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I've been printing small quantities from a PLA filament spool on a Craftbot printer for about two months now. Recently the printed objects have been coming out very brittle. Some structures that printed fine two months ago are now difficult to re-print. The print head gets clogged easily, and when the object does print, it's quite brittle and 1/4" to 1/8" rods will easily snap off if not handled gently.

I'll admit to not following the precautions for storage of PLA. It's much easier to just leave the filament installed rather than trying to remove it after each print, so this one spool has just been sitting on the back of the printer for all these weeks now. I'm sure it's been humid some of the days, we've had some rain here.

Has the spool of PLA been damaged just by leaving it exposed to room air for two months? Could that be the sole cause of the brittle prints, or are there other possible causes? Is there any way to fix the spool or future prints from this spool, or do I have to scrap it and get a new spool?

1 Answer 1

PLA absorbs moisture, so keeping the filament dry is a key factor. Aside from that, PLA is naturally more brittle than other plastics like ABS and Nylon Sorry, tried to find a graph to prove it, but couldn't find one.

There's a good Google Group discussion and many other resources that go over good storage habits, but as for fixing the existing filament.

Try the following:

  • Place PLA in an enclosure (plastic bin, Zip-loc bag, etc.)
  • If you have some, add some moisture absorber(s)
  • Place the tub in a warm environment (naturally or artificially) and make sure the area is dry as possible (not in the shed in the back, by the woods...). Possibly next to a heater vent or space heater in your house?

Essentially, you're trying to treat the material. When the material goes through a heat treatment (aka the heat block in the extruder), the mechanical properties are beginning to change. The brittleness can be set by how quickly the material cools. I'm speculating that the moisture does any of the following:

  1. Keeps the filament from heating up to the desired extrusion temperature.
  2. Burns the filament.
  3. The moisture is evaporated, leaving gaps in the extruded filament (under microscope).

I looked into this a few years ago and have forgotten most of what I found out, but I'll keep looking and update my answer here.