Why does my PLA filament keep snapping?

Question

Why does my 3 mm PLA filament always snap near the spool some time (usually a few hours) after a print has finished?

I have had it snap mid print but this is quite rare.

I have 3 rolls of 3 mm PLA filament from Velleman and 3DPrima. They all exhibit this behavior. They are between 2 and 3 years old. Each roll is stored in a foil bag with some desiccant, although this was not the case in the first year. The temperature in the room ranges between 14 °C and 18 °C in winter and rarely up to 30 °C in summer. Current relative humidity is 49 %. In summer I think I have seen it as high as 70 %. I baked one spool in the oven at about 50 °C for a few hours but this does not seam to have helped. When I remove the loose end of filament from the extruder after it has broken, some length of it seams to snap easily and some of it will just bend.

The printer has a direct extruder so there is no bowden tube.

cmm · Answer

PLA snapping is pretty universal.  The best countermeasure is to always remove the filament from the printer after a print and place it in a low-humidity chamber.  Other answers speak of moisture-induced micro-cracks, for which I have no evidence for or against.

This has occurred for me when I've been lazy about putting filament away after a print.

I've seen it with several brands of PLA, perhaps more often with filament with a glassy surface finish.

My filament is 1.75 mm.  3 mm filament may be more susceptible because of higher stresses on the surface for the same radius bend, or straightening from the same radius spool.

user77232 · Answer

When water gets absorbed into the filament, it causes some of the long chain polymers to break. This is a permanent reaction that cannot be fixed by baking the filament, which typically results in the embrittlement of the material. This is true for both the filament and the printed part.

Answered by user77232 on August 26, 2021

Mick · Answer

PLA seems to become brittle with age. Micro-fractures develop on the surface, and they will grow if filament is taken off the spool and held straight. This behaviour does not seem to be linked to moisture content. The best course of action is probably to use PLA quickly. Don't give it time to age.

Answered by Mick on August 26, 2021

Why does my PLA filament keep snapping?

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