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How do I fit a schrader valve to a Plastic bottle

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 14, 2007 12:20 pm

it's all a matter of preparation - you have to roughen up the surfaces in order to ensure proper adhesion, by doing this I've made epoxy connections that can take in excess of 400 psi without failure.
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Unread postAuthor: SpudBlaster15 » Sat Apr 14, 2007 12:24 pm

I can assure you that no epoxy to polyethylene connection could hold 400psi, no matter the surface preparation.
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 14, 2007 12:37 pm

Well, there are no threaded sections in my mini-piston design:

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It's made purely of epoxy-to-PVC connections and I regularly take it 400 psi and beyond, never had a failure.
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Unread postAuthor: SpudBlaster15 » Sat Apr 14, 2007 1:28 pm

Epoxies adhere very well to PVC. I don't doubt that an epoxy to PVC connection would hold 400psi. Polyethylene is a different story.
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 14, 2007 1:58 pm

It doesn't depend on the material in this case but the structure Epoxy does not naturally stick well to PVC, indeed I can make an epoxy piston by filling a section of smooth PVC with epoxy then tapping it out with a hammer. A PET bottle's walls are thin and flimsy, but if there were thick like PVC pipe (like the will be when the material is at its preform stage it's just as easy and effective to glue if the surfaces are properly roughened.
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Unread postAuthor: SpudBlaster15 » Sat Apr 14, 2007 2:06 pm

No, you are wrong.

Go ahead, roughen up the surface of a solid piece of polyethylene, apply some epoxy, allow it to cure, then observe as you are able to peel off the epoxy using your fingertips.

Regardless of how well the surface is prepared, epoxy cannot bond polyolefins effectively. If you cut large grooves into the PET stock, then applied epoxy, the joint would most likely hold pressure in the same manner a union does, but it certainly wouldn't seal.
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 14, 2007 4:07 pm

SpudBlaster15 wrote:If you cut large grooves into the PET stock, then applied epoxy, the joint would most likely hold pressure in the same manner a union does, but it certainly wouldn't seal.


If it's holding pressure, then it's sealed. I have tried this before, an peel away like magic it didn't.
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Unread postAuthor: SpudBlaster15 » Sat Apr 14, 2007 4:52 pm

It seems we have opposing definitions of "holding pressure". By holding pressure, I mean that the joint can sustain pressure without separating. Whether of not this joint leaks is a different matter.

Epoxy isn't magical as you seem to believe, it cannot bond everything, and polyolefins are good examples of this.
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