Does the PLA crack easily?

[zmaster 0]

I'm going to design a box which is composed by some parts screwed togheter.

I will use PLA and i have read that it is hard, which also means it is fragile.

I think that if i'll make the screw hole - the one where the the screw grips, don't know the correct english term - too tight then the PLA will crack.

Is it like so? Do you have some experience/suggestions?

[joergen 2]

I'm going to design a box which is composed by some parts screwed togheter.

I will use PLA and i have read that it is hard, which also means it is fragile.

I think that if i'll make the screw hole - the one where the the screw grips, don't know the correct english term - too tight then the PLA will crack.

Is it like so? Do you have some experience/suggestions?

PLA is really tough material, far from brittle as you described, and can be used with screws easily... I routinely make objects that use standard tripod mount screws (1/4in and 3/8in), and I have not seen a failure. smaller/finer threads are a bit more delicate by nature (i.e. M3) and should not be over-tightened. self-cutting screws should also not over-tightened.

The most important thing about PLA is the heat generated from friction. cutting a thread means you need to go suuuuuper slow, clean out the debris frequently, and check the tool temperature with your finger tips: if it is uncomfortable to touch, it gets too hot for PLA, which gets soft at 80C, and you can't cut into soft plastics.

[zmaster 0]

The most important thing about PLA is the heat generated from friction. cutting a thread means you need to go suuuuuper slow, clean out the debris frequently, and check the tool temperature with your finger tips: if it is uncomfortable to touch, it gets too hot for PLA, which gets soft at 80C, and you can't cut into soft plastics.

I think you're talking about drilling the holes with a tool, right? I'm going to create them directly in the design and use self-cutting screws, so there should be no problem.

Thank you!

[joergen 2]

I think you're talking about drilling the holes with a tool, right? I'm going to create them directly in the design and use self-cutting screws, so there should be no problem.

I had explicitly mentioned self-cutting screws for that reason... just keep in mind that small holes become much smaller during printing... on my UM, holes for self-tapping screws that are designed as 1-1.5mm are filled completely and need pilot drilling for self-tapping screws, while 3mm holes turn out to be almost 3mm... so 1-3mm is a difficult grey area.

[mm_build 1]

I routinely make parts with 3mm holes intended for a M3 screw. The holes are modelled at exactly 3mm diameter, but when I print them they are a snug fit for the M3 screw (which I measure an outside thread diameter of 2.90mm). The advantage of this is that you don't need a nut in most cases, and can simply thread the screw into the plastic. I have only used hex hand drivers to insert the screws, and I have never used a special 'self-cutting' screw, which is something I did not even know existed apart from 'self-tapping' screws used for sheet metal work. I have been using button head machine screws of the type that come with the Ultimaker, and they work well. If you don't want the threads to catch on the hole I generally use 4mm diameter which is a pretty good fit after printing.

For the brittleness of PLA, I haven't done any scientific tests, but I know from experience that when it breaks it does tend to 'snap' apart, where as ABS will kink but stay connected. I don't think ABS is necessarily any stronger than PLA, the point where they fail seems about the same, but PLA fails differently. In the case of a screw thread, I think PLA is better suited because it's not as soft.