Methods for controlling the tension of flexible filament entering the extruder?

I've just finished upgrading my Prusa i3 (the now defunct DIYTechShop/Xssink i3x variety) with a flexible filament extruder (a Flexystruder-style design). My first print came out reasonably well:



However, I found that I needed to constantly unwind filament from my spool so that there was no tension on the filament entering the extruder. Otherwise, the volume of filament being extruded drops noticeably, as can been seen in this photo of the bottom layer:



The reason for this is pretty clear. When the filament is stretched, it's diameter is reduced, so a smaller volume is extruded per motor step. What I'm surprised by is that I haven't found any mention of this issue on these forums, and only one passing reference to it elsewhere.

I did not use a bearing-based spool roller for this print because these Filaflex spools are narrower than my current roller. But, based on feel, the friction of my improvised setup for this print was not that much greater, such that I doubt a bearing spool roller will solve the problem.

My question is, does this problem already have a solution? My thought is to add something on top of the extruder that applies a constant frictional force greater than that required to unwind the spool (e.g. a piece of foam lightly clamped around the filament), but I don't want to waste time reinventing the wheel.

I would think that the pressure the pinch wheel puts on the filament at the drive gear is going to have a much bigger effect than a little tug back on the feeder side. When you print with hard filaments like ABS and PLA you set the pinch wheel tension very high so the drive gear teeth bite deep. I don't think that much pressure is necessary with softer filament. If the pinch wheel is smashing the soft filament against the drive gear it will tend to thin it out a bit. Try reducing the spring tension on the pinch wheel and see if that fixes the problem.

I've been printing with TPU filament and it has been very good, but I reduced the spring tension when I saw how soft the filament was. I have a straight line path from the filament spool, which is on top of the machine, to the extruder, so there are no bends to put additional strain on the filament as it comes off the spool. I do not use a ball bearing feeder- just a piece of PVC pipe over which the spool fits loosely.

Edited 1 time(s). Last edit at 02/06/2016 08:35PM by the_digital_dentist.

---

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

Quote
the_digital_dentist
I would think that the pressure the pinch wheel puts on the filament at the drive gear is going to have a much bigger effect than a little tug back on the feeder side.

Then why is less material extruded when the filament between the spool and extruder is in tension, compared to when it's slack? I can't see how your argument would explain this.


But, since you don't have this problem, I suspect there must be something else unusual about my setup. When printing, how much tension is in your filament between the spool and the extruder? Is it pulled into a straight line? How soft is your TPU, and is it 1.75mm or 3mm? Filaflex is the softest filament I've come across, so at 1.75mm, it's probably the worst case for this scenario.

Edited 1 time(s). Last edit at 02/06/2016 09:55PM by kh.

I'm using 1.75mm TPU from Foxsmart. There is only a little tension on the filament between the spool and the extruder, mostly because of the self-stickiness of the TPU on the spool. Here's what it looks like:



Unless you've got a LOT of tension on the filament coming off the spool, or you've got the filament running through a tube with a lot of bends, I can't imagine the tension on the feed side being high enough to stretch the filament. Maybe your extruder motor current is too low and/or pinch wheel tension is too high...

---

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

No problems with TPU here either. I use bearings on the spool holders and have the spool above the printer with the filament exit more or less centred over the middle of the X axis. TPU is still quite a robust filament (at least in 3mm). TPE is a whole lot softer, so if I can get that printing I'll keep an eye out for any tension issues.

Thanks for the feedback and info @the_digital_dentist and @JamesK. I'll report back when I've tested this some more.