DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] Certain preferred embodiments of the present invention will be described below in greater detail with reference to the accompanying sheets of drawings wherein like or corresponding parts are designated by the same reference characters throughout several views.

[0036] Referring now to FIG. 1, there is shown in perspective a brake disc or rotor of a disc brake formed by injection molding using an MMC material according to one embodiment of the present invention.

[0037] The brake disc 10 has a generally hat-like configuration including a cup-like central portion (hub portion) 11 and an annular disc portion 18 formed integrally with the hub portion 11.

[0038] The hub portion 11 includes a tubular peripheral wall 12 and a circular end wall 13 formed integrally with one end of the peripheral wall 12. The end wall 13 has a central hole or opening 14, a plurality of bolt holes 15 and a plurality of stud holes 16 arranged around the central opening 14 in concentrical that are arranged concentrically at equal angular intervals about the center of the opening 14.

[0039] Though not shown, bolts are inserted in the bolt holes 15 of the hub portion 11 to attach the brake disc 10 to a drive shaft side. Similarly, wheel studs are press-fitted in the stud holes 16 so that a wheel can be bolted to the brake disc 10.

[0040] The disc portion 18 has on its opposite surfaces pad rubbing areas which are adapted to be gripped by brake pads (not shown) of a caliper (not shown) disposed astride the brake disc 10, and so this portion 18 requires a high mechanical strength and an excellent wear resistance. Thus, the disc portion 18 is preferably formed solely of an MMC material. In the illustrated embodiment, the MMC material has an aluminum alloy matrix and silicon carbide particles dispersed in the aluminum alloy matrix as a reinforcing material. The MMC of this type is generally called “dispersion reinforced alloy”. Unlike the disc portion 18, the hub portion 11 does not require such a high mechanical strength and an excellent wear resistance because this portion is merely attached to the drive shaft side and free from abrasive wear caused due to frequent frictional engagement with the brake pads. Thus, at least the end wall 13 of the hub portion 11 is formed solely of an aluminum alloy which is inexpensive as compared to the aluminum-based MMC material. As shown in FIG. 2, the brake disc 10 has a first part E1 formed of aluminum alloy (corresponding to the hub end wall 13) and a second part E2 formed of aluminum-based MMC (corresponding to the disc portion 18). At a boundary between the two parts E1 and E2, the aluminum alloy and the aluminum-based MMC material are united together.

[0041] The brake disc 10 shown in FIGS. 1 and 2 are formed by injection molding as described below.

[0042] As shown in FIG. 3, an injection molding apparatus 20 used for carrying out the injection molding method of the present invention generally comprises a molding die composed of a movable die 21 and a fixed die 23 jointly defining therebetween a die cavity 22, an injection cylinder 26 attached to the fixed die 23, a piston 30 reciprocally movably received in the injection cylinder 26, an aluminum alloy feed unit 34 for supplying a desired quantity of aluminum alloy through an aluminum alloy supply passage or line 35 into the injection cylinder 26, and an MMC feed unit 37 for supplying a desired quantity of MMC material through an MMC supply passage or line 38 into the injection cylinder 26. The aluminum alloy supply line 35 is provided with a first shutoff valve 36 located near an outlet 35a of the supply line 35. Similarly, the MMC supply line is provided with a second shutoff valve 39 located near an outlet 38a of the supply line 38.

[0043] The injection molding apparatus 20 is of the vertical type wherein the movable die 21 is vertically movable relative to the fixed die 23, and the injection cylinder 26 is disposed vertically.

[0044] The fixed die 23 is formed with a gate 24 through which the die cavity 22 and the hollow interior (injection chamber) of the injection cylinder 26 communicate with each other. The term “gate” as used herein encompasses a sprue, a gate, a runner and so forth which form a non-product part of a molded article.

[0045] The piston 30 is slidably received in the injection cylinder 26 and has one end connected to an end of a rod 332 of a cylinder unit 31. When the cylinder unit 31 is operated to extend and contact the rod 32, the piston 30 reciprocates within the injection cylinder 26 as indicated by the profiled arrow shown in FIG. 3 so that an aluminum-based MMC material and an aluminum alloy fed into the injection cylinder 26 can be forced or injected into the die cavity 22 through the gate 24, as described below. The piston 30 is normally disposed in a retracted position which is located remotely from the gate 24, as shown in FIG. 3.

[0046] The aluminum alloy feed unit 34 is constructed to supply a desired quantity of aluminum alloy in a semi-solidified state into the injection cylinder 26 while the shut-off valve 36 is open.

[0047] Similarly, the MMC feed unit 37 is constructed to supply a desired quantity of aluminum-based MMC material (containing silicon carbide particles added or dispersed in an aluminum alloy matrix) in a molten or liquid state into the injection cylinder 26 while the shut-off valve 39 is open.

[0048] Turning now to FIGS. 4 through 6 inclusive, there is shown a sequence of operations achieved on the injection molding apparatus 20 for carrying out the injection molding method of the present invention. In these figures, the shutoff valves 36, 39 are dark-shaped when they are in a closed position.

[0049] As shown in FIG. 4A, the second shut-off valve 39 is closed and the first shut-off valve 36 is open, then a desired quantity of aluminum alloy 40 is supplied in a semi-solidified state from the aluminum alloy feed unit 34 through the aluminum alloy supply line 35 into the injection cylinder 26, as indicated by the arrow shown in this figure. In this instance, the quantity of supplied aluminum alloy is set to be substantially equal to the sum of a cubic capacity of the gate 24 (FIG. 3) and a cubic volume of an end wall of the hub of a brake disc blank (namely, a product part of a molded article).

[0050] Then, as shown in FIG. 4B, the first shut-off valve 36 is closed and the second shut-off valve 39 is opened, and after that a desired quantity of aluminum-based MMC material 42 is supplied from the MMC feed unit 37 through the MMC supply line 38 into the injection cylinder 26. In this instance, the quantity of supplied aluminum-based MMC material is set to be substantially equal to the sum of a cubic volume of a disc portion 18 of the brake disc blank and a cubic volume of a peripheral wall of the hub of the brake disc blank. The supplied aluminum-based MMC material quantity can be also determined by subtracting a cubic volume of the end all of the brake disc blank from the cubic capacity of the die cavity 22.

[0051] Since the aluminum alloy 40 is supplied in a semi-solidified state and the aluminum-based MMC material 42 is supplied in a molten state, the aluminum-based MMC material 42 is placed on the aluminum alloy 40 in a separated condition. Within the injection cylinder 26, the aluminum alloy 40 is situated on a piston side adjacent to the piston 30 and the aluminum-based MMC material 42 is situated on a gate side adjacent to the gate 24 (FIG. 3).

[0052] Subsequently, as shown in FIG. 5A, the second shut-off valve 39 is closed and the rod 32 of the cylinder unit 31 is extended to advance the piston 30 upward toward the fixed die 23 as indicated by the profiled arrow whereupon the semi-solidified aluminum alloy 40 is forced to move upward, thereby forcing the molten aluminum-based MMC material 42 to pass through the gate 24 and then enter the die cavity 22.

[0053] A further upward movement of the piston 30 causes the semi-solidified aluminum alloy 40 to move across the gate 24 and then enter the die cavity 22, whereby the molten aluminum-based MMC material 42 which is already existing inside the die cavity 22 is forced by the aluminum alloy 40 to flow in a radial outward direction to thereby fill in an annular disc portion 22a of the die cavity 22. When the piston 30 comes in contact with a lower surface of the fixed die 23, as shown in FIG. 5B, a hub end wall portion 22b of the die cavity 22 and the gate 24 are filled with the aluminum alloy 40. In this instance, respective positions of the aluminum-based MMC material 42 and aluminum alloy 40 correspond to the positions of the second and first parts E2 and E1 of the brake disc 10 shown in FIG. 2. The aluminum-based MMC material 42 and the aluminum alloy 40 are united together at a boundary therebetween.

[0054] After a pre-set dwell time for solidification, the movable die 21 is moved upward away from the fixed die 23 to open the die cavity 22 and a molded article 45 is removed from the die cavity 22, as shown in FIG. 6. Subsequently, a non-product part (gate portion) 47 of the molded article 45 is cut-off or removed from a product part (cavity portion) 46 of the molded article 45. The product part 46 is subsequently finished into a brake disc 10 shown in FIGS. 1 and 2 through appropriate working processes.

[0055] The non-product part 47 is formed solely of the aluminum alloy and can readily be recycled by being melted down in the aluminum alloy feed unit 34. With this recycling of the aluminum alloy, the manufacturing cost of the brake disc 10 can be reduced.

[0056] Turning now to FIG. 7, there is shown a brake disc 50 formed by injection molding using an aluminum-based MMC material according to another embodiment of the present invention. The brake disc 50 has a hat-like configuration including a generally cup-like hub portion 51 and an annular disc portion 54 formed integrally with each other. Unlike the brake disc 10 of the first embodiment shown in FIGS. 1 and 2, the brake disc 50 as a whole is formed solely of an MMC material.

[0057] To form the brake disc 50, the injection molding apparatus 20 shown in FIG. 3 is also used.

[0058] Firstly, a semi-solidified aluminum alloy 40 and a molten aluminum-based MMC material 42 are supplied respectively from the aluminum alloy feed unit 34 and the MMC feed unit 37 into the injection cylinder 20 in the same manner as done in the first embodiment described previously with reference to FIGS. 4A and 4B. In this instance, the quantity of supplied aluminum alloy 40 is set to be slightly greater than the cubic capacity of the gate 24, and the quantity of supplied aluminum-based MMC material 42 is set to be substantially smaller than the cubic capacity of the die cavity 22. Then, the first and second shut-off valves 36, 39 are closed.

[0059] Subsequently, the piston 30 is moved upward until it arrives at a lower surface of the fixed die 23, as shown in FIG. 8. With this upward movement, the piston 30 forces the molten aluminum-based MMC material 42 into the die cavity 22 via the semi-solidified aluminum alloy 40. In this instance, because the quantity of aluminum alloy 40 supplied in the injection cylinder 26 is slightly greater than the cubic capacity of the gate 24, a small part of the aluminum alloy 40 moves into the die cavity 22 and then united with the aluminum-based MMC material 42 at a boundary between the aluminum-based MMC material 42 and the aluminum alloy 40.

[0060] After a pre-set dwell time for solidification, the movable die 21 is moved upward away from the fixed die 23 to open the die cavity 22 and a molded article 55 (FIG. 9) is removed from the die cavity 22. The molded article 55 includes a product part (cavity portion) and a non-product part (gate portion) formed integrally with each other. The non-product part 57 is subsequently cut off or removed from the product part 56, as shown in FIG. 9. The product part 56 is subjected to subsequent working processes through which the product part 56 is finished into a brake disc 50 shown in FIG. 7. The non-product part 57 removed from the product part 56 is formed solely of aluminum alloy, and so this part can readily be recycled by melting it down in the aluminum alloy feed unit 34 shown in FIG. 8. With this recycling of the aluminum alloy, the manufacturing cost of the disc brake is reduced.

[0061] In the embodiments described above, the aluminum-based MMC material 42 is comprised of an aluminum alloy matrix and silicon carbide particles added in an dispersed condition to the aluminum alloy matrix as a reinforcing material. The present invention is not limited to processing of the MMC of this class and is equally applicable to other types of MMCs including a metal matrix and a reinforcing material added to the metal matrix. The term “metal matrix” as used herein encompasses metals, metal alloys and other metallic materials which can be converted in a molten or liquid state and processed in an injection molding system. Similarly, the reinforcing material may include ceramics, such as silicon carbide, aluminum oxide, aluminum nitride, magnesium oxide, silicon oxide, zirconium oxide, titanium carbide, tungsten carbide or combinations thereof. The reinforcing material may be added in the form of particles, fibers or frames. The aluminum alloy 40 may be replaced with another metal, an alloy of other metallic materials which can be processed in a semi-solidified state in the injection molding apparatus.

[0062] The molded article of the present invention should by no means be limited to brake discs as described in the illustrated embodiments but may include other articles which can be formed by injection molding. However, it is to be noted that the injection molding method of the present invention is particularly advantageous when used for forming a molded article having two parts adjoining with each other wherein one part is required to have a high mechanical strength than the other part. In such application, quantities of MMC material 42 and metal 42 to be supplied in the injection cylinder 26 are appropriately changed according to volumes of the first part (requiring a high mechanical strength) and second part (not requiring such a high mechanical strength) of a molded article to be formed.

[0063] It may be appreciated that according to the injection molding method of the present invention, a metallic material in a semi-solidified state and an MMC material in a molten state are supplied into an injection cylinder in the order named, so that these two different molding materials are separated into two layers within the injection cylinder. Thus, by merely advancing a piston toward a die cavity in a molding die, the molten MMC material is forced into the die cavity via the semi-solidified metallic material. When the quantity of supplied MMC material is set to be slightly smaller than the quantity of supplied metallic material, at least a gate of the molding die is filled with the metallic material. A gate portion (non-product part) of a molded article is cut off or removed from a cavity portion (product part). However, since the gate portion is formed solely of metallic material, it can readily be recycled by being melted down again in a feed unit of the injection molding apparatus. With this recycling of the gate portion, the overall cost of the molded article can be reduced.

[0064] In addition, a molded article, such as a brake disc, having two adjoining parts one of which is required to have a higher mechanical strength than the other part can readily be formed by appropriately changing the proportions of the MMC material and metallic material with respect the capacity of the die cavity. Due to a limited use of the MMC material which is expensive as compared to the metallic material, the molded article can be produced at a relatively low cost while maintaining a desired degree of mechanical strength which is achieved by the MMC material.

[0065] Obviously, various changes and modifications of th4e present invention are possible in the light of the above teaching. It is therefore to be noted that the invention may be practiced otherwise than as specifically described.