When Is A Clog Not A Clog?

My printer has been a little fussy lately. I was printing some PETG and didn’t notice any problems, but as soon as I switched to PLA the flow of the plastic out the nozzle would slow down for a bit, then go back to normal, then slow down again.

I’ve been running a 0.6mm nozzle for a while now and while it’s bigger than the stock nozzle, it can still clog, so I went through all of the usual steps – pulls at various temperatures, using the cleaning needle, trying to force the filament through by hand, but nothing seemed to fix the problem.

It was acting different than the clogs I was accustomed to, too. Normally a clog is kind of consistent – filament comes out in a spiral or diagonally or some other strange pattern. This time it was coming out nice and straight, and I didn’t notice any problems when running the extruder 10mm at a time.

Once I tried to extrude more than about 20mm, things started to act weird. Filament would come out the nozzle properly, then stop for a second or two while the extruder motor teeth turned and ground against the filament, then flow normally again. I was out of clean nozzles to try and figured that I was going to have to order another one, but the pausing had me thinking.

There are some differences between PLA and PETG. PETG usually runs at a higher temperature than PLA, but I’d even tried doing pulls with PETG to clean the nozzle and had no luck. But… I usually print PETG at about 30mm/s, while I print PLA around 50mm/s.

I thought about it some more and the next thing I came up with was that the hotend on the printer wasn’t heating enough, or was heating and cooling in cycles. It’s the same one that came with the printer, but after confirming the temperature on the display with a non-contact thermometer, I was pretty sure that wasn’t the problem.

Then I thought about the filament path from the extruder motor to the nozzle tip. On the CR-10s (and a lot of other printers), a PTFE Bowden tube connects the extruder motor assembly to the hotend. I wondered if the hotend and nozzle weren’t the problem at all. I slowly extruded filament until it was flowing out of the nozzle, then marked on the filament where it went into the extruder, then pulled it out. Then I reinserted it by hand and sure enough, there was resistance before the line on the filament had made it back to where it had been. So something was jamming it before it got to the nozzle.

I did some more pondering and then figured it out – the filament was jamming just as it was entering the hotend. As the filament heated up and melted, it went past the jam very easily, until unmelted plastic got to the same spot and jammed. A second or two later there was enough heat to melt it again, so it moved again. The reason PETG printed and PLA didn’t was because the hotend could melt the filament back far enough at the low print speed of the PETG to keep it moving, but with PLA there was too much cold filament too quickly.

A bit of swearing later (I no longer have a fingerprint on my left index finger), I had removed the Bowden tube from the hotend. This is what I found:

Melted Bowden Tube
That doesn’t look right…
Melted Bowden Tube
Ah-HA! Melted inwards. A little darkened, too.

The Bowden tube had melted inward, partially blocking the tube. I don’t know how long this has been going on, but a quick check of my notes showed that since I got the printer I’ve been running it more and more slowly, so it may have been a while.

A quick (and carefully measured) straight cut to freshen up the end and it went back into the printer… and it’s been printing beautifully since then. I haven’t tried upping the speed back to 70-80mm/s with PLA but I may do some experimenting with that soon.

Not bad for a year-old piece of plastic tubing that’s constantly exposed to temperatures above 230C. The printer came with a spare just in case, but I did some looking around and there are aftermarket tubes that are rated for higher temperatures. Might be something to look into at some point.

Long story short: if your printer is printing in a weird stop/go pattern and the extruder motor is running properly, it may be the tube, not the nozzle.

Oh, and this is as good a point as any for a reminder – if you have a 3D printer, INSTALL A SMOKE DETECTOR NEARBY. Melting plastic is generally good when printing, as long as it’s the plastic you intended to melt!

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