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Investment coring question


#1

I’m working on making a mold for a large brass casting, about one
pound of material finished. The interior is quite complicated. I
will be making a two piece aluminum mold for the exterior.

Can I make the core out of cast investment, then shoot wax around
it? And then continue to invest that assembly?

Or should I stop thinking and make my core out of water soluble wax?
Though I am concerned about getting the investment into the hidden
voids.

Hans Stangier


#2
Can I make the core out of cast investment, then shoot wax around
it? And then continue to invest that assembly? 

How will you keep the core properly positioned in the empty mold
before/while you inject the wax? And after casting, how will you get
the investment out of the hollow casting? Traditionally, investment
cores are held in place with pins which extend from outside the wax
to inside the core, so after the whole thing is invested, the pins
hold the core in place in the mold. Placing that core into the
injection mold means you need to provide the mold with someplace to
anchor the pins so after injection, they extend out past the wax. If
you don’t, then after the wax burns out, the core drops down out of
place. After casting, the pins (steel, or something higher melting
than your casting metal, or of the casting metal but sturdy enough so
entering molten metal during casting won’t melt the pins can be
pulled and the holes plugged, or if of the casting metal, simply
trimmed.

But this still doesn’t address how you will get the investment
filling out of the casting. There needs to be some sort of opening
somewhere. If several openings are large enough for strength, then
the resulting investment bridges can hold the core without pins. And
if you’ve got this sort of setup, then you don’t need a sepearte core
for casting, as the normal invesment process will fill the mold. Have
one hole, at least, positioned so it’s up during the investing
process, to aid getting all the air out.

Now, that’s for casting. If you’re wax model has these holes, so you
don’t need a premade core inside the wax during wax injection (which
will be the case if you want to remove the invesment from inside the
casting without having to drill holes into it to remove the
investment (in which case you might as well have had them in the wax
in the first place), then your problem is simply how to inject a mold
so as to get a hollow wax. That’s fairly easy, especially with a
metal mold. The mold will chill the wax from the outside in, so as
the wax cools after filling the mold, you get a skin of solidified
wax in the surface of the mold, while the center is still molten. So
fill the mold, without a core, wait an appropriate amount of time for
the wax to start to solidifiy at the surface, and then pour the wax
back out of the mold. The center will empty, leaving a hollow wax
model. The process is similar to what ceramics people do when slip
casting some types of plaster of paris molds. Pour in the slip, wait
a predetermined time while the plaster absorbs water at it’s surface,
then pour out the slip that’s still liquid enough, leaving a fairly
uniform thickness of clay slip layer inside the mold. For wax, the
layer is cooling not water absorbtion, but it works the same.

Or should I stop thinking and make my core out of water soluble
wax? Though I am concerned about getting the investment into the
hidden voids. 

If you could dissolve the water soluable wax out of the middle of
the wax model, then you could get investment in too. And you still
would have to address the issue of holding the core in position while
injecting the wax.

Hope that helps.
Peter Rowe


#3

While not strictly on topic I wonder whether this development has
applicability in preparing an investment mould.

http://tinyurl.com/yfouorw

Kind regards
Don Iorns