Sand
casting is done in a mold composed of fine casting sant. The sand used
is very fine. The sand is mixed (tempered) with of a glycerin to hold it
together.
The
sand is packed into a flask. The flask has two parts: one with pins,
the drag; one with holes to receive the pins, the cope. Packing is best
done on a flat shed of metal or marble called the mold surface.
Procedure : When
packing the flask, first place a half round piece of rod or a piece of
tubing cut in half on the steel or marble surface. Place the cope
section of the flask, hole side down, so that the half round piece is in
the center.
Now
pack the sand firmly into the cope, first with your finger, especially
in the corners, and then lightly with a hammer. Smooth the sand by
running a flat piece of steel over the top of the cope.
Turn
the cope over and remove the half round piece of metal. Piece the ring
to be cast on a round rod. The rod length should be slightly shorter,
than the length of the half round piece. Now place the ring an rod over
the depression in the sand formed by removing the half round piece. The
top of the ring should be up, and the shank down. Push the shank and the
rod down into the sand until the rod is halfway down.
Dust
a very light layer of powder, over the sand and the ring. Talc powder
may used. It is best to keep the powder in a small, semi porous cloth
bag. By tapping the bag lightly it is easy to spread a think, even layer
of powder. The powder acts as a separator, that is, it prevents the
sand in the drag from sticking to the sand in the cope.
Now
place the drag part of the flask on the cope, sieve some sand into it
and finally pack it entirely with sand and smooth the top.
The
flask must now be separated and the model ring removed. To separate tap
the sides of the flask lightly with a hammer to vibrate the model and
thus release adhering sand from it. Then slowly lift the drag from the
cope and remove the model ring. A sharp impression of the ring should
remain in the sand.
A
core a round piece of sand the same diameter as the rod the ring fits
on – must be made. It is made by packing sand into a small tube with the
same inside diameter as the outside of the rod. Push the packed sand
out of the tube with a rod.
A
sprue opening and gate is formed so that the molten metal may be poured
into the flask. The easiest way to do this is to take the back of a
needle file and push it through the sand from the shank impression side
of the cope.
Finally,
with a scriber, scratch several light lines from the ring impression to
the edge of the flask in order to permit the air to escape from the
hollow ring impression in the sand when the molten metal enters.
The
metal may be melted in a furnace crucible. The weight of the required
metal can be determined by weighing the model and then adding a little
extra metal to this weight. When melting the metal add a little borax to
keep it from oxidizing. Pour the molten metal as quickly as possible
into the gate in the sand. The flask can be opened almost immediately to
remove the cast ring.
The
finest castings are obtained from dried, water tempered sands. After
the flask has been assembled it is baked over a gas stove until all the
moisture in the sand evaporates. Where as a wet sand will from steam
when the molten metal enters and the steam will interfere with the
casting, a dry sand will not and therefore a better casting can be had. A
good castings can be obtained by merely heating the sand for a moment
or two before the flask is assembled.
