I have included a video above for casting spheres using plastic Molds. The second video is bellow and it explains how to mortar the two halves together. I hope this is helpful for some of you!
One key step you want to be sure not to miss is—- About one hour after the concrete has started to settle you can rotate the inside mold ( if you are creating a hollow center.) This process will lossen the inside mold, preventing it from being difficult to remove since the outside of the sphere has a bit of a texture to the plastic. It is good to do this another 6-12 hours after to make sure you will have an easy un-molding process.
The release agent recommended is Pam Cooking Spray. You will find that it is thin light so you can completely avoid residue on your finished piece. The most amazing part… The concrete pops right out!