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Old 05-12-2017, 01:09 PM   #1
Zebra

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Default Advantages of autoclave over vacuum over or bagging?

I broke my curing oven recently so I started the process of building a new stronger one. It prompted me to find out what my other options might be.

It got me wondering what, if any, advantages there might be for me in building a high pressure curing vessel instead of a vacuum oven?

I have an old 3000 psi large steel gas tank that would probably fit my part inside if I cut a door at one side. I also have the means to add pressure (I have a high pressure air compressor here).

I am trying to understand how the presssure affects the carbon fiber and mold. I.e. With a vacuum, I know the cf fabric is sucked in one direction which is great for open molds. If you use high pressure instead of a vacuum, is the cf forced out towards the edges of the pressure vessel? I.e. If it was used with a two-part mold, would it force the cf fabric Into both sides of the mold at the same time, allowing you to cure both sides together as one part (as you would with bladder molding)?

If an autoclave is not suitable for curing dimensional parts in two-part molds, what sort of part is it the right process for?

Has anyone here built a diy autoclave or is it typically considered too dangerous?
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Old 05-13-2017, 02:14 AM   #2
f1rob

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You can make your own,but remember it's. Basically a UXB with any sort of pressure in !
Hardest problem is the door,most people weld a flanges onto the tube an endcap after slicing an end off and bolt up every cook
Right pain but I know two guys who did everything for a WSB team with such a set up on a old gas tank
Really need to speak to someone who knows their stuff as repeated heat and pressure cycles can lead to failure
Cooking-everything still has to be vacuum bagged and is under vac in the clave,when you pressure the clave that pressure is pushing on the outside of the bag pushing it into the corners
Even at 15 psi you will notice a MASSIVE difference.carbon doesn't move about under pressure
Hollow parts ( on a bike frame for example)
You would have a through bag joined into your main bag
Bit weird to explain but a internal through bag when vac,d down expands
If the parts bigger or its a blind hole you can join your tube bag Into your main bag but you only join one end to the bag the other is sealed.
You push your tube bag into your part then turn it in on itself and pull it out the end that's joined to your main bag that will work
VERY hard to explain but easy to do once you know
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Old 05-13-2017, 10:10 AM   #3
Zebra

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Thanks.

I understand what you are saying mechanically. The part I am still not 100% clear on is the advantage of applying high pressure if you still have to vacuum bag parts anyway? The vacuum bag already a applies pressure to hold the cf against the mold surface and an inflatable bladder does the same job for tube-like parts, so why cure it in a high pressure atmosphere?

Is it similar to benefit you get from a pressure cooker? I.e. Does it make parts cure faster? If that's the primary purpose then I can probably live with just building another regular vacuum oven instead. I have super fast cure resins for when I need parts finished quickly.

I know a little about working with high pressure air from my PCP airgun hobby, so I understand what is involved and the potential dangers of getting it wrong. I was thinking that I would get the guy that does my hydro testing to take a look for me when it was done, to make sure I had created something safe and not an air-missile in my garage. Testing is the only way to know for sure....

It would be good if I could handle the door adding threads and have It screw on with plumbers tape to get a good seal but I doubt I could pull that off on a large tank. In my head, the door would be a bolt on / screw down end piece with a deralin seal. I would add a lip to prevent it blowing out at high pressure. With the lip, the pressure would hold the door firmly against the deralin seal. That's kinda how the valve works on high pressure air rifles to keep it sealed at 4500psi while still allowing it to release air when needed.

I don't think I would trust a door opening on external hinges in a high pressure device built by me...
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Old 05-13-2017, 10:18 AM   #4
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Vac bag applies a negative pressure of approx 15psi(negative) even in a clave at 15psi its doubleded it
90 psi is 600% increase !
Trust me I use a titan bearcat rifle at 3000psi it doesn't compare to a clave
Know of a door that failed on a 6 ft clave,they found it 1/4 mile from the clave after blowing off and going through the side of the building
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Old 05-13-2017, 10:40 AM   #5
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Quote:
Originally Posted by f1rob View Post
Vac bag applies a negative pressure of approx 15psi(negative) even in a clave at 15psi its doubleded it
90 psi is 600% increase !
Trust me I use a titan bearcat rifle at 3000psi it doesn't compare to a clave
Know of a door that failed on a 6 ft clave,they found it 1/4 mile from the clave after blowing off and going through the side of the building

That doesn't surprise me at all. I see what the rapid release of 3000-4500 psi does to the ammo in my air rifles. The strike of a hammer valve on a 1.5" tube filled to 3000 psi can generate well over 1000ft lb which is more than enough to kill any animal. That kind of pressure rapidly released from a 6 ft autoclave is a cannon and a half.

If that's what the door did when the autoclave was filled with air, imagine the damage that would have occurred if it was filled with a lighter gas like helium...

Don't they have laws about hydro testing autoclave like they do with scuba and scba tanks for safety?
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Old 05-13-2017, 12:56 PM   #6
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Normally insurance tested yearly
All have an over pressure blow off valve
Normally when they do a first off commission test they fill a clave with water and pressurise that
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