Vacuum Infusion: Increasing Fiber Volume Fraction

[mapinto121]

Hello,

I am currently working on making natural fiber composites using woven jute fabric as reinforcement and FiberGlast 2000series epoxy as a matrix. I have recently begun experimenting with vacuum infusion to reduce my void content and give better control over the volume fraction of the constituents. I just finished my first "good" sample, but found the fiber volume fraction disappointingly low, ~28%. I was wondering if anyone here could give me some advice on what parameters of my set-up to vary to increase the fiber volume fraction.

I attached a picture of my set up.
The laminate stack (6-layers) is put on a glass panel(coated in mold release) and then covered by peel-ply and a FiberGlast flow media. Both the resin inlet and vacuum outlet tubes are connected to spiral tubing to get a more even infusion. Vacuum was drawn to about -29in Hg for the infusion step, and was reduecd to about -20in Hg after infusion was complete to prevent resin boiling, which was a problem I had in my first few castings.

I would really appreciate any help I can get here, and please let me know if any more information from me would be helpful.

Thank you,
Michael Pinto

 

[mmhaynes]

Michael,
Typically during a VARTM process, there are a number of things that can be done to ensure the desired FVF. One consideration is the rate at which you allow the resin transfer into your part, if you open the gate too wide, it allows the bag to overfill resulting in a low FVF. Reducing the flow rate into the bag and restricting the amount of resin to something close to your fiber weight will help you achieve your desired goals. You can also use a layer of flow media to help with this, but it can result in a dry laminate (too high FVF). If you use a consistent design for your inlet and outlet, you can establish just how much resin is required at these points, then its a matter of doing it the same each time.

Mark Haynes
Senior Engineer
Composite Technology Development

 

[Compositepro]

It sounds to me that you have done all the right things. It is unlikely you will do much better with jute fibers and a vartm process. Ultimately the fiber packing will determine to fiber volume. Carbon and glass fibers can achieve high fiber volumes because the fibers are very straight and parallel. Jute fibers are soft and compressible so you can achieve far higher fiber volume if you use a high pressure press to consolidate the fibers. Prepregs for press cures is made from high viscosty resin that will not squeeze-out. The resin will be low viscosity during prepregging and then staged to a high viscosity in an oven prior to press-curing, typically at 1000 psi or more.

 

[cpinz]

What's that Soric for ? Is it intended as flow media ? If yes ,remove it. 6 layer can be easily infused with a normal flow media meshe available in different size an permeability.Of course your resin is intended for infusion and not for hand layup
Then, starting from the mould:
- mold release
- reinforcement
- peelply
- flow media
-vacuum bag

for the size of you panel inlet/exit are correctly positioned; I agree with mmhaynes, resin inlet is too large..squeeze it to reduce the flow if you want to use the same tube.
At the exit, place the tube more distant from the stacking,placing a peelply strip between the tube and plies in order to slow down the resin flow as it approaches the vent.
0.7-0.8 bar are enough to infuse such a small panel. Once infusion is completed, close the resin first and let the vacuum on, until (at least )resin has gelled.
For better final product consider the idea to degass the resin.

 

[mapinto121]

Thank you all for your helpful input. Using the recommendations you have given, I was able to increase the Fiber Volume Fraction to ~35%, which for the given loose fabric is likely the best I will achieve. My next step will be applying surface treatments to the fibers to attain better matrix/fiber adhesion.

Thank you again,
Mike Pinto