Brittle PLA

I have two identical prusa i3, side by side. I have on the left one PLA transparent and on right PLA Orange.

I have always the same behavior. The orange filament has no problem, though is on the printer several days, but the transparent one became brittle after 2- 3 days, always in or over the extruder..

I've seen peanut brittle.

On my CEL Robox I find the PLA or ABS will break if I leave it connected but not used for a few weeks, it is always right at the entry to the machine, not the roll, and the roll remains fine.I think perhaps it is interacting with materials in the build of the machine ???

it is because there is moisture in the air.

The quality of the material can be degrading.

try buying it from a Canadian source. Their products are better. Not from China.

if you need a supplier, let me know.

You are true Peter, but the reason is more easy; it's the air humidity that tend to make the filament fragile. I use several colours and obvious I alternate them. So maybe one roll I will use if after a month. If I leave it on the machine it spontaneously break and the first 2-3 meters become fragile and I should discard when I restart the machine. This happens mostly if there is circulating air in the room (here it is as it is hot during the day).

When I unpack a new roll from its plastic envelop I take apart both the envelop and the silica gel (for humidity absorbing) that is in the package. When I change colour I put the partially finished roll in its original pack and seal it with adhesive tape. Rolls last for months without problems.

The 3D Printing factory

The filaments roll storage, new and partially used.

I stumbled across this post in a web search . Here is my opinion 

It seems that the idea that moisture causes brittleness is an assumption that may have no proof . If I read through the post I do see a common thread of it possibly being caused by heat . Wet material may produce poor quality prints but it may not be the root cause of the brittleness . Material stored in the summer in warehouses is exposed to heat that may enhance the brittle conditions . It seems that the community may have used the web post to validate the cause and copied them to answer others leaving a trail of bogus misinformation . I would suggest we validate our the theory before making the assumption that other have .

Interesting topic. I don't know if this is useful, but there's a long article here:

https://onlinelibrary.wiley.com/doi/full/10.1111/j.1541-4337.2010.00126.x

some snippets like this seemed interesting to me, but probably worth reading in full:

One problem is that most information out there concerns PLA for non-3D-printing applications, like food packaging, or medical use inside humans since it biodegrades safely.

So for example, the above snippet refers to PLA sheet, but I don't know if bendy PLA filament is acceptable to consider as a sheet.. I think it is, but it's a guess for me.

I don't have any hard conclusions (and I don't own a 3D printer or PLA so I have no personal experience, but it's still an interesting topic for me to learn more from others) but from some of the research from non-3D-print applications, it seems that for PLA, moisture, or moisture and heat together, can cause some irreversible effects, and some reversible effects. Some things can be controlled, but realistically many people are working in environments that easily approach or exceed 50% RH, and cooling is to room temperature fairly quickly unlike a controlled environment like an oven.

It's very cool that people have found out some mitigation like removing unused filament and wrapping it up, or cutting off the heated end, etc. Even if the precise underlying physics isn't know, that experience is valuable. For example, when I cut acrylic, I can see that the melted acrylic is more brittle, but I don't know the physics to it. I just know I really need to avoid it getting toward melting stage, otherwise I can't machine that area further (apparently additives can reduce brittleness with such heat scenarios with some plastics, according to https://www.bpf.co.uk/plastipedia/additives/default.aspx  - anyway it's a digression). Also maybe if PLA in filament form is really just considered as a consumer hobby material by some manufacturers, there could be less controls, or alternatively might have their own mixes with additives to help with the brittleness? Just speculation : (

Also, as an idea, maybe design-level things can be done to mitigate too, e.g. have 3D printed shapes with thicker regions to change cooling rate (if that is an issue) or curves in multiple planes, to minimize flex of the completed item during use, i.e. perhaps design the item slightly differently if using a specific plastic.

I have made brackets for outdoor Christmas lights that have been out in the rain and snow for several years and still work. This is an application where strength gets tested every season. (They have lasted longer than commercial injection molded plastic brackets I had been using)

Thanks Douglas for the input

wall sections and design may impact the brittleness as well . I wonder if 3 mm as opposed to 1.75 mm has less brittleness and if there is a pattern to the brittleness based on size of the wall or diameter of the filament. The other issue is the static condition of the printed objects may not be inducing any stress to cause the brittleness to be a factor

As you point out mechanical design of the parts needs to take into consideration the strength of the plastic and how it is bonded together. Those brackets I mentioned hold the weight of outdoor lights plus the wind is blowing them around all the time. I did make them out of clear material so they wouldn't get too hot in the sun, but water didn't kill them.