (Nylon is ideal for this. Preheat to nylon temps, run through some nylon, let the extruder cool to ~180C, and then forcefully pull out the nylon. If you can't pull it out, pull hotter. If it leaves nylon behind, pull colder.)
- If you have a high-melting plastic like PET or nylon in the hot end, purge with a lower-melting solid filament like PLA first. This will reduce the viscosity of the old material when inserting the TPE.
- If your printer has a fixed loading speed, consider making a gcode file with nothing but a preheat and very slow extruder move as a custom loading script.
- If you fail to load and jam up the TPE, try again! Sometimes it takes a few load/unload cycles to get the old material purged and a clean feed going.
The final issue is print settings. You ever hear the saying, "you can't push a rope"? That's kind of what you're fighting with TPEs. With a properly constrained feed path, you can push rubber, but not very hard. So minimizing extrusion force is the name of the game.
- Print SLOW to start. Like 10mm/s. You can dial it up once you're getting good results. This will minimize nozzle back-pressure and reduce the amount of "pushing a rope" that the extruder drive must do.
- Retract as little as possible. Turn off "optional" retractions in your slicer, such as during layer changes. Some people even print with no retraction at all, and use high travel speeds and coast/wipe features to minimize stringing. That's overkill, but it can help with marginal extruders. I personally add about 50% to my normal ABS/PLA retraction distance.
- Print on the hotter side of the recommended range to start, then dial down the temp as needed to reduce stringing and oozing. Printing hot will reduce nozzle back-pressure.
With all that, it should be possible to print the softest TPEs with reasonable success. But if you really can't get it working right, plenty of higher-durometer TPEs are available now that are significantly easier to extrude.