Hollow casting present special problems.
1. The hole must be large enough to allow investment to enter the
core of the model. The model must be mounted in the flask so that
investment will be poured into the hollow section through the hole.
If spruing of the model is such that the opening is near the sprue
end of the flask a small vent hole must be located as high in the
model as possible. This small hole will allow trapped air to leave
the core as investment enters and raises up into the hollow. A large
mass of metal will form in the core if trapped air is not removed
from the area 2. The investment in the hollow portion of the wax
model will be supported only by the amount of investment that is
between the inside and outside of the model. If that portion of the
investment is small in comparison to the size of the inside
investment the investment might break free and float in the casting.
In large sculptures pins are place through the wax model into the
center of the core of the model. These pins will act as re-bars and
hold the core investment. The pins are removed when finishing the
There is a brief description with drawings of casting hollow items
on my casting blog: http://ganoksin.com/blog/leessilver/