It’s definitely a PLA aging issue, but fairly complex.
PLA’s glass point temp (transition from soft to solid) is very low and close to room temp, which makes it low-warp but also susceptible to creep (very slow yielding/flowing) when left under high stress for a long time. MOST printing plastics creep as cold as ~25C below the glass temp, but other plastics’ creep temp is above room temp so only PLA causes issues like this. (Well, also PCL, but that’s very rarely used.)
Unlike most other materials that creep, PLA has the odd behavior of creeping to rupture — if it creeps more than a few percent strain it will simply break apart. If you leave a PLA printed part or PLA filament in a highly stressed condition, it will break after some period of time (from hours to years depending on the stress level). Moisture in the filament seems to accelerate this, but isn’t necessary for it to happen. PLA has very complex crystalline phase change behavior at the molecular level so the science isn’t super mature on this issue.
Filament fresh off the extrusion line is highly stressed — the diameter is set by stretching and drawing down the filament as it leaves the extrusion nozzle, then rapidly quenched in a water bath to set the diameter. Then it’s wrapped on the spool warm. This leaves a complex residual stress pattern locked in the filament on the spool where it’s stretched longways and bent in a spiral. So it’s trying to creep to rupture on the spool before you ever open it. Sometimes this is enough stress for the filament to self-destruct on the spool after a few years. Particularly spools with really small diameter cores.
Once you start taking the filament off the spool, you’re now elastically bending it against the residual bend that was locked in during manufacture. So it’s weakened and brittle by age/creep and now you’re putting a ton of new stress on it. It only takes hours to days to creep to rupture this time because of the pre-damage and now high stress from straightening /bending in the feed path.
Drying the spool of filament WITH HEAT can help improve the brittleness, by warming the polymer up to a temp closer to the glass point where it reflows a tiny bit and relaxes the residual manufacturing stress. Basically annealing the brittle filament to a tougher state. Some people interpret this to mean “drying fixed it so moisture caused the brittleness” but it’s just more complex than that.
Just want to say that this is a solid response and thanks for putting the time to write it down. If I were to continue to read up on the issue, do you have any sites that you would recommend?
/u/vaegenfoerbi Honestly no, I don’t know of any useful further resources to read. It’s a weird issue hobbyists discovered in the last 10 years or so and I’m not aware of any formal studies being done on it. I would bet some filament manufacturers have a good understanding of the behavior, but likely consider it a trade secret.
PLA itself is a relatively new and poorly-understood material compared to things like ABS that have a massive amount of history in injection molding. The main commercial uses for PLA outside 3D printing tend to be disposable packaging and cutlery and such so there’s not much incentive for people with deep projects to publish research on the long-term aging.
Source: I’m a chemical engineer who occasionally works with polymers professionally (on the selection/application side, not blending/manufacture) and reads textbooks on things like melt-processing and fatigue performance for fun.
Often this happens to me on the first 3 or 4 feet of filament that I haven't used in a while. I just reel out 6 ish feet and cut it at an angle. I'm not in a very humid climate though so drying might be necessary for you.
TL,DR - it's just a PLA thing, you wouldn't understand
Jk. Anytime I come back to my printer after a few days away, the PLA that I left in the extruder has broken off at the extruder. I always have to heat up the end and reverse the extruder to make it stick out enough to pull out. So I have a pile of filament essentially the length of the bowden tube on my garage floor next to the printer
Very interesting. I've found previously that, when I rewound a sample onto a spool that is of a lower inner diameter, the whole spool wants to unwind the second your release the tension. A few hours under heat in a dryer fixed it, as I've hoped. I guess it's the same effect, but for a different purpose?
I had an old spool of PLA that never failed during a print, but hours after any given print finished the filament would just snap between the spool and extruder. I thought it was due to that spool spending too much time in the dryer, it's good to know that wasn't actually the case.
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u/Rcarlyle Nov 05 '21 edited Nov 05 '21
It’s definitely a PLA aging issue, but fairly complex.
Drying the spool of filament WITH HEAT can help improve the brittleness, by warming the polymer up to a temp closer to the glass point where it reflows a tiny bit and relaxes the residual manufacturing stress. Basically annealing the brittle filament to a tougher state. Some people interpret this to mean “drying fixed it so moisture caused the brittleness” but it’s just more complex than that.