Filament grinding



  • I have a BMG clone feeding 1.75 mm filament through a roughly 800 mm bourdon tube.
    A particular part that I am printing seems to print fine with a 0.2 mm layer height but fails repeatably when printed with 0.1 mm layer height.
    Odd thing is, the two runs I did failed at pretty much the same height (give or take).
    The nozzle is not clogged. If I give the filament a bit of a push it moves past the apparently ground down portion and carries on as per usual.
    The filament is PETG.

    I am theorizing that with lower layer height, less material goes through the extruder gears but the number of retractions and un-retractions stays the same. I am thinking that maybe the filament is being worn down a bit to the point where the diameter is reduced a bit and the extruder can't grip it securely enough and starts grinding. Once it starts grinding it can't recover.

    Four questions:

    1. does this sound like a plausible scenario?
    2. are there other possibilities (I tried upping the extruder temperature by 10 degrees to 250)
    3. would the real BMG extruder likely improve things or am I likely to spend a wad of cash for no improvement? There doesn't seem to be anything real high tech there to screw up for the Chinese.
    4. How would tension on the spring for the feed gear affect this situation? It seems to me that lower pressure on the feed gear would reduce wearing of the filament but would also reduce the amount of feed pressure that can be applied to the filament before grinding starts. I am currently set up about 3 turns of the screw off of fully screwed in.
      How does one find the sweet spot (is there a sweet spot?) between pressure on the feed wheel, traction and not wearing down the filament?

    In case it matters, pressure advance is at about 1.3, retraction is at 1 mm, retraction and un-retraction is at 30 mm/sec



  • @jens55
    IMO,

    1. It's plausible.
    2. Heat creep (because the filament is moving forward much more slowly - in which case upper the temperature may make it worse)
    3. I can tell you that a genuine Bondtech BMG is capable of retracting and un-retracting the same piece of filament in excess of 23,000 cycles, after which it is still possible to extrude filament as normal. https://somei3deas.wordpress.com/2018/05/11/bondtech-bmg-vs-e3d-titan-extreme-retraction-torture-test/
    4. In my experience, the spring tension does practically nothing on a genuine BMG because of the way the gears mesh which prevents excess "squeezing" of the filament (unlike say an E3D titan).


  • Thanks deckingman!

    Good point re heat creep, I had forgotten about that and I am using a clone Chimera hot end which is prone to this according to some people. The long term plan is to switch to water cooling.
    If it is related to heat creep, wouldn't I see a plugged up nozzle? I can easily get the filament to go again after giving it a bit of a push.

    Maybe I need to bring in a genuine BMG just to see the difference.



  • @jens55 said in Filament grinding:

    If it is related to heat creep, wouldn't I see a plugged up nozzle?

    No - Because the heat creeps up through the heat break (via the filament itself as well as via the metal "jacket" surrounding it), which softens the filament so that it swells when pushed by the extruder, causing it to "stick" inside the heat break.

    I can easily get the filament to go again after giving it a bit of a push..............

    .........Which is exactly what I would expect to happen with swollen filament inside the heat break (caused by heat creep).

    If it was me, I would try a lower temperature and/or bump the print speed up if possible.

    Edit. I don't know what your clone is like but examples that I've heard about don't have such aggressive filament gripping "teeth" as the genuine article. So it's possible that a genuine BMG will have a better grip which will push the swollen filament without the external push that you are having to apply.

    I used to have exactly those sort of problems with 5 colour diamond hot ends (which also suffer from heat creep in their standard configuration) and E3D Titan extruders. After changing to genuine BMGs, I never needed to use the "external push" technique.



  • @deckingman said in Filament grinding:

    @jens55 said in Filament grinding:

    If it is related to heat creep, wouldn't I see a plugged up nozzle?

    No - Because the heat creeps up through the heat break (via the filament itself as well as via the metal "jacket" surrounding it), which softens the filament so that it swells when pushed by the extruder, causing it to "stick" inside the heat break.

    I can easily get the filament to go again after giving it a bit of a push..............

    .........Which is exactly what I would expect to happen with swollen filament inside the heat break (caused by heat creep).

    I am a bit confused by that because previously, when dealing with apparent swollen filament, the system would be plugged solid and require some major effort to get it going again.
    In my current case, only a slight push is required to get things going again even after the printer had been extruding nothingness for an hour or so.
    My saying 'plugged nozzle' is not correct as I realize that heat creep grabs the filament higher up and doesn't plug the nozzle itself.
    Let me rephrase that there appears to be no blockage because only a very minor push is required to give the extruder something to grip. The push is about the same amount as if you would need to push filament through the bourdon tube.

    If it was me, I would try a lower temperature and/or bump the print speed up if possible.

    I will give lower temperature a try but there is little I can do to increase speed. I might be able to up it by 10%

    I think it will be best for me to go ahead and spend $110 for the real thing to clear up the question of 'is the real thing worth the price'. In comparison, I think the clone was less than $20.
    Proper feeding is priceless .....

    Thanks



  • Real BMG extruder has been ordered to clear this up once and for all 🙂



  • @jens55 said in Filament grinding:

    I am a bit confused by that because previously, when dealing with apparent swollen filament, the system would be plugged solid and require some major effort to get it going again.
    In my current case, only a slight push is required to get things going again even after the printer had been extruding nothingness for an hour or so................

    There are degrees of softening and swelling. For sure, if the filament softens enough, then you'll get a complete blockage. If doesn't soften at all, you'll get no blockage. But if it softens slightly, you'll get moderate swelling and a partial blockage which is easy to push back through the heat break.

    For info, the next iteration of my 6 input mixing hot end will have the Bowden tubes going right through the heat breaks (similar to the "lite" version of a V6). Long story short but there are inevitable heat creep issues when some filaments are loaded but not moving forward as a print progresses. The PTFE liners help because it is a low friction surface for the swollen filament(s) and is the reason why "all metal" hot ends are much more prone to blockages due to heat creep.


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