Well, after multitudinous fine tunings after casting each sand core, we finally got the last 4 to slip out of the core box "relatively" easy. There's still one minor major undercut which we've been trying to fix with some epoxy glue. We had a splinter shave off of the box that fills in the bottom core print. After 3 tries at fillng the shaving, we approached reasonable success. One last attempt. Only problem is we've got a dozen sand cores all cast which is exactly what we need.
However, if we decide to go into full-scale production, we'll fine tune them even more. Next up, we're going to test the sand cores in the green sand to make sure that they slide easily into the mold and rest where they should rest. But that's another story.