Very low extrusion rate after PLA clog

Hi all,

A couple weeks ago I was trying to print something using PLA at very low speeds, and I was hit by the somewhat infamous heat creep issue where filament ends up melting in the PTFE tube. I took my extruder apart and was able to clear the clog after soaking the tube in some boiling water to soften the PLA that was stuck in the tube.

A couple of weeks passed and life intervened so I havent gotten back to printing. But today I was finally able to try again. I wanted to print out some very simple calibration parts to try end better understand some of the various settings in the SLIC3R profiles, since blindly setting things to be very slow is probably what ended up causing the clog in the first place.

I am finding today that the extruder is barely laying down any material. I’m just getting a very thin, wispy strand laying down on the bed (or rather, that’s mostly failing to adhere to the bed because there’s so little of it). If I manually force filament in to the head it flows, but lacking a force probe in my hand I cant say for sure whether I had to push this hard in the past. Is the problem likely to be due to some error I made in reassembling the extruder head, or will I have to take things apart and do a more thorough job of cleaning the PTFE tube?

If I have to better clean the tube, how should I go about doing that? Everyone always says “soak it in acetone” but my understanding is that acetone is not effective on PLA. I would give it a shot but I dont have any on hand. My research has never really turned up what would be an effective solvent for PLA. I use isopropyl alcohol to clean my print bed but I think that’s mostly just removing my finger oils from the bed. Is the lack of a good, relatively safe solvent one of the reasons why some people stay away from PLA?

Would it be easier to just replace the stupid tube? I probably would have ordered a new one already if I had seen a 3mm tube on the Lulzbot shop.


Did you clean the teeth out of the hobbled bolt?

You probably still have something in the hot end impeding flow. I would disassemble and verify. I have run an 1/8" drill bit BY HAND, through the ptfe tube on my hot end after a really bad jam and it has worked out well for me. Just make sure no debris remains in your hot end. Also make sure your hobbled bolt is clean as previously mentioned and make sure the tension on your idler is good. I cranked the thumb screws down while printing the bed level gcode until I had good extrusion. This was after I had made sure my nozzle was cleaned.

Thanks for the replies! I’m just getting back to my printer after the holiday.

I think that I have mostly resolved the problem, but I do still have a question about idler tension aimed at trying to avoid this problem again in the future. I spent a bunch of time trying various things to clean up and make sure I have properly re-assembled the head today, and I think that part is looking good.

However, after doing all of that I was still having issues. After a short time through a test print I would hear a popping noise from the head followed by a dramatic drop in flow rate. Looking at the filament after stopping the print revealed that the hobbed bolt had chewed a small indent in the filament, slackening the idler and making it impossible to feed forward.

I have had better results with a different color filament that seems to be a little less soft, but I think the real variable is the idler tension. How tight do I really want to make this thing? Everything I’ve seen has been pretty qualitative in describing the tension needed (tighten it until you get a “good” flow rate? Easy to do if you know what you’re looking for, not so much if you’re a noob like me). So far I have assumed that tighter is better, so I have been nearly completely compressing the springs. But after my testing today it is looking like that is probably a contributor to, if not the main cause, of the hobbed bolt chewing up my filament. I set the tension much looser and so far it’s looking good.

Thanks again for the suggestions, so far the Lulzbot community has been a big help in getting me up to speed on how to use this beast of a printer. It says something about the quality of the hardware that even though I’m very new at this and have had several issues, the prints I have made are typically of much higher quality than other pictures I have seen posted online from people who are new at 3d printing and looking for assistance.

For my idler tension I started with the recommended 8-10mm, but with the PLA I have the extruder would chew it up after a few minutes. This happened almost immediately after clearing out yet another jam about a week ago. So I sat and watched my extruder while manually extruding through Pronterface. It would go good until I heard a click then start to chew it up. So after cleaning that out for about the 5th time in an hour I marked my idler bearing with a sharpie, and tried again. I found that it was not turning at nearly the same rate as the nut on the end of the hobbed bolt. In fact it was barely moving at all. It was like lightning struck my brain, I loaded up the bed leveling print and adjusted the idler screws in and out until I had good, solid lines on the print bed. They ended up a lot tighter than I thought they should be, but since I have had no issues with retractions eating up the filament I keep them where they are. The springs are almost at full compression and I measure about 6 or 7mm, but it has been working so far. My ABS on the other hand likes the tension a little looser, I would say about 10-11mm.

Now I wouldn’t get caught up on the numbers, I have noticed that 1/2 a turn on the thumb screws can make all the difference between not enough and too much tension. Just like with adjust the Z end stop for getting the first layer dialed in, it will take some guessing and tweaking.

I hope it helps you out.