This is the worst possible setup to print living hinges: PLA isn’t terribly flexible, and here the flexible bits have to be printed vertically, which really isn’t ideal to withstand cracking.
Yet when the application isn’t heavy-duty like in this clamp-on connector, even PLA printed “against the grain” works well too: this connector can spread its hooks linearly 3 mm apart and the living hinges don’t crack or weaken even after many cycles.
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I’ve Printed LOTS of PLA living hinges and even springs. They do work just fine for the limited to moderate cycles they last. I’ve printed 100’s of toy catapults and other small toys in PLA and then handed them out to 8 and 9 year old children. They held up amazingly well under that abuse.
The toys held up under the abuse right? The toys?
At some point the toys would fail, they were 8 and 9 year olds after all, but it could take a month or more in the hands of children. I was pretty shocked. I did have a paper wad cannon that had a spring that was printed all in one inside the barrel. The first few had issues with the compression spring breaking when over compressed by the kids. But a bit of a redesign of the spring by making it beefier, it not only lasted nearly the whole school year but the kids got more range. Much to the other teachers and parents disappointment.
As long as you stay within its plastic deformation limit, “many cycles” should not matter.
PLA’s downfall for flexible design elements is permanent deflection. It cannot be used on anything that is expected to stay in its tensioned state for anything more than a few seconds. This is why PLA works for catapults and toothpick guns and latches that have a single position rest state where the flexible element is relaxed. If you leave it under tension, though, even just for a few hours, it will not spring back fully. Eventually, it just won’t spring back at all.
Through much testing (read: slowly pissing myself off) I determined in the course of developing my Rockhopper that ABS is the best commonly available choice for permanently or semi-permanently loaded printed spring fixtures – at least out of what most normal and sane people can print with their hobby level machines. Even PETG is better than PLA in that respect, but PLA was useless for me.
