I performed an experiment yesterday which I hope will add clarity to the recent thread on the light weight nature of carbon fiber. Two conflicting ideas popped up on that thread. One was that carbon fiber fabric of the same weight as glass will generate a laminate of heavier weight simply because carbon fiber being less dense makes a fabric that starts out thicker and thus soaks up more resin. The other idea was than carbon, maybe because it has fibers packed together better, does not soak up more resin. I am talking about applications using hand lay-up, not vacuum bagging. And the superior stiffness of carbon fiber is not in question.

I measured and weighed three samples of 6 oz. woven CF, 6 oz open weave E glass, and 6 oz. satin weave S glass, both before and after applying resin. The samples were placed between two layers of plastic film where I squeezed out excess resin from the cloth with a roller. The final samples contained no substrate, only cloth and resin.

The 2 inch square cloth dry samples of CF, E glass, and S glass weighed 8.2, 8.2, and 8.0 grains respectively, and were .16. .07, and .06 inches thick (using my cheap caliper). So they were all very close to being the same weight and all very close to the advertised 6 oz. cloth weight, regardless of thickness.

After cure the samples were cleaned of squeeze out and peeled from the plastic wrap. The same samples now weigh 23.5, 18.5, and 16.0 grains. The CF sample weighs 47% more and the E glass sample 16% more than the S glass sample. The S glass sample yielded a 50:50 ratio by weight. I don't assume that these numbers represent the absolute numerical relationship between these fabrics due to possible errors in my methods, but the different weights among the samples in too large to ignore and also matches well with my experience. Carbon fiber is really stiff, and has many good applications, but it may not be as light as you expect without using advanced techniques like vacuum bagging. Satin cloth yields a lighter lay-up than open weave cloth, but not by a great deal.

One question is still nagging at me. Is it possible that unidirectional carbon fiber, with fewer and or smaller gaps between the fibers, yields a much less resin rich lay-up?

Ted