Phillip,
I defer to your knowled since mine is 38 years ago since last doing wax casting. It was also for jewelry, but since we did the pasters in wax blanks and wax rods and didn't use centerfugal methods, I think we did it more primitively and the instructor didn't tell us about plastic.
We did do something that was interesting though, in stead of centerfugal forces we used steam pressure to inject the mold. Supposedly this was a process that the Egyptians used. The plaster mold was first placed in a kiln so that all the wax was evaporated. Next the cooled down mold was placed on a stone, and metal ingots (gold or silver) were placed in a bowl on top of the mold and melted. The mold was made in an old tin can (I'm sure the Egyptians didn't use those) and prior to melting the ingots a peanut butter jar lid had a wooden handle screwed to it and an absorbant fire retardant material was placed inside the lid (Don't think the Egyptians had those either) the whole lid was set in a bowl of cold water while the ingots were being melted. Once the ingots were heated the lid was slammed down on the mold and held firmly in place. The resulting steam forced the molten silver/gold into the mold instantly.
We had two choices for removing the piece, dunk in water and let it melt, or simple remove the plaster from the can and crack it open. I did about a dozen rings using this method and a couple of pendants and charms. I got pretty good at it, but like I said that was 38 years ago.