Autoclave Cure - To vent or not to vent. 10

[glevans]

When autoclave curing parts it is normal to vent the vacuum when pressure gets to 15 - 20 psi. Why is this ? Are there any good reasons to leave the vacuum connected ??

 

[berkshire]

glevans (Aerospace)
Once you get above about 15 psi the vacuum pump is not doing any good, remember the pressure for one standard atmosphere.
The only reason you might want to leave a hose connected, is if you had a pollution control device connected to the output of the pump, and even that is not a good reason.
It is much easier to just plug the hose into a dump manifold.
B.E.

 

[Compositepro]

You ask a very good question. I may write a book on the subject some day. The complication is that there are many different types of prepregs or polymers out there that have different processing requirements. Often inappropriate cure cycles are selected and this makes it difficult for engineers in the future to be able to understand what is appropriate.

The main reason for venting vacuum is to prevent void formation due to excessive resin bleed. This is a problem with resin systems that drop to a low viscosity during cure. In such cases the resin flows out of the fibers to the vacuum in the bleeder/breather until the bag pressure is entirely supported by the fibers and the resin is actually under vacuum. Under these conditions most resins contain enough volatile material that it will boil and continue to push more resin out of the fibers, resulting in voids. Venting the vacuum ensures that the resin pressure never drops below 15 psia (absolute pressure).

However, many prepregs are not fully impregnated. This may be intentional or not. Dry fibers in prepregs are beneficial for improving permeability to air flow while pulling vacuum, but they will also allow air to flow back into a part when vacuum is vented. Putting 20 psig on the autoclave will not prevent this. Vacuum should not be vented, if at all, until the resin in the part has flowed enough that the fibers are fully impregnated and consolidated so that air cannot re-enter the part. This is usually at some elevated temperature like 150F. There is also nothing relevant about 15-20 psig. Venting can wait until 85 psig.

For many resin sytems venting is not required or can be replaced with a bagging techique that prevents bleed while allowing good breathing.

One other reason to vent vacuum is for condensation cure polymers where a lot of water or solvent is removed during cure. It isn't good for the vacuum pumps to run these through. Also traps to condense and collect these volatiles do not work under vacuum.

One of the biggest problems I see in many new cure cycles is that full pressure is applied at the beginning of the cure cycle. The good reasons to do this is that you find out immediately if your bag is bridged anywhere so that when it blows when you can still abort the cure cycle. Also, heat transfer is better at higher pressure. However, one of the greatest benefits of the autoclave is lost. Large amounts of volatiles can be removed very quickly by allowing boiling to occur before autoclave pressure is applied. this boiling of volatiles (such as residual solvents and absorbed moisture) is also very effecive at striping air (which is not a volatile but a gas) out of the part. When pressure is applied there is no air left in the part and volatile bubbles will instantly collapse and go into solution. It is then possible to get zero voids rather that the typical 1/2%

 

[edrush]

Excellent post Compositepro! I have never seen a good reason to vent but you have presented some good arguments. My gut feel is that if a bag is fully supported by fiber at 100 psi or so and at the minimum of the viscosity profile, the laminate is too dry. But I have no evidence to back this up.

 

[Compositepro]

Edrush, in a way you may be correct, but "too dry" is an ambgiguous term. I guess you mean that after cure it visually appears dry. This is caused by the lack of a uniform resin layer on the part surface, which reflects light differently than exposed fiber. Almost by defintition that means there are voids in the part. This can be caused by lack of sufficient resin in the prepreg, which, I think, you are implying. But more often it is caused by excess bleed during cure, or by a blown vacuum bag, or by a tool leak.

Yes, a laminate where the fibers are fully supporting the bag will usually have a very high fiber volume because you have 100 psi of compaction pressure on the fibers. It will appear "dry" and may or may not have voids. However, it is also possible that the fibers have been entangled and "bulked-up" so that even 100 psi will give 60% fiber volume. I have encountered this with carbon fabrics. Airweave is an extreme example of what I mean.

 

[JacobL]

Compositepro,

Do you have any good material reference on the manufacturing process of composites? (in electronic format preferable).
I'm starting work at a composite manufacturer, and can use all the help I can get.

TIA

Jake.

 

[Compositepro]

I used to have a collection of articles that had some useful information but you would still have to dig to extract the information. The important thing is to understand your basic chemistry and physics and which principles to apply when. The hard part is that they all apply at all times but some are significant and some are not. The ones that are important will change from situation to situation. I have never come across one general reference on composite processing. Most handbooks are cookbook recipies for working with one material. The key to solving problems is often just having the correct "mental picture" (or "model") of the situation. Starting with the wrong model (or no model) can result in moths of wasted effort. In my experience, some of the more important points are:
A vacuum-bag is not a vacuum chamber.
A volatile is not a gas but a liquid with a vapor-pressure.
Air flow requires a physical path.
Fiber compaction pressure and resin hydrostatic pressure is equal to bag pressure at any one point.
Differences in resin presure leads to resin flow.

These are just a few of many.

 

[GTwigg]

Excellent post Compositepro. Please do write a book on the subject as I think the events going on inside of the vacuum bag are the most mis-understood (by me at least) keys to making good composite parts.

In one of the earlier posts on this thread you stated: "For many resin sytems venting is not required or can be replaced with a bagging technique that prevents bleed while allowing good breathing."

Can you elaborate on what bagging techniques would give you this combination of breathing with restricted resin flow? I've had reasonable success with non-perforated release film and using dry fiber tows tying the (pre-preg or semi-preg) laminate to the breather. Are there more tricks which can be employed to that end? Thanks.

 

[Compositepro]

The technique I refered to is called "string breathe" and is similar but distinct from "string bleed". It works well with prepregs that are not fully impregnated and therefore contain a network of dry fibers that allow air to be drawn out of the edges of laminates. You create your own "semi-permeable" material using fiberglass strings or strips of cloth. Other fibers can also work. Fiberglass cloth is often used as a breather material because the space between the filaments and yarns allow gas flow and cannot be closed by compaction of the fibers.

There a two different types of pores in woven cloth or threads. The larger and highly permeable pores are created by yarn bundles that cannot pack any closer together. But there are much smaller pores inside the yarn bundles created by how the individual filaments pack together. The larger pores acount for almost all of the permeability of fiberglass and these pores can easily be sealed with strips of vacuum-bag sealant on both sides of a cloth or string. The sealant is to viscous to penetrate and seal the the pores in the threads but gas molecules can easily flow through these pores. Resins have a viscosity somewhere between that of air and sealant tape. Most high-flow resins will not flow more than 1/2" though a glass string that is encapuslated with sealant tape, and has vacuum on one end and 85 psi of resin (bag) pressure on the other end, before the resin gels. This is a very low flow rate and therefore essentially zero bleed.

This can start to sound complicated but it is actually simple and robust one you get used to it. You very reliable airflow paths which also very reliably block resin flow. W.L. Gore has a new membrane that is amazingly permeable to gas but doesn't let any resin through. Same principle but I'm sure will cost more.

 

[Prabbit]

The use of vacuum in the manufacturing of composite laminate structures (Epoxoxies, Phenolics & BMI) is beneficial for the consolidation and removal of gases during the cure cycle.

Vacuum is used to remove the following

Air Entrapped during Layup Process
Moisture absorbed by the prepreg (Solvents, Acetone absorbs moisture
Solvents expelled during Cure, 180-220 F Range
Gases emitted during the cross linking process, (Water, NH3 and Etc)

After the material is completely Gelled, vacuum is dumped.

Again, this is not true for all systems. Suppliers of the material add special additives to assist and prevent accumulation of gases and allows them to rise to the surface.

Hence, understand your material. Perform the following tests

DSC, TGA, Rheometrics and optical microscopy. These tests can be performed by your supplier.

With this information yous should be able to tell how to efficiently cure your material. You will be able to determine how to cure over 10-20 different materials (Epoxies, Phenolics and BMI) using Hot melts or solution coated materiasl all within the same cure. These are tricks of the trade, kept close to the chest. People who do not understand the philosphy must have X amount of cure profiles to control. Many times it is diffcult to explain to some customers, but have proven this many times. It is your choice.

As an added feature you will be able to determine if this resin system may be cured outside of the Autoclave with vaccum only

Last pressure has a tendency of pushing your gases to the location of least pressure. These are bridging areas and sharp corners or they locate between fabric weaves. They will acculumate in these regions. The use of vacuum allows the removal of these gases and fills them with resin. sharp corners.