[bleucamaro]

What is the design objective for this part, and why are you wanting to consider composites? Are you looking for a specific weight, strength (tensile, shear or compressive), conductivity, aesthetics, hardness, surface finish???

When you break it down, "composites" just means more than one material. Carbon fiber, like fiberglass is just a reinforcement material that provides tensile strength, like re-bar in reinforced concrete. So, when you say you want a solid sphere, from composites, why not just a ball of resin?

Also, why do you want it cast or injection molded? If you went with metal, you could bond 2 or more pieces of plate together then machine them into a sphere, same with wood or other plastics.