Can MJF watertight parts replace machined or molded fluid components? In an r/3Dprinting thread on making engineering prints watertight, a designer needed parts that could handle pressure spikes up to 10 bar and connect to NPT fittings. Bureaus get versions of this question whenever a customer wants a manifold, housing or reservoir printed.
Is MJF PA12 watertight as printed?
HP says yes, within limits. Its white paper “Fluid tightness characterization I” states that parts printed in HP 3D High Reusability PA 12 achieve watertight properties without additional post-processing. HP’s PA 12 data sheet lists the same claim.
What wall thickness does HP recommend?
HP tested 1.25, 2.5 and 4 mm walls with water at 25°C for 7 hours. Its recommendations:
- 1.25 mm: recommended at 3 bar, not at 10 or 20 bar.
- 2.5 mm: recommended at 3 and 10 bar, not at 20 bar.
- 4 mm: recommended at 3, 10 and 20 bar.
Shape matters too. HP says spheres handle pressure best because they have no flat surfaces. In its tests, cubic vessels broke at 10 and 20 bar, while spherical and cylindrical vessels were recommended at all three pressures.
Does orientation matter?
Yes. A 2018 study in Materials (MDPI) found that wall thickness and print orientation decide watertightness. For the same wall, vertical orientation needs more layers and is more likely to have air gaps that leak. The authors held 4.2 MPa of water pressure for one hour on a part with its cylindrical section oriented in the XY plane.
What about threads for fittings?
Commenters in the thread suggested modeling the hole to the minor diameter and tapping the NPT thread afterward. Treat that as one operator’s approach, and test the joint, since the thread is often the weakest point.
How should you test MJF watertight parts?
One commenter warned never to pressure test with air: compressed air stores a lot of energy and releases it all at once if the part fails. Hydrostatic (water) testing is the safer approach. Follow your customer’s test standard and your shop’s safety procedures.
What should the bureau tell the customer?
- Ask for working pressure, fluid, temperature and any test standard up front.
- Recommend wall thickness and shape based on HP’s guidance.
- Orient pressure-bearing walls deliberately and record it on the job.
- Agree on who does pressure testing before you quote.
Related on the Solvi blog: Should You Dye MJF Parts Black or Vapor Smooth Them?; MJF Threads: Print Them, Tap Them, or Use Inserts?; PA11 vs PA12 on MJF: When Is PA11 Worth Running?.
Sources: HP white paper “Fluid tightness characterization I” (4AA7-2720ENW); HP PA 12 data sheet; MDPI Materials 11(8):1472; r/3Dprinting watertight thread.