whats the other 5% ? I use 94 lead 5% antimony.
It could be the lead;
more info would help. a pic of the mold showing the problem. I'm not sure what a tube insert is, but I am assuming it has a hook in it. do you have other molds that pour with the same lead? what percent of rejects are you getting? with my experience it could be any one of these things or a combination of, lead temp, rpms, spin direction, gate location, the speed the way you pour the lead in, etc....