I know some companies run their autoclave at 50psi, some at 90psi, etc. I imagine there are… health risks involved with raising the pressure of a heated tank. But in general, theoretically, only with respect to the carbon fiber laminate, how does part quality compare to pressure applied to it? Are there limits other than the autoclave limits?
The quality is not the issue. The issue is what resin system is being used. Each resin manufacturer has a different cure schedule for their resin. It is a Time temperature and pressure graph essentially that must be followed.
I must say, do not try to build your own autoclave, you might kill yourself, honestly. unless the tank has a hinge on it with bolts that look they belong on a bank safe, you will blow the door off of it. Also you have to figure out a way to get your vacuum line in it.
I have access to an autoclave. I didn’t mean to insinuate that I was going to do anything unsafe. I do also have an idea for which I would like to explore the possibility of, but I will discuss that with my mech. engineering professors. Right now I was just wondering about the titled topic.
As a general rule of thumb: the greater the pressure the better the results. However you get diminishing returns as pressure goes up, there is a big gain from vacuum to 4bar, less gain 4 to 7bar, last 0.01% above 7bar.
The higher the pressure the denser the air so it will transfer heat better and help to prevent exotherm too.
The improvements are better surface finish, ie no pin hole. The pressure creates nearly zero voids and any gases that are trapped dissolve into the epoxy which also eliminates voids. You get better consolidation of fibres giving a better packing density which means better volume fraction = light, stronger parts.
Commercially the higher the pressure the high the running costs as you need more ‘gas’ to reach that pressure and it will take longer to pressurise and dump.
We run ours at 3.5bar with vacuum so the part sees 4.5bar as it gives a good balance.
Right. I figured it would be a diminishing returns sort of deal… Is there some sort of theoretical limit to fiber volume fraction?
100 percent volume fiber?
I would assume that it’s hard to say exactly what the functional limit it. You just need enough resin to transmit the load between fibers.
haha, I came back to omit the 100 percent option, but it looks like you beat me to it. So it’s a trial and error sort of thing…
There is a limit as the fibres are round so there is always a gap (I can give you a number tomorrow, my book is at work). 70% fibre is as low as some manufactures of prepreg are willing to go, 65% is common.
In compression your matrix is doing most of the work, thus less matrix = less compressive strength but better tensile strength. It’s a balance.
that makes sense and is what I was looking for. Possibly it is the ratio of the circle(radius r) to a square unit cell(assuming smallest square possible having the length of one side 2r)… that would give you 78.5% Vf. Thanks.
Kind of an off topic question, but when you run your vacuum and get the part how you like it…14psi, then you put it in the autoclave and start upping the pressure.
What happens to the vacuum? Does the pump still pull even though the pressure inside is higher than what it can normally pull? Do you turn off the pump (disconnect) and then let the autoclave push out any air etc?
I don’t have any experience with autoclaves, (just more of a curiosity of how the process works).
I have a friend that does a lot of molding with heated molds and inflatable bladders. He goes to 150 psi.
vacuum lines run inside of the autoclave and the vacuum aspect is a totally different system then the outside pressure exerted on the bag by the autoclave. I think that answers your question correctly…
DY: It’s a debated topic on what exactly the vacuum pump does when you are exerting 100psi on the bag. Some people say 85 psi plus vacuum equals the 100psi, but yet some people vent the bag to outside atmosphere once the pressure reaches, say 20psi.
I understand that the 100psi pressure, along with the 14psi relative vacuum pressure will still end up with the 100psi on the bag, because the vacuum isn’t really doing anything in the higher pressure atmosphere. However, if you have something that outgases, the vacuum pump can draw out those gases easier.
Might be some journal topics on this actually…
Yeah it is much debated. We have seen better results from leaving the vacuum on. I have had bags pop when I restart the pump when it is up to pressure, so that would lead me to think that the vacuum adds a little something more.
As Riff said it helps if there is out gassing or a small leak and if there is any moisture in the laminate (we cook parts for people that don’t have temp controlled rooms, and sometimes they piss out water).
the part, or the people? your autoclave room must smell weird.
so companies sell autoclave time?
Little bit of both. Our autoclave is in the main factory area (its 3m dia x 4m, so to big for it’s own room) so it’s not too bad.
Yeah we offer cooks to third parties. Means that pre-pregs are a better option as you don’t need to buy an autoclave, which brings done the cost a lot.
Consider this
Everyday objects are subjected to 14.7 psia (sea level) at all times. The trouble is that since this force is equally distributed, there is no net effect.
Likewise without a vacuum the force on a vacuum bag is 14.7 psia on both sides so there is no compression.
When the air inside the bag is removed it can no longer push up and so the section between the top of the bag and the bottom of the plate now balance and everything in between squishes.
In an autoclave this is the same.
If an autoclave is pushing 100 psi absolute (atmosphere is 14.7 psi absolute) and a vacuum is being pulled then the total pressure is 100 psia
If the bag is vented to atmosphere then its 100 psia - 14.7 psia or 85.4 psia. This is because the air in the bag is able to push back some of the force from the autoclave.
I think this is the source of confusion. Hopefully I answered someone’s question
If you want I can talk to my boss. Our autoclave is sitting idle for sometime now. 
I’m in San Diego. If anyone had autoclave time around here I might be interested