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Using 3D Printed Molds To Cast Custom Seals (e.g. to keep old guns alive)

Diving Gecko

shooter & shooter
Jun 24, 2008
1,366
326
138
Shanghai
Just An Idea For Now
As the title implies, I think we can use 3D printing to make seals. The basic idea is to design or reverse engineer the seal and then 3D print a mold to cast silicone or PU rubber into. I have thought about it for a while now - but while I have seen a few other people do it on Youtube for non speargun purposes, I have not yet ventured into it myself. But I think it could be a viable method to e.g. keep old airguns with "strange", propriatary seals alive.

Here's a video showing an example but you could really make quite complex geometry. E.g. I have no doubt, I can make a vacuum cuff to my own design but whether I can find a PU or silicone rubber which wont tear after a few shots is the real question:


In reg. to the mold materials, you could use most cheap and easy-to-print 3D printing filaments unless you pick a rubber which needs an elevated cure temp, but many don't. As for the seals, while silicone is often very soft, you can actually get 2-component silicones that go up to hardnesses of around 60A which is almost NBR O-ring territory (O-rings are mostly 70A Shore). Some silicones might be even harder but the hardest and easily available one I found with a bit of searching was 60A hardness. Also, silicone would likely release easily from a 3D printed mold. If that same mold was surface treated and with proper use of release agents it could be possible to use 2-component polyurethanes which come in hardness from soft to very hard.

I am still not back to my 3D printer in China and no idea when I will have time to run some test in this but wanted to plant the seed for others. If I do end up doing this, it would be to try to make a custom vacuum cuff a la Dima/Salvi/Pelengas that would work with shafts larger than 8mm. But while I actually do think we can use this method to make static seals, I am not 100% sure I can find a PU rubber or a silicone that will stand up to the abuse a vacuum cuff takes.

I'll update this if I have more news - but as said, just wanted to share the idea for now:)
 
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Diving Gecko

Diving Gecko

shooter & shooter
Jun 24, 2008
1,366
326
138
Shanghai
Some More Info
I've looked a bit more at the data on Smooth-on website and it seems that going with PU might offer some advantages. For the same hardness, their PU seems to have better tear strength, higher elongation @ break and lower viscosity. For a "dynamic" seal like a vacuum cuff that has a shaft run over it and blow it open ever so often, I think both tear strength and how much it can elongate before it breaks could be important. The elongation figures are almost double for the PU but it could also be important that the PU formulations are way less viscous than the silicones. This will make PU easier to degas and pour and get into the nooks and crannies of the mold.
On the flip side, I think PU sticks incredibly well to most materials so there would be more work to prepare the mold for casting in terms of making sure the seal will actually release from the mold.

Here's a silicone with shore 60A hardness:

(No idea if it would work for our uses)

The same vendor has a range of PU rubbers, some going up to 85A. Here's another 60A hardness one here that I have actually seen a guy online make DIY car engine mounts out of:

(Again, no idea if it would work for our uses)

And a shore hardness chart:
Shore-Hardness-Scale_Polytek copy.jpg
 
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popgun pete

Well-Known Member
Jul 30, 2008
3,227
665
153
Australia
I worked in the tyre industry as a tyre engineer and carbon black gives strength to rubber, in fact there were different grades of carbon black that provided varying properties to rubber depending on what purpose the rubber was for. In an automotive tyre there are many different compounds used such as tyre tread rubber and bead area rubber and many more throughout the composite structure. The problem with vacuum cuffs is the spear tail nibbling and eroding the edge of the sealing lip, any damage there such as chips and nicks and the vacuum seal can start to leak. You could use other materials with a thicker or wider contact area, the problem is the friction may go up as the shaft slides through it.
 
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Diving Gecko

Diving Gecko

shooter & shooter
Jun 24, 2008
1,366
326
138
Shanghai
One more video, this time of a guy casting PU rubber feet in a 3D printed PLA mold. It's a long video, but my take-away from it is that he obtained a very easy release with a bit of mold prep but nothing major. Also, don't mind the bubbles in his final parts. They are easy to get rid of by degassing the PU/silicone in a vac chamber before pouring and for seals, I would certainly do that:

If releasing PU was an issue, then the way around it is to 3D print a male copy of the final seal and make a female silicone mold from that male tool/plug and cast the PU into the silicone mold. PU will release from a silicone mold without issues, I think.
 
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popgun pete

Well-Known Member
Jul 30, 2008
3,227
665
153
Australia
Although never common some pneumatic guns had diaphragms and bellows type fittings that were used to keep water out, or rubber parts that were external hooks, gun sights and line wrapping points. Examples of the latter are Mordem guns that had a penchant for using rubber moldings all over the gun. It is not uncommon for these guns to be totally devoid of these fittings so that new owners of second-hand guns have no clue as to what the gun originally looked like. This is why it is important that complete guns are documented to maintain a record of what needs to be duplicated if technology can be brought to bear on this problem of restoring guns to at least working order even if they are never to be used again. However performance evaluations can be never ruled out as claims of yesteryear are put to the test for a less gullible market who have a much better idea of what a speargun needs to be able to actually deliver.
 
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