Chester L. Lucas

“ The greatest advantage of die-casting is the fact that the castings produced are completely and accurately finished when taken from the dies. When we say completely, we mean that absolutely no machining is required after the piece has been cast, as it is ready to slip into its place in the machine or device of which it is to be a part. When we say accurately, we mean that each piece will come from the die an exact counterpart of the last one ”

“ The possibilities of die-casting are numerous. By this method of manufacturing it is possible and practical to cast pieces that could not possibly be machined. It is an every-day occurrence to make castings with inserted parts of another metal, as, for instance, a zinc wheel with a steel hub. It is also possible to make babbitt bearings that are harder and better than can be made in any other way. Often there are two or more parts of a device that have formerly been made separately, machined and assembled, that can be die-cast as one piece. In such cases the saving in production is very great. ”

“ Taken as a whole, from ten to sixty pieces per hour are the maximum limits for speed in die-casting, and with a well-working die, of simple construction, a speed of forty pieces per hour is considered good production. It is possible, however, when the castings to be produced are small in size and simple in shape, to gate a number of them together, or rather to construct the dies so that six or more castings may be made at once. By this means it is often possible to cast five or six thousand pieces per day of ten hours, on a hand die-casting machine. ”

“ Generally speaking, casting-dies are made of machine steel; the parts which are exceptions are the heavy bases and frames, which are made of cast iron, and the dowel pins and small cores, usually made of tool steel. Except in rare instances, there are no hardened parts about a casting-die; this is the case because the melting points of some of the alloys that are die-cast are high enough to draw the temper from any hardened parts of the dies.
The ideal die is simple in construction, with as few parts as practicable ”

“ From a metallurgical standpoint a die-casting is superior to a sand-casting on account of its density, strength and freedom from blow-holes. Also, when the hot metal comes in contact with the cool dies, it forms a “skin” similar to the scale on an iron sand-casting. As the die-casting requires no machining after leaving the dies, this skin increases the wearing qualities of the casting. ”

“ The speed at which die-castings may be produced varies with the size of the castings being made, the composition of the metal being cast, and the style of dies that must be employed. In many cases, in die-casting, separate brass or steel pieces are used, that must be placed in the dies before each operation so that they will be inserted in and become a part of the finished casting. The dies may be difficult of operation on account of draft problems or pins and screws that must be inserted in the dies and removed from each casting before another can be made. ”

“ We have now taken up the description and general operation of the die-casting machine, but like every other machine, there are numerous little kinds and practices in its working the observing of which makes the difference between good and poor die-casting. ”

“ The fitting of the parts of a die-casting mold is one of the most important parts of the work. It demands the highest type of workmanship, for a poorly fitted die means a die which works hard in addition to producing poor castings. It is very important that all movable parts should work freely. Fig. 42 shows the assembling operation on a die-casting mold, the casting which is to be duplicated being shown in the immediate foreground. ”

“ One of the first experiments in the direction of die-casting was undertaken to get out some rubber mold parts cheaply enough to leave a profit on a job that was beginning to look dubious from the financial side. ”

“ For this latter reason a part that necessarily must be made of brass, iron or steel, cannot be die-cast, although mixtures approximately equal in strength to iron and brass are readily die-cast. To give added strength to a die-cast part it is often advisable to add webs and ribs or to insert brass or iron pins at points that are weak or subject to hard wear. Roughly speaking, it is the part that has required a good deal of accurate machine operations that shows the greatest difference in cost when die-cast, and sometimes the saving is as great as 80 per cent. ”

“ Generally speaking, a part should not be considered for die-casting if there are but few pieces required, because the cost of the dies would usually be prohibitive. Often, however, it happens that because of the large amount of accurate machine work being done on a machine part, it is economical to make a die for the comparatively small number of parts required and die-cast them. ”

“ Aluminum alloys have been cast in France and Germany in limited quantities, but very seldom in this country on account of their high melting point, as well as their effect upon the die. After aluminum alloys have been run in the dies for a short time, the surfaces of the molds become pitted. Through some unexplained cause, the metal seems to flake out particles of the steel in the molds. When an aluminum alloy is to be used, a good mixture is 80 per cent aluminum, 3 per cent copper and 17 per cent zinc. ”

“ For making die-castings that are to have pieces of another metal inserted, it is necessary to have a die with provisions for receiving the metal blank and holding it firmly in position while the metal is being cast around it, and of course the piece must be held in such a manner that it can be easily withdrawn from the die with the finished casting. ”

“ The metal for the die-casting machine is mixed in the proper proportions for the work in hand by means of a separate furnace, before being poured into the tank of the machine itself. The burners are lighted and the dies are set up on the two die-plates. As soon as the machine has “warmed up,” so that the metal is in a thoroughly melted condition, the sprue-cutting lever T is thrown back, leaving a clear passageway to the die cavities. ”

“ In some machines pressure is applied to the metal by hand, in others power is used, and in still another class the metal is forced into the dies with compressed air. The provisions for opening and closing the dies vary in the different machines; there are various means employed for cutting the sprue, and the styles of heaters are numerous.
One or two of the largest firms in the die-casting industry have automatic casting machines for turning out duplicate work in large quantities very rapidly. ”

“ Half-round bearing dies are usually made to cast two bearings at a time, for the reason that it is just as easy to cast two pieces of such a shape as it is to cast one, and, in addition, the die is balanced in a better manner. ”

“ In view of the advances that have been made in die-casting, it is singular that there are to-day only5 about a dozen concerns in the business in this country, but as the subject becomes better understood, and the possibilities of the process are realized, the demand for this class of castings will result in many other firms going into the work, and it is not improbable that a large number of factories will install die-casting plants of their own to aid them in producing better work in a more economical way. ”

“ One of the great advantages of the use of die-castings is that no finishing is required after the pieces leave the molds. Finish requirements should be plainly stated in ordering die-castings, as the alloy must be suited to these requirements. ”

“ While it requires three men to operate a die-casting machine in the best manner, the man who attends to the compression lever has a good deal of spare time between strokes, and if two or even three of the machines are conveniently placed, one man can easily pull levers for all three. ”

“ A rough part that would require little or no machining should not be die-cast, because pound for pound, the die-casting metals cost more than cast iron or steel. The casting machine cannot make parts as rapidly or of as hard metals as the punch press or the automatic screw machine. ”

“ So many different styles and sizes of bearings are made by a concern doing much die-casting that it is essential that the die-cast bearings should bear some distinguishing number to identify them. As this number is of no consequence to the user it is well to have the number in an inconspicuous place, but it must be where it will not be effaced by scraping, etc. Bearing in mind that it is much easier to produce raised lettering by die-casting than to produce sunken lettering, it will be readily seen26 that the oil groove affords a good place in which to put the bearing number. ”

“ There are two methods of preventing air from being trapped in die-casting molds; either by constructing the dies so that the air may be46 exhausted from the mold cavity before admitting the metal, or by venting the die so that the air may be forced out by the inrushing metal. In the first of these methods it is necessary that the joints in the mold be made as close as possible, otherwise it will be impossible to produce anything like a vacuum in the mold cavity. ”

“ The hot metal, of course, “shoots” through them in thin ribbons, but not enough escapes to affect the pressure on the metal which goes into the casting.
Fig. 41. Fitting Ejector-pins
No matter how carefully a die may have been constructed, or how carefully it has been assembled, there is always a certain amount of “babying” to be done before it will work satisfactorily. The casting may stick a little here, or there may be a rough spot there, and it is the successful elimination of these troubles which constitutes the production of a good die-casting. ”

“ It is doubtful if die-casting, properly speaking, was originated until about fifteen years ago, and it is certain that it is only during the past few years that the activities in this line have been very noticeable. ”

“ It does not pay to cast rough heavy work that can be made just as efficiently by sand casting. Generally speaking, the greatest saving can be effected by die-casting small pieces which have previously required a large amount of machining to produce. On large plain work the amount of metal required for the casting makes the cost excessive on account of the difference in cost of the metals. If, however, the large work must be finely finished by polishing, etc., it is oftentimes found of advantage to die-cast. Corners, especially those joining thick and thin sections, as at B, Fig. ”

“ The outside corners and edges of the faces of both die-halves should be well rounded off so as to insure the absence of slight dents or rough places that might prevent the dies from fitting perfectly. ”

“ The metals that produce the best die-castings are alloys of lead, tin, zinc, antimony, aluminum and copper, and the bulk of the die-castings made at the present time are mixtures of the first four of these metals. ”

“ It now remains to describe the toggles used for operating the cores which form the holes through the three lugs in the casting. One of these toggles, of which there are three, is shown at I, in Fig. 31, and also in Fig. 32.45 These toggles consist of brackets which are attached to the die-plate, and levers which are fulcrumed at the ends of the brackets so that their operation works the core pins. It is necessary to remove these core pins after each casting has been made and position them before another casting can be produced. ”

“ The metal should be kept just above the melting point and at a uniform temperature. If the metal is worked too cold, the result will manifest itself in castings that are full of seams and creases, and it will be difficult to “fill” the thin places in the dies. ”

“ The pressure is regulated to suit the particular casting or die, the proper amount being determined by experiment. Similarly, an air exhaust pipe F, which may be seen directly above the supply pipe, sub-divides into two tubes which extend34 to the die cavity to exhaust the air before the metal is admitted. There are two methods of overcoming the presence of air in the die cavity—the exhaust method and the venting method, and it is the former that is here described. ”

“ If, however, it has many parts which must be fitted, it is usually considered advisable to provide the die with vents consisting of milled recesses a few thousandths inch deep. Several vents are provided, from which the air can escape when the metal is admitted to the dies. ”

“ The first essential to good die-casting is a good casting machine. Perhaps the best known types of casting machines are of the familiar33 plunger type, of which there are several varieties, the pneumatic type and the rotary or automatic type. ”

“ The die-half with the slides thus clamped in the inner or closed position, is set up on the lathe faceplate and the die-cavity indicated up and bored out to the finish size, which operation also finishes the ends of the slides to the proper radius. The die may now be taken down and the slides removed to engrave the letters upon their concave ends. The engraving can be done in the best manner on a Gorton engraving machine, but if such a machine is not available they may be cut in by hand. ”