I'll think about the foam core material tomorrow. Minimizing confusion, it's not actually modulus that you affect by putting the foam in the middle. Modulus for a given material is a constant. It's a property of a material, not the shape of a piece of it. Specifically, modulus is the ratio of stress (in psi) divided by the elongation or contraction of the material under that stress.

What you get when you take two E/G plates in a beam that were sitting on top of one other and make them farther apart by adding foam in the middle is a change in something called the moment of inertia. The moment of inertia is a measure of how close the material is to the center of the object.

The moment of inertia is one of the most important terms in the beam bending equation. Larry is right however that the farther away from the center of a beam a piece of material is, the more effect it has in helping to resist bending.

This is similar to the calculation I did for brunog a while back in one of the pdf's I posted although I suspect there could be typo and I don't know the post number off the top of my head.

Designing the proper shape for what you want to build is a function of what it is you want to build so without more details, it's tricky to be helpful. For a gantry that you want to be light, a hollow or foam core beam is great. For a machine base, the heavier solid might be more advantageous due to the extra weight to keep vibration down.

Yours Sleepily,

Cameron