Vacuum bag vs bolt force (fabricating a carbon fiber wing)

In summary, the speaker is trying to make a strong carbon fiber wing by vacuum bagging the top and bottom halves separately and then using a strong epoxy to bond them together. They are considering using either a vacuum bag or a series of bolts and washers to apply pressure. The speaker's calculations for the total force are 2170 lbs for the vacuum method and 2280 lbs for the bolt method. They also mention that the excess resin seeping out should not be a concern. The speaker believes that the vacuum method is preferable as it distributes the load evenly. They also share their experience with using a vacuum bag and pressure for fabricating aluminum honeycomb wing sections in the past.
  • #1
lovethepirk
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Hello, thanks in advance.

I'm trying to make a near 100% solid carbon fiber wing. I'm vacuum bagging the top half of the wing and the bottom half of the wing separate.
After making sure there is a thin 2mm gap b/w the center of both halves the goal is to put a very strong, thick epoxy between them and clamp the mold together.

I'm wondering if I should use the vacuum bag for this process or use a series of bolts and washers as seen in the attached picture?
zkkA9k7.jpg
<Moderator's note: post edited to include the image. Please do not use external image servers.>

Questions:
1) The surface area of the top side of the mold is 80 sq in and the bottom is 75 sq in. If I draw a vacuum strong enough for 14psi is my total force 155 sq in x 14psi = 2170 lbs?
2) If I use 20 bolts with the a surface area of the washers touching the flange of .38 square inches each and each washer can apply a force to the flange of 300 psi is my
total force on the part theoretically 20 bolts x .38 sq in x 300psi = 2280 lbs?

*The above questions are assuming a near perfectly rigid mold. I'm just trying to see if my math is right.3) Will only the excess resin seep out of the seems of the parts if I vacuum bag the halves together or will the vacuum cause more resin to escape the mold? I think in theory the vacuum shouldn't cause excess to seep out.Thanks.
 
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  • #2
I would think the vacuum method is preferred as it distributes the load evenly over the entire surface; whereas the bolt method would result in concentrated forces at/near the bolt locations only.
I don't think you need to worry about the resin seep-out as it will only need a thin coating to bond the two together. Generally, on a glue line...the thinner the better. (as long as all imperfections are filled).
In fabricating aluminum honey-cone wing sections for the F-86 and F-100 Fighter jets, we used a vacuum bag and applied pressure as well, using an autoclave pressurized to 100 psi (that was back in 1955).
I think your math is OK.
 
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1. What is the difference between using a vacuum bag and bolt force when fabricating a carbon fiber wing?

The main difference between using a vacuum bag and bolt force when fabricating a carbon fiber wing is the method of applying pressure to the composite materials. In vacuum bagging, a vacuum is used to compress the materials and remove any excess air, while in bolt force, mechanical pressure is applied using bolts or clamps.

2. Which method is more commonly used in the industry?

Vacuum bagging is the more commonly used method in the industry due to its ability to produce consistent and uniform pressure, resulting in a higher quality product. It is also a more cost-effective and efficient method compared to bolt force.

3. What are the advantages of using a vacuum bag?

Using a vacuum bag in carbon fiber wing fabrication has several advantages. It allows for better consolidation of the composite materials, resulting in a stronger and more lightweight product. It also eliminates the need for additional tooling, reduces the risk of voids or air pockets, and can be used for larger and more complex shapes.

4. Are there any disadvantages to using a vacuum bag?

One potential disadvantage of using a vacuum bag is the need for a vacuum pump and equipment, which can be expensive. It also requires a certain level of skill and experience to properly set up and monitor the vacuum pressure. Additionally, the process can be time-consuming, as the materials need to be left under vacuum for a specific amount of time to cure properly.

5. Can bolt force be used in conjunction with vacuum bagging?

Yes, bolt force can be used in conjunction with vacuum bagging to provide additional pressure and ensure proper consolidation of the materials. This method is often used in more complex or critical applications to achieve the desired strength and quality of the carbon fiber wing.

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