Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/512

 480 FOUNDING being only used with these, for the production of ingots or masses that are brought to shape by other means, or when a special quality of surface is required, as in chill casting. In most cases castings are hollow, the thickness being determined by the empty space included between the mould proper which represents the external surface of the object, and a false mould or core, which may also reproduce a finished surface, as in cylinders, pipes, &c., or be rough and uneven, as in statuary castings, where only the external surface is exposed. The material generally used in mould ing from patterns is fine sand, either &quot; green,&quot; i.e., slightly damp, or dry, that is, dried by artificial heat the first method being adopted for all castings of moderate size and weight, while dry sand mouldings are chiefly used for heavy castings where great solidity and strength are required. The principal requisites of a good foundry sand are fine and uniform grain, a certain amount of cohesiveness without being sticky, infusibility at the temperature of the metal poured, and freedom from combustible or other substances giving off gases when heated. These are best fulfilled by a nearly pure quality of siliceous sand, with at most 3 or 4 per cent, of clay and a slight proportion of hydrated peroxide of iron ; the particles when moulded should allow free passage for gases to escape, while perfectly impermeable to the melted metal. Good foundry sands are easily procured in most parts of the United Kingdom, the best being those obtained from reconstructed sandstones in the alluvia of the Thames and other large rivers, and the drift of the New Red Sandstone districts of central England. In other countries not so well provided, foundry sands are often im ported or brought from considerable distances inland. The same sand is used continuously, the moulds after use being emptied into a pit in the foundry floor, whence the supply for new moulds is taken as required. Fresh sand is added from time to time to make up the waste and to maintain the required plasticity, which diminishes by constant heating. A proportion of ground coal or char coal is mixed with the sand, so that, although the latter is actually red or brown when fresh, it is reduced to a dark grey or black in the foundry. The sand forming the mould is held together by an outer frame or box called a flask, as many flasks being used as there are separate parts in the mould. These are united by lugs and cotters, the top one being sometimes loaded when the object is large to prevent it moving under the pressure of the fluid metal. A proper division of the mould is one of the chief points to be attended to in foundry work ; where the object is divisible by a central plane into two FIG. 1. Arrangement of Patterns in casting Railway Chairs. equal and similar halves, two flasks are usually sufficient ; but in complex and irregular forms three, four, or even a larger number are required, its divisions being so arranged that no portion of the pattern overhangs within any section, so that it may be withdrawn by a straight pull without shaking the sand. The ordinary operation of moulding is as follows. A flask laid with its lugs uppermost is rammed up with old sand to a smooth surface. In this the lower half of the pattern is imbedded, and the surface is covered with dry or facing sand to prevent adhesion. Upon this a second flask is placed, and sand is carefully rammed upon the pattern until the box is completely filled, when the whole is turned over, and the first or false part is emptied, the surface of the upper half smoothed down or faced with sand or finely ground coal or charcoal, and a runner stick, which forms the passage or ingate for the metal, inserted. The second half is then similarly moulded in a second flask, and when finished the upper box is lifted by a crane, leaving the pattern in the lower one or drag, from which it is lifted by spikes or rods screwed on temporarily, a slight vibrating motion being set up by striking it rapidly with a piece of wood or iron in order to start it more easily. This is an operation of some nicety, as the blows must be moderate so as not to risk injury to the sand. Provision is made for the exit of gases by piercing vent holes through the sand by a fine wire during ramming. The surface of the mould is finished by dusting it over with charcoal or graphite, In moulding railway chairs and similar objects of an irregular form required in great numbers, metal patterns are used with loose pieces united by spikes and dove-tails for the overhanging parts, such as the inner faces of the jaws, the joints being so arranged that the straight parts of the pattern may be withdrawn, leaving the loose parts behind in the mould, whence they are afterwards removed by hand. Fig. 1 represents in section an arrangement of this kind, as applied to moulding railway chairs. The right hand figure shows the pattern in place with the sand rammed, and the left the mould with the pattern D with drawn, the loose jaws or &quot; core prints &quot; a, b, c, remaining in the sand, but in such a position as to be easily removed when the flask is turned over. The pattern is with drawn by a straight pull on the handle H. The stop P gives support to the cores, *tc., which represent the trenail holes in the finished casting (fig. 2), and prevent them being dragged away with the pattern, as they might be if left unsupported. The regular descent of the pattern is ensured by the deep sides A and the guides they move in.