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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.

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    $\begingroup$ LEGO(R) is from ABS. $\endgroup$ – Trish Mar 29 at 13:47
  • $\begingroup$ I'm not sure "press fit" is the best description here. $\endgroup$ – Caleb Mar 30 at 21:35
  • $\begingroup$ @Caleb Because the objects are repeatedly connected and deconnected and not just connected once? I am open for suggestions. I did do some searching around before asking here, but had problems finding stuff. Probably because I didn't know the right terms (if there are any). $\endgroup$ – Jan van der Laan Mar 31 at 6:59
  • $\begingroup$ Note, if you print comes out with elephant foot, initially the lower part may be too wide, after use it might be the normal fit. Please check for elephant foot. $\endgroup$ – 0scar Mar 31 at 12:51
  • $\begingroup$ @0scar that's why I added a small bevel to the (long) edges of the holes. If you look carefully they are just visible on the photo. $\endgroup$ – Jan van der Laan Mar 31 at 13:06
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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.

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  • $\begingroup$ I printed some test parts using PETG; using them for a day and a half as fidget tools, connecting and disconnecting them probably over a 100 times, they have become less tight but still hold well. $\endgroup$ – Jan van der Laan Mar 30 at 19:31
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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.

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  • $\begingroup$ The page you link to seems to suggest that PLA should actually be ok. I would expect this to be more of a problem of wear/abrasion. That still doesn't really explain why PETG does better as PETG is, as far as I know, softer and more prone to abrasion. Perhaps because PLA is harder is loses tiny slivers of its surface while PETG (and probably ABS/ASA) just deforms a little bit $\endgroup$ – Jan van der Laan Mar 31 at 7:04
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    $\begingroup$ No that page shows that PLA permanently deforms much more than PETG or ABS, meaning that the holes will get slightly wider if you leave the pieces fit together (like LEGO's), and they won't be tightly held anymore. I updated the link the original article, the one above was not showing some pictures to me. $\endgroup$ – FarO Mar 31 at 8:18
  • $\begingroup$ Thanks. This is much clearer. The original page you linked to focused more on the fact that PLA didn't do as bad as expected. $\endgroup$ – Jan van der Laan Mar 31 at 10:29
  • $\begingroup$ I have added a picture of the current version of the pieces to the question. Seeing those, do you still think that creep is the cause of the PLA pieces not fitting after a while and not wear/abrasion? $\endgroup$ – Jan van der Laan Mar 31 at 12:28
  • $\begingroup$ Yes abrasion could be the main issue here. PETG will be better, and ABS even more because it's less flexible and your part seems to benefit from a stiff material. $\endgroup$ – FarO Mar 31 at 13:38
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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.

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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.

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  • $\begingroup$ PETG seems to work well. And indeed a simple enclosure might be something to try (instead of waiting to find enough time to build a proper one). $\endgroup$ – Jan van der Laan Mar 30 at 19:34

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