PLA use outdoors?
I have been using PLA filament for two years now and have had good prints. ABS on the other hand has not been so good, so my choice of filament is PLA.
I am getting ready to do a sign for the American Legion and the colors are black, blue, and red and are 0.8 mm thin. The black letters are 4" x 2.5", blue are 3" x 2" and the red are 7.75" x 5.5". I plan to treat them with UV protection spray and attach them with clear epoxy to white back lit Plexiglass.
As the letters are quite thin, my question is how well will this hold up in the weather? The sign hangs on a pole that points east & west so the letters will be facing north and south. The original was painted with spray paint and the red paint south side faded to the point you could hardly see it at all. The sign had been there for some time and was done at a professional sign company.
Possible duplicate of 3d printing for outdoor use: what types of filament are most weather resistant?.
Desert SW (USA) summer days inside a car can be 150F or more. PLA is not so good. I use ABS for outdoor here.
Why does everyone keep mentioning a car? It's an outdoor sign, it does not matter how hot it gets in your car. Your car is almost always going to be hotter than the ambient so I fail to see how it's relevant to keep mentioning it.
I'll stuff this here as I don't have a full answer yet but the glass transition of PLA is 60C, if your letters are supported (glued) well then I see don't see them failing even at the glass transition temperature. I imagine color retention would depend on the supplier and pigment used but I would also have to guess that the same/similar pigments are used in ABS/PLA so they would have similar color retention.
@tjb1 I guess I could have added on the sidewalk or side of a building, or just in direct baking sun for generalized purposes. I have seen a lot of people print items for a car hence the most common reference. I have seen an egg fry on a sidewalk and just yesterday I was at an auction where they left a couple large screen TVs in the sun, by days end the back on the one was warped and curled.
For context: If you consider paint, ink, etc. red tends to fade worse than other colors outdoors, especially blue, unless the red is specially made for UV resistance. The reason is the blue reflects most damaging UV but red at the opposite end of the spectrum tends to absorb it. Also, when I was looking for info on UV resistance of HDPE/UHMW, one site said that the best UV resistance additive was plain old carbon black which is what is typically used to make things black; apparently it soaks up the UV without damage. However, black parts may get hot and PLA doesn't like heat.
I printed a handle for a rather big rolling door in natural PLA (From Fabberparts) - no UV protection. It's on the weather side of the house and is exposed to direct sun half the day.
And after three years cycling to all the German seasons it's still absolutely fine. Also, Wikipedia told me that PLA has good UV resistance - so you should be fine IMHO.
Here is a good blog post about your question: Using PLA for Long-Term Outdoor Applications.
Unfortunately, the original link no longer worked and the blog had been moved, with an updated URL and the URL would not work (for me at least). I tracked down the article and have amended your answer with the correct URL. However, as links tend to invariably die over time, you may wish to consider including and quoting the relevant and salient points from the article, directly in your answer. It would be a shame to lose information from the blog, should their website die some day.
In case the blog post gets ever removed again, I saved it at https://web.archive.org/web/20190708121636/https://www.iepas.ucar.edu/using-pla-for-long-term-outdoor-applications/
Keep in mind that PLA has a much lower temperature point, where is starts getting flexible. I once had PLA-printed parts in my car in the summer for three hours and when I came back, they where bent.
I don't know about the weather conditions in your local environment, but if you experience hot temperatures and your sign is hanging in direct sunlight, I would suggest to make sure you secure the letters against bending (e.g. cover them with a coat of epoxy or something like this).