This invention relates in general to die-casting machines and in particular to high-speed die-casting machines for the production of precision miniature castings where the weight of the produced casting generally does not exceed two ounces and the face of the die generally does not exceed approximately ten square inches. While these size limitations provide a suitable focus, many of the features of the present invention are applicable to much larger machines.
Heretofore, machines for the die casting of precision miniature pieces have been generally of the Dynacast type, a die-casting machine developed in the mid-1960's and the subject of several United States patents, including U.S. Pat. No. 3,364,981. This machine, at the time of its development, drastically changed the manufacture of miniature die-casting pieces which prior to the time of the U.S. Pat. No. 3,364,981 were manufactured in large conventional die-casting machines using multiple-cavity dies. However, the state of the art has remained practically unchanged during the last 20 years. At the expiration of the above-mentioned patent, the machine has been duplicated by several manufacturers with more or less the same features but without eliminating the various shortcomings.
One shortcoming of the Dynacast machine is the fact that the machine can use only two vertical cores and only occasionally a frontal core. The zinc injection system of the Dynacast machine is made by a gooseneck partially submerged into the liquid zinc of the melting pot. The gooseneck has a swiveling movement in order to bring the nozzle in contact with the die face and to move the nozzle away from the die face to break the runner from the liquid metal. Besides the mechanical elements needed for this movement (which adds complexity to the machine), this system reduces the speed at which the machine can operate. The position of the gooseneck precludes the use of a fourth core.
Another shortcoming of the Dynacast machine includes the way the level of the zinc in the neck of the gooseneck is controlled. This system leaves a certain amount of air ahead of the zinc in the neck of the gooseneck, which amount of air is variable and depends upon several parameters which are not easily controlled, such as the temperature of dies, size of the shots, speed of the machine and temperature of the nozzle. Another drawback of the Dynacast machine is the way the toggle mechanism operates in order to close the die halves and to create the force between them. This force generated between the dies varies, depending on the wear of the mechanical components of the toggles, temperature of the dies, temperature of the plate of the machine, and so forth.
A further drawback of the Dynacast machine is the fact that it cannot produce parts with what is known in the industry as a "hardware" finish; in other words, parts that do not need to be plated. The reason for such inability involves the way the zinc is injected into the die, the gating (which has to be small because of the subsequent separation from the part with the tumbling operation) and the system of heat transfer on the die. The parts from the Dynacast machine are ejected with the gates attached to them, and this requires a subsequent operation (tumbling) in order to separate the parts from the gate, with all of the negative attributes of such operation including surface pitting, dimensional alteration, and breaking of sharp corners.
The present invention provides a machine which is designed to overcome the above shortcomings and disadavantages. In order to achieve this object, the disclosed machine produces castings which may require up to four cores along the plane of the dies (substantially two vertical and two horizontal) and two cores in a plane perpendicular to the surface of the die with a total of six cores. The machine of the present invention produces castings with much less porosity than any other machine used today and with surface finish of the type as previously mentioned which is known in the industry as a hardware finish. The cast pieces can be trimmed automatically in the machine so that the pieces are separated from the gate, the gates being discharged in a separate container for a subsequent remelt.
Another reference of possible interest to the heat transfer aspects of the present invention is U.S. Pat. No. 4,248,289 which issued to Perella et al. This patent discloses a means to heat the heat-exchange medium, water, by electric immersion heaters to preheat the die aove the normal boiling point of water by controlling the internal pressure of the die-cooling cavity. Die cooling is accomplished by the heat of evaporation at the temperature and pressure required for best casting quality. The wall thickness of the die cavity to water cavity is sized according to the heat-transfer rate or the die-cavity material. As steam is produced within the cavity, make-up water is introduced to maintain a wetted surface.
In die-casting practice, the "coring" of a cast part encompasses many forms. The core may be in the form of an entry hole, round, square or otherwise, a hollow space, a countersunk or extended boss, an embossed face, or any shape that is required that is not formed by the two-part mold cavity. Some core requirements are parallel to the line of separation of the mold halves but are independent in movement. Other cores not in the line of mold separation movement must be positioned and supported either by devices integral with the die blocks or as a function of the die-casting machine. Those arrangements which are integral with the molding dies add substantially to the cost of the dies and are not practical for miniature casting work. Since the hydraulic pressure area of some cores can be equal to that of the die cavity, it is necessary to have equal support to that of the die cavity to lock the core in a fixed position. This may be accomplished by toggle movement to rapidly position the core in the die and then lock the position as securely as the main die-closing action.