Patent Publication Number: US-11648711-B2

Title: Multilayer core molding method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a multilayer core molding method. 
     This application is based on and claims priority to Japanese patent application No. 2019-220554, filed on Dec. 5, 2019, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     Known methods for molding multilayer cores of golf balls include techniques to arrange, on spherical inner cores, outer core materials, which are discrete from the inner cores and molded to have recesses for receiving the inner cores, and subsequently close molds to thereby compression-mold the outer core materials around the inner cores in accordance with substantially hemispherical cavity surfaces designed to mold outer cores (e.g., Patent Literature 1, which is hereinafter referred to as PTL1). 
     CITATION LIST 
     Patent Literature 
     PTL 1: U.S. Pat. No. 6,436,327 B1 
     SUMMARY 
     The aforementioned existing technique, however, may cause eccentricity in the inner cores. 
     It would be helpful to provide a multilayer core molding method capable of preventing the eccentricity in the inner cores. 
     A multilayer core molding method for molding a multilayer core of a golf ball according to an embodiment of the present disclosure includes: 
     an upstream process of obtaining an intermediate molded body; and 
     a downstream process of performing molding using a downstream process molding apparatus, wherein 
     the intermediate molded body includes:
         an inner core; and   an unvulcanized or semi-vulcanized first outer core material that covers only part of a surface of the inner core and is integrated with the inner core, and       

     the downstream process molding apparatus includes:
         a first downstream process mold including a substantially hemispherical first downstream process mold cavity surface;   a second downstream process mold including a substantially hemispherical second downstream process mold cavity surface; and   an intermediate plate including a substantially hemispherical intermediate plate cavity surface configured to be arranged facing the first downstream process mold cavity surface, and a substantially hemispherical intermediate plate projecting surface configured to be arranged facing the second downstream process mold cavity surface, and wherein       

     the downstream process includes:
         a second outer core arrangement step of arranging a second outer core material on the second downstream process mold cavity surface or the intermediate plate projecting surface;   an intermediate molded body arrangement step of arranging the intermediate molded body on the intermediate plate cavity surface or the first downstream process mold cavity surface; and   a preparatory molding step, performed after the second outer core arrangement step and the intermediate molded body arrangement step, of compression-molding the second outer core material and the intermediate molded body, in a state in which the first downstream process mold, the second downstream process mold, and the intermediate plate are closed against each other.       

     In the multilayer core molding method according a preferred embodiment, 
     the downstream process further includes:
         an intermediate plate removal step, performed after the preparatory molding step, of removing the intermediate plate; and   a raw core forming step, performed after the intermediate plate removal step, of assembling the second outer core material to the intermediate molded body to form a raw multilayer core.       

     In the multilayer core molding method according to another preferred embodiment, 
     in the intermediate plate removal step, while the intermediate plate is removed, the second outer core material is held on the second downstream process mold cavity surface, and the intermediate molded body is held on the first downstream process mold cavity surface, and 
     in the raw core forming step, the first downstream process mold and the second downstream process mold are closed against each other to assemble the second outer core material to the intermediate molded body. 
     In the multilayer core molding method according to still another preferred embodiment, 
     in the preparatory molding step, part of the first outer core material of the intermediate molded body sticks out from between the first downstream process mold cavity surface and the intermediate plate cavity surface to form a first flash, and part of the second outer core material sticks out from between the intermediate plate projecting surface and the second downstream process mold cavity surface to form a second flash. 
     In the multilayer core molding method according to still another preferred embodiment, 
     in the preparatory molding step, a temperature T 1  of the first downstream process mold, a temperature T 2  of the second downstream process mold, and a temperature T 3  of the intermediate plate satisfy the relation T 1 ≥T 2 ≥T 3 . 
     In the multilayer core molding method according to still another preferred embodiment, 
     in the preparatory molding step, a temperature T 1  of the first downstream process mold, a temperature T 2  of the second downstream process mold, and a temperature T 3  of the intermediate plate satisfy the relation T 1 =T 2 &gt;T 3 . 
     In the multilayer core molding method according to still another preferred embodiment, in the preparatory molding step, a temperature T 1  of the first downstream process mold satisfies 80° C.&lt;T 1 &lt;170° C. 
     In the multilayer core molding method according to still another preferred embodiment, 
     in the preparatory molding step, a temperature T 1  of the first downstream process mold, a temperature T 2  of the second downstream process mold, and a temperature T 3  of the intermediate plate satisfy the relation T 1 &gt;T 2 &gt;T 3 . 
     In the multilayer core molding method according to still another preferred embodiment, 
     the downstream process further includes
         a vulcanization step, performed after the raw core forming step, of vulcanizing the raw multilayer core in a state in which the first downstream process mold and the second downstream process mold are closed against each other.       

     In the multilayer core molding method according to still another preferred embodiment, 
     a temperature T 1 ′ of the first downstream process mold and a temperature T 2 ′ of the second downstream process mold in the vulcanization step are respectively equal to a temperature T 1  of the first downstream process mold and a temperature T 2  of the second downstream process mold in the preparatory molding step. 
     In the multilayer core molding method according to still another preferred embodiment, 
     a time period t of the compression molding in the preparatory molding step is shorter than a time period t′ of the vulcanization in the vulcanization step. 
     The present disclosure provides a multilayer core molding method that prevents the eccentricity in the inner cores. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG.  1    illustrates an inner core arrangement step in an upstream process of a multilayer core molding method according to a first embodiment of the present disclosure; 
         FIG.  2    is a partially enlarged view of  FIG.  1   ; 
         FIG.  3    illustrates a covering step in the upstream process of the multilayer core molding method according to the first embodiment of the present disclosure; 
         FIG.  4    is a partially enlarged view of  FIG.  3   ; 
         FIG.  5    illustrates an intermediate molded body removal step in the upstream process of the multilayer core molding method according to the first embodiment of the present disclosure; 
         FIG.  6    is a perspective view of the intermediate molded body of  FIG.  5   ; 
         FIG.  7    illustrates a second outer core arrangement step in a downstream process of the multilayer core molding method according to the first embodiment of the present disclosure; 
         FIG.  8    illustrates an intermediate molded body arrangement step in the downstream process of the multilayer core molding method according to the first embodiment of the present disclosure; 
         FIG.  9    illustrates a preparatory molding step in the downstream process of the multilayer core molding method according to the first embodiment of the present disclosure; 
         FIG.  10    illustrates an intermediate plate removal step in the downstream process of the multilayer core molding method according to the first embodiment of the present disclosure; 
         FIG.  11    illustrates a raw core forming step and a vulcanization step in the downstream process of the multilayer core molding method according to the first embodiment of the present disclosure; 
         FIG.  12    illustrates a multilayer core removal step in the downstream process of the multilayer core molding method according to the first embodiment of the present disclosure; 
         FIG.  13    is a perspective view of a continuous molded body including multilayer cores of  FIG.  12   ; 
         FIG.  14    is a sectional view of one of the multilayer cores of  FIG.  12   ; and 
         FIG.  15    illustrates the second outer core arrangement step and the intermediate molded body arrangement step in the downstream process of the multilayer core molding method according to a second embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of a multilayer core molding method according to the present disclosure will be described by illustration with reference to the drawings. 
     The same components in the figures are denoted by the same reference numerals. 
     With reference to  FIGS.  1  to  14   , a description is given of the multilayer core molding method according to a first embodiment of the present disclosure. 
     The multilayer core molding method of the present embodiment is used, for example, to mold a multilayer core  3  of a golf ball as illustrated in  FIG.  14   . The multilayer core  3  is configured to constitute part of the golf ball. The multilayer core  3  illustrated in  FIG.  14    includes a spherical inner core  31  and an outer core  32 , which is arranged adjacent to the inner core  31  on an outer side in a circumferential direction of the inner core  31 . 
     Although the multilayer core  3  in the example of  FIG.  14    is configured as a dual-layer core consisting of two layers, namely, the inner core  31  and the outer core  32 , the multilayer core  3  molded by the multilayer core molding method of the present embodiment may be configured as a multilayer core consisting of three or more layers. For example, the inner core  31  may consist of a plurality of core layers. Further, the multilayer core  3  may include additional one or more core layers on the outer side in the circumferential direction of the outer core  32 . 
     The inner core  31  and the outer core  32  are each formed of rubber. For example, butadiene rubber is preferably used as rubber that forms the inner core  31  and the outer core  32 . Preferably, rubber that forms the inner core  31  and rubber that forms the outer core  32  have compositions different from each other. Preferably, rubber that forms the inner core  31  and rubber that forms the outer core  32  have hardnesses different from each other. 
     Additionally, the golf ball (which is not illustrated) having the multilayer core  3  may have any configuration, and examples of configurations include one or more intermediate layers arranged on the outer side in the circumferential direction of the multilayer core  3 , and a cover arranged on the outer side in the circumferential direction of the one or more intermediate layers. The intermediate layers are formed of, for example, resin. The cover is formed of, for example, urethane or ionomer. 
     To start with, steps included in the multilayer core molding method according to the first embodiment of the present disclosure will be schematically described. 
     The multilayer core molding method according to the first embodiment of the present disclosure includes an upstream process ( FIGS.  1  to  6   ) and a downstream process ( FIGS.  7  to  13   ). 
     [Upstream Process] 
     Firstly, the upstream process ( FIGS.  1  to  6   ) will be schematically described. In the upstream process, molding is performed using an upstream process molding apparatus  1 . In the upstream process, intermediate molded bodies  33 , which will be described later, are obtained. The upstream process includes an inner core arrangement step, a covering step, and an intermediate molded body removal step. 
     As illustrated in  FIG.  1   , the upstream process molding apparatus  1  includes a first upstream process mold  11  and a second upstream process mold  12 . 
     The first upstream process mold  11  and the second upstream process mold  12  are configured to be arranged facing each other. In the example of  FIG.  1   , the first upstream process mold  11  and the second upstream process mold  12  are configured to be arranged facing each other in a vertical direction, and the first upstream process mold  11  is configured to be positioned on an upper side in the vertical direction with respect to the second upstream process mold  12 . However, the first upstream process mold  11  and the second upstream process mold  12  may be configured to be arranged facing each other in any direction other than the vertical direction, and the first upstream process mold  11  may be configured to be positioned on any side other than the upper side in the vertical direction with respect to the second upstream process mold  12 . 
     Herein, a direction (e.g., the vertical direction in the example of the figure) in the upstream process molding apparatus  1  in which the first upstream process mold  11  and the second upstream process mold  12  are arranged facing each other is referred to as an “upstream process mold axial direction (EAD).” On the other hand, a direction (e.g., a horizontal direction in the example of the figure) perpendicular to the upstream process mold axial direction (EAD) is referred to as an “upstream process mold perpendicular-to-axis direction (EPD).” 
     The first upstream process mold  11  has one or more (e.g., in the example of the figure, a plurality of) first upstream process mold cavity surfaces  110 . The first upstream process mold cavity surfaces  110  are configured to be arranged facing second upstream process mold cavity surfaces  120  of the second upstream process mold  12 . The first upstream process mold cavity surfaces  110  are each configured in a hollow shape which is open on a side thereof closer to the corresponding second upstream process mold cavity surface  120  (e.g., on the lower side in the vertical direction in the example of  FIG.  1   ). 
     The second upstream process mold  12  has one or more (e.g., in the example of the figure, a plurality of) second upstream process mold cavity surfaces  120 . The second upstream process mold cavity surfaces  120  are configured to be arranged facing the first upstream process mold cavity surfaces  110  of the first upstream process mold  11 . The second upstream process mold cavity surfaces  120  are each configured in a hollow shape which is open on a side thereof closer to the corresponding first upstream process mold cavity surface  110  (e.g., on the upper side in the vertical direction in the example of  FIG.  1   ). 
     As illustrated in  FIG.  3   , the upstream process molding apparatus  1  is configured, in a state in which the first upstream process mold  11  and the second upstream process mold  12  are closed against each other, to define upstream process mold cavities  14  between the first upstream process mold cavity surfaces  110  and the second upstream process mold cavity surfaces  120 . 
     —Inner Core Arrangement Step— 
     In the inner core arrangement step, the inner cores  31  that have been vulcanized are arranged on the second upstream process mold cavity surfaces  120  of the second upstream process mold  12  ( FIGS.  1  and  2   ). 
     Herein, the term “on” in “ . . . arranged on the second upstream process mold cavity surfaces  120 ” refers to arrangement that causes contact with the second upstream process mold cavity surfaces  120 . 
     The inner cores  31  are formed of rubber, as described above. The inner cores  31  are vulcanized in advance before the inner core arrangement step, and the entire inner cores  31  have been vulcanized. The inner cores  31  are each configured in a spherical shape. Preferably, each inner core  31 , after being vulcanized and before subjected to the inner core arrangement step, is polished into the spherical shape. 
     Note that the term “vulcanized” herein refers to a state in which vulcanization is completely completed. One method of determining, for a certain member (e.g., one of the inner cores  31 ), the “state in which vulcanization is completely completed” is, for example, to subject a sample piece cut out from the member to differential scanning calorimetry (DSC) measurement. When substantially no exothermic reaction is observed before and after the temperature at which organic peroxide added is to be decomposed, the member may be determined to be in a state in which the organic peroxide is not left, that is, the “state in which vulcanization is completely completed. 
     On the other hand, the term “unvulcanized” refers to a state in which no vulcanization has occurred at all. The term “semi-vulcanized” refers to a state between “vulcanized” and “unvulcanized”, that is, a state in which vulcanization has progressed halfway. 
     —Covering Step— 
     In the covering step, which is performed after the inner core arrangement step, the inner cores  31  are covered with first outer core materials  321  to obtain the intermediate molded bodies  33  ( FIGS.  3  and  4   ). 
     The intermediate molded bodies  33  obtained in the covering step each include the inner core  31 , and the first outer core material  321 , which covers only part of a surface of the inner core  31  and is integrated with the inner core  31 . In the example of the figure, each intermediate molded body  33  obtained in the covering step is composed of the inner core  31  and the first outer core material  321 , which covers only part of the surface of the inner core  31  and is integrated with the inner core  31 . The inner core  31  and the first outer core material  321  are closely contacted (adhered) to each other and accordingly, prevented from being detached from each other or hardly detached from each other. 
     The first outer core material  321  included in each intermediate molded body  33  has a body portion  321   m , which covers the inner core  31 . The inner core  31  includes a protruding portion  31   p  ( FIG.  4   ), which is not covered by the body portion  321   m  of the first outer core material  321  and protrudes to the outside of the body portion  321   m . The protruding portion  31   p  is substantially hemispherical. 
     The first outer core materials  321  constitute part of the outer cores  32  included in the multilayer cores  3  ( FIG.  14   ), which are final products obtained through vulcanization in a later-described vulcanization step in the downstream process. The first outer core materials  321  are rubber, as described above with respect to the outer cores  32 . 
     The entire first outer core materials  321 , immediately before and while covering the inner cores  31 , are preferably in an unvulcanized state. In this case, in the covering step, the first outer core materials  321 , after covering the inner cores  31 , are preferably maintained in the unvulcanized state even when heated for a predetermined time period or when not heated. That is, the entire first outer core materials  321  of the intermediate molded bodies  33  obtained in the covering step are preferably in the unvulcanized state. In the covering step, however, the first outer core materials  321 , after covering the inner cores  31 , may be at least partially brought into a semi-vulcanized state (in the case of the partially semi-vulcanized state, a remaining part of the first core materials  321  are preferably in an unvulcanized state). In this case, the first outer core materials  321  of the intermediate molded bodies  33  obtained in the covering step are at least partially in the semi-vulcanized state (in the case of the partially semi-vulcanized state, the remaining part of the first core materials  321  are preferably in the unvulcanized state). 
     Additionally, the first outer core materials  321 , immediately before and while covering the inner cores  31 , may be at least partially in the semi-vulcanized state (in the case of the partially semi-vulcanized state, the remaining part of the first core materials  321  may be in the unvulcanized state). In this case, the first outer core materials  321  of the intermediate molded bodies  33  obtained in the covering step are preferably at least partially in the semi-vulcanized state (in the case of the partially semi-vulcanized state, the remaining part of the first outer core materials  321  are preferably in the unvulcanized state) 
     An outer surface of the first outer core material  321  of each intermediate molded body  33  obtained in the covering step may be configured in any shape. In the example of the figure, the outer surface of the first outer core material  321  of the intermediate molded body  33  is configured in a substantially truncated cone shape. 
     —Intermediate Molded Body Removal Step— 
     In the intermediate molded body removal step, which is performed after the covering step, the intermediate molded bodies  33  are removed from the upstream process molding apparatus  1  ( FIGS.  5  and  6   ).  FIG.  6    illustrates one of the intermediate molded bodies  33  obtained after the intermediate molded body removal step. 
     [Downstream Process] 
     Secondly, the downstream process ( FIGS.  7  to  13   ) will be schematically described. In the downstream process, molding is performed using a downstream process molding apparatus  2 . In the downstream process, the intermediate molded bodies  33  obtained in the upstream process are used to obtain the multilayer cores  3  ( FIG.  14   ). The downstream process includes a second outer core arrangement step, an intermediate molded body arrangement step, a preparatory molding step, an intermediate plate removal step, a raw core forming step, the vulcanization step, and a multilayer core removal step. 
     As illustrated in  FIGS.  7  and  8   , the downstream process molding apparatus  2  includes a first downstream process mold  21 , a second downstream process mold  22 , and an intermediate plate  23 . 
     The intermediate plate  23  is arranged between the first downstream process mold  21  and the second downstream process mold  22 , and is configured to be arranged facing the first downstream process mold  21  and the second downstream process mold  22  ( FIG.  8   ). 
     In the examples of  FIGS.  7  and  8   , the first downstream process mold  21 , the intermediate plate  23 , and the second downstream process mold  22  are configured to be disposed along the vertical direction, and the first downstream process mold  21  is configured to be positioned on the upper side in the vertical direction with respect to the second downstream process mold  22 . The first downstream process mold  21 , the intermediate plate  23 , and the second downstream process mold  22 , however, may be configured to be disposed along any direction other than the vertical direction, and the first downstream process mold  21  may be configured to be positioned on any side other than the upper side in the vertical direction with respect to the second downstream process mold  22 . 
     Herein, a direction (e.g., the vertical direction in the examples of the figures) in the downstream process molding apparatus  2  in which the first downstream process mold  21 , the intermediate plate  23 , and the second downstream process mold  22  are arranged facing each other is referred to as a “downstream process mold axial direction (OAD).” On the other hand, a direction (e.g., the horizontal direction in the examples of the figures) perpendicular to the downstream process mold axial direction (OAD) is referred to as a “downstream process mold perpendicular-to-axis direction (OPD).” 
     The first downstream process mold  21  has one or more (e.g., in the examples of the figures, a plurality of) first downstream process mold cavity surfaces  210 . The first downstream process mold cavity surfaces  210  are configured to be arranged facing intermediate plate cavity surfaces  230  of the intermediate plate  23  ( FIG.  8   ). The first downstream process mold cavity surfaces  210  are each configured in a hollow shape that is open on a side thereof closer to the corresponding intermediate plate cavity surface  230  (e.g., on the lower side in the vertical direction in the example of  FIG.  8   ). The first downstream process mold cavity surface  210  is configured in a substantially hemispherical shape. The first downstream process mold  21  further includes, on a surface thereof closer to the intermediate plate  23  (e.g., on the lower side in the vertical direction in the example of  FIG.  8   ), flat surfaces  215 , which extend parallel to the downstream process mold perpendicular-to-axis direction OPD on the outer side in the circumferential direction of the first downstream process mold cavity surfaces  210 . In the examples of the figures, each flat surface  215  continues from the corresponding first downstream process mold cavity surface  210  to the outer side in the circumferential direction of the first downstream process mold cavity surface  210 , and extends continuously over the entire circumference around the first downstream process mold cavity surface  210 . The first downstream process mold  21  further includes, on the surface thereof closer to the intermediate plate  23  (e.g., on the lower side in the vertical direction in the example of  FIG.  8   ), flat surfaces  214 , which are positioned closer to the intermediate plate  23  than the flat surfaces  215  are (e.g., on the lower side in the vertical direction in the example of  FIG.  8   ) and which extend parallel to the downstream process mold perpendicular-to-axis direction OPD on the outer side in the circumferential direction of the first downstream process mold  21 . 
     The second downstream process mold  22  has one or more (in the examples of the figures, a plurality of) second downstream process mold cavity surfaces  220 . The second downstream process mold cavity surfaces  220  are configured to be arranged facing intermediate plate projecting surfaces  231  of the intermediate plate  23  ( FIG.  8   ). The second downstream process mold cavity surfaces  220  are each configured in a hollow shape that is open on a side thereof closer to the corresponding intermediate plate projecting surface  231  (e.g., on the upper side in the vertical direction in the example of  FIG.  8   ). The second downstream process mold cavity surface  220  is configured in a substantially hemispherical shape. The second downstream process mold  21  further includes, on a surface thereof closer to the intermediate plate  23  (e.g., on the upper side in the vertical direction in the example of  FIG.  8   ), flat surfaces  225 , which extend parallel to the downstream process mold perpendicular-to-axis direction OPD on the outer side in the circumferential direction of the second downstream process mold cavity surfaces  220 . In the examples of the figures, each flat surface  225  continues from the corresponding second downstream process mold cavity surface  220  to the outer side in the circumferential direction of the second downstream process mold cavity surface  220 , and extends continuously over the entire circumference around the second downstream process mold cavity surface  220 . The second downstream process mold  22  further includes, on the surface thereof closer to the intermediate plate  23  (e.g., on the upper side in the vertical direction in the example of  FIG.  8   ), flat surfaces  224 , which are positioned closer to the intermediate plate  23  than the flat surfaces  225  are (e.g., on the upper side in the vertical direction in the example of  FIG.  8   ) and which extend parallel to the downstream process mold perpendicular-to-axis direction OPD on the outer side in the circumferential direction of the second downstream process mold  22 . 
     The intermediate plate  23  includes, on a surface thereof closer to the first downstream process mold  21  (e.g., on the upper side in the vertical direction in the example of  FIG.  8   ), the one or more (e.g., in the examples of the figures, the plurality of) intermediate plate cavity surfaces  230 . The intermediate plate cavity surfaces  230  are configured to be arranged facing the first downstream process mold cavity surfaces  210  of the first downstream process mold  21  ( FIG.  8   ). The intermediate plate cavity surfaces  230  are each configured in a hollow shape that is open on a side thereof closer to the corresponding first downstream process mold cavity surface  210  (e.g., on the upper side in the vertical direction in the example of  FIG.  8   ). The intermediate plate cavity surface  230  is configured in a substantially hemispherical shape. The intermediate plate cavity surfaces  230  define cavities that are to receive the protruding portions  31   p  of the inner cores  31  of the intermediate molded bodies  33 . The intermediate plate  23  further includes, on the surface thereof closer to the first downstream process mold  21  (e.g., on the upper side in the vertical direction in the example of  FIG.  8   ), first flat surfaces  233 , each of which continues from the corresponding intermediate plate cavity surface  230  to the outer side in the circumferential direction of the intermediate plate cavity surface  230  and extends parallel to the downstream process mold perpendicular-to-axis direction OPD. In the examples of the figures, each first flat surface  233  continuously extends over the entire circumference around the corresponding intermediate plate cavity surface  230 , and continuously extends between adjacent intermediate plate cavity surfaces  230  so that the plurality of intermediate plate cavity surfaces  230  is coupled. 
     The intermediate plate  23  further includes, on a surface thereof closer to the second downstream process mold  22  (e.g., on the lower side in the vertical direction in the example of  FIG.  8   ), the one or more (in the examples of the figures, the plurality of) intermediate plate projecting surfaces  231 . The intermediate plate projecting surfaces  231  are configured to be arranged facing the second downstream process mold cavity surfaces  220  of the second downstream process mold  22  ( FIG.  8   ). The intermediate plate projecting surface  231  are each configured in a projecting shape that protrudes toward the corresponding second downstream process mold cavity surface  220  (e.g., toward the lower side in the vertical direction in the example of  FIG.  8   ). The intermediate plate projecting surface  231  is configured in a substantially hemispherical shape. The intermediate plate projecting surface  231  has the same shape as the intermediate plate cavity surface  230 . The intermediate plate  23  further includes, on the surface thereof closer to the second downstream process mold  22  (e.g., on the lower side in the vertical direction in the example of  FIG.  8   ), second flat surfaces  234 , each of which continues from the corresponding intermediate plate projecting surface  231  to the outer side in the circumferential direction of the intermediate plate projecting surface  231  and extends parallel to the downstream process mold perpendicular-to-axis direction OPD. In the examples of the figures, each second flat surface  234  continuously extends over the entire circumference around the corresponding intermediate plate projecting surface  231 , and continuously extends between adjacent intermediate plate projecting surfaces  231  so that the plurality of intermediate plate projecting surfaces  231  is coupled. 
     The downstream process molding apparatus  2  is configured, in a state in which the first downstream process mold  21  and the intermediate plate  23  are closed against each other, to define first downstream process mold cavities  24   a  between the first downstream process mold cavity surfaces  210 , and the intermediate plate cavity surfaces  230  and the surrounding first flat surfaces  233  ( FIG.  9   ). Further, the downstream process molding apparatus  2  is configured, in a state in which the second downstream process mold  22  and the intermediate plate  23  are closed against each other, to define second downstream process mold cavities  24   b  between the second downstream process mold cavity surfaces  220 , and the intermediate plate projecting surfaces  231  and the surrounding second flat surfaces  234  ( FIG.  9   ). Moreover, the downstream process molding apparatus  2  is configured, in a state in which the first downstream process mold  21  and the second downstream process mold  22  are closed against each other directly without the intermediate plate  23 , to define third downstream process mold cavities  24   c  between the first downstream process mold cavity surfaces  210  and the second downstream process mold cavity surfaces  220  ( FIG.  11   ). 
     —Second Outer Core Arrangement Step— 
     In the second outer core arrangement step, the second outer core materials  322  are arranged on the second downstream process mold cavity surfaces  220  of the second downstream process mold  22 , or the intermediate plate projecting surfaces  231  of the intermediate plate  23  ( FIG.  7   ). In the example of  FIG.  7   , in the second outer core arrangement step, the second outer core materials  322  are arranged on the second downstream process mold cavity surfaces  220 . 
     Here, the term “on” in “arranged on the second downstream process mold cavity surfaces  220  of the second downstream process mold  22 , or the intermediate plate projecting surfaces  231  of the intermediate plate  23 ” refers to arrangement that causes contact with or close facing to the second downstream process mold cavity surfaces  220  or the intermediate plate projecting surfaces  231 . Note that, however, when arranged on the second downstream process mold cavity surfaces  220 , the second outer core materials  322  are preferably arranged to contact the second downstream process mold cavity surfaces  220 . Further, when arranged on the intermediate plate projecting surfaces  231 , the second outer core materials  322  preferably closely face the intermediate plate projecting surfaces  231 . 
     An example of arranging the second outer core materials  322  on the intermediate plate projecting surfaces  231  will be described later with reference to  FIG.  15   . 
     The second outer core arrangement step may be performed before, in the course of, or after the above-described upstream process, and is preferably performed before or in the course of the upstream process because this reduces a total time required for the multilayer cores  3  to be molded. Further, the second outer core arrangement step may be performed before, in the course of, or after before the intermediate molded body arrangement step which will be described later, and is preferably performed before or in the course of the intermediate molded body arrangement step because this reduces the total time required for the multilayer cores  3  to be molded. Moreover, the second outer core arrangement step is more preferably performed immediately before the intermediate molded body arrangement step which will be described later, because this makes the vulcanized state of the second outer core materials  322  equivalent to the vulcanized state of the first outer core materials  321  of the intermediate molded bodies  33  arranged in the intermediate molded body arrangement step. 
     By being vulcanized in the subsequent vulcanization step, the second outer core materials  322  constitute part of the outer cores  32  included in the multilayer cores  3  ( FIG.  14   ), the final products. The outer core  32  of each multilayer core  3  is formed of the first outer core material  321  and the second outer core material  322  described above. The second outer core materials  322  are rubber, as described above with respect to the outer cores  32 . 
     The entire second outer core materials  322 , in the second outer core arrangement step, are preferably in the unvulcanized state. 
     Additionally, the second outer core materials  322 , in the second outer core arrangement step, may be at least partially in the semi-vulcanized state (in the case of the partially semi-vulcanized state, a remaining part of the second outer core materials  322  may be in the unvulcanized state). 
     In the example of  FIG.  7   , each second outer core material  322  is configured in a cylindrical shape in the second outer core arrangement step. In the second outer core arrangement step, however, the second outer core material  322  may be configured in any shape, such as a hemispherical, spherical, or a rectangular parallelepiped shape. 
     —Intermediate Molded Body Arrangement Step— 
     In the intermediate molded body arrangement step, which is performed after the above-described upstream process, the intermediate molded bodies  33  obtained in the upstream process are arranged on the intermediate plate cavity surfaces  230  of the intermediate plate  23 , or the first downstream process mold cavity surfaces  210  of the first downstream process mold  21  ( FIG.  8   ). In the example of  FIG.  8   , the intermediate molded bodies  33  are arranged on the intermediate plate cavity surfaces  230  in the intermediate molded body arrangement step. 
     Here, the term “on” in “arranged on the intermediate plate cavity surfaces  230  or the first downstream process mold cavity surfaces  210  of the first downstream process mold  21 ” refers to arrangement that causes contact with the intermediate plate cavity surfaces  230  or the first downstream process mold cavity surfaces  210 . 
     An example of arranging the intermediate molded bodies  33  on the first downstream process mold cavity surfaces  210  will be described later with reference to  FIG.  15   . 
     In the intermediate molded body arrangement step, as illustrated in  FIG.  8   , the protruding portions  31   p  of the inner cores  31 , which are included in the intermediate molded bodies  33 , are arranged facing the intermediate plate cavity surfaces  230 , and the body portions  321   m  of the first outer core materials  321 , which are included in the intermediate molded bodies  33 , are arranged facing the first downstream process mold cavity surfaces  210 . This allows the protruding portions  31   p  of the inner cores  31 , which are included in the intermediate molded bodies  33 , to face the second outer core materials  322  via the intermediate plate  23 , after the second outer core arrangement step and the intermediate molded body arrangement step ( FIG.  8   ). In the example of  FIG.  8   , in the intermediate molded body arrangement step, the intermediate molded bodies  33  are arranged on the intermediate plate cavity surfaces  230 , so that the protruding portions  31   p  of the inner cores  31  of the intermediate molded bodies  33  come into contact with the intermediate plate cavity surfaces  230  (in other words, so that the protruding portions  31   p  are received in the cavities defined by the intermediate plate cavity surfaces  230 ). 
     —Preparatory Molding Step— 
     In the preparatory molding step, which is performed after the second outer core arrangement step and the intermediate molded body arrangement step, the second outer core materials  322  and the intermediate molded bodies  33  are compression-molded in a state in which the first downstream process mold  21 , the second downstream process mold  22 , and the intermediate plate  23  are closed against each other ( FIG.  9   ). 
     Additionally, the process of closing the second downstream process mold  22  and the intermediate plate  23  against each other may be performed at any time after the second outer core arrangement step and before the preparatory molding step. Further, the process of closing the first downstream process mold  21  and the intermediate plate  23  against each other may be performed at any time after the intermediate molded body arrangement step and before the preparatory molding step. 
     In the preparatory molding step, the second outer core materials  322  are compression-molded in the second downstream process mold cavities  24   b  between the second downstream process mold cavity surfaces  220 , and the intermediate plate projecting surfaces  231  and the surrounding second flat surfaces  234 . Each second downstream process mold cavity surface  220  molds a substantially hemispherical outer surface in the corresponding second outer core material  322 . Each intermediate plate projecting surface  231  molds a substantially hemispherical inner surface  322   r  in the corresponding second outer core material  322 . 
     Further, in the preparatory molding step, the intermediate molded bodies  33  (specifically, the first outer core materials  321 ) are compression-molded in the first downstream process mold cavities  24   a  between the first downstream process mold cavity surfaces  210 , and the intermediate plate cavity surfaces  230  and the surrounding first flat surfaces  233 . Each first downstream process mold cavity surface  210  forms a substantially hemispherical outer surface in the first outer core material  321  of the corresponding intermediate molded body  33 . 
     —Intermediate Plate Removal Step— 
     In the intermediate plate removal step, which is performed after the preparatory molding step, the first downstream process mold  21 , the second downstream process mold  22 , and the intermediate plate  23  are released from each other, and the intermediate plate  23  is removed from the downstream process molding apparatus  2  ( FIG.  10   ). 
     —Raw Core Forming Step— 
     In the raw core forming step, which is performed after the intermediate plate removal step, the second outer core materials  322  are assembled to the intermediate molded bodes  33  to form raw multilayer cores  3 R ( FIG.  11   ). 
     In the raw core forming step, the protruding portions  31   p  of the inner cores  31  of the intermediate molded bodies  33  are fitted to the inner surfaces  322   r  of the second outer core materials  322  (in other words, the protruding portions  31   p  are received in the cavities defined by the inner surfaces  322   r  of the second outer core materials  322 ), and the first outer core materials  321  and the second outer core materials  322  of the intermediate molded bodies  33  are combined to form the outer cores  32 . 
     Each raw multilayer core  3 R includes the inner core  31 , and the outer core  32 , which is arranged on the outer side in the circumferential direction of the inner core  31 . The outer core  32  covers the entire surface of the inner core  31 . The outer core  32  is formed of the first outer core material  321 , which is included in the intermediate molded body  33 , and the second outer core material  322 . 
     Note that the term “raw” in the “raw multilayer core  3 R” means that at least part of the raw multilayer core  3 R (specifically, the outer core  32 ) is in the unvulcanized and/or semi-vulcanized state. 
     —Vulcanization Process— 
     In the vulcanization step, which is performed after the raw core forming step, the raw multilayer cores  3 R are vulcanized in the state in which the first downstream process mold  21  and the second downstream process mold  22  are closed against each other ( FIG.  11   ). 
     Additionally, the process of closing the first downstream process mold  21  and the second downstream process mold  22  against each other may be performed at any time in the course of the raw core forming step, or after the raw core forming step and before the vulcanization step. In the example of  FIG.  10   , the process of closing the first downstream process mold  21  and the second downstream process mold  22  against each other is performed in the course of the raw core forming step ( FIG.  11   ). 
     In the vulcanization process, the raw multilayer cores  3 R are vulcanization-molded in the third downstream process mold cavities  24   c  between the first downstream process mold cavity surfaces  210  and the second downstream process mold cavity surfaces  220 . 
     By vulcanization-molding the raw multilayer cores  3 R in the vulcanization step, the vulcanized multilayer cores  3  are formed. 
     —Multilayer Core Removal Step— 
     In the multilayer core removal step, which is performed after the vulcanization step, the first downstream process mold  21  and the second downstream process mold  22  are released from each other, and the multilayer cores  3  formed in the vulcanization step are removed from the downstream process molding apparatus  2  ( FIGS.  12  and  13   ).  FIG.  13    illustrates the multilayer cores  3  removed in the multilayer core removal step. 
     In the examples of  FIGS.  12  and  13   , the plurality of multilayer cores  3  is coupled to each other by joining flashes  36 , which will be described later. A section of the multilayer cores  3  and the joining flashes  36  in  FIG.  12    corresponds to a section taken along the line A-A in  FIG.  13   . The joining flashes  36  are removed after the multilayer core removal step (in a flash removal step). Subsequently, the individual multilayer cores  3  are finally obtained. 
     The above-described upstream process and the downstream process are conducted to finally obtain the multilayer cores  3  ( FIG.  14   ). 
     Now, operational advantages of the present embodiment will be described. 
     As described above, the multilayer core molding method of the present embodiment includes, in the upstream process, the inner core arrangement step ( FIGS.  1  and  2   ) of arranging the vulcanized inner cores  31  on the second upstream process mold cavity surfaces  120  of the upstream process molding apparatus  1 , and the covering step ( FIGS.  3  and  4   ), performed after the inner core arrangement step, of covering the inner cores  31  with the first outer core materials  321  to thereby obtain the intermediate molded bodies  33 . Further, the intermediate molded bodies  33  obtained in the covering step ( FIGS.  3  and  4   ) each include the inner core  31 , and the unvulcanized or semi-vulcanized first outer core material  321  that covers only part of the surface of the inner core  31  and is integrated with the inner core  31 . Moreover, the multilayer core molding method of the present embodiment includes, in the downstream process, the second outer core arrangement step ( FIG.  7   ) of arranging the second outer core materials  322  on the second downstream process mold cavity surfaces  220  or the intermediate plate projecting surfaces  231 , the intermediate molded body arrangement step ( FIG.  8   ) of arranging the intermediate molded bodies  33  on the intermediate plate cavity surfaces  230  or the first downstream process mold cavity surfaces  210 , and the preparatory molding step ( FIG.  9   ), performed after the second outer core arrangement step ( FIG.  7   ) and the intermediate molded body arrangement step ( FIG.  8   ), of compression-molding the second outer core materials  322  and the intermediate molded bodies  33  in the state in which the first downstream process mold  21 , the second downstream process mold  22 , and the intermediate plate  23  are closed against each other. 
     In this regard, suppose that the inner cores  31  and the unvulcanized or semi-vulcanized first outer core materials  321 , in a state of being discrete from each other (i.e., in a state of not being integrated [adhered] to each other), are stacked to be arranged on cavity surfaces of the downstream process molding apparatus  2 . In this circumstance, since the first outer core materials  321  are discrete from the inner cores  31 , the first outer core materials  321 , when being arranged, are likely to be inclined or misaligned with respect to to the inner cores  31  (and thus with respect to the cavity surfaces). When the first outer core materials  321  are inclined or misaligned with respect to the inner cores  31 , the first outer core materials  321  are unlikely to flow evenly around the inner cores  31  in a subsequent step (e.g., the preparatory molding step), and this increases the potential of the eccentricity in the inner cores  31  of the multilayer cores  3  ( FIG.  14   ). Further, in this circumstance, since being discrete from the inner cores  31 , the first outer core materials  321  have a shape that is likely to deform gradually over time due to residual stress of the rubber that forms the first outer core materials  321 , and thus, the first outer core materials  321 , when being arranged, are even more likely to be inclined or misaligned with respect to the inner cores  31  (and thus with respect to the cavity surfaces). This, in turn, further increases the potential of the eccentricity in the inner cores  31  of the multilayer cores  3  ( FIG.  14   ). 
     Herein, the eccentricity in the inner cores  31  refers to that a center C 31  of the inner core  31  of any multilayer core  3  ( FIG.  14   ) finally obtained is misaligned from a center C 32  of the outer core  32 . 
     In contrast, the present embodiment forms the intermediate molded bodies  33  in advance in the covering step by integrating the first outer core materials  321  to the inner cores  31 , before arranging the inner cores  31  and the first outer core materials  321  on the cavity surfaces (specifically, the intermediate plate cavity surfaces  230  or the first downstream process mold cavity surfaces  210 ) of the downstream process molding apparatus  2 . Accordingly, when the inner cores  31  and the first outer core materials  321  are subsequently arranged on the cavity surfaces of the downstream process molding apparatus  2 , the first outer core materials  321  are prevented from being inclined or misaligned with respect to the inner cores  31  (and thus with respect to the cavity surfaces), and this prevents the potential of the eccentricity in the inner cores  31  of the multilayer cores  3 . Further, according to the present embodiment, the inner cores  31 , which are integrated on the inner side in the circumferential direction of the first outer core materials  321 , prevent the first outer core materials  321  of the intermediate molded bodies  33  from deforming due to the residual stress, thereby preventing the first outer core materials  321  from being inclined or misaligned with respect to the inner cores  31 . This, in turn, further prevents the eccentricity in the inner cores  31  of the multilayer cores  3 . 
     Further, as described above, the preparatory molding step ( FIG.  9   ) of compression-molding the first outer core materials  321  in the first downstream process mold cavities  24   a  in the state in which the protruding portions  31   p  of the inner cores  31  in the intermediate molded bodies  33  are received in the cavities defined by the intermediate plate cavity surfaces  230  prevents the inner cores  31  from being misaligned in the first downstream process mold cavities  24   a . This, in turn, prevents the eccentricity in the inner cores  31 . 
     Hereinafter, more detailed explanation, preferred configurations, modifications, etc., of the multilayer core molding method of the present disclosure will be described. 
     In the examples described herein, in the downstream process molding apparatus  2  as described above, the first downstream process mold  21 , the intermediate plate  23 , and the second downstream process mold  22  may be configured, as in a second embodiment illustrated in  FIG.  15   , to be disposed along the vertical direction, and the second downstream process mold  22  may be configured to be positioned on the upper side in the vertical direction with respect to the first downstream process mold  21 . 
     In this case, in the downstream process, the second outer core materials  322  are preferably arranged on the intermediate plate projecting surfaces  231  of the intermediate plate  23  in the second outer core arrangement step, and the intermediate molded bodies  33  are preferably arranged on the first downstream process mold cavity surfaces  210  of the first downstream process mold  21  in the intermediate molded body arrangement step. 
     In this case, each second outer core material  322  arranged in the second outer core arrangement step may have any shape. Preferably, however, the second outer core material  322  includes, for example, the substantially hemispherical inner surface  322   r  and is arranged so that the inner surface  322   r  covers the corresponding intermediate plate projecting surface  231 , whether out of or in contact, because this allows stable arrangement of the second outer core material  322 . 
     Further, in this case, in the intermediate molded body arrangement step as illustrated in  FIG.  15   , the protruding portions  31   p  of the inner cores  31  in the intermediate molded bodies  33  are arranged facing the intermediate plate cavity surfaces  230 , and the body portions  321   m  of the first outer core materials  321  in the intermediate molded bodies  33  are arranged facing the first downstream process mold cavity surfaces  210 . More specifically, in the intermediate molded body arrangement step, the intermediate molded bodies  33  are preferably arranged on the first downstream process mold cavity surfaces  210 , so that the main body portions  321   m  of the first outer core materials  321  in the intermediate molded bodies  33  come into contact with the first downstream process mold cavity surfaces  210  (in other words, so that the body portions  321   m  are received in the cavities defined by the first downstream process mold cavity surfaces  210 ). 
     In the examples described herein, as in the example illustrated in  FIG.  2   , each second upstream process mold cavity surface  120  in the upstream process molding apparatus  1  preferably includes a substantially hemispherical receiving recessed surface portion  121 , which defines the cavity configured to receive the corresponding inner core  31 . Since the receiving recessed surface portion  121  is configured in a substantially hemispherical shape, the inner core  31  is positioned without difficulty by simply arranged on the receiving recessed surface portion  121  in the inner core arrangement step. This, in turn, prevents the eccentricity in the inner cores  31 . 
     In this case, as illustrated in  FIG.  4   , the receiving recessed surface portion  121  of each second upstream process mold cavity surface  120  in the upstream process molding apparatus  1  does not mold the outer surface of the body portion  321   m  of the corresponding first outer core material  321  in the covering step in the upstream process. The outer surface of the body portion  321   m  of the first outer core material  321  is molded at least by part of the second upstream process mold cavity surface  120  other than the receiving recessed surface portion  121 . 
     Note that the term “substantially hemispherical” herein refers to a perfectly hemispherical shape, or a shape that is not perfectly hemispherical but close to hemispherical. 
     In the examples described herein, as in the examples illustrated in  FIGS.  1  to  6   , the covering step in the upstream process preferably uses extrusion molding to cover the inner cores  31  with the first outer core materials  321 . In this case, the upstream process molding apparatus  1  is configured to have a function as an extrusion molding machine. As illustrated in  FIGS.  1  and  3   , for example, the upstream process molding apparatus  1  having the function as the extrusion molding machine preferably includes, in the first upstream process mold  11 , a storage  16  storing the first outer core material  321 , a pressing member  15  configured to press the first outer core material  321  stored in the storage  16  toward the upstream process mold cavities  14  ( FIG.  3   ), and one or more (e.g., in the examples of the figures, a plurality of) extrusion ports  17  arranged between the storage  16  and the upstream process mold cavities  14 . In this case, in the covering step, firstly, the first upstream process mold  11  and the second upstream process mold  12  in the upstream process molding apparatus  1  are closed against each other. Secondly, the first outer core material  321  stored in advance in the storage  16  is pressed toward the upstream process mold cavities  14  by the pressing member  15 . Being pressed by the pressing member  15 , the first outer core material  321  is extruded into the upstream process mold cavities  14  through the one or more extrusion ports  17 . The first outer core materials  321 , after being extruded into the upstream process mold cavities  14 , cover the inner cores  31  so that only part of the surface of each inner core  31  is covered. As in the example of  FIG.  3   , the first outer core material  321  is extruded into the upstream process mold cavities  14  preferably in the upstream process mold axial direction EAD. 
     However, the covering step may also use compression molding to cover the inner cores  31  with the first outer core materials  321 . In this case, the upstream process molding apparatus  1  is configured to have a function as a compression molding machine. In this case, for example, in the covering step, firstly, in a state in which the first upstream process mold  11  and the second upstream process mold  12  in the upstream process molding apparatus  1  are released from each other, the first outer core materials  321  of any shape may be arranged between the inner cores  31  on the second upstream process mold cavity surfaces  120  and the first upstream process mold cavity surfaces  110 , and subsequently, the first upstream process mold  11  and the second upstream process mold  12  may be closed against each other to thereby compression-mold the first outer core materials  321  in the upstream process mold cavities  14 . Thus, the first outer core materials  321  cover the inner cores  31  so that only part of the surface of each inner core  31  is covered. 
     In the examples described herein, in the covering step in the upstream process, as an example illustrated in  FIG.  4   , the inner cores  31  are preferably covered with the first outer core materials  321  inside the upstream process mold cavities  14 , in the state in which the first upstream process mold  11  and the second upstream process mold  12  are closed against each other. This permits each first outer core material  321  to achieve a shape, a position with respect to the corresponding inner core  31 , or the like, as desired, compared with a case of the covering step using, for example, extrusion molding, assuming that the inner cores  31  are covered with the first outer core materials  321  in the state in which the first upstream process mold  11  and the second upstream process mold  12  are released from each other. Accordingly, the eccentricity in the inner cores  31  may be prevented. 
     In this case and when the upstream process molding apparatus  1  includes the receiving recessed surface portion  121  in each second upstream process mold cavity surface  120 , the outer surface of the body portion  321   m  of the first outer core material  321 , in the covering step, is molded by part of the second upstream process mold cavity surface  120  other than the receiving recessed surface portion  121  and the first upstream process mold cavity surface  110  inside the corresponding upstream process mold cavity  14 . 
     Further, in this case and when the upstream process molding apparatus  1  includes the receiving recessed surface portion  121  in each second upstream process mold cavity surface  120 , as illustrated in  FIG.  4   , the upstream process molding apparatus  1  is preferably configured to allow a central axis O 14  (or an extension thereof, which similarly applies hereinafter) of the upstream process mold cavity  14  to pass through a center C 121   e  of an open end surface  121   e  of the receiving recessed surface portion  121  of the second upstream process mold cavity surface  120 , in the state in which the first upstream process mold  11  and the second upstream process mold  12  are closed against each other. Herein, the opening end surface  121   e  of the receiving recessed surface portion  121  is a circular imaginary surface. The central axis O 14  of the upstream process mold cavity  14  is preferably parallel to the upstream process mold axial direction EAD. 
     The above configuration facilitates a central axis O 321   m  of the body portion  321   m  of the first outer core material  321  to pass through the center C 31  of the inner core  31  ( FIG.  4   ) in the intermediate molded body  33  obtained by the covering step. This, in turn, facilitates the intermediate molded body  33  to be arranged, with the central axis O 321   m  of the body portion  321   m  of the first outer core material  321  being parallel to the downstream process mold axial direction OAD and passing through the center C 230   e  of the opening end surface  230   e  of the intermediate plate cavity surface  230 , when the protruding portion  31   p  of the intermediate molded body  33  is received in the cavity defined by the corresponding intermediate plate cavity surface  230  of the intermediate plate  23  in a subsequent step (e.g., in the example of  FIG.  8   , in the intermediate molded body arrangement step, and in the example of  FIG.  15   , when the first downstream process mold  21  and the intermediate plate  23  are closed against each other after the intermediate molded body arranging step and before the preparatory molding process), thereby preventing the eccentricity in the inner cores  31 . Herein, the opening end surface  230   e  of the intermediate plate cavity surface  230  is a circular imaginary surface. 
     Similarly, as in the example illustrated in  FIG.  4   , the upstream process molding apparatus  1  is preferably configured to allow the central axis O 14  of the upstream process mold cavity  14  to pass through the center C 31  of the inner core  31  arranged on the second upstream process mold cavity surface  120  (more specifically, in the example of  FIG.  4   , the receiving recessed surface portion  121 ), in the state in which the first upstream process mold  11  and the second upstream process mold  12  are closed against each other. 
     The above configuration facilitates the central axis O 321   m  of the body portion  321   m  of the first outer core material  321  to pass through the center C 31  of the inner core  31  ( FIG.  4   ) in the intermediate molded body  33  obtained by the covering step, thereby preventing the eccentricity in the inner cores  31 . 
     In the examples described herein, as in the example illustrated in  FIG.  4   , when each second upstream process mold cavity surface  120  has the receiving recessed surface portion  121  in the upstream process molding apparatus  1 , the second upstream process mold cavity surface  120  preferably includes a flat surface portion  122  ( FIG.  4   ), which continues from the receiving recessed surface portion  121  (specifically, the opening end surface  121   e  of the receiving recessed surface portion  121 ) toward the outer side in the circumferential direction and which extends parallel and flat with respect to the upstream process mold perpendicular-to-axis direction EPD. The flat surface portion  122  preferably extends annularly over the entire circumference around the receiving recessed surface portion  121 . In this case, the flat surface portion  122  of the second upstream process mold cavity surface  120  forms a flat surface portion  321   f  ( FIGS.  4  and  6   ) on the body portion  321   m  of the first outer core material  321  of the intermediate molded body  33  in the covering step in the upstream process. The flat surface portion  321   f  constitutes an end surface of the body portion  321   m  of the first outer core material  321  of the intermediate molded body  33  that is located at one end in the axial direction (i.e., direction parallel to the central axis O 321   m ). Part of the inner core  31  of the intermediate molded body  33  that is located beyond the flat surface portion  321   f  and closer to the one end of the main body  321   m  of the first outer core material  321  in the axial direction constitutes the protruding portion  31   p , and a remainder that is located beyond the flat surface portion  321   f  and closer to another end of the main body  321   m  of the first outer core material  321  in the axial direction is buried inside the main body  321   m  of the first outer core material  321 . 
     With the above configuration, when the protruding portion  31   p  of the intermediate molded body  33  is received in the cavity defined by the corresponding intermediate plate cavity surface  230  of the intermediate plate  23  in a subsequent step (e.g., in the example of  FIG.  8   , in the intermediate molded body arrangement step, and in the example of  FIG.  15   , when the first downstream process mold  21  and the intermediate plate  23  are closed against each other after the intermediate molded body arranging step and before the preparatory molding process), the flat surface portion  321   f  of the first outer core material  321  of the intermediate molded body  33  is brought into contact with the first flat surface  233  of the intermediate plate  23 , whereby the position and orientation of the intermediate molded body  33  can be easily and reliably adjusted so that the flat surface portion  321   f  of the first outer core material  321  is parallel to the first flat surface  233  of the intermediate plate  23  (and thus is parallel to the downstream process mold perpendicular-to-axis direction OPD). This facilitates the central axis O 321   m  of the first outer core material  321  of the intermediate molded body  33  to be parallel to the downstream process mold axial direction OAD, thereby preventing the eccentricity in the inner cores  31 . 
     In the examples described in this specification, as in the example illustrated in  FIG.  2   , when each second upstream process mold cavity surface  120  in the second upstream process mold  12  includes the substantially hemispherical receiving recessed surface portion  121  that defines the cavity configured to receive the corresponding inner core  31  in the upstream process molding apparatus  1 , a diameter D 1  of the open end surface  121   e  of the receiving recessed surface portion  121  is preferably larger than a diameter D 2  of the inner core  31 . In this case, a gap  121   g  is formed between the receiving recessed surface portion  121  and the inner core  31  in a state in which the inner core  31  is received in the cavity defined by the receiving recessed surface portion  121  through the inner core arrangement step in the upstream process. 
     Consequently, even when the dimensional accuracy of the inner core  31  prepared in advance is not as high as expected and the inner core  31  is not perfectly spherical, the inner core  31  may be easily arranged in the inner core arrangement step so that the center C 31  of the inner core  31  is positioned at the center C 121   e  of the opening end surface  121   e  of the receiving recessed surface portion  121  or faces to the center C 121   e  of the opening end surface  121   e  in the upstream process mold axial direction EAD, simply by dropping the inner core  31  onto the receiving recessed surface portion  121 . This prevents the eccentricity in the inner cores  31 . 
     Further, the above configuration results in formation of a thin portion  321   s  in the subsequent covering step, since part of the first outer core material  321  enters the gap  121   g  ( FIGS.  4  and  6   ). In this case, the first outer core material  321  of the intermediate molded body  33  includes the thin portion  321   s , in addition to the body portion  321   m . The thin portion  321   s  is continuous from the body portion  321   m  and extends on the surface of the protruding portion  31   p  of the inner core  31  away from the body portion  321   m . The thin portion  321   s  covers only part of the surface of the protruding portion  31   p  of the inner core  31 . Due to the thin portion  321   s  formed in the covering step, the inner core  31  is held not only by the body portion  321   m  of the first outer core material  321  but also by the thin portion  321   s , and the degree of adhesion between the inner core  31  and the first outer core material  321  may be increased. This, in turn, effectively prevents the first outer core material  321  from being misaligned or detached from the inner core  31  in the covering step or in a subsequent step. Accordingly, durability of the multilayer cores  3  may be improved, and the eccentricity in the inner cores  31  may be prevented. 
     In this case, in the downstream process molding apparatus  2 , the cavity defined by each intermediate plate cavity surface  230  of the intermediate plate  23  ( FIGS.  8  and  15   ) is preferably configured to receive the protruding portion  31   p  of the inner core  31  of the corresponding intermediate mold  33  and the surrounding thin portion  321   s . From this perspective, in the downstream process molding apparatus  2 , a diameter of the opening end surface  230   e  ( FIGS.  8  and  15   ) of the intermediate plate cavity surface  230  of the intermediate plate  23  is preferably larger than the diameter D 2  of the inner core  31 . 
     Preferably, the diameter D 1  of the open end surface  121   e  of the receiving recessed surface portion  121  in the upstream process molding apparatus  1  is from 102 to 105% of the diameter D 2  of the inner core  31 . 
     Similarly, the diameter of the open end surface  230   e  of the intermediate plate cavity surface  230  of the intermediate plate  23  of the downstream process molding apparatus  2  is preferably 102 to 105% of the diameter D 2  of the inner core  31 . Further, the diameter of the open end surface  230   e  of the intermediate plate cavity surface  230  of the intermediate plate  23  of the downstream process molding apparatus  2  is preferably equal to the diameter D 1  of the open end surface  121   e  of the receiving recessed surface portion  121  in the upstream process molding apparatus  1 . 
     However, in the examples described herein, the diameter D 1  of the open end surface  121   e  of the receiving recessed surface portion  121  may be equal to (or substantially equal to) the diameter D 2  of the inner core  31 . In this case, the gap  121   g  is not formed between the receiving recessed surface portion  121  and the inner core  31  in the state in which the inner core  31  is received in the cavity defined by the receiving recessed surface portion  121  through the inner core arrangement step ( FIG.  2   ), and the thin portion  321   s  is not formed in the subsequent covering step ( FIG.  4   ). Accordingly, the first outer core material  321  of the intermediate molded body  33  obtained by the covering step does not include the thin portion  321   s  but only includes the body portion  321   m . In this case, the diameter of the open end surface  230   e  of the intermediate plate cavity surface  230  of the intermediate plate  23  in the downstream process molding apparatus  2  is preferably equal to (or substantially equal to) the diameter D 2  of the inner core  31 . 
     Additionally, in the examples described herein, the diameter D 1  of the open end surface  121   e  of the receiving recessed surface portion  121  may be smaller than the diameter D 2  of the inner core  31 . 
     In the examples described herein, as in the example illustrated in  FIG.  2   , when, in the upstream process molding apparatus  1 , each second upstream process mold cavity surface  120  of the second upstream process mold  12  includes the substantially hemispherical receiving recessed surface portion  121  that defines the cavity configured to receive the corresponding inner core  31 , a depth of the receiving recessed surface portion  121  may be equal to (or substantially equal to), smaller than, or larger than a radius (half the diameter D 2 ) of the inner core  31 . 
     Herein, the “depth of the receiving recessed surface portion ( 121 )” refers to a distance from a deepest point of the receiving recessed surface portion  121  (i.e., a point farthest from the opening end surface  121   e  of the receiving recessed surface portion  121 ) to the opening end surface  121   e  of the receiving recessed surface portion  121 . 
     The depth of the receiving recessed surface portion  121  will correspond to a height of the protruding portion  31   p  of the inner core  31  of the intermediate molded body  33  obtained in the covering step. The smaller the depth of the receiving recessed surface portion  121 , the smaller the height of the protruding portion  31   p  of the inner core  31  of the intermediate molded body  33 , and thus, the larger the part of the surface of the inner core  31  in the intermediate molded body  33  that is covered by the body portion  321   m  of the first outer core material  321 . 
     From the perspective of increasing the degree of adhesion between the body portion  321   m  of the first outer core material  321  of the intermediate molded body  33 , and the inner core  31 , and thus improving the durability of the multilayer core  3 , the depth of the receiving recessed surface portion  121  is preferably equal to (or substantially equal to) or smaller than the radius of the inner core  31 , more preferably smaller than the radius of the inner core  31 . 
     On the other hand, from the perspective of ease of positioning the intermediate molded body  33  in the downstream process or ease of molding or the like in the downstream process, the depth of the receiving recessed surface portion  121  is preferably substantially equal to (more preferably, equal to) the radius of the inner core  31  as in the example of  FIG.  2   . In this case, the body portion  321   m  of the first outer core material  321  of the intermediate molded body  33  obtained by the covering step will cover substantially half the surface of the inner core  31  ( FIG.  4   ). 
     In the examples described herein, at least part of the first outer core material  321  that is located on the inner side in the circumferential direction in each intermediate molded body  33  obtained in the covering step in the upstream process is preferably in the unvulcanized or the semi-vulcanization state. This increases the degree of adhesion between the first outer core material  321  and the inner core  31 , thereby effectively preventing the first outer core material  321  from being misaligned or detached from the inner core  31  in a subsequent step. Accordingly, the durability of the multilayer cores  3  may be improved, and the eccentricity in the inner cores  31  may be prevented. 
     In the examples described herein, part of the first outer core material  321  that is located on the outer side in the circumferential direction in the intermediate molded body  33  obtained in the covering step is preferably in the semi-vulcanized state. This allows the part of the first outer core material  321  that is located on the outer side in the circumferential direction to be moderately cured, whereby the shape, the position or the orientation with respect to the inner core  31 , or the like of the first outer core material  321  may be effectively maintained until immediately before the subsequent preparatory molding step. Accordingly, the eccentricity in the inner cores  31  may be prevented. 
     In the examples described herein, the first outer core material  321  of the intermediate molded body  33  obtained in the covering step preferably does not include a vulcanized part. 
     In the examples described herein, in the covering step in the upstream process, a temperature of the upstream process molding apparatus  1  is preferably higher than a temperature of each first outer core material  321 . More specifically, at a beginning of the covering step (i.e., when the first outer core material  321  starts to cover the corresponding inner core  31 ), the temperature of the upstream process molding apparatus  1  is preferably higher than the temperature of the first outer core material  321 . Further, throughout the covering step, the temperature of the upstream process molding apparatus  1  is preferably higher than the temperature of the first outer core material  321 . 
     Additionally, as described above, the entire first outer core material  321  of the intermediate molded body  33  obtained in the covering step is preferably in the unvulcanized state. 
     From a similar perspective, the temperature of the upstream process molding apparatus  1  is preferably, for example, 50 to 80° C. in the covering step (more specifically, for example, at the beginning of the covering step, or throughout the covering step). Further, from the similar perspective, the temperature of the first outer core material  321  is preferably, for example, 40 to 60° C., in the covering step (more specifically, for example, at the beginning of the covering step, or throughout the covering step). 
     Note that these temperatures are particularly preferred when the covering step is carried out by extrusion, as in the examples of  FIGS.  3  and  4   . 
     Further, from the similar perspective, when the covering step is performed in the state in which the first upstream process mold  11  and the second upstream process mold  12  in the upstream process molding apparatus  1  are closed against each other, as in the example of  FIGS.  3  and  4   , it is preferable to maintain the state in which the first upstream process mold  11  and the second upstream process mold  12  in the upstream process molding apparatus  1  are closed against each other for, for example, 20 to 40 seconds in the covering step, and subsequently perform the intermediate molded body removal step. 
     In the examples described herein, in the preparatory molding step ( FIG.  9   ) in the downstream process, in the state in which the first downstream process mold  21  and the intermediate plate  23  are closed against each other as in the example of  FIG.  9   , it is preferable that the central axis O 321   m  of the body portion  321   m  of the first outer core material  321  in each intermediate molded body  33  is parallel to the downstream process mold axial direction OAD and also passes through the center C 230   e  of the opening end surface  230   e  of the corresponding intermediate plate cavity surface  230 , the center C 31  of the inner core  31 , and a center C 210   e  of an opening end surface  210   e  of the first downstream process mold cavity surface  210  ( FIG.  8   ). This prevents the eccentricity in the inner cores  31 . Herein, the opening end surface  210   e  of the first downstream process mold cavity surface  210  ( FIG.  8   ) is a circular imaginary surface. 
     In the examples described herein, the intermediate plate removal step ( FIG.  10   ) in the downstream process, while the first downstream process mold  21 , the second downstream process mold  22 , and the intermediate plate  23  in the downstream process molding apparatus  2  are released from each other and the intermediate plate  23  is removed from the downstream process molding apparatus  2 , as in the example of  FIG.  10   , it is preferable to hold the second outer core materials  322  on the second downstream process mold cavity surfaces  220  and hold the intermediate molded bodies  33  on the first downstream process mold cavity surfaces  210 . Further, in this case, in the raw core forming step ( FIG.  11   ), as in the example of  FIG.  11   , the second outer core materials  322  are preferably assembled to the intermediate molded bodies  33  by closing the first downstream process mold  21  and the second downstream process mold  22  against each other. 
     With the above configuration, the second outer core materials  322  may be assembled to the intermediate molded bodies  33  by closing the first downstream process mold  21  and the second downstream process mold  22  against each other (in the raw core forming step), immediately after the intermediate plate  23  is removed (in the intermediate plate removal step). This reduces the total time required for the multilayer cores  3  to be molded. Further, in a period after the intermediate plate  23  is removed from the downstream process molding apparatus  2  and before the second outer core materials  322  are assembled to the intermediate molded bodies  33 , the second outer core materials  322 , and the first outer core materials  321  of the intermediate molded bodies  33  may be prevented from being deformed by the residual stress of the rubber, and thus, the eccentricity in the inner cores  31  may be prevented. 
     In the examples described herein, in the preparatory molding step ( FIG.  9   ) in the downstream process, as in the example of  FIG.  9   , part of the first outer core material  321  of each intermediate molded body  33  preferably sticks out from between the corresponding first downstream process mold cavity surface  210  and the corresponding intermediate plate cavity surface  230  (i.e., from the corresponding first downstream process mold cavity  24   a ) to form a first flash  321   b  between the first downstream process mold  21  and the intermediate plate  23 , and part of the second outer core material  322  preferably sticks out from between the corresponding intermediate plate projecting surface  231  and the corresponding second downstream process mold cavity surface  220  (i.e., from the corresponding second downstream process mold cavity  24   b ) to form a second flash  322   b  between the intermediate plate  23  and the second downstream process mold  22 . 
     With the above configuration, in the subsequent intermediate plate removal step ( FIG.  10   ), the first flashes  321   b  attach to the first downstream process mold  21 , thereby facilitating the intermediate molded bodies  33  to be held on the first downstream process mold cavity surfaces  210 , and the second flashes  322   b  attach to the second downstream process mold  22 , thereby facilitating the second outer core materials  322  to be held on the second downstream process mold cavity surfaces  220 . In other words, when the first downstream process mold  21 , the intermediate plate  23 , and the second downstream process mold  22  in the downstream process molding apparatus  2  are released from each other, a situation may be prevented in which the intermediate molded bodies  33  and the second outer core materials  32  attach to the intermediate plate  23  and fail to detach from the intermediate plate  23 . 
     Further, with the above configuration, in the intermediate plate removal step ( FIG.  10   ), while the intermediate molded bodies  33  are held on the first downstream process mold cavity surfaces  210  and the second outer core materials  322  are held on the second downstream process mold cavity surfaces  220 , the first flashes  321   b  attached to the first downstream process mold  21  pull the first outer core materials  321  of the intermediate molded bodies  33  toward the outer side in the circumferential direction, thereby preventing the first outer core materials  321  from contracting due to the residual stress of the rubber, and the second flashes  322   b  attached to the second downstream process mold  22  pull the second outer core materials  322  toward the outer side in the circumferential direction, thereby preventing the second outer core materials  322  from contracting due to the residual stress of the rubber. Accordingly, the eccentricity in the inner cores  31  may be prevented. 
     Additionally, in this case, the first flashes  321   b  and the second flashes  322   b  are assembled to each other in the raw core forming step ( FIG.  11   ). Subsequently, in the vulcanization step, the first flashes  321   b  and the second flashes  322   b  are vulcanized and integrated with each other to form the joining flashes  36  ( FIGS.  11  and  12   ). In the vulcanization step, a continuous molded body  37 , which includes the integrally coupled multilayer cores  3  and joining flashes  36 , is molded. Subsequently, in the multilayer core removal step ( FIGS.  12  and  13   ), the continuous molded body  37  formed in the vulcanization step is removed from the downstream process molding apparatus  2 . After that, the joining flashes  36  are removed from the continuous molded body  37  (in the flash removal step), and the multilayer cores  3  are finally obtained. 
     Additionally, as in the example of  FIG.  9   , the first flashes  321   b  preferably have parts in which the first outer core materials  321  of the plurality of intermediate molded bodies  33  are coupled to each other. Further, as in the example of  FIG.  9   , the second flashes  322   b  preferably have parts in which the plurality of second outer core materials  322  are coupled to each other. 
     A total volume of the joining flashes  36  in the continuous molded body  37  is preferably 5 to 20% of a total volume of the plurality of multilayer cores  3  in the continuous molded body  37 . 
     In the preparatory molding step ( FIG.  9   ), however, the formation of the first flashes  321   b  and the second flashes  322   b  may be omitted. 
     In the examples described herein, when the first flashes  321   b  are formed in the preparatory molding step ( FIG.  9   ) as described above, the first downstream process mold  21  preferably includes, on the surface thereof that is closer to the intermediate plate  23   c , one or more (e.g., in the example of  FIG.  9   , a plurality of) grooves  212  that are located on the outer side in the circumferential direction of the first downstream process mold cavity surfaces  210 . In this case, the first downstream process mold  21  preferably includes, on the surface thereof that is closer to the intermediate plate  23 , the flat surfaces  215  that extend between the first downstream process mold cavity surfaces  210  and the grooves  212  in parallel to the downstream process mold perpendicular-to-axis direction OPD to couple the first downstream process mold cavity surfaces  210  and the grooves  212 . The flat surfaces  215  are located at the same positions in the downstream process mold axial direction OAD, with respect to the open end surfaces of the first downstream process mold cavity surfaces  210 . Due to this configuration, the first flashes  321   b  formed in the preparatory molding step ( FIG.  9   ) include thick portions  321   bk , which are molded between the grooves  212  of the first downstream process mold  21  and the first flat surfaces  233  of the intermediate plate  23 , and thin portions  321   bn , which are molded between the flat surfaces  215  located between the first downstream process mold cavity surfaces  210  and the grooves  212  in the first downstream process mold  21 , and the first flat surfaces  233  of the intermediate plate  23 . The thin portions  321   bn  are smaller in thickness than the thick portions  321   bk  in the downstream process mold axial direction OAD. The thin portions  321   bn  couple the first outer core materials  321  of the intermediate molded bodies  33  and the thick portions  321   bk . Since having the thick portions  321   bk , the first flashes  321   b  may be more firmly attached to the first downstream process mold  21 , and this facilitates the intermediate molded bodies  33  to be held on the first downstream process mold cavity surfaces  210 . Further, since having the thin portions  321   bn , the first flashes  321   b  may be easily broken at the thin portions  321   bn  in the flash removal step, and thus, the operation of removing the first flashes  321   b  is simplified. 
     The grooves  212  preferably extend continuously, on the outer side in the circumferential direction of the first downstream process mold cavity surfaces  210 , over the entire circumferences. This case implies that the thick portions  321   bk  of the first flashes  321   b  that are molded by the grooves  212  continuously extend, on the outer side in the circumferential direction of the intermediate molded bodies  33 , over the entire circumferences. This allows the first flashes  321   b  to be more firmly attached to the first downstream process mold  21 . 
     Further, the flat surfaces  215 , which are located between the first downstream process mold cavity surfaces  210  and the grooves  212 , preferably extend continuously over the entire circumferences around the first downstream process mold cavity surfaces  210 . This case implies that the thin portions  321   bn  of the first flashes  321   b  that are molded by the flat surfaces  215  extend continuously over the entire circumferences between the intermediate molded bodies  33  and the thick portions  321   bk . Thus, the operation of removing the first flashes  321   b  is even more simplified. 
     In this case, at least one of the one or more grooves  212  in the first downstream process mold  21  preferably includes, on a groove bottom surface thereof (i.e., on a surface thereof that is located opposite to the open end surface of the groove  212 ), a convexity  213  ( FIG.  9   ). In the example of  FIG.  9   , only one or more grooves  212   a  in the plurality of grooves  212  that are located between the plurality of first downstream process mold cavity surfaces  210  include, on the groove bottom surfaces thereof, the convexities  213 . The grooves  212  including the convexities  213  will provide the thick portions  321   bk  of the first flashes  321   b  formed in the preparatory molding step ( FIG.  9   ) with concavities  321   bg  molded in accordance with the convexities  213 . A resulting anchor effect allows the first flashes  321   b  to be even more firmly attached to the first downstream process mold  21 , thereby further facilitating the intermediate molded bodies  33  to be held on the first downstream process mold cavity surfaces  210  in the intermediate plate removal step ( FIG.  10   ). 
     Additionally, in the example of  FIG.  9   , in the preparatory molding step ( FIG.  9   ), in part of the first downstream process mold  21  that is located on the outer side in the circumferential direction of the one or more (e.g., in the example of  FIG.  9   , the plurality of) grooves  212  included in the first downstream process mold  21 , the flat surfaces  214  of the first downstream process mold  21  and the first flat surfaces  233  of the intermediate plate  23  are in abutment contact with each other, without forming any first flash  321   b.    
     Similarly in the examples described herein, when the second flashes  322   b  are formed in the preparatory molding step as described above ( FIG.  9   ), the second downstream process mold  22  preferably includes, on the surface thereof that is closer to the intermediate plate  23 , one or more (e.g., in the example of  FIG.  9   , a plurality of) grooves  222  that are located on the outer side in the circumferential direction of the second downstream process mold cavity surfaces  220 . In this case, the second downstream process mold  22  preferably includes, on the surface thereof that is closer to the intermediate plate  23 , the flat surfaces  225  that extend between the second downstream process mold cavity surfaces  220  and the grooves  222  in parallel to the downstream process mold perpendicular-to-axis direction OPD to couple the second downstream process mold cavity surfaces  220  and the grooves  222 . The flat surface  225  are located at the same positions in the downstream process mold axial direction OAD, with respect to the opening end surfaces of the second downstream process mold cavity surfaces  220 . Due to this configuration, the second flashes  322   b  formed in the preparatory molding step ( FIG.  9   ) include thick portions  322   bk , which are molded between the grooves  222  of the second downstream process mold  22  and the second flat surfaces  234  of the intermediate plate  23 , and thin portions  322   bn , which are molded between the flat surfaces  225  located between the second downstream process mold cavity surfaces  220  and the grooves  222  in the second downstream process mold  22 , and the second flat surfaces  234  of the intermediate plate  23 . The thin portions  322   bn  are smaller in thickness than the thick portions  322   bk  in the downstream process mold axial direction OAD. The thin portions  322   bn  couple the second outer core materials  322  and the thick portions  322   bk . Since having the thick portions  322   bk , the second flashes  322   b  may be more firmly attached to the second downstream process mold  22 , and this facilitates the second outer core materials  322  to be held on the second downstream process mold cavity surfaces  220  in the intermediate plate removal step ( FIG.  10   ). Further, since having the thin portions  322   bn , the second flashes  322   b  may be easily broken at the thin portions  322   bn  in the flash removal step, and thus, the operation of removing the second flashes  322   b  is simplified. 
     The grooves  222  preferably extend continuously, on the outer side in the circumferential direction of the second downstream process mold cavity surfaces  220 , over the entire circumferences. This case implies that the thick portions  322   bk  of the second flashes  322   b  that are molded by the grooves  222  continuously extend, on the outer side in the circumferential direction of the second outer core materials  322 , over the entire circumferences. This allows the second flashes  322   b  to be more firmly attached to the second downstream process mold  22 . 
     Further, the flat surfaces  225 , which are located between the second downstream process mold cavity surfaces  220  and the grooves  222 , preferably extend continuously over the entire circumferences around the second downstream process mold cavity surfaces  220 . This case implies that the thin portions  322   bn  of the second flashes  322   b  that are molded by the flat surfaces  225  extend continuously over the entire circumferences between the second outer core materials  322  and the thick portions  322   bk . Thus, the operation of removing the second flash  322   b  is even more simplified. 
     In this case, at least one of the one or more grooves  222  of the second downstream process mold  22  preferably includes, on a groove bottom surface (i.e., on a surface thereof that is located opposite to the open end surface of the groove  222 ), a convexity  223  ( FIG.  9   ). In the example of  FIG.  9   , only one or more grooves  222   a  in the plurality of grooves  222  that are located between the plurality of second downstream process mold cavity surfaces  220  include, on the groove bottom surfaces thereof, the convexities  223 . The grooves  222  including the convexities  223  will provide the thick portions  322   bk  of the second flashes  322   b  formed in the preparatory molding step ( FIG.  9   ) with concavities  322   bg  molded in accordance with the convexities  223 . A resulting anchor effect allows the second flashes  322   b  to be even more firmly attached to the second downstream process mold  22 , thereby further facilitating the second outer core materials  322  to be held on the second downstream process mold cavity surfaces  220  in the intermediate plate removal step ( FIG.  10   ). 
     Additionally, in the example of  FIG.  9   , in the preparatory molding step ( FIG.  9   ), in part of the second downstream process mold  22  that is located on the outer side in the circumferential direction of the one or more (e.g., in the example of  FIG.  9   , the plurality of) grooves  222  included in the second downstream process mold  22 , the flat surfaces  224  of the second downstream process mold  22  and the second flat surface  234  of the intermediate plate  23  are in abutment contact with each other, without forming any second flash  322   b.    
     In the examples illustrated in  FIGS.  11  and  12   , in the vulcanization step ( FIGS.  11  and  12   ), and the thick portions  321   bk  of the first flashes  321   b  and the thick portions  322   bk  of the second flashes  322   b  are joined to form thick portions  36   k  of the joining flashes  36 , the thin portions  321   bn  of the first flashes  321   b  and the thin portions  322   bn  of the second flashes  322   b  are joined to form thin portions  36   n  of the joining flashes  36 . 
     In the preparatory molding step ( FIG.  9   ) in the downstream process, at least part of the first outer core materials  321  and the second outer core materials  322  that are located on the inner side in the circumferential direction in the intermediate molded bodies  33  are maintained preferably in the unvulcanized or the semi-vulcanized state, more preferably in the unvulcanized state. This increases the degree of adhesion between the first outer core materials  321  and the second outer core materials  322 , and the inner cores  31  in the subsequent vulcanization step, and thus, the durability of the multilayer cores  3  may be improved. 
     Further, in the preparatory molding step ( FIG.  9   ) in the downstream process, part of the first outer core materials  321  and the second outer core materials  322  that are located on the outer side in the circumferential direction in the intermediate molded bodies  33  are preferably brought into the semi-vulcanized or the vulcanized state. This allows the part of the first outer core materials  321  and the second outer core materials  322  that are located on the outer side in the circumferential direction to be moderately cured, and thus, the spherical outer surfaces of the outer cores  32  may be molded with higher precision, and the eccentricity in the inner cores  31  may be prevented. 
     From the above perspective, in the examples described herein, in the preparatory molding step ( FIG.  9   ) in the downstream process, when a temperature of the first downstream process mold  21  is T 1  and a temperature of the second downstream process mold  22  is T 2 , T 1  and T 2  preferably satisfy the relation T 1 ≥T 2 . 
     Further, in the preparatory molding step ( FIG.  9   ) in the downstream process, when a temperature of the intermediate plate  23  is T 3 , T 1  and T 3  preferably satisfy the relation T 1 ≥T 3 , and more preferably satisfy the relation T 1 &gt;T 3 . Further, T 2  and T 3  preferably satisfy the relation T 2 ≥T 3 , and more preferably satisfy the relation T 2 &gt;T 3 . 
     Further, T 1 , T 2 , and T 3  preferably satisfy the relation T 1 ≥T 2 ≥T 3 . More preferably, T 1 , T 2 , and T 3  satisfy the relation T 1 =T 2 &gt;T 3  or satisfy the relation T 1 &gt;T 2 &gt;T 3 . 
     Further, T 1  preferably satisfies 80° C.&lt;T 1 &lt;170° C., and more preferably satisfies 150° C.&lt;T 1 &lt;170° C. T 2  preferably satisfies 80° C.&lt;T 2 &lt;170° C., and more preferably satisfies 150° C.&lt;T 2 &lt;170° C. T 3  is preferably 50 to 100° C. 
     Additionally, these magnitude relations and numerical ranges of the temperatures T 1 , T 2 , and T 3  are also preferable from the perspective of enabling the second outer core materials  322  to be held on the second downstream process mold cavity surfaces  220  and also enabling the intermediate molded bodies  33  to be held on the first downstream process mold cavity surfaces  210 , while the first downstream process mold  21 , the second downstream process mold  22 , and the intermediate plate  23  in the downstream process molding apparatus  2  are released from each other and the intermediate plate  23  is removed from the downstream process molding apparatus  2  in the subsequent intermediate plate removal step ( FIG.  10   ). 
     In the examples described herein, it is preferable that a temperature T 1 ′ of the first downstream process mold  21  and a temperature T 2 ′ of the second downstream process mold  22  in the vulcanization step ( FIG.  11   ) in the downstream process are respectively equal to the temperature T 1  of the first downstream process mold  21  and the temperature T 2  of the second downstream process mold  22  in the preparatory molding step ( FIG.  9   ) in the downstream process. This omits the need for changing the temperature of the downstream process molding apparatus  2  in the preparatory molding step ( FIG.  9   ) and the vulcanization step ( FIG.  11   ) and improves workability, and also reduces the total time required for the multilayer cores  3  to be molded. 
     T 1 ′ and T 2 ′ may be, however, respectively different from T 1  and T 2 . 
     In the examples described herein, a time period t of the compression molding in the preparatory molding step ( FIG.  9   ) in the downstream process is preferably shorter than a time period t′ of the vulcanization in the vulcanization step ( FIG.  11   ) in the downstream process. This prevents the first outer core materials  321  and the second outer core materials  322  from excessively vulcanized before the vulcanization step. 
     The time period t is preferably, for example, 1 to 3 minutes. The time period t′ is preferably, for example, 8 to 15 minutes. 
     A multilayer core molding method according to the present disclosure may be used to mold a multilayer core of a golf ball.