Bowden Worm Hole

Just finished a lot of Print testing with my Anycubic Vyper Bowden tube FDM printer. As I suspected much of the pros and cons about bowden tube systems and retraction issues is pure misinformation or speculation.

Some of the “gospel” may have been true years ago when systems were built up from available tube ID materials. But I have proved to my own satisfaction most of todays machine are built for purpose with correct hardware.

The bowden tube Inside diameter (I.D.) is closely sized to freely guide 1.75mm filament without excessive “side slap” (looseness) or binding (too small). 

Movement amount on the input end is almost exactly the same as the output. So close, it is not worth measuring. Certainly not 4 or 5 millimeters difference. 

I used bowden type (tube-in-tube) plastic push rods for Radio Control model aircraft for years. With both outside tube ends secure, they work flawless with no lost motion between servos and critical flying control surfaces.

The problem I have experienced  with 3D printing is when either end of the bowden tube is not absolutely firmly restrained, huge amounts of excessive retraction movement is required. This, I am certain, is the basis of the “Urban Legend” of long retractions are necessary with bowden tubes in FDM printing.

The published Cura standard for the AC Viper retraction is 6.5 mm. It works but my testing proves that much is not necessary. I am currently using 3 mm retraction with no issues such as stringing, and have created outstanding printed surface finish.

THIS IS NOT MY RECOMMENDATION but I am suggesting ones needs to run their own tests and retract only the necessary amount.

There are many variables, and nozzle extrude temperature is a big one. Higher temperatures reduces viscosity and produce more flow with less pressure. So there can be no single correct “magic” setting for retraction.

All that is necessary is to drop the pressure in the nozzle to zero or slightly negative to stop flow. Fluids (melted filament) DO NOT compress or act like a spring unless they contain air. This is why air is bled from hydraulic systems such as car brakes.

Sucking back filament inside the nozzle many millimeters is not what happens. That would require sucking air in and out of the nozzle tip.  The printer would be “blowing bubbles” into the extruded flow pattern or at least vacuuming up plastic flow from the print surface. That is not happening in my experience.

What I think is happening:

I believe is excessive retraction may pump air from cold end of the extruder into the hot mix. Over retraction pulls, thins, and strings out the molten filament at the cold end at the cold to hot melt boundary, allowing room for air. The bowden tube contains air at atmospheric pressure

What does one often see on the end of the filament when doing a filament change. Either a blob with a cold pull or a long thin strand with a hot pull. What is a retract? It’s a hot pull…

All FDM users know we certainly do not pull out all the old material when changing filament.

Then slamming the filament forward on a re-feed move, could trap that air. It could be the real cause of snapping and bubbles (blamed on moisture) in the hot end. And why direct feed short retract extruders seem to perform so much better. With short retractions, they do not pump air into the top end the extruder. 

One reasonable retraction standard I have seen printed is, “retraction no more than the nozzle length.” I think that is a good reference, but nothing to do with what is happening in the nozzle.

Retraction speed is another variable. Reaction time is important. “Coasting” at end (stoping flow early) and using less than retraction length when starting again are variables included in some control codes.

Beyond the “stringing” caused by inadequate retraction there are also issues with “zits” and “blobs” when ending and starting. Especially when using “random” start/stop locations. One solution is to designate a “seam” area for all layer starts and stops and not use random start/stop.

No simple answers here. Just total awareness of all that is going on. 

My current mindset is that huge 8 mm to 12 mm retractions just because the system uses a bowden feed are unnecessary if the bowden tube is firmly secured at both ends. 

This “urban ‘Bowden’ legend” needs carefully evaluation of the science that is going on. The bowden tube is a kind of galactic “worm hole” in 3D printing space… 🙂

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