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Vibration induced casting defects


#1

I have experienced problems for some time with rough surfaces on
heavy castings. I was treated by Orchidians to several lengthy and
informative pieces that provided several tips that have been
implemented.

It seems that light and medium weight castings seem to be allright
and do not exhibit surface problems. I recently cast two medium belt
buckles that are considerbly heaver than the successful pieces. The
surface is very rough and details are covered with a fine rough
texture. The surface of the mold seems to have broken down during the
casting process.

In a complete evaluation of my casting process, I discovered that
the vacuum pump induces considerable vibration into the vacuum
casting table and have taken steps to reduce the induced vibration to
near 0. It does not slop around but movement is quite apparent.at the
button before it freezes.

The mold was dewaxed and calcined at 1250 F. The investment was
precisely mixed to the gram of investment and mililiter of water, The
dewaxed flask was cooled and held at 750 F for 30 minutes before
vacuming and pouring the silver. The investment is less than 1 year
old and is stored dry and turned once in a while.

Does it seem reasonable that a heavy charge of liquid silver in a
mold subject to vibrational movement cause the mold surface to
degrade?


#2

Is 750 degrees hot enough?


#3

You do not say whet temperature your molten metal is, you need a
considerable superheat if your mould is going to be as cool as it is.
I suspect the problem is the metal is chilling as soon as it hits the
mould and starting to freeze. the bulk of the metal prevents it from
adhering to the mould and you are getting turbulence in the flow
which leaves microcrystals in the melt that appear at the surface as
a rough texture. Your metal needs to be 200 degress hotter than its
melting point with cool moulds.

Nick royall