Venting the Dies
There are two methods of preventing air from being trapped in die-casting molds; either by constructing the dies so that the air may be 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. 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 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.