I am going to divide this section into 3 parts. Shell building, dewaxing and burnout and pouring. We will start with building a ceramic shell.
Ceramic shell is a type of investment mold. Investments have two things in common. First, they must be very refractory to handle the heat of molten bronze and second they must be able to be broken away easily. Before the advent of ceramic shell, various types of plaster mixtures were used to surround the wax. Some foundries still use plaster investment and in some cases it is a better choice.
But the advantages of ceramic shell, to me, outweigh plaster investments. A few of the advantages are:
1. Faster burnout. 1 hour versus days. Savings in energy costs, premature furnace degradation and labor costs from tending the furnace 24/7.
2. Control of pouring temperature and cool down. I pour shell molds at 1200+F. Improvements in surface, ability to cast thinner sections and elimination of cold shut are the result. During cooling, Fiberfrax can be used to slow cooling in sections where thickness variations result in differential cooling flaws such as heat tear and excessive shrinkage.
3. Wax recovery near 100%. Clean wax recovery saves money through reuse.
4. Less disposal cost. Shell molds are 1/10 the size of plaster molds.
Ceramic shell molds are made of two parts, a liquid slurry made of coloidal silica and fused silica flour and fused silica or zircon sand. The process is dip the wax in the slurry, drain, coat with sand, dry. Repeat above 9 or 10 times. The first two coats of sand (called stucco in the trade) are fine grained to pick up detail. The remaining coats are courser to build volume for increased strength. A final dip in the slurry seals the mold.
This is a shot of my slurry tank. It is mixed
with a prop type mixer. When the slurry is up, it must be mixed to keep the flour
from settling to the bottom. The mixer is on a timer that mixes the slurry around 2
minutes in every 10. Contrary to popular belief, a slurry can be left to settle and
rejuvenated by plunging in a mixer. Since we do batch casting, we shut our slurries
down when not in use.
Here I am dipping the wax into the
slurry. The pattern is dipped just long enough to completely wet it.
Draining the slurry from the wax. This
step takes about 30 seconds.I hang them from hooks as the weight increases. This is
where the metal conduit holders come in handy. They have a hole drilled in the end
for this purpose. Next they go to the fluidized bed for application of stucco.
A fluidized bed is a container of stucco with a
provision for air to flow through it. As the air flows through the stucco it
separates and lifts the particles making it much like a fluid. The mold is immersed
in the floating particles and cling to the wetted surface. the mold must now be
completely dried before the next coat is applied.
This is the drying room. Drying
conditions are important for making good shells. I try to maintain a constant 72
degrees and a relative humidity of around 50 percent. This allows the shells to dry
quickly without wax expansion and contraction due to heat cycles. This can cause
shell cracking in the beginning stages of the process and lead to weak shells. There
are large air fans mounted above the drying rack to move a lot of air for even drying.
The green plastic pipe assembly is an air manifold used for running small tubing
into the cores for drying them quickly. There is a small air blower mounted to it.
The conduit is inserted into holes drilled in the wooden cross member to hold the
molds during drying.