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I've got a direct drive extruder printer, that I have equipped with a Bowden-tube to give it a nice and defined filament path from my spool and to protect it from moisture when it exits my drybox. I've noticed that this 1 m Bowden tube adds quite a bit of friction to the system. I am keenly aware of it as feeding filament form the drybox into the tube by hand is quite a chore already, especially with high friction materials like PETG. Also I notice that I can sustain only lower volumetric flow without skipping or underextrusion after adding the tube.

So I figured, why not have a motor on both sides of the bowden tube? A big and heavy one at the entrance, providing a baseline pressure on the filament. And a comparatively light one on the other end, adding that fine control you want for your retraction, rather than trying to implement that via a 1 m wet noodle full of static friction.

Seems like it would give you the best of both worlds. Not quite as light as a full Bowden system, but you could have all the control of a direct drive system, with much less of the weight/flow rate tradeoff.

Anyone aware of this being done before? Or any good arguments as to why it is a stupid idea? Seems like a fairly straightforward mod; in the simplest implementation, just add an additional extruder of the same spec on the other side of your Bowden tube, and split the control signal to be the same amongst both motors.

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A Bowden tube is by design fairly small diameter to match the filament within, constraining the forces applied by the remote extruder mechanism. As you've noted, friction is a consideration.

For your application, you would not have to have such a tightly constrained diameter. You could use a Bowden tube for 2.85 mm filament, if your direct drive extruder is made for 1.75 mm filament.

You'd have the environmental control of the smaller tube, the path control and all of the other benefits, but none (or little) of the friction.

Additionally, you would not have the complexity of managing retraction or synchronizing feed that a dual motor system presents.

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  • $\begingroup$ Thats what I thought; but my polybox dry box came with some 2.85 sized tubing, and the friction in running 1.75 mm filament though was absolutely horrible. Maybe just a bad batch, or something stuck inside? Didn't have any kinks or obviously deformed openings. Regardless of the impact of the tube though, a small extruder on the hontend to provide fine control combined with one at the inlet for baseload pressure might make sense. $\endgroup$ – Eelco Hoogendoorn Sep 7 '20 at 22:05
  • $\begingroup$ Sounds like bad tubing, probably not PTFE. $\endgroup$ – R.. GitHub STOP HELPING ICE Sep 8 '20 at 1:59
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I have a similar setup with one of my printers (that is using 1.75 mm filament), this answer is therefore my own experience with guiding tubes from spool to extruder (my previous was a direct drive extruder, the latter uses a Bowden setup, but both with a tube leading up to the extruder).

Initially I used smaller diameter tubing (inner diameter of 2 and 3 mm, that is commonly used for Bowden setups for respectively for 1.75 mm and 2.85 mm filament), but when I changed to 4 mm inner diameter (and 6 mm outer diameter) all friction was gone. The benefit of using 4 mm inner diameter is that it fits over the 2/4 mm ID/OD tubes, and as such I slide the larger inner diameter tube over a piece of 4 mm outer diameter, forme, this also simplifies feeding the filament to the extruder.

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  • $\begingroup$ Yeah that is the tube that came with my polybox; 4/6 mm. Perhaps I only tried with PETG but the friction there was insane. $\endgroup$ – Eelco Hoogendoorn Sep 8 '20 at 8:44
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    $\begingroup$ @EelcoHoogendoorn I seldomly print in PLA, so my experiences are with PETG as well. I guess your tubes might not be so low friction, are you sure they are PTFE? PTFE tubes are soft, pliable, the harder tubes are often not PTFE. See also this comment. $\endgroup$ – 0scar Sep 8 '20 at 8:53

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