Very Specific Loading issue
Im having a very unusual filament loading issue with my MMU2S. First of all, it apears only to impact filament tube Number 2, but its slowly starting to reach the other tubes. Its not a filament issue as I have exchanged them around.
Which is that it can load filament properly, but when it comes time for the print it gets stuck getting it to the hot-end. It either fails to load it to the end, or unload it from the end. And the loading grip-thingy is showing signs of damaging the filament itself. I have tested the sensors and they seem to work fine.
RE: Very Specific Loading issue
Are all of the hobbed gear sections in the MMU clean?
'Slowly starting' caught my eye, as a buildup of filament residue can cause all kinds of filament grip issues.
RE: Very Specific Loading issue
Whats the hobbed gear section? And how do I clean it?
RE: Very Specific Loading issue
Prusa literature refers to what some call 'hobbed gears' as 'pulleys', and the screen scrape below shows them in the MMU2S.
When filament residue builds up in the teeth, the friction of the teeth on the filament is reduced significantly.
I use a combination of alcohol, Q-tips, a dental explorer, and canned air to clean those. A larger sewing needle or almost any kind of a pick can substitute for the dental explorer, which I use to go between the teeth to remove any impacted filament residue.
I also recommend (among other things) using those idler tension bolt knobs, which makes it much easier to open up the unit and to adjust the tension without using the hex key. One version of this is shown at the link below, there are others as well.
https://www.printables.com/model/75244-mmu2s-idler-tension-screw-retainer-thumbscrew
Having said all of that, I must admit that I don't use the MMU2S much anymore, since I got a second printer with a dual extruder. I can only remember using it once in the past year when I did a three-color print. When I built the MMU2S kit I did, however, get it working very reliably.
RE: Very Specific Loading issue
You might have more resistance in the filament path on slot 2. To check: Move the selector out of the way and load slot 1 and slot 2 through the MMU manually. Then you can compare the two path's resistance by pulling the filament in front of the MMU. If 2 is harder to pull out, this might be the problem.
High resistance will cause the gears in the MMU start to grind on the filament which causes the damage you see on the filament and the loading errors.
Solution: Reduce friction in the filament path. Depending on how you feed the filament to your MMU there are several ways to do that. (Higher ID PTFEs, pass through adaptor, screws on buffer to tight, path to long...)
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RE:
I did check it out and it is a loading issue. Part of it is the MMU2 tubes (which I could fix), but another part is the plastic tunnels that lead to the 'pulleys'. Not sure how to fix that.
Edit: Wait, I figured out the issue: I wasn't inserting the tubes all the way.