Japanese Patent Publication Nos. 48-7570 and 49-36093 disclose die-casting methods in which the molten metal is injected into a die cavity and is squeezed also at a portion of the mold cavity other than the portion through which the molten metal has been injected into the die cavity. In these known methods, however, the squeezing of the injected molten metal is effected only after the solidification of the metal in a restricted part of the injection passage or in the sprue through which the die cavity is communicated with the injection passage or runner. Therefore, it has been impossible to carry out a good die-casting operation with these known methods for the following reasons.
Namely, the restricted part of the injection passage or the sprue in the die-casting machine is liable to be more heated than other portions of the machine because of a high flow velocity of the molten metal through such portions of the machine. Therefore, if the restricted part or the sprue is positioned at the same level as the die cavity, the solidification of the metal in such a restricted part of sprue is delayed from the solidification of the metal in the die cavity. With the die-casting method in which the molten metal is squeezed after the metal is solidified in the sprue, the metal in the thin-walled parts of the die cavity will be solidified before the squeezing is commenced. It is, therefore, impossible to prevent cavities or voids from being formed in such thin-walled parts. If such cavities or voids are once formed, a considerably high pressure is required to squash and fill up the cavities or voids. With the prior art discussed above, therefore, it has been practically difficult to prevent the formation of cavities or voids. Further, since the squeezing is effected after the metal in the die cavity is partially solidified, various defects such as segregation, which would adversely affect the quality of the product, a surface fault like a sheared surface and so forth are inevitably caused.
In the past, it has also been proposed in Japanese Patent Laid-Open Publication No. 51-129817 to squeeze the molten metal immediately after the filling of the die cavity with the molten metal. This method, however, merely squeezes the molten metal without delay after the filling of the die cavity and does not suggest any consideration as to the level of the pressure to be applied and to the amount of molten metal forced by the pressure back into the die cavity (referred to hereunder as "squeezing displacement of metal"). According to the result of the studies made by the present inventors, it is impossible to effectively suppress the formation of cavities or voids and to avoid the fluctuation of quality of the products by simply squeezing the molten metal immediately after the filling of the die cavity with the metal.
Namely, it has been known by the inventors that, in this die-casting method, the squeezing pressure is directly transmitted also to the injection plunger because the squeeze is commenced while the metal in the sprue is still in the molten state. Thus, if the level of the squeezing pressure is considerably higher than the injection pressure, the injection plunger would be forced back by the pressure, so that the molten metal of an amount corresponding to the squeezing displacement of metal is moved back toward the injection side. It is, therefore, impossible to effect a substantial squeezing unless the squeezing displacement of metal is unduely increased. It has also been found that a sufficient squeezing cannot be achieved by a level of pressure as low as the level of the injection pressure imparted by the injection plunger. Thus, in order to surely prevent generation of cavities or voids, it is essential to maintain the squeezing pressure at a proper level.
As to the amount of the squeezing displacement of metal, a displacement which is too small would not be able to satisfactorily squash and fill up the cavities or voids. On the other hand, a squeezing displacement which is too large would require an impractically large size of die-cast machine, although such a large squeezing displacement will be effective to prevent the formation of voids. Moreover, with a displacement too large, the ratio of the amount of molten metal constituting the product to the total amount of the molten metal injected by the injection plunger becomes unacceptably small, which is quite impractical from the view point of industrial utility.