I agree with James.
This is going to be similar to arguing politics or religion, you
will have advocates for both processes.
I am sure there are people on the list that can compare the density
of centrifugally cast items vs vacuum cast items.
The shape of a sprue button is not a good way to compare the density
of a casting made with either method. The shape can be a reflection
of the mold and metal temperature.
I read someplace that a concave sprue button indicates the
temperature combination of metal and mold was slightly too cold. A
convex sprue indicates the metal and mold temperature was too hot.
Someone told me, when I first started vacuum casting, that my mold
temperature was too cold. I increased the temperature of the mold of
my next casting. The sprue button blew up like a balloon. I cut it in
half with my band saw. The thing looked like a sponge.
So much for the comments on sprue buttons. I will comments on some
of the merits of vacuum casting. Others who do centrifugal casting
can better list the advantages of centrifugal casting.
For me the most important feature of vacuum casting is the ability
to prevent fire scale when casting sterling silver. See "tips from
the jewelers bench" for the method of preventing fire scale. Vacuum
casting allows you to cast MUCH larger objects. You will be able to
control the temperature of the mold. The time between taking the mold
out of the oven and pouring will be less. You will need to use more
investment when centrifugal casting. The head space (amount of
investment above the project) must be greater. The force of the metal
hitting the end of the mold can cause the end to blow out. The rapid
input of the metal hitting parts of the mold in centrifugal casting
can break off thin sections of investment.
Just a comment about my own experience with centrifugal casting. I
had a lot of my jewelry cast by a shop using centrifugal casting. I
supplied the wax, they did the sprueing. Most all of the buckles and
larger pendants had stretch marks on the back side. I don't gets
these when I vacuum cast.