1. Field of the Invention
The present invention relates in general to techniques associated with a pre-plasticizing injection apparatus in which a process of plasticizing a resin material and a process of ejecting or injecting the resin material is separately performed, more precisely, techniques relating to a screw type pre-plasticizing injection apparatus including a heat-plasticizing cylinder having a screw, and a plurality of heating and injecting cylinders each having a plunger, and an injection molding method using the pre-plasticizing injection apparatus.
2. Discussing of the Related Art
As one type of the pre-plasticizing injection apparatus, there is known an apparatus of the screw type which includes a single heat-plasticizing cylinder having a screw and a single heating and injecting cylinder having a plunger, as disclosed in JP-A-48-18351 and JP-U-2-121913. Such a screw type pre-plasticizing injection apparatus has been widely used because of its excellent capacity of plasticizing a resin material and its excellent uniformity of quality of the plasticized resin material.
In such a pre-plasticizing injection apparatus, as disclosed in the above mentioned documents, the resin material is supplied to the heat-plasticizing cylinder from one axial end portion of the heat-plasticizing cylinder. The resin material is transferred to the other axial end portion of the heat-plasticizing cylinder, as the screw is rotated about its axis of rotation, while the resin material is heated to be molten or plasticized. The plasticized resin material is ejected from the above indicated other axial end portion of the heat-plasticizing cylinder and introduced into the heating and injecting cylinder through one of its axial ends. As the resin material is delivered into the heating and injecting cylinder, the plunger of the heating and injecting cylinder is moved from its fully advanced position toward its fully retracted portion, whereby the heating and injecting cylinder is filled with the resin material. Thus, the resin material is stored in the heating and injecting cylinder. Then, the plunger of the heating and injecting cylinder is moved toward the fully advanced portion for injecting the resin material through an injection nozzle into a mold cavity of a mold which is clamped by a suitable clamping device.
However, the conventional pre-plasticizing injection apparatus requires a relatively long cycle time of injection molding, since the resin material is injected into the mold cavity after the resin material is plasticized in the heat-plasticizing cylinder and is then fed into and stored in the heating and injecting cylinder. In particular, when the resin material is injected from the heating and injecting cylinder into the mold cavity, ejection of the resin material from the heat-plasticized cylinder is necessarily interrupted, resulting in difficulty of improving injection molding efficiency of the injection apparatus.
Moreover, in the conventional pre-plasticizing injection apparatus, the resin material is fed into the heating and injecting cylinder through a passage formed in its end portion. In this arrangement, a mass of the resin material which is initially introduced into the heating and injecting cylinder is finally ejected from the heating and injecting cylinder, leading to unfavorable staying of the initially introduced mass in the heating and injecting cylinder, resulting in deterioration of the quality of an article produced by injection molding. In this respect, it is noted that an amount of the resin material stored in the heating and injecting cylinder is determined to be larger than that of the resin material required for filling the mold cavity by a predetermined surplus amount, for preventing possible insufficient supply of the resin material to the mold cavity. Accordingly, the surplus amount of the resin material tends to remain in the heating and injecting cylinder after the completion of the injecting operation of the resin material. The quality of the resin material remaining in the heating and injecting cylinder is significantly deteriorated.