I have experienced the same problem twice now and wonder as to
its cause. I use a centrifugal caster and vacuum well before
casting. I pour the investment into the flask by pouring slowly
down a wooden rod.
Both times I have cast a ring with a reasonably hollow ( domed )
head. I have calculated the metal needed carefully as I have
done on successful casts. When I remove the cast rings there is
no button and the space (hollow) on the underside of the dome is
full of metal that I assume was for my button. A realmess!
Is it due to the fact that the investment did not fill the
hollow space and air remained there and hence a space for molten
metal existed? How do I avoid (cure) this problem?