metal mould sand iron pattern surface cast foundry required moulds
FOUNDING, the art of reproducing solid objects in metal or other fusible substances by pouring the melted substance into moulds. It is also known as casting, and objects so produced are said to be of cast metal. Works where founding or casting is carried on arc termed foundries, and their proprietors founders. The verb to found is not, however, in current use, being almost entirely replaced by cast. The root of the word is the Latin fundus.
Three principal operations are involved in founding: - (1) moulding, or the production of a hollow mould to receive the melted metal; (2) melting, or running down the metal; and (3) pouring, or filling the mould with the liquid metal. The preparation of the original object or pattern from which the mould is made is not strictly part of foundry work proper, the founder receiving the pattern prepared in wood from the original drawings from the engineer's patternmaker, except in those cases where no pattern is required, and the model is built up on the foundry floor by the moulder by the use of revolving templates, dividing engines, or other contrivances.
The metals best suited for foundry work are those that possess the property of increasing in volume at the moment of passage from the liquid to the solid state so that its particles may be pressed into and fill up the finest cavities of the mould in setting. This property is best developed in bismuth, the alloys of copper with tin and zinc (bronze and brass), and cast iron. Lead does not take a sharp impression unless alloyed with tin or antimony, as in type metal. Copper also does not give sound castings.
Patterns for moulding require to be made somewhat larger than the cast required, the difference being determined by the linear dilatation of the metal between the ordinary temperature and that at the moment of solidification. This varies for different metals; for cast iron it is about -D16-; for hard bronze, ; soft bronze, brass, A.; zinc, h; lead,-; tin, yr; and bismuth, T. Patterns for iron founders are therefore made larger than the finished size required in the proportion of one-eighth of an inch to the foot in their linear measurement, an allowance known as " shrinkage " - the patternmaker's rule being longer by that quantity than the ordinary'engineer's rule. Patterns are usually made of wood, except when the object is intended to be reproduced in great numbers, when brass or iron ones are often used. The more easily fusible alloys, such as pewter, type metal, Britannia metal, &c., are cast in metallic (iron or brass) moulds, which are used indefinitely; but with metals having a higher melting point, a separate mould is required for each cast, metal moulds 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 moulding from patterns is fine sand, either " green," 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 imported 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 charcoal 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 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 boles 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 dovetails 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 withdrawn, the loose jaws or "core prints" 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 withdrawn by a straight pull on the handle H. The stop P gives support to the cores, &c., 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.
hydraulic press or other mechanical arrangement. The lower mould forming the base of the chair is a nearly flat plate moulded in another machine.
In loam moulding, as used for large pipes or cylinders, a hollow core of iron or brick is placed in the centre of the foundry, and around it a layer of loam, that is, a mixture of sand and clay rendered plastic by mixture with water, is laid on by trowels and finished up by a revolving template working round a central vertical spindle to the dimension of the interior, forming the " nowel" or core, which when dried is washed with finely ground charcoal and water. Upon this a loam pattern i3 made up by another template representing the outer surface of the cylinder to the thickness of the finished work. This in like manner is dried and black-washed, and finally a shell of brickwork is built outside, leaving a few inches space between it and the second moulded surface, which is carefully filled up with loam, and forms the " cope " or mould proper. This when dried is lifted by a crane, and either separates from the pattern or " thickness " or drags it away with it, but in either case the latter is broken away, and when the cope is replaced the mould is ready for use as soon as the necessary air-vents, ingates, runner passages, &c., have been provided. In many large foundries, however, gas and water pipes of large size are now produced from permanent moulds of cast iron faced with a thin layer of sand or loam ; the outer moulds, being divided into two parts, are brought up to the work and removed by trucks running upon railways.
The method of moulding for bell-founding and statuary is generally similar in principle to that of a loam moulding, with this difference, that the thickness representing the finished object is made up not of loam but of wax, and in the case of statuary, where the object is to use as little metal as possible, it is usually very thin. A plaster cast divided into sections, taken from the original work, forms the matrix within which the wax is moulded of the proper thickness, the inside core being formed of clay with some metal bars to give support, when the work is large. When the plaster mould is removed, the waxen surface is finished up by the sculptor, and the outer mantle or mould proper is formed by coating it with a porous clay mixture. This when dried is carefully baked or burned in a furnace, and the wax melting at the same time leaves the hollow to receive the metal. It is sometimes necessary to leave holes in the casting to allow of the withdrawal of the surface; these are afterwards stopped with plugs of the same metal. Great care is required in the placing of the ingates and runners so as to allow the mould to be regularly and rapidly filled, and prevent any part of the metal setting or chilling before the proper moment.
Melting. - This may be effected either with or without contact with the fuel. In the former case the metal is charged alternately with coke, and occasionally a little flux, into a cylindrical or slightly conical blast furnace known as a cupola, and accumulates in a hollow or sump at the bottom below the tuyeres or blast pipes, whence it is tapped out from time to time, either directly into the mould or more generally into a ladle, for conveyance to the moulds arranged upon the foundry floor. In the second case the fusion takes place either upon the bed of a reverberatory furnace or in crucibles in air furnaces heated by coke or by gaseous fuel. Of these methods the first or cupola is only fitted for iron founding, the reverberatory furnace is used for bronze and iron, and in special forms for steel ; while crucible melting is most general for brass and bronze small castings, as well as for the finer kinds of steel, or generally for any metal that is likely to be altered by direct contact with the fuel. The description of these appliances belongs more properly to the article FURNACE. The founder's ladle or " shank " is a bucket or cup-shaped vessel of wrought iron lined with a shell of fire-clay, with a lip for pouring, having two projecting handles. One of these is straight and serves as a pivot; the other with a cross bar called a crutch is used as a tipping handle when pouring. When of small size the filled ladle can be carried from the cupola to the work and poured by two men, but when of large size containing several tons of metal they are slung from a crane and tipped by a tangent screw and worm wheel, manipulated by a man standing at a distance. The perfection of ladle arrangement is to be seen in Bessemer's process of steel making, where several tons of melted steel are distributed into a ring of ingot moulds in a circular pit by two or three men in a very few minutes. In Krupp's arrangements for making large steel castings from crucibles an intermediate or equalizing ladle is used. The crucibles, which contain about 70 lb each, are drawn from the furnaces in regular order, and poured in such a manner that an uninterrupted stream of metal is kept up from the ladle to the mould.
Large castings when filled from above are liable to be spongy or unsound in the upper part of the mould, or that last filled. In such cases an extra length is given at the top of the mould, as the unsound portion or dead-bead is afterwards removed. This plan is usually followed in casting bronze guns. Sound and dense castings can be obtained by filling with a vertical side runner, so that the metal enters the mould from below and solidifies under the hydrostatic head represented by the vertical height of the runner. A method employed by Sir Joseph Whitworth, of applying hydraulic pressure to the metal in the mould until it solidifies, has been adopted with great success by the inventor in the prevention of blow holes and similar imperfections in steel ingots.
In "chill casting" a portion of the surface of the whole or a part of the mould is made of cast iron, so that the metal brought in contact with it is rapidly cooled. It is adopted in the production of Palliser's cast iron projectiles for penetrating armour plates, rolls for boiler and other iron plates, and paper glazing, and in America for hardening the treads of railway wheels. The iron where in contact with the chill surface becomes white, of a platy crystalline structure, and intensely hard, while such portions as are cooled in contact with the sand remain finely granular, dark grey, and comparatively soft. (H. B.)