MJF threads are a frequent design question for parts that need fasteners. In an r/3Dprinting thread, a designer preparing parts for HP MJF in PA12 asked whether to model an M6 thread on a 4.8 mm tap hole like in metal, or make the hole larger. The replies covered the three real options: print the thread, tap it, or install an insert.
Can MJF print threads directly?
Forerunner 3D, an MJF service, lists M3 (6-32) as the smallest straight thread it prints in MJF Nylon 12, and M6 (1/4-20) for MJF TPU and for SLS. One commenter suggested modeling threads slightly oversize, about 0.2 mm on the radius, because material gets pulled inward during printing. Another warned that metal screws in plastic threads don’t offer much strength.
Should you print a hole and tap it instead?
Another commenter said they wouldn’t model threads at all, just print a suitable hole and tap it. Xeon NC, another MJF service, says PA12 can be tapped after printing for M3 and larger, with a boss and the proper pre-drill size. It recommends inserts for repeated assembly.
What does HP say about inserts?
HP’s MJF Handbook section on inserts recommends heat-staking or ultrasonic inserts for its thermoplastic parts because of their high overall performance. Press-in and self-threading inserts may also work in some applications. HP says to follow the insert supplier’s hole size and depth: oversized holes reduce joint strength, and undersized holes can crack the part. For bosses, HP notes the traditional rule that the boss diameter is twice the insert’s external diameter for inserts under 6 mm, with a 3 mm wall for larger inserts.
Does installation direction matter?
In pull-out testing by Avid Product Development, an MJF service, long heat-set inserts installed from the back held 705 lbf versus 452 lbf from the front. The back-installed insert also pulled through slowly instead of failing all at once. These are one shop’s results for its own test setup.
Which MJF threads option should you recommend?
- Low-cycle, light loads: printed or tapped threads.
- Repeated assembly or higher loads: heat-set or ultrasonic inserts sized to the supplier’s spec.
- Customer choice unclear: ask how often the joint will be opened and what load it carries.
Insert installation is a separate labor step, so list it on the quote and track it as its own operation on the floor.
Related on the Solvi blog: MJF Tolerances: Clearance for Holes and Nut Pockets; MJF Watertight Parts: Can PA12 Hold Water Under Pressure?.
Sources: r/3Dprinting MJF threads thread; HP MJF Handbook, Inserts (4AA7-4111ENW); Forerunner 3D threads guide; Xeon NC PA-12 MJF page; Avid Product Development threaded connections study.