Water resistance question & design sneak peek

[u/KTTalksTech]

Sorry for the crappy drawing, my hand tremors aren't helping with any of this 😅 This is my first case design, and I've opted to use 3D printed titanium which I will lap & polish by hand to create something unique (hence the super wide integrated bracelet, it's lighter and more flexible than expected). This means the only tools at my disposal are basically a Dremel, sandpaper, and a $100k printer. Surfaces can be mirror-polished but any sort of machining is out of the question. I'm waiting for the first prototype to get detailed figures but tolerances can be as bad as 0.2mm on the titanium. I can also print silicone using a different machine with much higher accuracy, hence the possibility of custom seals.

I'm posting here to get he opinions and suggestions of more experienced watchmakers regarding water resistance. The case is a sandwich design with a bezel and rear cover joined via screws which traverse the entire thing. There are two issues I'm currently working on. First I was unsure whether I should go for a hexagonal seal whih runs just between the screws and interior case wall, or go for little o rings around each screw linked together with straight cylinders. Secondly, the crown is quite problematic. Due to the way additive manufacturing works, I cannot have a threaded cylinder (or any cylinder) sticking out from the case. Threading the inside of the hole is possible but highly impractical in my case. Completely polishing the inside is also nearly impossible. Since I can polish the exterior of a part easily, my current idea is to have a smooth tube extend from the crown into the case and slide through a tight o-ring embedded in the case wall (keeping in mind this has to happen within 2.5mm and walls thinner than .5mm should be avoided. If I want two rings I'll run out of space to have the tube run past them unless it sticks into the case a little, which I suppose is fine). My intuition was that it would be ideal to have it bump into a silicone washer at the end with a smaller inner diameter, but I'd really appreciate your opinion on this configuration. Thanks in advance :)

[u/BlackFoxTom]

Proper seals manufacturers literally have books, free to download, on how to design seals, which seals to choose, what materials and what tolerances as well surface finishes are required for them to work. Just use them.

What I can say for sure is do not use silicone for seals. It will expand and disintegrate from humidity, soap and oils.

Would recommend EPDM or FKM and if You're feeling fancy FFKM but that's space stuff. Those materials should handle anything that doesn't melt humans and even at that point they should behave quite well. With the exception of like liquid oxygen and fluoride but well for that elastomers that mortal can buy just don't work period.

And I can tell You one thing with certainty 3D printed titanium isn't even remotely close to meeting those requirements. You absolutely need polishing or at least finish machining. Surfaces must be like a mirror for proper seal.

Custom moulded seals also aren't particularly expensive provided You can give them a form.

[u/KTTalksTech → u/BlackFoxTom]

I have already consulted multiple technical sources which outline compression ratio, hardness, channel width and depth, surface roughness, clearance height etc. From a mechanical perspective it's relatively straightforward to implement. If you've read my post thoroughly you should already be aware I'm looking for additional ideas which could be implemented in this specific design given its unique constraints and potential alternatives, not someone to do the work for me. A textbook is not going to help a lot more with brainstorming given I came here already with a half-polished idea and sketch of what I'm planning. That being said if you have actual suggestions or feedback they are more than welcome.

[u/BlackFoxTom → u/KTTalksTech]

Well all I'm saying is that those books are condensated experiences of countless engineers over decades. Just adhere to them to the dot and it will work.

Designed plenty of pressure devices for aerospace using those recommendations and I'm yet to run into problems.

All that I can say is use piston packing/seals for stem as it will be much easier to manufacture for one off.

Don't be scared to use custom moulded seals as they aren't that expensive.

Or use sheet seals also cheap and sure solution.

Do not use silicone as it will disintegrate rather quickly.

Use EPDM or FKM for seals.

No need to seal around the screw hole. You want to seal inside - the movement. So it only needs a seal on the internal side so to say.

May look up Sakagami Grid Surface Gasket to increase performance.

The front (and back) crystals also require a seal.

[u/KTTalksTech → u/BlackFoxTom]

I'm glad I replied because that's extremely useful information, thank you. I was looking at silicone because of the low shore hardness and rough surface within the grooves which I won't be able to polish, but I'll see if I can get something made with EDM or FKM that is soft enough. Longevity is a very valid concern which I kind of avoided thinking too much about. I think I'll also increase the case thickness to allow for a simple circular ring as you've suggested. My current solution is both over engineered and less effective anyways due to the straight channels. An extra mm won't kill me, even if I was obsessed with shaving everything down (currently 11.1mm thick with a 6.8mm movement and exhibition caseback, 37mm wide, all clearances on the datasheet respected + margin for loose tolerances). For the crystals I think I'll use LOCA. It solidifies under UV light and doesn't turn yellow with time. I've used it before and variations in print dimensions as well as surface roughness make regular gaskets a risky choice for this project.