# Minimum dimensions for an o-ring water-tight seal?

I’m looking for advice on designing a water-tight seal. I have a tube that gets an end-cap screwed into it. Both parts are in Tough v04. The threads that cinch up the cap are further “inside” than the o-ring, and they are working. The problem is that the bottom of the tube flares out from the force of the o-ring when I insert the cap. The OD measures close to 63mm with the o-ring inserted. I can fix it with a hose-clamp on the outside of the tube, but I’d like to avoid that.

The tube forms the “bore” for the system, it is 60mm OD and 55.7mm ID (2.15mm thick wall). The cap that gets inserted is the “piston” and has is 54.318mm OD and 45mm ID (2.5mm wall thickness). The o-ring gland is a groove 49.793mm which compresses the -225 o-ring 16%. The o-ring is 50A durometer, and I estimated 7psi along the circumference of the o-ring at 20% compression, or 7.15psi total (o-ring is 1/8" high) [sorry for the mixed units… o-ring is imperial, the rest of the design is metric… I’m no worse than 1990’s Ford cars. ]

Right now, the outer surface of the tube is a simple cylinder. I was thinking of adding a thicker ring around where the o-ring gets inserted (like a blister around the circumference on the outside where the oring sits on the inside), or a lattice structure around the circumference to add rigidity, or simply just make the whole outer tube thicker— but by how much?

Any help and advice would be greatly appreciated!!!

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It looks like you’re calculating this like it’s a metal part for a simple o-ring application in which the tube and end-cap are considered rigid bodies, this is not the case with 3D printed parts especially with Tough which is quite a bit less rigid than standard resins.

2.15mm wall is probably a bit low given the (relatively) enormous forces applied to the tube (even with a 50 shore-A ring), so increasing that might help but keep in mind that due to the deformation of the tube you’re definitely not getting your 16% of compressions. Really I would instead consider some other method to guarantee waterproofness. I would be looking at what is being done for plastics parts such as PP or PE parts.

If you really need to make the parts functional as designed I would try to print it with thicker walls in a standard resin which despite being much brittle is also a bit stiffer.

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Thanks so much for your reply. I designed a first go of a compression fitting. This first version needs a little refinement, but it is already performing better. When I have a working model I’ll be sure to upload it.

Thanks again!

How are you curing your parts printed in tough resin? I ask because the FL curing guide shows that exposure to UV is only part of the recipe: complete curing is only possible with heat as well (I think at 60C). If you don’t have heat, you’ll probably have to cure much longer (see the graphs in that cure guide).

I print parts for an engineering prototype in Tough (and I’m awaiting my Form Cure so only have UV curing now) so I cure for an extended period (a few hours) to get reasonable rigidity out of my Tough parts. There’s a distinct color change once the parts cure. If you’re not seeing this color change, consider a longer cure and you may seem less deformation in your parts.

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