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fluxline

Jam, Bowden tube shrinking

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So I've had the UM about a month now, initial assembly went ok beside a few defects like z axis wobble, extruder feeder motor gear seated incorrectly/rough edges, stripped threads on end stop sensor, burs on threads for extruded tube, ... So the delivered stuff isn't perfect but can be fixed ( besides the z axis spindle rod, I'm guess I need to live with the wobble).

But something I cannot fix is shrinking of the Bowden tube at the hot end, and I'm not willing to dismantle and cut off part that has shrunk once a month. I measured the tube at original install at greater than 3mm, now it is at 2.8. I had a jam over the weekend with nozzle being clean but filament end to big to, pass through Bowden tube at hot end. Got the filament out, clipped the end and started printing only to have the jam return. Clipped the end again, put it back through with nozzle and peek off and it does not fit through. Even a non feeder fed piece does not fit through without a hard push.

I have never run higher than 230, and I think this should be a very save temperature to run on. It is also mounted inside the peek with a correct distance as measured from the top of the peek inside to the brass tube (5.6mm), Bowden sticking out from bottom of wood housing (tube pulled up as if blue retainer installed) 6.6mm. The 1mm gap between the aluminum plate and wood shows as in the assembly pictures.

So is one of the monthly maintenance tasks to cut off the end of the Bowden because of shrinkage? I've seen other posts of tube shrinkage but nothing resolved. Any advice on where to go from here, because I don't think the solution is clipping the tube once a month and I doubt a new tube will solve the problem. Thanks in advance for any tips

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besides the z axis spindle rod, I'm guess I need to live with the wobble).

almost every acme rod has some wobble, and that's why the acme nut on the z stage has so much play. wobble doesn't affect the print much/at all.

 

But something I cannot fix is shrinking of the Bowden tube at the hot end, and I'm not willing to dismantle and cut off part that has shrunk once a month. I measured the tube at original install at greater than 3mm, now it is at 2.8. I had a jam over the weekend with nozzle being clean but filament end to big to, pass through Bowden tube at hot end. Got the filament out, clipped the end and started printing only to have the jam return. Clipped the end again, put it back through with nozzle and peek off and it does not fit through. Even a non feeder fed piece does not fit through without a hard push.

the PFA tube doesn't shrink. if anything, it gets soft (past 250C), and will deform from too much pressure from the 4 screws on the head. under normal operation, there is no need to cut the bowden more than once or twice in a year, probably less.

I would suggest you dial back a bit on the pressure with the 4 screws, making sure the bowden-brass interface is tightly sealed, without overcompressing it.

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First, thanks for the reply. The current recommendation is a 7mm protrusion from the bottom of the wood. I measured the distance of the peek wood interface to the brass tube inside the peek at 5.6mm, added the 1mm gap as shown and so protruded the Bowden from the bottom of the wood 6.6mm with tube pulled up as if the blue clip was in place. The 4 screws were hand tightened and wood alu plate gap remained at about 1mm. I really don't see how I could have over tightened things, any less and I think I would get leakage at the Bowden peek interface.

Unless I see some other ideas, i'll cut and go again with your suggestion but now that I look more I see several other similar threads with jams but no obvious clogs in the tube nor leakage ... Just no mor extrusion.

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I also experienced deformation of the end of the bowden.

I _may_ have had too much pressure due to over-tightening the 4 screws - pretty understandable when you are a new user and trying to stop plugs. Unfortunately over-tightening probably increases the chance of the white clip giving way.

I was only printing at 210, although there is all the unknown inaccuracy in the temperature measurement - and I don't have a reliable way to measure the temperature via a different device.

This is the first I've heard of having to trim the bowden on a regular basis though - does anyone else do this?

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It is VERY unwise to trim your bowden tube.

It must be 100% perfect flat, like: |_|

If it is somewhat crooked it automatically always has a gap what

will cause leakage. We take real good measurements to make sure every bowden tube is flat on the bottom but if

you are experiencing leakage, it could never hurt to check. You could check by placing the bottom on a flat surface like a table. If you decide to cut it, make sure it is 100% flat. But if you would go to support for a new bowden tube, we will most likely consider your bowden tube as unreliable. (we have to have some sort of guidelines).

When you are assembling your extrusion head, and you are leaving a small gap at the side where your hot end is, make sure this is because you can not tighten it any further, instead of leaving a gap because it says so in the manual.

If you feel like you can tighten the 4 long screws all the way, then you most likely have a gap between in your PEEK.

Try to lower your bowden tube by 0,5mm/1mm and see if this makes a difference.

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With a Bowden tube that no longer fits for whatever reason, the only solution is to cut the best you can. I made a jig with an aluminum plate and 6.3mm hole and sliced it with a scalpel. The end appears square and clean. Lightly finger tightened the 4 head bolts, a bit more on the bolt over the Bowden tube to make sure they mated. The results are little different than the original. I failed to measure the inside/diameter of the resulting tube, felt like an idiot when all was assembled and ran for 30 seconds before no more extrusion. Assumed a jam, but after tearing apart nothing inside. Filament was sticking inside Bowden tube and could not pass through. What I have now is after running the rebuild:

- PEEK (2 samples) ID 6.32mm/6.38mm

- Bowden OD at feeder: 6.48mm

- Bowden OD at hot end, directly under alu plate: 6.48mm

- Bowden OD at hot end, at very end: 6.23mm

- Bowden ID at hot end, at very end (what was inside peek), last night at fail: 2.89mm

- Bowden ID at hot end, at very end (what was inside peek), now: 3.00mm

I've also measured the filament and see variance from approx 2.9 - 3-1. I had measured while running at various times over the last month and was always around 2.9, nothing above 3, so that seems to have increased towards the middle of the real. Maybe they only quality check at the beginning of the reel and this one passed but had some larger diameter inside.

So, i will cut again, measure this time and and see if it works. Contacted UM and they will replace the Bowden tube and filament. They recommend that I wait for the new head being released and buy that as it is more forgiving of multi-diameter filament.

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I won't go through all the numbers here but for my system the difference in the OD of the Boden tube and the ID of the PEEK causes the ID of the Bowden tube inside the PEEK to shrink enough that even tolerant PLA will cause the filament to jam in the Bowden tube and not enter the brass tube of the heating element.

I have 2 PEEKs , the inside diameter of 1 is 6.32mm and the other is 6.38mm, even leaving a PEEK on the tube without bolting the head together for a day causes is to shrink. Not enough to cause a jam but down to an inside diameter of 3.1mm. With it heated for use, the tube shrinks to the point that even tolerant PLA will not pass through the Bowden opening. I’ve seen in the forums that UM used PTFE for the Bowden tube earlier but switched to PFA at some point and PFA has a deflection temperature much lower than what it would receive at the brass tube interface. Seems I am going through what was already discovered before as can be seen on the forums. Tribal knowledge I guess. Maybe there was also a change to the ID of the PEEK as well.

I see on the forums that there are many threads with ‘Filament grinding’, ‘jamming’, and such. In reading through the posts their issues seem to be what I and many others had problems with and that is a design failure. I think people with such issues should measure their tubes and PEEKs and post to both see what what varience we have in the PEEKs inside diameter as well as find if this is the root cause of the issues people are seeing. I would like to know from UM what the inside diameter of the PEEK should be, as well as what the deflection temperature of the PFA tubing they supply is?

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Hi Fluxline,

The ID of the PEEK should be 6.35/6,40mm

The deflection temperature i don't know from the top of my head.

But with the new hot end there is an extra piece included to create some extra space between the heater block and the peek, so the temperature will be significantly lower in the PEEK. And therefore it shouldn't effect the bowden tube.

I wouldn't say it is a design flaw, but it has a weak spot. The assembly is quite delicate, and this is one of the issues

that is being improved with the new hot end. It is fail proof, meaning it can only be assembled 1 (correct) way.

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