This article was originally published in Practical Engineering 1940 Vol No1. Information within this article is therefore correct as of 1940. The publication of this material aims to provide historical insight on the subject and its place in industry.
The rapid strides made in mass-production methods in the machine shops and other departments of a modern factory have been reflected in the almost complete mechanisation of the larger modern foundry; a change which has also been brought about to a great extent by developments in metallurgy, necessitating precise control over the whole sequence of operations, from the melting of the metal to the knocking out of the finished casting.
Most readers will probably be familiar with the traditional method of moulding from a wooden pattern — two boxes filled with damp sand being used to take the impression of the pattern, with suitable core pieces made from baked sand inserted where a hollow section is required. The moulder was — and is, in a small foundry — essentially a craftsman, trained by years of experience to produce intricate castings from the wooden patterns, making good any imperfections by skilled use of his trowel. He usually mixed his own sand, and was generally called on to cast his own moulds.
The first step in speeding up the process was the substitution of metal patterns for the wooden type, enabling a large number of moulds to be prepared with absolute accuracy, as compared with taking perhaps a dozen moulds from a wooden pattern. Further developments were the mechanical ramming of the moulding box and the automatic mixing of the sand.
The layout of the modern foundry is naturally influenced by the type of casting to be produced, but assuming it is a fairly complex item, such as a motor-car cylinder block, the general scheme is usually to arrange the coresand plant at one end of the shop, with the green-sand plant at the opposite end. The following sequence of events is fairly typical of that in large scale modern foundries. The various types of core sand required for different castings, such as dry sand, sea sand, or red sand, are dried in a rotary drier before use. The sand is generally broken up by vanes in the drier while hot gases from a coke firebox are passed through it. The rate of feed is mechanically controlled according to the moisture content of the sand.
Mixing the Sand
The next step is the accurate mixing of the sand; this is generally effected by overhead conveyors, which discharge the sand from buckets into the sand mixers, of which the Pneulec type is widely used. In this type of mill a positive mixing action is obtained by the action of a stirring mechanism and steel bars bolted to a rotating disc, in addition to the usual runner. In large mills two runners and two sets of stirrers are employed, the entire mechanism being carried on ball and roller bearings, and being driven by a 30 h.p. electric motor.
The core sand is then transferred by overhead conveyors to the core-blowing machines. In the latest types, an impellor in the head of the machine gives a swirling movement to the sand, which, in conjunction with the usual air blast, rules out any risk of a part of the core box being incompletely filled. The machine is controlled by a lever, which is held over for a specific number of seconds, determined by the capacity of the core box. The cores then pass on a roller conveyor through a continuous slow-drying oven. After cooling, the cores are inspected and sprayed, finally passing through a drying oven until all moisture has evaporated.
Preparation of Moulds
Meanwhile, the moulds are being prepared. The green-sand plant is usually designed to prepare a continuous supply of backing and facing sand, according to predetermined requirements. When the sand is wetted, ample time is allowed to ensure that the water permeates it thoroughly. Previously used sand is passed on a belt conveyor through a magnetic separator, to remove any metallic particles from previous castings for which the sand has been employed, and is fed to the green-sand mill. Facing sand also passes through a separator and disintegrator, and the two types of sand are passed to overhead hoppers which serve the moulding stations on the roller track.
Filling the moulding boxes is carried out mechanically by machines which operate in pairs, one ramming the top half of the box, and the other the bottom or "drag" half. A typical machine of the jar, roll-over, pattern-drawn type, is provided with hydro-pneumatic oil control for the roll-over and pattern-drawing movement. Other types of machine may be of the straight-draw type, or may be provided with pneumatic-electric operation.
From the moulding machines the top and bottom halves of the moulds pass to the inspection section before reaching the pouring station. Here the molten metal, which is usually transferred from the cupolas in ladles slung from overhead carriers, is poured, and the mould boxes pass through fume tunnels and are allowed to cool before reaching the knock-out station, in which the moulds are separated and the sand allowed to fall through a grating on to a return belt conveyor, which carries it back to the sand plant. It only remains to transfer the casting to the dressing and fettling sections, where, after being subjected to shot-blasting, they undergo the usual inspection and gauging processes, while hammer and punch tests may be made to detect blow holes in the casting.
While the foregoing is typical of the procedure followed in the case of the average industrial castings, specialised components, such as those designed for internal - combustion - engine cylinder blocks, may require different handling.
Whereas the baked sand core is warm and dry, the water-bound mould sand is damp. The metal coming into contact with the damp sand is chilled, and chilled cast iron is not only extremely hard but is also brittle. The metal in contact with the baked core, on the other hand, retains its normal characteristics. In the case of a cylinder block the result is that the exterior of the casting, including the crankcase walls, is hard and brittle, whereas it is the cylinder bores and valve seatings which require the maximum hardness to resist wear.
Advantages of the "All-core" Method
The "all-core" method is employed in the Morris engine factory in order to overcome the disadvantages of the ordinary method. This involves building up the whole of the mould from baked core pieces, bound together with oil or possibly with molasses. Each core piece, of which there may be over thirty, dovetails into those adjacent to it, and is further located by metal dovetails which slide in grooves in the sides and base of the corebox.
Among the advantages of this method is the fact that each core can be produced accurately in a metal mould. The least inaccuracy will prevent a core from assembling with the remainder. Again, control over chilling of the metal can be obtained by varying the sand mixture in order to produce a greater hardness at particular points. Alternatively, the same sand mixture can be used throughout, as in the Morris foundry, enabling a harder cast iron to be employed, owing to the elimination of the risk of local chilling.
In America, considerable experiment has been devoted to the baked-sand mould method of casting, since in the majority of factories in which cylinder blocks are cast in wet-sand moulds by ordinary pouring under atmospheric pressure, defective castings are often obtained due to movement of the cores, the presence of sand holes, and other causes. In the case of a manufacturer producing V-type cylinder blocks, the high proportion of defective castings, which at times exceeded 40 per-cent., led to the adoption of baked-sand moulds, which are virtually dies in which limits can be maintained to within a few thousandths of an inch. This process also enables the moulds to be poured from large hoppers with the result that head of molten iron produces in effect, a form of pressure casting.