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Background

The bowden fittings used to carry filament around were originally pneumatic parts, used to carry around compressed air. The fitting had teeth to keep the tube from being withdrawn unless the ring was depressed, and while you can depress the ring with a pliers or even your fingers, it might be better to use a tool like this one http://www.thingiverse.com/thing:1777398from thingiverse.

So I used to think that there was only one kind of bowden fitting and that kind of bowden fitting had a lip on it, and that lip stopped the tube from passing through the fitting. Then came the "all metal" hot end. And for the first time (I a not a pneumatics guy) I saw a different kind of Bowden fitting, and this one allowed the bowden tube to pass through the fittings. The all metal hotends use this kind of fitting and you shove the bowden tube way into the hotend. If you don't get it to the problem you will have many problems, slumping and underextrusion.

When I got my K280, HE3D gave me two such fittings, and I put one in my extruder driver and one into the extruder.

So what is the problem?

Well, suppose you are changing filament, and the printer is cold. You need to pull out the old filament, but it is melted right to the darn hotend. So you start the hotend heating. Now, here is the issue. You need to pull the filament as soon as it starts to get plastic - if you let it melt completely, it will leave gunk in the hotend, and it is even possible that a pull will get gunk out of the nozzle. My practice is to set the extruder to 200C, grab the filament, hold the extruder motor release down, and pull on it - as soon as the filament gets plastic, it pulls out.

And when I do that, the bowden tube pulls through the fitting and jams in the drive gear. You have to release the ring to get it back to the position. Even if you run the extruder backwards, it can pull the tube through and jam the extruder. And by the time you do that the extruder is too hot and the plastic is molten. A bunch of gunk is potentially left in the hotend.

What is the fix?

Simple. Get one of the old bowden fittings, make sure that the teflon tube can't be pushed through. I can't help with a link because I happened to have one in spare parts. Make sure it is the right threads - it might be M6. Install it on the extruder motor side only. Now shove the tube in and rejoice. The bowden tube can't be pulled into the extruder again.

It might be my imagination but I think that the better support of the tube gives you a better print, the bowden tube does not wiggle when you extract the filament for a hop. You can extract farther and faster.

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