Abstract:
A casting die device and a casting method. The casting die device has a core pin for forming an inner hole in a casted article. The core pin is a hollow body, and a pressurizing pin is inserted into a hollow inner part of the core pin. Vibrations from a vibrator of a micro vibration machine are imparted to the pressurizing pin via a vibration transmission member. The vibrations further propagate to the core pin from the pressurizing pin, and then propagate to the area surrounding the core pin in a molten metal that has been poured into a cavity.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a casting die device and a casting method for obtaining a cast product in which an inner hole, at least one end of which is open, is formed. 
       BACKGROUND ART 
       [0002]    High pressure casting (die casting) is known as a method of obtaining, e.g., cast products of aluminum alloy. In the high pressure casting, the obtained cast products have excellent dimensional accuracy, and the high pressure casting enables mass production advantageously. Therefore, the high pressure casting method has been adopted widely. 
         [0003]    In high pressure casting, molten metal poured into a plunger sleeve is extruded by a plunger tip, and the molten metal is supplied to a cavity. That is, an injection process is performed in the casting method. 
         [0004]    In the process, the molten metal passes through a narrow runner and a gate, and is supplied into a cavity. In this case, for example, the molten metal staying in the gate may be solidified earlier than the molten metal which has reached the cavity. In such a situation, molten metal for a rise is not poured sufficiently. Therefore, this is one of factors which may cause occurrence of casting defects such as blow holes or cracks in the cast product. 
         [0005]    In an attempt to avoid the occurrence of such defects, in a technique proposed in Japanese Laid-Open Patent Publication No. 07-001102, a pressurizing pin for applying pressure to molten metal in a cavity is provided. Further, vibrations are applied to the pressurizing pin from a vibration device such as a mechanical vibration generator or an ultrasonic vibrator. 
       SUMMARY OF INVENTION 
       [0006]    For example, in the case of obtaining a valve body of a spool valve as a cast product, it is required to form a valve hole (inner hole) for slidably inserting a spool as a valve member. The valve hole of this type is formed by a core pin, for example. That is, the core pin is inserted into the cavity beforehand. In this state, the molten metal is poured into the cavity. After the molten metal is solidified and the cast product is obtained, the core pin is removed or separated away from the cast product, whereby a hollow portion having a shape corresponding to the shape of the core pin is formed. The hollow portion serves as the valve hole. 
         [0007]    An inner wall surface (casting surface) of the valve hole normally has casting defects such as blow holes or flow lines. Application of vibrations to the pressurizing pin as described in Japanese Laid-Open Patent Publication No. 07-001102 is effective in reducing casting defects on outer surfaces of the cast product. However, in this method, it is difficult to reduce casting defects in the inner hole formed by the core pin, such as the valve hole. This is because the pressurizing pin never contacts the surface of the inner hole. 
         [0008]    A main object of the present invention is to provide a casting die device having a simple structure which makes it possible to obtain a cast product with reduced casting defects in an inner wall surface of an inner hole of the cast product. 
         [0009]    Another object of the present invention is to provide a casting method which makes it possible to obtain the above cast product. 
         [0010]    According to one embodiment of the present invention, a casting die device is provided, for obtaining a cast product, an inner hole being formed in the cast product, at least one end of the inner hole being open. The casting die device includes a core pin having a hollow structure and configured to form the inner hole, a pressurizing pin inserted into a hollow interior portion of the core pin, and configured to be displaced by operation of a displacement drive source and apply pressure to molten metal introduced into a cavity, a vibration generating unit configured to generate vibrations applied to the pressurizing pin, and a vibration transmission member configured to transmit the vibrations generated by the vibration generating unit to the pressurizing pin. 
         [0011]    Further, according to another embodiment of the present invention, a casting method is provided for obtaining a cast product, an inner hole being formed in the cast product, at least one end of the inner hole being open. The method includes the steps of forming a cavity into which a core pin is inserted, the core pin having a hollow structure and being configured to form the inner hole, introducing molten metal into the cavity, and applying pressure to the molten metal introduced into the cavity, by a pressurizing pin inserted into a hollow interior portion of the core pin. Vibrations generated by a vibration generating unit are applied to the pressurizing pin through a vibration transmission member to thereby apply the vibrations to the molten metal in the cavity. 
         [0012]    It should be noted that the term “inner hole” includes the meanings of a through hole both ends of which are open, and a bottomed hole one end of which is closed. Further, the term “sound surface” and the term “sound layer” as used below refer to a surface and a layer where casting defects, such as blow holes or flow lines, etc., of a size that results in leakage of internal substance inside the inner hole cannot be recognized. 
         [0013]    That is, in the present invention, the core pin has a hollow structure, and the pressurizing pin is inserted into the hollow interior portion of the core pin. Therefore, even though the core pin and the pressurizing pin are used in combination, it is possible to simplify the structure. 
         [0014]    Further, since vibrations are transmitted to the core pin, the inner wall surface of the inner hole where casting defects are not easily reduced only by the pressurizing pin, can be formed as a sound surface. That is, in the inner wall surface of the inner hole, casting defects, such as blow holes or flow lines having a size of a degree that causes leakage of internal substance (e.g., hydraulic oil, etc.) inside the inner hole cannot be recognized. Further, the inner wall surface has a good appearance. 
         [0015]    Therefore, it is possible to directly use the inner wall surface as it is, i.e., the casting surface, as the inner wall, without the need to carry out a grinding treatment, a mirror finishing treatment, etc. Consequently, the number of steps required for processing the cast product into the finished product is reduced, and cost reduction is achieved. Further, in this case, since grinding dust is not generated, improvement in the material yield is achieved. 
         [0016]    Moreover, in this case, the amount of burrs is also reduced. Additionally, since no grinding treatment or the like is required, grinding dust is not generated. For these reasons, improvement in the material yield is achieved. 
         [0017]    Further, an internal portion of the cast product from the casting surface up to a predetermined depth forms substantially a sound layer. That is, no casting defects having a size of a degree that causes leakage of internal substance can be recognized in the internal portion of the cast product from the casting surface up to the predetermined depth. Therefore, for example, about half of the predetermined depth (i.e., half of the sound layer) may be removed by a grinding process, and a newly exposed surface (processed surface) may be used as the inner wall of the inner hole. 
         [0018]    Preferably, the displacement drive source for displacing the pressurizing pin has a hollow structure. In this case, by inserting a vibration transmission member into a hollow interior portion of the displacement drive source, it becomes easy to apply vibrations to the pressurizing pin through the vibration transmission member. 
         [0019]    As a suitable example of this type of displacement drive source, there may be presented a double rod type cylinder including two displacement rods each having a hollow structure. 
         [0020]    As the vibration device, for example, a micro-vibration generator (air vibrator, etc.) for generating mechanical vibrations at the vibration frequency of one hundred to several hundred Hz may be adopted. Alternatively, the vibration device may be an ultrasonic vibration generator for generating ultrasonic vibrations. 
         [0021]    Further, at the time of pouring the molten metal into the cavity, preferably, pressure is applied to the molten metal. That is, preferably, the casting die device is a high pressure casting die device, and the casting method is a high pressure die casting (HPDC) method. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0022]      FIG. 1  is a vertical cross-sectional view taken along a thickness direction of a spool valve having a valve body (cast product), obtained by a casting method according to an embodiment of the present invention; 
           [0023]      FIG. 2  is a high magnification laser microscopic photograph of an inner wall of a valve hole (inner hole) formed in the valve body; 
           [0024]      FIG. 3  is a low magnification laser microscopic photograph of an inner wall of a valve hole (inner hole) formed in the valve body; 
           [0025]      FIG. 4  is a vertical cross-sectional view of main parts of a casting die device according to an embodiment of the present invention; 
           [0026]      FIG. 5A  and  FIG. 5B  are views showing a process flow in the case of displacing a vibrated pressurizing pin in a hollow interior portion (slide hole) of a core pin, in the casting die device. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    Hereinafter, a preferred embodiment of a casting method according to the present invention will be described in detail in connection with a casting die device for carrying out the casting method, with reference to the accompanying drawings. In the embodiment of the present invention, a valve body of a spool valve is shown as an example of a cast product. 
         [0028]    Firstly, the spool valve will be described with reference to  FIG. 1 .  FIG. 1  is a vertical cross-sectional view taken along a thickness direction (the direction indicated by arrow Z in  FIG. 1 ) of a spool valve  12 . The spool valve  12  has a valve body  10  as a cast product. In the valve body  10 , a valve hole  14  is formed as an inner hole extending in an axial direction, e.g., in a longitudinal direction (the direction indicated by arrow X in  FIG. 1 ). 
         [0029]    The valve hole  14  opens on one end in the direction of the arrow X. The opened end is closed by a cap member  16 . The other end is closed by an inner wall of the valve body  10 . The inner wall functions as a stopper wall for blocking a spool  18  (valve member). 
         [0030]    The valve body  10  has an inlet port  36  through which a hydraulic oil is introduced into the valve hole  14 , an outlet port  38  through which the hydraulic oil is led out from the valve hole  14 , a drain port  40 , and a hydraulic oil supply port  42  through which the hydraulic oil is supplied from another valve (not shown).  FIG. 1  shows a state where the spool  18  is biased elastically by a pressure regulating spring  34 , and one end surface of the spool  18  abuts against (contacts or is blocked by) the stopper wall. In this state, the inlet port  36  and the outlet port  38  are placed in communication with each other through an annular groove  20  of the spool  18 . On the other hand, the drain port  40  is closed or sealed by a large diameter portion  22 . 
         [0031]    The inner wall of the valve hole  14  defines a casting surface that exhibits a metallic luster. Further, as can be seen from  FIG. 2  which is a high magnification laser microscopic photograph of the inner wall (casting surface), blow holes or flow lines, etc., having a size of a degree that causes leakage of the hydraulic oil, are not recognized on the inner wall (casting surface). That is, even though the inner wall is a casting surface that is not subjected to a grinding treatment or a mirror finishing treatment or the like, the inner wall forms a sound surface in which casting defects cannot be recognized, and moreover, the surface has a good aesthetic appearance. 
         [0032]    Further, as shown in  FIG. 3 , on the casting surface that forms the inner wall, a plurality of fine lines  44 , which are visible when observed at low magnification by a laser microscope, extend in a direction perpendicular to a longitudinal direction (indicated by an arrow X). Such lines  44  cannot be observed on the inner wall of a valve hole formed without applying vibrations. That is, the lines  44  are believed to be formed as a result of application of vibrations. It should be noted that the lines  44  do not cause leakage. 
         [0033]    As will be described later, the valve hole  14  is formed by a core pin  92  (see  FIG. 4 ) to which vibrations are applied. It is presumed that the distance between the adjacent lines  44  correspond to the frequency of vibrations. 
         [0034]    Further, casting defects having a size of a degree that causes leakage of hydraulic oil, cannot be recognized in an inner portion from the inner wall surface of the valve hole  14  that forms the casting surface, up to a depth of at least 1 mm. That is, in the valve body  10 , the inner portion thereof from the inner wall surface of the valve hole  14  to the depth of 1 mm is a so-called a sound layer. 
         [0035]    Therefore, the casting surface can be used directly as it is, as the inner wall of the valve hole  14 . Stated otherwise, there is no particular need to carry out a complex operation such as grinding or the like with respect to the casting surface of the valve hole  14 . Further, as a result, the number of steps required for obtaining a practically usable valve body  10  is reduced, and a commensurate reduction in the cost is achieved. However, grinding treatment may be applied to the inner wall of the valve hole  14 , as will be described later. 
         [0036]    The valve body  10 , in which the valve hole  14  (inner hole) having such an inner wall (casting surface) is formed, can be produced by the casting operation to be described below. 
         [0037]    Firstly, the casting die device  50  will be described. The casting die device  50  is, for example, a high pressure casting die device for applying a pressure of 35 to 100 MPa to molten metal  66 . The casting die device  50  includes a fixed die  52  whose position is fixed, and a movable die  54  which is displaceable in directions to approach toward or separate away from the fixed die  52 . A first insert  56  is disposed in the fixed die  52 , and a second insert  58  is disposed in the movable die  54 . By closing the dies  52 ,  54 , a cavity  60  is formed by the first insert  56  and the second insert  58 . 
         [0038]    A fitting hole  62  is formed to penetrate through the fixed die  52 , and a plunger sleeve  64  is fitted into the fitting hole  62 . A molten metal supply port (not shown) is formed at an upper position of the plunger sleeve  64 . Molten metal (e.g., molten aluminum alloy)  66  is supplied from the molten metal supply port into the plunger sleeve  64 . 
         [0039]    A plunger tip  70  is slidably arranged in the plunger sleeve  64 . The plunger tip  70  is coupled to an injection rod  68  of an injection cylinder (not shown). Therefore, the molten metal  66  supplied into the plunger sleeve  64  is pushed out by the plunger tip  70 . Further, a runner  72  is formed from a front end of the plunger sleeve  64  up to the cavity  60 . The runner  72  is a passage for guiding the molten metal  66  outflowing from the plunger sleeve  64  into the cavity  60 . 
         [0040]    Further, in the casting die device  50 , a core  74  is disposed. The core  74  includes a pin retaining member  76  and a strut supporting member  78  connected to the pin retaining member  76 . The core  74  is displaceable in the vertical direction in  FIG. 4  under operation of a sliding mechanism (not shown) provided on the strut supporting member  78 . 
         [0041]    A stepped hole  80  extending toward the cavity  60  is formed so as to penetrate through the pin retaining member  76 . The diameter of the stepped hole  80  is expanded on the strut supporting member  78  side to thereby form a support step  82 . A guide hole  84  is formed so as to penetrate through the strut supporting member  78 . The guide hole  84  is connected to the stepped hole  80 . The diameter of the guide hole  84  is expanded on the strut supporting member  78  side, to thereby form a blocking step  86  in the guide hole  84 . 
         [0042]    A core pin  92  is inserted into the stepped hole  80 . The core pin  92  includes a shaft  88  and a head  90  having a slightly large diameter. The head  90  of the core pin  92  is supported by the support step  82  of the stepped hole  80  to thereby retain the core pin  92  by the pin retaining member  76 . Therefore, the core pin  92  is displaced integrally with the core  74 , and the front end of the shaft  88  of the core pin  92  enters into the cavity  60  at the time of die closing. The front end of the shaft  88  forms the valve hole  14  (see  FIG. 1 ). 
         [0043]    It should be noted that clearance in a range of about 0.01 to 0.1 mm is formed between the core pin  92  and the inner wall of the stepped hole  80 . Therefore, the core pin  92  can sway or rotate inside the stepped hole  80 . 
         [0044]    The outer circumference of the shaft  88  of the core pin  92  has a straight shape without any draft angle. Accordingly, the valve hole  14  has a straight shape as well. In this case, in comparison with a tapered valve hole having a draft angle, machining of the valve hole  14  can be performed easily, and it becomes possible to reduce the amount of machining. 
         [0045]    In this regard, the core pin  92  has a hollow structure where a slide hole  94  penetrates and extends through the core pin  92  in the longitudinal direction. A lower end of an elongated pressing shaft  98  of a pressurizing pin  96  is inserted into the slide hole  94 . Clearance in a range of about 0.01 to 0.1 mm is formed between the slide hole  94  and the lower end of the pressing shaft  98 . 
         [0046]    A large diameter flange  100  is formed at a substantially intermediate position of the pressing shaft  98  of the pressurizing pin  96  in the longitudinal direction thereof so as to protrude outward in the diameter direction. The flange  100  abuts against the blocking step  86 , whereby further downward movement of the pressurizing pin  96  is blocked. It should be noted that clearance in a range of about 0.01 to 0.1 mm is also formed between the guide hole  84  and the lower end of the pressing shaft  98 , and between the guide hole  84  and the flange  100 . 
         [0047]    The pressurizing pin  96  is displaced (raised or lowered) by a double rod type cylinder  102  as a displacement drive source. The double rod type cylinder  102  has a cylinder main body  106  supported by a strut  104  provided upright in the strut supporting member  78 . The cylinder main body  106  is equipped with a lower rod  108  and an upper rod  110  (displacement rods). The lower rod  108  and the upper rod  110  move back and forth cooperatively such that the lower rod  108  and the upper rod  110  are protruded from or retracted in the cylinder main body  106 . All of the cylinder main body  106 , the lower rod  108 , and the upper rod  110  have a hollow structure. 
         [0048]    A rod-shaped vibration transmission member  112  of a vibration device is inserted into a hollow interior portion of the double rod type cylinder  102  (i.e., an inner hole extending from the lower rod  108  to the upper rod  110 ). A threaded portion  114  having a small diameter protrudes from a lower end of the vibration transmission member  112 , and the threaded portion  114  is screwed into a screw hole  116  formed in an upper end of the pressurizing pin  96 . In this manner, the vibration transmission member  112  is coupled to the pressurizing pin  96 . 
         [0049]    A micro-vibration generator  118  (vibration generating unit) of the vibration device is supported at an upper end of the upper rod  110 . The vibration transmission member  112  and the micro-vibration generator  118  jointly form the vibration device. Therefore, the micro-vibration generator  118  is displaced such that the micro-vibration generator follows the forward movement/backward movement, i.e., upward/downward movement, of the upper rod  110 . As the micro-vibration generator  118 , for example, an air vibrator may be used. 
         [0050]    The upper end of the vibration transmission member  112  faces a vibration element  120  of the micro-vibration generator  118 . When the micro-vibration generator  118  is not actuated, the lower end surface of the vibration element  120  is separated from the upper end surface of the vibration transmission member  112  by a predetermined distance. 
         [0051]    When the micro-vibration generator  118  is actuated, the vibration element  120  moves up and down at a predetermined cycle. The stroke of the vibration element  120  is slightly larger than the distance between the vibration element  120  and the vibration transmission member  112 . Therefore, when the vibration element  120  is lowered, the vibration element  120  abuts against the vibration transmission member  112 . It is a matter of course that when the vibration element  120  is raised, the vibration element  120  is separated from the vibration transmission member  112 . In this manner, by repeatedly carrying out abutment and separation of the vibration element  120 , vibrations at a predetermined frequency are applied to the vibration transmission member  112 . 
         [0052]    In this regard, since the vibration element  120  is separated from the vibration transmission member  112  by a predetermined distance, when the vibration element  120  abuts against the vibration transmission member  112 , collision energy is generated. It is presumed that vibrations of a predetermined frequency to which such collision energy is added are applied to the vibration transmission member  112 . 
         [0053]    The casting operation for obtaining the valve body  10 , i.e., the casing method according to the embodiment of the present invention, is carried out in the following manner, using the casting die device  50  having the above structure. 
         [0054]    Firstly, the movable die  54  is displaced toward the fixed die  52 . Then, the core  74  is lowered, and the dies  52 ,  54  are closed. As a result, the core pin  92  enters into the cavity  60  formed by the first insert  56  and the second insert  58 . At this time point, the lower rod  108  and the upper rod  110  of the double rod type cylinder  102  are positioned at raised positions. Therefore, the pressurizing pin  96  is positioned at a raised position as well. In  FIG. 4 , the position of the front end of the pressurizing pin  96  and the position of the flange  100  at this time point are shown by imaginary lines. 
         [0055]    Next, the micro-vibration generator  118  is actuated to move the vibration element  120  up and down. As described above, when the vibration element  120  is lowered, the vibration element  120  comes into abutment against the vibration transmission member  112 , and when the vibration element  120  is raised, the vibration element  120  is separated from the vibration transmission member  112 . Therefore, vibrations at a predetermined frequency are applied to the vibration transmission member  112 . For example, the vibrations are mechanical vibrations, the frequency of which is in a range of one hundred to several hundred Hz. 
         [0056]    As described above, the lower end of the vibration transmission member  112  is coupled to the upper end of the pressurizing pin  96 . As a result, vibrations are transmitted to the pressurizing pin  96 . Therefore, the pressurizing pin  96  is vibrated in the slide hole  94 , and repeatedly carries out collision and separation with respect to the inner wall of the slide hole  94 , and consequently, the core pin  92  is vibrated. In this manner, vibrations are transmitted to the core pin  92 . Since clearance is present between the core pin  92  and the inner wall of the stepped hole  80 , when the core pin  92  is vibrated, the core pin  92  can sway in the diameter direction, or rotate in the circumferential direction. 
         [0057]    In this state, next, the molten metal  66  (e.g., molten metal of aluminum alloy) is supplied from a molten metal supply port formed on the plunger sleeve  64 . After a predetermined quantity of the molten metal  66  is introduced into the plunger sleeve  64 , an injection cylinder (not shown) is actuated, and accordingly an injection rod  68  moves forward. Following this movement, the plunger tip  70  slides in a direction to push the molten metal  66 . 
         [0058]    As a result, the molten metal  66  supplied into the plunger sleeve  64  is extruded from the plunger sleeve  64  by the plunger tip  70 , and guided by the runner  72 , so that the molten metal  66  reaches the cavity  60 . That is, the molten metal  66  is supplied to the cavity  60 , and the cavity  60  is filled with the molten metal  66 . Thus, in the embodiment of the present invention, pressure is applied to the molten metal  66  in the plunger sleeve  64 , whereby the molten metal  66  is introduced into the cavity  60  to perform high pressure die casting (HPDC). 
         [0059]    In this regard, the core pin  92  is inserted into the cavity  60 . In the embodiment of the present invention, as described above, vibrations are applied to the core pin  92 . Therefore, the vibrations are reliably applied to a portion that surrounds the core pin  92 , of the molten metal  66  supplied into the cavity  60  (hereinafter referred to as a “core pin surrounding region”) through the core pin  92 . That is, the core pin surrounding region, which eventually becomes the inner wall of the valve hole  14 , can be vibrated directly. 
         [0060]    In this case, the pressurizing pin  96  repeatedly moves forward (protrudes from the core pin  92 ) and backward (enters the core pin  92 ), through the opening at the front end of the slide hole  94  formed in the core pin  92 . At this time, the pressurizing pin  96  abuts against and is separated away from the core pin surrounding region. Also by this movement, vibrations are transmitted to the core pin surrounding region. 
         [0061]    When the vibration element  120  is separated from the core pin  92 , the core pin  92  is pushed by the viscoelasticity of the core pin surrounding region (molten metal  66 ), and returns to substantially the original position. 
         [0062]    Application of the vibrations continues until the dies are opened. Therefore, vibrations continue to be applied to the core pin surrounding region, i.e., a portion forming the inner wall of the valve hole  14 , from when the molten metal contacts the core pin  92  until when the molten metal is placed in a solid state (solidified). Since the core pin  92  sways in the diameter direction easily, and rotates in the circumferential direction easily, the vibrations can be transmitted, in particular, to the diameter direction and/or the circumferential direction of the core pin  92  easily. 
         [0063]    Further, since a tiny gap (clearance) is formed between the inner wall of the slide hole  94  of the core pin  92  and the circumferential side wall of the pressurizing pin  96 , when the vibrations are applied, frictional heat is produced between the core pin  92  and the pressurizing pin  96  by sliding/vibrating movement. In the structure, since heat is produced in the core pin  92 , the core pin surrounding region of the molten metal  66  is heated. In the structure, improvement in the running performance of the molten metal  66  in the core pin surrounding region is achieved advantageously. 
         [0064]    Further, when vibrations are applied to the core pin surrounding region in the molten metal  66 , the sizes of bubbles in the molten metal  66  are reduced by cavitation phenomenon, and the bubbles move in a direction away from the vibration source (core pin  92 ). It should be noted that the reduced bubble sizes are about 0.1 mm. 
         [0065]    As described above, in the embodiment of the present invention, the core pin  92  has a hollow structure, and the pressurizing pin  96  is inserted into the hollow interior portion of the core pin  92 . Therefore, while the structure is simplified, it is possible to use the core pin  92  and the pressurizing pin  96  in combination in a single casting die device. 
         [0066]    After the cavity  60  is filled with the molten metal  66 , the double rod type cylinder  102  is actuated. Accordingly, when the lower rod  108  and the upper rod  110  are lowered, the pressurizing pin  96  is pushed by the lower rod  108 , and the lower end of the pressurizing pin  96  is lowered from a position indicated by an imaginary line to a position indicated by a solid line in  FIG. 4 , and protrudes slightly beyond the lower end of the core pin  92 . The pressurizing pin  96  is lowered in this manner, whereby pressure is applied to the molten metal  66  in the cavity  60 . It should be noted that, following the downward movement of the lower rod  108  and the upper rod  110 , the micro-vibration generator  118  supported by the upper rod  110  is lowered as well. 
         [0067]    During the downward movement, the lower end of the pressing shaft  98  of the pressurizing pin  96  slides inside the slide hole  94 , as illustrated in a process flow of  FIGS. 5A and 5B . At this time, vibrations from the micro-vibration generator  118  are applied beforehand to the pressurizing pin  96  through the vibration transmission member  112 . In this case, the sliding resistance against the pressing shaft  98  is small in comparison with the case where non-vibrated vibration transmission member  112  slides in the slide hole  94 . Therefore, it becomes possible to avoid galling in the inner wall of the slide hole  94  and in the outer surface of the pressurizing pin  96 . 
         [0068]    The movement of the pressurizing pin  96  is blocked by the flange  100  of the pressurizing pin  96  abutting against the blocking step  86  in the guide hole  84  formed in the strut supporting member  78 . That is, further downward movement of the pressurizing pin  96  is blocked or prevented. 
         [0069]    Thereafter, the molten metal  66  in the cavity  60  becomes solidified. Thus, the valve body  10  having a shape corresponding to the shape of the cavity  60  is obtained. The valve hole  14  is formed at a position corresponding to the core pin  92 . 
         [0070]    After elapse of a predetermined time from the end of supplying the molten metal  66  to the cavity  60 , the core  74  is raised, and the movable die  54  is separated away from the fixed die  52 , whereby the dies  52 ,  54  are opened. As a result, the valve body  10  is exposed. 
         [0071]    As described above, vibrations are applied to the pressurizing pin  96  and the core pin  92 , whereby the core pin surrounding region is vibrated sufficiently. Further, the sizes of the bubbles in the core pin surrounding region are reduced sufficiently. Therefore, in the valve body  10 , the inner wall of the valve hole  14  shows metallic luster, and is formed as a casting surface (sound surface) where no blow holes or flow lines (casting defects) having a size of a degree that causes leakage of hydraulic oil can be recognized. Further, the maximum surface roughness of the casting surface is about 1.5 μm. Further, the internal portion of the inner wall in the depth direction in a range of 1 mm is also formed as a sound layer where no blow holes or flow lines (casting defects) having a size that causes leakage of hydraulic oil can be recognized. 
         [0072]    Further, in the casting surface, a plurality of lines  44  (see  FIG. 3 ) are formed in a direction perpendicular to the axial direction (direction in which the core pin  92  is pulled out). It is presumed that the distance between the adjacent lines  44  corresponds to the vibration frequency of the vibration element  120 . 
         [0073]    In a general casting technique where applying of vibrations is not carried out, casting defects tend to be present in the inner wall (casting surface) of the valve hole  14  immediately after the core pin  92  has been pulled out. Therefore, if the casting surface is directly used as the inner wall without any processes, there is a concern that leakage of the hydraulic oil may occur. 
         [0074]    In contrast, in the embodiment of the present invention, as described above, the casting surface is formed as a sound surface where no casting defects are recognized. Therefore, the inner wall can function as the valve hole  14  in which the valve member is accommodated, without the need to carry out an operation such as grinding or the like with respect to the inner wall (casting surface) of the valve hole  14 . That is, there is no particular need to perform a grinding process. Accordingly, the number of process steps required for obtaining the valve body  10 , and thus the spool valve  12 , is reduced. For this reason, it is possible to achieve cost reduction. 
         [0075]    Further, in the case where casting is carried out while vibrations are applied to the core pin surrounding region, there is an advantage in that burrs that are formed in the valve body  10  are made smaller in size. Additionally, since no grinding process is required, and no grinding dust is produced, portions of material that become scrap material are reduced. Therefore, improvement in the material yield is achieved. 
         [0076]    Further, since vibrations are applied to the core pin surrounding region, the surface roughness of the inner wall (casting surface) of the valve hole  14  becomes small. More specifically, the maximum surface roughness was measured at a plurality of arbitrary positions on the inner wall of the valve hole  14 , and it was found that the maximum surface roughness was not more than 1.5 μm. 
         [0077]    Though it is difficult to avoid casting defects in the inner wall surface of the inner hole such as the valve hole only by the pressurizing pin  96 , as described above, by inserting the pressurizing pin  96  into the core pin  92 , the inner wall surface of the inner hole can be obtained as a sound surface. Further, the molten metal  66  is pressed by the pressurizing pin  96 , and this point also contributes to reduction in the casting defects. 
         [0078]    Moreover, while the outer circumference of the shaft  88  of the core pin  92  has a straight shape, it is possible to pull out the core pin  92  from the valve hole  14  without causing scoring or galling in the valve hole  14 . Additionally, improvement in the circularity or roundness of the valve hole  14  is achieved. 
         [0079]    The present invention is not limited to the above described embodiment, and various changes can be made without departing from the scope of the present invention. 
         [0080]    For example, in the above-described embodiment, though mechanical vibrations are applied at the vibration frequency of one hundred to several hundred Hz, it is a matter of course that ultrasonic vibrations may be applied. In this case, instead of the micro-vibration generator  118 , an ultrasonic vibrator may be adopted. Vibrations may be applied in a state where the front end of the vibration element  120  of the ultrasonic vibrator is not separated away from the upper end surface of the vibration transmission member  112 , and are in abutting contact with the upper end surface of the vibration transmission member  112 . 
         [0081]    Further, the cast product, which is obtained in the above manner, is not limited to the valve body  10  of the spool valve  12 , as long as the cast product has an inner hole formed by the vibrated core pin  92  or the like. As another example of such a cast product, a body of an actuator may be presented. In this case, for example, the inner hole is a slide hole for a piston. 
         [0082]    Further, as yet another example, there may be presented a throttle body or a carburetor body. In this case, the inner hole is an air intake path, and the internal substance is air or an air-fuel mixture.