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Help! Print turns to junk after a few cm


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Posted · Help! Print turns to junk after a few cm

I was doing some testing of surface finish and ringing recently, printing fairly fast (150mm/s and thickish layers - 0.15mm). With a normal nozzle, that would be a bit marginal, due to the high volume per second (9mm³/s), but with my larger nozzle there's more capacity headroom there, and it should have been a pretty comfortable print at 230ºC.

And suddenly I started getting problems - just like the title suggests, and this picture shows:

Massive under-extrusion after a few cm

I cleaned everything, checked the mechanics, and tried again. Everything seemed fine, then suddenly - same thing. Two or three more attempts... everything starts fine, then fails.

For those of that have been here for a while, this sounds like a familiar situation. And for me, the first suspect is usually the total throughput, and the resultant back pressure. If that's ok, then we might talk about extruder drive health, spring tension etc. Or something unhappy at the hot end - maybe the teflon got damaged, or the fan slipped and started blowing on the nozzle, or, or....

And yeah, I thought of all those things... and no, it wasn't any of them.

It was something I don't remember ever seeing suggested on here as a possible cause for this kind of recurring print failure....

I'd changed over to an older spool of filament, that was about half used up. As a result the filament I was using was quite tightly wound around the spool, with a tight radius of curvature, and because it had been moved around a lot in the past year, the loops were intertwined quite a bit. As a result, the filament was tending to get caught on itself on the reel as it unwound. If I pulled it off the spool by hand, it would resist slightly, and then unwind a few turns, creating enough slack for the print to carry on for a while. But once that was used up, the extruder (already working to overcome quite a bit of back pressure from the nozzle) couldn't overcome the sticktion of the filament on the reel. Worst cases it would stutter briefly, and then come loose - resulting in just a brief area of weakness on a layer, or part of a layer. But if it got stuck for longer, then the print would continue with very poor extrusion... not totally failed, but massively under-extruding.

So, now there's another suspect for these sorts of problems....

 

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    Posted · Help! Print turns to junk after a few cm

    That same thing happened to me a few weeks ago. I fixed it by pulling the filament out of the bowden, undoing the tangle and feeding it back through. But it ruined a print. The problem was relatively obvious when it happened.

     

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    Posted · Help! Print turns to junk after a few cm

    It wasn't even really tangled as such... Just that the winding pattern, allied to the tight radius of curvature, and the tendency of the filament to find the shortest possible path around the spool meant that the filament tended to get caught on other loops. The extra friction needed to dig it out of any loops that it was catching on was enough to tip the extruder drive over the edge.

    In the end I moved the reel to the floor on a turntable, and pulled off a bunch of loops of plastic to hang in mid air, so the print could finish without needmimg to unwind more.

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    Posted · Help! Print turns to junk after a few cm

    With 1.75mm filament, the standard extruder might not get a really good grip, due to the smaller diameter between the drive bolt and the bearing?

     

    that, and there is much more room for play within the bowden tube, which may cause inconsistent extrusion i think, and i don't know how the hot end will function during extrusion with this different amount of pressure.

     

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    Posted · Help! Print turns to junk after a few cm

    I had the same thing happen when I first got my Ultimaker. It was purchased used and came with 3 spools of filament. After upgrading everything I possibly could and trying everything I could find on the forum, I was still getting something similar to the picture you have posted.

    Once I switched over to new filament everything was fine. In my case, the filament was also fairly ovalized.

    Nice to see independent validation of an issue!

     

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    Posted · Help! Print turns to junk after a few cm

    that, and there is much more room for play within the bowden tube, which may cause inconsistent extrusion i think, and i don't know how the hot end will function during extrusion with this different amount of pressure.

     

    Just for information about 1.75 mm filament, the extruder just make it fine, only thing what I did to get not bad overall experience is put inside hot-end teflon tube 2 mm inner diameter.

     

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