Any clever technique to detect bowden problem before blowout?

As I work on my newly built printer doing calibration and adjustment, sometimes I happen to print the first layer without any room between printbed and nozzle, and "pop" the bowden tube comes out of the top of the extruder to give the filament some place to go :-).

Are there any clever techniques to notice filament not moving and automagically emergency stop the print? A loop in the filament path with a strain gauge or something? Or maybe a coupling on the line that is outside the extruder and deliberately weaker than anything else so it can blow out in a easy to fix location? (Something like that might make it easier to change filament at that point as well).

Presumably I'll eventually stop doing this once I get things properly adjusted.

The Bowden connectors are really pneumatic tube connectors. They certainly shouldn't pop out under pressure.

Check that the little plastic piece is lifted up... pressing it down releases the teeth in the connector to remove the tube. Some people use a small spacer under the tab, because it might get pulled down and release the tube when printing.

It might also be that the teeth have ripped up the end of your tube and are no longer gripping it. You might want to try cutting the last 5mm or so off the end of the tube.

If neither of those works, you probably have a faulty connector.

Probably the weakest link in the system should be the stepper motor power (unless you have extra big stepper motors). If there's too much resistance, the motor will make a loud clattering noise.

You can try reducing motor current/torque so that it starts slipping instead of pushing the tube out. I'm not sure if that would have any effect on normal printing. The tube is most likely to be pushed out when there's a lot of resistance to the strong push from the motor. That can be low temperature for the filament and/or speed and/or layer thickness printed.

Or use two filament motion sensors, one at the motor and the other at the input to the hot end. If the distance difference (M-HE) is positive, the motor is pushing faster than the filament is moving into the hot end. That means it's going to push out the tube. Ideally you want zero difference, or an average of zero calculated in a specific window of time. It might be desirable to have brief blips where the difference is positive, but short enough duration that they never push out the filament, followed by a negative difference where the coiled filament inside the tube pushes through the extruder faster than the motor pushes, "unwinding" the filament spring in the tube. Other than short blips like that it should not be possible to have negative differences.

Edited 1 time(s). Last edit at 04/27/2021 09:37PM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

In all the years I've never found a ptfe tube to release when pushing down on the bit that looks like it does something...in fact maybe DD can solve this one, the Hex hotend...as I pulled the tube out something went ping!! I assume it was the disc of teeth, but I never found it, now that tube has no anchor & I believe the hotend is now goosed, as that part isnt replacable...it's never even been used, so while some of its design was ok, I won't be buying another.

I've seen a ring of phosphor bronze(?) blades pushed down into some hot-ends, presumably to help grip the PTFE tube. They also have the little plastic piece at the entrance to the hot-end with the little lock ring. My printer has a very short Bowden tube (maybe 70 mm total length). I managed to get the extruder to push out the tube once or twice, but always from the extruder side, never the hot-end. If you want to get the PTFE tube out of the hot-end, you have to take out the heat break (and heater block, of course) and then push the tube through the hot-end, toward the heater block, otherwise you either chew up the PTFE or you hear "ping" when you bend/break the blades inside the hot-end (if it has them).

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

The PTFE can blow out of my extruder because the whole thing is 3D printed, no pneumatic connectors involved, just a 3D printed "collet" which grips the tube. It is definitely the weakest link at the moment, but I think I've solved my problem: I had some bad custom startup code I botched which was always moving the bed closer to the nozzle at the start of a print. No blowouts since I fixed that stupidity :-).

Yeah I just gave it a good yank...and heard the ping as the small part flashed past my vision, into the crevices of the room, tried to find something by sweeping a magnet on a stick, but no bite. So will either have to figure another way to constrain it, easier in the short direct method, or ditch the hotend, or find some circlip or ring from another & try fit it on the tube and pull back up the heatsink.