Autoclave use in composites is something of a threshold – above it and you’re practically “aerospace” – below it and you’re just regular folks. It’s a really effective way to make very high quality parts in a repeatable way – the compaction of laminates is ideal and the temperature and pressure control is necessarily refined. Process control is necessary with an autoclave. You can get away with dodgy stuff with just atmospheric pressure, but with high pressure and heat, you have got to have a well-sorted process or it goes sideways in a big way. The only real problem with autoclave processing is that it’s expensive!
In the last decade or so, the aerospace industry has realized that autoclaves are expensive and time-consuming. Resin and processing technology has improved enough to build more structures “out-of-autoclave.” Airplanes have to be just right, so this is a big step. It requires validation of new processing and material technology. Its only worth it because good-enough parts can be made at a much more profitable (but still safe) price-point.
Note that I am only discussing pre-preg construction here. You can do fun stuff with wet-layup and an autoclave but you might make a mess and I would avoid it if I were you – except for special occasions!
But do you need one?
So if you’re not building aerospace composites, why might you want an autoclave? In my opinion, there are four good reasons:
- You are processing thick (wicked structural) pre-preg laminates. My boat-builder mind jumps to masts, foils, rudder stocks, etc.
- You are using a complicated female mold, and need lots of pressure to mold highly detailed features. Autoclave = super vacuum bag!
- You are making parts on mandrels and want to compact without bleeding resin.
- You have demanding parts that aren’t huge and you need a degree of control that doesn’t allow for resin bleed. Or you need to use higher viscosity resin.
- You are making cored parts and worry about volatiles or moisture in the (honeycomb mainly) core causing bubbles between the skins and the core.
Wait that’s five! Last one is a maybe. You should probably try to solve this another way, but if you have an autoclave it will certainly help!
First: What does more pressure get you?
First and foremost, more pressure gets you more compact laminates. Your laminates have fewer voids in them when cured at higher pressures. This makes sense – it’s essentially a clamping pressure and it squeezes more air out. It also compresses the air down to a smaller volume (because air is compressible) so the voids become smaller. This has a bigger impact on compression performance than tension performance – often a significant amount – or so I’m told. Do tests!
More pressure also gets you better laminate compaction against the mold in areas of complex geometry. Bridged vacuum bags or process materials are more readily crushed into place with 5 atmospheres of pressure than with one. This doesn’t mean you can cut corners, but it can help.
Better surface finish is another benefit of high pressure cures. This is for the same reason that there are fewer voids – pressure on the laminate as a whole and pressure on the individual air molecules bushing them closer together. Surfaces are often more pin-hole free when parts are cured at elevated pressure – especially if tool surfaces aren’t super shiny.
More pressure means that resin viscosity is less of an issue. Making sure to get the needed flow at atmospheric pressure becomes less of an issue with elevated pressure. Higher temperature resins often have higher viscosity and benefit from the additional pressure of autoclave cure.
Thick Laminates
When you lay up plies of pre-preg in a mold, it is common practice to “debulk” them. This just means that every one (or two, or 5 – depends on what you’re up to) ply you put in the mold, you use a vacuum bag or other method to pull air out from between the plies and firmly compact the newest plies against the ones already in the mold. This keeps air from being trapped in the part causing voids. It also keeps wrinkles from forming on outside corners as multiple loose plies stack up.
Even with careful debulking, it is possible for air to get trapped in thick stacks of material – especially stacks of unidirectional material all pointing the same way. Curing these thick laminates at high pressure means that more of the air is either forced to the surface or compressed so it is much smaller. Either way, this is a win!
Complicated Geometry
So you’re building a part and it’s for – well you can’t tell me what it’s for – but it is like part of a bike frame, tubular but bigger with these tight radii in the corners. It’s got to be molded in one piece in a split mold – the whole thing would just fit across a ping-pong table. You’re going to lay up each half in a composite tool, with good alignment features and a stagger of the off-axis plies up one half, while lapping them back in the other. Its not crazy-thick, but there’s core in some of the flat parts of it.
So there’s going to be a bag – an assemblage of bags really – inside, and a bag around the tool perimeter instead of a gasket. You’ve slip-jointed the layup in the corners inside, but you’re still worried about getting the bag pressure you need to really nail the corners. Good thing you have an autoclave!
The extra pressure on the internal bag will compact the laminate with several times more authority than just atmospheric pressure. So those corners will be really in there! The same pressure that can compact your laminate can pop your bag if there are any places where it is “bridging” and the bag is stretched across an outside corner or feature. Might be good to look into a bag with some higher elongation properties just to be safe. Either way, with vacuum pressure you’d get no compaction and (best case) a repair area in that corner – the autoclave just ruins your day faster and before you get your hopes up!
Mandrel Molding
Wrapping pre-preg around a tubular (and ideally maybe a little tapered) aluminum mandrel is a great way to make seamless tubular parts. These are often compacted or debulked using a “pull tape” – a disposable tape that is wrapped tightly around the laminate every few layers to compact it. The pull tape is removed and the next layers go on. It works better for rounder things and is not great for things with corners. If you are making a part like this, wrinkles in the laminate will be your enemy. When it comes time to cook your creation, if you use a bag stack that allows resin to bleed off, it will slightly reduce the diameter of your part and let small wrinkles form. If you actually wanted that resin to stay in the part but at the same time wanted to have a low void laminate, maybe you need an autoclave!
No Resin Bleed
When curing pre-preg under vacuum bag at atmospheric pressure, it is common to use a perforated release ply and some kind of bleeder/breather to manifold the vacuum and allow air out of the laminate – and usually some resin too. With an elevated pressure cure (in an autoclave) it is common to eliminate the “bleeder” part by using non-perforated release film – so air from inside the part can’t bleed out. This is to keep all (or too much of) the resin from getting bled out of the laminate.
Where does this air go, you ask? As air is pressurized, its volume decreases – obvious to scuba divers and people who own air compressors and stop to think about it. Temperature, pressure and volume (see “Ideal Gas Law” – PV = nRT) are all related. For a given number of air molecules, if you increase the pressure, they fit in a smaller space. So your voids will be smaller in actual volume. This is the principle behind “Pressure Casting” – which is a neat way to reduce the voids in cast resins. Higher pressure also keeps liquids from “boiling” into gas as readily – so your moisture is more likely to stay liquid and not turn into a gas with its associated huge increase in volume.