HOW TO WORK WITH HYDROCAL
AND OTHER MODELING TIPS -
LOST WAX BRASS CASTING
Lost wax investment casting is a process that turns a wax pattern into metal.
It was developed ages ago to create jewelry and art objects. We model railroaders
are most familiar with those incredible details on our brass locomotives. I'll be
using the process to make brass production masters for white metal details since
they need to withstand heat and pressure during the vulcanizing process of those
Making the Patterns
The essential elements of this process are the master patterns. Fortunately
(and this is the reason to do this process) you can use just about any material you
like to construct your master. It can be paper, wood, plastic, metal- whatever is best
or easiest to work with. We'll make a mold of the pattern and reproduce as many wax
copies as are required. In the end they will be turned into brass.
The lost wax part comes from using a wax model to form the desired parts. They
can be hand-built or cast. They are placed in a flask in a tree-like fashion and
then investment plaster specifically formulated to withstand high temperatures is
mixed with water and poured into the flask surrounding the wax tree. The flask is
then placed in an oven and slowly brought up to temperature. Along the way the wax
melts and drips down into a pan which is removed before the oven is brought up to
casting temperature. By then all of the wax is burnt out and only the hollow shape
of our wax tree and the patterns is left in the cavity.
A brass casting alloy is melted and poured into the cavity. To make sure it
filled every nook and cranny we'll use centrifugal force or a vacuum. In a moment
the brass solidifies and the flask is dunked into water. The boiling shock helps
break apart the investment plaster, which has done its job. If we are lucky we'll
soon pull a brass tree full of nice new parts.
Here are some new HO scale patterns I'm working on. I'm using wood, paper,
modeling clay (some good, some bad), plastic and metal. Also included above are some
of the Builders In Scale white metal castings.
Below we have Glenn Farley's master patterns for the Durango roundhouse. Most are
for the stove.
Making Wax Molds
A room temperature vulcanized (RTV) silastic rubber mold is made for each part.
We'll use the same Dow Corning 3110 rubber I use for my Hydrocal castings. The masters
are mounted on sprues and placed in a U-shaped mold frame with a cone base designed
to match up with the wax injector nozzle. Plastic sides complete the mold frame
and the RTV is mixed with a catalyst and poured in. Once cured the molds are cut
in half with a jagged surface so they will mate back together in good alignment.
The mold frames are ready to receive the rubber. The mold making is described
in another article.
Below we have the two mold halves. It was cut with a scalpel in a rippled
fashion so they will align properly.
Making Wax Duplicates
Wax is heated in a pressurized pot and injected into the molds making wax duplicates.
Investing the Waxes
These are stuck to a wax trunk by a heated tool forming
a wax tree. The tree is built to fit inside a flask with about 1/4" to spare.
The next step is to seal off the perforated flask with a sleeve and make it
ready for the investment plaster. It is carefully mixed, vacuumed, poured into
the flask, and vacuumed again. Drawing a vacuum expands any tiny air bubbles and
the bubble up to the surface. Otherwise they could leave voids that would fill with
metal and ruin the casting. All this must be done in a limited work window of 5-10
The investment sets in a few minutes and then the base cap and sleeve are removed.
Any excess plaster is cleaned off and the flask is ready for the oven.
Burning Out the Wax
The burn out takes 8-10 hours. The temperature must rise slowly allowing time
for all the moisture to escape, the wax to melt and incinerate all remnants. This
means watching it with care. The door is left cracked and inch for the first couple
hours as the wax burns. Generally it's checked every hour or so and the temperature
is manually controlled by a knob. Finally 1275 degrees is reached and the burn out
is complete. The temperature is dropped to the casting temperature of 1150.
Casting the Brass
There are several ways to introduce the brass. It is melted either by
using a torch (there's something about fire and flames) or as shown here we're
now using an electric melting furnace. We're also using a vacuum chamber and
perforated flasks to pull the liquid brass into the cavity and hopefully fill
every corner and detail. Another way is to use centrifugal force with a spin
casting machine where the flask is flung on a spring loaded arm. It too is a
little more fun- probably because of the better possibility of disaster, flying
hot metal, and so on.
By now we have the brass heated to its casting temperature. The flask is pulled
from the oven and placed in the vacuum chamber and the pump is turned on. The gage
shows it is pulling the air out and the brass is poured in. In a moment the brass
solidifies, the vacuum is shut off and the flask is removed and dunked in water.
It boils like crazy and the investment having done its job is knocked apart revealing
our new brass parts.
The brass parts are cut from the sprues and cleaned up. Each is inspected for
flaws. Remember, if you have a flaw anywhere along the line it is going to show up
on all the subsequent duplicates so we only want the good ones. These look good.
We did have a few screw-ups. One tree was ruined as we failed to get a good vacuum.
May fault for taking pictures! Another was prone to surface berries so we must have
had a poor vacuum when we invested that one. Good thing we did twice as many as we
needed. In the end we had two full sets of the required parts which was just
enough to fill the white metal mold.
All of these steps take good timing. It's best to begin the burnout soon after
investing the flasks. The burn out takes 8 to 12 hours. And then the casting
a couple more. You are almost stuck with doing this overnight or finishing at
Next step, we make molds for white metal spin casting!
Copyright C. C. Crow, 2009, all rights reserved