I normally remove the flask from the vacuum pump immediately after
pouring the metal. The button is usually still molten when I remove
the flask from the vacuum table. I let the flask cool for between 6
to 15 minutes depending on the amount of metal poured.
On the first large castings I did I quenched too soon and had jagged
holes in the casting. I assume the holes were caused by metal that
had not solidified and blew up when quenched. I no longer have that
problem when I let the large castings cool for 15 minutes.
Peter is correct. The vacuum pulls air out of the mold. It does
not pull the metal into the mold. Gravity pulls the metal into the
mold. I cast lar ge pieces of pottery. I try to sprue the items so
that gravity pulls the metal down against the design in the mold
rather that have the metal forced upward into the design.
Button button who has the button. I have seen small buttons shrink
into the flask leaving a concave button with a hole in it. I believe
that indica tes that the metal in the sprue button may still be pulled
into the mold before it freezes. I would never pour a casting with
having a substantial sprue button.
As I have always said the casting art is not consistent between
casters. What works for one may not work for others. Generalizations
about casting without getting down into the nitty gritty of each cast
may not give the best results on all castings.