I have a machine part printed in 100% infill PETG HF and it has a couple of shouldered socket head cap screws sticking out to connect to another part. Although I modeled this part with tight screw tolerances and had to chase the threads with a tap, the shoulder bolts work themselves out over time and have to be snugged down periodically. I can’t share pictures or be too specific about the part as it is proprietary equipment. From what I’ve searched online it seems that standard thread-locker will degrade certain 3D printed materials. I haven’t been able to determine if PETG HF is one of them. Anybody know if there is a good brand/type they have had success with?
Anaerobic thread locker (the most common type) will damage plastic. There are thread lockers intended specifically for plastics, usually cyanoacrylate based. That is basically Super Glue (aka Krazy Glue). Try a very small spot of glue near the top of the thread, enough to stop loosening, but not so much that you could not break the bond if necessary.
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It would be best if you didn’t have to chase the threads at all. Can you adjust the thread in your model?
If you have to chase the thread, know that taps come in three common fit classes. G1, G2 and G3. Very basically: G2 is standard, G1 is tight, and G3 is loose fit. Use a G1 tap if you have to chase the threads. This will give you a snug fit. You’ll need to get the G1 tap from a machine shop supply house.
Even with the snug thread, screws are likely to loosen. Anaerobic thread lockers could damage the plastic, but I’d recommend experimenting with ‘red’ or ‘blue’ threadlocker to see what happens. If badness occurs, then CA glue may be what works.
There are many different thread forms, and machine screws are far from optimum for plastic. There are special screws designed for use in plastic and these work way better. I use machine screws in 3D printed objects as little as possible.
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Thank you. Initially I could screw the bolts in part way, but I could tell it was too close to the point of no return (30 years experience as industrial machinery maintenance). It’s probable that it was a standard G2. I will try to get a G1 and try on the next one. I will likely try a small spot of CA as well. This part was first made in standard PETG and worked well but the tolerances seemed to change a tiny bit in the PETG HF for the same file.
Wood screws with a drop of CA glue works pretty well, but you must provide the pilot hole. 3+ walls. Machine screws are 100% useless IMO, unless there’s a captive nut in there they are threading into.
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I assume the model doesn’t allow for a heat set insert? I try to design my models to use heat set inserts for frequently used threads.
These are fairly large coarse thread screws so not so bad. Yes, I like to allow for a captive nut when possible as well. I’ve had good luck with sheet metal screws on smaller prints with a pilot hole only. Unfortunately, the part I’m dealing with right now can’t use heat inserts or captive nuts due to the design of the other parts theses fit with. So, back to a thread locking compound or glue… with tighter tolerance threads.
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