Foundry facing is the term given to materials applied to or mixed with the sand which comes in contact with the melted metal. The object is to give a smooth surface to the casting. They accomplish this in two ways: (1) by filling in the pores between the sand, thus giving a smooth surface to the mold face before the metal is poured; and (2) by burning very slowly under the heat of the metal, forming a thin film of gas between the sand and iron during the cooling process. This prevents the iron burning into the sand and causes the sand to separate from the casting when cold.
Different forms of carbon are used for this purpose because carbon will glow and give off gases, but it will not melt. The principal facings are graphite, charcoal, and sea coal.
Graphite is a mineral form of carbon. It is mined from the earth and shipped in lumps which are blacker than coal and are soft and greasy like a lump of clay. The purest graphite comes from the Island of Ceylon, India. There are several beds, however, in the coal fields of North America.
Charcoal is a vegetable form of carbon. It is made by forming a shapely pile of wood, covering this over with earth and sod, with the exception of four small openings at the bottom and one at the top. The pile is set on fire and the wood smoulders for days. This burns off the gases from the wood, leaving the fibrous structure charred but not consumed. Charcoal burning is done in the lumbering districts. The charcoal for foundry facings should be made from hard wood.
Although sea coal contains a high per cent of carbon, it is less pure than the other facings and gives off much more gas. Sea coal is made from the screenings from the soft-coal breakers. The coal should be carefully selected by the manufacturer and be free from slate and very low in sulphur.
All facings are manufactured by putting the raw materials through a series of crushers, tumbling mills, or old-fashioned burr stone mills, and then screening them. The finest facings are bolted much as flour is.
In the shop the molder distinguishes between facings or blackings, and facing sand. Blacking consists of graphite or charcoal, and is applied to the finished surface of a mold or core. Facing sand is the name given to a mixture of new sand, old sand, and sea coal, which in the heavier classes of work forms the first layer of sand next the pattern.
The use of the different facings will be clearly seen from the tabulation on page 8.
Good facing for light molds; dusted on from bag after pattern is drawn.
Burns at low enough temperature to be effective before thin work cools.
Mixed with molasses water for wash for small cores and dry-sand work.
Resists moisture; prevents sand surfaces from sticking together.
May be used as a parting dust on joint of bench molds.
Good facing for bench molds; dusted on from bag; good for medium and heavy green-sand work. Applied with camel's hair brush, and slicked over with tools.
Good on heavier green sand because it is more refractory than charcoal, but still forms gas enough to keep metal from burning into sand.
As heavy blacking for dry-sand and loam work, used as above.
Mixed with facing sand in proportions from 1:6 to 1:12. See section on Molding.
Helps to force vents through sand when mold is first poured, and prevents strong sand of the facing from caking, because it continues to throw off gas after casting has solidified.