This invention relates to a process and an apparatus for manufacturing a cast product and, more particularly, to a cast forging process and apparatus capable of manufacturing a machine component to which a high-strength is required, such as a scroll of a scroll compressor and a VTR drum without any fear of defect such as shrinkage cavities.
FIGS. 7a and 7b are schematic sectional views illustrating a conventional indirect insertion-type squeeze casting apparatus disclosed in "Illustrated Dictionary of Casting, Second Edition" P. 121, edited by Japanese Association of Casting, FIG. 7a illustrating a state before casting and FIG. 7b illustrating a state immediately after casting. In these figures, reference numeral 1 is a top die, 2 is a bottom die, 2a is a molten metal reservoir disposed in the bottom die 2, 2b and 2c are a plurality of gates and cavities (a single cavity may be used), respectively, disposed between the top and the bottom dies 1 and 2, 3 is an extrusion plunger, 4 is a die-clamping ram attached to the top die 1, 5 is a knock-out punch, 6 is a molten metal and 7 is a solidified casing.
The operation will now be described. The top die 1 is firmly urged against and fastened to the bottom die 2 by the die-clamping ram 4. The molten metal 6 is supplied to the molten metal reservoir 2a formed in the bottom die 2 immediately before the casting. In this state, the injection plunger 3 is moved downward to pressurize the molten metal 6. The injection plunger 3 continues to pressurize the molten metal 6 at a high pressure equal to or more than 500 atmospheric pressure until the molten metal 6 solidifies and casting is completed. Then, the injection plunger 3 is moved rearward (upward) and the top die 1 is moved upward by the die-clamping ram 4 to open the die and the cast product 7 can now been taken out from the die by moving the knock-out punch 5 upward. The gate sections 2b are removed by a separate pressing step to obtain a cast product corresponding to the cavity portion 2c.
FIGS. 8a and 8b illustrate a scroll member for use in a scroll compressor having a hollow boss portion, FIG. 8a being a front view and FIG. 8b being a sectional view taken along B--B line in FIG. 8a. In these figures, reference numeral 40 is a scroll member for a scroll compressor, which has a scroll-shaped spiral teeth portion 40a on one side of a base plate portion 40b and a boss portion 40c axis-symmetrically formed on the other side of the base plate portion 40b. This is a component which requires a high precision of the order of .mu.m.
FIG. 9 illustrates a schematic sectional view of an indirect insertion cast forging apparatus for casting the scroll member for the scroll compressor illustrated in FIGS. 8a and 8b based on the disclosure of "Illustrated Dictionary of Casting, Second Edition" P. 121, edited by Japanese Association of Casting, the left half illustrating the state before casting and the right half illustrating after casting. In the figure, reference numeral 1c is a gate disposed in the top die 1 and 1d is a cavity.
The operation will now be described. The top die 1 is firmly urged against and fastened to the bottom die 2 by the die-clamping ram 4. The molten metal 6 is supplied to the molten metal reservoir 2a formed in the bottom die 2 immediately before the casting. In this state, the injection plunger 3 is moved upward to pressurize the molten metal 6. The molten metal 6 is introduced by the injection plunger 3 to the cavity 1d through the gate 1c. The injection plunger 3 continues to pressurize the molten metal 6 at a high pressure equal to or more than 500 atmospheric pressure until the molten metal 6 solidifies and casting is completed. Then, the die-clamping ram 4 is moved-upward to open the die 1, and the injection plunger 3 is moved upward and a connected cast product in the cavity 1d and the molten metal reservoir 2a can now been taken out from the die. The gate sections 1c are removed by a separate step of sawing to obtain a cast product 40 corresponding to the cavity portion 1d.
As is well known, the cast forging process is a process in which a molten metal is injected into a die cavity at a low speed capable of maintaining a laminar flow to prevent the generation of cast cavities due to the trapped gas and in which a high pressure about 500 atmospheric pressure or more is kept being applied to the molten metal until it is solidified to prevent the generation of shrinkage cavities due to the volume shrinkage upon the solidification of the molten metal, the process being known as a process for obtaining a defect-free, quality cast product. However, while it is necessary to keep applying a high pressure until each corner of the molten metal in the die solidify in order to prevent the generation of the shrinkage cavities, in the case of the indirect insertion cast forging process illustrated in FIGS. 7a and 7b, the molten metal 6 flows through the gate 2b to reach the cavity 2c and solidifies therein, so that it is necessary that the molten metal within the gate 2b does not solidify and stays in a molten state until the molten metal within the cavity 2c solidifies in order that the high pressure applied to the molten metal 6 within the molten metal reservoir 2a from the injection plunger 3 keeps being transmitted to the molten metal within the cavity 2c. Also, the molten metal within a recessed portion at the center of the cavity 2c in the illustrated example is required to solidify at a later time than the molten metal surrounding it. That is, in order to prevent the generation of the shrinkage cavity, it is necessary that a directional solidification in which the solidification is achieved progressively from the outer portion of the cavity 2c toward the molten metal reservoir 2a can be achieved. In order to achieve this, it is necessary that the gate 2b has large dimensions, which leads to problems of requiring a machine cutting of the gate 2b to obtain the cast product corresponding to the cavity 2c after the casting 7 is taken out from the die because the press cutting provides a very poor surface conditions and dimensional accuracy of the cut surface. The second problem is the limited configuration of the product, in which the dimension of the thin-wall portion for example is limited because the molten metal which becomes a product in the cavity 2c must be solidified from the outer portion toward the inner portion.
Further, in order to prevent generation of shrink cavities in the scroll member of the scroll compressor illustrated in FIGS. 8a and 8b, it is necessary that the directional solidification in which the solidification of the molten metal progresses from the outer portion of the cavity 1d toward the molten metal reservoir 2a can be achieved as in the apparatus illustrated in FIG. 9. In order to achieve this, firstly, it is necessary that the gate 1c has large dimensions, which leads to problems of requiring a machine cutting of the gate 1c to obtain the cast product corresponding to the cavity 1d after the casting is taken out from the die because the press cutting provides a very poor surface conditions and dimensional accuracy of the cut surface. Secondly, with this configuration of the product, the shrink cavities in the base plate portion near the gate cannot completely be eliminated even when the gate thickness is made similar to the thickness of the base plate portion.