To withstand the high heat, pure silica sand is used as the basis of the facing mixtures for steel molds. Pure quartz or silica rock is quarried, and reduced to sand form through a series of rock crushers. At the foundry the necessary bond is given by the addition of fire clay and molasses water. These are thoroughly mixed with the sand in a facing mill and mixer, Figs. 135 and 136. A typical mixture is as follows:
1 barrow silica Band.
3 pails powdered fire clay.
Temper with molasses water.
Where quartz sand is very expensive, the following mixtures, I, II, III, or IV, will reduce the cost. The old crucibles and fire brick should be crushed separately in the mill before mixing.
Fig. 143. Shrinkage Webs.
The holes for pins should be drilled to template in all flasks of the same size, so that copes and drags may be interchanged. Flask pins are slipped through the holes temporarily when the flask is being closed or opened.
On large work the cope is bolted to the drag while being rammed. The bottom plate is of cast iron, and is clamped to the flange of the drag with short clamps and steel wedges. The same tools are used for packing and finishing the mold as those described in connection with iron molding.
Flasks of from 18 inches to 48 inches in length have two handles bolted on the ends to lift them with. Larger flasks have trunnions, rockers, or U-shaped handles cast on the sides.
Fig. 145 shows type of convenient small flask built up of channel and angle iron, size 14 by 20 to 24 by 48 inches.
In packing the mold, place the pattern on the board and cover with 1 1/2 to 3 inches of facing, depending on the size of the job. Tuck well with the fingers. The facing is used as prepared by the mixer, not sifted. Set the drag on the board, shovel in heap sand, and ram the mold somewhat harder than for iron. Strike off, and seat the bottom plate, fastening it firmly to the flange of drag with clamps or bolts. When this is done, roll the mold over, and remove the moldboard.
Press with the fingers all over the joint surface, especially around the pattern, to make sure of firm packing. If soft places are found, they should be tucked in with facing sand. When needed repairs are made, slick the joint all over. Use burnt core sand fur making the parting.
Try on the cope and adjust bars to fit the pattern. Clay-wash the cope before packing. Put on necessary facing over the joint and the pattern. Set the gate on the joint, but place risers directly on the pattern. Set the necessary gaggers, shovel in heap sand, and ram the cope. Vent well, lift the cope, moisten the edges, and draw the pattern.
In finishing the mold, nails are used freely, about 1 1/2 to 2 inches apart, driven in till the heads are flush with the surface of the sand. This is to prevent the cutting of the surface by the rush of hot metal when the mold is poured.
It is at this stage that the thin webs previously mentioned, are cut into the corner fillets where needed. The whole surface of the mold must be smoothly slicked over with the trowel and convenient slicks. When this is done, paint on the facing wash with a very flexible long-bristled brush.
Molds for steel should be more than dried; they should be thoroughly baked to drive off every particle of moisture. This prevents the steel boiling in the mold and causing imperfections in the casting.
Where but little machine work is to be done on small work up to 1 1/4 inches thick, the molds may be made up in wooden flasks and poured green. For this class of work, only pure quartz sand and fire clay are used, tempered with molasses water. These may be made up and poured on the same day.
Cores for steel molds are made up in boxes similar to those used in the iron foundry. Although using special sands, the cores are strengthened with iron rods, vented with cinders, and provided with convenient hangers for lifting, as described in previous paragraphs.
Where cores must be made in halves, one set of half-cores may be made and baked. The other half is then made and rolled over directly on the baked half. Fire-clay wash is used to cement the joint. This method allows the joint between the halves of a core to be nicely slicked down.
Steel cores must be more collapsible than those for iron, on account of the excessive shrinkage of the metal. This is provided for in the mixture of the sand used, and by thoroughly baking the core to reduce the effect of the binding materials to a minimum.