Just to follow up on a couple of previous postings with regards to lost wax
casting, I thought that I would add a few comments based on my experience.
About 10 years ago, I produced ( imported ) a series of HO scale brass
passenger cars - after the initial importing I decided to learn quite a bit
more about the lost wax casting process. I was able to successfully talk my way
into taking a couple of courses at a community college about two hours away.
The courses were part of a Jewelers program. Specifically I took an
introductory course, and then in a later term took a production casting course.
The metal of choice for these courses was actually silver, and I was prohibited
from casting brass for the first term - but the basics are very much the same.
The first potential source of shrinkage comes when you decide what your "
master " is made of, and wether or not more than 1 copy is required.
For one of a kind pieces, jewelers and model makers will typically either carve
a wax, or assemble wax pieces to produce the model or master. At this point a
sprue is attached, the plaster cast is made, and then into the kiln to lose the
wax ( burnout ) to yield the female cavity into which your molten metal is
poured or injected. In that process, the only measurable shrinkage occurs when
the molten metal cools. Molten brass depending on the alloy, melts in the 1600
to 1700 degree f range - as it cools it shrinks - the heavier the mass of the
piece, the greater the shrinkage and usually as a % of the original dimension,
and again depending on the actual brass alloy, the percentage of shrinkage
changes, but 3% to 4% is not uncommon.
If multiple pieces of the same master are required, it gets a little more
complicated, and likely a second opportunity for shrinkage presents itself.
In this case, the master ( not wax ) is usually inserted between layers of
casting rubber, and the rubber is vulcanized around the master to capture all
of the detail - this is done with heat and pressure. Once cooled, the rubber is
slit open to retrieve the master, and the resulting cavity is clamped back
together. This rubber mold / cavity is then repetitively injected with hot wax
to produce multiple parts. These parts are then treed or sprued and cast like
the example above. The second level ( actually now the first level ) of
shrinkage has actually already occured, when the injected wax cools, it shrinks.
The vulcanizing process can take as little as an hour to complete. If you have
a lot more time, Room Temperature Vulcanizing ( RTV ) rubber can be used, but
this can take up to 2 days before the mold can be used. But because heat is not
used, other materials like styrene can be used for the master - potentially
reducing an opportunity for shrinkage. More about this later.
A good caster will take this all into account.
That is the simplest part of the whole process. I initially thought that I
might try some of this at home for smaller production runs of some s scale
details.....If we were casting our details in silver or gold, actually casting
at home would not be that big of a challenge, but brass is another story. The
term " Brass " is used to describe a whole family of alloys - all with
different desireable, and sometimes undesireable properties....the most
concering of which is Zinc along with its twin - Zinc Oxide when you melt it.
Zinc Oxide is never good for you. Another challenge, is that when you melt
alloys, they tend to seperate into their component parts, and depending on
their different melting temps, tend to evaporate. In the case of melting brass,
if you don't replenish the Zinc, to the correct percentage, ( by weight ) you
can drastically change the composition of the alloy producing some undesireable
characteristics.
As a result, I have opted to leave the actual casting to the pros. Fortunately,
their is a very very good caster in the local area, which I had the opportunity
of touring as a result of a meeting my production casting instructor set up.
The actual caster is a large commercial co, casting for the jewelers trade
mainly in gold and other precious metals. I had to have a background check
before I could visit this high security facility - somehow I passed that!
When I was there, they showed me their production casting facilities, and they
actually melt the components of the allow seperately and they are reblended in
their melted state just prior to injecting under vacum into the plaster molds.
The melting of these component metals is done with electric induction heating
in an oxygen deprived enviroment - this is done to reduce oxidation etc.
We had a small machine at school based on these principles, and of course I had
a ball with it, and learned a lot. I have since discovered that the couple in
Texas that provide a casting service for model railroaders use the same machine.
Most production casters prefer that the masters or models are wax - they
actually recover it during the burnout process and sell it for reuse. The
bottom of the burnout kilns have holes for the wax to run out, and drip down
into troughs of water - as the wax hits the water, it solidifys, and is then
retrieved for reuse.
The caster that I have found in the local area, will accept and burnout masters
made from other materials....like styrene. So in my course, to test this
process, I used some commercial styrene detail parts ( Grandt Line ) to see if
I could cast them in brass.....and was successful. The next step was to make my
own styrene masters and cast them, which also worked quite well. The real
motivation for styrene masters is that process eliminates one level of
shrinkage....the injected molten wax part of the process.
Bill Lane's use of Rapid prototyping also serves to reduce the shrinkage, as
the 3D printer can put down layers of wax or acylic in predetermined dimensions
that could compensate for shrinkage - there may be very very small amounts of
shrinkage as the liquid solidifys, but you would need some very good equipement
to measure it.
So why is shrinkage so important in what we want from a decent model?
If a casting is going to be machined after the casting process, then managing
the shrinkage is not as bis an issue - for instance cast driver centers for a
steam engine can and do have their finished diameters, thicknesses, axle holes,
and crankpin holes all machined to much closer tolerances than the casting
process can produce.
If an air tank or stack has a sllightly different dimension due to shrinkage,
within reason, that may not pose a significant issue.
In my view, and experience - the biggest challenge or issue with shrinkage
occurs in sideframes of trucks. In a perfect world, axle holes on any pair of
sideframes would always be the same dimension from each other so that both
axles are parallel to each other, and the truck bolster. Most of us I am sure
have models with brass trucks that have the axles skewed because the axle holes
on each sideframe are different dimensions apart from its mate. This is as a
result of the shrinkage in the casting process being poorly managed.
Thanks for reading, and I hope I haven't bored you to death!
Dan Kirlin
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