In case a cover plate must be bedded down against a flat surface, as in the example just mentioned, or must take the impression of an irregular surface on the top of a mold or pattern, as illustrated in Fig. 123, the method to pursue is as follows: After casting, invert the plate and carefully lower it into position, and make sure that all fingers clear the surface by at least 1/2 or 3/4 inch. Now set the plate with the fingers up, fill in with loam enough to just clear their tops, leaving the proper openings for runners, risers, tie bolts, etc., and dry thoroughly in the oven. Upon removal from the oven, invert and try this loam cover again on the surface it must fit; scrape away any portions which project too much. Now hoist away the cover and coat the face with clay wash. Having previously prepared the surface of the pattern with oil and any loam joint with oil and parting sand, spread an even thickness of fresh loam all over and bed the plate down upon this. The cover plate, being still hot, will, by the aid of the clay wash, cause the thin layer of fresh loam to dry out and stick fast to the dry loam forming the body of the plate.
As an example of a simple loam mold let us consider the details of a large casting, having the shape of the frustrum of a cone, with a flange at the top and bottom and a flanged nozzle projecting from one side, such as the section clearly shown in Fig. 122.
Set the sweep, level up the building plate, and, building the brickwork as shown in A, Fig. 120, sweep the seat, joint, and bottom surface of flange, as shown at A, Fig. 121. The lower flange may be formed by a wooden pattern furnished by the pattern maker, but it is more common to have the sweep made with the small board x, which may be removed. By doing this the exact shape of the flange may be swept up without changing the main sweep, as shown at B, Fig. 121. This dummy flange, as it is called, is swept-up from fairly stiff mud.
The next step is to seat the cope ring and set the cope sweep, as shown at C, Fig. 121. This sweep shapes the mold for the outside of the casting, for the top flange, and for the top joint of the mold. Loam is thrown, a handful at a time, against the joint and dummy flange, and the engaging faces of bricks are rubbed with loam and pressed into position.
When the top of the lower flange is reached in this way, the courses are laid-up for about 2 feet before the loam is spread upon their inner surface and struck off. This method is pursued until the mold is built to its full height.
The projecting nozzle is formed by a wooden pattern; this should be well oiled, and the brickwork and loam laid-up under it to support it at the proper level, as given by the center line on the pattern and corresponding line on the sweep. Such projections frequently must be supported in their exact position with reference to the main pattern by temporary wooden framework or skeleton work until the mold is built up under them.
A finger y nailed to the top member of the cope sweep, shapes the guide surfaces on the outside of the mold which are used to center the cover plate in closing the mold. A similar finger exactly the same distance from the spindle, is fastened to the sweep used to form the cover plate.
Then the whole cope is lifted off and set on iron supports where it may be conveniently finished with black wash and slicks. It is then baked over night in the oven.
The dummy flange is now entirely removed from the first part swept, the core sweep is set, D, Fig. 121, and the center core is struck up. This core is then blackened, slicked off, and baked. The cover plate is struck off with the stickers up, and baked so. This cover carries six 1-inch round holes through it, which will be just over the shell of the metal when the mold is closed. Five of them connect with the pouring basin and serve as runners, while the sixth serves as a riser.
In assembling the mold for pouring, the core is first set on a level bed of sand, the cope is accurately closed over it by the aid of the guide marks, and lastly the cover plate is closed in position. Now the whole mold is firmly clamped by blocking under the spider, from which wrought-iron loops or strings connect under the lugs of the building plate, as shown in Fig. 122.
The small core for the nozzle is now set, resting on stud chaplets. The cover plate D is slid over the end of this core and thus holds it firmly in position.
The casing is now placed around the mold and molding sand rammed in to support the bricks against the casting pressure. At the level of the nozzle core cinders are placed, and a pipe leads off to carry away the vent gases. The sand is rammed to about 12 inches over the cover plate and in it are cut the channels connecting the pouring basin and runners. A couple of bricks are set in the bottom of the basin to receive the first fall of metal from the ladle.
In pouring, the runners must be flooded at once and kept so until the mold is full.
In heavy cylindrical castings it was formerly thought necessary to carry the shell of the casting some 6 inches higher than the top flange. This head served to collect all dirt and slag that perchance entered the mold with the iron, and it was cut off in the machine shop and returned to the foundry as scrap. With the increased knowledge of iron mixtures this head is now done away with in most instances.
Where a large casting is to finish practically all over, and very clean metal is therefore necessary, overflow channels, connecting with pig beds, are often constructed in modern practice. Then, when pouring, the metal is not stopped until a certain per cent of it has been flowed entirely through the mold. This of course tends to wash out any dirt which may have gotten into the mold when pouring began.
When the casting is cold, the casing and packing sand as well as the blocking under the spider are removed. Then the whole mold is carried to the cleaning shed where the bricks are removed and the casting cleaned.