In looking at the sample end castings, it seems as though air pockets got trapped inside the mold.
This is a problem even with one part, flat molds sometimes, let alone two part molds.
It could be also that the resin is setting up too fast in the mold, becoming thicker and not allowing entrapped air to rise out of it.
Somehow, relief holes need to be put into the mold to let any the air trapped inside escape during the pour.
Any rods or whiskers on the casting from those relief holes can be cut off and smoothed down later.