SAND CASTING

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.