I am starting a new thread on this topic because I am building a 24” x 48” CNC router for the local high school Mercury 1089 FIRST robotics team. Yesterday I cast 5 plates for mounting casters onto the router frame and they all came out quite well. The improvement over previous efforts came from a thinner mud coating on the foam, and a longer sprue. All but one had porosity when I cut off the sprue. I believe it was the last one without porosity. I had a burner overheat problem and was trying to restart it but it wouldn’t fire up. So I went ahead and poured it to empty the crucible. So likely it was colder than the others that I poured at 1300F. The aluminum was bright and shiny under the mud. What to do about porosity?