Bryan D

(Similar to a comment I made yesterday you can also minimize warping if you remove the piece from the mold and THEN cut the rim. My cup forms come out of the mold with the sprew still in place. I place the clay form on a banding wheel and cut the sprew with an exacto knife. I then place the cone form back into the cup and allow it to set up for awhile. )
This would be pretty slow for production casting .
When we had hundreds of molds going we designed the spew to be longer (you must do this always anyways) and cut the top inside spew with a plastic slip knife.(which does dot injuge the plast as much. Then demolded the piece and finished the cut with a sharp metal blade many at a time. This mold was a three-piece mold. We where making thousands of aroma therapy lamps for a decade
The  fill hole in the front was also the spew. The top is cupped to hold the oils the bottom was flat.These where all cone 10 porcealian.Still have the masters if anyone is interested.

Being a slip caster myself I sometimes run into this.  From my experience, 30+ years, its mostly a process problem.  (A picture of your mold would help.) Placing a conical cup form into the casting, right after demolding, often reduces this problem. You can eyeball round but having an actual round cone form makes it much easier. Similar to a comment I made yesterday you can also minimize warping if you remove the piece from the mold and THEN cut the rim. My cup forms come out of the mold with the sprew still in place. I place the clay form on a banding wheel and cut the sprew with an exacto knife. I then place the cone form back into the cup and allow it to set up for awhile.