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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.

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    $\begingroup$ Welcome to 3DPrinting.SE! $\endgroup$ – Pᴀᴜʟsᴛᴇʀ2 Jan 1 at 15:41
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    $\begingroup$ I believe this is one of the disadvantages of 3 mm (vs 1.75 mm) filament - coiling or uncoiling it puts it under a lot more strain. $\endgroup$ – R.. GitHub STOP HELPING ICE Jan 1 at 20:16
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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.

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  • $\begingroup$ Therefore PLA is a perishable good? $\endgroup$ – user77232 Jan 1 at 17:26
  • $\begingroup$ @user77232 Some user claim to be able to store it for years. Mine seems to last about a year on the roll. $\endgroup$ – Mick Jan 1 at 18:22
  • $\begingroup$ I know that moisture breaks the long chains, but I never heard of micro cracking,do you have a source for that. Note that all materials have defects that can act as start points for cracks, but what drives the cracking when it is stored? $\endgroup$ – 0scar Jan 2 at 14:22
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    $\begingroup$ "age" is better described as "poor storage conditions". Some homes can store PLA without enclosure, others need to store it in airtight boxes. $\endgroup$ – Trish Jan 2 at 21:09
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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.

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  • $\begingroup$ it can be prevented by proper storing in a dry place. brittle filament also can be used with the proper setup that avoids bending and unbending. $\endgroup$ – Trish Jan 2 at 21:09
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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.

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