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I recently had a print failure/crash, where the print stuck to the nozzle and forced molten filament back into the print head, fans, and heater block.

I changed the nozzle for a new 0.4mm same as existing. I shortened the bowden tube by ~8mm to remove some crispiness, and the push-lock connector on top of the print head, which was full of solidified PLA.

The hotend was scraped clean of PLA, and the wiring was inspected. The silicon sock was unhappy but I managed to get it to stay in place.

The part-cooling fan duct was deformed, but I have reshaped it as well as I could. The part-cooling air is probably slightly less than it was.

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I'm printing some Gridfinity bins, and the base just isn't filling in completely and there is also more stringiness.

The sides are not joining up at all, and are just a series of separate strands. They do merge somewhat at the corners.

enter image description here

My printer worked much better before the crash - what do I have to focus on to improve this?

All print jobs since the reassembly are lower in quality, with one in three showing these large "wire bundles" look but all of them are not as good as before-prints.

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Judging from the picture, you have serious inconsistent/under extrusion, not just on the lines that aren't adhering but everywhere, and this is almost surely the source of your problems:

I shortened the bowden tube by ~8mm to remove some crispiness, and the push-lock connector on top of the print head, which was full of solidified PLA.

Ability to correctly extrude material is highly dependent on having an unconstrained path for the filament from the extruder gear to the melt zone. Cutting the hotend side of the bowden tube, or even just removing it from the hotend and reinstalling it, is very error-prone even if you're experienced with doing it. Here are some of the things that can go wrong:

  • End of tube not cut square (perpendicular): There will be a gap between the tube and the nozzle mating surface on one side, allowing material to squeeze into the gap and jam.

  • Tube not tensioned against nozzle: Again, material can squeeze into a gap and jam.

  • Tube overly tensioned against nozzle: Tube will compress, reducing the inner diameter so that the filament has a lot more friction going through it. Especially at the end, it may bend inwards and make a lip that's very hard to pass and that interferes with retraction.

Here's the procedure I recommend:

  1. Use a miter-box type tool to hold the PTFE tube for cutting clean perpendicular cut with a razor blade. There are printable tools for this but you may need to find something else if your printer isn't working well enough to print one.

  2. Carefully use a razor blade or x-acto knife to chamfer the inner and outer surfaces at the end of the tube just slightly so it can't get a lip from compression against the nozzle. The amount of chamfer should be very slight, maybe 0.25 mm at most. This probably isn't strictly necessary but helps make sure you have more margin for error. I've also seen folks do this with an appropriate drill bit, but I don't know how to do it that way. Make sure any debris is cleaned up when you're done so bits of PTFE don't clog the nozzle.

  3. Thread the tube fitting all the way into the heatsink, then back it off by about 1/4 to 1/3 of a turn (0.25 to 0.33 mm) and press the tube in until it hits the nozzle. Then, tighten it the rest of the way to compress the tube against the nozzle by the distance you backed it out.

Another option if you already want to do this is switching to an all-metal hotend (can be done just by replacing the heat break, not the whole hotend), but that comes with its own set of gotchas so I wouldn't recommend it unless you have other reasons to want one already.

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  • $\begingroup$ Well spotted! The issue was much more blatant than your description, once I looked in the right place. $\endgroup$
    – Criggie
    May 22 at 2:20
  • $\begingroup$ Another suggestion from elsewhere was that the extruder motor was slipping - apparently enders are known for cracking the pressure arm over time. Not my issue, but worth including for future readers. $\endgroup$
    – Criggie
    May 22 at 2:21
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SOLVED!

enter image description here

Turns out I hadn't pushed the bowden tube completely home into the hotend on the print head. So there was a space where molten plastic was spreading out, interfering with both extrusion and retraction.

I've flipped the bowden tube end-for-end now, and my test job is well-underway.

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