Founding

I

Introduction

Founding, process of producing metal objects, called castings, by pouring molten metal into a hollow mold, usually made of sand. The founding process itself is sometimes called casting. It is an ancient art that is still used extensively, although it has been supplanted to some extent by other methods, such as die-casting, forging, extrusion, machining, and rolling. See Iron and Steel Manufacture; Metallurgy.

Founding involves three separate operations: first, a wood, plastic, or metal pattern intended as a replica of the finished object is made; a hollow mold is then made by packing sand around the pattern and removing the pattern; finally, molten metal is poured into the mold.

In cases where a small number of castings are to be made the foundry pattern is usually of varnished wood, but in the production of a large number of castings, it may be plastic, cast iron, steel, aluminum, or other metal. The pattern differs in two important ways from the original: its dimensions are slightly enlarged to compensate for the shrinkage of the casting as it cools, and patterns of hollow objects have projections that correspond to the cores (described below). Although patterns can be made in one piece, a complicated casting is easier to remove from the mold if it consists of two or more parts. Patterns of objects with straight sides are usually made with a slight taper for the same reason. Pattern parts are fitted with matching pegs and holes to ensure precise alignment when assembled. Molding a pattern and pouring a casting can be understood through a description of a simple casting such as a pipe with a flanged end (Figure 1). The pattern for this casting is shown in Figure 2. The molds for most castings are prepared in flasks (wooden boxes without top or bottom) equipped with pegs or other devices that enable the boxes to occupy the same relative position when fitted together (Figure 3). The lower box is called the drag, and the upper one the cope. In making the mold, the flat portion of half of the pattern is placed on a flat surface and the drag is invested and placed around it. Molding sand is poured into the flask and rammed down until the entire flask is filled. The flask is then turned over and the other half of the pattern is set in place, as in Figure 4. A layer of special dry sand, called parting sand, is sprinkled on the surface of the flask; then the cope is placed in position, filled with sand, and rammed. The two halves of the mold are then taken apart and the pattern is drawn or removed. One or more gates or pouring holes are then pierced through the sand of the cope, as are smaller holes called risers that carry away part of the steam formed when the hot metal is poured into the mold.

Finally, the core, the part of the mold that forms the hollow within the casting, is prepared. In the pipe shown in Figure 1, this core takes the form of a simple cylinder, but a complex casting may require one or more intricately shaped cores. The cores are formed in divided core boxes that serve as patterns. After forming they are baked in an oven until they are strong enough to be handled. The core is placed within the mold (Figure 5), and the other half of the mold is replaced. It is now ready for pouring. Having been melted in a furnace, the metal is hand-poured from a crucible for small castings or, in most cases, from a large dipper or bucket carried by a crane or special car until the mold is completely filled to the top of the gate.

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After the casting has cooled within its mold, they both are shaken out of the flasks, and the mold is broken. The rods of metal formed in the gates and risers must be sawed off or otherwise removed.

The sand used in founding contains sufficient clay to make it cohesive when slightly moistened before use. Parting sand is used to make the flasks come apart cleanly when separated; this sand is dry and contains little or no clay. Sand is used for molds because it permits a certain amount of vapor and gas to escape when the casting is poured. For metals with low melting points, such as brass, it is possible to use solid molding materials such as plaster of Paris. Such molds—and metal molds—have smoother surfaces than sand molds, and thus produce castings with finer detail and finish. They cannot, however, be used in casting iron or steel. Many variations and special techniques are involved in ordinary founding. In manufacturing, making two or more castings in a single mold is frequently desirable. Patterns of objects with overhanging parts may be made with removable pattern portions so that the pattern may be drawn from the mold piece by piece without disturbing the sand. In casting such machine parts as gears, in which the rim must be as tough as possible, pieces of iron or steel called chills are sometimes placed in the mold around the rim. The chills conduct heat rapidly, permitting the portion of the casting near them to harden quickly, thus toughening the metal. Large wheels and gears are sometimes cast without flasks, in beds of sand directly on the foundry floor. In such cases, the form of the wheel rim is carved directly from sand, and cores are placed in the mold for forming the hub and spokes.

II

Modern Founding Methods

Centrifugal casting is a method for casting objects of circular form. A circular mold is revolved rapidly during casting; no core is required because the rotation of the mold holds the metal against it by centrifugal force. This technique is useful in the production of metal pipe. In investment casting, an adaptation of the cire perdue (lost-wax) process of ornamental casting, the pattern is constructed of wax, often in a die-casting machine, and coated with a watery paste of refractory material that is allowed to dry. This refractory shell, with the wax pattern still inside, is packed in sand, and the mold is baked. The wax melts and runs out, leaving the mold ready to receive the metal. Castings made by the lost-wax process produce close tolerance and reproduce fine detail accurately. In the continuous casting process, widely used in steel production, molten steel is poured at a steady rate into the top of a water-cooled mold of uniform cross section, and solid steel is continuously withdrawn from the bottom of the mold.

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