This is a quick blog to show some images of the tests we made on 3mm PLA filament to see how much tension it can take from a self-threaded M3 nut. The idea is that it could be used to post-tension light structures printed with RepRap-type printers. This structures are being investigated by Forrest Higgs:
Printing Light Structures (Forrest Higgs)
The test itself was simple. First, we used an M3 die to thread one end of a 350mm long PLA filament. The die cut very well through the PLA, and the nut screwed on it very smoothly. We added a washer, and hang the filament between the jaws of a vice:
The jaws are open, so the filament hangs from the nut and the washer. Then, with a piece of metal we have on hand, we made a loop in the other end of the filament, and used it to hang different sizes of "calibrated" weights...
First, a jug of 12 kg. Then, another one with 6 kg, to make a total of 18 kg:
The filament hold that fine. Then we tried to hang 8 kg more (some iron tools we have on hand), but the filament failed. The nut stripped the thread. So, the failure point of the die cut thread is somewhere between 18 kg and 26 kg.But, as we said, the nut screwed very smoothly over the threaded PLA. We supposed the die cut too deeply into the plastic, making a nice but weak thread. So, we clamped the threaded PLA end in the vice, and screwed the M3 nut over the (stripped) tread. Then we continued screwing the nut, cutting with it some 8 mm of new thread in the PLA. The force needed to do that was relatively high, and the final thread worked well, and was much more "solid" than the previous one:
The image is out of focus, but you can see the upper end of the yellow PLA, with the ripped thread, and below it the new thread, nice and clean, where the M3 nut is screwed. With this stronger union, we went back to the weight hanging process. Now the 12+6+8 kg hung fine:
The next step was trying to add 8 kg more. Again, the filament failed. But this time it was not the thread. The PLA broke at the loop where the weights were hung. The fold in the PLA and the tension of the weight added enough stress, and cracked the filament. We could not make more testing, but we have the impression that the threaded end of the PLA could withstand those 32 kg.
In the next image you can see the filament, still in good condition, and the intact threaded end. The clean fracture point can be barely seen at the other end. We tortured the filament to force it hold the form you see in the photo. Even then it showed no signs of fatigue:
The PLA has lower tensile strength than ABS (20 instead of 40 MPa) (although this numbers can be quite different depending on the exact formulation of the polymers). So, it seems possible to hold somewhere near 40 kg from a self threaded nut in ABS filament.
