subject: Vertical Injection Machine Using Three Chambers [print this page] A vertical injection machine having a first chamber for supplying liquid metal, a second chamber located lower than the first chamber and a first conduit connecting the first chamber to the second chamber. The first conduit has a shutter rod disposed within and a valve head attached to a lower portion of the shutter rod.
The valve head is adapted to seal a lower portion of the first conduit. A metering rod is disposed within the second chamber. The vertical injection machine also includes a vertically oriented third chamber and a second conduit connecting the second chamber to the third chamber. An injection rod is disposed vertically within the third chamber and a injection nozzle is located at an upper portion of the third chamber.
Conventional die casting apparatus are classified into cold chamber and hot chamber. In cold chamber die casting apparatus, molten metal is poured into a sleeve which is secured on a die plate and connected to an inlet opening to the mold cavity. Molten metal is injected by a plunger into the die. The molten metal in the sleeve is easily cooled down when it spreads at the bottom of the sleeve as the plunger moves forward slowly to discharge air or gas. Cooled molten metal in the sleeve forms a chilled fraction and semi-solid or solid particles. The chilled fraction and particles are injected into the molding die causing the physical properties of molded parts to be deteriorated.
Cooled molten metal increases the viscosity of the molten metal and makes it difficult to fill the mold cavity. Further, it causes blemishes on surface of a molded part. This is a serious problem particularly for magnesium alloys for which the latent heat of solidification is small (smaller than aluminum, lead and zinc). Because of the small latent heat of solidification, magnesium solidifies quickly when it comes in contact with materials having a lower temperature.
Hot sleeves have been used, but the heated sleeve is not as hot as liquidus temperature of the metal because the sleeve is connected to a molding die whose temperature has to be below the solidus temperature of the metal. The molding die temperature must be sufficiently below the solidus temperature of the molten metal to produce an adequate solidification rate. That is, a solidification rate which reflects the required time for an operation cycle. Molten metal poured into the sleeve has a substantially higher temperature than the liquidus temperature of the metal to counter the cooling in the sleeve. This is a disadvantage in energy cost for heating.
The cold chamber apparatus forms a thick round plate as a part of the casting, often called a biscuit, in the sleeve between a plunger head and an inlet of a die. After the casting is pulled away from the molding dies when the dies are opened, the biscuit is cut away from the casting and recycled. However, sometimes the biscuit is larger than the product. This is a disadvantageous use of metal which has a substantial recycling cost.
In hot chamber die casting apparatus, an injection mechanism is submerged in molten metal in a furnace. The temperature of the molten metal to be injected is maintained above its liquidus. The injection mechanism has a shot cylinder with a plunger, gooseneck chamber and a nozzle at the end of thereof. The molten metal is injected through a gooseneck-type passage and through a nozzle into the die cavity without forming a biscuit. This is an advantage of hot chamber die casting apparatus.
Another advantage of a hot chamber apparatus over a cold chamber apparatus is the time for an operation cycle. As mentioned above, in cold chamber apparatus, the casting is formed by injecting molten metal into a mold cavity between closed dies and cooling to until the casting is solid. The dies are separated and the molded part is pulled away, lubricant is sprayed onto the opened dies, and the dies are closed again. Then, the dies are ready to start the next operation cycle. The molten metal is poured into the injection sleeve when the molding dies are closed, i.e., when the dies are ready to start the next operation cycle, so that the molten metal does not spill out from the inlet opening of the die because the injection sleeve directly communicates with a die.
On the other hand, hot chamber die casting apparatus fill molten metal in the gooseneck and a shot cylinder system by returning an injection plunger to its fill up position. Molten metal is supplied through an opening or fill port on a shot cylinder. While cooling the injected molten metal in the dies, the nozzle is positioned by inclining the gooseneck chamber. The molten metal in the nozzle gooseneck system tends to flow back into the furnace through the fill port on the shot sleeve, reaching a hydrostatic level when the dies are opened. By simultaneously filling molten metal into the gooseneck and a shot cylinder system and cooling injected metal in the closed dies, time for an operation cycle of the hot chamber apparatus is shortened compared with the cold chamber die casting apparatus.
However, solidification of the molten metal in the nozzle section of the gooseneck and dripping of molten metal from the nozzle and the cast sprue are problems for hot chamber die casting apparatus. It is known that in hot chamber die casting apparatus a vacuum is created in the injection mechanism when the plunger is withdrawn. However, the vacuum is instantaneously destroyed once the plunger passes the opening or fill port on the shot cylinder supplying molten metal from the furnace because the furnace is at atmospheric pressure. Thus, the molten metal is sucked into the shot cylinder, and the gooseneck and the nozzle are completely filled at the time that the casting is solidified and the dies are separated.
There is molten metal in the nozzle for most of the time that the casting is cooling. When the cooling at the tip of the nozzle is properly controlled, it is understood in the industry that the metal in the nozzle tip becomes semi-solid. The formed semi-solid metal works as a plug which prevents molten metal from dripping out of the nozzle when the dies are separated. If the cooling is insufficient, the metal in the tip of the nozzle and the cast sprue is still liquid when the dies are separated and dripping occurs. On the other hand, when too much cooling is applied, the metal in the nozzle tip solidifies and freezes together with the cast sprue. The casting will stick in the stationary die after the dies open.
Problems in the hot chamber die casting apparatus are caused because a heavy injection mechanism is submerged in the molten metal in the furnace. The injection mechanism with a gooseneck chamber and a shot cylinder system is difficult to clean up. It is also difficult to replace worn plunger rings and sleeves. A worn plunger ring and sleeve decreases injection pressure due to leakage and makes shot volume inconsistent in filling the mold cavity. The inconsistent shot volume produces inconsistent molded parts.
Die casting apparatus are also classified according to the arrangement of the injection system, that is, horizontal and vertical. In a horizontal die casting apparatus, an injection system is horizontally arranged for horizontally injecting molten metal into molding dies. A vertical die casting apparatus has a vertically arranged injection system for vertical injection of molten metal.
One embodiment of the present invention includes a vertical injection machine comprising a first chamber for supplying liquid metal, a second chamber located lower than the first chamber, a first conduit connecting the first chamber to the second chamber, the first conduit having a shutter rod disposed therein, a valve head attached to a lower portion of the shutter rod, the valve head adapted to seal a lower portion of the first conduit, a metering rod disposed within the second chamber, a vertically oriented third chamber, a second conduit connecting the second chamber to the third chamber, an injection rod disposed vertically within the third chamber and an injection nozzle located at an upper portion of the third chamber.
Another embodiment of the present invention includes a vertical injection machine comprising a vertical injection machine comprising a first chamber containing a heating means for melting metal into a liquid state in a first chamber, a second chamber, a passing means for passing a portion of the liquid metal from the first chamber into the second chamber and a sealing means for sealing the passing means. The machine also comprises a vertically oriented third chamber, a metering means for drawing in liquid metal from the first chamber into the second chamber and for forcing the portion of metal from the second chamber to the third chamber, an injection means for sealing an opening between the third chamber and the second chamber and for injecting liquid metal from the third chamber into a mold, and a lifting means for lifting the vertical injection machine toward the mold prior to injecting the liquid metal into the mold and for lowering the vertical injection molding machine away from the mold after injecting the liquid metal into the mold.
