MOULDING AND CASTING

§ 57. The valuable property of moderate fusibility, which many metals possess, enables articles of various kinds to be produced from a pattern, by impressing upon sand or other mobile material a copy of the pattern, and subsequently pouring molten metal into the cavity thus obtained. The solidified metal will be an exact counterpart of the impression in the sand.

The term "moulding" is used to signify the various operations concerned in preparing the impression to receive the metal, and the term "casting" is applied to the whole operation of producing the object, and includes both the moulding and running in the metal. The principal materials used in moulding are:-sand of various kinds, loam, plaster of Paris, blackening, pea-flour, etc.

Sand is by far the most common, and certainly the most perfect and convenient moulding material. The properties which make it so valuable are its porosity, adhesiveness, mobility, practical infusibility, and unalterability. By means of its porous nature the gases generated during pouring of the metal can freely escape; by its adhesive property a perfect impression is produced from any given pattern; the metal is firmly retained when run into the mould; by its mobility it gives way sufficiently, and flows into the finest markings of the pattern when pressure is applied; and in virtue of its non-fusibility and chemical unalterability, the heat of the molten metal does not fuse it, or change its chemical composition. The best kinds of moulding sand employed for casting brass have been found to have an almost uniform chemical composition, varying only in size of grain or aggregate form. It contains

Sand containing other metallic oxides, such as lime and

L

magnesia, is too weak or too close-that is to say, it will not retain its form, or it will cause the metal to boil by its closeness, the gases not having a free means of escape. When the oxide of iron is greater than the above proportions the sand is liable to fuse, and unite with and blister the surface of the casting, generate gases, and cause blowholes in the metal. Different kinds of castings require different kinds of sand. One class necessitates the use of a porous and yet adhesive sand; in another the sand must be very fine, free from grit, and very adhesive, so as to conform to the finest parts of the pattern.

The best moulding sand is often found along the banks of large rivers, in the vicinity of granite or slate mountains; or in coal districts, where the river flats are largely composed of sand. In such localities it sometimes contains too much oxide of iron, and is liable to melt; but this may be modified by mixing it with coke-dust. Kampmann states that a good sand for moulds may be artificially made from the following mixture:

Fine quartzose sand

Red English ochre

93

326

Aluminous earth, the least possible calcareous 5

A valuable casting sand is obtained from the new red sandstone at Birmingham. The value of the quarry of this sand at the Old Cemetery was estimated at £20,000 sterling.

Core-sand. This sand should be coarse, porous, and very adhesive, such as rock-sand, the fine material from abraded rocks; free-sand from river-banks, or from the sea-shore, and pounded blast-furnace cinder, etc., are often mixed with fine, strong sand, and a little clay to make it adhesive. In each case fresh sand must be used for a core, as old sand, burnt sand, or sand mixed with coal is not advisable. One part of clay mixed with nine parts of free-sand is sufficiently strong for small and simple cores, but for large and complicated ones a stronger sand is required.

Parting-sand.-A substance which does not retain moisture is required for this purpose. Red brick-dust is preferred, but free-sand, sea and river sand, and blast-furnace cinder are also used.

Facing-sand.-If molten metal is allowed to come in immediate contact with some kinds of fresh sand, a surface fusion of the sand takes place, with consequent roughness of the casting. If the raw sand is too coarse the metal will penetrate the sand to some extent, and also produce a rough casting. To avoid this defect the sand is coated with carbon or carbonaceous matter in the form of fine dust. Carbon and coal dust do not adhere well to old sand, so that in such a case the mould is first dusted with pea-flour and then with charcoal. Charcoal powder is also mixed with one-tenth of its volume of fine sand, and used as a facing for small castings.

The success in casting is not only dependent on the manipulative skill of the founder, but also on that of the pattern-maker. If a wood-pattern is required, pine, mahogany, oak, and other kinds of wood are sometimes used. In the brass trade boxwood is most commonly employed. The pattern is made larger than the size of the required casting, about inch per foot being allowed for shrinkage and finishing. Patterns with their edges at right angles do not leave the sand without disturbing the impression, so that they are made to taper in the parts which enter the sand. Sharp angles in a pattern should be avoided as much as possible, as they leave an edge of sand which is liable to break off on removing the pattern, and produce a defect in the casting. Such a defect in the mould is remedied by mending the broken places with sand before pouring. Wood patterns should either be varnished or brushed with black-lead to prevent absorption of moisture, and enable them to part from the sand more freely. Bees-wax or plaster of Paris is used for stopping up holes and cracks in the wood. It often happens, when a permanent pattern is required, that a metal pattern is cast from a wooden one,

and the former then used as a pattern from which castings are obtained. In such a case the wood pattern should be made much larger than the finished articles, to be subsequently produced, are required. This is to allow for the double shrinkage, and for the dressing of the cast work. Sometimes patterns must have pegs of wire attached to enable the caster to lift them easily out of the sand. Besides wood and metal, casting patterns are often made of clay, plaster of Paris, or wax.

Moulding-sand is kept in position by means of shallow

iron frames, open at top and bottom, and called flasks. These are of various sizes, each side of a frame having a depth of about 3 inches. A flask consists of two

parts, as represented in Fig. 25, where A is the upper and B the lower frame; or they may be termed the peg-side and the eye-side respectively. In addition to the two parts shown in the figure, a third frame is often used, termed the odd-side, which is subsequently described.

§ 58. To make the odd-side.-The sand is first tempered with water, and passed through a rough sieve with about five meshes to the linear inch. It should be noted here that black or old sand does not hold together so well as raw sand, the black sand containing burnt flour, charcoal, and brick-dust; and probably the sharp edges of the particles of sand are partly worn off by frequent use, and cannot be wedged so firmly together. If black sand only be used the castings are liable to be blown, the material not being sufficiently porous to allow the air and other gases to escape.

The moulding is commenced by placing one of the frames, often termed the she-side, on the top of a flat board, then dusting the inside with parting sand, then adding some raw

sand, and filling up with black sand.

The sand is rammed hands, then with the

down tightly with the palms of the knuckles, and finished with a mallet. The surface is next scraped level with a straight-edged piece of wood, a board placed on the top, and the whole frame with its contents inverted.

The patterns are now carefully laid on the mould and the dust bag shook over them; this will leave a clear outline of the patterns upon the sand, which is then cut away in order to let them in half-way. The "odd-side," which is always the "peg-side," is now fixed to the lower frame, parting sand added, and then a mixture of raw sand and black sand, and finally black sand to complete the mould as before. The facing-sand, which is sometimes mixed with black sand, half and half, must be pounded in a mortar and passed through the meshes of a fine sieve. The odd-side must be rammed down as tightly as possible, as upon the compactness of the sand the future success of the operations depends. A board is then placed on the top and the "flask" turned over. The board which now forms the top is hammered to loosen the patterns, and the "she-side" taken off and broken up.

The "odd-side" contains the patterns, and is now ready to mould from. A frame is fixed to the "odd-side," sand pressed in as before, then covered with a board, and the whole inverted. The patterns are then loosened by hammering the top board, and the odd-side, which now contains perfect impressions of half of each of the patterns, is removed, leaving the patterns in the she-side. A frame containing pegs is now fixed to the she-side, sand added and pressed as before. This peg-side is then removed and placed aside, leaving the patterns in the "she-side." The "odd-side " is then placed on the "she-side," the flask again inverted, the patterns loosened with the hammer, and the she-side removed, leaving the patterns in the "odd-side" ready to start again. It will thus be seen that the "odd-side" is used over and over again to make other moulds from, a group of five to six moulds constituting a heat. About six