Autoclave processing 7

[ScottW]

Is it important to maintain vacuum during an autoclave pressure cycle? Nobody I have asked can supply a definite for or against answer. Your opions would be very welcome.

 

If the part will fall out of the mold if left unattended, of course it is very important to maintain vacuum. I have heard of venting once full pressure is reached in the autoclave. By venting, I mean open the vacuum lines to the atmosphere. I guess this can allow volatiles to be driven off more effectively? Also, it may aid in minimizing voids, since the lower the pressure on the resin, the larger the air bubbles within it will be. If possible, you should probably try it both ways to see which works best. If consolidation is more important to you than driving off volatiles or minimizing void content, you should probably stick to both vacuum and pressure.

 

[ClaireBaker]

The principle of dropping vacuum when full pressure is reached was developed in the early stages of composite manufacture, when resin systems typically had a high volatile content. Maintaining vacuum with such systems during cure is actually likely to increase void content and size by continuing to "boil" those volatiles off. When you vent the bag to atmosphere, the air in the spaces inside the bag equilibrates with atmospheric pressure, so you get a few molecules of air flowing into the bag, but only where wthere are spaces left after consolidation.

A vac bag consolidates the composite by excluding air, thus subjecting it to atmospheric pressure. Autoclave pressue is over and above that presssure, therefore consolidation can be maintained, so the vacuum is not needed for consolidation, provided of course that the bag remains sealed. Some autoclaves require that a minimal vacuum level (e.g. 5 "Hg) be maintained as a way of checking whether a bag has started to leak.

New resin systems don't tend to evolve as many volatiles as the old ones, so it is possible to maintain vacuum during cure without ill effects. For resin systems such as cyanate esters, for which moisture present during cure can pose a problem, it may be advisable to maintain vacuum (although I'm not sure of the effectiveness of this approach: prevention is better than cure).

This is as near to an explanation as I've ever managed to extract from people on the subject during my 4 years in composites engineering.

 

[IOW]

Yes, as said before the "Reduced" vacuum in an Autoclave is used to remove the "Volatiles", produced during the curing process, this must be matched to correct location of the vacuum ports to allow the volatiles to be removed from all of the component!
If not, and the component suffers "Port Shut off" the result will give the component the same "Look" as what would happen with a vacuum bag failure during cure.

 

[Compositepro]

Autoclave cure cycles are often developed in a cookbook fashion without a clear understanding of what each step does and how it relates to other steps. Venting vacuum during cure is a good example. There are many complicating factors in composite processing but they can and must be addressed. Resins can be addition curing like epoxy or condensation curing where water or other volatile is created by the cure reaction. Also resin can behave like a solid or liquid depending on temperature.

There are really only two good reasons to vent vacuum. For high flow resin systems it helps to reduce bleed and to maintain a minimum pressure on the resin in the composite material. Second it is easier to condense and recover volatile materials at ambient than under vacuum. Some volatiles are toxic and its not a good practice to vent them out of the exhaust of your vacuum pump into the plant.

Prepreg lay-ups are a mixture of fibers, resin, voltiles, and air. Note that I list volatiles and air separately. For the purpose of composite processing air is not a volatile but a gas. Volatiles are liquids with significant vapor pressure or, another way to look at it gasses that are condensible. Volatiles and air behave very differently so they cannot logically be grouped together. Given the starting materials listed the goal is to finish with a solid laminate containing only resin and fiber.

Vacuum bags are used to consolidate laminates and remove air. However, a vacuum bag is not a vacuum chamber and it is a lot more complicated how air flows in a vacuum bag. The bag is just a thin film (usually 3 mil nylon). If there is event the slightest difference in pressure the bag moves (this is most apparent when first evacuting the bag). The pressure on both sides of the bag are always equal! Ambient air pressure applies a force of 15 pounds on every square inch of bag area. The bag then applies a force of 15 pounds on every square inch of laminate. It misleading to say that the laminate is under vacuum. Dry fabrics are used as "breathers". These fabrics support the bag to keep open porosity between filaments to allow air flow.

So, what is the bag for? It is an impermeable membrane to gases. As air is removed from the breather fabric by a vacuum pump it cannot be replaced by ambient air due to the bagging film. When the film contacts the breather it encounters resistance that the air pressure under the bag starts to drop and the breather starts to compress. the sum of the air pressure under the bag and the fabric compaction pressure is always equal to the pressure on top of the bag. These are fairly basic concepts and easy to understand. But, it only applies to permeable breather fabric.

When the breather fabric contacts the edge of a prepreg laminate the permeability becomes a big unknown. Some manufacturers purposely leave dry fibers in prepregs to act as breathers. Some do it accidentally. Some do everything possible to have no dry fiber. But mainly there is little or no control. If the prepreg is tacky and has no permeability it has exactly the same characteristics as vacuum bag sealants. Prepreg layers will seal together at the laminate edges and no futher air flow is possible. Large area interlaminar voids can be the result.

Applying autoclave pressure will make these voids smaller but they will not go away. Bleeding resin from the laminate into the breather can remove significant amounts of this air. Bleeding can occur at laminate edges or at pin holes in release films on the surface of the laminate. Keep in mind that the resin pressure is equal to the bag pressure minus the fiber compaction pressure. As resin bleeds out in certain areas the fiber compaction increases and therefore the permeability of the fibers decreases. The result is that when bleed occurs the resin pressure near a pin hole will quickly drop to full vacuum while the resin pressure further away will be close to bag pressure. The pressure gradient is not linear as many would assume. There is a pressure front that starts at the pinhole and moves away from the hole with time. An air bubble will be completely unaffected by bleed until this pressure front reaches it. At this point the higher pressure air in the bubble will quickly create a channel through the lower pressure resin in the compacted fibers.

Venting the vacuum limits how low the resin pressure drops to. All resins contain some volatiles and will boil if the resin pressure drops too low. So, bleed cures can remove air bubbles but can create volatile bubbles.

So why not always vent vacuum. Because there are much better techniques available to remove air bubbles that don't have the drawbacks of bleed cures. It is impossible to accurately control resin content (i.e. part thickness and weight). And bleed cures rarely achieve 0% void content. Most cure cycles that call for vacuum to be vented say "vent vacuum when clave pressure reaches 20 psi". But the autoclave is usualy at or near room temperature at this point. Vaccuum should never be vented until the clave pressure is at 20 psi AND the resin is fluid. If the resin is still essentially solid and the prepreg contains dry, unimpregnated fibers then air will move back into the laminate by the same paths it came out. It will be like there never was any vacuum.

The best method for removing both air and volatiles is the elevated temperature dwell before before applying clave pressure. Heating a laminate to as close to final cure temperature as possible but where the reaction rate of the resin is slow enough to not gel before pressure application. This allows the volatiles in the resin to actually boil. Prepreg plies will actually lift and separate under the bag and the volatile vapors will "burp" out the edges of the laminate into the breather. This process will also strip all of the air out of the laminate. When pressure is the applied the laminate will go from frothy to fully consolidated because all the non-condensible air has been removed and the volatile bubbles will condense and go in to solution in the resin.

I don't know how many readers will make it through this posting but I hope it generates some activity on the composites forum