Hi Emma, the nature of the SLS printing process does not lend itself well to being watertight. This is because there will always be some space between fused particles making the parts slightly porous. On top of that, nylon is hygroscopic, i.e. it will absorb moisture over time. SLS is probably not the best option.

Hi, Emma.

You can make watertight SLS parts by paying special attention to the sealing surfaces. Typically wall thickness around ~2 mm will be watertight. Leaks most likely will be at the junction where you are sealing. E.g., where you may have a cap or gasket. This is because of the inherently rough surface area of SLS nylon. Here’s what you can do:

1. Design wise, mitigate sealing points to focus on specific areas. Try to have all sealing surfaces as circular vs. irregular, and don’t have any weird T-joint seals because they’re so tough to guarantee a seal. Make sure you have features like screws on the outside of the seal so you don’t have to seal each screw hole. Seals should be given even pressure and be backed up by some sort of mechanical restraint (screws, caps, clips, etc.)
2. Smooth the part and sealing surface:
   1. Easiest, but risky: Chemical vapor smooth will help give a smoother surface across the part. I’d do this even if I’m doing the other suggestions below.
   2. Expensive, but less risky: Post-machining, however, is the best way to get a sealable surface (you need to add extra material to your design and post machine away. 
   3. Tedious, but doable: Sand, epoxy fill, and sand again the sealing surfaces.
3. Use multiple o-rings, if possible, with silicone grease to help fill the voids. If it’s an irregular surface, use a silicone caulk.
4. Test with a light submersion and water indicating strips (or just paper) on the inside.

I have made SLS parts hit IP69, which was 1 hr, 10-meter saltwater submersion. Don’t worry about the hygroscopic nature of nylon for prototyping unless the total life of the product is underwater.

Thanks!

Hi Emma, there are definitely other options. For example, SLA (resin printing) can produce highly complex watertight parts. The one downside is that this process doesn’t use standard thermoplastics but instead makes use of photopolymers. Most common photopolymers do not have great mechanical properties, especially when compared to the nylons used in SLS printing. That’s not to say there aren’t high strength photopolymers out there, they are just more costly.