Abstract:
A cold-chamber die-casting apparatus and method for making a die-cast part with an open space within the geometry of the part. The apparatus includes an injection shaft which receives molten material for casting the part. The molten material is pushed with a plunger through a gate and into a tool cavity corresponding to the part. The gate is disposed at an end of the injection shaft and adjacent the tool cavity at a position that corresponds to the open space of the part and is inside the geometry of the part.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to U.S. Provisional Application Ser. No. 60/948,668 filed Jul. 9, 2007, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to die casting and, more specifically, relates to a method and apparatus for cold chamber die casting of relatively thin-walled parts with an open space for receiving components such as display panels or key pads within the part geometry. 
     BACKGROUND OF THE INVENTION 
     Die casting has long been known as a method of forming parts with complex geometries and/or surface ornamentation. Historically, the die casting of aluminum parts was commonplace in the automobile industry and many of the known methods have arisen from the needs of automobile manufacturers. Recently, the need to produce smaller, and more intricate, aluminum parts has arisen in the cell phone and electronics industries because such casings have excellent resistance to wear and work well to insulate internal components from the environment (heat, shocks, wetness, etc.). Aluminum parts also provide a smooth, metallic finish that allows for additional surface treatments, such as electroplating to enhance the quality and aesthetics of the parts. However, current methods of die casting aluminum parts do not adequately and consistently produce good results when being used to form smaller, more intricate parts. 
     Currently, the die casting of aluminum parts involves: pouring molten aluminum from a ladle into an injection shaft, plunging the molten aluminum through an external biscuit, up through a runner into the tool cavity. The tool cavity is located above the injection shaft in order to prevent the gravitational flow of molten aluminum into the tool cavity. 
     If the die casting machine is configured such that the injection shaft is located at the center of the tool cavity, some of the molten aluminum will flow though the force of gravity into the tool cavity prior to plunging the melt into the cavity. The resulting parts would have a poor surface finish and less dense microstructure due to the cooling of the molten aluminum which had leaked into the cavity prior to plunging the rest of the melt. 
     The aforementioned conventional method is shown in  FIG. 3 . The injection shaft is located beneath the tool cavity and the melt is plunged through a biscuit and travels upwards through a runner and then into the tool cavity. This casting method works well for larger parts, but results in a low yield when casting thin-walled parts. For such parts, as the melt travels upwards through the runner and into the tool cavity, it cools and loses both speed and pressure, thus causing flow marks and resulting in incomplete parts and parts with a poor microstructure and surface finish when forming smaller, more intricate parts. Many of these parts will either be scrapped and re-melted or will require secondary processing to make them acceptable. 
     U.S. Pat. No. 7,025,114, incorporated herein by reference in its entirety, also shows a similar method of die casting, but uses a three piece mold in order to obtain two-part mold structures. With reference to FIG. 3 of U.S. Pat. No. 7,025,114, a melt is poured into pouring port  343  which is then pressed upwards through a runner  33  by a plunger  341  before entering into cavity  32  via gates  312 . Similarly to the aforementioned conventional method, the melt cools and loses both speed and pressure as it travels upwards through the runner and into the gates resulting in the same types of defects when casting smaller, more intricate parts. An additional problem with this method is that the melt will be cooler when it enters into the top gates than when it enters into the lower gates as it will have had to travel a greater distance, thus resulting in parts having a non-uniform density and poor microstructure. Therefore, there is a need for a method for die casting thin-walled parts with an open space within the part geometry that will result in a higher yield. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a cold-chamber die casting machine and method that utilize a gate located in an open space inside the part geometry which prevents the gravitational flow of molten material, e.g., aluminum, zinc, or magnesium, into the tool cavity and also serves as an inlet to the tool cavity. In the present die casting method, molten aluminum, or melt, is poured via a ladle into an injection shaft. Then, a plunger located inside the injection shaft presses the melt out of the injection shaft and through a gate which is located inside the tool cavity in an area corresponding to an open space of the part. The gate contains outlets near the top of the gate, which allow the melt to fill the tool cavity and thus create the part. At this point, the mold halves will open, typically by pulling back a movable mold half from a stationary mold half, and the part will be ejected, preferably by ejector pins located in the outside structure of the gate. Excess material on the part will then be broken off, cut or trimmed. 
     By providing outlets only along the upper regions of the gate, the melt is prevented from leaking into the tool cavity prior to pressurizing the melt into the tool cavity via the plunger. The tool, or the mold halves, is preferably designed such that the gate is located in a portion of the tool cavity that corresponds with an empty space or open space of the part. For example, if a hand held device casing is being formed, the gate should be located inside the opening where an LCD, or liquid crystal display, is to be mounted. This is because no aluminum will fill that portion of the tool cavity as it corresponds to an open space of the part. This method of forming parts will result in a higher yield and parts formed thereby will require less secondary processing. Since the melt is pressed into the center of the tool cavity directly forming an internal biscuit, rather than through an external biscuit and long external runner, the fill time decreases and less excess material remains. Thus, the overall cycle time decreases and part production increases. Furthermore, tool life is extended because the melt can be injected into the tool cavity at a lower speed and pressure as it has less distance to travel before filling the tool cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       These and other objects and features of the invention will become more apparent by referring to the drawings, in which: 
         FIG. 1  is a side view of a cold-chamber die casting machine in accordance with the present invention; 
         FIG. 2  is a rear view of the gate surrounded by the aluminum cast part; and 
         FIG. 3  is a side view of a conventional cold-chamber die casting machine. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a side view of the cold-chamber die casting machine, generally at  1 , according to the present invention and  FIG. 2  shows a rear view of the internal biscuit  6 , runner  7 , and gate opening  12  or passage of the present invention connected to a finished part  11 . A molten material  2 , typically aluminum, is poured into an injection shaft via a ladle  3 . Next, a plunger  5  pressurizes the melt or molten material  2  into the tool cavity  8  through at least one gate opening  12  located near an upper portion of the internal biscuit  6 . The plunger  5  begins moving toward the tool cavity  8  first at a low speed, and then, as it gets closer to the area where an internal biscuit will be formed, it accelerates the molten material  2  at a higher speed. This is done to prevent turbulent flow of the molten material  2  which could cause flow marks, bubbles or other defects in the final part. The mold halves  9  and  10  that form the interior tool cavity  8  are clamped together by a clamping force of 85 to 200 tons to ensure that they do not separate from each other during the injection phase. For example, two suppliers of cold-chamber die casting machines, Toyo and Toshiba, produce machines that have a tool clamping force of 125 tons and 135 tones, respectively. Once the tool cavity  8  has been filled with molten material  2 , the molten material  2  will cool and harden and the mold halves  9  and  10  will separate revealing a finished cast part  11 . The part may be ejected through use of ejector pins located in the gate structure, preferably, near the runner  7  and/or excess overflow material  14  to minimize the number of ejector pin burrs left on the part  11 . 
     As can be seen in  FIGS. 1 and 2 , the molten material  2  can only enter the tool cavity  8  through at least one gate opening  12  or passage located adjacent the runner  7  near the top of where the internal biscuit  6  is formed. Thus, prior to the plunger  5  pressurizing the molten material  2  into the tool cavity  8 , no molten material  2  is able to leak into the tool cavity  8  through the force of gravity. 
     The molten material  2  is pressed from the injection shaft  4  into the tool cavity  8  through a gate opening  12  through the use of a plunger  5 . The tip size of the plunger  5  is approximately equal in diameter to and concentric with the internal biscuit  6 . The tip size of the plunger  5  is preferably smaller than those used in conventional machines so that the internal biscuit  6  is able to fit into an open space  13  of the part  11  to be molded. The open space may correspond to an electronic component, such as a display panels or key pad that is included in an electronic device using the finished molded part. 
     Conventional machines that utilize a clamping force between 80 and 200 tons have a plunger tip size of 45 mm or more while the plunger tip size in the present invention is preferably less than 30 mm in diameter and in a preferred embodiment is equal to 25 mm. Currently, the only machines available with a smaller tip size are those that also have a smaller clamping force. According to an embodiment of the invention, when casting smaller, more intricate parts, it is preferable to maintain a relatively high clamping force of 80 to 200 tons between the mold halves, despite decreasing the size of the plunger tip in order to ensure that the cast parts will consistently have a good surface quality. 
     Once the molten material  2  is pressed to the end of the injection shaft where the internal biscuit  6  will be formed after completion of injection, it is pressed upwards through the runner  7 , through at least one gate opening  12 , and then into the tool cavity  8  where the finished part  11  is formed therein. The gate opening  12  may be just a single aperture, a plurality of apertures or may be a tunnel or multiple tunnels from the area where the internal biscuit  6  is formed to the beginning of the part geometry. The size and shape of the gate opening  12  may vary as necessary to control the flow of molten material  2  into the part  11  in order to obtain the greatest yield for a particular part geometry. 
     The flow of the molten material  2  into the tool cavity  8  forming part  11  is shown in  FIG. 2  through a series of flow lines. The molten material  2  exits the area where the internal biscuit  6  is formed, upwards through a runner  7  and gate opening  12 , and then begins filling the tool cavity  8 . In the case of the particular cavity illustrated in  FIG. 2 , the molten material  2 , after flowing upward, will flow outward to the two sidewalls of the tool cavity  8 . Subsequently, it will continue flowing downward along the two sides, and then across the bottom in an inward direction where the two flows will meet. The particular flow for a particular part will depend on the geometry of the part. 
     The mold halves  9  and  10  communicate to form the interior tool cavity  8  that is in the shape of part  11 . It is preferable to provide some excess molten material  2  to ensure that the tool cavity  8  becomes completely filled without gaps and forms a full part  11  having a good microstructure and surface finish. Therefore, an outlet for excess overflow material  14  is provided. The size and location of the outlet for excess material overflow  14  will vary depending upon the part geometry. In a preferred embodiment, the excess material is shown entering into a second aperture  15  of part  11 . After the part  11  has been ejected from the tool cavity  8 , metal corresponding to the internal biscuit  6 , runner  7  and gate opening  12 , as well as any other excess material may be easily broken off or trimmed. 
     Although the preferred form of the invention has been shown and described, many features may be varied, as will readily be apparent to those skilled in this art. Thus, the foregoing description is illustrative and not limiting.