Vacuum bagging large part

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#1

Ok guys,

I have a fiberglass side fender I need to fix. While I am at it I thought I will just lay some 2x2 twill carbon fiber to give it a bit of strength but mainly for cosmetic purposes. I would like however to vacuum bag the whole fender (and maybe lay another layer of bi axial carbon on the back of it for extra strength potentially).
My main concern is bending the fender too much (even to the point of braking) so I am reaching out for some advice. My thinking is that I could do one at the time (obviously) and apply less psi to the part and release it before it fully cures. Also, I am thinking that enough vacuum bagging should be allowed to shrink

#2

you should be able to bag it and not distort anything. If you do envelope bag it, then be sure you have plenty of extra bag so you can pleat it well enough to avoid crushing or warping it. If you have any doubts than just do one side at a time. I don’t see why you’d want to add carbon to a fender though? Is it that weak to begin with?

#3

Thanks a lot. I thought that giving enough extra bag should help me avoid the crushing of the part but i thought there might be something else I should be aware of. As for being weak, well, one of them has been damaged a bit, so i will need to fix it. Doing a layer on the inside will give it a bit more strength, but the outside one is more for cosmetic purposes than structural strength. Plus it will hide the repaired areas well! :slight_smile:

thanks for your suggestion.

#4

Why not just do this as a hand laid job, assuming the shape is ok to do then this would be easier and likely a better finish for cosmetic sake than a vacuum process.

#5

If you are just laying carbon fiber fabric on top of an existing part as if you are skinning it to get that carbon fiber look, there are a number of potential problems with vacuum bagging it…

Vacuum bagging works best when making cf parts in a rigid female mold. The vacuum bag material and the release film crinkle up under the vacuum pressure against the rear surface of the part. You don’t worry about the imprint pattern of the bag and film because they aren’t visible in the surface. If you use an existing part as a male mold, the bag and release film will leave their imprint on the presentation side and spoil that look if you use a regular bagging set up.

One of the key considerations when bagging is to make a mold that is rigid enough not to warp under the vacuum. I don’t know how stiff the existing part is but you can test it first by putting it in the vacuum bag without the cf to see if it hold it’s shape.

You don’t want to cure carbon fiber over a warping part as the cf will hold the fiberglass in it’s warped form once it has cured.

It might be easier to add the cf skin without a vacuum bag. You can apply a black epoxy surface coat to the surface using a roller (to ensure the coat is flat and even). When the black surface coat has partially cured to the point that it is hard but still tacky, you carefully apply dry cf fabric and smooth out any trapped air with a (clean) roller and let it set. You then apply you laminating resin as usual, cure, sand, clear coat etc.

Or…

You can create a custom silicone vacuum bag. Smooth-on sells a brush on silicone bagging rubber and trial sized pots are $30. The surface of the custom bag will mirror the part so it will leave a nice finish on the cf if positioned correctly. You may have to use sheet lining wax to correct for part thickness though and you will have trouble finding a single length of sheet wax for a part that long.

If the original fiberglass part is hollow, you can potentially increase strength by filling the void with something stronger. For example, chopped cf strands wetted out with epoxy, or even a high density urethane foam.