My New Pressure Chamber

[JD Combs Sr]

THIS IS "NOT" A SUGGESTED METHOD. IT IS ONLY A NARRATIVE OF SOMETHING I MADE. ALL RISK IS MINE.

I cast pen barrels on occasion and have always done the onesy towsy thing but I wanted the ability to cast multiple barrels on a 10 inch length of tubing in one shot so I made a long round mold out of 5/8ID PVC tubing. However the overall length of the mold did not allow use of my 2gal paint pot I normally use as a pressure pot for my castings. Needed something longer then the paint pot turned on its side. Three inch PVC to the rescue.

The mold. Its about 13 to 14 inches long, in any case much longer then the depth of the paint pot. More on it in another post.


A comparison of the new one to the paint pot.


The pressure end and the access end with the mold inside.


I used a 3" clean-out adapter for the access end but could not get it to seal. Had two of the threaded plugs so used a section off the bottom of one and glued into the bottom of the thread female section to act as a seat for a gasket. The first pic shows the plug with the psa foam I used as a gasket. The second pic shows the trimmed down plug being used with PSA sand paper to sand the seat smooth.


Installation of the seat. I took a saw kerf section out of the seat section so that it would collapse a little and go far enough down into the female fitting to allow the plug to use all of it's threads when screwed in. Glued the saw kerf together and filled the back side of the seat with PVC glue to seal it and keep it in place.


The only negative to the assemble is that the clean-out is the only part that is not schedule 40 and is not intended for pressure use. However, it does have almost 1/8" deep threads and is nearly 1/4" thick through it's cross section. Although schedule 40 is rated at around 160psi I am running at most 70psi. I tested it to 80psi and noted no creaks or groans so I am not worried about it not holding up. It still leaks a little, drops from 60psi to 40 in about 30 minutes but with a compressor attached overnight that is not a problem.

Appreciate all comments.

 

[mark james]

I have no issue with the basic set-up and design. But the "chemist in my wife" (she's the chemist) says a drop in pressure from 60 PSI to 40 PSI in 30 minutes is not a "little" leak.

If the basic set-up is sound, a double-check (soap tests) on the joints may help.

Yes, the compressor can keep it inflated, but there are still leaks.

Have FUN! Stay safe!

 

[Skie_M]

Oh, OK ... I see now how you made your mold fit in you pressure pot ... you built it custom to fit!

 

[liljohn1368]

What's the advantages of laying your pressure pot on the side? Other than longer molds..

 

[Mach4]

I've read lots of stories of people getting seriously injured from PVC exploding. I've thought that if I ever decided to go that route I'd do a carbon fiber/epoxy layup over the PVC to add strength and, more importantly, contain the shrapnel in the event of an "event". The other concern is the "access port" - I'm not sure how best to contain that, but at least keep it pointed away from any life forms.

....or maybe weld up a "scatter shield" like what is mandated in auto racing to contain the shrapnel from exploding flywheels.

 

[Skie_M]

I notice that the paint pot has considerably higher volume than the new rig ... I assume the new rig pressurizes a good deal faster due to this lower capacity...

 

[Rounder]

IF using PVC I would suggest going with Schedule 80 to increase the safety factor. You can not be TOOOOOOOO safe. PVC shrapnel would be deadly.

 

[KCW]

I am by no means, an expert, and I don't know where you are using this, but I would caution using this in the summer, in your garage, or any other place, where this will see higher temperatures. The strength of PVC degrades considerably with temperature. At 110 degrees, PVC is approximately half as strong.

 

[JD Combs Sr]

New Pressure Chamber Responses

Before I respond to your posts let me show you some data from my research on Sched 40 PVC I did. Straight PVC not CPVC has a working pressure of 156psi(130psi for threaded fittings), is static tested at 590psi and has a burst pressure rating of 840psi. I am sure that "threaded fittings" does not include clean-out plugs but it does include the female section. I have tested it at 80psi with no signs, creaks, groans, swelling etc. I will be operating it at less then 70psi and yes, it leaks a little but I am not after a hermetically sealed container, just something to apply a consistent and steady pressure to Alumilite castings. If my shop compressor has to come on a few times over the night to maintain that pressure it is not a big deal to me.
Now I will respond individually to each post with some responses referring to info in the above.

I have no issue with the basic set-up and design. But the "chemist in my wife" (she's the chemist) says a drop in pressure from 60 PSI to 40 PSI in 30 minutes is not a "little" leak.
If the basic set-up is sound, a double-check (soap tests) on the joints may help.
Yes, the compressor can keep it inflated, but there are still leaks.
Have FUN! Stay safe!

Hi Mark, thanks for responding. As I mentioned above it does leak a little but is insignificant to me and the application. Definitely intend to stay safe. Thanks again.

I notice that the paint pot has considerably higher volume than the new rig ... I assume the new rig pressurizes a good deal faster due to this lower capacity...

Oh, OK ... I see now how you made your mold fit in you pressure pot ... you built it custom to fit!

Hi Skie, exactly my intention. :biggrin: Yes there is probably a difference in charge time but it is so slight as to not even being noticeable.

What's the advantages of laying your pressure pot on the side? Other than longer molds..

Thanks JP, good question. It is just a thing on my part, I want to see what is going on in the mold, all of my molds are made in the horizontal. For me it is just easier. No other reason then personal preference.

I've read lots of stories of people getting seriously injured from PVC exploding. I've thought that if I ever decided to go that route I'd do a carbon fiber/epoxy layup over the PVC to add strength and, more importantly, contain the shrapnel in the event of an "event". The other concern is the "access port" - I'm not sure how best to contain that, but at least keep it pointed away from any life forms.
....or maybe weld up a "scatter shield" like what is mandated in auto racing to contain the shrapnel from exploding flywheels.

Hi Mach4, appreciate the suggestions but based on the info I found(see above) on PVC I believe the chamber is safe, however, I do tend to stay away from the access end when it is charged. :wink:

IF using PVC I would suggest going with Schedule 80 to increase the safety factor. You can not be TOOOOOOOO safe. PVC shrapnel would be deadly.

Thanks for the suggestion Rounder but I believe the safety margin is more then sufficient operating at only 70psi or less.

I am by no means, an expert, and I don't know where you are using this, but I would caution using this in the summer, in your garage, or any other place, where this will see higher temperatures. The strength of PVC degrades considerably with temperature. At 110 degrees, PVC is approximately half as strong.

Hi KCW, It is being used in an air-conditioned shop that rarely sees a temperature on the hottest days over 80*F. If it goes above that I shut the shop down and head to the house:wink:. I am retired and don't "need" to be in the shop so if it gets a little uncomfortable I leave until the cool of the next morning.:biggrin: Also in most cases the chamber will only be pressurized overnight when the shop is unoccupied.
The pressure specs I quoted above are based on 73 degrees, if it ever got up to 85 in the shop while the chamber was charged two things can be assumed 1st I want be there and 2nd the pressure de-rating factor would drop the 156psi to 124psi, still well over my operating pressure.

Again thanks every one for your comments and suggestions. I will make a major safety post if the "big blow" ever happens.

 

[JD Combs Sr]

What's the advantages of laying your pressure pot on the side? Other than longer molds..

Liljohn, I realized I didn't answer your question thoroughly enough, at least not enough for me. The main reason I like horizontal entry into the pressure chamber/pot is that it is simpler and has a less likely hood of spilling the mold. I use to load the paint pot from its top but it seem overly cumbersome to have to load it like my arm was one of those claw devices used to pickup and drop toys down a shut. It is much easier to simple slide the mold into the chamber/pot, just like putting a pie into an oven. Yes I even cook on occasion.:biggrin:
Oh and BTW I have rotor-cuff and carpel-tunnel issues to work around also.

 

[lorbay]

On the 7th picture is that a 2" x 72" belt sander in the back ground.???

Lin.

 

[Gregf]

I assume you have read the articles on the dangers of using pvc with compressed air?


Can PVC or CPVC pipe be used for compressed air lines? | US Plastic Corporation

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