That is why I want it to be several molds that can be baked separately and then layered together later to form a smooth sphere when covered with icing. For a 12 inch diameter I would need the bottom to be 2.51 inches in depth. The middle and upper sections of the sphere would need to be an additional 3.49 inches total (2 sections) to make it a radius total of 6.00 inches. The bottom 2.51 inches would be used again to make the base of the sphere. If anybody needs further explanation I can provide a drawing and verbal explanation on video.
Silicon is really expensive. Would be a lot easier to just bake different sizes of round cakes, assemble them and simply bevel / trim them into a sphere.
The weight of the top half of the sphere is going to turn the whole thing into an oblate spheroid without support.