It seems like if I leave the filament (PLA) in the AMS tubes for several days they always break on withdrawal when I resume use.
I’m guessing it is some kind of chemical reaction between filament and tube.
It is more likely to be moisture in the filament that causes it to break.
I have an AMS for my A1 mini and I have left filament on there for weeks and I have never experienced this.
The PTFE tube do not react to the filament in use or at rest.
As your post indicates you have an A series printer, you AMS is the lite version.
You can add a printed AMS lite enclosure, known as the “Ultimate Filament Spool Enclosure for AMS Lite” which also allows for embedded Thermometer & Hygrometers to monitor the ambient levels that can affect your spool’s current levels.
If your filaments have moisture in them, you will need to dry the filaments to stop them being brittle. Your print quality should also increase and other related problems should reduce.
I recommend reading this, the examples shown are extremese, yours may be different.
https://wiki.bambulab.com/en/filament-acc/filament/dry-filament
For those with any other version currently sold, the following is true for the AMS or AMS 2 Pro.
If you have not added a humidity display to your AMS, I recommend you do so.
You should also add desiccant containers in the printer as well and fill them with silica gel. This will not dry out your filament, but it will slow down any moisture being added.
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It’s actually just the mechanical change of being unwound, no tube needed.
This is 100% moisture. PLA becomes brittle when moist and it can happen relatively quickly. When I had an A1 this was a completely regular occurrence when I didn’t put away the spools when I was done. The AMS Lite does not protect against moisture at all unless you modify it as Malc said, and the filament will snap if bent a bit. Make sure your filament is kept dry and you won’t have this issue with it breaking.
EDIT: In fact, a quick trick to see if PLA is moist is just to bend it and see if it snaps easily
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