Patent Publication Number: US-2022227960-A1

Title: Method of manufacturing rubber product

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method of manufacturing a rubber product, and in particular to a method suitable for manufacturing rubber products that can flexibly respond to changes in the shape of the products. 
     The present application claims priority to Japanese Patent Application No. 2019-112255 filed on Jun. 17, 2019, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     Known methods of manufacturing rubber products include a method of manufacturing a product by placing unvulcanized rubber in a mold that matches the shape of the product and vulcanizing the rubber by applying heat and pressure, as in Patent Literature (PTL) 1, and a method of placing unvulcanized rubber in a mold that is larger than the product, vulcanizing the rubber by applying heat and pressure, and then machining the rubber by cutting or the like to yield the shape of the product. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP2004-107514A 
     SUMMARY 
     Technical Problem 
     However, in the method of using a mold that matches the shape of the product, the mold needs to be modified or made anew when a change in the shape of the product occurs for a reason such as a change in product specifications. The production costs and the time and effort for processing have thus been problematic. Furthermore, in a method of using a mold larger than the product, there has been room for improvement in the manufacturing accuracy, such as dimensional accuracy during machining, depending on the physical properties of the rubber. 
     It is an aim of the present disclosure to provide a method of manufacturing a rubber product that can flexibly respond to changes in the shape of a product and can achieve high manufacturing accuracy. 
     Solution to Problem 
     We carefully studied how to solve the aforementioned problem and concluded that a desired product shape can be obtained without using a mold by considering the shape of the desired product as a laminated body of thin pieces yielded when cutting on a number of planes orthogonal to a predetermined axis, and by then stacking and bonding the thin pieces together to manufacture the product. After further investigating the aforementioned additive manufacturing, we discovered that the additive manufacturing can be applied to rubber products by performing appropriate treatment on the powder of uncrosslinked rubber to be used as a material and further crosslinking the powder of the uncrosslinked rubber, thereby completing the present disclosure. 
     We provide the following. 
     A method of manufacturing a rubber product according to the present disclosure is a method of manufacturing a rubber product by bonding a plurality of thin pieces on a manufacturing table, the method including: 
     an adhesion prevention treatment step of treating uncrosslinked rubber powder, which is a material of the rubber product, to prevent mutual adhesion of the uncrosslinked rubber powder; 
     a first raw rubber powder supply step of supplying raw rubber powder, on which the adhesion prevention treatment has been performed, onto a surface of the manufacturing table uniformly in a layer of a thickness corresponding to a thin piece; 
     a first crosslinking step of irradiating a portion of the layer of raw rubber powder with an electron beam according to a shape corresponding to the thin piece to crosslink the portion that is irradiated; 
     a post-treatment step of adjusting relative positions of a surface of a crosslinked portion in the layer of raw rubber powder and a surface of the manufacturing table to align the surface of the crosslinked portion with the surface of the manufacturing table; 
     a second raw rubber powder supply step of supplying the raw rubber powder onto the surface of the manufacturing table, including the surface of the crosslinked portion, uniformly in a layer of a thickness corresponding to a next thin piece to be stacked on the thin piece; and 
     a second crosslinking step of irradiating a portion of the layer of raw rubber powder with an electron beam according to a shape corresponding to the next thin piece to crosslink the portion that is irradiated; 
     wherein the post-treatment step, the second raw rubber powder supply step, and the second crosslinking step are sequentially repeated to bond the plurality of thin pieces together. 
     Advantageous Effect 
     According to the present disclosure, a method of manufacturing a rubber product that, without use of a mold, can flexibly respond to changes in the shape of a product and can achieve high manufacturing accuracy can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a flowchart illustrating an outline of a method of manufacturing a rubber product according to an embodiment of the present disclosure; 
         FIG. 2  is a diagram illustrating an example of a rubber product manufactured by the method of manufacturing a rubber product according to an embodiment of the present disclosure; 
         FIG. 3  is a diagram illustrating thin pieces; 
         FIG. 4A  is a perspective view schematically illustrating the configuration of an additive manufacturing apparatus; 
         FIG. 4B  is a cross-sectional view schematically illustrating the configuration of the additive manufacturing apparatus; 
         FIG. 5A  is a perspective view schematically illustrating a first raw rubber powder supply process; 
         FIG. 5B  is a cross-sectional view schematically illustrating a first raw rubber powder supply process; 
         FIG. 6A  is a perspective view schematically illustrating a first cross-linking process; 
         FIG. 6B  is a cross-sectional view schematically illustrating a first cross-linking process; 
         FIG. 7A  is a perspective view schematically illustrating a post-treatment process; 
         FIG. 7B  is a cross-sectional view schematically illustrating a post-treatment process; 
         FIG. 8A  is a cross-sectional view schematically illustrating a second raw rubber powder supply process; 
         FIG. 8B  is a cross-sectional view schematically illustrating a second cross-linking process; 
         FIG. 8C  is a cross-sectional view schematically illustrating a post-treatment process; 
         FIG. 8D  is a diagram illustrating a state in which a plurality of raw rubber powder layers are stacked; 
         FIG. 9  is a perspective view transparently illustrating another example of a rubber product manufactured by the method of manufacturing a rubber product according to an embodiment of the present disclosure; and 
         FIG. 10  is a perspective view transparently illustrating yet another example of a rubber product manufactured by the method of manufacturing a rubber product according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Methods of manufacturing rubber products according to embodiments of the present disclosure are described below.  FIG. 1  is a flowchart illustrating an outline of a method of manufacturing a rubber product according to an embodiment of the present disclosure. 
     As illustrated in  FIG. 1 , the method of manufacturing a rubber product according to the present disclosure is a method of manufacturing a rubber product by bonding a plurality of thin pieces on a manufacturing table, and as illustrated in  FIG. 1 , the method includes treating uncrosslinked rubber powder, which is a material of the rubber product, to prevent mutual adhesion of the uncrosslinked rubber powder (an adhesion prevention treatment step S 1 ), acquiring data on a plurality of thin pieces of the rubber product (thin piece data acquisition step S 2 ), setting a manufacturing table to a state in which manufacturing can start (preparation step S 3 ), supplying raw rubber powder, on which the adhesion prevention treatment has been performed, onto a surface of the manufacturing table uniformly in a layer of a thickness corresponding to a thin piece (first raw rubber powder supply step S 4 ), irradiating the raw rubber powder layer with an electron beam according to a shape corresponding to the thin piece to crosslink the irradiated portion (first crosslinking step S 5 ), adjusting relative positions of a surface of a crosslinked portion in the raw rubber powder layer and a surface of the manufacturing table to align the surface of the crosslinked portion with the surface of the manufacturing table (post-treatment step S 6 ), supplying the raw rubber powder onto the surface of the crosslinked portion uniformly in a layer of a thickness corresponding to the next thin piece to be stacked on the thin piece (second raw rubber powder supply step S 7 ), and irradiating the raw rubber powder layer with an electron beam according to a shape corresponding to the next thin piece to crosslink the irradiated portion (second crosslinking step S 8 ). The post-treatment step S 6 , the second raw rubber powder supply step S 7 , and the second crosslinking step S 8  are sequentially repeated to bond the plurality of thin pieces together and manufacture the rubber product. 
       FIG. 2  illustrates a rubber product  10 , which is an example of a rubber product to be manufactured by the method of manufacturing a rubber product according to the present embodiment. As illustrated in  FIG. 2 , the rubber product  10  can, for example, have a cylindrical shape. 
     Here, thin pieces refer to layers that are divided by cutting at numerous planes orthogonal to the axis along the stacking direction of the rubber product to be manufactured. 
     In the example of the rubber product  10 , when the Z-axis direction is the stacking direction (height), a plurality of sliced layers are formed by cutting the cylindrical three-dimensional shape at numerous planes orthogonal to the Z-axis direction. Each layer in the plurality of sliced layers illustrated in  FIG. 3  is a thin piece of the rubber product  10 . The stacking direction in additive manufacturing of the rubber product  10  refers to the Z-axis direction in  FIGS. 2 and 3 , but a suitable direction can be used as the stacking direction according to the shape, size, and the like of the rubber product to be manufactured. 
     Although the thin pieces L 1  to L 3  from the first layer to the third layer in the Z-axis direction of the rubber product  10  are illustrated in  FIG. 3 , the rubber product  10  is actually converted into data on thin pieces L 1  to LN (N is a natural number) for the number of layers required to manufacture the rubber product  10 . 
     In this way, the shape of the rubber product  10  is converted into the data on a plurality of thin pieces, and the thin pieces are then stacked and bonded together based on this data to manufacture the rubber product  10 . 
     Each step is described in detail below. 
     [Adhesion Prevention Treatment Step] 
     In the adhesion prevention treatment step S 1 , uncrosslinked rubber powder, which is a material of the rubber product  10 , is treated to prevent mutual adhesion of the uncrosslinked rubber powder. 
     A material suitable for rubber products can be used as the material forming the uncrosslinked rubber powder in the present disclosure. Powders of polyisoprene rubber (IR), styrene-butadiene copolymer rubber (SBR), polybutadiene rubber (BR), ethylene propylene diene rubber (EPDM), chloroprene rubber (CR), halogenated butyl rubber, acrylonitrile butadiene rubber (NBR), and other synthetic rubbers, in addition to natural rubber (NR), can be used as the rubber powder. Among these, powders of natural rubber (NR), styrene-butadiene copolymer rubber (SBR), and polybutadiene rubber (BR) are preferred. These rubber powders may be used alone, or a combination of two or more types thereof may be used. 
     The uncrosslinked rubber powder may also contain other materials other than natural rubber and synthetic rubber. As the other materials, fillers such as carbon black and silica, softeners, age resistors, zinc oxide, crosslinking accelerators, and the like may be selected as appropriate within a range not detrimental to the aim of the present disclosure. The other materials may be added not only by inclusion in the uncrosslinked rubber powder in advance but also after the treatment to prevent adhesion of the uncrosslinked rubber powder. 
     Furthermore, as an example of other materials, calcium oxide may be added. By the addition of calcium oxide into the uncrosslinked rubber powder, even when the rubber is heated to a temperature higher than the melting point during the crosslinking of the rubber, bubbles that might be generated in the rubber by heat can be suppressed, and cracks or tears can be prevented from occurring at the areas with bubbles. 
     The treatment on the uncrosslinked rubber powder to prevent mutual adhesion of the powder refers to the treatment for preventing the powder particles from adhering to each other and is suitably performed by coating the surface of the uncrosslinked rubber powder with an adhesion prevention agent. As the adhesion prevention agent, any of synthetic resin, talc, silica, calcium carbonate, calcium stearate, zinc stearate, carbon black, and the like, for example, or any combination thereof, can be used. 
     While the means for coating the surface of the uncrosslinked rubber powder with the adhesion prevention agent is not particularly limited, a powdered adhesion prevention agent having a diameter smaller than that of the uncrosslinked rubber powder can be stirred and mixed with the uncrosslinked rubber powder to coat the surface of the uncrosslinked rubber powder, for example. The surface of the uncrosslinked rubber powder may also be coated by applying a dissolved solution or an aqueous dispersion of the adhesion prevention agent to the surface of the uncrosslinked rubber powder. When a dissolved solution or aqueous dispersion of the adhesion prevention agent is used, a coating layer of uniform thickness can easily be formed on the surface of the uncrosslinked rubber powder, but a step of drying the uncrosslinked rubber powder is required after the application. A powdered adhesion prevention agent is preferably used to prevent an increase in the number of production steps. 
     The adhesion prevention treatment step may be a step included in the preparation of the uncrosslinked rubber powder that becomes the material of the rubber product  10  or may be a separate step performed after the preparation of the uncrosslinked rubber powder. 
     According to the adhesion prevention treatment step, mutual adhesion of the uncrosslinked rubber powder can be prevented. That is, lumps are not formed due to mutual adhesion of the uncrosslinked rubber powder. This facilitates handling of the uncrosslinked rubber powder. In particular, when the uncrosslinked rubber powder is stored or applied to the additive manufacturing apparatus  100  described below, the manufacturing accuracy of the rubber product can be improved, and the uncrosslinked rubber powder can be easily stored and supplied. 
     The average diameter of the uncrosslinked rubber powder (raw rubber powder) to which the adhesion prevention treatment is applied is preferably from 1 μm to 500 μm. By the average diameter being 1 μm or more, floating during storage and other times can be suppressed, and the mutual adhesion of the raw rubber powder can more effectively be suppressed. By the average diameter being 500 μm or less, the manufacturing accuracy of the rubber product can be further improved. 
     In the below-illustrated steps from the preparation step onwards, the raw rubber powder is used to manufacture the rubber product  10 . The manufacturing apparatus is not particularly limited as long as the method of manufacturing a rubber product according to the present disclosure can be implemented, but an additive manufacturing apparatus  100  with the following form can, for example, be used. 
       FIG. 4A  is a perspective view schematically illustrating the configuration of the additive manufacturing apparatus  100 , and  FIG. 4B  is a cross-sectional view schematically illustrating the configuration of the additive manufacturing apparatus  100 .  FIG. 4B  schematically illustrates a cross-section along line II-II of  FIG. 4A . 
     As illustrated in  FIGS. 4A and 4B , the additive manufacturing apparatus  100  includes a manufacturing table  101 , a recoater  140 , and electron beam irradiation means  150 . The manufacturing table  101  includes a manufacturing unit  110 , a supply plate  120 , and a raw material supplier  130 . 
     The manufacturing unit  110  includes a manufacturing base  111  and a raising/lowering mechanism  112 , and the manufacturing base  111  includes a surface  111   a  for manufacturing rubber products. The manufacturing base  111  has a circular cylinder shape in the illustrated example but may have a polygonal cylinder shape or the like. The manufacturing base  111  can be raised and lowered in the up-down direction (Z-axis direction) by the raising/lowering mechanism  112 . The raising/lowering mechanism  112  can, for example, be provided with a piston member using hydraulic pressure or pneumatic pressure, or with a ball screw. 
     The supply plate  120  is disposed around the manufacturing unit  110  and includes a plate-like portion  121  and a support  122  (not illustrated). The plate-like portion  121  includes a surface  121   a  onto which raw rubber powder can be supplied. The plate-like portion  121  has a shape in which a part of a plate-like member is perforated according to the outer circumferential shape of the cylindrical manufacturing base  111  in the illustrated example, but the shape is not particularly limited. The support  122  can be any columnar or plate-like shape that supports the plate-like portion  121 . The support  122  may be provided with a raising/lowering mechanism, but is not coupled to raising/lowering operations by the raising/lowering mechanism  112  of the manufacturing unit  110 . 
     The raw material supplier  130  has a tank-shaped or box-shaped raw material housing  131 , which opens to the surface  121   a  of the supply plate  120  and extends in the thickness direction (Z-axis direction) of the supply plate  120 , and a raising/lowering mechanism  132 . The raw material housing  131  can contain raw rubber powder. A bottom portion  131   b  of the raw material housing  131  can be raised and lowered in the Z-axis direction by the raising/lowering mechanism  132 , and the raw rubber powder housed in the raw material housing  131  can be pushed out onto the surface  121   a  by the raising/lowering operation of the bottom portion  131   b.  The raising/lowering mechanism  132  can, for example, be a mechanism provided with a piston using hydraulic pressure or pneumatic pressure, or with a ball screw. 
     The arrangement and configuration of the raw material supplier is not limited to the example of the raw material supplier  130 . A configuration may be adopted in which a raw material housing and a nozzle are included, the raw material supplier is arranged at a position (upper side) separated in the Z-axis direction from the surface  121   a,  and the raw rubber powder is discharged onto the surface  121   a.    
     The recoater  140  has a roller shape and is disposed on the surface  121   a.  The recoater  140  is connected at both axial ends to a drive mechanism (not illustrated) and can be moved while rolling in the X-axis direction. The recoater  140  can be separated from the surface  121   a  by a certain distance in the Z-axis direction, and this distance can be adjusted according to the thickness of the layer to be supplied onto the surface  121   a.  As illustrated in  FIG. 4A , the axial length of the recoater  140  is larger than the longitudinal length of the opening in the surface  121   a  of the raw material supplier  130 . The recoater  140  is not limited to a member having a roller shape or a rotating member and may, for example, be a plate-like member (blade). 
     In the additive manufacturing apparatus  100 , the electron beam irradiation means  150  is arranged at a distance in the Z-axis direction with respect to the surface  111   a.  Although any electron beam irradiation means  150  can be used, at least one electron beam source  151  and an electron beam adjusting means  152  are preferably used. 
     The electron beam source  151  can be a thermal electron emission type electron gun. More specifically, for example, thermal electrons are generated by heating a cathode formed by tungsten, LaB 6 , CeB 6 , or the like, and an electron beam is generated by accelerating the thermal electrons. 
     The electron beam adjusting means  152  can, for example, include a magnetic field generator  152   a  and a focus controller  152   b.  Using the magnetic force of a permanent magnet, an electromagnet, or the like, the magnetic field generator  152   a  can adjust the convergence, deflection, and the like of the electron beam generated by the electron beam source  151 . The focus controller  152   b  can also adjust the focus of the electron beam at the irradiation target using an optical lens, an electromagnetic lens, or the like. Although one electron beam source  151  is depicted in the illustrated example, a plurality of electron beam sources  151  may be included, each of which may be controlled under different irradiation conditions. 
     By being housed in a chamber (not illustrated) or the like, the additive manufacturing apparatus  100  can be used in a high vacuum environment during operation of the electron beam irradiation means  150 . 
     Furthermore, the additive manufacturing apparatus  100  can be connected over a network to a control device  200 . The control device  200  includes a hardware processor, such as a central processing unit, and includes a thin piece data acquisition interface  201  and a manufacturing controller  202 . 
     The thin piece data acquisition interface  201  can acquire the data on the thin pieces necessary for manufacturing the rubber product  10 . 
     The manufacturing controller  202  can control the additive manufacturing by providing instructions and information, via a network, to each component of the additive manufacturing apparatus  100  based on the data on the thin pieces. That is, the manufacturing controller  202  can control the operations of the manufacturing table  101 , the recoater  140 , the electron beam irradiation means  150 , and the like, which configure the additive manufacturing apparatus  100 . 
     Each of the steps from the thin piece data acquisition process S 2  onwards in the method of manufacturing a rubber product according to the present embodiment will be described below in detail with an example using the additive manufacturing apparatus  100 . 
     [Thin Piece Data Acquisition Step] 
     In the thin piece data acquisition step S 2 , the thin piece data on the rubber product  10 , which is necessary for manufacturing the rubber product  10 , is acquired by the thin piece data acquisition interface  201  of the control device  200 . 
     The thin piece data can, for example, be acquired by converting three-dimensional manufacturing data on the rubber product  10 . More specifically, the thin piece data acquisition interface  201  acquires the three-dimensional manufacturing data from an apparatus such as another computer that is connected to the control device  200  or that can transmit information to the control device  200 . Here, the three-dimensional manufacturing data for manufacturing the rubber product  10  is, for example, three-dimensional manufacturing data designed by three-dimensional CAD, or three-dimensional manufacturing data captured by a three-dimensional scanner, digitizer, or the like. The three-dimensional manufacturing data may be converted into Standard Triangulated Language (STL) format, in which the surface of the three-dimensional rubber product  10  is represented as a collection of triangles. This three-dimensional manufacturing data on the rubber product  10  is acquired by the thin piece data acquisition interface  201  and converted into data on thin pieces L 1  to LN (N is a natural number). 
     Three-dimensional manufacturing data may also be converted to thin piece data by an apparatus such as another computer that is connected to the control device  200  or is capable of transmitting information to the control device  200 , and the thin piece data may then be acquired by the thin piece data acquisition interface  201 . 
     [Preparation Step] 
     Next, the preparation step S 3  will be described with reference to  FIGS. 4A and 4B . 
     To start manufacturing based on the thin piece data, the manufacturing table  101  is set to a state in which the manufacturing of the rubber product can be started in the preparation step S 3 . That is, the relative positions of the surface  111   a  of the manufacturing unit  110  and the surface  121   a  of the supply plate  120  are adjusted to align the surface  111   a  with the surface  121   a.  In other words, the surface  111   a  and the surface  121   a  are arranged so that they extend to the same position and are substantially flush. Extending to the same position refers to being at the same position in the Z-axis direction and extending in the X-axis direction. 
     Of the two ends, in the X-axis direction, of the surface  121   a  of the supply plate  120 , the recoater  140  is preferably arranged to be located at the end on the raw material supplier  130  side before the first raw rubber powder supply step S 4 . 
     A base material (not illustrated) made of a material that can be removed from the rubber product  10  may be disposed beforehand on the surface  111   a  of the manufacturing unit  110  so that the rubber product  10  can be easily removed from the surface  111   a  of the manufacturing unit  110  after all the steps are finished and the bonding of all the thin pieces is completed. 
     [First Raw Rubber Powder Supply Step] 
     Next, the first raw rubber powder supply step S 4  will be described with reference to  FIGS. 5A and 5B . In  FIGS. 5A and 5B  and subsequent figures, the control device  200  is omitted. 
     In the first raw rubber powder supply step S 4 , the raw rubber powder is supplied onto the surface of the manufacturing table  101  uniformly in a layer of a thickness corresponding to the thin piece L 1 . 
     In the example using the additive manufacturing apparatus  100 , first, the raw rubber powder is stored in the raw material housing  131  of the raw material supplier  130 , and the bottom portion  131   b  is raised by the raising/lowering mechanism  132  in a direction approaching the surface  121   a  in the Z-axis direction, so that an amount of raw rubber powder capable of forming a layer of a thickness corresponding to the thin piece L 1  is disposed on the surface  121   a  of the supply plate  120 . After the raw rubber powder is disposed on the surface  121   a,  the recoater  140  is rolled to pass over the surface of the manufacturing table  101 , i.e., the surface  111 a of the manufacturing unit  110  and the surface  121   a  of the supply plate  120 , thereby supplying the raw rubber powder onto the surface  111 a of the manufacturing unit  110  and the surface  121   a  of the supply plate  120  uniformly in a layer of a thickness corresponding to the thin piece L 1  to form a raw rubber powder layer ml. At this time, it is essential that the recoater  140  be rolled while maintaining a distance, from the surface  111   a  of the manufacturing unit  110  and the surface  121   a  of the supply plate  120 , of a thickness corresponding to the thin piece L 1 . 
     The thickness corresponding to the thin piece L 1  can be controlled by the control device  200  based on the thin piece data. The thickness corresponding to the thin piece L 1 , that is, the thickness t 1  of the raw rubber powder layer m 1 , can be adjusted according to the irradiation conditions of the electron beam described below, the average particle size of the raw rubber powder, and the like, but is preferably 500 μm or less for uniform crosslinking over the thickness direction. 
     [First Crosslinking Step] 
     Next, the first crosslinking step S 5  will be described with reference to  FIGS. 6A and 6B . 
     In the first crosslinking step S 5 , the raw rubber powder layer ml formed by the first raw rubber powder supply step S 4  is irradiated with an electron beam according to a shape corresponding to the thin piece L 1  to crosslink the irradiated portion. 
     The electron beam irradiation means is not particularly limited, but the electron beam irradiation means  150 , for example, can be used. The electron beam generated by the electron beam source  151  of the electron beam irradiation means  150  is converged and deflected by the magnetic field generator  152   a,  and the irradiation position is moved based on the thin piece data to scan the raw rubber powder layer ml. Furthermore, the focus controller  152   b  enables adjustment of the position where the electron beam achieves just focus, i.e., the focus. 
     When the raw rubber powder layer ml is irradiated with the electron beam, the rubber molecules of the raw rubber powder are provided with energy by high-speed electrons. The high-speed electrons cleave the molecular bonds, which generates radicals, and the radicals react between molecular chains to form a three-dimensional structure, yielding a crosslinking reaction. 
     According to the electron beam crosslinking means, the location to be crosslinked can be changed by controlling the position where the electron beam is irradiated, thus eliminating the need to prepare a mold and other such hassle, and enabling a flexible response to changes in product shape. Furthermore, a high manufacturing accuracy can be achieved. Rubber products with complex shapes that are difficult to remove from a mold, or that cannot be manufactured with a mold, can also be manufactured. 
     The irradiation conditions of the electron beam can be controlled by the control device  200  based on the thin piece data on the rubber product  10  that is to be manufactured. The specific irradiation conditions of the electron beam can be adjusted appropriately according to factors such as the thickness of the raw rubber powder layer and the average diameter of the raw rubber powder. 
     For example, the focusing of the electron beam is preferably set to achieve just focus on the surface of the raw rubber powder layer to generate a sufficient crosslinking reaction in the rubber. 
     Although the temperature at the time of electron beam irradiation is not particularly limited, the first crosslinking step S 5  is preferably performed in a temperature environment lower than the melting point of the rubber. According to the above configuration, the crosslinking reaction can be produced at a lower temperature than with means for vulcanizing rubber by heating. Bubbles that might be generated in the rubber by heating can therefore be suppressed, and cracks or tears can be prevented from occurring at the areas with bubbles. 
     Furthermore, the electron beam irradiation is preferably performed in a high vacuum environment to prevent deactivation of the radicals by oxygen. For example, when the additive manufacturing apparatus  100  is housed in a chamber or the like, a high vacuum is created in the chamber. 
     By the first crosslinking step S 5 , a crosslinked portion M 1  can be formed in the raw rubber powder layer ml as illustrated in  FIGS. 6A and 6B . In the illustrated example, a ring-shaped crosslinked portion M 1  is formed, corresponding to the thin piece L 1  of the rubber product  10 . 
     [Post-Treatment Step] 
     The post-treatment step S 6  after the first crosslinking step S 5  will be described with reference to  FIGS. 7A and 7B . In the post-treatment step S 6 , after the first crosslinking step S 5 , the relative positions of the surface of the crosslinked portion M 1  in the raw rubber powder layer ml and the surface of the manufacturing table  101  are adjusted to align the surface of the crosslinked portion M 1  with the surface of the manufacturing table  101 . That is, in the illustrated example, the relative positions in the Z-axis direction of the surface of the crosslinked portion M 1  and the surface  121   a  of the supply plate  120  are adjusted to align the surface of the crosslinked portion M 1  with the surface  121   a  of the supply plate  120 . To adjust the relative positions, the manufacturing unit  110  can be lowered, by the thickness of the crosslinked portion M 1  of the raw rubber powder layer m 1 , away from the surface  121   a  in the Z-axis direction using the raising/lowering mechanism  112 , for example, to align the surface of the crosslinked portion M 1  with the surface  121   a  of the supply plate  120 . 
     The relative positions of the surface of the crosslinked portion M 1  and the surface  121   a  of the supply plate  120  may be adjusted by raising and lowering the supply plate  120  relative to the manufacturing unit  110 . 
     Before or after the post-treatment step S 6 , a step of removing the uncrosslinked raw rubber powder remaining on the surface  121   a  of the supply plate  120  may be provided. The means for removing the uncrosslinked raw rubber powder is not particularly limited, but suction means, for example, may be used. Alternatively, a collection hole (not illustrated) opening to the surface  121   a  may be provided on the outer periphery of the manufacturing unit  110  in the supply plate  120 , and the uncrosslinked raw rubber powder may swept by a brush or blown by air to the collection hole and removed. A passage connecting the collection hole to the raw material housing  131  may be formed to reuse the uncrosslinked raw rubber powder in a subsequent step. 
     [Second Raw Rubber Powder Supply Step]The second raw rubber powder supply step S 7  will be described with reference to  FIG. 8A .  FIG. 8A  is a schematic cross-sectional view illustrating the second raw rubber powder supply step S 7  and schematically illustrates the manufacturing table  101 , the recoater  140 , and the raw rubber powder layer m 1 . 
     In the second raw rubber powder supply step S 7 , the raw rubber powder is supplied onto the surface of the manufacturing table  101  including the surface of the crosslinked portion M 1 , i.e., the surface of the crosslinked portion M 1 , the surface of the raw rubber powder layer m 1 , and the surface  121   a  of the supply plate  120 , uniformly in a layer of a thickness corresponding to the next thin piece L 2  to be stacked on the thin piece L 1 . 
     As in the first raw rubber powder supply step, first, the bottom portion  131   b  of the raw material housing  131  is raised by the raising/lowering mechanism  132  in a direction approaching the surface  121   a  in the Z-axis direction, so that an amount of raw rubber powder capable of forming a layer of a thickness corresponding to the thin piece L 2  is disposed on the surface  121   a  of the supply plate  120 . After the raw rubber powder is disposed on the surface  121   a,  the recoater  140  is rolled to pass over the surface  121   a  of the supply plate  120  and the surface of the crosslinked portion M 1  to supply the raw rubber powder onto the surface of the crosslinked portion M 1  as a raw rubber powder layer m 2 . At this time, it is essential that the recoater  140  be rolled while maintaining a distance, from the surface  121   a  of the supply plate  120  and the surface of the crosslinked portion M 1 , of a thickness corresponding to the thin piece L 2 . In the illustrated example, the raw rubber powder layer m 2  is supplied on the crosslinked portion M 1 , the raw rubber powder layer ml containing the crosslinked portion M 1 , and the surface  120   a  of the supply plate  120 . 
     The layer of a thickness corresponding to the thin piece L 2  can be controlled by the control device  200  based on the thin piece data. 
     [Second Crosslinking Step] 
     Next, the second crosslinking step S 8  will be described with reference to  FIG. 8B . In the second crosslinking step S 8 , the raw rubber powder layer m 2  formed by the second raw rubber powder supply step S 7  is irradiated with an electron beam according to a shape corresponding to the next thin piece L 2  to crosslink the irradiated portion. 
     The electron beam irradiation means  150 , for example, can be used for the electron beam irradiation. The electron beam generated by the electron beam source  151  of the electron beam irradiation means  150  is converged and deflected by the magnetic field generator  152 a, and the irradiation position is moved based on the thin piece data to scan the raw rubber powder layer. Furthermore, the focus controller  152 b enables adjustment of the position where the electron beam achieves just focus, i.e., the focus. 
     The irradiation conditions of the electron beam may be the same as the irradiation conditions in the first crosslinking step S 5 , or may be different. 
     For example, in the first crosslinking step S 5 , the focusing of the electron beam may be controlled so that the electron beam achieves just focus on the surface of the raw rubber powder layer, and in the second irradiation step, a plurality of electron beam supply sources may be used and the electron beams controlled to achieve just focus on the surface of the raw rubber powder layer and on the boundary with the adjacent raw rubber powder layer immediately below the raw rubber powder layer. According to the above configuration, the raw rubber powder layer can be sufficiently crosslinked, and the crosslinked portions M 1  and M 2  of the adjacent raw rubber powder layers m 1  and m 2  can be bonded together. 
     The focusing of the electron beam can be controlled so that the focusing is always the same, or the focusing can be changed. 
     In the second crosslinking step S 8 , although the temperature at the time of electron beam irradiation is not particularly limited, the crosslinking is preferably performed in a temperature environment lower than the melting point of the rubber, as in the first crosslinking step S 5 . According to the above configuration, the manufacturing accuracy can be further enhanced, and deterioration of the rubber product due to heating can be prevented. 
     The second crosslinking step S 8  enables the formation of a crosslinked portion, in the raw rubber powder layer, corresponding to the next thin piece. In  FIG. 8B , the crosslinked portion M 2 , which is a ring-shaped layer corresponding to the thin piece L 2  in the second layer from the bottom in the Z-axis direction of the rubber product  10 , is stacked onto and bonded to the crosslinked portion M 1 . 
     After the second crosslinking step S 8  is performed, the post-treatment step S 6 , the second powder supply step S 7 , and the second crosslinking step S 8  are sequentially repeated to bond the plurality of thin pieces together. 
     That is, after the second crosslinking step S 8 , as illustrated in  FIG. 8C , the relative positions of the surface of the crosslinked portion M 2  in the raw rubber powder layer m 2  and the surface of the manufacturing table  101  are adjusted to align the surface of the crosslinked portion M 2  with the surface of the manufacturing table  101 . In other words, in the illustrated example, the relative positions in the Z-axis direction of the surface of the crosslinked portion M 2  and the surface  121   a  of the supply plate  120  are adjusted to align the surface of the crosslinked portion M 2  with the surface  121   a  of the supply plate  120 . 
     After the surface of the crosslinked portion M 2  and the surface  121   a  of the supply plate  120  are aligned, the raw rubber powder is supplied onto the surface of the raw rubber powder layer m 2 , which is at the same position in the Z-axis direction as the surface  121   a  of the supply plate  120 , uniformly in a layer of a thickness corresponding to the next thin piece L 3 . The layer is then irradiated with an electron beam according to a shape corresponding to the next thin piece L 3  to crosslink the irradiated portion. The raw rubber powder layer can thus be sufficiently crosslinked, and the crosslinked portions of the adjacent raw rubber powder layers can be bonded together.  FIG. 8D  illustrates a state in which raw rubber powder layers ml to m 8  are stacked and aligned after a crosslinked portion M 8  is formed by electron beam irradiation of the raw rubber powder layer m 8 . In this way, by repetition of the post-treatment step S 6 , the second raw rubber powder supply step S 7 , and the second crosslinking step S 8 , a rubber product  10  having a plurality of thin pieces L 1  to LN bonded together is manufactured. 
     The method of manufacturing a rubber product including each of the above steps S 1  to S 8  can flexibly respond to changes in the shape of the product and achieve high manufacturing accuracy. 
     After the last layer in the Z-axis direction is crosslinked, a step of removing the rubber product  10  from the surface  111   a  of the manufacturing unit  110  and removing the uncrosslinked raw rubber powder remaining at the outer and inner periphery of the rubber product  10  is preferably included. 
     The means for removing the uncrosslinked raw rubber powder is not particularly limited, but the powder can, for example, be removed by blowing water or air onto the rubber product  10 . 
     Another example of the rubber product is a rubber product  11  having a shape such that a cylindrical hollow  11   a  is formed inside a cylinder, with a hole  11   b  penetrating from the hollow  11   a  to the outer periphery of the cylinder, as illustrated in  FIG. 9 . After crosslinking to yield the shape of the rubber product  11  by steps S 1  to S 8 , the uncrosslinked raw rubber powder remaining in the hollow  11   a  can be discharged from the hole  11   b.  At this time, after the uncrosslinked raw rubber powder is discharged from the hole  11   b,  the hole  11   b  may be plugged with another crosslinked rubber material. In this way, a hollow rubber product can be easily formed. 
     Another example of the rubber product is a rubber product  12  having a shape such that the entire outer periphery is curved, a hollow is formed inside, and a hole  12   b  penetrates from the hollow to the outer periphery, as illustrated in  FIG. 10 . After crosslinking to yield the shape of the rubber product  12  by steps S 1  to S 8 , the uncrosslinked raw rubber powder remaining inside can be discharged from the hole  12   b.  At this time, after the uncrosslinked raw rubber powder is discharged from the hole  12   b,  the hole  12   b  may be plugged with another crosslinked rubber material. In this way, a hollow rubber product can be easily formed. 
     INDUSTRIAL APPLICABILITY 
     The method of manufacturing a rubber product of the present disclosure can be suitably applied to the manufacturing of rubber products such as anti-vibration rubber, seismic isolation rubber, tires, and tire treads. 
     REFERENCE SIGNS LIST 
       10 ,  11 ,  12  Rubber product 
       100  Additive manufacturing apparatus 
       101  Manufacturing table 
       110  Manufacturing unit 
       111  Manufacturing base 
       111   a  Surface 
       112  Raising/lowering mechanism 
       120  Supply plate 
       121  Plate-like portion 
       121   a  Surface 
       130  Raw material supplier 
       131  Raw material housing 
       131   b  Bottom portion 
       132  Raising/lowering mechanism 
       140  Recoater 
       150  Electron beam irradiation means 
       151  Electron beam source 
       152  Electron beam adjusting means 
       152   a  Magnetic field generator 
       152   b  Focus controller 
       200  Control device 
       201  Thin piece data acquisition interface 
       202  Manufacturing controller 
     L 1 , L 2 , L 3  Thin piece 
     m 1 , m 2 , m 3 , m 4 , m 5 , m 6 , m 7 , m 8  Raw rubber powder layer 
     M 1 , M 2 , M 8  Crosslinked portion