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Hollow wax from casting


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
cast item.

There is a brief description with drawings of casting hollow items
on my casting blog:

Lee Epperson


Hey Lee,

Hollow casting present special problems. 

You could use a core for the inside of the wax, then investment
doesn’t have to go inside the wax model.

Kindest regards Charles A.


A friend who works with metal clay suggested using the cork that
they use when making hollow forms. However, as she does not do any
lost wax casting, I am not sure this would work with wax, but am
passing the along in hopes that someone is familiar with
using the cork for lost wax hollow forms and can let us know if this
is workable.

My own preference would be to cast the hollow form in two pieces and
then solder them together as was already suggested.



About hollow forms, there is also another product in the metal clay
world other than the cork — a new metal clay works like clayhollw
form and can be formed into the shape desired — however when fired,
instead of sintering into metal, it turns to dust and can be poured
out of a small opening in the hollow form.