Watertight MJF Parts: Wall Thickness and Orientation

Watertight MJF parts aren't a given. Powder-bed nylon can leak through pores between layers, and whether it does depends a lot on how thick the wall is and which way it was printed.

Can a Powder-Printed Nylon Part Hold Water or Air Pressure?

It can, if the walls are thick enough and the part is oriented well, but test it before you trust it. In the study below, thin walls printed vertically were the ones that leaked.

An r/3Dprinting user ordered an SLS part, expected it to be airtight, and found air leaking out. He wanted it to hold about 100 PSI with printed threads. One reply said SLS should be airtight and suspected a thin spot, a corner or the threads, with plenty of thread tape as the fix. Another said SLS prints are typically watertight above 3 mm walls and seals parts with Dichtol AM. Others suggested vapor polishing or a coating. One said nothing printed would survive half that pressure.

That thread was about SLS. MJF also fuses powder layer by layer, and the study below tested MJF directly.

What one study found about watertight MJF parts

A 2018 open-access study on MJF PA12 vessels printed test vessels on an HP Jet Fusion 4200 with walls from 0.4 to 0.7 mm. Thinner walls leaked more. At the same thickness, vertical prints leaked more than horizontal ones, and sections of the vertical samples showed more pores.

The numbers are stark. With a leak rate of 3 L/h as the cutoff, a 0.5 mm wall printed horizontally held past 0.4 MPa, while the same wall printed vertically hit the cutoff at 0.044 MPa. At 0.6 mm, vertical reached it at 0.357 MPa. At 0.7 mm, both orientations held past 0.4 MPa. The team also printed a ball valve that held 4.2 MPa of water for an hour. That's one study on one machine, so treat it as a direction, not a design value.

Design moves that help

Make sealing walls thicker than you think they need to be. Going by that table, a 0.5 mm wall printed upright is asking for trouble. Where you can, have the pressure-holding walls built flat.

Don't make the printed surface do all the sealing either. A real seal, like an O-ring groove, is easier to trust, and you should pressure test a sample before a run. MJF parts also come out rougher than SLS as-built, which matters on a sealing face, and thin-wall tolerances read differently too.

At DigiForge

MJF Nylon and SLS Nylon PA12 are both on our Materials page. Our 3D printing service lists a tolerance of ±0.3 mm or ±0.3%, whichever is greater. For a part that has to hold water or air, tell us the pressure and the fluid, and ask us when you quote about sealing or testing.