Skin Dried Molds. (To obtain the advan tages of dry sand molds, and of green sand bedded in work.)— Ordinary floor sand is used for the mold except the two inches of sand which faces the mold which is as rich in loam as for dry sand work. If the surface of an or dinary mold is to be only slightly dried, mo lasses water or thin clay water may be sprayed on the surface to make it strong when dry. A mold when finished is generally skin dried to a depth of one or more inches. Iron baskets filled with burning charcoal are hung in differ ent parts of the mold or heated plates may be held near the surfaces or gas flames may be directed against the surfaces.
Loam Molding.— The body of the mold is built on iron plates with ordinary red brick laid in black mud, which is ordinary floor-sand mixed with clay wash. Some mix sawdust with the mud to make it porous and to help in dry itiF the mold. Straw is sometimes laid in the joints for the same purpose. Pushing a vent wire between the joints also gives vent. This body is built about three-quarters of an inch away from the surface of the finished mold. On the brick surface is spread loam which is a mixture of sand, clay and water. This is faced off with trowels or sweeps. If the mold is cir cular such as a cylinder or a kettle, a spindle is located at the centre of the mold. On this is fixed arms on which are fastened sweeps or templates. Made up cores are also used for giving shape to parts of the mold.
The outside or the inside of the mold with the bottom can be finished first as is most con venient. It is then taken to the oven by the crane. While this is drying the other part is built up and finished and dried. All parts are then put together, the bottom and top plates being bolted together with rods. A crib is built about the mold with steel or cast plates bolted together. Sand is rammed between the brick work and the casing, as hard as possible, form ing also the gates for taking iron to the bottom of the mold. Water pipes and other specialty castings are made in iron molds fitted for the purpose and molding machines are used to a great extent for such work. Every different pattern presents problems which require differ ent treatment.
Cores are used in all classes of molds. An iron anchor may be coated with green sand and set in a mold. Dry sand cores are made of silica sand mixed with a binder such as floor, rosin or oil. When baked these binders make the core hard and strong, and the melted iron burns the binder so that the core sand falls out when the casting is cold. Cores are generally made in boxes. Wires or iron frames are molded in to give strength. Cores are vented by making channels with wires or by molding in a core and then drawing it out. For in tricate cores cords of beeswax are molded in which melt out in baking. Cores are located by core prints in the mold left by the pattern and are held in place by chaplets of various shapes placed beneath to prevent sagging and above to prevent the iron from raising the cores. Nails
forced into the mold are often used, the core resting on the heads. Tinning chaplets prevents rusting and helps the iron to fuse their surfaces.
Chilling.— To produce a hard face or sur face on castings, chilling is resorted to as in cast iron car wheels. In this instance the cir cumference of the mold is made of a heavy ring or iron which tends to cool quickly the sur face of the casting. The hardening mky extend to a depth varying from 1-32 to 1-2 inch. In some forms of chills a circulation of cold water is maintained to continue the resistance to heat.
Melting Iron.— Iron is melted in a cupola, a firebrick lined vertical cylinder in which is charged layers of coke and iron. A blower forces air through tuyeres near the lower end. The melted iron is tapped out at the front into fire clay lined ladles and taken to the mold by hand ladles or by crane. For very large cast ings several cupolas are used and melted iron may be held in large ladles for some time while more is being melted. Air furnaces are also used for heavy castings.
Quality of Iron for Castinv.— Silicon softens cast iron and decreases shrinkage, there fore a standard measure of shrinkage tells whether more or less silicon is needed. Strength depends very largely upon the size of grain, which is not wholly dependent upon the chem ical composition. A thin portion of a casting shrinks more and sooner than a thick part which often causes fracture. It has been erroneously said that cast iron expands at the instant of solidification.
Iron becomes rigid at the surface of the mold while the central part is fluid. When all is solid, and just about as it ceases to be red hot, the casting expands more or less at the same time that carbon crystallizes into graphite.
In a casting one inch square this expansion occurs about 15 minutes after the iron enters the mold, but in a casting four inches square it occurs two hours after. This expansion at dif ferent times of parts of a casting of varying thickness, at the same time that the casting is shrinking unevenly from the loss of heat, pro duces severe strains which often fracture the casting. The remedy is to make all parts of equal thickness and to uncover thick portions as soon as solid. There are 50 factories in the United States making. foundry supplies, giving employment to about 500 men, and adding more than $1,000,000 to the value of the products they handle. They supply various mixtures of sand and plumbago and sea-coal, as well as charcoal facings to keep the metal from adher ing to the molding sand. They also sell parting compounds to facilitate separation of the parts of the molds; for this purpose day and hydro carbons are employed. Their other products are brazing and welding compounds, foundry (flour,* plumbago and founders' small supplies, as brush-wheels, rammers, sieves and flasks.