Why does PLA crack when left sitting in the bowden?

[Anders Olsson]

I have noticed lately with several printers and several brands of PLA that the filament cracks into small pieces when left sitting in the bowden.
The way this happens surprises me, is there some chemistry involved, or just initial cracks from the feeding that slowly gets larger when left under stress?

Here is an example from out Delta Tower, where PLA from Kjell & Co was left in the printer over Christmas/New year:

We have had the same thing happening with other types of pure PLA in that printer.

Recently I saw this with Ultimaker gray PLA in an UM2Extend which had also been sitting unused for some time.

I have not seen this happening with Colorfabb PLA/PHA (which is much more flexible) or with any other type of filament that I have used.
Haven't heard reports of printed parts (put of pure PLA) cracking when sitting under load for some time too, as I can recall.

So I am curious, what are your theories on this, why does PLA seem to self destruct when left inside the bowden?

[Neotko]

The only time I seen something like that (but not that extreme) was when I used 1.75 on the 6-3mmdia bowden, due hysteresis the preassure made it twist inside and snapped by 3 points.

[Izzy]

Interesting, the only one I had problems with so far was the UM blue, but that was still on the reel. I keep all my filaments in the sealable plastic bags with the small silicon bags inside but not come across this above.

[Neotko]

Oh unused, if not sealed it looses water and cracks. afaik

[PeggyB]

I have a whole batch of Colorfabb filaments from 2012 before they changed their recipe (shiny, now their filaments look matt on the roll). Cannot leave them overnight in the printer or they will crack. Some colors worse then others.

[martin-bienz]

I have had mixed results with various types / manufacturers of PLA. There are some UM filament roles, 3 years old which are still fine. With Diamondage I have not had any issues so far (but not that old). Others are brittle as hell after a year (German RepRap). I keep them (whenever I can) all in their bags with the silicon bags plus in a plastic container with a lid that can be closed (not airtight).

[Anders Olsson]

Interesting, thanks for the replies, seems like this is a fairly common issue then.
Haven't seen any of my spools crack on the spool independent of how I store them, but most of my filaments are not PLA, so I have little experience with this.
It still puzzles me that the filament breaks when sitting inside the bowden for some, I am thinking if I should ask the polymer chemists at work about this
Would be great if one could somehow treat older PLA to make it less brittle.

[Neotko]

I can't find the post but

I remember a post where ultimaker users tried this method and worked but not for long.

Basically it needs water, but there's a point where too much it's a problem and the heat temp should be controlled. I remember ultiarjan tested this (I think) but I can't find it.

[jonnybischof]

I can confirm that this is usually not an issue with DiamondAge PLA (probably the highest quality "regular" PLA out there). Only when I pull the filament out of the bowden with too much force and twist it at the same time, it sometimes snaps.
Note that your local climate can make a big difference - if you live by water you will make a different experience to someone in a dry area.

[Titus]

There was a post on UMforums, someone taped the filament loosely on a table overnight and it cracked. Can't remember what the clue was though xD