Well done, Dan.
I was on a shoot where bronze sclptures were being cast. Two guys,
wearing those silver suits with face masks, carried the pot and then
simply poured the molten metal into the mold. It went so fast, I barely
got the shot. This artist made western themed items, but the casters
were contract.
Wish I could have hung around, but they wanted to keep me away and it
was very hot working.
Bob Werre
PhotoTraxx
On 6/25/12 11:57 AM, dan.kirlin wrote:
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
