PVC pipe works great as molds for round castings. No, it does not need a mold release. Just seal one end (a lot of us just use tape to seal the end) and pour the polyester resin in. At the PR hardesn, it shrinks slightly. For me, after an overnight wait, I just knock the molds together and the blank slides out. If the molds were not very clean, the blank may stick a bit. A metal rod let me force the blank out with hand pressure.
I do not put tubes inside the PVC pipe molds but it might make things simpler? But maybe not. Depends on what you are doing.
I like to mix ans swirl the colors in the blank and do not think a tube would allow me to do that.
Bubble and voids have not been a problem for colored PR blanks for me and not pressure or vacumn is used. Just be easy with the mixing and pouring.
Rich S.