Abstract:
A method of producing a die-cast article by die-casting, the die-cast article having at least one of an inner and an outer circumferential surface, the method comprising the steps of: preparing a mold assembly including a hollow portion having a molding surface for molding one of the inner and outer circumferential surfaces of the die-cast article; closing the mold assembly to define therein a mold cavity having a configuration corresponding to that of the die-cast article; subjecting the hollow portion to an elastic deformation in a direction toward the one of the inner and outer circumferential surfaces of the die-cast article to be produced; introducing a molten metal into the mold cavity while the hollow portion is subjected to the elastic deformation; opening the mold assembly and permitting the elastic deformation to be removed from the hollow portion after the molten metal has solidified; and removing the die-cast article from the mold assembly. An apparatus for practicing the method is also disclosed.

Description:
[0001]    This application is based on Japanese Patent Application No. 2001-083085 filed Mar. 22, 2001, the contents of which are incorporated hereinto by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates in general to a method of producing a die-cast article having at least one of an inner and an outer circumferential surface which does not have an inclination corresponding to a draft provided on a casting mold, or which is provided with a relatively small angle of inclination. The present invention is also concerned with a die-casting apparatus suitable for practicing the method.  
           [0004]    2. Discussion of the Related Art  
           [0005]    One example of a method of producing a die-cast article having at least one of an inner and an outer circumferential surface comprises steps of: positioning a slide core within a mold cavity formed in a casting mold, so that an outer circumferential surface of the slide core functions as a part of a molding surface partially defining the mold cavity; and introducing a molten metal into the mold cavity, so that the inner circumferential surface of the die-cast article is formed by the outer circumferential surface of the slide core. The slide core is retracted from the mold cavity after the molten metal has solidified to give the intended die-cast article, so that the slide core is removed from the die-cast article. Since the molten metal shrinks upon its solidification, the slide core is subjected to a holding force caused by shrinkage due to the solidification of the molten metal, so that the slide core cannot be easily removed from the die-cast article. For easy removal of the slide core from the die-cast article, the slide core is generally provided with a draft, so that the die-cast article produced by using the slide core has an inclined inner circumferential surface corresponding to the draft of the slide core. In general, the die-cast article which is produced by using the slide core desirably has a straight inner circumferential surface having a constant diameter. Accordingly, the inclined inner circumferential surface of the die-cast article is subjected, after the die-casting process, to a machining operation to provide the straight inner circumferential surface. For minimizing the required amount of the stock removal by the machining operation, it is desirable to minimize the angle of the draft of the slide core. To this end, EP 0642855 A (corresponding to JP-A-7-60399) discloses a casting mold having a slide core which is formed of a material whose thermal expansion coefficient is equal to or higher than that of a molten metal to be introduced into the mold cavity. According to this arrangement, the slide core has been heated to a temperature substantially equal to that of the molten metal before the metal molten solidifies into a die-cast article. Thereafter, the temperature of the slide core is lowered with a decrease of the temperature of the die-cast article. Since the thermal expansion coefficient of the slide core is equal to or higher than that of the molten metal, the slide core is subjected to shrinkage whose amount is equal to or larger than that of shrinkage of the molten metal. Therefore, the slide core is easily removed from the die-cast article. In the proposed method, however, the material of the slide core is inevitably limited to that which is suitable for a casting mold and which has a thermal expansion coefficient equal to or higher than the molten metal for forming the die-cast article. In addition, the proposed method is not available for eliminating or reducing an angle of inclination to be provided on an outer circumferential surface of the die-cast article.  
         SUMMARY OF THE INVENTION  
         [0006]    It is therefore an object of the present invention to provide a method of die-casting which permits reduction in the angle of inclination provided on an inner or an outer circumferential surface of a die-cast article, and a die-casting apparatus suitably used for the method. This object may be achieved according to any one of the following modes of the present invention, each of which is numbered like the appended claims and depends from the other mode or modes, where appropriate, to indicate and clarify possible combinations of elements or technical features of the present invention, for easier understanding of the invention. It is to be understood that the present invention is not limited to the technical features or any combinations thereof which will be described for illustrative purpose only. It is to be further understood that a plurality of elements or features included in any one of the following modes of the invention are not necessarily provided all together, and that the invention may be embodied without some of the elements or features described with respect to the same mode.  
           [0007]    (1) A method of producing a die-cast article by die-casting, the die-cast article having at least one of an inner circumferential surface and an outer circumferential surface, the method comprising the steps of: preparing a mold assembly including a hollow portion which has a molding surface for molding one of the inner and outer circumferential surfaces of the die-cast article; closing the mold assembly so as to define therein a mold cavity having a configuration which corresponds to that of the die-cast article; subjecting the hollow portion to an elastic deformation in a direction toward the one of the inner and outer circumferential surfaces of the die-cast article to be produced; introducing a molten metal into the mold cavity while the hollow portion is subjected to the elastic deformation; opening the mold assembly and permitting the elastic deformation to be removed from the hollow portion after the molten metal has solidified in the mold cavity; and removing the die-cast article formed in the mold cavity from the mold assembly.  
           [0008]    If the molten metal is introduced into the mold cavity and solidified therein with the hollow portion of the mold assembly being elastically deformed in the direction toward one of the inner and outer circumferential surfaces of the die-cast article to be produced, the die-cast article and the hollow portion engage each other with an interference fit therebetween. Thereafter, the hollow portion is freed or released from the elastic deformation and is restored to its original state, that is, deformed in a direction opposite to the above-indicated direction. Described in detail, where the outer circumferential surface of the hollow portion of the mold assembly serves as the molding surface, the hollow portion which has been expanded by the elastic deformation contracts. Where the inner circumferential surface of the hollow portion serves as the molding surface, the hollow portion which has been contracted by the elastic deformation expands. Accordingly, the die-cast article and the hollow portion, which have been held in an interference fit with each other, are positioned relative to each other such that there is a clearance therebetween, so that the hollow portion and the die-cast article can be easily removed away from each other. According to the present method, it is possible to minimize or eliminate a draft provided on the molding surface of the hollow portion, so that the inner or outer circumferential surface of the die-cast article has a minimum inclination corresponding to the minimized draft of the molding surface, or no inclination.  
           [0009]    (2) A method according to the above mode (1), wherein the hollow portion has a non-molding surface which is opposite to the molding surface, the non-molding surface being a tapered surface, a dimension of which in a direction perpendicular to a direction parallel to a centerline of the hollow portion gradually changes in the direction, the step of subjecting a hollow portion to an elastic deformation comprising steps of preparing a tapered member having a tapered surface which corresponds to the tapered surface of the hollow portion, and causing the elastic deformation of the hollow portion by an interference fit between the tapered surface of the hollow portion and the tapered surface of the tapered member.  
           [0010]    According to the above mode (2), the hollow portion can be easily elastically deformed when the hollow portion and the tapered member engage each other with an interference fit between the respective tapered surfaces of the hollow portion and the tapered member.  
           [0011]    (3) A method according to the above mode (1) or (2), wherein the hollow portion is a hollow cylindrical member having a circular shape in transverse cross section.  
           [0012]    The transverse cross sectional shape of the hollow portion is not particularly limited. While the principle of the present invention is advantageously applicable to an arrangement wherein the hollow portion has an axsymmetric shape in transverse cross section such as a regular polygonal shape, the present invention provides a particularly desirable effect where the hollow portion has a circular shape in transverse cross section.  
           [0013]    (4) A method according to any one of the above modes (1)-(3), wherein the die-cast article is a cylinder block which is used for a swash plate type compressor and which includes a plurality of cylinder bores, the at least one of the inner and outer circumferential surfaces of the die-cast article being an inner circumferential surface of each of the cylinder bores.  
           [0014]    The present method is suitably used for producing a cylinder block for the swash plate type compressor. Where a plurality of cylinder bores are formed in the cylinder block such that the cylinder bores are adjacent to each other, the roundness of each cylinder bore may be deteriorated when the hollow portion has a complete circular shape in transverse cross section. In this case, the hollow portion is preferably arranged to have a transverse cross sectional shape which slightly deviates from a complete circular shape at least while the hollow portion is subjected to the elastic deformation, so that each cylinder bore to be formed in the cylinder block as the die-cast article has a complete circular shape in transverse cross section.  
           [0015]    (5) A method according to any one of the above modes (1)-(3), wherein the die-cast article is a front housing which is used for a swash plate type compressor and which includes a hollow cylindrical recess, the at least one of the inner and outer circumferential surfaces of the die-cast article being an inner circumferential surface of the hollow cylindrical recess of the front housing.  
           [0016]    The hollow cylindrical recess of the front housing for the swash plate type compressor has an inner circumferential surface which is slidable relative to a rotation preventing part of a piston of the compressor, for preventing a rotary motion of the piston about its centerline. Accordingly, it is required that the inner circumferential surface of the hollow cylindrical recess of the front housing has a high degree of dimensional accuracy. According to the present method, it is possible to reduce the required amount of stock removal from the inner circumferential surface of the hollow cylindrical recess of the front housing in the machining operation conducted thereon. Alternatively, the machining operation can be eliminated.  
           [0017]    (6) A die-casting apparatus for producing a die-cast article having at least one of an inner circumferential surface and an outer circumferential surface, comprising: a mold assembly including a hollow portion which has a molding surface for molding one of the inner and outer circumferential surfaces of the die-cast article; and deforming device for elastically deforming the hollow portion such that the hollow portion is subjected to an elastic deformation in a direction toward the one of inner and outer circumferential surfaces of the die-cast article to be produced.  
           [0018]    The method of producing a die-cast article according to the above mode (1) can be practiced by using the die-casting apparatus according to the above mode (6).  
           [0019]    (7) An apparatus according to the above mode (6), wherein the mold assembly includes a first mold and a second mold which are moved toward and away from each other, so that the first mold and the second mold are opened and closed, the hollow portion extending in a direction parallel to a direction in which the first mold and the second mold are opened and closed.  
           [0020]    The hollow portion may extend in a direction which intersects the direction in which the first mold and the second are opened and closed. (The direction in which the first and second molds are opened and closed is hereinafter referred to as a “parting direction” of the two molds.) The present arrangement wherein the hollow portion extends in the direction parallel to the parting direction of the two molds permits the hollow portion to be easily subjected to the elastic deformation, or permits the hollow portion to be easily removed from the die-cast article after the hollow portion has been freed from the elastic deformation.  
           [0021]    (8) An apparatus according to the above mode (6) or (7), wherein the mold assembly has a main body in which an engaging portion is formed, the hollow portion being provided by a member separate from the main body, and wherein at least a part of the hollow portion, which part is adjacent to the molding surface, and the engaging portion are positioned relative to each other such that there is a clearance in a radial direction therebetween.  
           [0022]    The engaging portion has an engaging hole and an engaging protrusion. Where the molding surface is an outer circumferential surface of the hollow portion, the engaging portion is provided by the engaging hole. Where the molding surface is an inner circumferential surface of the hollow portion, the engaging portion is provided by the engaging protrusion. In either case, if at least a part of the hollow portion, which part is adjacent to the molding surface of the hollow portion, is positioned relative to the engaging portion such that there is a clearance in a radial direction therebetween, the above-indicated part can be easily subjected to the elastic deformation, so that the hollow portion including the molding surface can be subjected to substantially uniform elastic deformation over an entire axial length thereof.  
           [0023]    (9) An apparatus according to any one of the above modes (6)-(8), wherein the molding surface of the hollow portion for molding the one of the inner and outer circumferential surfaces of the die-cast article is an outer circumferential surface of the hollow portion, and wherein the deforming device for elastically deforming the hollow portion includes: an expanding member which engages an inner circumferential surface of the hollow portion; and a pushing device which forces the expanding member onto the inner circumferential surface of the hollow portion, so that the hollow portion is expanded.  
           [0024]    The expanding member according to the above mode (9) may be provided by a tapered member or a collet described below, for instance. Where the expanding member is provided by the tapered member, the pushing device is a device for effecting an interference fit described below. Where the expanding member is provided by the collet, the pushing device is a collet-diameter changing device described below.  
           [0025]    (10) An apparatus according to any one of the above modes (6)-(8), wherein the molding surface of the hollow portion for molding the one of the inner and outer circumferential surfaces of the die-cast article is an inner circumferential surface of the hollow portion, and wherein the deforming device for elastically deforming the hollow portion includes: a contracting member which engages an outer circumferential surface of the hollow portion; and a pushing device which forces the contracting member onto the outer circumferential surface of the hollow portion, so that the hollow portion is contracted.  
           [0026]    The contracting member according to the above mode (10) may be provided by a tapered member or a collet described below, for instance. Where the contracting member is provided by the tapered member, the pushing device is a device for effecting an interference fit described below. Where the contracting member is provided by the collet, the pushing device is a collet-diameter changing device described below.  
           [0027]    (11) An apparatus according to any one of the above modes (6)-(8), wherein the hollow portion has a non-molding surface which is opposite to the molding surface, the non-molding surface being a tapered surface, a dimension of which in a direction perpendicular to a direction parallel to a centerline of the hollow portion gradually changes in the direction, the deforming device for elastically deforming the hollow portion including: a tapered member having a tapered surface which corresponds to the tapered surface of the hollow portion; and a device for effecting an interference fit between the tapered surface of the hollow portion and the tapered surface of the tapered member.  
           [0028]    The die-casting apparatus according to the above mode (11) is suitably used for practicing the method according to the above mode (2).  
           [0029]    (12) An apparatus according to the above mode (11), wherein the tapered member is held by the main body of the mold assembly such that the tapered member and the hollow portion are axially movable relative to each other, the device for effecting an interference fit including an axial moving device for moving the tapered member and the hollow portion relative to each other in an axial direction of the tapered member.  
           [0030]    The present arrangement is particularly advantageous when the hollow portion and the tapered member are located such that they extend in a direction which intersects the above-indicated parting direction of the two molds. The present arrangement is applicable to an arrangement wherein the hollow portion and the tapered member extend in a direction parallel to the parting direction of the two molds.  
           [0031]    (13) An apparatus according to the above mode (12), wherein the axial moving device includes a hydraulic cylinder which is fixed to the mold assembly.  
           [0032]    (14) An apparatus according to the above mode (11), wherein the tapered member is fixed to one of the first and second molds, which one mold is opposite to the other of the first and second molds which is equipped with the hollow portion, the first and second molds being opened and closed by an opening and closing device which also functions as the device for effecting an interference fit.  
           [0033]    (15) An apparatus according to any one of the above modes (7)-(14), wherein the other of the first and second molds which is equipped with the hollow portion includes an ejecting device which pushes the die-cast article in a direction away from the other mold to remove the die-cast article from the hollow portion.  
           [0034]    The ejecting device permits the die-cast article to be easily removed from the hollow potion. The die-cast article can be easily removed from the hollow portion especially when the ejecting device is actuated after the hollow portion has been freed from the elastic deformation. It is particularly advantageous to employ the feature of this mode (15) and the feature of the above mode (14) in combination. In this case, at the same time when the first and second molds are opened, the tapered member and the hollow portion are separated away from each other, so that the hollow portion is freed from the elastic deformation for permitting easy removal of the die-cast article from the hollow portion. In this state, the ejecting device is actuated, whereby the die-cast article can be easily removed from the hollow portion.  
           [0035]    (16) An apparatus according to the above mode (6), wherein the hollow portion is a hollow cylindrical portion having an annular shape in transverse cross section, and wherein the deforming device for elastically deforming the hollow portion includes: a collet which engages, at one of inner and outer circumferential surfaces thereof, a non-molding surface of the hollow portion, which non-molding surface is opposite to the molding surface for molding the one of the inner and outer circumferential surfaces of the die-cast article; and a collet-diameter changing device for changing a diameter of the collet so as to force the collet onto the non-molding surface.  
           [0036]    In the above mode (16), the collet is a hollow cylindrical member which consists of a plurality of segments that are arranged in a spaced-apart relation with each other in the circumferential direction. The thus constructed collet is easily expanded or contracted in its radial direction. If the spacing between the adjacent segments of the collet is relatively large, the amount of the elastic deformation of the hollow portion is undesirably small at portions thereof corresponding to the relatively large spacing of the segments of the collet. In this case, the roundness of the outer or inner circumferential surface of the hollow portion is deteriorated, leading to deterioration of the roundness of the inner or outer circumferential surface of the die-cast article provided by the corresponding outer or inner circumferential surface of the hollow portion. In view of this, the spacing between the adjacent segments of the collet is preferably minimized. While the segments of the collet may be completely separated from each other, it is desirable that the segments of the collet are partially connected to each other so as to constitute an integral unitary member.  
           [0037]    (17) An apparatus according to the above mode (16), wherein the other of the inner and outer circumferential surfaces of the collet, which the other circumferential surface is opposite to the one circumferential surface which engages the non-molding surface of the hollow cylindrical portion, is tapered to give a first tapered surface whose diameter gradually changes in an axial direction of the hollow cylindrical portion, and wherein the collet-diameter changing device includes: a tapered member having a second tapered surface which corresponds to the first tapered surface; a multiplicity of balls interposed between the collet and the tapered member such that the balls maintain a constant position relative to each other, and such that the balls are rotatable independently of each other, at least while the first and second tapered surfaces engage each other; and an axial moving device which moves the tapered member and the hollow cylindrical portion relative to each other in an axial direction of the tapered member and the hollow cylindrical portion, so that the first and second tapered surfaces engage each other with an interference fit therebetween via the balls.  
           [0038]    The multiplicity of the balls are accommodated and held in a recess formed in one of the first and second tapered surfaces, such that each ball is rotatable and such that a part of each ball projects outwardly from the recess. In this case, the ball is held in contact with the other of the first and second tapered surfaces at its projecting portion. Alternatively, the balls may be held by a retainer which is a separate member from the tapered member and the hollow cylindrical portion. The balls are held by the retainer such that the balls are rotatable and such that the balls project from the inner and outer surfaces of the retainer, respectively, so that the balls are held in rolling contact with and between the first and second tapered surfaces. Either of those arrangements are effective to reduce the friction caused between the tapered member and the hollow cylindrical portion when the tapered member and the hollow cylindrical portion engage each other with an interference fit therebetween. Accordingly, the present arrangement improves the durability of the die-casting device.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    The above and optional objects, features, advantages and technical and industrial significance of the present invention will be better understood and appreciated by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which:  
         [0040]    [0040]FIG. 1 is a front elevational view in cross section of a swash plate type compressor produced according to one embodiment of a die-casting method and apparatus of the present invention;  
         [0041]    [0041]FIG. 2 is a perspective view showing a cylinder block of the swash plate type compressor of FIG. 1;  
         [0042]    [0042]FIG. 3 is a front elevational view partly in cross section schematically showing a casting system including the die-casting apparatus;  
         [0043]    [0043]FIG. 4 is a front elevational view in cross section showing a principal part of the die-casting apparatus;  
         [0044]    [0044]FIG. 5 is a front elevational view in cross section showing a die-casting apparatus constructed according to another embodiment of the present invention;  
         [0045]    [0045]FIG. 6 is a front elevational view in cross section showing a die-casting apparatus constructed according to still another embodiment of the present invention; and  
         [0046]    [0046]FIG. 7 is a side elevational view showing a part of the die-casting apparatus of FIG. 6.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]    Referring to the accompanying drawings, there will be described presently preferred embodiments of a die-casting method and apparatus according to the present invention as applied to the production of a swash plate type compressor.  
         [0048]    Referring first to FIG. 1, there is shown a compressor of swash plate type used for an air conditioning system of an automotive vehicle. In FIGS. 1 and 2, reference numeral  10  denotes a cylinder block having a plurality of cylinder bores  12  (seven cylinder bores in this embodiment) formed so as to extend in its axial direction such that the cylinder bores  12  are arranged along a circle whose center lies on a centerline L of the cylinder block  10  and such that the cylinder bores  12  are equiangularly spaced from each other in the circumferential direction of the cylinder block  10 . Single-headed pistons generally indicated at  14  (hereinafter simply referred to as “piston  14 ”) are reciprocably received in the respective cylinder bores  12 . To one of the axially opposite end faces of the cylinder block  10 , (the left end face as seen in FIG. 1, which will be referred to as “front end face”), there is attached a front housing  16 . To the other end face (the right end face as seen in FIG. 1, which will be referred to as “rear end face”), there is attached a rear housing  18  through a valve plate  20 . The front housing  16 , rear housing  18  and cylinder block  10  cooperate to constitute a housing assembly of the swash plate type compressor. The rear housing  18  and the valve plate  20  cooperate to define a suction chamber  22  and a discharge chamber  24 , which are connected to a refrigerating circuit (not shown) through an inlet  26  and an outlet  28 , respectively. The valve plate  20  has suction ports  32 , suction valves  34 , discharge ports  36  and discharge valves  38 .  
         [0049]    A rotary drive shaft  50  is disposed in the cylinder block  10  and the front housing  16  such that the axis of rotation of the drive shaft  50  is aligned with the centerline L of the cylinder block  10 . The drive shaft  50  is supported at its opposite end portions by the front housing  16  and the cylinder block  10 , respectively, via respective bearings. The cylinder block  10  has a central bearing hole  56  formed in a central portion thereof, and the bearing is disposed in this central bearing hole  56 , for supporting the drive shaft  50  at its rear end portion. The front end portion of the drive shaft  50  is connected, through a clutch mechanism such as an electromagnetic clutch, to an external drive source (not shown) in the form of an engine of an automotive vehicle. In operation of the compressor, the drive shaft  50  is connected through the clutch mechanism to the vehicle engine in operation so that the drive shaft  50  is rotated about its axis.  
         [0050]    The rotary drive shaft  50  carries a swash plate  60  such that the swash plate  60  is axially movable and tiltable relative to the drive shaft  50 . The swash plate  60  has a central hole  61  through which the drive shaft  50  extends. The diameter of the central hole  61  of the swash plate  60  gradually increases in the axially opposite directions from its axially intermediate portion towards the axially opposite ends. (In other words, the inner dimension of the central hole  61  as measured in a vertical direction of FIG. 1 is larger at the axially opposite ends than the axially intermediate portion.) To the drive shaft  50 , there is fixed a rotary member  62  as a torque transmitting member, which is held in engagement with the front housing  16  through a thrust bearing  64 . The swash plate  60  is rotated with the drive shaft  50  by a hinge mechanism  66  during rotation of the drive shaft  50 . The hinge mechanism  66  guides the swash plate  60  for its axial and tilting motions. The hinge mechanism  66  includes a pair of support arms  67  fixed to the rotary member  62 , guide pins  69  which are formed on the swash plate  60  and which slidably engage guide holes  68  formed in the support arms  67 , the central hole  61  of the swash plate  60 , and the outer circumferential surface of the drive shaft  50 .  
         [0051]    The piston  14  indicated above includes an engaging portion  70  engaging the radially outer portion of the opposite surfaces of the swash plate  60 , and a head portion  72  formed integrally with the engaging portion  70  and fitted in the corresponding cylinder bore  12 . The head portion  72 , cylinder bore  12 , and valve plate  20  cooperate with one another to define a pressurizing chamber  79 . The engaging portion  70  engages the radially outer portion of the opposite surfaces of the swash plate  60  through a pair of hemispherical shoes  76 .  
         [0052]    A rotary motion of the swash plate  60  is converted into a reciprocating linear motion of the piston  14  through the shoes  76 . A refrigerant gas in the suction chamber  22  is sucked into the pressurizing chamber  79  through the suction port  32  and the suction valve  34 , when the piston  14  is moved from its upper dead point to its lower dead point, that is, when the piston  14  is in the suction stroke. The refrigerant gas in the pressurizing chamber  79  is pressurized by the piston  14  when the piston  14  is moved from its lower dead point to its upper dead point, that is, when the piston  14  is in the compression stroke. The pressurized refrigerant gas is discharged into the discharge chamber  24  through the discharge port  36  and the discharge valve  38 . A reaction force acts on the piston  14  in the axial direction as a result of compression of the refrigerant gas in the pressurizing chamber  79 . This compression reaction force is received by the front housing  16  through the piston  14 , swash plate  60 , rotary member  62  and thrust bearing  64 . The engaging portion  70  of the piston  14  has an integrally formed rotation preventive part (not shown), which is arranged to contact the inner circumferential surface of the front housing  16 , for thereby preventing a rotary motion of the piston  14  about its centerline to prevent an interference between the piston  14  and the swash plate  60 .  
         [0053]    The cylinder block  10  has a supply passage  80  formed therethrough for communication between the discharge chamber  24  and a crank chamber  86  which is defined between the front housing  16  and the cylinder block  10 . The supply passage  80  is connected to a solenoid-operated valve  90  having a solenoid coil  92 . The solenoid-operated valve  90  is selectively energized and de-energized by a control device (not shown) constituted principally by a computer. During energization of the solenoid coil  92 , the amount of electric current applied to the solenoid coil  92  is controlled depending upon the air conditioner load, so that the amount of opening of the solenoid-operated valve  90  is controlled according to the air conditioner load.  
         [0054]    The rotary drive shaft  50  has a bleeding passage  100  formed therethrough. The bleeding passage  100  is open at one of its opposite ends to the central bearing hole  56 , and is open to the crank chamber  86  at the other end. The central bearing hole  56  communicates at its bottom with the suction chamber  22  through a communication port  104 .  
         [0055]    The present swash plate type compressor is of variable capacity type. By controlling the pressure in the crank chamber  86  by utilizing a difference between the pressure in the discharge chamber  24  as a high-pressure source and the pressure in the suction chamber  22  as a low pressure source, a difference between the pressure in the crank chamber  86  and the pressure in the pressurizing chamber  79  is regulated to change the angle of inclination of the swash plate  60  with respect to a plane perpendicular to the axis of rotation of the drive shaft  50 , for thereby changing the reciprocating stroke (suction and compression strokes) of the piston  14 , whereby the displacement capacity of the compressor can be adjusted. Described in detail, the pressure in the crank chamber  86  is controlled by controlling the solenoid-operated valve  90  to selectively connect and disconnect the crank chamber  86  to and from the discharge chamber  24 . The maximum angle of inclination of the swash plate  60  is limited by abutting contact of a stop  106  formed on the swash plate  60 , with the rotary member  62 , while the minimum angle of inclination of the swash plate  60  is limited by abutting contact of the swash plate  60  with a stop  107  in the form of a ring fixedly fitted on the drive shaft  50 .  
         [0056]    Between the rotary member  62  and one of the opposite major surfaces of the swash plate  60  which is remote from the rear housing  18 , an elastic member in the form of a compression coil spring  108  is disposed to function as biasing means for biasing the swash plate  60  toward the stop  107  so that when the compressor is in its off state, the swash plate  60  is positioned substantially at right angles with respect to the centerline of the cylinder block  10 , in abutting contact with the stop  107 . When the compressor is turned off, the swash plate  60  is moved to its minimum inclination position by the biasing force of the compression coil spring  108  and is kept at the position until the compressor is re-started.  
         [0057]    The cylinder block  10  and each piston  14  are formed of an aluminum alloy. The piston  14  is coated at its outer circumferential surface with a fluoro resin film which prevents a direct contact of the aluminum alloy of the piston  14  with the aluminum alloy of the cylinder block  10  so as to prevent seizure therebetween, and makes it possible to minimize the amount of clearance between the piston  14  and the cylinder bore  12 . Other materials may be used for the cylinder block  10 , the piston  14 , and the coating film.  
         [0058]    The cylindrical wall of each of the cylinder bores  12  of the cylinder block  10  is formed with an extension  150  (FIG. 2) at a first circumferential part thereof which corresponds to a radially outer portion of the cylinder block  10  and which is more distant from the centerline L of the cylinder block than a second circumferential part which corresponds to a radially inner portion of the cylinder block  10 . The extension  150  extends from the above-indicated first circumferential part of each cylinder bore  12  in the axial direction toward the crank chamber  86 . Front end faces  152  of the extensions  150  are connected to each other in the circumferential direction of the cylinder block  10  so as to be flush with each other. The front housing  16  is attached to the front end faces  152  of the extensions  150 . The inner circumferential surface of each cylinder bore  12  has a complete cylindrical surface  154 , and a part-cylindrical surface  156  on the side of the front housing  16 . Owing to provision of the extensions  150 , the cylindrical wall of each cylinder bore  12  has a larger axial length at the above-indicated first circumferential part of the cylinder bore  12  which corresponds to the radially outer portion of the cylinder block  10  and which is distant from the centerline L of the cylinder block  10 , than the second circumferential part of the cylinder bore  12  which corresponds to the radially inner portion of the cylinder block  10  and which is near to the centerline L of the cylinder block  10 . Accordingly, the piston  14  placed at its end of the compression stroke engages the inner circumferential surface of the cylinder bore  12  over a larger axial distance corresponding to the axial dimension of the extension  150 , at the above-indicated first circumferential part of the cylinder bore  12  corresponding to the radially outer portion of the cylinder block  10 . This arrangement is effective to prevent the engaging portion  70  of the piston  14  from being inclined toward the radially outer portion of the cylinder block  10 . Therefore, the piston  14  can be smoothly retracted into the cylinder bore  12  without being adversely influenced by an excessively large resistance of friction which would be otherwise caused between the inner circumferential surface of the cylinder bore  12  and the outer circumferential surface of the piston  14 . Accordingly, the swash plate  60  can be moved to its minimum inclination position without being adversely influenced by the piston  14 . Since the extension  150  is not formed at the above-indicated second circumferential part of each cylinder bore  12  which corresponds to the radially inner portion of the cylinder block  10  and which is near to the centerline L of the cylinder block  10 , the swash plate  60  is not inhibited from moving from its maximum inclination position to its minimum inclination position.  
         [0059]    There will be next described a method of producing, by die-casting, the cylinder block  10  constructed as described above, according to a first embodiment of the present invention. Referring to FIG. 3, there is schematically shown a casting system  200  which includes a die-casting apparatus for producing the cylinder block  10 . The casting system  200  includes a pair of stationary platens  204 ,  206  which are located on a main frame  202  of the system  200  in opposed relation to each other. Four guide rods  208  extend between the two stationary platens  204 ,  206 , such that each guide rod  208  connects one of four corners of the stationary platen  204  to the corresponding one of four corners of the stationary platen  206 . The four guide rods  208  are parallel to one another. A movable platen  210  is slidably supported by the four guide rods  208 . The stationary platen  204  is provided with a hydraulic cylinder  214  which is adapted to open and close the mold assembly described below. The hydraulic cylinder  214  is a kind of a hydropneumatic cylinder, and includes a housing  216  which is fluid-tightly fixed to one of the opposite major surfaces of the stationary platen  204  which is remote from the movable platen  210 . The hydraulic cylinder  214  further includes a piston  218  which is carried by a piston rod  220  and which is slidably and fluid-tightly received in the housing  216 . The piston rod  220  of the hydraulic cylinder  214  extends through the stationary platen  204  toward the movable platen  210 , and is connected at its distal end to the movable platen  210 . The movable platen  210  is advanced toward and retracted from the stationary platen  206  by the hydraulic cylinder  214  while being guided by the guide rods  208 . A maximum distance of retracting movement of the movable platen  210  away from the stationary platen  206  is determined by suitable limiting means not shown.  
         [0060]    A stationary mold  224  is removably attached to one of the opposite major surfaces of the stationary platen  206  on the side of the movable platen  210 , while a movable mold  226  is removably attached to the other major surface of the movable platen  210  on the side opposite to the hydraulic cylinder  214 . As described above, the casting system  200  includes the mold assembly of the stationary and movable molds  224 ,  226 . The stationary mold  224  consists of a plurality of plate members which are superposed on one another. The plate members comprise a mold plate, and a fixing plate at which the stationary mold  224  is fixed to the stationary platen  206 . Similarly, the movable mold  226  consists of a plurality of plate members which are superposed on one another. The plate members comprise a mold plate, and a fixing plate at which the movable mold  226  is fixed to the movable platen  210 . The stationary and movable molds  224 ,  226  are fixed, with a high degree of positional accuracy, to the stationary platen  206  and the movable platen  210 , respectively, by engagement of engaging grooves formed in the respective stationary and movable platens  206 ,  210  with engaging protrusions provided on the respective stationary and movable molds  224 ,  226 , for instance. Alternatively, the stationary and movable molds  224 ,  226  are fixed to the stationary and movable platens  206 ,  210 , respectively, while the two molds  224 ,  226  are positioned relative to each other such that a positioning pin provided on one of the two molds  224 ,  226  is fitted in a pin hole formed in the other of the two molds  224 ,  226 .  
         [0061]    The two molds  224 ,  226  are butted together and are spaced apart from each other at their contact surfaces  230 ,  232 , as shown in FIG. 4. The movable mold  226  is moved toward the stationary mold  224  by a drive force of the hydraulic cylinder  214 , so that the two molds  224 ,  226  are closed together with the contact surfaces  230 ,  232  being held in close contact with each other. The two molds  224 ,  226  have respective molding surfaces  240 ,  242  which cooperate with each other to define therebetween a mold cavity  236  whose configuration follows a profile of the cylinder block  10  to be obtained. A molten metal (a molten aluminum alloy whose major component is aluminum, in the present embodiment) is injected into the mold cavity  236  for die-casting the cylinder block  10 .  
         [0062]    The lower end of the mold cavity  236  is held in communication with a sleeve  250  (FIG. 3) via a runner (not shown) which extends in a direction parallel to the contact surfaces  230 ,  232 . The sleeve  250  is provided with a molten metal inlet. The runner has a gate provided at one of its opposite open ends on the side of the mold cavity  236 . The gate has a cross sectional area smaller than that of the other portion of the runner. The sleeve  250  is a cylindrical member which extends through the stationary platen  206 , so that one of opposite end portions of the sleeve  250  remote from the mold cavity  236  is located outside the two molds  224 ,  226 . A plunger chip  254  formed at one end of a plunger  252  and having a diameter larger than that of the plunger  252  is slidably fitted in the above-indicated one end portion of the sleeve  250  located outside the two molds  224 ,  226 . The plunger  252  is fixed to a piston rod  258  of a plunger drive cylinder  256  as a plunger drive device. The plunger drive cylinder  256  is a hydraulically operated cylinder, and is fixedly supported by the main frame  202 . The sleeve  250 , plunger  252 , plunger chip  254 , plunger drive cylinder  256 , and piston rod  258  cooperate with one another to constitute an injecting device for injecting the molten aluminum alloy into the mold cavity  236  via the molten metal inlet of the sleeve  250 .  
         [0063]    Within the movable mold  226 , there is provided an ejecting device  260  which includes a pushing cylinder  262  (FIG. 3) and a pushing member  266  (FIG. 4) having a plurality of eject pins  264 . The pushing cylinder  262  is a hydraulically operated cylinder, and fixed to the movable mold  226  such that the pushing cylinder  262  does not interfere with other members. When the pushing cylinder  262  is actuated, the piston rod  268  of the pushing cylinder  262  is advanced, for thereby pushing the pushing member  266  toward the main body  283  of the movable mold  226 . Accordingly, the distal end of each eject pin  264  is moved from its retracted position in which the distal end of the eject pin  264  cooperates with the molding surface  242  to partially define the mold cavity  236 , to its advanced position in which the distal end of the eject pin  264  projects into the mold cavity  236  so as to eject the die-cast article therefrom. When the piston rod  268  of the pushing cylinder  262  is retracted, the pushing member  266  is also retracted. A maximum distance of advancing movement of the pushing member  266  is determined by abutting contact of its front surface with one of two stops provided on the movable mold  226  while a maximum distance of retracting movement of the pushing member  266  is determined by abutting contact of its rear surface opposite to the front surface, with the other stop.  
         [0064]    The hydraulic cylinder  214 , plunger drive cylinder  256 , and pushing cylinder  262  are controlled by a control device not shown principally constituted by a computer. More specifically described, directional control valves provided in the fluid passages which are connected to those cylinders are controlled by the control device.  
         [0065]    The movable mold  226  is provided with a hollow cylindrical portion  280 . Described in detail, a hollow cylindrical member  282 , which is provided by a separate member from a main body  283  of the movable mold  226 , is fixed to the main body  283  by suitable fixing means, such that the axial direction of the hollow cylindrical member  282  is parallel to the parting direction of the stationary and movable molds  224 ,  226 , and such that the hollow cylindrical member  282  is not movable relative to the main body  283  of the movable mold  226 . The distal end portion of the hollow cylindrical member  282  which projects from the molding surface  242  into the mold cavity  236  by a predetermined axial length functions as the hollow cylindrical portion  280  that is subjected to an elastic deformation to change (to increase or decrease) its diameter. The distal end face of the hollow cylindrical member  282  is flush with the contacting surface  232  of the movable mold  226 . The hollow cylindrical member  282  is suitably formed of alloy tool steels (e.g., SKD  61  of SKD tool steels specified according to the Japanese Industrial Standard), which are usually used for forming the casting mold. It is desirable that at least portions of the stationary and movable molds  224 ,  226 , which portions define the mold cavity  236 , are formed of the alloy tool steels.  
         [0066]    The proximal end portion of the hollow cylindrical member  282 , which is opposite to the hollow cylindrical portion  280 , is fitted in an engaging hole  284  formed in the main body  283  of the movable mold  226 . The engaging hole  284  extends in a direction parallel to the parting direction of the two molds  224 ,  226 . An engaging pin  288  having a large-diameter engaging portion  290  at its distal end is fitted in the proximal end portion of the hollow cylindrical member  282 , such that the large-diameter portion  290  is held in engagement with a shoulder  286  formed in the inner circumferential surface of the hollow cylindrical member  282 . The proximal end portion of the engaging pin  288 , which is opposite to the large-diameter engaging portion  290  and which extends through the main body  283  of the movable mold  226  toward the pushing member  266 , is externally threaded, and two nuts  292 ,  293  are engaged therewith, whereby the hollow cylindrical member  282  is fixed to a fixing member  294 , and the fixing member  294  is in turn fixed to the main body  283  of the movable mold  226 .  
         [0067]    The engaging hole  284  formed in the main body  283  of the movable mold  226  has, at one of its axially opposite ends which is nearer to the molding surface  242 , a large-diameter portion  296  having a diameter slightly larger than the other portion of the engaging hole  284 . While the hollow cylindrical member  282  is not elastically deformed, the outer circumferential surface of the hollow cylindrical member  282  has a constant diameter over an entire axial length thereof as indicated by a two-dot chain line in FIG. 4. A portion of the outer circumferential surface of the hollow cylindrical member  282 , which portion is adjacent to a molding surface  300  of the hollow cylindrical portion  280  (which will be described), is fitted in the large-diameter portion  296  of the engaging hole  294 , such that there is a small clearance in a radial direction therebetween. The outer circumferential surface of the hollow cylindrical portion  280  functions as the molding surface  300  for forming the inner circumferential surface of the cylinder bore  12  of the cylinder block  10  to be produced. Namely, the molding surface  240  of the stationary mold  224 , the molding surface  242  of the movable mold  226 , and the molding surface  300  of the hollow cylindrical portion  280  cooperate to define the mold cavity  236  having a configuration which follows that of the cylinder block  10 . In FIG. 4, one of a plurality of the hollow cylindrical portions  280  (seven in the present embodiment) is shown for forming one of a plurality of the cylinder bores  12  (seven in the present embodiment). The inner circumferential surface of the hollow cylindrical portion  280  is a tapered surface  304  whose diameter linearly decreases in the axial dimension of the hollow cylindrical portion  280  from its open end on the side of the stationary mold  224  toward the movable mold  226 .  
         [0068]    The stationary mold  224  is provided with a tapered member  310  such that the tapered member  310  is coaxial with the hollow cylindrical portion  280  of the movable mold  226 . The tapered member  310  is fixedly attached to a main body  312  of the stationary mold  224  by suitable fixing means. The main body  312  of the stationary mold  224  is formed with an engaging hole  314  which extends in the axial direction of the stationary mold  224 . At one of opposite axial ends of the engaging hole  314  which is remote from the movable mold  226 , there is formed an internally threaded portion  316 . The tapered member  310  is formed, at one of its opposite ends which is remote from the movable mold  226 , an externally threaded portion  320 . The externally threaded portion  320  of the tapered member  310  is held in engagement with the internally threaded portion  316 , whereby the tapered member  310  is fixed to the stationary mold  224 . The tapered member  310  may be otherwise fixed to the stationary mold  224 . For instance, the tapered member  310  may be fixed to the stationary mold  224  such that the tapered member  310  is press-fitted into an engaging hole formed in the stationary mold  224 . The tapered member  310  has a head potion  322  formed at its proximal end adjacent to the externally threaded portion  320 . The head portion  322  of the tapered member  310  has a diameter larger than the other portion. With the externally threaded portion  320  being engaged with the internally threaded portion  316  such that one of the opposite end faces of the head portion  322 , which end face is adjacent to the externally threaded portion  320 , is held in abutting contact with the end face of the stationary mold  224 , as shown in FIG. 4, the distal end portion of the tapered member  310  (which is opposite to the externally threaded portion  320 ) projects a suitable axial distance from the contact surface  230  and the molding surface  240  of the stationary mold  224  in a direction toward the movable mold  226 . The outer circumferential surface of the distal end portion of the tapered member  310  is a tapered surface  326  corresponding to the tapered inner circumferential surface  304  of the hollow cylindrical portion  280 .  
         [0069]    The movable mold  226  is moved toward the stationary mold  224 , so that the two molds  224 ,  226  are closed together with the contact surfaces  230 ,  232  being held in close contact with each other. In this state, the tapered inner circumferential surface  304  of the hollow cylindrical portion  280  and the tapered outer circumferential surface  326  of the tapered member  310  engage each other with an interference fit therebetween, so that the hollow cylindrical portion  280  and an axial part of the hollow cylindrical member  282 , which part is adjacent to the hollow cylindrical portion  280 , are elastically deformed in a radially outward direction, whereby the diameter of the hollow cylindrical portion  280  and the above-indicated axial part of the hollow cylindrical member  282  is increased, as shown in FIG. 4. For easier understanding, the amount of the elastic deformation is exaggerated in FIG. 4. The above-indicated axial part adjacent to the hollow cylindrical portion  280  is radially outwardly expanded by the elastic deformation, so that the outer circumferential surface of the axial part adjacent to the hollow cylindrical portion  280  is held in sealing contact with the inner circumferential surface of the large-diameter portion  296  of the engaging hole  284 . Accordingly, the engaging hole  284  is fluid-tightly closed at its open end on the side of the molding surface  242 , for inhibiting fluid communication with the mold cavity  236 . The axial dimensions of the hollow cylindrical portion  280  and the tapered member  310  are determined such that there is left an axial clearance between the front end faces of the tapered member  310  and the large-diameter engaging portion  290  of the engaging pin  288  when the tapered member  310  is entirely press-fitted into the hollow cylindrical portion  280 . When the movable mold  226  is moved away from the stationary mold  224 , the tapered member  310  is retracted from the hollow cylindrical portion  280 , so that the hollow cylindrical portion  280  is freed from the elastic deformation and restored to its original state.  
         [0070]    There will be next explained a method of die-casting the cylinder block  10  by using the die-casting apparatus constructed as described above. Initially, the hydraulic cylinder  214  is actuated so as to move the movable mold  226  toward the stationary mold  224 , so that the two molds  224 ,  226  are closed together with the contact surfaces  230 ,  232  being held in close contact with each other. When the two molds  224 ,  226  are closed together so as to define therebetween the mold cavity  236 , the tapered inner circumferential surface  304  of the hollow cylindrical portion  280  and the tapered outer circumferential surface  326  of the tapered member  310  engage each other with an interference fit therebetween, so that the hollow cylindrical portion  280  is elastically deformed in a direction toward the inner circumferential surface of the cylinder bore  12  of the cylinder block to be produced, namely in a radially outward direction. Since the engaging hole  284  formed in the movable mold  283  is fluid-tightly sealed as described above, the engaging hole  284  is inhibited from communicating with the mold cavity  236 . While the two molds  224 ,  226  are closed together, the sleeve  250  is held in fluid communication with the mold cavity  236  via the runner, and the plunger chip  254  is placed in its retracted position at which the molten metal inlet of the sleeve  250  is held in communication with the mold cavity  236 . In this state, the molten metal (e.g., the molten aluminum alloy) is introduced from the molten metal inlet into the sleeve  250 . Subsequently, the plunger chip  254  is advanced toward the two molds  224 ,  226 , so that the level of the molten metal in the sleeve  250  is raised, whereby the molten metal is introduced into the runner. Thereafter, the advancing speed of the plunger chip  254  is increased, so that the molten metal is jetted into the mold cavity  236  through the narrow gate provided at the end of the runner. The plunger chip  254  is kept actuated after the mold cavity  236  has been filled with the molten metal, and the molten metal in the mold cavity  236  solidifies under a sufficiently high pressure.  
         [0071]    The molten metal in the mold cavity  236  solidifies into the cylinder block  10  a predetermined time after the mold cavity  236  has been filled with the molten metal. The inner circumferential surface of the cylinder bore  12  is formed by the molding surface  300 , i.e., the outer circumferential surface of the hollow cylindrical portion  280  which is radially outwardly expanded by the elastic deformation. Thereafter, the movable mold  226  is moved away from the stationary mold  224 . At the same time when the two molds  224 ,  226  are opened, the hollow cylindrical portion  280  is moved away from the tapered member  310 , so that the tapered inner circumferential surface  304  of the hollow cylindrical portion  280  and the tapered outer circumferential surface  326  of the tapered member  310  which have been held in an interference fit with each other are disengaged from each other. Accordingly, the elastically deformed hollow cylindrical portion  280  is restored to its original state. That is, the hollow cylindrical portion  280  which has been radially outwardly expanded by the elastic deformation is radially inwardly contracted. Accordingly, the molding surface  300  of the hollow cylindrical portion  280  and the inner circumferential surface of the cylinder bore  12  of the cylinder block  10 , which have been held in an interference fit with each other, are positioned relative to each other such that there is a radial clearance therebetween. Thereafter, the pushing cylinder  262  is actuated to advance the eject pins  264 , whereby the die-cast cylinder block  10  held by the movable mold  226  is pushed in a direction away from the movable mold  226 . Since there exists a radial clearance between the molding surface  300  of the hollow cylindrical portion  280  and the inner circumferential surface of the cylinder bore  12  as described above, the cylinder lock  10  can be easily removed from the movable mold  226 .  
         [0072]    In the present embodiment, the molding surface which forms the inner circumferential surface of the cylinder bore  12  of the cylinder block  10  is provided by the outer circumferential surface of the hollow cylindrical portion  280 . The hollow cylindrical portion  280  corresponds to a hollow portion. The tapered inner circumferential surface  304  of the hollow cylindrical portion  280  is a tapered surface, a dimension of which in a direction perpendicular to a direction parallel to the centerline of the hollow portion gradually changes in the direction. The tapered outer circumferential surface  326  of the tapered member  310  is a tapered surface which corresponds to the above-indicated tapered surface of the hollow portion. The engaging hole  284  is an engaging portion. The stationary mold  224  and the movable mold  226  correspond to a pair of molds of the die-casting apparatus, which molds are moved toward and away from each other. The hydraulic cylinder  214  constitutes an axial moving device for moving the tapered member and the hollow portion relative to each other in the axial direction of the tapered member. The axial moving device is one example of a device for effecting an interference fit between the tapered surface of the hollow portion and the tapered surface of the tapered member. The hydraulic cylinder  214  corresponds to an opening and closing device for opening and closing the stationary and movable molds  224 ,  226 . In the present embodiment, the hydraulic cylinder  214  also functions as the device for effecting an interference fit described above. The tapered member  310  is one example of an expanding member which engages the inner circumferential surface of the hollow cylindrical portion  280 . The hydraulic cylinder  214  functioning as the above-described device for effecting an interference fit constitutes a pushing device which forces the expanding member onto the inner circumferential surface of the hollow cylindrical portion  280  for expanding the hollow cylindrical portion  280 .  
         [0073]    According to the present embodiment, the cylinder block  10  formed in the mold cavity  236  can be easily removed from the movable mold  226  without any problem even where the hollow cylindrical portion  280  which forms the inner circumferential surface of the cylinder bore  12  is provided with a relatively small angle of draft or no draft. Therefore, the present arrangement permits a reduction in the required amount of stock removal by the machining operation to be conducted on the inner circumferential surface of the cylinder bore  12 , or eliminates the machining operation, resulting in a reduction in the cost of manufacture of the compressor. The outside diameter and the amount of the elastic deformation of the hollow cylindrical portion  280  are suitably determined depending upon the dimension of the intended inner circumferential surface of the cylinder bore  12 .  
         [0074]    The hollow cylindrical portion  280  may be provided on the stationary mold  224  while the tapered member  310  may be provided on the movable mold  226 . The tapered member  310  may be arranged to be axially movable relative to the two molds  224 ,  226  by a suitable drive device such as a hydraulically operated cylinder which is provided independently of the hydraulic cylinder  214 .  
         [0075]    The present die-casting apparatus and the die-casting method may be employed in producing a die-cast article other than the cylinder block  10  described above. Referring next to FIG. 5, there will be described a die-casting apparatus constructed according to a second embodiment of the invention for producing the front housing  16  of the swash plate type compressor. The front housing  16  has a hollow cylindrical recess, as shown in FIG. 1. Since the structure of the casting system which includes the die-casting apparatus of this second embodiment (FIG. 5) is similar to that of the casting system  200  shown in FIG. 3, a detailed explanation of which is dispensed with. Like the die-casting apparatus of FIG. 4 of the above-described first embodiment, the die-casting apparatus of the second embodiment includes a stationary mold  400  and a movable mold  402 , which are moved toward and away form each other. Each of the stationary and movable molds  400 ,  402  consists of a plurality of plate members which are superposed on one another. The movable mold  402  is moved toward and away from the stationary mold  400  by a suitable opening and closing device such as a hydraulic cylinder, and the two molds  400 ,  402  are closed together at their contacting surfaces  404 ,  406 . The stationary mold  400  is provided with a hollow cylindrical member  408  such that the hollow cylindrical member  408  extends in a direction parallel to the axial direction of the two molds  400 ,  402 , in other words, in a direction parallel to the parting direction of the two molds  400 ,  402 . The hollow cylindrical member  408  includes a hollow cylindrical portion  410  which projects from the contact surface  404  in the axial direction of the hollow cylindrical member  408 , and an engaging portion  414 . The engaging portion  414  is adjacent to the hollow cylindrical portion  410  and fitted in an engaging hole  412  formed in a main body  411  of the stationary mold  400 , such that the engaging portion  414  is axially unmovable with respect to the stationary mold  400 . At one of the opposite axial ends of the engaging portion  414  which is remote from the hollow cylindrical portion  410 , there is formed a flange  416  having a diameter larger than that of the hollow cylindrical portion  410  and the engaging portion  414 . The engaging hole  412  includes a large-diameter section on the side which is remote from the contact surface  404 , and a small-diameter section on the side which is nearer to the contact surface  404 . The diameter of the inner circumferential surface of the small-diameter section of the engaging hole  412  is made smaller than that of the outer circumferential surface of the flange  416  of the engaging portion  414  of the hollow cylindrical member  408 . The hollow cylindrical member  408  is inserted from its distal end into the large-diameter section of the engaging hole  412 , until the flange  416  of the hollow cylindrical member  408  is brought into abutting contact with a shoulder surface formed between the large- and small-diameter sections of the engaging hole  412 . Thus, an amount of protrusion of the hollow cylindrical portion  410  from the contact surface  404  is determined by the abutting contact of the flange  416  with the shoulder surface of the engaging hole  412 . Further, a fixing member  417  is inserted into the large-diameter section of the engaging hole  412  until the end face of the fixing member  417  is brought into abutting contact with the end face of the flange  416 . The fixing member  417  is fixed to the main body  411  of the stationary mold  400  by suitable fixing means in the form of bolts, so that the hollow cylindrical member  408  is inhibited from moving in the axial direction away from the main body  411  of the stationary mold  400 . The thus fixed fixing member  417  serves as a part of the main body  411  of the stationary mold  400 . While the hollow cylindrical member  408  is not subjected to the elastic deformation, the engaging portion  414  of the hollow cylindrical member  408  is fitted in the engaging hole  412  such that there is a clearance therebetween in the radial direction, as indicated by the two-dot chain line in FIG. 5. The outer circumferential surface of the hollow cylindrical portion  410  has a constant diameter over an entire axial length thereof, and function as a molding surface  420  for forming the inner circumferential surface of the front housing  16 . The molding surface  420  cooperates with a molding surface  422  of the stationary mold  400  and a molding surface  424  of the movable mold  402  to define a mold cavity  426  whose configuration follows that of the front housing  16 . The molding surface  424  of the movable mold  402  which forms the outer surface of the front housing is provided with a draft in its axial direction. The inner circumferential surface of the hollow cylindrical member  408  is a tapered surface  428  whose diameter gradually decreases in a direction parallel to the centerline of the hollow cylindrical member  408  from the stationary mold  400  toward the movable mold  402 .  
         [0076]    As shown in FIG. 5, the lower end of the mold cavity  426  is held in communication with a sleeve (not shown) having a molten metal inlet, via a runner  430 . The runner  430  extends in a direction parallel to the contact surfaces  404 ,  406 , and is provided, at one of its opposite open ends on the side of the mold cavity  426 , with a gate having a cross sectional area smaller than the other portion of the runner  430 . In this second embodiment, too, the injecting device which includes the sleeve, plunger, plunger chip, and plunger drive device is employed. The structure of the injecting device is similar to that of the injecting device used in the first embodiment described above, and a detailed explanation of which is dispensed with. The movable mold  402  is provided therein with an ejecting device (not shown) whose structure is similar to that of the ejecting device  260  used in the above-described first embodiment.  
         [0077]    The stationary mold  400  is provided with a tapered member  440 . The tapered member  440  is supported by the stationary mold  400  such that the tapered member  440  is axially movable within the inner space of the hollow cylindrical member  408 . The tapered member  440  extends in the axial direction of the hollow cylindrical member  408  such that the axes of the tapered member  440  and the hollow cylindrical member  408  are aligned with each other. The outer circumferential surface of the tapered member  440  at its distal end which is on the side of the hollow cylindrical portion  410  is a tapered surface  444  corresponding to the tapered inner circumferential surface  428  of the hollow cylindrical member  408 . The tapered member  440  is connected to a piston rod  446  of a tapered-member-moving cylinder (not shown) which is fixed to the stationary mold  400 . The tapered-member-moving cylinder is a hydraulically operated actuator.  
         [0078]    The movable mold  402  is moved toward the stationary mold  400 , so that the two molds  400 ,  402  are closed together at the contact surfaces  404 ,  406 . After the two molds  400 ,  402  have been closed together, the tapered-member-moving cylinder is actuated so as to move the tapered member  440  in the axial direction toward he hollow cylindrical member  408 , so that the tapered inner circumferential surface  428  of the hollow cylindrical portion  410  and the tapered outer circumferential surface  444  of the tapered member  440  engage each other with an interference fit therebetween. Accordingly, the hollow cylindrical portion  410  and an axial part of the hollow cylindrical member  408  adjacent to the hollow cylindrical portion  410  (in other words, the entirety of the hollow cylindrical member  408 ) are elastically deformed in a radially outward direction, whereby the outside diameter of the hollow cylindrical member  408  is increased. The above-indicated axial part adjacent to the hollow cylindrical portion  410  is radially outwardly expanded within the engaging hole  412 , so that the outer circumferential surface of the axial part is held in close contact with the inner circumferential surface of the engaging hole  412 . Accordingly, the open end of the engaging hole  412  on the side of the molding surface  422  is fluid-tightly sealed for inhibiting the fluid communication with the mold cavity  426 . At the same time, the advancing movement of the tapered member  440  is stopped, and the end faces of the tapered member  440  and the hollow cylindrical portion  410  are flush with each other. With the hollow cylindrical portion  410  being elastically deformed, the molten metal such as a molten aluminum alloy is introduced into the mold cavity  426 . After the molten metal in the mold cavity  426  has solidified and before the stationary mold  400  and the movable mold  402  are opened, the tapered member  440  is retracted in the axial direction away from the hollow cylindrical portion  410  toward the stationary mold  400 . Accordingly, the tapered outer circumferential surface  444  of the tapered member  440  and the tapered inner circumferential surface  428  of the hollow cylindrical portion  410  are disengaged from each other, whereby the elastically deformed hollow cylindrical member  408  including the hollow cylindrical portion  410  is restored to its original shape. In this state, there is a radial clearance between the molding surface  420  and the inner circumferential surface of the hollow cylindrical recess of the front housing  16  to be obtained. Thereafter, the movable mold  402  is moved away from the stationary mold  400  with the front housing  16  being held by the movable mold  402 . After the two molds  400 ,  402  have been fully opened, the front housing  16  is pushed by the ejecting device in a direction away from the movable mold  402 .  
         [0079]    The front housing  16  produced as described above is formed with a through-hole through which the rotary drive shaft  50  extends. This through-hole may be formed by a machining operation after the die-casting process. Alternatively, the through-hole may be formed in the die-casting process. In this case, the through-hole may be formed by an axial member provided with a draft having a suitable taper angle. The through-hole may be formed by using a mold assembly which is equipped with a hollow cylindrical portion and a device for elastically deforming the hollow cylindrical portion, which are similar to those described in the present embodiment. In this case, the hollow cylindrical portion is provided with a relatively small angle of draft, or the hollow cylindrical portion does not have a draft.  
         [0080]    The hollow cylindrical portion  410  may be fixed to the main body  411  of the stationary mold  400  by the fixing means similar to that as described above with respect to the first embodiment of FIGS.  1 - 4 . On the contrary, in the above-described first embodiment, the hollow cylindrical portion  280  may be fixed to the movable mold  226  by the fixing means as described with respect to the second embodiment of FIG. 5. The hollow cylindrical portion  410  and the tapered member  440  may be provided on the movable mold  402 . The ejecting device may be provided on either the stationary mold  400  or the movable mold  402 .  
         [0081]    The present die-casting method and the die-casting apparatus permit formation of the inner circumferential surface of the axial through-hole such as the inner circumferential surface of the cylinder bore  12 , as described with respect to the first embodiment of FIGS.  1 - 4 , and the inner circumferential surface of the recess having a closed end such as the inner circumferential surface of the front housing  16 , as described with respect to the second embodiment of FIG. 5.  
         [0082]    Referring next to FIG. 6, there is explained a die-casting apparatus constructed according to a third embodiment of the present invention, which die-casting apparatus is arranged to form an outer circumferential surface of a die-cast article by an inner circumferential surface of a hollow cylindrical portion. Like the die-casting apparatuses of the above-described first and second embodiments, the die-casting apparatus of this third embodiment includes a stationary mold  500  and a movable mold  502  which are moved toward and away from each other, so that the two molds  500 ,  502  are opened and closed. The structure of the casting system including the die-casting apparatus in this embodiment is similar to that of the casting system  200  shown in FIG. 3, a detailed description of which is dispensed with. Each of the stationary and movable molds  500 ,  502  consists of a plurality of plate members which are superposed on one another. The movable mold  502  is moved toward and away from the stationary mold  500  by an opening and closing device for opening and closing the two molds  500 ,  502  (not shown) in the form of a hydraulically operated cylinder, for instance. The two molds  500 ,  502  are closed together at their contact surfaces  504 ,  506 .  
         [0083]    The movable mold  502  is provided with a hollow cylindrical member  512  which extends in a direction parallel to the parting direction of the two molds  500 ,  502 , in other words, in the axial direction of the two molds  500 ,  502 . The hollow cylindrical member  512  is fixed at its flat fixing plate portion  514  to a main body  511  of the movable mold  502  by suitable fixing means such as bolts, such that the hollow cylindrical member  512  is not movable relative to the main body  511  of the movable mold  502 . The hollow cylindrical member  512  includes a hollow cylindrical portion  510  which axially extends from an inner peripheral portion of the fixing plate portion  514  toward the stationary mold  500 . The hollow cylindrical portion  510  has an annular shape in transverse cross section. An engaging member  520  is fitted in the hollow cylindrical member  512 . The engaging member  520  includes, at its proximal end which is opposite to the hollow cylindrical portion  510 , a fixing portion  522  having a diameter larger than the other portion of the engaging member  520 . Like the hollow cylindrical member  512 , the engaging member  520  is fixed to the main body  511  of the movable mold  502  at the fixing portion  522 .  
         [0084]    The engaging member  520  has a tapered outer circumferential surface  524  at its distal end portion which is opposite to the fixing portion  522 . The hollow cylindrical potion  510  is subjected to an elastic deformation such that the hollow cylindrical portion  510  is radially outwardly expanded or radially inwardly contracted. While the hollow cylindrical portion  510  is not subjected to the elastic deformation, there is a radial clearance between the inner circumferential surface of the hollow cylindrical portion  510  and the tapered outer circumferential surface  524  of the engaging member  520 . The inner circumferential surface of the hollow cylindrical portion  510  serves as a molding surface  530  while the front end face of the engaging member  520  serves as a molding surface  532 . The stationary mold  500  has a protrusion  536  which protrudes in the axial direction of the stationary mold  500  from the contact surface  504  toward the movable mold  502 . The protrusion  536  is located within a space defined by the molding surfaces  530 ,  532  when the stationary and movable molds  500 ,  502  are closed together at the contact surfaces  504 ,  506 . The outer circumferential surface, and the front end face of the protrusion  536  which is remote from the stationary mold  500 , serve as molding surfaces  538 ,  539 , respectively. The molding surfaces  530 ,  532 ,  538 ,  539  cooperate to define a mold cavity  540  having a configuration which follows that of an intended die-cast article. The protrusion  536  is provided with a draft such that the diameter of the protrusion  536  gradually decreases in the axial direction from its proximal end toward its distal end.  
         [0085]    A collet  550  is fitted on the outer circumferential surface  544  of the hollow cylindrical portion  510 , which outer circumferential surface  544  is opposite to the molding surface  530 . As shown in FIG. 7, the collet  550  consists of a plurality of segments  551  (preferably, six or more segments) which are equiangularly spaced from each other in the circumferential direction of the collet  550 . The diameter of the colet  550  is mechanically changed (i.e., decreased) by a collet-diameter changing device  552 , so that the inner circumferential surface  554  of the collet  550  is forced onto the outer circumferential surface  544  of the hollow cylindrical portion  510 , for thereby elastically deforming or contracting the hollow cylindrical portion  510  in a radially inward direction. The collet  550  has a tapered outer circumferential surface  556  whose diameter gradually decreases in the axial direction of the collet  550  from the stationary mold  500  toward the movable mold  502 .  
         [0086]    The collet-diameter changing device  552  includes as major components a tapered member  560 , and an axial moving device for axially moving the tapered member  560 . The tapered member  560  is a generally cylindrical member, and has a tapered inner circumferential surface  568  which corresponds to the tapered outer circumferential surface  556  of the collet  550 . A multiplicity of balls  570  are interposed between those tapered inner and outer circumferential surfaces  568 ,  556 . The balls  570  are held by a retainer  572 , such that the balls  572  maintain a constant position relative to each other, and such that the balls are rotatable independently of each other. The retainer  572  is a member separate from the collet  550  and the tapered member  560 . Each of the balls  572  projects from the retainer  572  in both of the radially inward and outward directions of the retainer  572 , and cooperates with the retainer  572  to constitute a rolling bearing. Namely, the rolling movement of the balls  570  between the tapered outer circumferential surface  556  of the collet  550  and the tapered inner circumferential surface  568  of the tapered member  560  of the collet-diameter changing device  552  effectively reduces friction caused when the tapered member  560  and the collet  550  engage each other with an interference fit therebetween by the axial moving device  564 . The rolling bearing constituted by the balls  570  and the retainer  572  is prevented from moving apart from the collet  550  and the tapered member  560 , by abutting contact with stops  576 ,  578  which are formed at two axial portions of the tapered member  560 . The axial moving device  564  for axially moving the tapered member  560  includes a hydraulic actuator in the form of a hydraulically operated cylinder (not shown) as a drive source, a piston rod  580  of the hydraulically operated cylinder, and a connecting member  582  for connecting the piston rod  580  and the tapered member  560 .  
         [0087]    An ejecting device  590  is provided within the movable mold  502  such that the ejecting device  590  does not interfere with other components of the movable mold  502 . Like the ejecting device  260  in the above-indicated first embodiment, the ejecting device  590  includes a pushing cylinder  592  fixed to the movable mold  502  and a pushing member  596  equipped with a plurality of eject pins  594 . When the pushing cylinder  592  is actuated, a piston rod of the pushing cylinder  592  is advanced so as to move the pushing member  596  toward the stationary mold  500 , so that the front end face of each eject pin  594  is moved from its retracted position in which the front end face of each eject pin  594  is flush with the molding surface  532  so as to partially define the mold cavity  540 , into its advanced position in which the front end face of each eject pin  594  projects into the mold cavity  540  so as to push the die-cast article in a direction away from the movable mold  502 .  
         [0088]    The mold cavity  540  is held in fluid communication with a sleeve  602  having a molten metal inlet, via a runner  600 . The runner  600  is provided, at one of its opposite open ends on the side of the mold cavity  540 , with a gate having a cross sectional area smaller than the other portion of the runner  600 . In this third embodiment, too, the injecting device which includes the sleeve  602 , a plunger  608 , a plunger chip  610 , and plunger drive device is employed. The structure of the injecting device is similar to that of the injecting device used in the first embodiment described above, and a detailed explanation of which is dispensed with. The movable mold  402  is provided therein with an ejecting device (not shown) whose structure is similar to that of the ejecting device  260  used in the above-described first embodiment.  
         [0089]    The movable mold  502  is moved toward the stationary mold  500 , so that the two molds  500 ,  502  are closed together with their contact surfaces  504 ,  506  being held in close contact with each other. After the two molds  504 ,  506  have been closed together, the axial moving device  564  is actuated to move the tapered member  560  toward the stationary mold  500 , whereby the tapered inner circumferential surface  568  of the tapered member  560  and the tapered outer circumferential surface  556  of the collet  550  engage each other with an interference fit therebetween. Accordingly, the hollow cylindrical portion  510  and an axial part of the hollow cylindrical member  512  adjacent to the hollow cylindrical portion  510  are elastically deformed in a radially inward direction, so that the diameter of the inner circumferential surface of the hollow cylindrical portion  510  (the molding surface  530 ) is decreased, as shown in FIG. 6. The above-indicated axial part of the hollow cylindrical member  512  adjacent to the hollow cylindrical portion  510  is radially inwardly deformed such that its inner circumferential surface is held in close contact with the tapered outer circumferential surface  524  of the engaging member  520 , for thereby inhibiting a fluid communication between the mold cavity  540  and the inside of the movable mold  500  in which the axial moving device  564  and other components are disposed. With the hollow cylindrical portion  510  being elastically deformed, a molten metal such as a molten aluminum alloy is introduced into the mold cavity  540 . After the molten metal has solidified in the mold cavity  540 , the stationary mold  500  and the movable mold  502  are separated away from each other with the die-cast article being held by the movable mold  502 . Thereafter, the tapered member  560  is retracted in a direction away from the stationary mold  500 , and the tapered inner circumferential surface  568  of the tapered member  560  is disengaged from the tapered outer circumferential surface  556  of the collet  550 , so that the elastically deformed hollow cylindrical portion  510  is restored to its original state, namely, the diameter of the hollow cylindrical portion  510  which has been reduced is increased to the original value. In this state, there is a radial clearance between the molding surface  530  of the hollow cylindrical portion  510  and the outer circumferential surface of the die-cast article. Accordingly, the die-cast article held by the movable mold  502  is easily pushed by the ejecting device  590  in a direction away from the movable mold  502 . The die-casting apparatus according to this embodiment is suitably used in die-casting a blank for a head portion of a compressor piston, for instance.  
         [0090]    In the present embodiment wherein the tapered surfaces are formed on the tapered member  560  and the collet  550 , the thickness of the hollow cylindrical portion  510  can be made constant over an entire axial length thereof, so that the hollow cylindrical portion  510  can be uniformly subjected to the elastic deformation in the radially inward direction. Since the tapered member  560  and the collet  550  engage each other with an interference fit via the rolling bearing constituted by the retainer  572  and the balls  570 , the tapered member  560  and the collet  550  have a relatively small degree of mutually frictional resistance. Accordingly, the tapered member  560  can be axially moved by the axial moving device  564  with high efficiency for the interference fit with the collet  550 , resulting in a reduced size of the axial moving device  564 . Further, the present arrangement wherein the tapered member  560  and the collet  550  engage each other via the rolling bearing assures a reduction of the wear and an improved durability of the two members.  
         [0091]    For elastically deforming the hollow cylindrical portion  510  while keeping its roundness, the number of the segments  551  of the collet  550  is desirably maximized. It is desirable that the circumferential clearance between the adjacent ones of the plurality of segments  551  is minimized while the collet  550  is radially inwardly contracted for reduction of its diameter. On the other hand, it is desirable that the circumferential clearance between the adjacent ones of the plurality of segments  551  is constant while the collet  550  is in its original state. To this end, as in an ordinary collet, the segments of the collet  550  are connected to each other by an elastically deformable connecting member, rather than completely separated from each other. This elastically deformable connecting member is provided at one of opposite axial ends of the collet  550 , e.g., at an axial end portion of the collet  550  which is remote from the hollow cylindrical portion  510  and which is nearer to the movable mold  511 .  
         [0092]    As is apparent from the foregoing description, in the present embodiment, the molding surface which forms the outer circumferential surface of the die-cast article is provided by the inner circumferential surface of the hollow cylindrical portion  510 . The hollow cylindrical portion  510  corresponds to the hollow portion. The tapered outer circumferential surface  556  of the collet  550  is a first tapered surface whose diameter gradually changes in the axial direction of the hollow portion, while the tapered inner circumferential surface  568  of the tapered member  560  is a second tapered surface which corresponds to the first tapered surface. The collet  550  and the collet-diameter changing device  552  cooperate to constitute a deforming device for elastically deforming the hollow portion. The collet  550  is a contracting member which engages the outer circumferential surface of the hollow cylindrical portion  510 . The collet-diameter changing device  552  is a pushing device which forces the contracting member onto the outer circumferential surface  544  of the hollow cylindrical portion  510  for contracting the hollow cylindrical portion  510 . The collet and the collet-diameter changing device for mechanically changing the diameter of the collet used in the present third embodiment may be employed in the above-described first and second embodiments of FIGS.  1 - 4  and FIG. 5, respectively, wherein the molding surface which forms the inner circumferential surface of the die-cast article is provided by the outer circumferential surface of the hollow portion.  
         [0093]    The present die-casting apparatus and the die-casting method using the apparatus are suitably employed in producing articles such as the cylinder block  10  having the cylinder bores  12  and the front housing  16  having the hollow cylindrical recess, which articles are formed of a material whose major component is aluminum. The present die-casting apparatus and the die-cast method can be employed in producing articles other than the described above.  
         [0094]    While the presently preferred embodiments of this invention have been described above, for illustrative purpose only, it is to be understood that the present invention may be embodied with various changes and improvements such as those described in the SUMMARY OF THE INVENTION, which may occur to those skilled in the art.