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Epoxy casting/use

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Unread postAuthor: Gun Freak » Tue Jun 01, 2010 8:54 pm

Me either, but when the barrel is supported onto the chamber it ends up looking pretty darn straight.
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Unread postAuthor: jackssmirkingrevenge » Wed Jun 02, 2010 12:26 am

Slauma wrote:How high have you taken the pressure on that cannon and what do you think it could handle?


The material can take a lot *if used in the correct manner* as explained in my tutorials.

Some examples of incorrect use where I was reckless and didn't follow my own principles can be found here :roll:

On the other hand, the majority of my properly built high pressure projects have taken 800 psi+ without exploding ;)
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Unread postAuthor: jrrdw » Wed Jun 02, 2010 5:30 am

To the front of the line. 3
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Unread postAuthor: Slauma » Thu Jun 03, 2010 5:39 pm

So here's the plan as of now. I've modified the elbow and the tee so they have a tight fit. After thinking about it, I've decided not to use a temporary mold to cast the epoxy in, rather construct an encasing out of pvc and simply fill it with epoxy and seal it. Here's a little sketchup I did to give an idea

Image

The parts pictured will be constructed by boiling and bending sections of 4" PVC. I'll flatten a couple sections for the ends and I'm going to try to "wrap" it in another section. I'll most likely fit a pressure gauge into [what's left of] the elbow and have it sticking out of the "wrapped section". I'm not exactly sure how I'll do this yet.

JSR,
Thanks for the links, it's always nice to have some successes AND failures to learn from. I still have a bit of uneasiness about the strength though, maybe you can help soothe it. What do you think about using some carbon fiber thread to reinforce this? You can get 100yards of the stuff on ebay for like 10 bucks. I also thought it would be a good idea to really roughen up (scratches, deep gouges, everything but holes) the elbow and the tee with a dremel before encasing them in the epoxy. The more uneven the surface, the more surface area, the more bonding area, the stronger the gun, right? This might all be overkill though, I don't plan to use this cannon over 150psi, simply because I'm not interested in getting a face full of pvc shrapnel. I do have higher pressures at my disposal so it would be nice to use it at high pressures, I would only do so if I could do so comfortably though.

It seems to me like carbon fiber+epoxy+pvc fittings could yield some very good weight/strength ratio parts. I don't know enough on the subject to know whether this could produce high pressure vessels, but it seems like it would add at least some strength to it. Carbon fiber-reinforced polymer (CFRP) pressure vessels have been made to hold over 10,000 psi. []

There was a thread I remember seeing not long ago where someone was asking about carbon fiber. I can't find it now though. If I remember correctly many were quick to dismiss the idea and a lot of it was about cost. Unless there's something that I'm missing it doesn't seem like the cost would be that terrible. There is plenty of carbon fiber cloth and thread available on ebay and other places on the net. It's sold for automotive work, bicycling, and for model aircrafts.
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 04, 2010 12:42 am

The only thing I don't like about your design is the lip-to-lip joint of the tee and the elbow, in this situation I would want to the mould casing to have structural strength. Wrapping with CF thread would put my mind at ease though.

As for roughing up, I usually use a broken section of file or drill bit to really shred the surface ;)
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Unread postAuthor: Slauma » Fri Jun 04, 2010 10:00 pm

Right on, the current design includes a ~6mm gap between the fittings and the side walls of the enclosure. This will allow epoxy to pretty much encase the fittings, except for the front and back. Would you consider this sufficient without CF? It's always an option though. Hell, for 10 bucks, I might just go ahead and order some. I really wish I had the time to do some tests with it. CF seems like it could be a very useful material for strengthening PVC. It would cost less than $50 to cover this entire cannon in it. If that meant I could use it with sch40PVC to safely withstand pressures up to 350 psi even that would be well worth it to me. The thread I mentioned early on CF is here. I'm interested what others might have to say.

Maybe I'll post some pictures up soon of what I have done so far.
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Unread postAuthor: jackssmirkingrevenge » Sat Jun 05, 2010 10:40 am

The issue is that epoxy on its own has relatively little tensile strength, and for say a 1 inch fittings at 150 psi, that's over 115 lbs of force trying to pull the elbow and tee apart. Without any stength in the mould lining, I personally would not feel safe without some kind of wrapping keeping the parts together, and in this case the CF thread sounds ideal.
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