Abstract:
Disclosed is a cast pin equipped with circular grooves which are provided at any location. The cast pin ( 10 ) is equipped with: an outer tube ( 11 ) in the shape of a hollow body the tip of which is closed; an inner tube ( 20 ) inserted into the outer tube ( 11 ); and a cooling medium pipe ( 30 ) that is inserted into the inner tube ( 20 ) and supplies a cooling medium to the interior of the inner tube ( 20 ). Three circular grooves ( 22 ) are formed at prescribed intervals in the longitudinal direction, for example, on the outer circumferential surface ( 21 ) of the inner tube ( 20 ). The circular grooves ( 22 ) are formed in the outer circumferential surface ( 21 ) by applying a cutting tool from the radial outward direction of the inner tube ( 20 ).

Description:
TECHNICAL FIELD 
     The present invention relates to an improved cooled core pin. 
     BACKGROUND ART 
     A core pin is used for making a cast hole in a casting simultaneously with a casting process. Finishing a cast hole can reduce a machining allowance and the number of machining steps but also increase a material yield, as compared to machining a hole by means of a drill or the like. 
     However, because the core pin is inserted into a cavity and surrounded by high-temperature molten metal, a thermal load on the core pin would become great. As a measure for reducing the thermal load, a cooled (type) core pin is recommended which is cooled by a cooling medium, such as water (see, for example, Patent Literature 1).  FIG. 18  hereof is a sectional view of an outer pin in the core pin disclosed in Patent Literature 1. 
     Referring to  FIG. 18 , the outer pin  100  has an annular groove  102  in its inner peripheral surface  101 . Generally, such an annular groove  102  is formed by a boring method. Namely, a central hole is made in the material by means of a drill or the like. Then, a bore  105  having a blade section  104  at the distal end of a rod  103  is inserted through an inlet  106  and rotated relatively to shave off the material so as to form the annular groove  102 . 
     It is essential that a maximum length L at the distal end of the bore  105  be smaller than a diameter of the inlet  106 . The smaller the diameter of the inlet  106 , the smaller becomes an outer diameter of the rod  103 . As the outer diameter of the rod  103  becomes smaller, flexure is more likely to occur at the distal end of the rod  103 . Therefore, with the boring method, a finishing accuracy of the annular groove  102  tends to be low. Additionally, it is difficult to provide the annular groove near the distal end  107  (remote from the inlet  106 ) of the outer pin  100 . 
     However, depending on the core pin, it may sometimes be required that the annular groove  102  be also provided near the distal end  107 . Thus, there has been a demand for a structure which allows the annular groove  102  to be provided at a desired position. 
     PRIOR ART LITERATURE 
     Patent Literature 1: Japanese Patent Application Laid-open Publication No. 2000-94114. 
     SUMMARY OF INVENTION 
     Technical Problem 
     It is therefore an object to provide an improved core pin which allows a annular groove to be readily provided at a desired position. 
     Solution to Problem 
     According to the present invention, as defined in a first aspect hereof, there is provided a core pin comprising: an outer tube in the form of a hollow tube closed at the distal end thereof; an inner tube inserted in the outer tube with the outer peripheral surface thereof contacting the inner peripheral surface of the outer tube; and a cooling medium pipe inserted in the inner tube, with a predetermined distance kept between the inner peripheral surface of the inner tube and the outer peripheral surface of the cooling medium pipe, for supplying a cooling medium into the inner tube, characterized in that the core pin includes a heat insulating chamber provided between the outer tube and the inner tube, and the heat insulating chamber is defined by an annular groove formed in the outer peripheral surface of the inner tube and the inner peripheral surface of the outer tube covering the annular tube. 
     Preferably, as recited in a second aspect hereof, in addition to the first aspect, the outer tube is formed of an iron-based material while the inner tube is formed of a copper based material, and a gap is provided at a normal temperature between the inner peripheral surface of the outer tube and the outer peripheral surface of the inner tube such that the outer peripheral surface of the inner tube is brought into close contact with the inner peripheral surface of the outer tube in response to pouring of a molten metal. 
     Preferably, as recited in a third aspect hereof, the inner tube is segmented in a zone where heat transfer is required and a zone where heat retention is required, and the zone where heat transfer is required is formed of a material of a higher thermal conductivity than a material of the zone where heat retention is required, the zone where heat transfer is required and the zone where heat retention is required being integrally joined to each other. 
     Preferably, as recited in a fourth aspect hereof, in addition to the third aspect, the outer tube is formed of an iron-based material, and the zone of the inner tube where heat transfer is required is formed of a copper-based material. A gap is provided at normal temperature between the inner peripheral surface of the outer tube and the outer peripheral surface of the zone where heat transfer is required such that the outer peripheral surface of the zone where heat transfer is required is brought into close contact with the inner peripheral surface of the outer tube in response to pouring of a molten metal. 
     Preferably, as recited in a fifth aspect hereof, in addition to the first aspect, the core pin of the present invention is adapted to be mounted to a mold for forming, around the outer tube, a small thickness portion of a product and a general thickness portion greater in thickness than the small thickness portion, and the heat insulating chamber is provided near the small thickness portion of the product. 
     Preferably, as recited in a sixth aspect hereof, in addition to the first aspect, the core pin of the present invention is adapted to be mounted to a mold for forming, around the outer tube, a small thickness portion of a product and a general thickness portion greater in thickness than the small thickness portion, the outer tube being inserted in a cavity of the mold in partial contact with the mold. The heat insulating chamber is provided near the small thickness portion and in a region of the outer tube where the outer tube contacts the mold. 
     In the invention recited in the first aspect hereof, the annular groove is formed in the outer peripheral surface of the inner tube. Such an annular groove can be formed in the outer peripheral surface of the inner tube by applying a cutting tool from radially outside of the inner tube. Unlike the conventional boring method, this method can provide the annular groove at a desired position. Also, the present invention can eliminate a need to care about flexure of the cutting tool, and a satisfactory finishing accuracy of the annular groove can be achieved. 
     In the invention recited in the second aspect hereof, in addition to the first aspect, the outer tube is formed of an iron-based material while the inner tube is formed of a copper-based material, and the gap is provided at normal temperature between the inner peripheral surface of the outer tube and the outer peripheral surface of the inner tube such that the outer peripheral surface of the inner tube is brought into close contact with the inner peripheral surface of the outer tube in response to pouring of the molten metal. The close contact and the gap are achieved or implemented by virtue of the thermal conductivity of the copper being about 1.5 times the thermal conductivity of the iron. 
     In response to the pouring of the molten metal, the inner tube is brought into close contact with the outer tube except for the annular tube, so that heat of the molten metal can be sequentially transmitted smoothly to the outer tube and then to the inner tube to be absorbed by the cooling medium. 
     After the molten metal solidifies, the core pin is removed from the casting as part of mold release operation. Because the inner tube continues to be cooled by the cooling medium, a gap is formed again between the outer tube and the inner tube. After that, the outer tube is not cooled any longer by the cooling medium although the inner tube continues to be cooled by the cooling medium. Thus, the cooling of the outer tube becomes much slower, so that the outer tube is supplied to a next casting process while still remaining at high temperature. 
     Prior to the casting, a liquid mold release agent is applied to the outer tube. This liquid mold release agent is sufficiently dried, prior to next pouring of the molten metal, by potential heat of the outer tube. If the outer tube is low in temperature, then the liquid mold release agent is scarcely dried. If the molten material is poured in this state, a liquid component included in the mold release agent would be evaporated by the heat of the molten metal, so that casting defects, such as blow holes, may be undesirably produced. The present invention can avoid such defects because there is no fear of gas being produced from the mold release agent, with the result that casting quality can be significantly enhanced. 
     In the invention recited in the third aspect hereof, the inner tube is segmented in the zone where heat transfer is required and the zone where heat retention is required, and the zone where heat transfer is required is formed of a material of a higher thermal conductivity than the material of the zone where heat retention is required. The zone where heat transfer is required and the zone where heat retention is required are integrally joined to each other. Because the zone where heat retention is required has a low thermal conductivity, it can achieve a desired heat retaining effect. Further, because the zone where heat transfer is required has a high thermal conductivity, it can achieve great heat transfer. 
     In the invention recited in the fourth aspect hereof, in addition to the third aspect, the outer tube is formed of an iron-based material, and the zone of the inner tube where heat transfer is required is formed of a copper-based material. The gap is provided at normal temperature between the inner peripheral surface of the outer tube and the outer peripheral surface of the zone where heat transfer is required such that the outer peripheral surface of the zone where heat transfer is required is brought into close contact with the inner peripheral surface of the outer tube in response to pouring of the molten metal. In response to the pouring of the molten metal, the inner tube is brought into close contact with the outer tube except for the annular tube, so that heat of the molten metal can be sequentially transmitted smoothly to the outer tube and then 5 to the inner tube to be absorbed by the cooling medium. 
     After the molten metal solidifies, the core pin is removed from the casting as part of mold release operation. Because the inner tube is cooled by the cooling medium, a gap is formed again between the outer tube and the inner tube. After that, the outer tube is not cooled any longer by the cooling medium although the inner tube continues to be cooled by the cooling medium. Thus, the cooling of the outer tube becomes much slower, so that the outer tube is supplied to a next casting process while still remaining at high temperature. 
     Prior to the casting, a liquid mold release agent is applied to the outer tube. This liquid mold release agent is sufficiently dried, prior to next pouring of the molten metal, by potential heat of the outer tube. If the outer tube is low in temperature, then the liquid mold release agent is scarcely dried. If the molten material is poured in this state, a liquid component included in the mold release agent would be evaporated by the heat of the molten metal, so that casting defects, such as blow holes, may be undesirably produced. The present invention can avoid such defects because there is no fear of gas being produced from the mold release agent, with the result that casting quality can be significantly enhanced. 
     In the invention recited in the fifth aspect hereof, in addition to the first aspect, the heat insulating chamber is provided near the small thickness portion of the product. In case a blow hole or the like has been formed in the general thickness portion of the product, greater in thickness than the small thickness portion of the product, at the time of machining of a screw hole or the like, inconveniences, such as bending of a drill during machining and pressure leakage, would be introduced. Thus, it is desirable that a final solidification portion be formed in a thicknesswise middle region of the great thickness portion of the product. For that purpose, it is necessary to rapidly cool a surface layer that contacts the mold. On the other hand, it is difficult to fill the molten material into the small thickness portion of the product, and thus, a heat insulating layer is provided to keep warm the small thickness portion. Thus, the present invention can cause cooling performance to differ around a single cooling pin although the thickness of the product varies. 
     In the invention recited in the sixth aspect hereof, in addition to the first aspect, the core pin of the present invention is a device which is mounted to the mold for forming, around the outer tube, a small thickness portion of a product and a general thickness portion greater in thickness than the small thickness portion, and in which the outer tube is inserted in the cavity of the mold in partial contact with the mold. The heat insulating chamber is provided near the small thickness portion and in the region of the outer tube where the outer tube contacts the mold. 
     In case a blow hole or the like has been formed in the general thickness portion of the product, greater in thickness than the small thickness portion of the product, at the time of machining of a screw hole or the like, inconveniences, such as bending of a drill during machining and pressure leakage, would be introduced. Thus, it is desirable that a final solidification portion be formed in a thicknesswise middle region of the great thickness portion of the product. For that purpose, it is necessary to rapidly cool a surface layer that contacts the mold. On the other hand, it is difficult to fill the molten material into the small thickness portion of the product, and thus, a heat insulating layer is provided to keep warm the small thickness portion. Thus, the present invention can cause cooling performance to differ around a single cooling pin although the thickness of the product varies. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded view showing a preferred embodiment of a core pin of the present invention; 
         FIG. 2  is a sectional view of the core pin shown in  FIG. 1 ; 
         FIG. 3  is an enlarged sectional view taken along line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a sectional view showing a state where a gap has been formed between an outer tube and an inner tube after pouring of a molten metal; 
         FIG. 5  is an exploded view of a modification of the core pin shown in  FIG. 1 ; 
         FIG. 6  is a sectional view of the modification of the core pin shown in  FIG. 5 ; 
         FIG. 7  is an enlarged sectional view taken along line  7 - 7  of  FIG. 6 ; 
         FIG. 8  is a sectional view showing a state where a gap has been formed between the outer tube and the inner tube after pouring of the molten metal; 
         FIG. 9  is a perspective view of a cylinder block; 
         FIG. 10  is a partly enlarged sectional view of a cylinder block, 
         FIG. 11  is a partly enlarged sectional view of a cylinder block casting mold; 
         FIG. 12  is a sectional view showing a state where the molten metal has been poured into a cavity of the mold shown in  FIG. 11 ; 
         FIG. 13  is an exploded sectional view showing a state where the mold has been released from the state of  FIG. 12 ; 
         FIG. 14  is an enlarged sectional view taken along line  14 - 14  of  FIG. 13 ; 
         FIG. 15  is a sectional view of a cylinder head; 
         FIG. 16  is a sectional view of a mold for casting the cylinder head shown in  FIG. 15 ; 
         FIG. 17  is a partly enlarged sectional view of the cylinder head casting mold shown in  FIG. 16 ; and 
         FIG. 18  is a sectional view of an outer pin in a conventionally-known core pin. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, preferred embodiments of the present invention will be described hereinbelow with reference to the accompanying drawings. Inventions recited in claims  1  and  2  are based on  FIGS. 1 to 4 , inventions recited in claims  3  and  4  are based on  FIGS. 5 to 8 , an invention recited in claim  5  is based on  FIGS. 9 to 14 , and an invention recited in claim  6  is based on  FIGS. 15 to 17 . 
     Embodiment 
     As shown in  FIG. 1 , a preferred embodiment of a core pin  10  of the present invention comprises: an outer tube  11  in the form of a hollow tube closed at its distal end; an inner tube  20  inserted in the outer tube  11  with its outer peripheral surface  21  contacting the inner peripheral surface  12  of the outer tube  11 ; and a cooling medium pipe  30  inserted in the inner tube  20 , with a predetermined distance (i.e., gap  32  indicated in  FIG. 3 ) kept between the inner peripheral surface  23  of the inner tube  20  and the outer peripheral surface  33  of the cooling medium pipe  30 , for supplying a cooling medium into the inner tube  20 . 
     The inner tube  20  has a plurality of, e.g. three, annular grooves  22  formed in the outer peripheral surface  21 . Such annular grooves  22  can be formed in the outer peripheral surface  21  by applying a cutting tool from radially outside of the inner tube  20 . Unlike the boring method, this method can provide the annular grooves  22  at desired positions. Also, the instant embodiment can eliminate a need to care about flexure of the cutting tool, and thus, a satisfactory finishing accuracy of the annular grooves  22  can be achieved. 
       FIG. 2  shows a finished form of the core pin  10 . The annular grooves  22  formed in the outer peripheral surface of the inner tube  20  are each closed or covered with the inner peripheral surface of the outer tube  11  so that heat insulating chambers  24  each of a rectangular sectional shape are formed. A cooling medium, such as water, is caused to flow through the interior of the central cooling medium pipe  30  toward a distal end portion  31 , so that the cooling medium is supplied through the distal end portion  31  into the inner tube  20 . Then, the cooling medium flows backward through the gap  32  between the cooling medium pipe  30  and the inner tube  20  to thereby compulsorily cool the inner tube  20 . 
     At normal temperature, a gap  25  is provided between the inner peripheral surface  12  of the outer tube  11  and the outer surface  21  of the inner tube and a gap  32  is provided between the inner peripheral surface  23  of the inner tube  20  and the outer peripheral surface  33  of the cooling medium pipe  30 , as shown in  FIG. 3 . The inner tube  20  is preferably formed of copper alloy, and a heat expansion coefficient of the copper alloy is 17.7×10 −6  (mm/mm·K) while a thermal conductivity of the copper alloy is 372 (W/m·K). 
     The outer tube  11  is preferably formed of steel, and a heat expansion coefficient of the hot tool steel is 12.1×10 −6  (mm/mm·K) while a thermal conductivity of the hot tool steel is 372 (W/m·K). 
     In  FIG. 3 , if the outer tube  11  is surrounded by high-temperature molten aluminum of 660° C. or over, the outer tube  11  gets hot, in response to which the temperature of the inner tube  20  increases. Let it be assumed that the outer tube  11 , whose inner diameter is 10 mm at normal temperature, has reached 400 C. 
     The inner peripheral surface of the outer tube  11  has a circumference (peripheral length) of 10p (mm) at normal temperature (25° C.). At 400° C., the inner peripheral surface has a circumference of 10.045π (mm), which can be determined by performing a calculation of 10π (1+12.1×10 −6 ×(400-25))=10π×1.0045=10.045π. By converting the circumference into a diameter, it is determined that the inner diameter of the outer tube  11  is 10.045 mm at 400° C. 
     The inner tube  20 , on the other hand, is cooled by the cooling medium, but it is expected that, at a time point immediately after pouring of the molten metal, the temperature of the inner tube  12  increases up to about 400° C. that is generally the same temperature as the inner peripheral surface of the outer tube  11 . Let&#39;s assume here that the outer diameter of the inner tube  20  is 9.98 mm at normal temperature and the inner tube  20  has reached a temperature of 400° C. 
     The outer peripheral surface of the inner tube  20  has a circumference of 9.98π (mm) at normal temperature (25° C.). At 400° C., the outer peripheral surface has a circumference of 10.046π (mm), which can be determined by performing a calculation of 9.98π(1+17.7×10 −6 ×(400−25))=9.98π×1.0066=10.046π. By converting the circumference into a diameter, it is determined that the outer diameter of the inner tube  20  is 10.046 mm at 400° C. Such an outer diameter of the inner tube  20  is very approximate to the inner diameter (10.045 mm) of the outer tube  11 . 
     By a calculation of (10−9.98)/2=0.01, a gap  25  of 1/100 mm is secured between the outer tube  11  and the inner tube  20  at normal temperature. 
     After the pouring of the molten metal, the gap disappears due to a difference between the thermal expansion coefficients, so that heat transfer from the outer tube  11  to the inner tube  20  becomes active or is promoted and thus a temperature increase of the outer tube  11  can be suppressed. 
     The following describe, with reference to  FIGS. 5 to 8 , a modification or modified embodiment of the core pin of the present invention. As shown in  FIG. 5 , the modification of the core pin  10 B comprises: the outer tube  11  in the form of a hollow tube closed at its distal end an inner tube  20 B inserted in the outer tube  11  with its outer peripheral surface  21  contacting the inner peripheral surface  12  of the outer tube  11   i  and a cooling medium pipe  30  inserted in the inner tube  20 B, with a predetermined distance (i.e., gap  32  indicated in  FIG. 7 ) kept between the inner peripheral surface  23  of the inner tube  20 B and the outer peripheral surface  33  of the cooling medium pipe  30 , for supplying a cooling medium into the inner tube  20 B. 
     The outer tube  11  is formed of hot tool steel whose heat expansion coefficient is 12.1×10 −6  (mm/mm·K). Further, because of requirements of a casting, the outer tube  11  is segmented in a zone Z 1  where heat transfer is required in an axial direction of the tube and a zone Z 2  where heat retention is required. Of the inner tube  20 B, a portion of the zone Z 1  where heat transfer is required is in the form of a cap  26  formed of copper, and a part corresponding to the zone Z 2  where heat retention is required is in the form of a stainless pipe  27 . More specifically, the cap  26  is fitted over and brazed to an end portion of the stainless pipe  27 , so that the cap  26  and the stainless pipe  27  are integrated together. The other structural elements in the modification are identical to, and thus depicted by the same reference numerals as, those in the embodiment of  FIG. 1  and will not be described here to avoid unnecessary duplication. 
       FIG. 6  shows a finished form of the core pin  10 B. The annular grooves  22  formed in the outer peripheral surface of the inner tube  20 B are each closed or covered with the inner peripheral surface  12  of the outer tube  11  so that the heat insulating chamber  24  of a rectangular sectional shape is formed. A cooling medium, such as water, is caused to flow through the interior of the central cooling medium pipe  30  toward the distal end portion  31 , so that the cooling medium is supplied through the distal end portion  31  into the inner tube  20 . Then, the cooling medium flows backward through the gap between the cooling medium pipe  30  and the inner tube  20 B to thereby compulsorily cool the inner tube  20 B. The outer tube  11  is cooled by the inner tube  20 B. 
     The copper alloy forming the cap  26  has a thermal conductivity of 372 (W/m·K), and the stainless tube  27  has a thermal conductivity of 16.7 (W/m·K) and is SUS304. Because the thermal conductivity of the stainless tube  27  is 1/20 (one twentieth) or less of the thermal conductivity of the cap  26  and additionally the stainless tube  27  has the heat insulating chambers  24 , the stainless tube  27  has a low thermal conductivity property. Namely, the stainless tube  27  has a superior heat retention performance and thus is well suited as the zone Z 2  where heat retention is required. Further, because the thermal conductivity of the cap  26  is twenty times or more of the thermal conductivity of the stainless tube  27 , the cap  26  has a superior thermal conductivity property and thus is well suited as the zone Z 1  where heat transfer is required. 
     At normal temperature, a gap  25  of about 1/100 (0.01 mm) is provided between the outer tube  11  and the cap  26 , as shown in  FIG. 7 . Further, in response to pouring of the molten metal, the cap  26  is brought into close contact with the outer tube  11  due to a difference between the thermal expansion coefficients as shown in  FIG. 8 , so that heat transfer from the outer tube  11  to the cap  26  becomes active and thus a temperature increase of the outer tube  11  can be suppressed. 
     Further,  FIG. 9  shows a cylinder block  40  that is a typical example of a casting. The cylinder block  40  includes a water jacket  42  around the periphery of a cylinder liner  41 , a plurality of (ten in the illustrated example) of bolt holes  43 , and an oil passage  44  located outside the bolt holes  43 . 
     Further, as shown in  FIG. 10 , each of the bolt holes  43  has an internal thread portion  45  formed in a distal end portion of the bolt hole  43 . Thus, the distal end portion of the bolt hole  43  has a smaller diameter than the other portion of the bolt hole  43 . Consequently, a thickness T2 in the neighborhood of the internal thread portion  45  is greater than a thickness T1 of the other portion. 
     Next, a description will be given about a construction of a mold for casting the aforementioned cylinder block  40 . As shown in  FIG. 11 , the cylinder block casting mold  50  includes a side mold  51  surrounding the side surface of the cylinder block, and a movable mold  52  put over the side mold  51 . The movable mold  52  has a water-jacket forming section  53  and an oil-passage forming section  54  each projecting from the body of the mold  52 , and the core pin device  10 B is provided between the water-jacket forming section  53  and the oil-passage forming section  54 . The movable mold  52  also has a cavity  55  surrounding the core pin device  10 B, and a width T2 of a gap in a distal end portion of the cavity  55  is greater than a width T1 of the other portion of the cavity  55 . 
     Because the heat insulating chambers  24  are provided between the outer tube  11  and the inner tube  20 B, heat transfer is limited in a region of the gap width T1 when molten aluminum is poured into the cavity  55 . In a region of the gap width T2, however, heat transfer is promoted because the cap  26  is formed of copper having a high thermal conductivity. 
     Generally, if a blow hole exists near a surface layer of a great thickness portion, the following inconveniences would occur. Namely, if a screw hole or the like is machined, the screw hole would communicate with the blow hole to cause an unwanted pressure leakage. Also, a drill would bend during the machining. 
     Therefore, according to the present invention, the great thickness portion, i.e. general thickness portion, is cooled rapidly. Then, a chill layer is formed in the surface layer. The chill layer has not only good workability but also fine density, and thus, even if a blow hole exists in a thicknesswise middle region, there is no fear of the blow hole undesirably communicating with a hole. Besides, there is no fear of the drill undesirably bending. Thus, in the present invention, the great thickness portion, i.e. general thickness portion, is cooled rapidly with a view to causing the thicknesswise middle region to become a final solidification portion. 
     On the other hand, it is difficult to fill the molten metal into a small thickness portion because a cavity space is narrow. If the solidification progresses before the molten metal is filled into every corner of the cavity space, unwanted underfill tends to occur. Thus, the present invention is constructed to keep warm a small thickness portion of a product by means of the heat insulating chambers and thereby suppress a temperature decrease of the molten metal. Keeping warm the small thickness portion as above can secure a molten metal flow and thereby prevent occurrence of underfill. 
     Namely, in case a blow hole or the like has been formed in a general thickness portion of a product, greater in thickness than a small thickness portion of the product, during machining of a screw hole or the like, introduce inconveniences, such as bending of a drill during machining and pressure leakage, would be introduced. Thus, it is desirable that a final solidification portion be formed in a thicknesswise middle region of a great thickness portion of the product. For that purpose, it is necessary to rapidly cool a surface layer that contacts the mold. On the other hand, it is difficult to fill the molten material into a small thickness portion of a product, and thus, a heat insulating layer is provided to keep warm the small thickness portion. Thus, the present invention can cause cooling performance to differ around a single cooling pin although the thickness of the product varies, for example, in the range of T1-T2. 
     After the molten metal has solidified, the side mold  51  and the movable mold  52  are detached from the cylinder block  40  as indicated by arrows in  FIG. 13 . 
     For a period from the time of molten metal pouring to an initial cooling stage, heat of the molten metal actively transfers to the outer tube  11  and the cap  26 , and then the cap  26  is kept in close contact with the outer tube  11  due to a difference between the thermal expansion coefficients. 
     For a period from an end stage of the casting cycle to mold opening, the heat transfer (i.e., heat absorption) to the outer tube decreases dramatically due to temperature decrease or solidification of the molten metal. The cap  26 , on the other hand, is cooled by the cooling medium. 
     Let&#39;s now assume that the temperature of the inner peripheral surface of the outer tube  11  has decreased to 300° C. At 300° C., the inner peripheral surface has a circumference of 10.033π (mm), which can be determined by performing a calculation of 10π(1+12.1×10 −6 ×(300−25))=10π×1.0033=10.033π. The circumference can be converted into a diameter of 10.033 mm, which is indicative of an inner diameter of the outer tube  11  at 300° C. 
     Because the cap  26  is cooled by the cooling medium, the cap  26  is expected to have a temperature of about 100° C. At 100° C., the outer peripheral surface of the cap  26  has a circumference of 9.993π (mm), which can be determined by performing a calculation of 9.98π(1+17.7×10 −6 ×(100−25))=9.993π. By converting the circumference into a diameter, it is determined that the outer diameter of the cap  26  is 9.993 mm at 100° C. 
     By a calculation of (the inner diameter of the outer tube—the outer diameter of the cap)/2=(10.033−9.993)/2=0.02, a gap  25  of 0.02 mm is formed as shown in (b) of  FIG. 14 . Because this gap  25  performs a heat insulating function or action, only the cap  26  is cooled by the cooling medium, so that the gap  25  gets bigger. However, the outer tube  11  does not decrease in temperature so much because of the presence of the gap  25 . 
     In  FIG. 13 , the outer tube  11  is supplied to a next casting process while still remaining at high temperature. Prior to the casting, a liquid mold release agent is applied to the outer tube  11 . This liquid mold release agent is sufficiently dried, prior to next pouring of the molten metal, by potential heat of the outer tube  11 . 
     If the outer tube  11  is low in temperature, then the liquid mold release agent is scarcely dried. If the molten material is poured in this state, a liquid component included in the mold release agent is evaporated by the heat of the molten metal, so that casting defects, such as blow holes, may be undesirably produced. 
     With the present invention, however, the mold release agent can be sufficiently dried by the potential heat of the outer tube prior to next pouring of the molten metal and thus there is no fear of gas being produced from the mold release agent, with the result that casting quality can be significantly enhanced. 
     In  FIG. 5 , the modified inner tube  20 B comprises the cap  26  formed of copper alloy, and the stainless pipe  27 . The heat expansion coefficient of the copper alloy is 17.7×10 −6  (mm/mm·K), while the heat expansion coefficient of the stainless pipe  27  is 17.6×10 −6  (mm/mm·K). There is almost no difference in heat expansion coefficient between the stainless pipe  27  and the cap  26 . 
     As a consequence, the same action as described above in relation to (a) and (b) of  FIG. 14  occurs between the iron-based outer tube  11  and the stainless pipe  27 . Namely, the iron-based outer tube  11  and the stainless pipe  27  are brought into close contact each other in response to pouring of the molten metal as shown in (a) of  FIG. 14  and the gap  25  is formed again after solidification of the casting as shown in (b) of  FIG. 14 , so that a high temperature of the outer tube  11  can be maintained. 
     The following describe an instance where the basic principles of the present invention are applied to a cylinder head that is another typical example of a casting. As shown in  FIG. 15 , the cylinder head  60  includes first to fifth shaft support sections  61  to  65  for supporting cam shafts. As shown, the first shaft support section  61  and the fifth shaft support section  65  have a great volume and thus will hereinafter be referred to as “general thickness portions”. The second to fourth shaft support sections  62  to  64 , on the other hand, have a smaller volume than the general thickness portions and thus will hereinafter be referred to as “small thickness portions of a product” or “product&#39;s small thickness portions”. 
     A cylinder head casting mold  70  shown in  FIG. 16  is used to cast such a cylinder head  60 . Namely, the cylinder head casting mold  70  comprises lower and upper molds  71  and  72 , and first to fourth protrusions  73  to  76  are provided on the upper mold  72 . 
     A first (leftmost in  FIG. 16 ) cavity  81  defined by the first protrusion  73  and a fifth (rightmost in  FIG. 16 ) cavity  85  defined by the fourth protrusion  76  are used to form the general thickness portions. Further, a second cavity  82  defined between the first protrusion  73  and the second protrusion  74 , a third cavity  83  defined between the second protrusion  74  and the third protrusion  75  and a fourth cavity  84  defined between the third protrusion  75  and the fourth protrusion  76  are used to form the small thickness portions of a product. 
     Further, core pin devices  10 C and  10 D are inserted through the cylinder head casting mold  70  from left and right sides respectively of the cylinder head casting mold  70  so as to pass through the first to fifth shaft support sections  61  to  65 . 
     The following detail, with reference to  FIG. 17 , the left core pin  10 C and the mold  70 . However, the right core pin  10 D and relationship between the right core pin  10 D and the mold  70  will not be described here because the right core pin  10 D is identical in construction to the left core pin  10 C. 
     As shown in  FIG. 17 , the core pin  10 C comprises the outer tube  11 , the inner tube  20  and the cooling medium pipe  30  similarly to the aforementioned, but the annular groove  22  is provided in regions corresponding to the second cavity  82  and contacting the first and second protrusions  73  and  74  without being provided in a region corresponding to the first cavity  81 . 
     Namely, the core pin  10 C is mounted to the mold  70  capable of forming, around the outer tube  11  of the core pin  10 C, a product&#39;s small thickness portion (formed by the second cavity  82 ) and a general thickness portion (formed by the second cavity  81 ) greater in thickness than the product&#39;s small thickness portion, and the outer tube  11  is inserted in the mold cavity in partial contact with the mold (first and second protrusions  73  and  74 ). Further, the heat insulating chamber  24  is provided near the small thickness portion corresponding to the second cavity  82  and in a region of the outer tube where the outer tube contacts the mold (more specifically, the first and second protrusions  73  and  74 ). 
     In case a blow hole or the like has been formed in a general thickness portion of a product, greater in thickness than a small thickness portion of the product, during machining of a screw hole or the like, inconveniences, such as bending of a drill during machining and pressure leakage, would be introduced. Thus, it is desirable that a final solidification portion be formed in a thicknesswise middle region of the great thickness portion of the product. For that purpose, it is necessary to rapidly cool a surface layer that contacts the mold. On the other hand, it is difficult to fill the molten metal into the product&#39;s small thickness portion, thus, the present invention is constructed to keep warm the product&#39;s small thickness portion by means of the heat insulating layer. As a result, the present invention can cause cooling performance to differ around the single cooling pin although the thickness of the product varies. 
     Whereas the embodiments of the core pin of the present invention have been described as applied to a casting process of a cylinder block or cylinder head, the present invention may be applied to casting processes of other castings. 
     INDUSTRIAL APPLICABILITY 
     The core pin of the present invention is well suited for application to casting of cylinder blocks. 
     LEGEND 
     
         
           10 ,  10 B,  10 C,  10 D . . . core pin,  11  . . . outer tube,  12  . . . inner peripheral surface of the outer tube,  20 ,  20 B . . . inner tube,  21  . . . outer peripheral surface of the inner tube,  22  . . . annular groove,  23  . . . inner peripheral surface of the inner tube,  24  . . . heat insulating chamber,  25  . . . gap between the outer tube and the inner tube,  30  . . . cooling medium pipe,  32  . . . gap between the inner tube and the cooling medium pipe,  33  . . . outer peripheral surface of the cooling medium pipe,  50  . . . mold (cylinder block casting mold),  70  . . . mold (cylinder head casting mold), Z 1  . . . zone where heat transfer is required, Z 2  . . . zone where heat retention is required