This is a consistent problem for me, that on my 8 cav aluminum MP molds I get semi lunar accumulations of lead around the sprue holes, all on the underside trailing edge. This happens after a few hundred cycles and causes finning on the boolit bases.
The pot is PID controlled at 720° and I take care to let the sprue harden up prior to cutting. The mold top and the plate underside are pretreated and during casting retreated with a rubdown using a 2 stroke oiled rag. The sprue bolt tension is maintained by a wave washer (there's also a wave washer on the stop bolt) and the bolt is turned in enough so the the plate and its handle don't quite open by themselves when the blocks are rotated handle side down. There is also a set screw for the bolt so that it doesn't turn, for which I filed a flat into the bolt threads to set the position.
So, any suggestions on how to keep the build up from happening and how to get rid of it now that it has? Unlike lead specks and other smears on the hot blocks or sprue plate, I can't remove these build ups with a rag or small alloy ingot. Do I need to sand the underside of the plate?
Thanks in advance.
Kevin C