Prepreg - inflatable bladder voids

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#1

Hello everybody,

I am currently working on a closed tubular CF project and I would need some help.

The part that I’m working on, will be molded into female plugs. 2 halves. For this I normally have 2 options:
a. - mold it in separate halves. Layup carbon on the female mold, vacuum, oven. After curing, I put strips of carbon arround a bladder. Put the bladder inside, close the molds, inflate and oven.
b. - wrap the layup carbon arround the bladder, but the whole thing into the molds, close the molds, inflate the bladder and put it in the oven.

Method b. sounds better, but the question that I have is how to remove the possible voids. Even if I’m using prepreg, there will be voids to some extenct in the part. What is the process that solves this problem?
Method a. in my case, would eliminate most of the voids in the part, but the second part of the process, with the bladder join, would still have the same problem.

Thank you in advance,
Mihai

#2

Method B will create lots of surface voids. You will never win if you are expecting a pin-hole free finish. Bladders will only remove voids if the air can move with resin that migrates from the mold. Method A takes too long (two curing cycles) and the seam can be weak or add too much extra weight.

You need to option C. :smiley:

Watch this video of how I make bladder molded parts with co-cured seams. You can ask any questions after watching the video. BTW, you need to use some juicy prepreg with bladder molding. You need to create a prepreg layup that is void free before the bladder pressure is applied. The excess resin needs to migrate out of the closed mold via vents or along the seaming flanges. Some air voids may migrate with this resin. Typically, voids that are far from the flanges or vents will remain. You will also need to engineer the seam so that everything gets joined and bladder remains out of the way. It is possible to create that shape in one shot and have the part come out of the mold pre-painted or clear-coated.

https://vimeo.com/35648020
https://vimeo.com/37883424

#3

We do make smaller tubes using method B, with prepreg in the autoclave. If the bladder is leak free we don’t get pin holes. We did learn that you can’t just wrap the whole lot up (like a toilet roll) and hope that it will expand into the mould. You need to do the layers more like an onion where each is independent of the others so that they slip as the bladder expands. But this method is best with more than just vacuum. At the higher pressures the air can dissolve into the resin. if you have a bag on the outside you can get a greater differential pressure.

It is also important to make sure there are no twists in the bag as they will make voids. Plus make the bag big enough.

#4

First of all thank you for the answers.
I watched the glider videos few days earlier. Great video, RC man myself too :slight_smile:
Well, in my case that option wouldn’t work. The shape is too complex. I don’t have that kind of patience to deal with it.
The part, needs to be very strong and also very light, so it seems I need option D. :slight_smile:

My basic ideea is yes, juicy prepreg. I am thinking to use dry carbon with the poor man’s prepreg solution. I wet the fibers, pull them under some mylar, cut to shape. Remove mylar, and wrap onto the bladder.

From here onwords, I was thinking for the following scenario: Put the fibers on the bladder (yes, onion explains it best), then put the whole thing into the moulds. Now, my solution is this: the moulds will be punctured with 0.5mm dia wholes. Let’s say 4 holes per square cm, evenly all accross the surface. After, I closed the moulds, I wrap the whole thing in breather cloth, and vacuum bag.

The molds that I will be using are fiberglass, so drilling them shouldn’t be an issue.
The bladder pressure pushing outside the mold, and the vacuum pulling again outside the mold, it should do the trick.
Please see attached drawing.

The finished part should look all with spikes here and there. But, in the end all carbon parts need some post molding work. To remove some spikes would be easy.
I am not interested in the finish of the part. It will be painted either way.
Probably, or surely, I’ll have to redrill the molds for every next pull that I’m doing, because resing will get stuck in those holes.

Some details are:

  • home made prepreg. Some long curing resin. Oven dry. Not sure 60 or 120 degrees C.
  • either latex bladder formed to shape via water solubile compounds/ either some elastic plastic wrap. If I can pull it out, it’ll be latex, if not, than some plastic sheet wrap, which will remain inside for good.
  • breether cloth and vacuum bag. Normal pressure.
  • oven, 60 or 120 degrees. I could go for 60 degrees, if I’m using room temp curing resin. The whole layup process, is 2hrs plus.
    Do you guys think this ideea would work?

One more time, thanks
Cheers,
Mihai

#5

Bladder molding works because there is a pressure differential. In your bagging scenario the pressure on the outside of the vent holes is the same and the pressure inside them. Very little resin or air is going to migrate. You will need to add perfply and and breather material to the inside of the layup if you are going to bag the whole mold. Oh wait…if there is bleeder on the outside the resin and air would migrate through the holes in the mold and into the bleeder (rather than have to go all the way to the vacuum line). So never mind. :o

It could work. :slight_smile:

Putting the layup on a floppy bladder will have it’s challenges. You could add wax sheet to the inside of your mold that equals the thickness of the layup. You can then gently inflate the bladder in the mold and fill it with sand while under pressure. You then can very gently apply vacuum. You now have a firm bladder that will receive the layup. In a production scenario you would make second undersized mold for filling the bladder. I’ve always had pinhole problems with this technique with woven fabrics (UD fibers can come out looking alright).

#6

That would work. There may be a moment before the resin spikes on the sides break that the part will feel like it is stuck in the mould but that should pass as the part should be stronger than the resin. You may wish to put perforated plastic or peel ply against the outside of the mould under the breather or it will stick to the mould.

#7

I think getting it out of the mold is going to be rather difficult with all those holes. You will also have to drill the resin out of all those holes after each use.

#8

Yes, there is a breether cloth between the molds and the vacbag.
UD carbon will be used.
And yes :slight_smile: drilling and redrilling…
Is there an E option?

#9

Option e: salt pour and aluminum tooling. Very expensive.

Aluminum tooling to cast a salt part. Layup carbon over salt part. Cure. Soak in hot water and the salt dissolves.

#10

There are other ways to get this method. There is some powder (don’t recall it’s name), that I can pour into the mold. It gets solid. Afterwords I can disolve it in water anytime.

How do you look at this? carbon over this, and inside the female molds? or straight vacuum over it and that’s it, without female molds?

In my case, the exterior has to be exact, so I deffinetly need the 2 female molds. And to continue furthermore, how will this solve the voids problem?

Or, am I just making too big of an issue with the voids?

#11

Hmmm. If outside tolerances are a concern a soluble mandrel will probably not work.

#12

There is always the suck it and see method. Take your best guess and run with it. During the process you will see what works and what doesn’t and adjust accordingly. Sure there is the risk of failure but if you’ll learn a lot.

#13

Just thinking…we are all hung up on vacuum. When what we need is a pressure dimmerentioal from the inside to the outside. What about building the two ffemale molds. Layup the two halves separately vacuum bag these two parts. Trim flash and insert into mold again. Fill cavity of part with silicone or urethan rubber rotate to coat inside surfaces of part. Remove rubber bag split into top tube of part and bottom tube of part seal ends. Inflate slightly so to layup carbon around rubber tubes. Return to mold and clamp halves together. Inflate rubber bladder to 20psi and cure mold. Bladder will streach slightly ot cure void problem . pull bladder from top and bottom tube. Think innertube in a tire type of process.

#14

Dsho’s idea is similar to what we do. We use silicone inserts. With their thermal expansion coefficient and ability to be poured into any shape, you can achieve insane amounts of pressure with them. FYI, You would need a pretty flexible insert to be able to remove it from your part, it looks extremely complex.

#15

Hey,

I know this thread is kind of old, but I have a question regarding the co-cure two-halves method, or option C. Is there any discernible difference in strength between this and, say, the pinhole option D? I’m looking at making a bike frame, so strength and fatigue durability are a must.

Sorry for piggybacking your thread, pupet! :o Hope your design came out well.