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I'm not sure how to ask it but i'll try

I want to 3d-print 2 half spheres with a screw cap that then can be clipped once (can't un-clip later) And can be filled with water (lets say its food safe, i'll also coat the internal to waterproof it)

enter image description here

So, is there any known mechanical way to create a joint that will prevent water leakage?

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  • $\begingroup$ it might be easier if you don't split the opening with the bottle cap. Then you could have a cone type seal with external snaps (maybe a bit clunky looking). The idea with conical seals is that the male angle is slightly different from the female angle, and there is a slight interference and flex resulting in a circular contact line. $\endgroup$
    – Pete W
    Jul 31 at 2:03
  • $\begingroup$ finely polished steel surfaces will weld themselves together. $\endgroup$
    – Tiger Guy
    Jul 31 at 4:46
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For a static joint, there are basically three options:

  1. Use an adhesive of some sort to overcome irregularities in the mating surfaces.
  2. Use a gasket to overcome irregularities in the mating surfaces.
  3. Use sufficient clamping force to overcome irregularities in the mating surfaces.

For a purely 3D-printed object, option (3) is out: you can't get sufficient precision on the clamping and mating surfaces to prevent leaks. That leaves whichever of (1) or (2) is better for your application.

But if you're 3D printing, why print the object in pieces? One of the strengths of additive manufacturing is that you can make objects such as hollow spheres that can't be made in a single piece using subtractive techniques.

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  • $\begingroup$ Thanks! this way I can stack more in a package and ship :) thats why... $\endgroup$
    – Simon
    Jul 31 at 1:07
  • $\begingroup$ @Simon Then I would assume you won’t be 3D printing. 3D printing is great for low volume prototyping, but bad for mass production (generally). If you are injection molding, you may be able to achieve a sealing snap fit. $\endgroup$
    – Eric S
    Jul 31 at 1:27
  • $\begingroup$ Yes, that's the ultimate goal, but what i wanted to know is how to design a sealing snap :) $\endgroup$
    – Simon
    Jul 31 at 1:35
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My advice is to change the design if possible from vertical split to horizontal. i.e.

horizontal vertical
enter image description here enter image description here
Better Not so good

The reason is that :

  1. in the vertical solution the internal pressure will tend to separate the halves

enter image description here

figure 2: Forces developing on a half of cylinder due to internal pressure (source: Mathalino

  1. you could go for a design that is hemicylindrical (lower part cylinder, top flat). In that version, you wouldn't need to worry so much about sealing if the vessel was static.

enter image description here

figure 4: variation with lower cylinder and top flat

Finally in general the dimensional accuracy of 3D printing does not lend itself for precision sealing. IMHO, it would be best to design perimetrical grooves around the sealing surface and then use an o-ring.

If you are feeling particularly adventure you could design the parts in such a way that they need to slightly deform when you put them together (i.e. they are pushing together). That will help the O-ring perform better.

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Do you need the rigidity of the part? The seal is always the problematic detail and if you are going to waterproof it you might as well use a separate membrane.

This reduces your problem into putting paintballs / laundry pods / liquid filled satches into a rigid ball.

https://www.alibaba.com/product-detail/8-Nozzles-Sachet-Juice-Drink-Liquid_60804464117.html https://www.wsj.com/articles/three-tide-pods-a-wash-procter-gamble-pushes-more-doses-1465407495 https://www.bzpaintball.co.uk/

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