1.  Field of the Invention
The present invention relates to a method for injection molding metallic materials to obtain a product of desired configuration by supplying a metallic material contained in an injection cylinder into a mold cavity. 
2.  Description of the Related Art
A typical example of known disk brakes for motor vehicles is shown in cross section in FIG.  32 hereof.  As shown in FIG.  32, a disk rotor 503 of the example disk brake 502 is mounted to a drive shaft 500 via a hub 501.  A peripheral portion of the disk rotor 503 is placed in a caliper 506. 
A wheel 509 is braked by transmitting an oil pressure from a feed flow path 508 to a cylinder, not shown, of the caliper 506 and pressing brake pads 507, 507 onto a disk portion 505 of the disk rotor 503. 
Since the brake pads 507, 507 are pressed hard onto the disk portion 505 of the disk rotor 503, the disk rotor 503 is required to be formed of a high-strength material. 
On the other hand, in order to reduce the total weight of a motor vehicle, the disk rotor 503 is preferably manufactured of a lightweight material. 
Metal matrix composite material (MMC) materials are well known for their high strength and lightweight.  For example, employing aluminum (Al) alloy as a metal matrix can save the weight, and adding silicon carbide (SiC) to Al alloy matrix can realize increase in strength. 
A method for manufacturing the disk rotor 503 from an Al alloy matrix composite material will now be described with reference to FIG.  34 hereof. 
Such a method is carried out in a conventional apparatus for injection molding a metal matrix composite material.  An injection cylinder 515 is brought into communication with a cavity 512 defined by a movable die 510 and a stationary die 511 via a gate 513.  A plunger 516 is mounted to the injection cylinder 515 so as to be capable of upward and downward movements, an MMC feeding means 518 is connected to the injection cylinder 515 via a feed path 517, and a shut-off valve 519 is mounted to an exit side of the feed path 517. 
A shut-off valve 519 is opened and an AL alloy matrix composite material is fed from the MMC feeding means 518 into the injection cylinder 515 as shown by arrow a. The plunger 516 is moved upward as shown by arrow b, and Al alloy matrix composite material is filled into the cavity 512 through the gate 513.  Then, the movable die 510 is moved upward as shown by the arrow c to open the die and the cast product is taken out of the die.  The cast product taken out will be described below. 
Reference is made next to FIG.  34 showing a cast product taken out from the die.  The cast product 520 is cut into a product portion 521 and a non-product portion 522. 
The product portion 521 is a member formed of Al alloy matrix composite material molded in the cavity 512 and is to be processed to obtain the disk rotor 503 as shown in FIG.  32. 
The non-product portion 522 is a member formed of Al alloy matrix composite material remained at the gate 513 (See FIG.  33). 
The non-product portion 522 remained at the gate 513 is also of Al alloy matrix composite material obtained by adding SiC grains to Al alloy matrix.  Therefore, since Al alloy matrix composite material cannot be reused as it is, it is necessary to separate SiC grains from Al alloy matrix in order to reuse it.  However, such separation is technically difficult, and if possible, it costs much.  Therefore, the non-product portion 522 is put on a shelf in the existing circumstances and this contributes to increase in the cost of the product portion 521 molded by Al alloy matrix composite material (or a metallic material). 
On the other hand, there are products that do not require high strength among injection molded articles.  Since such products are not required to contain SiC grains for increasing strength, they may be formed of normal aluminum alloy material (or a metallic material) by injection molding.  Therefore, there is a tendency that they are considered to be easily reusable because it is not necessary to separate SiC grains as in the case of Al alloy matrix composite material when reusing the non-product remained at the gate. 
However, in order to reuse the non-product portion as a molten material in subsequent injection molding, it is necessary to melt the non-product portion, which requires much heat energy for melting the non-product portion.  Therefore, in the existing circumstances, the non-product portion is shelved, thereby contributing to increase in cost of the product molded of Al alloy material (metallic material). 
FIG.  35 shows a conventional injection molding apparatus.  The injection molding apparatus 600 includes an injection apparatus 601 opposed to a die 602.  A molten metallic material 605 is poured into a cavity 604 through a gate 603.  When the molten material 605 is solidified in the cavity 604, a disk rotor 608 for the disk brake is obtained. 
FIG.  36 shows an example disk rotor including casting defects.  The disk rotor 608 is a defective apparatus having a defect that is appeared on an opposite side of the gate 603 (See FIG.  35) as a boundary of imperfect integrity at the joint due to lowering of the temperature of the molten material, that is, cold shuts 606 and scabs 607.  In other words, referring to FIG.  35, when the molten material 605 is injected into the cavity 604 through the gate 603 upwardly at the lower portion and then the molten material 605 collides against the upper portion of the cavity 604, the cold shuts 606 and scabs 607 are generated. 
Referring now to FIG.  37A to FIG.  37C, a conventional injection apparatus will be described. 
In FIG.  37A, a molten metal matrix composite material 703 is fed from the molten material feeding apparatus 704 into the injection cylinder 702 of the injecting apparatus 701. 
In FIG.  37B, the injection cylinder 702 is connected to the forming die 705.  The molten material 703 is injected into the cavity of the forming die 705 by the plunger 706 moving up and down in the injection cylinder 702, as shown by the arrow. 
In FIG.  37C, the plunger 706 moves to the upper limit and injecting operation terminates.  After that, when the injected molten material 703 is solidified in the forming die 705, and the cast product 707 is completed. 
However, slugs 708 may enter into the cast product 707, which is a defect of the cast product.  The slug 708 is a residue 709 generated by the oxide of the molten material 703 of metal matrix composite material, and is generated on the surface layer portion 711 of the molten material 703 and may adhere to the inner wall surface 712 of the injection cylinder 702.  The adhered residue 709 causes a defect on the cast product 707 by being mixed in the molten material 703.  When the defect is developed, the cast product has to be disposed, which leads to lowering of manufacturing efficiency.