Hi, I have been casting now for over 20 years and I teach casting and
wax carving at the Craft Guild of Dallas. Here are some pointers:
Several things can cause porosity. Here are some rules I go by. Go by
the following rules for spruing, always have enough metal for a
good-sized button, never allow the flask to drop below 900 degrees.
The sprue has three important parts in the casting process.
It holds your wax at a proper angle in the flask
It is the funnel for your metal to get to the model.
It is an important part of the process when the molten metal is
solidifying. (This is where plays the most important part of the
casting process to keep porosity down.)
As your metal is forced into the open cavity of the investment, it
must have a smooth pour so that there is no turbulence. If the
joints of the sprues are not smooth this can cause turbulence, or
even some obstruction.
When molten metal solidifies it shrinks. When it shrinks, it will
either pull more metal from the sprues, or if it can=92t it will pull
in air, causing porosity. In an ideal casting, the item being cast
should solidify first, and as it shrinks, it pulls the molten metal
from the sprue. Then as the sprue solidifies and shrinks, it pulls
molten metal from the button. The button solidifies and shrinks
unable to pull molten metal from the sprue, it pulls in air. All
porosity should be in the button.
As a rule the button should have the same weight in it as the rest
of the casting therefore allowing the button to stay molten during
the solidification process, and also adding the weight to help force
the metal into the open cavity of the investment.
Improper spruing can cause porosity.
Always attach your sprue to the thickest part of your wax, so that
when the thick area of the model is solidifying, it can pull metal
from the sprue. The sprue must be as large as the area you are
attaching it to.
When attaching the sprue make sure the contact point is secure with
no gaps. Create fillets or curve the joint from the sprue to the wax
so that there are no harsh corners for the molten metal to pass
through.
The sprue should be as short as possible, but no shorter than 1/2 an
inch. A sprue that is too long can start cooling down before it gets
to the wax model and can solidify first, before the wax model.
Always place the sprue on the model so that when the metal is
funneled into the open cavity it always is going in one direction.
Check this by holding the sprue attached to the wax model upside
down; visually look at it to make sure that nothing is pointed
upward or above the attachment area of the sprue junction to the
model.
Check with the manufacture of the investment, some manufactures will
state not to allow the flask to dip below 900 degrees after firing.
Your flask may have been too cold.
Also you did not say whether the metal you were using was new casting
grain or a mixture of used and new metal. When casting, the metal
must be new or a mixture of 1/2 new and 1/2 use metal (used metal
being re-melting a button and sprues.)
I hope this helps
Janet Alexander
www.ornettes.com