There is a lot of advice to dry most filaments before printing, but how does ambient humidity affect finished prints? As I understand things, printing does not cause a chemical change to the polymer so I would think it is just as hydroscopic after printing as before.
I have a bone pla model getting rain and everything for 12 months. It is perfect.
Moisture absorption is a diffusion - based process, it starts on the outer surfaces and moves inward with time. That means it can happen faster in thinner walls, slower in thicker walls, and will generally get a bump up in how fast it happens in printed materials due to imperfect layer bonding and porosity.
A printed block of a material will absorb humidity faster than a cast block of the same material of the same size.
The question is always “does this matter?” - and I’d say in many cases it doesn’t, but designers still need to be aware of how their specific material changes with time and how it will matter in their application - which speaks to both material choice but also design choices and printing choices.
Yes, a 1.75mm piece of filament can change when exposed to humidity, but how often are we printing features with 1.75mm cross section area, parallel to the print bed?
I say parallel to the print bed because vertical or diagonal sections of the same cross section are already much weaker due to interlayer weakness.
If you’re printing PLA for a strength - requiring application, yes it will become more brittle over time - but why are you printing with PLA if you need strength in the first place? If you’re printing with nylon for stiffness and toughness, it is going to become less stiff over time - and potentially a bit tougher - and that may need to be accounted for, either in a heavier design or a different material selection.
I’ve always thought that the main concerns with drying filament before printing were things like “pop-corning” (my term, not official), what water does when it is suddenly heated in a small confined space (like a hot end).So you dry them to prevent that as much as possible.
If you keep PLA prints out of UV light (direct sunlight) and hot cars, they should last a few years at least. I doubt we have data yet on the longevity as the hobby is quite new and filaments are changing fast.
However, bio-pla, being bio-degradable, may break apart faster. Airsoft bio pellets, for example, are bio-PLA and will crumble into white powder within a few weeks. However, they are designed to do that, and PLA for printers is not.
(Conversely this means regular PLA for printers is not really as bio-degradable as they claim it is; it will degrade but only in an industrial composter)
I read some article some where(lost in the archives) that said PLA was actually something like compost available (can’t remember exact wording) which refered to the fact that it wouldn’t break down below 160 degrees (again I may be remembering the exact figure wrong), the industrial compost set-up you mention.
Nylons get mildly stronger post-printing, a requirement that they absorb moisture back from the environment. I’m sure PLA etc do absorb more moisture again but you’d never really know it. I imagine if you had a filament recycler you’d have to process it down in moisture again.