Fig. 31. A Typical Die-casting Mold
The Dies Used
Next to the casting machine, the dies or molds are the most important necessary factor. A general view of the Van Wagner Co.’s die-making department is shown in [Fig. 30]. In order to gain a proper conception of the work required in producing a high-grade die-casting mold, we will follow the different steps which are necessary in making the mold. The first and most important step is the proper planning of the die. Before any work at all can be done, it is necessary to plan the die, i. e., to decide just where the parting lines will come; just what method will be used for ejecting the piece; what alloy will be used; where the casting will be gated; and a hundred and one minor points, all of which have a direct bearing upon the performance of the finished dies. All these decisions have to be made by the diemaker, and in [Fig. 37] he is shown, micrometer in hand, computing the shrinkage allowances that he will make in the dies. This is a very important factor on accurate work as the shrinkage varies from 0.001 to 0.004 inch, according to the alloy and the general shape of the piece.
Fig. 32. Die-casting Mold shown in [Fig. 31], disassembled
Before taking up the actual machining operations of the mold-making as conducted in this factory, it will be well to take a typical die-casting mold and note its general construction. [Fig. 31] shows a typical die-casting mold closed, while [Fig. 32] shows the same mold disassembled on the bench to show its construction. The piece for which the mold has been made is also shown. [Fig. 33] shows a similar die in section. From the three illustrations a good idea of an average die-casting mold can be obtained. Referring to these illustrations, the principal parts of this die are the ejector box A, and the ejector plate B which is operated by the racks C. For operating the ejector plate, the pinion shaft D having a handle suitable for turning, is furnished. This, of course, fits into a bored hole in the ejector box, bringing the pinion into mesh with the racks for raising the ejector plate. In the ejector plate are three ejector pins E for removing the casting from the mold. The ejector pins operate through holes F. Beyond the pinion shaft may be seen the casting for which this mold has been made. It will be noticed that the top side of the casting has three projecting lugs through which are small holes. Provision for forming this side of the die-casting is made in the lower half of the mold G, while the upper half of the die-casting is taken care of by the top plate H. One of the toggles for operating the core pins through these three lugs is shown at I. These parts will be described more fully later. The sprue cutter is shown in position in the die at J.
Machining the Die Cavities
As will be noticed from [Fig. 30], the machinery in the die-making department is of modern design, for no other class of work demands as good tool equipment and as much skill in the making as die-casting molds. The die-blocks are made of machinery steel. [Fig. 34] illustrates the first step in making a die-casting mold after the die-block has been shaped approximately to size. This operation consists in carefully facing off the die surfaces on a vertical-spindle grinding machine. This, of course, is a quick method of surfacing the die-block, and it insures that the top and bottom surfaces of these plates will be parallel, permitting the die-faces to come together properly.
Fig. 33. Section through a Die-casting Mold