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Casting quality - Centrifugal vs. Vacuum


#1

Hello,

Since liquids like molten gold do not compress easily, I am certain
that any difference in density has to do with other factors. The
factors as to which are better have more to do with what you are
making, and how your shop is or is to be arranged.

Even the density difference between castings and stampings is
insignificant in terms of volume and weight. The properties change,
due to the crystal structure, rather than density.

We see just about every kind of item well made on either vacuum or
spin casting. The best of the best is all arranged around the
equipment available.

Great castings come from a well run system of methods and machines.
Its just not about spin vs. vacuum.

Daniel Ballard
Precious Metals West
National Sales Manager


#2

Hi all,

Since liquids like molten gold do not compress easily, I am
certain that any difference in density has to do with other
factors. The factors as to which are better have more to do with
what you are making, and how your shop is or is to be arranged. 

You can’t compress liquids at all no matter whether you’re talking
about molten gold or water. That is what the principle of hydraulics
exploits. The only state of matter that can be compressed is a gas.
There may be solids and liquids which have air spaces but any
compression will be in the pockets of air and not the solid or
liquid matrices.

From what I’ve read so far (as I’ve no casting experience yet),
porous castings are due to poor technique rather than one technique
verses another, although if one were to guess then vacuum casting
would suggest (that’s the name VACUUM suggesting, not me suggesting)
a better quality casting in the absence of air.

Helen Hill
Preston, UK


#3

I have no practical experience regarding this, but can discuss based
on theory.

As far as I know there are 3 principles of casting that are
relevant:

  • Centrifugal casting where liquid metal is “thrown” SIDEWAYS into
    the flask by centrifugal generated gravity. Might be assisted by
    vacuum “under and outside” the flask.

  • Pouring liquid metal DOWN into the flask, assisted by vacuum under
    and outside the flask.

  • Sucking molted metal UP into the flask by vacuum over and outside
    the flask.

To protect against oxidation some prosesses might take place in a
vacuum orprotected by inert gas.

Here is my understanding of the relevant physical laws:

  • Air pressure and gravity. These two are not the same things…

  • Internal pressure in the liquid. The “height” of it generates
    internal pressure. Like when diving 10 meters down into the sea you
    experience the pressure of 2 atmosferes.

  • Accelleration/decelleration. The “speed” of a liquid might
    generateinternal pressure inside the liquid if it is suddenly
    "stopped" inside a confined area.

The various casting techniques make use of these principles to
"stuff" the liquid metal into the mold before it freezes.

Like when pouring liquid into a container:

  • Gravity pulls the liquid down into it.

  • The height and weight of the liquid (being affected by gravity)
    generates liquid pressure on the walls inside the container.
    Pressure is higher “deeper” into the liquid.

  • If the container is purous and there is vacuum under and on the
    sides of it, the weight of 1 atmosphere of air can be added to the
    liquid pressure.

So… what makes the liquid move down into the flask, out into the
sprue and up inside the molds?

Basically - the difference between the internal pressure in the
liquid and the pressure in the mold.

Having no experience in casting, I will not speculate in what
casting tecnique is best. To me it looks like investment cost, size
of series and what technique is well known to the caster is most
important.

For you specific problem, I can only summarize based on theoretical
knowledge and lurking in this forum. A “clean” mold (well burnt out
wax) seems important. Also protecting the liquid metal from
oxydation by using inert gas or a propane torch while pouring might
be relevant.

My 2 cents :)))