This is a fully parametric quick-change filament filter designed to keep dust on the filament from getting into your extruder and clogging it. This was inspired by the CreativeTools Universal Filter, but trying to shove a sponge into that closed filter didn't make sense to me. So instead I used OpenSCAD to design a two-piece threaded filter that screws together, and put slits on the side so it could be slipped onto and off of the filament without even needing to disconnect from the extruder. Then I added tabs on the side so it could still easily be unscrewed even if it ends up with a little lubricating oil on the side. After creating this I found Kezat's filter which follows the same philosophy, so I listed this as a remix since he had the idea first.
I added a 1.75mm and a 3mm version, since it is only one variable to change. I only tested the 1.75mm version, but if there are any problems with the 3mm version let me know in the comments.
It is important to select a soft sponge for inside so that it tightly wraps the filament. You then should add a little lubricating oil to the sponge before inserting it, to help it flow smoothly and grip the filament tightly. CreativeTools recommended mineral oil, but I had run out of that and found that grapeseed oil worked fine at PLA-printing temperatures. I recommend intentionally staying above the oil's smoke point if you experiment with alternates. After the first print, very little oil passes through into the print. After you have it assembled, make sure it slides freely up and down on the filament, but with an obvious grip of the sponge around the filament.
Note: The OpenSCAD file uses my threading library for the threaded rod feature that forms the basic structure of this. It is just a hollowed out threaded rod with a slit and tabs. http://www.thingiverse.com/thing:1686322
Update 2016-07-28: My wife noted that the top and the bottom of this were not obvious during filament changes. Therefore version 2 is now labeled "IN" and "OUT" on the two sides. It makes a difference if you turn it around and push the dirt you filtered back out the input side, so this is an important distinction. Also, change your sponges once in a while if you want to see evidence that these actually work.
Update 2017-12-24: Something changed in Cura that made the connection of the external threads to the base print out much thinner than before, so version 3 has this thickened slightly.
0.5 x/y, 0.2 z, 1.5 perimeter, 1.0 top/bottom
Infill does not matter much since this object is mostly perimeter anyway. But I recommend at least 2 layers for the perimeter so that the threads do not snap off if you tighten it down too tightly. I used 3 layers and 50% and found it very sturdy.