As has already been said, it is necessary that the pattern maker should have some knowledge of molding in order that he may construct his patterns so that they can easily be removed from the sand. A brief description of the general method employed will suffice.
Ordinarily, a casting is made in a flask, consisting of two parts, each containing its complement of sand - the upper part called the cope, and the lower part the nowel or drag. The pattern is sometimes made in two pieces that separate or part along the line separating the cope and the drag. Thus, in Fig. 104 the pattern separates with the flask on the line A B, and, when so separated, the cope is turned upside down, and the portion C of the pattern is lifted out. The portion D is lifted out of the drag in the same way.
In the case of molding a hollow object, the internal cavity in the casting is formed by means of a dry-sand core which rests in impressions made in the sand by core prints attached to, and forming a part of, the pattern.
To illustrate this, let it be required to cast the hollow cylinder shown in Fig. 105. The wooden pattern necessary to produce this hollow cylinder is shown in Fig. 100, which, as will be seen, represents the cylinder only externally by the part A. The core prints, one on each end of A, are represented by x and y. These projections form part of the pattern, and make their impressions in the sand with the part A, which alone represents the required cylinder. For making the core, the length of the inside of the core box, in which the dry-sand core is formed, will be the extreme length of the pattern including x and y, and the inside width will be the exact diameter of the core prints. In this case, the core being a cylinder, only a half-core box. Fig. 107, is used. In it are made two semi-cylindrical cores, which, after being dried, are cemented together, thus forming the complete cylindrical core required.
Fig. 105. Hollow Cylinder.
Fig. 106. Cylinder Split Pattern with Core Prints.
To mold this halved or split pattern, as it is called, the upper half of the pattern is laid on the molding board, and the drag is turned over it with the bottom side of the drag up and the parting side on the molding board, as shown in Fig. 108. After being rammed up, the drag and molding board are turned over and the board removed, when the parting of the pattern is exposed, the half-pattern being imbedded in the sand.
The second half of the pattern is now placed in position on the first, and dry parting sand is spread over the surface of the wet or green sand; the cope is put in position on the drag, as shown in Fig. 109, and rammed up. Upon the cope and the drag being separated, the sand separates on the line to which the parting sand has been applied, which, as may be seen, is the line of parting of the cope and the drag, one-half of the pattern remaining in each.
After the halves of the pattern have been removed from the cope and from the drag, respectively, the completed dry-sand core is placed in the molds made by the core prints x and y. This core B is shown in position in Fig. 110, and entirely fills the parts of the mold made by x and y, leaving between itself and the surface of the mold made by A room for the metal to be poured which is to form the required cylinder.