Hi, can someone answer this question regarding overall pressure requirements when autoclaving?
There is a difference of opinion at my workplace which is if you input 50psi of clave pressure and keep a part under vacuum throughout then the reality is approx 65psi. Or, does the clave pressure make the vacuum pressure redundant when it goes above it so the true pressure remains at 50psi?
I would appreciate a definitive answer to solve this once and for all,
Thanks
Vac is negative clave is positive
So when you have your part sitting ready to bag its at atmospheric ( thou it has no affect on it) an your gauge is effectively at zero
Once bagged an the airs pulled out an dependant where you are in the world an what altitude you will pull -15 psi pressure
When you fill the clave you start from zero an once you get above +15 vac is redundant an not doing anything
50psi is 50psi
Lots of places vent the vac to atmosphere once the jobs got pressure on doesn’t affect the pressure on the job
If the 50 psi in the autoclave is relative then you add 50 psi pressure over the 15 psi from the atmosphéric pressure ending with 65 psi absolute over the bag. If the 50 psi is absolute, you add opnly 35 psi ending with 50 psi on the bag.
I"d go with 50 psi is 50 psi… considering there is no atmosphere inside the autoclave, you don’t get the 15psi from the atmosphere. As the clave ramps up to 20 psi, it would then be at 20 and there would be no atmospheric pressure in teh chamber at that point and as such, it would be a straight 20psi; or 50 or whatever.
Most of the recipes I"ve seen on autoclave parts, have the vacuum on only during the beginning and then the vacuum is turned off… I would imagine this is so any volatiles can migrate out of the laminate but once the resin is flowing and air/water is removed, it’s not necessary.
Thank you for your responses. It just goes to show that although opinions are similar, there is indeed a difference in interpretation of what actually happens inside the autoclave and sort of validates the discussions going on at my workplace. In a sense, both answers are correct but i think the key might well be whether the autoclave pressure starts at absolute or is relative. It is something i will look into when back at work tomorrow. Thanks for all your thoughts on this puzzling matter.
Sammymatik, I understand your point of view.
Here’s my way to see it. When outside the clave you apply vacuum, your parts will be compressed by 14.5 psi (atmospheric pressure). You put the parts in the clave. Before closing it, the gauge read '0’psi. Now you close the clave. The pressure inside is still the atmospheric pressure so there’s still 14.5 psi acting on the part. Now you start addidng pressure inside the clave until the gauge read 50 psi. That pressure is added to the initial 14.5 psi giving 64.5 psi.
I’m not sure but it seems logical.
Vacuum is a view of the human mind because we consider atmospheric pressure being ‘0’ psi. In reality, vacuum is only a pressure difference between the pressure inside a container and atmospheric pressure. Absolute pressure is always positive.
If you don’t vaccum the bag then you have 14.5 psi inside and outside the bag. When under 50 psi in the calve your parts will see 50 psi.
Pressure differential. Read the internal pressure of the part using a digital absolute pressure gauge. What ever external pressure you use, whether it be atmosphere or above atmosphere using an autoclave. the pressure force on the part will be external pressure minus internal pressure. If you vent the vacuum to atmosphere in a autoclave, the pressure on the part will be autoclave pressure minus atmosphere pressure.
From testing at work, unless you have a female radius or an extremely low resin content stack, the pressure resin sees is 95%-100% of what the autoclave pressure is, regardless of vacuum level. The pressure drops noticeably when you get within about 1/2" from the edge of laminate (depending on resin viscosity). So if the AC is at 50psig, the resin will be about 64.7psia (or 50psig). When you have a female radius or bridging, that reduces the pressure under the bag or on the tool side, which is why you’ll often see voids in the female radii but not in the middle of a part.
haha… yah vacuum is indeed a weird thing, it’s so un natural I find these sorts of discussions come up a lot since it’s not necessarily a common sense thing. I think this is really a gauge issue though.
If you put a part in the clave, close the door the part is still seeing atmospheric pressure, so the part is under 14.5 psi of pressure from 1 atmosphere. If your gauge is referenced against 1 atmosphere and it reads 0 PSI then when you pump in N2 and it goes up to 50PSI on the gauge, then yes you would have 50 PSI plus the one atmosphere. This assuming that your at sea level with ideal barometric pressure.
If you had a gauge that was referenced against absolute pressure, then the gauge should read 1 atmosphere even with the door open, and as you add pressure, it would then give you the appropriate reading, in the example you specify, something like 65 psi. Which if the order called for 50 psi, you would have gone over what was required. I would think more pressure wouldn’t be bad but could incur extra cost in N2 and increased wear in the autoclave over time.
So I’d say if your gauge reads 0 to start with the door open, then you are probably correct in assuming that the actual pressure when it reads 50psi is actually 65psi.
I’m not a clave expert so this is my 2cents of wisdom.
Sammymatik, I agree completely.
Gents, really enjoying the discussion here and yes, i agree it is a very weird subject to get your head round. I haven’t been able to get any info regarding the starting pressure point for the AC yet ie, is it absolute or relative. It would seem to me that this subject polarises opinion no matter where we are in the composites world. I will continue to take both into account until i can prove one is correct or at least more likely than the other. Once again, much appreciate all your thoughts, thanks
well if you want to know for sure, it’s pretty easy to figure out…
put a load cell in a vacuum bag, put in the clave, run it, and see the results…
we just like bsing about stuff around here, all in good fun! 