Filament diameter vs Bowden performance

Just had an academic thought...

Wouldn't 3mm filament be better suited to use with a Bowden style extruder, due to its increased rigidity?

Yes and no.
Yes it's more rigid, but if it has the same clearance in the tube them the increase in extruder hysteresis will be larger.
That supposes that similar length retracts are required for both filaments, which may not be true.
Where the rigidity does matter is between the extruder and the entrance to the Bowden tube, most people run very aggressive retracts, as much as 10mm at 100mm/s on 1,75 filament and softer 1.75mm filament (abs and worse nylon) will buckle into almost any gap as it primes. It's pretty much. None issue for PLA because it's much more rigid.

I'm going to get my first taste of a Bowden soon on a Rostock. I'm looking forward to it. Will be using 1.75mm and I'm switching my prusa to 1.75 to avoid having to stock different diameters here. We shall see how it goes.

Polygon, Well, I was really talking about buckling stresses more than the inherent issues of 1.75 vs 3mm fiber and precision. But your points are well taken, especially with regards to hysteresis. I hadn't considered that. Thanks for responding.

I would think hysteresis would be far worse on 1.75 since the filament itself can compress. The play in the tube seems to not effect the printing or retraction at all. I have lots of Bowden machines with 3mm filament and some have 3.1mm ID tubing and others have 4mm ID tubing and it makes zero difference on the settings or print quality.

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Sublime Wrote:
-------------------------------------------------------
> I would think hysteresis would be far worse on
> 1.75 since the filament itself can compress. The
> play in the tube seems to not effect the printing
> or retraction at all. I have lots of Bowden
> machines with 3mm filament and some have 3.1mm ID
> tubing and others have 4mm ID tubing and it makes
> zero difference on the settings or print quality.


Interesting, I've been going under the assumption that the bulk off e hysteresis if a function of the filament moving in the tube, I can't imagine there is a lot of compression in the filament itself.
My experiments on 1.75 filament seem to imply you need some retract and above some threshold it really doesn't make a difference, but speed or rather rate of change of hotend pressure is critical, and I can see 3mm having an advantage there.
I've only used 1.75 on a Bowden extruder, so I can't do any direct comparisons.
I've seen good prints out of both 3mm bowdens and 1.75mm bowdens.

Polygonhell Wrote:
-------------------------------------------------------
> Sublime Wrote:
> --------------------------------------------------
> -----
> > I would think hysteresis would be far worse on
> > 1.75 since the filament itself can compress.
> The
> > play in the tube seems to not effect the
> printing
> > or retraction at all. I have lots of Bowden
> > machines with 3mm filament and some have 3.1mm
> ID
> > tubing and others have 4mm ID tubing and it
> makes
> > zero difference on the settings or print
> quality.
>
>
> Interesting, I've been going under the assumption
> that the bulk off e hysteresis if a function of
> the filament moving in the tube, I can't imagine
> there is a lot of compression in the filament
> itself.
> My experiments on 1.75 filament seem to imply you
> need some retract and above some threshold it
> really doesn't make a difference, but speed or
> rather rate of change of hotend pressure is
> critical, and I can see 3mm having an advantage
> there.
> I've only used 1.75 on a Bowden extruder, so I
> can't do any direct comparisons.
> I've seen good prints out of both 3mm bowdens and
> 1.75mm bowdens.

I can't actually compare 1.75 to 3 either. The only reason I have the 4mm tubing on some machines is because 3.1mm+ filament won't go through the normal tubing so I thought I would try it and it worked well. I recommend the proper size tubing simply because you can load it with short segments of filament that will cause a jam in the 4mm tubing, otherwise I see no difference. But in the Developers or RepRappers sub section there was a discussion about this a little while ago and someone said the some testing had been done and that person found that 1.75 worked better for them. But this may have had to do with other elements like plastic and hotend.

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3mm takes significantly more force to extrude which is why all 3mm designs are geared.. I have always assumed 1.75 is more suited to bowden for that reason.

just for the record, the springiness of abs and pla is far less than the elasticity of the tubing used. shorter tubing means less pressure built up. also it means retraction is needed to remove that pressure. a polyvinyl tubing will have about 15mm stretch over a length of 1meter. ptfe tubing seems more rigid, only expanding about 3mm over a 1 meter length. this has been my experience only. you may get different results.

also it takes twice as much force to go around a 90deg angle. I'm not sure why but i think it has to do because of the feedstock resistance to bending. perhaps a temp change may have an effect? i don't know.

Edited 1 time(s). Last edit at 02/18/2013 03:06AM by jamesdanielv.

"also it takes twice as much force to go around a 90deg angle. I'm not sure why but i think it has to do because of the feedstock resistance to bending. perhaps a temp change may have an effect? i don't know."

That makes sense. I'd also think that there would be direct contact by the filament with the tubing wall on almost and bend in the feed, so sliding friction is also an issue.

"just for the record, the springiness of abs and pla is far less than the elasticity of the tubing used. shorter tubing means less pressure built up. also it means retraction is needed to remove that pressure."

Good points. I didn't think about the difference between PLA and ABS and the tube. And what you said about retraction...yeah, the longer the run, the more retraction would seem to be necessary to relieve the pressure.

The more I read and think about this, the more I start to think that the benefits of one size are offset by it's own limitations. 3mm is stiffer, which is good as far as the retraction is concerned, but it means it can't conform as easily to the interior of the tubing over longer runs or when the tubing is flexing, increasing sliding friction. The 1.75 is more flexible, so it has more problems with regards to compression, but it's sliding friction would - I think - be lower, because it can conform to the interior more easily)

I don't see why 1.75 would actually compress more, sure it's more compressible, but at the same time it takes significantly less extrusion force so these should roughly cancel out.

What I can see is the 1.75 buckles in the Bowden tube more than the 3mm since the bending stiffness would be much lower.

Edited 1 time(s). Last edit at 02/19/2013 03:24PM by konwiddak.

That's what I meant by "compression", ie the difference between the length of the relaxed filament vs when it is being driven forward. In this case, it doesn't literally mean that the filament has been reduced in length, but that the amount of filament in the tube is more when it's being driven than when relaxed. I probably should have said that a bit better in the first place. sorry...