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.
Improved technique, and the fact that alloys have a low melting point and remarkable physical properties coupled with stability, is leading to the increased use of die-castings. In many instances they are even being used to supplant parts fabricated from pressings with great success, as not only is assembly time reduced or completely cut out, but a much cleaner and neater product possessing great strength is the result. It is hard to find a branch, at all events in the lighter types of engineering, in which die-castings are not or could not be employed with success. This is borne out by the fact that, to quote even a few instances, such castings are now used in large numbers in the automobile, motor-cycle, electrical, small mechanical, gramophone and wireless, clock, instrument making, typewriter and calculating machine, and toy industries.
Although die-castings have only come into use to any great extent generally within recent times, the origin of the process dates back a considerable number of years. As a matter of fact, it is close on one hundred years since printer's type was first cast by machine on practically the same lines that are in use for general die-casting at the present moment. Doubtless, at that time, the need for such a revision of methods was partly brought about by the natural desire for increased production, but in all probability the change was necessitated by the difficulty or impossibility of making sound and clear-cut small type by ordinary gravity-casting methods.
Die-casting is essentially a process suited to the production of certain types of castings in quantities. In ordinary foundry practice, a pattern is used to form a cavity in sand into which the molten metal is poured, an individual mould being prepared for each casting required. On account of technical difficulties it is improbable that this method will ever be superseded for metals that have a high melting point.
Sand-castings, except for very rough work, must be finished, at all events so far as important dimensions are concerned, by machining, and the preparation of unmachined surfaces to a condition suitable to receive a deposit which afterwards has to be polished, is a costly business.
By the adoption of the die-casting process, finishing operations are almost entirely eliminated, but whereas for sand-casting the preparation of a suitable pattern is in most cases an expensive item, the provision of a suitable die is a considerable one and therefore cannot be considered as a paying proposition unless the cost can be spread over a large number of castings. In comparing the cost of the castings, it has to be remembered that a die-casting represents a finished machined part except perhaps for, say,a tapping operation, so that where intricate machining operations have to be carried out on a part, the high cost of the work so involved might make a die more than worth while on a relatively small quantity.
Although within the space of a single article little more than a brief outline of the method can be given, sufficient can be said to convey to the reader the general principles of what is a really interesting process.
Whilst aluminium and certain types of bronze can be die-cast, by far the greater proportion of die-castings are made in zinc-base alloy. This metal casts well and with a good finish. Earlier die-castings generally were confined to lightly stressed or purely ornamental parts that could be made from lead or tin-base alloys. This was on account of structural changes taking place in the then used zinc-base metals with ageing, resulting in serious changes in the formation of the castings or in disintegration.
Comparatively. recent developments have led to the introduction of zinc-base alloys in which these serious defects have been overcome, and further, the mechanical and physical properties are such that r castings made from them can satisfactorily replace those formerly made of iron or brass for certain purposes. These facts have probably had much to do with the popularisation of the process.
Machines for the purpose of die-casting follow more or less the same lines as regards the method used for filling the dies. It will be seen that this consists of a gas-heated metal pot, arranged within which, in such a manner that it is almost completely submerged in molten metal, is a cylinder having a bottom outlet terminating in a "gooseneck." The cylinder is fitted with a piston or plunger, and when this is at the top of its stroke a port is uncovered through which the molten metal flows into the cylinder and gooseneck.
The means vary on different machines for bringing the dies up to the nozzle, but when the dies are in position the plunger is depressed, either by a hand lever or air pressure, to force the molten metal into the die cavity. As the dies are comparatively "cool, the solidification of the metal therein is almost instantaneous, so that the plunger can be raised again at once when the surplus metal in the nozzle "drops back into the gooseneck. The die is then withdrawn from the nozzle, opened, and the casting ejected. With modern hand-operated or semi-automatic machines, the labour involved in performing the sequence of. operations has been reduced to a minimum, and the rate at which castings are produced must be seen to be believed.
As may be imagined, the most interesting part of die-casting lies in the dies themselves, for seldom are two jobs alike, and from a die-maker's point of view, each has to be dealt with in strict accordance with its peculiarities.
A die for producing a part full of cored holes, such as a carburetter body, must of necessity be a complicated piece of work, because for holes that are other than at right-angles to the parting line, core-drawing mechanism has to be incorporated. The principle on which the dies are made and operated will be best explained by taking simpler forms of castings as an example. Perhaps it would be as well to explain that the dies are made in halves so that the casting can be removed, and it is known as the parting line where the faces of the half dies meet.
The die in which this particular part is made is shown separated at the right-hand side. When together, the halves are kept in alignment by the locating pins protruding from the fixed half of the die. In this half is sunk an impression of the top portion of the casting. The raised part on the opposite half corresponds in reverse to the inside of the casting as seen in the top left-hand corner, the blind holes in the boss centres being formed by slightly tapered pins fixed in the die. On the left-hand side of the dies is seen the runner, which" matches up with the nozzle of the machine and through which the metal is admitted to the die-cavity. Leading from the impression in the fixed die, and seen at the right-hand side, is another shallow channel a few thousandths of an inch in depth. This is a vent through which the air in the cavity of the die is forced out by the incoming metal.
Removing the Finished Casting
The lay-out of the die must be carried out in such a manner that the casting comes away on the movable half when the dies are opened. Usually, on account of shrinkage, or to the presence of cores, the casting is firmly attached to this half, and some means of pushing the casting off is required. It will be noticed that there are three holes, arranged triangular fashion, drilled through the face of the movable die. Two smaller holes are also in the die adjacent to the small core-pins. The purpose of these holes is to receive the pins set in the ejector plate seen on the left-hand side of the fixed die. This device functions as follows: when the dies are opened, the travel of the movable die is continued until the plate reaches a stop, which pushes it forward. The small pins do the actual work of ejecting. The purpose of the larger pins is merely to return the small pins to a position flush with the surface of the die when ready for casting.
As previously mentioned, this die presents no real difficulties on account of the parting line of the casting being absolutely straight. It may be remarked that for the castings to be made with an absence of fin or "flash" at the parting line the opposing faces of the die must fit perfectly and, therefore, flat surfaces are most easily produced with accuracy. The port-like openings in the sides of the castings call for careful work in the making of the die. These are provided by making the projecting portion on the face of the movable die to fit perfectly, the sides of the opposite cavity in such a manner, of course, as not to prevent the die, faces from bedding, the sides of the plug, as it were, being milled away in places to leave the bars between. When ejected from the die the castings appear as those shown in the top left-hand, that is, with the runner included. The runner channel or gate in the die is cut fairly deep up to within about Jin. of the cavity where the depth is reduced to approximately 1/32in. Thus, the unwanted metal is joined to the casting by a very thin section only and is, therefore, easily broken off.
Although still a simple proposition as a die-casting, the one shown in the bottom right-hand corner presents more difficulties when the dies are considered. This will be apparent when the contour of the parting line of the dies at the left-hand side is examined. These dies are made to cast two pieces at a time, the cavities being gated together. The ejector plate and pins are in the position that they occupy in the moving half of the die after a casting has been ejected. This view clearly shows how the plate is returned by the four large pins as the dies are closed.
While it may be said that the examples chosen are not strictly accurate jobs, the products from the dies are absolutely uniform as regards dimensions. The surface finish of the casting depends upon the manner in which the die cavities are made free from scratches, and afterwards polished, plus the matter of keeping the molten metal at the correct temperature.