Efficient way of forming cf/honeycomb/cf sandwich

General Composites Discussions Composites Talk
#1

I apologize if this post appears twice, there was some weirdness with the forum software.

Anyhow,

I am creating parts with a slight compound curve (think skateboard) with one skin of cf, a core of aramid honeycomb, and a bottom skin of cf. My current work flow is painfully inefficient.

First, I wet layup the top skin on my mold and vacuum bag it over night. Then, I wet layup the bottom skin and bag it over night. Finally I lay the honeycomb between the two skins in the mold and bag that over night. I am really a novice at all this, and am following a procedure recommended to me by an acquaintance who makes CF parts for the medical industry.

This three day process seems terribly inefficient. What I would like to do is wet layup the top skin, lay the honeycomb on that, and lay a wetted bottom skin on that all into the mold together and bag once. I was told that I needed to bag each skin separately to “squeeze the excess epoxy” out first and then laminate them together.

I’m not sure why I need to vacuum squeeze out excess epoxy. If it is a matter of weight saving, it really doesn’t matter if my part is 10 or 20 grams heavier (over a surface area of about 350sq.in). If it is a strength issue, then yes I’m right at the failure threshold so I can’t weaken the design much further.

If doing it all at once is OK, what is the proper procedure? Presentation skin directly on the mold, or should I put peel ply between the top skin and mold? Peel ply on the bottom of the sandwich? No peel ply?

If I really do need to do each skin individually, and then laminate, is there a more efficient way to do it? I have only one vacuum pump, one mold, and one bag. I can make more molds and buy another bag, but I can’t really afford to buy another vacuum pump. This is for a hobby/hobby business. Not really a commercial endeavor.

Suggestions?

#2

I’d be inclined to do away with the Kevlar honeycomb and go with just plain with the outer skin on both sides. This way u can bag it all in one shot with no fear of resin getting into the cells.

Plascore is the manufacture

#3

Sorry, just plain… what? Just the carbon fiber without a core? It’s my understanding that the core is giving me stiffness that would take many layers of carbon fiber to match. I honestly don’t know for a fact, but I understand that to match the strength of two 5.4oz twill 3K single layer skins and one layer of .060" honeycomb I would need six or seven layers of 5.4oz twill. That would triple the weight of the part for the same stiffness and strength. I can stand to gain 20 or 30 grams, but not 450 grams.

Plascore as a substitute for nomex is an interesting idea (not sure if that is what you meant by “just plain”) I have never seen it in the thicknesses I am working (1/16"-1/8"). Do you know if it available thin? Also, will it conform to a compound curve like the nomex does?

#4

Are you using dry fibers which you are applying resin to by hand or are you using prepregs? If it’s the first then I’m not 100% sure on the process but I imagine doing everything at once might cause the resin to flow into the core which is not desirable.

Regarding the peel ply. The main ideas behind peel ply is to protect the surface after mold extraction and to prepare the surface for adhesion application. If you’re only concerned about appearance then I would put the outer surfaces against the mold without any peel ply since this will give the best surface quality/appearance.

#5

Sure , just lay it up all in one shot. The difference in weight will be bugger all. 50% resin to cloth ratio , and lay up both sides on the mould surface , lay in your core and clamp it shut.

I do it every day like that , but with rc wings. I use a foam core that is .2mm over size , so once layed up I just clamp mould together wet. No bagging needed.

If you core is perfect size , I can not see why you couldn’t do it like that as well.
Maybe some pics of what you got might help

Tim

#6

I also think do it in one shot if weight isn’t an issue. So long as you aren’t absolutely soaking the fabric, you’re not going to have a huge excess of resin. Just weigh your fabric, and measure out the same amount of resin for the weight (add 10% for wastage). Then you know you won’t have a lot of excess resin.

#7

Thanks for the replies! I am not using prepreg. I would love to have an autoclave, but I am basically one step above doing it on my kitchen table :smiley:

What I was doing currently was measuring out equal by weight epoxy to cloth and squeegeeing it out on plastic sheet BEFORE I added it to the mold, so it isn’t what I would call “soaking wet”.

Thank you Hanaldo and Timbuck! You gave me the confidence to just do it. This stuff is expensive :eek: so I hate to make dumb mistakes. The fact that Timbuck does wings this way is reassuring. I am making sound boards for musical instruments that have a similar level of curvature to a wing. I need it very stiff, very light, and NO DAMPENING. The nomex helps keep things stiff and light, and I certainly don’t want the voids filled with epoxy. I just didn’t think I was applying that much epoxy. My finished skins were only weighing ~15 grams less than the weight of the epoxy plus cloth, so it didn’t seem like that much was squeezing out.

Here is a picture of the finished instrument, for those who are interested. I don’t have any pictures of the construction.

#8

My experience with honeycomb sandwich panels is that they’re best done in 3 layups. 1)The first consists of wetting out the bottom reinforcement and placing the honeycomb on fabric. Then vacuum bag the entire part.

2)Once this part is cured you should use a filler such as microspheres to bridge any gaps you have in the honeycomb. Let this cure and sand the filler if necessary. For relatively flat parts you can skip this step. You can also get honeycomb that’s “over expanded” which means it conforms really well in certain directions. This may help your core material conform to complex shapes.

3)Wet out the top layer of reinforcement and vacuum bag the part. I would recommend doing this on a mylar or plastic sheet because otherwise you risk just pouring resin into the honeycomb cells.

Don’t worry too much about conserving resin because the bond between the core and fabric relies on a meniscus forming in each honeycomb cell. Too little resin especially on the top layer can cause delamination. With prepreg layups you use film adhesive instead to create this bond.

I once tried to conserve resin on a large sandwich panel part, but the part weighed almost exactly the same and had lots of delamination :frowning:

#9

Interesting to get more info about step#3
Maybe thin fiberglass layer ? Maybe some new laminating technics ?

#10

Timbuck is doing it all at once because he uses foam layer - with closed cell structure which do not absorb the resin. I am afraid if you do this with the honeycomb you will end up with a very poor result because the force of the vaccum will crush the upper layer of the CF witch will be soft exposing the honeycomb behind the Cf . If you want to minimize this you must cure the laminate between two back plates one for the bottom and one for the top layup …of corse the you will have maybe some problems with very reach and at the same time somewhere else very dry spots at your laminate . I doubt about the finish but there is only one way to find out really … : )

#11

and…:slight_smile: ?
This sounds like foreword.

#12

Try what you want to do having in mind everything that you red from here and depending of the results reevaluate your process . This the way …Also keep as informed about the results because what you try to do is interesting. :slight_smile:

#13


I’d better using the experience of other…

#14

Maybe something like this ?
https://www.heatcon.com/product/hcs2404-190-film/

#15

It needs 107 C cure … Can u do that ?
Even if you can can your epoxy handle this temp for 90 min ?

#16

Im sure that there are many same products ,including low temp. film.
As for epoxy…I order it for exact item and mould,so 107 C is not a problem.
Maybe you have another idea ? I’d like to know more methods how is it possible to make honeycomb laminate.