Surface Voids in Bladder Molded CF Tubes

General Composites Discussions Composites Talk
#1

Hello all,

Working on setting up a composites department for manufacturing various composite tubes (no, not cold air intakes…) We bought all of the equipment our supplier was using to make our parts for us to bring it in house but are having serious quality problems so this is probably the first of many, many questions.

First a little background about the process we inherited. We are using two piece aluminum molds, a heated press, and hi-temp silicone tubing as bladders. The pre-preg is wrapped loosely around the bladder, this assembly is inserted into warm mold, bending it not so gracefully around the curves, and the mold loaded into the press. Press lowered, bladder inflated, part cooks.

The biggest problem we are having is air trapped on the surface of the part. From searching through the archives here this seems to be a very common problem with this process.

So the question is: Is is possible to consistently get good surface finish using this method?

We’ve managed to improve the quality by lowering both the initial temperature of the mold and the temperature of the press, while extending the cure time, but they are still far from perfect.

Thanks in advance…

#2

could you layup the first ply on the mold and then just tie it together with the pieces laid up on the bladder? Not sure if a seam is allowable but if you layup on the tool and debulk, you should avoid any voids.

We just pulled a part like this, laid up on the tool with overlap to tie the two sides together. Perfect finish, shiny, nice, no voids.

#3

Are you taking one piece of cloth and roll wrapping it leaving one seam?

If so, it could be that its too tight too begin with and not allowing the bladder to compress fully leaving some voids.

Pay particular attention to the cloth measurements to obtain consistent results.

#4

I’ve thought about doing in mold layup but have not had time to try a test part. It would require the customer to sign off on the change as well. Hadn’t thought of a partial in mold layup though, that seems even better.

It is actually several pieces (4 to 6 glass, uni and twill) rolled up. There are times when material is pinched and it does not fully expand against the mold, which definitely leaves voids but generally the voids I am trying to remove are pretty small pinholes.

We removed the manual valve for inflating the bladders this week and replaced it with a solenoid valve and a couple of timer relays to automate pulsing the air into the bladders. Seems to have really helped improve quality and consistency but there are still some small voids and material distortion from inflating.

Thanks again.

#5

I’ve always had issues with that process when using woven fabrics. I have had success when using UD materials. UD materials are much less prone to holding air to begin with.

See this post for an understanding of evacuation techniques when bladder molding: http://www.compositescentral.com/showpost.php?p=53904&postcount=11

How much pressure are you using??? What is the fiber and format of the layer against the mold? Is there any way for the traped air to escape the system??? How far would any air have to travel before leaving the system? Have you tried using some perf-ply and bleeder (vented to the atmosphere) on the bladder first? How close in the bladder shape to the size of the part?

For what it’s worth, here is how I do it. There are some videos of the process in the first post of the thread. http://www.compositescentral.com/showthread.php?t=7007&highlight=Juicy

#6

We are using 90 psi, have tried ramping it up and regulating it down with no noticeable improvements. The fabric layer against the mold is 284 Twill oriented approximately 0 degrees with the length of the tube. I say approximately because it varies with the bends and distortion from the bladder not pushing it uniformly against the mold. The mold is vented at the parting line along the entire length of the part on both sides. This is allowing excess resin and trapped air to flow away from the part into adjacent grooves in the mold. The tube is approximately 1.125 inches in diameter so air would have to travel approximately .88 inches to exit the part surface. I have not tried perf-ply and breather, not something we have on hand yet. The bladder is straight silicone tubing quite a bit smaller than the ID of the finished part. Having the material rolled up smaller aids in placing a straight pre-roll of fabric into an s-curved mold without pitching it in the mold. So I would say that the bladder isn’t very close to the shape of the part both in size and contour. I’ve watched your videos before, good stuff…

Thanks again for all the suggestions…