Surface Defects/Void Formation 2

[samplesecond]

Hello All!

First off-ive been following the recent thread about pinholes/surface defects, and have tried some of the suggestions there..but still not having much success...so was looking for some more input.

The application is a simple pressure vessel to be constructed out of carbon prepreg. The net thickness will be in the neighborhood of .450". The tooling consists of a polished two part aluminum female cavity, with a solid silicone 'Plug' for compression/consolidation.

Interior/exterior surface finish of the part is critical, as is the net thickness.

I am still seeing surface defects/voids in the finished product.

My questions are:
1)Should I consider building thickness over several cure cycles? (ie-layup 1/3 thickness, cure, layup additional fiber, cure..etc etc..)?

2) I have been 'debulking' in a vaccuum bag after all plies have been laid at about 105F for between 2-12hrs. Should I instead debulk several times as I layup additional plies?? And, what are the pros/cons to sticking the whole thing in a vaccuum chamber vs applying vaccuum bag pressures to the part?

Thanks!

-OS-

 

[RPstress]

While repeated cures probably won't do too much harm, it is rather non-standard. It depends a lot on the cure temperature and material system. On 350 deg F cure epoxy we have done two cures in the past with no detectable degredation in the double-cured material. It's hard to see why it should degrade appreciably as long as the Tg is ahead of the cure temperature.

It was once recommended to me (I think in this forum, but I can't find the thread) to debulk every half a ply! We would normally debulk every third ply at the least, and every ply for some important parts.

However, these debulks would be under vacuum only at room temperature. We generally wouldn't do too many heated debulks.

Sometimes a very long debulk before curing can do the trick, but it sounds like you've tried this. (Was that up to 12 hours under vacuum at 105 deg F? What's your cure temperature?)

Getting in touch with your material/resin supplier is the usual recourse. They may be able to suggest an alternative cure profile.

The debulking is mainly effective for general porosity. Are your defects surface defects only, or have you got general porosity throughout the thickness?

If your defects are confined to the surface then a specialised surfacing film may help, and even just a layer of film adhesive can be effective.

You don't mention how pressure during cure is applied. Is this in an autoclave?

 

[samplesecond]

The recommended cure for this particular system was about 325F.. I have tried long debulks at elevated temps, but only after all plies were laid. As far as defects, I have cut a part in half to analyze the cross section-appears to be without any voids, but I have yet to perform any quantifiable tests.

Pressure is applied via silicone plug (thermal expansion)..should be seeing in the neighborhood of around 90-100psi at 325F

It seems as if this problem is due to improper debulking technique.
I will try debulking every several plies...Are there any other processing tips/starting points that will help the effectiveness of the 'debulk'?

Thanks for your help thus far RPstress!

-OS-

 

[Compositepro]

You may be having the same problem that I just recently encountered. If you are getting porosity only on the surface in contact with the aluminum tool and not on the other surfaces, or though the thickness, then your problem is almost certainly due to a galvanic reaction between the carbon fiber and the aluminum tool. Hydrogen is being generated. It takes very little conductivity in the resin to cause a problem and most monomers are reactive and therefore somewhat conductive.
You may be able to find another prepreg resin that works okay. Or you need to go with non-aluminum tooling or have a barrier between the aluminum and carbon. The barrier could be a non-conductive release coat (this may not be very robust) or even chome, gold, or nickel plating.

 

[samplesecond]

Do you have any product recommendations for a non-conductive release coat? And how durable are the various platings? Are there any other drawbacks aside from having to have the tooling plated?

Again, what is the standard protocall for debulking? If I debulk every several plys...how long should I keep it under vacuum? Should I be using specialized release plys/breather? To date, I have used standard release ply/breather that I normally use in a wetlayup process..

 

[RPstress]

That's interesting about a galvanic source of surface porosity. I haven't come across that before. And I do like the idea of gold plated tooling...

Our people tell me that for important parts they may debulk every two plies of UD under vacuum for five or ten minutes. Every ten or twenty plies we might do a heated debulk if we were seriously worried about consolidation. However, we try to keep heated debulks to a minimum.

They also tell me that, yes, you want a perforate and some sort of breather material in there to get as much air out of the stack as possible, but it's nothing fancy.

Can't help with the other stuff you ask, I'm afraid.

 

[Compositepro]

Most release coats are designed to be very thin and therefore would not be very effective as electrical insulation. As you go thicker there will be a problem with getting a good surface finish. There are a number of things that could work but I have not tried them. Your best bet is a non-aluminum or plated tool. Magnaplate has some offerings.
As for debulks you do have to understand the properties of your prepreg to develop the right proceedure. I've often seen where the debulk proceedure is inappropriate and makes the problem worse. Normally I only recommend debulks to compact the part during lay-up to prevent wrinkles that would occur if the part is compacted during cure. There are much better ways to eliminate voids during an autoclave cure.Since you are essentially doing closed cavity molding with prepreg,doing hot debulks would probably help. For the debulk to be effective you must get a small amount of resin bleed though the perforations. Debulking every two or three plies is usually adequate. If you aren't getting voids though the part thickness you shoudn't need to worry about debulking.
With the expanding silicone you will also be moving around any air that is trapped in the part. Depending on your part and silicone geometry, compaction of your part will start at some point and progress as the silicone expands. This will roll the air with the compaction front. Small, insignificant bubbles will join to make a bigger bubble. The final bubble will be a void and it could be inside or outside you part. This is a common effect in compression molding.

 

[CompositeGeek]

Samplesecond,

It is not clear from your original statement if you are seeing porosity on both the internal and the external surfaces of your part or if the condition is primarily on one surface only.

I have encountered similar problems with peculiar porosity conditions when using solid silicone plugs, in matched dies. Unfortunately our dies were not "polished" - Our surface finish was typically in the 32-64 range.

It is also "kinda" evident that this is a laminate and not a filiment wound vessel. Therefore I am "assuming" a laminate.

1. Is the laminate layed up in the shells then they close and you insert the mmandrel?

or

2. Are you laying up on the mandrel and then closing the mold around it?

In case of both 1 and 2, you may be suffering from some mandrel shrinkage which means your not getting uniform pressure in the laminate during cure which could be affecting gas and resin flow during cure. I have used sticky backed telfon coating on these kind of mandrels to promote air/gas escaping during cures for surface pinhole porosity. And I had to closely monitor the mandrel for shrinkage over time. The nubmer of cures before shrinkage impacted was a function of numerous conditions to address here.

If either 1 or 2, you may consider introducing some dry fiberglass strands connected to your breather path to also help with evacuating air/trapped gases.

Last but not least, If you are doing process 2 then you debulking is compressing material in the opposite direction from your cure pressures. You may actually be hurting your self with a debulk in this case.

As for galvanic corrosion between the tool and the carbon, thats new on my. I would eliminate the gas flow questions first.

Good Luck, I'm going back to livin the 787 Dream!

Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

 

[samplesecond]

Compositegeek-

I should have been a little more clear..

The part is a laminate with defects only on the outside surface (surface again female aluminum mould). Plies are laid into the female cavity, and the silicone mandrel inserted after all plies are laid. Im not sure if this is an issue of mandrel shrinkage..no part is over a few months old, and has only seen about 4 part runs.

 

[Compositepro]

A quick way to confirm that the problem is a galvanic reaction is to switch to glass prepreg with the same resin. You can also use a volt meter electrically connected to the carbon and aluminum. The resin needs to be hot and the carbon in contact with the mold but you don't have to close the mold. You will probably measure about 1.1 volts.

 

[CompositeGeek]

SS,
I think C-pro has the right idea. I would use his technique to eliminate the issue about galvanic responses.

Last suggestion, if the tooling is reacting to the resin, you might consider plating the tool with a nickel surface as an alternative to modifying the release agent you are using.

Good Luck!

Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

 

[JakeLiv]

with parts which require very good surface qualiy, we use a very thin glass ply on the exterior. In any case, the first ply is applied, and that vacuumed onto the mold. after that, every several plies we do the same bagging. no need to do it in temperature.
You can also use a glass ply on the interior surface.

 

[Compositepro]

JakeLiv, do you use a dry fiberglass ply or prepreg? I can see that dry fiberglass would be an excellent breather and prevent air entrapment but tack to the tool would be a problem.

 

[JakeLiv]

this method works only with prepreg, and preferable with higher resin content. Not to say that it need a very high resin content, just so it's not too dry. It's possible to do a test run of the process and see compared results using a plate where 1 piece is made this way, and the other is made with no intermediate vacuum bagging.

Also, we had 1 case where we autoclaved the first layer, and that proceeded in the same fasion, but thats a little overkill.