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I am designing some parts that should modular fit together. I am currently exploring a Lego-like design with octagonal holes and cylindrical pins.

I notice that (depending on the amount of clearance) that the fit is initially tight (to the extent that the pieces are very difficult to remove from each other), but that after a few dozen times connecting and disconnecting the parts the fit becomes very loose. I am currently using PLA. With what material would this occur less quickly/is more resistant to this kind of wear?

The sizes of the pins/holes are slightly bigger than Lego (probably similar to Duplo). Don't think that snap-fit would work in that size. Below of a picture of one of the test pieces (this one later printed in PETG).

enter image description here

I have an open printer so I prefer materials that don't require me to build an enclosure first. It is a Prusa i3 MK3S: Direct drive; 1.75 mm filament; max temp 300 °C; heat bed max temp 120 °C.

Jan van der Laan
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PLA and nylon deform over time under stress ("creep"), therefore they will get loose(r) and will snap less effectively together.

See what happens to clips after one day of stretching (respectively nylon, PLA, PETg, ABS):

enter image description here

And if you leave them unstretched, after a while:

enter image description here

PLA permanently deforms a lot. PETG is better than PLA, but ABS or ASA are the best choice for the purpose.

A material simply "flexible" is not necessarily the best choice.

FarO
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A flexible material, such as PETG or ABS, is probably the best. PLA is brittle, especially after absorbing moisture, and probably would crack under continued use.

Nylon is good, but not easy to use. With PETG in an enclosure, I end up running fans to avoid heat creep; so PETG may actually do better without an enclosure.

If you're printing to flex your print, keep in mind the the x and y axes are stronger than the z-axis, which depends on how well the layers stick together. PETG tends to stick to the print surface too well; so I use an Elmer's glue stick for it to pull up the glue layer instead of damaging my print surface.

Perry Webb
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If you're aiming for a Lego-like snap fit, look at Lego parts to see how they work. Using the right material is important, but using it the right way is at least as important. Lego bricks are made so that the walls and tubes can deform very slightly as the studs of another brick are pushed into the brick.

The walls of the octagonal holes in your part appear to be solid. Perhaps they're not really, and underneath the surface layer there's a lesser amount of infill, but infill can provide a lot of strength, and that solid surface will prevent deformation at the entrance of the hole.

Try removing the solid bottom layers and reducing or eliminating the infill in the part so that the walls of your holes can flex more.

Caleb
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PETG, ABS and NYLON would work great for press-fit or snap-fit connections. If you really can't build a (cardboard) enclosure I would go for PETG as it has good repeatable mechanical properties and does not require an enclosure.

Hacky
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