View Full Version : clogging due to 'pumping' action caused by quick succession retractions - solution?

04-17-2018, 10:53 PM

Didn't want to bump old threads so I thought I'd start a new one. So my Cube3 has been printing beautifully and consistently for months, but I am noticing that it is failing on tall thin prints (a 1 cm square pillar 13 cm tall). It will print fine for about 1/3rd of the print, then lots of stringing and under-extrusion, then no extrusion. I am printing at 200 microns, strong, criss cross infill. For example I tried printing four of these at once at it failed 2 cm into the pillar part https://www.thingiverse.com/thing:2400950

I read in previous post that lots of retractions in succession can lead to clogging (due to hot expanded filament being pulled up out of the meltzone and cooling in the expanded state), which I have noticed myself and I suspect is the reason. Is there any solution to this? Or must I just accept the limitations of the slicer...

My plastic roll is less than a month old, and I have printed about of half the roll no problems, and the prints look great. I am using the KISS method on a firmware hacked cube3, so it's the factory nozzle and tube and cartridge. Maybe the FTF extruder hub will dimish this problem?

<venting my frustrations over the internet> this really sucks because I was away from my printer for the print and after the extrusion stopped the printer kept going, cooking the plastic inside the nozzle for a few hours. Like cooking it reeeaaaallllly good. After 4 cold pulls (and some black stuff coming out of the nozzle) it still will not extrude properly :-( Looking inside with a flashlight I see plastic around the inside of the nozzle. I only have so many nozzles left... </venting>

04-18-2018, 07:05 AM
Ha, yes, I've been talking about this for quite some time.

I'll let you vent a bit more and I need some dinner... but after that, I will chime in.

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Okay, the problem is simple and the answer is "yes".

The melt in the nozzle is actually a much smaller problem than you'd think during.
Normally that is simply the barrel nut if you turn it "just a little further". I will explain.

You can also check the barrel nut at the filament driver but I suspect that was your first instinct.
For that I can certainly recommend the tapped M5 or M6 fitting directly into the aluminum housing.

The real culprit is two-fold, as you already guessed.
Not all prints have that "perfect storm" where the an entire length of filament has pumped back and forth a number of times for every millimeter of the filament in the tube.
That's a mouthful but it explains what I normally experience with stock carts and diamond fill. Perfectly good spool out of the box. Printed many things wonderfully!
This was the butter cream color. I pulled the full length of the tractor'd filament out of the stock Bowden boondoggle and noticed serious stiction, meaning it would bind with force, not difficult or impossible but also not free or minimal like it should be in Teflon. So the filament wasn't sliding through the tube anymore and the Bowden tube can stretch as much as 1/2" or better. So it is just pushing the tube, clicks to relieve the pressure, and you've lost a significant amount of material that should be on your print, not scooped into the drive gear. Not only that, even if it doesn't click/strip the filament, the compliance in the tube could drive the stretching effect of the tube to allow for maximum motor pressure by actually advancing the filament a bit, but by all rights, the machine did what it was suppose to do, manage extrusion by constant force, and on the next retract, no filament was actually "delivered".

All this is the symptoms of a marginal Bowden tube. A tube that has been over-heated and/or over-pressed to get the nylon spine on there. And if it's not, at least you know for sure if you do the simple test using the filament that failed from the very tube that I am saying it got "crowded" in because it was getting flatter and flatter every time the filament retracted. When it does nothing but dots to make pillars or fill a multitude of small corner fills, that is the real test of the stock cartridges. Many simply blow it off as a one-off failure. But when the same print fails over and over when everything else works fine, you know you have something you can analyze. The symptoms of a bound widened filament within a compromised factory Bowden tube is a recipe for failure. This is how I learned there are good over-mold tubes and there are bad ones. If the skin of the inner tube has been compromised (pinched reducing the ID), then it will not easily pass wide filament.

Also remember that plastic has a high rate of thermal expansion. Consider that the filament could be a couple of % bigger in diameter under service up and around the print head or over the build plate. Keep that in mind when you are testing the tube. The friction of the pumping action binding with the wall can generate its own heat and change the surface texture of the PLA. I only add this in that when you do this outside the cube, you might think it is not too bad, but it is actually a bit worse in practice, so to speak.

Oh, about the barrel nut: this is where I feel quite stupid in that I always needed to cinch that barrel nut up just a few threads. Little did I know all I was doing is choking the tube ID. It took a while to know to put in a 2mm drill shank or something and work the threads of the nut into the Teflon tube. Most casual attempts at this just go back to reducing the ID as soon as some heat get added. This is the case on the driver end too but there is no heat there so the problem is a little less severe.

So the melt that really bothers the hot end is not the nut so much. That melt will just neck down to a string at the barrel nut on pullbacks. And on forward drive, it has to refill the cavity it just stretched. This is not where to look... where it is more related to my previous paragraph (turning the nut a few turns more): the gap between the face of the aluminum nozzle (inside) and the Teflon tube you shove into it. If that gap final is much more than a 1/2mm or so, it will fill with plastic out to the OD of the tube making a donut pouch that never re-melts. This is where a perfectly good cartridge one day simply clogs the next after cooling and re-heating.

Now don't go thinking that a hub solution will solve all your problems. Although many never experience this level of annoyance with this, I will tell you from experience that there is a lot of 4mm OD and 2mm ID tubes out there that are too small on the ID. I will happily send you those that fail -my- inspection for the cost of postage. And along with that warning, I will also warn against other types of tubing that are not -quit- PTFE. I've tried PET, for instance. We don't need to extreme heat tolerance of PTFE, right? (affirmative!) But we do need its slickness. PLA just loves to leave skid marks on PET, which is a thermoplastic! I trust you get the analogy... it is simply excess stiction that creates higher forces in the filament drive motors. And it is that stiction which you are fighting, creating these "thin" prints.

There is one more failure mode I should put down while it is in my mind. Many cartridges had their hot end assembled with "assembly aids". I've seen black assembly aids and tan colored assembly aids. I know... "it is filament working its way along the tube!"...
Yes, filament will work its way around the threads in the stock piece of tube in the stainless steel tube and provide a thin coating of plastic.
The stuff I am talking about is gummy like tar. This stuff can clog a nozzle, and if the nozzle is run at ABS temps, it can create a carbon build-up in the nozzle. That is not something that is easily removed, if at all. They used a caulking to hold the tip on with the spring while the close up the assembly. But the stuff migrated... normally outward, but when you rebuild it, you shove this stuff into the nozzle and !hope! it comes out. The only thing worse than carbon buildup is a good side chunk of Teflon coming down the pipe. Good thing is, care of your nozzles will let them last a long time. One the flipside, run metallic or carbon fiber embedded filament and the nozzle will self destruct in a matter of a dozen ounces of filament rather than multiple 1 Kg spools. Poor care as in switching the same nozzle with PLA and ABS. PLA will carbonize at 260*C. I've seen marshmallow browning on 3DS white ABS for Pete's sake! Of course -any- remaining carbon will potentially build into an adhering carbon plaque. Good thing is, there are ways to clean and manage your valuable nozzles with some simple techniques.

If you plan to run your printer a lot, you'll make sense of every word on this page at some point.
And I welcome anyone to challenge any specifics of this post. I may learn something and I may have to correct something I said.
But my convictions as they stand today, with a lot of experience and backing (and expense), it does require some convincing.
I don't expect anyone to fully comprehend all these words, nor do I expect anyone to understand the words the same way I mean them.
Give me the change to clarify, and I will be happy to participate.
Tell me I'm flat out wrong; and I will wish you all the luck in the world with your printer.
I don't really expect anyone to report back that this information was on the right track after all.
But if you did, and we can learn something together, then I will whitewash the walls of the delusion happily!
If you cannot see above that I can be nothing but humble with my own mistakes, then we don't need a conversation.
After all, I once said all cartridges are salvageable until I learned about the subject of this thread.
I should say, "Most cartridges will make some good prints, but only 1/2 the stock cartridges will always make good prints.".
Now you see why people here and on the Open Build forum went on a rampage to solve this problem because the rest of the printer is a jewel.

Did you donate to the coders' fund on Open Builds yet? :p Kidding! Not affiliated! But I did give some.

05-25-2018, 08:29 PM
Hi, so a print failed again due to this 'pumping' problem. Parts I had printed many times before using the same settings and filament, failed on a tall, thin section (but really not that thin...).

The filament had been ground in the extruder hub, so I had to disassemble the hub and the print head to get this filament out, eventually I had just the tubing with the filament stuck inside. Lo and behold the filament is jammed right at the print headbarrel nut. I couldn't pull the filament out with pliers, I also could not push it out with a thin allen key. The filament was jammed in there good, so I had to unscrew the barrel the nut until it was past the problem zone and push out the filament.

Worth noting:
-the filament stuck in the barrel nut was smooth all around (no teeth marks) and was not discoloured. This indicates that it had been melted sufficiently before being retracted up.
- The filament was actually stuck in the 'up' position from a retraction, when I disassembled the head and pulled the nozzle off, there was no plastic in the nozzle at all, and looking at it (with a light source behind it) could see the aperture clearly as well as light reflecting off the inside wall. This indicates that the nozzle is not clogged. (I assume that any plastic in there had dripped out, since the printer kept on going for another 1.5 hours).

I figure a simple solution would be to drill out the tube inside the print head, especially on the inside of the barrel nut. A 5/64th drill bit fits perfectly inside the tube, so I put the bit all the way through the barrel nut, with the ends aligned (so I don't puncture the tube with the pointy end of the bit). Drilling at very low speed and gently wiggling the barrel nut against the side of the bit (VERY gently) seemed to work, some plastic tube shreds came out when I pulled the bit out. I could then easily slide the previously jammed filament through.

Hopefully this is help prevent the problem... but I still think that barrel nut so close to the melt zone is major design flaw.

05-25-2018, 10:45 PM
It is indeed a serious design flaw. I've been saying that for how long now? Drilling it may be a temporary fix.

It is positive that this is where your cartridge hung up. The melt bulb that got stuck is fairly normal in operation. The more it pumps, the higher up in the tube the heat travels. The fact that you has a completely clean nozzle is definitely proof that it was doing exactly that for some time.

Have you considered one of the many options for a different nozzle configuration?

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BTW, when I pull the Bowden tube from a nozzle, it always has that fully melted bulb. Even without the barrel nut, this melt bulb will not go back through the Bowden tube. I have to either snip it off or snip the filament before the driver and push it through.

Remember that this condition is slightly different. In my case is a given ID for my 4mm OD and 2mm ID Teflon tubing and another ID in the stock 2.8mm OD tubing.
One day I will get a true measurement, but for now I consider the stock tubing to have a slightly larger ID than common 2mm ID Bowden tubes.