Common red brick is best for making loam molds, Figs. 120 to 123. It should be free from glaze and have a uniform texture, so that the pieces will break clean when it is necessary to fit them to the shape. An old 12-inch half-round file makes a handy tool for cutting these. Sometimes brick is molded up from loam, and air-dried. It is much more fragile than red brick, and may be used in pockets, or where the shell of the casting is quite thin, and ordinary brick might resist the shrinkage strain to such an extent as to endanger cracking the casting.
For laying up the brickwork, mud is used, loam facing being applied only to those surfaces which come in actual contact with the iron. Mud is made from burnt loam or old floor sand, mixed with clay wash to the consistency of mortar.
The composition of loam facing and slip have already been given under the description of making a barrel core.
Cinders are an important material in this work. Their size will depend upon their position in the mold. For working in between brick, the cinders should be crushed if necessary, put through a No. 4 sieve to remove smallest pieces, then passed through a No. 2 sieve to remove the larger pieces.
The names of the main parts of a loam mold differ somewhat from those applied when molding in flasks. As will be seen from the section, Fig. 122, there are three main divisions in the mold: A, which corresponds to the drag in a three-part mold, is called the core. B, which corresponds to the cheek, is called the cope in loam work. And C, which serves the same purpose as the cope of a green-sand mold, is spoken of as the cover in loam molding. When the central core is actually made a separate piece, as in Fig. 123, the lower part of the mold is called the bed or foundation.
In laying-up a loam mold, Fig. 120, set the plate central with the spindle and approximately level. Then set the sweep and finish leveling the plate until repeated measurements at the four quarters of the circle show a uniform space between the lower edge of the sweep and the surface of the plate. For the building plate this measurement should be 5 inches; for a sticker plate the sweep should clear the sticker points by 1/2 to 1 inch according to the thickness of the casting.
The hands are used in spreading mud or loam upon the plates or brickwork when building the mold. The brick must always be set well apart, leaving a space at least the width of a finger between them. Fill in these spaces with fine cinders. The reason for this is fourfold. It facilitates drying; it provides good vent; it gives or crushes sufficiently when the casting shrinks not to cause undue strain; and it reduces the labor in cleaning. In each course of brick the joints should lead as directly as possible away from the casting, but the joints should be broken between courses. These points are illustrated in the sketch A, Fig. 120. As shown, the first two courses of the core are usually set edgewise. For the rest of the core and for the cope, the bricks are laid flat. These bricks run lengthwise around the circumference, with a course of headers about every four to six courses.
Cinders between brick form the ordinary means of leading the vent from the loam facing. In confined places or pockets, as, for example, between the flange D and the main casting, Fig. 122, additional provision is made by laying long wisps of straw between the courses of brick. The service of the straw is similar to that of the hay rope of a barrel core.
The joint in loam work is made by a plate lifting away from a loam seat, or by two loam surfaces separating one from another. In forming the first of these the loam seat is swept up and allowed to partially set, then the surface is brushed with oil, and parting sand is thrown over it. The seat should then be soft enough to allow the iron plate to sink into it sufficiently to find a good bearing, while the oil and parting sand will prevent the loam facing from adhering to the underside of the plate. For the loam-to-loam joint, the same method is used, but the loam is allowed to set somewhat harder before building the joint against it. The angle of the main joint should be about 1 in 4 inches.
To insure the different parts being put together for casting in exactly the same position in which they were built, a guide surface of loam is smoothed across the joint at three or four convenient points on the outside walls of the mold. These surfaces are each marked differently with the edge of the trowel, similar to the cut at C, Fig. 120.
To properly separate and finish some molds, it is necessary to lift away a portion of the mold before lifting the main part. Such a portion is called a drawback. The drawback is always built up in position against a pattern or sweep. With the cover plate, which on a smaller scale often serves the same purpose, as at D, Fig. 122, a flat joint is made on the outer wall of the mold, but the cover plate is swept up separately. At 3, Fig. 123, is shown a drawback which carries but a few courses of brick. It may be lifted away by lugs cast in the drawback plate with little danger of displacing its brickwork in handling.
If the shape of the drawback renders it impracticable to handle it by the lower plate alone, the brickwork should be bound together by means of hook bolts which clamp on a top plate set sufficiently below the upper joint to be entirely protected from the metal. This upper plate has staples cast in it by which the whole drawback may be lifted. At B, Fig. 120, the typical construction of such a piece is illustrated. The drawing shows one-half the length of the brickwork removed to bring out more clearly the rigging used. The upper end of the second lifting staple shows at a, with the loam cut neatly away to allow hooking into the staple.
Where the main core lifts away or is to be covered with metal over its top, it must be bound together in a similar manner. This is illustrated in the mold for the marine-engine cylinder, Fig. 123, in which both of these conditions occur.
If a casting has an internal flange requiring thickness of metal underneath the main core, the rigging will be altered to fit these conditions, as shown at D, Fig. 120. In this sketch a is a sticker plate and so will carry the loam necessary to face the bottom of the core. To this the small bearing plate b is securely bolted by the hook bolt c. This plate must set directly upon solid brickwork, as it carries the weight of the entire core. On this bearing plate are cast three studs which firmly support the sticker plate at the required height above the flange surface. The sticker plate carrying this print is filled with loam or dry sand and given a first baking, then swept to a finished surface before being inverted into position. Then the remainder of the core is built up on top and bound together, as in the previous example. Another way to form the bottom of this core is to sweep up a dummy flange d, in mud. Set the bearing plate b, and work the loam in around the studs to form the short neck to the level of the top of the flange. Then spread over this flange 1/2 inch of loam and bed down onto this the sticker plate which has been previously filled with loam and dried, as is described below. Be sure that the studs on b bring up to a firm bearing against the plate a, then clamp tight with hook bolts and proceed to sweep-up the body of the core.