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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?

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    $\begingroup$ Have you tried drying the filament (eg in an oven on "warm")? $\endgroup$
    – Ryan Carlyle
    Jun 21, 2016 at 19:58
  • $\begingroup$ Haven't tried anything yet. What temp is "warm"? Less than 400F I would guess :) $\endgroup$
    – emackey
    Jun 21, 2016 at 20:26
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    $\begingroup$ Yeah, you don't want to overly soften the filament or it will go out of round and jam. Maybe 120F / 50C for a couple hours. $\endgroup$
    – Ryan Carlyle
    Jun 21, 2016 at 20:27
  • $\begingroup$ I had this as an answer, but the question was about PRINTS, not FILAMENT. There is no question that PLA filament grows brittle with age. It grows so brittle that it will spontaneously break when subjected to stress. I experience this as PLA breaking along the path between the spool and the printer when left untouched for a few hours. This happens even during the depth of a New England winter when the humidity is very low. Part of this problem comes when the PLA is tightly coiled onto a spool with a small hub. ... $\endgroup$
    – cmm
    Jun 19, 2019 at 14:23
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    $\begingroup$ The PLA seems to anneal the strain induced by winding so that when the filament is unwound the filament is again strained. The strain may open tiny cracks, which develop into larger cracks and breakage. I have not had this problem with newer spools where the diameter of the hub is larger. Recently I threw out 4 spools that were 80% used because the remaining 20% broke several times during printing. I don't see this with open but less used rolls of filament I bought around the same time. I do not have hard data on when I bought each spool. Now, I avoid spools with small hubs. $\endgroup$
    – cmm
    Jun 19, 2019 at 14:26

2 Answers 2

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Increase nozzle temperature. When the filament is new it will print easier, requiring less heat to print well. So if you didn't store your filament properly to begin with, increasing print temperature will make it jam less and increase layer bonding.

The reason for this is because the moisture that accumulates in the filament will absorb heat and evaporate when printed, meaning that the filament itself isn't getting the same amount of heating as it used to.

That being said, the storage suggestions mentioned by tbm should be your first priority. I personally put my filament in Zip Lock plastic bags and store these in a dry location not exposed to sun or temperature changes.

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    $\begingroup$ Awesome tip! And yes I've been taking better care of my PLA these days, getting it off the printer and storing in a ZipLoc bag etc. But cranking up the temp seems to help a lot. $\endgroup$
    – emackey
    Jun 28, 2016 at 20:53
  • $\begingroup$ Throw some silica gel moisture absorber packs into the ziplocs. $\endgroup$
    – Technophile
    Nov 30, 2019 at 19:49
  • $\begingroup$ I save all of the little desiccant packages I get for exactly this reason! $\endgroup$
    – Tim S.
    Dec 30, 2019 at 17:56
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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.

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  • $\begingroup$ If you have some, add some moisture absorber(s) (desiccant packs). If you don't have some, buy some and then add some moisture absorber(s). $\endgroup$
    – Technophile
    Nov 30, 2019 at 19:52

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