Patent Publication Number: US-2018036958-A1

Title: Synthetic resin welded body and method of manufacturing the same

Description:
TECHNICAL FIELD 
     The present invention relates to a synthetic resin welded body formed by welding synthetic resin parts with each other and a method of manufacturing the same. 
     BACKGROUND ART 
     Conventionally, there is a known technique of a synthetic resin welded body formed by welding synthetic resin parts with each other. An example of the technique is disclosed in Patent Literature 1. 
     In Patent Literature 1, a cylinder head cover including a baffle plate in a rectangular flat plate shape and a flow passage forming member which is laid over the baffle plate from below to form an oil passage is disclosed. The baffle plate and the flow passage forming member are welded to each other on their laid-over faces. 
     However, in the structure described in Patent Literature 1, in radiating laser light from a side of the baffle plate to the flow passage forming member to weld them to each other, displacement of a laser scanning position and increase in a laser radiation diameter may cause radiation of the laser to a portion other than a portion of the oil passage forming member (absorption-side synthetic resin part) to be welded. As a result, the portion of the oil passage forming member which should not melt may melt unintentionally. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-open No. 2007-127014 
     SUMMARY OF INVENTION 
     Technical Problem 
     The present invention has been made with the above-described circumstances in view and it is an object of the invention to provide a synthetic resin welded body and a method of manufacturing the same, in which it is possible to make a portion of an absorption-side synthetic resin part to be welded likely to melt in laser welding. 
     Solution to Problem 
     The problem to be solved by the present invention is as described above and solutions to the problem will be described next. 
     A synthetic resin welded body according to an aspect of the invention includes: a laser light absorbing absorption-side synthetic resin part; and a laser light transmissive transmission-side synthetic resin part which is laid over the absorption-side synthetic resin part and laser-welded in this state. The transmission-side synthetic resin part is formed so that a welded area which is laser-welded to the absorption-side synthetic resin part is higher in laser transmittance than a non-welded area which is the other area. 
     In the synthetic resin welded body according to the aspect of the invention, in the transmission-side synthetic resin part, the welded area is formed to be smaller in thickness than the non-welded area so that the welded area is higher in laser transmittance than the non-welded area. 
     In the synthetic resin welded body according to the aspect of the invention, in the transmission-side synthetic resin part, the non-welded area is colored so that the welded area is higher in laser transmittance than the non-welded area. 
     In the synthetic resin welded body according to the aspect of the invention, in the transmission-side synthetic resin part, the welded area is made of material which is higher in laser transmittance than material for the non-welded area so that the welded area is higher in laser transmittance than the non-welded area. 
     In the synthetic resin welded body according to the aspect of the invention, the transmission-side synthetic resin part is a baffle plate which is disposed on a cylinder head cover and the absorption-side synthetic resin part is an oil passage forming member which includes a recessed portion recessed to an opposite side from the baffle plate and which is fixed to one face of the baffle plate to thereby form an oil passage with the recessed portion and the baffle plate. 
     A method of manufacturing a synthetic resin welded body according to another aspect of the invention includes: an absorption-side synthetic resin part preparing step of preparing a laser light absorbing absorption-side synthetic resin part; a transmission-side synthetic resin part preparing step of preparing a laser light transmissive transmission-side synthetic resin part; a disposing step of laying the absorption-side synthetic resin part and the transmission-side synthetic resin part over each other; and a laser welding step of radiating laser light from a side of the transmission-side synthetic resin part to melt the absorption-side synthetic resin part to thereby weld the absorption-side synthetic resin part and the transmission-side synthetic resin part to each other to obtain the synthetic resin welded body. The transmission-side synthetic resin part is formed so that a welded area which is laser-welded to the absorption-side synthetic resin part is higher in laser transmittance than a non-welded area which is the other area. 
     In the method of manufacturing the synthetic resin welded body according to the other aspect of the invention, in the transmission-side synthetic resin part, the welded area is formed to be smaller in thickness than the non-welded area so that the welded area is higher in laser transmittance than the non-welded area. 
     In the method of manufacturing the synthetic resin welded body according to the other aspect of the invention, in the transmission-side synthetic resin part, the non-welded area is colored so that the welded area is higher in laser transmittance than the non-welded area. 
     In the method of manufacturing the synthetic resin welded body according to the other aspect of the invention, in the transmission-side synthetic resin part, the welded area is made of material which is higher in laser transmittance than material for the non-welded area so that the welded area is higher in laser transmittance than the non-welded area. 
     In the method of manufacturing the synthetic resin welded body according to the other aspect of the invention, the transmission-side synthetic resin part is a baffle plate which is disposed on a cylinder head cover and the absorption-side synthetic resin part is an oil passage forming member which includes a recessed portion recessed to an opposite side from the baffle plate and which is fixed to one face of the baffle plate to thereby form an oil passage with the recessed portion and the baffle plate. 
     Advantageous Effects of Invention 
     As effects of the aspects of the present invention, the following effects are exerted. 
     In the synthetic resin welded body according to the aspect of the invention, it is possible to make only a portion to be welded of the absorption-side synthetic resin part likely to melt in the laser welding. 
     In the synthetic resin welded body according to the aspect of the invention, it is possible to make only the portion to be welded of the oil passage forming member likely to melt in the laser welding. 
     In the method of manufacturing the synthetic resin welded body according to the other aspect of the invention, it is possible to make only the portion to be welded of the absorption-side synthetic resin part likely to melt in the laser welding. 
     In the method of manufacturing the synthetic resin welded body according to the other aspect of the invention, it is possible to make only the portion to be welded of the oil passage forming member likely to melt in the laser welding. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of an inside of a cylinder head cover of an engine and shows a state of use of a baffle plate and an oil passage forming member according to an embodiment of the present invention. 
         FIG. 2  is a bottom view of the baffle plate. 
         FIG. 3  is a plan view of the oil passage forming member. 
         FIG. 4  is a plan view of a welded body formed by the baffle plate and the oil passage forming member. 
         FIG. 5  is a cross-sectional view taken along line A-A in  FIG. 4 . 
         FIG. 6  is a flowchart of a method of manufacturing the welded body formed by the baffle plate and the oil passage forming member according to the embodiment of the invention. 
         FIG. 7  is a partial cross-sectional view showing a state of the baffle plate and the oil passage forming member before laser welding. 
         FIG. 8  is a partial cross-sectional view showing a state of the baffle plate and the oil passage forming member after the laser welding. 
         FIG. 9  is a cross-sectional view of a welded body formed by a baffle plate and an oil passage forming member according to a second embodiment of the invention. 
         FIG. 10  is a cross-sectional view of a welded body formed by a baffle plate and an oil passage forming member according to a third embodiment of the invention. 
         FIG. 11  is a cross-sectional view of a welded body formed by a baffle plate and an oil passage forming member according to a fourth embodiment of the invention. 
         FIG. 12  is a cross-sectional view of a welded body formed by a baffle plate and an oil passage forming member according to a fifth embodiment of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Directions shown by arrows U, D, F, B, L, and R in the diagrams are respectively defined as an upward direction, a downward direction, a forward direction, a backward direction, a leftward direction, and a rightward direction and a description will be given below. 
     First, by using  FIG. 1 , a structure of an engine  1  including a baffle plate  110  and an oil passage forming member  120  according to an embodiment of the present invention will be described. 
     The engine  1  according to the embodiment includes valve gears  30 , which will be described later, on both of an intake side and an exhaust side. Because structures of the valve gears  30  on the intake side and the exhaust side are substantially the same, the structure on the exhaust side (the structure on a left side in  FIG. 1 ) will be mainly described and the structure on the intake side (the structure on a right side in  FIG. 1 ) will be omitted appropriately for convenience of description. 
     The engine  1  mainly includes a cylinder head  10 , a cylinder head cover  20 , the valve gear  30 , a cam cap  40 , the baffle plate  110 , and the oil passage forming member  120 . 
     The cylinder head  10  along with a cylinder block (not shown) forms a main structural body of the engine  1 . The cylinder head  10  is fixed to an upper portion of the cylinder block. The cylinder head  10  mainly includes cylinder head-side bearing portions  11  and oil galleries  12 . 
     The cylinder head-side bearing portion  11  supports an exhaust-side camshaft  32 A (described later) from below so that the exhaust-side camshaft  32 A can rotate. The cylinder head-side bearing portion  11  is formed in a left portion of the cylinder head  10  to form a semicircular recess with an upper side open in front view. 
     The oil gallery  12  is an oil passage for feeding lubricant to respective portions of the engine  1 . The oil gallery  12  is formed to pass through a left wall of the cylinder head  10  in a front-back direction. 
     The cylinder head cover  20  covers an upper portion of the cylinder head  10 . The cylinder head cover  20  is formed in a shape of a bowl with a lower side open. The cylinder head cover  20  is placed on the upper portion of the cylinder head  10  and suitably fixed with bolts or the like. The baffle plate  110  (described later) is mounted to an inner side of the cylinder head cover  20  to form an oil separator chamber  21 . The oil separator chamber  21  can store blow-by gas and cause the gas to flow back to an intake system after carrying out oil dropping. 
     The valve gear  30  is for opening and closing exhaust ports (not shown) of the engine  1  at predetermined times. The valve gear  30  mainly includes exhaust valves  31 A and the exhaust-side camshaft  32 A. 
     The exhaust valves  31 A are for opening and closing the exhaust ports (not shown) of the engine  1 . Each of the exhaust valves  31 A is disposed with its longitudinal direction oriented substantially in a vertical direction. A lower end of the exhaust valve  31 A extends to the exhaust port. A middle portion of the exhaust valve  31 A in the vertical direction is inserted through the cylinder head  10  to be able to slide. 
     The exhaust-side camshaft  32 A is for driving the valve gear  30  for opening and closing. The exhaust-side camshaft  32 A is placed on the cylinder head-side bearing portion  11  of the cylinder head  10  with its longitudinal direction oriented substantially in the front-back direction. The exhaust-side camshaft  32 A includes cams  33 . 
     Each of the cams  33  is a portion formed in a plate shape having an irregular distance from a center of rotation (center of the exhaust-side camshaft  32 A) to an outer periphery. The cam  33  is disposed in a position corresponding to each of cylinders in the front-back direction. The cam  33  is disposed above the exhaust valve  31 A. The cam  33  rotates about an axial center of the exhaust-side camshaft  32 A to thereby cause the exhaust valve  31 A to slide in the vertical direction with respect to the cylinder head  10 . 
     The cam cap  40  is fixed to the upper portion of the cylinder head  10  to retain the exhaust-side camshaft  32 A between the cylinder head  10  and the cam cap  40 . The cam cap  40  is formed in a substantially rectangular parallelepiped shape with its longitudinal direction oriented in the left-right direction. The cam cap  40  mainly includes a cam cap-side bearing portion  41 . 
     The cam cap-side bearing portion  41  supports the exhaust-side camshaft  32 A from above so that the exhaust-side camshaft  32 A can rotate. The cam cap-side bearing portion  41  is formed in a left portion of the cam cap  40  to form a semicircular recess with a lower side open in front view. The cam cap-side bearing portion  41  is formed in a position facing the cylinder head-side bearing portion  11  in the cylinder head  10  and cooperates with the cylinder head-side bearing portion  11  to rotatably support the exhaust-side camshaft  32 A. 
     Although it is not described concretely, the engine  1  having the above-described structure includes, as an intake-side structure (structure on a right side in  FIG. 1 ), the (intake-side) valve gear  30  for opening and closing intake ports (not shown) of the engine  1  at predetermined times. The intake-side valve gear  30  includes intake valves  31 B for opening and closing the intake ports (not shown) of the engine  1  and an intake-side camshaft  32 B for driving the intake-side valve gear  30  for opening and closing as shown in  FIG. 1 . 
     By using  FIGS. 1 to 5 , structures of the baffle plate  110  and the oil passage forming member  120  will be described below in detail. 
     The baffle plate  110  shown in  FIGS. 1, 2, 4, and 5  is a member for defining the oil separator chamber  21 . The baffle plate  110  is formed in a rectangular plate shape. The baffle plate  110  is mounted to the inner side of the cylinder head cover  20 . The baffle plate  110  is disposed with its longitudinal direction oriented in the front-back direction and its plate faces facing in the vertical direction. 
     The baffle plate  110  is made of laser light transmissive synthetic resin. It is essential only that the baffle plate  110  have transmittance of the laser light to be used (laser light of a predetermined wavelength) and the baffle plate  110  having the laser light transmittance not lower than 25% may be used, for example. To put it concretely, as material of the baffle plate  110 , polyester resin such as polybutylene terephthalate (PBT) and polyethylene terephthalate (PET), polyolefin resin such as polyethylene and polypropylene, polyamide resin, vinyl chloride resin, fluorine resin, and the like can be used. 
     The baffle plate  110  includes a welded area  111  and a non-welded area  112  (is divided into the welded area  111  and the non-welded area  112 ). Details of the welded area  111  and the non-welded area  112  will be described later. 
     The oil passage forming member  120  shown in  FIGS. 1 and 3 to 5  is a member for forming a lubricant feed path for feeding the lubricant to predetermined lubrication portions. The oil passage forming member  120  has such an outside shape in plan view as to surround a periphery of the lubricant feed path. The oil passage forming member  120  is formed in a plate shape with its central portion (portion corresponding to the lubricant feed path) protruding downward (see  FIG. 5 ). The oil passage forming member  120  is fixed to a lower face of the baffle plate  110 . 
     The oil passage forming member  120  is made of laser light absorbing synthetic resin. It is essential only that the oil passage forming member  120  have absorbance of the laser light to be used and the oil passage forming member  120  having the transmittance of the laser light not higher than 5% can be used, for example. To put it concretely, as material of the oil passage forming member  120 , the same synthetic resin as that used for the above-described baffle plate  110  into which an absorbent such as carbon black for, increasing the absorbance is mixed can be used. 
     The oil passage forming member  120  has a recessed portion  121 , an introduction port  122 , discharge ports  123 , and a welding portion  124 . 
     The recessed portion  121  shown in  FIGS. 3 and 5  is formed with its portion corresponding to a feed passage of the lubricant recessed downward (to an opposite side from the baffle plate  110 ). The recessed portion  121  is branched so as to form end portions in positions corresponding to predetermined lubrication portions. With the oil passage forming member  120  fixed to the lower face of the baffle plate  110 , the recessed portion  121  forms an oil passage  130  (see  FIG. 5 ) between the baffle plate  110  and the oil passage forming member  120 . In other words, the recessed portion  121  forms side walls and a bottom wall of the oil passage  130 . The oil passage  130  is formed in a hatched area in  FIG. 4  so as to feed the lubricant from the oil gallery  12  fed through a predetermined oil passage to the predetermined lubrication portions (e.g., the cams  33 ). 
     The introduction port  122  shown in  FIG. 3  is a portion for introducing the lubricant into the oil passage  130 . The introduction port  122  is formed to extend downward from a bottom face of the recessed portion  121  to pass through the oil passage forming member  120  in the vertical direction. The introduction port  122  is formed near a front left end portion of the oil passage forming member  120 . The introduction port  122  communicates with the oil gallery  12  through the predetermined oil passage. 
     The discharge ports  123  shown in  FIG. 3  are portions for discharging the lubricant flowing through the oil passage  130  to the predetermined lubrication portions. The discharge ports  123  are formed to extend downward from the bottom face of the recessed portion  121  to pass through the oil passage forming member  120  in the vertical direction. The discharge ports  123  are respectively formed at the respective end portions (positions corresponding to the predetermined lubrication portions) of the recessed portion  121  (oil passage  130 ). To put it concretely, the discharge ports  123  are formed above the cams  33 . 
     The welding portion  124  shown in  FIGS. 3 and 5  are portions where the oil passage forming member  120  is welded to the baffle plate  110 . The welding portion  124  is formed in a protruding shape on an upper face of the oil passage forming member  120 . The welding portion  124  is formed in a hatched area in  FIG. 3  to extend from a position near a front end to a position near a back end of the oil passage forming member  120  in plan view. To put it concretely, the welding portion  124  is formed to lay a closed trail on an outer side of the recessed portion  121  along the recessed portion  121  in plan view. In a state in which the baffle plate  110  and the oil passage forming member  120  are integrated with each other by laser welding, a height (vertical length) of the welding portion  124  is substantially equal to a depth of a recess-shaped portion  111   a  (described later) (see  FIG. 5 ). 
     The welded area  111  shown in  FIGS. 2 and 5  is an area at which the baffle plate  110  is welded to the welding portion  124  of the oil passage forming member  120 . The welded area  111  is formed in a hatched area in  FIG. 2  to extend from a position near a front end to a position near a back end of the baffle plate  110  in plan view or bottom view. To put it concretely, the welded area  111  is formed to lay a closed trail on an outer side of the oil passage  130  along the oil passage  130  in plan view or bottom view. The welded area  111  is formed to have substantially the same shape in bottom view as a shape of the welding portion  124  in plan view. The welded area  111  is formed to have a dimension in a width direction (dimension in a left-right direction of the welded area  111  in an enlarged cross-sectional view shown in  FIG. 5 ) which is substantially the same as or slightly greater than a dimension in a width direction of the welding portion  124  (dimension in the left-right direction of the welding portion  124  in the enlarged cross-sectional view shown in  FIG. 5 ). 
     The recess-shaped portion  111   a  is formed throughout the welded area  111 . Conversely, the area where the recess-shaped portion  111   a  is formed is the welded area  111 . 
     The recess-shaped portion  111   a  is a portion formed in a recessed groove shape throughout the welded area  111 . The recess-shaped portion  111   a  is formed in the lower face of the baffle plate  110 . A shape of the recess-shaped portion  111   a  in bottom view is the same as the shape of the welded area  111  in bottom view. The recess-shaped portion  111   a  houses the welding portion  124 . 
     In this manner, because the recess-shaped portion  111   a  is formed in the welded area  111 , the welded area  111  is formed to be smaller in thickness (vertical length) than the other area (non-welded area  112  described later). As a result, the welded area  111  is higher in laser transmittance than the non-welded area  112 . 
     The non-welded area  112  is the area other than the welded area  111 . The non-welded area  112  includes an area on an inner side of the welded area  111  (area surrounded with the welded area  111 ) and an area on an outer side of the welded area  111  in plan view or bottom view. The non-welded area  112  is not welded to the oil passage forming member  120 . A portion of the non-welded area  112  where the oil passage forming member  120  is disposed (portion around the welded area  111 ) comes in contact with the oil passage forming member  120 . 
     By laying the baffle plate  110  and the oil passage forming member  120  formed as described above over each other and laser-welding them to each other, a welded body (synthetic resin welded body) is formed. 
     By using  FIGS. 6 to 8 , a method of manufacturing the welded body formed by the baffle plate  110  and the oil passage forming member  120  will be described below in detail. In a manufacturing process of the manufacturing method according to the embodiment, the baffle plate  110  is disposed on a lower side and the oil passage forming member  120  is disposed on an upper side. 
     The method of manufacturing the welded body formed by the baffle plate  110  and the oil passage forming member  120  according to the embodiment of the invention includes a baffle plate preparing step, an oil passage forming member preparing step, a disposing step, and a laser welding step (see  FIG. 6 ). 
     In the baffle plate preparing step, the baffle plate  110  is prepared (step S 101 ). Because details of the baffle plate  110  are as described above, the step will not be described here. 
     In the oil passage forming member preparing step, the oil passage forming member  120  is prepared (step S 102 ). Because the welding portion  124  of the oil passage forming member  120  is melted by the laser light in the laser welding step (described later), the height of the welding portion  124  is greater than the depth of the recess-shaped portion  111   a . Because other details of the oil passage forming member  120  are as described above, they will not be described here. 
     In the disposing step, the baffle plate  110  and the oil passage forming member  120  are laid over each other (step S 103 ). At this time, the baffle plate  110  is disposed with its face having the recess-shaped portion  111   a  facing upward. The oil passage forming member  120  is disposed with its face having the welding portion  124  facing downward. The oil passage forming member  120  is disposed above the baffle plate  110 . The oil passage forming member  120  is disposed so that the welding portion  124  is housed in the recess-shaped portion  111   a  and that a top face of the welding portion  124  (face facing downward in  FIG. 7 ) comes in contact with a bottom face of the recess-shaped portion  111   a . At this time, the lower face of the oil passage forming member  120  (face on which the welding portion  124  is formed) is not in contact with the upper face of the baffle plate  110  (face in which the recess-shaped portion  111   a  is formed) and a clearance is formed between both the faces (see  FIG. 7 ). 
     In the laser welding step, the baffle plate  110  and the oil passage forming member  120  are laser-welded to each other (step S 104 ). The laser light is radiated toward the welding portion  124  from below the baffle plate  110  (see arrows in  FIG. 7 ). A light source of the laser light is not especially restricted and a semiconductor laser, a YAG laser, or the like may be used. 
     Because the baffle plate  110  is made of the laser light transmissive synthetic resin, most of the laser light radiated from below the baffle plate  110  is not absorbed by the baffle plate  110  and passes through the baffle plate  110 . Then the laser light is absorbed by the oil passage forming member  120  made of the laser light absorbing synthetic resin. To put it concretely, the laser light is absorbed by the welding portion  124 . 
     Energy of the laser light absorbed by the welding portion  124  is converted into heat. In this way, the welding portion  124  is heated. When a temperature of the welding portion  124  increases, a portion near the bottom face of the recess-shaped portion  111   a  in contact with the welding portion  124  is also heated due to heat transfer. As a result, a melted layer is formed at a contact portion between the oil passage forming member  120  and the baffle plate  110  (between the top face of the welding portion  124  and the bottom face of the recess-shaped portion  111   a ). The melted layer cools down and sets and, as a result, the oil passage forming member  120  and the baffle plate  110  become welded to each other. 
     The radiation of the laser light is carried out until the welding portion  124  is melted and the lower face of the oil passage forming member  120  comes in contact with the upper face of the baffle plate  110  (upper face of the non-welded area  112 ). 
     As described above, because the welded area  111  of the baffle plate  110  is thinner than the non-welded area  112 , the laser transmittance of the welded area  111  is higher than that of the non-welded area  112 . In other words, the welded area  111  is more likely to transmit the laser than the non-welded area  112 . Therefore, the laser light radiated toward the welding portion  124  from below the baffle plate  110  can easily reach the welding portion  124  through the welded area  111 . Therefore, it is possible to shorten the time required to melt the welding portion  124 . 
     Here, displacement of a laser light scanning position and increase in a laser light radiation diameter may cause displacement of a laser light radiation position in the left-right direction with respect to the welding portion  124 . At this time, the laser light displaced from the welding portion  124  tries to pass through the non-welded area  112  first. Because the non-welded area  112  is thicker than the welded area  111 , the non-welded area  112  is lower in laser transmittance than the welded area  111 . Therefore, the laser light is less likely to pass through the non-welded area  112 . As a result, it is possible to suppress the radiation of the laser light to a portion of the oil passage forming member  120  other than the welding portion  124  (portion around the welding portion  124 ), which suppresses melting of the portion of the oil passage forming member  120  other than the welding portion  124 . Consequently, it becomes likely to melt only the welding portion  124 . 
     As described above, the welded body (synthetic resin welded body) according to the embodiment includes the laser light absorbing oil passage forming member  120  (absorption-side synthetic resin part) and the laser light transmissive baffle plate  110  (transmission-side synthetic resin part) which is laid over the oil passage forming member  120  and laser-welded in this state. The baffle plate  110  is formed so that the welded area  111  which is laser-welded to the oil passage forming member  120  is higher in laser transmittance than the non-welded area  112  which is the other area. 
     With this structure, it is possible to make only the welding portion  124  (portion to be welded) of the oil passage forming member  120  likely to melt in the laser welding. 
     In the baffle plate  110  in the welded body according to the embodiment, the welded area  111  is formed to be smaller in thickness than the non-welded area  112  so that the welded area  111  is higher in laser transmittance than the non-welded area  112 . 
     With this structure, it is possible to make only the welding portion  124  (portion to be welded) of the oil passage forming member  120  likely to melt in the laser welding. 
     In the welded body according to the embodiment, the baffle plate  110  is the baffle plate  110  which is disposed on the cylinder head cover  20  and the oil passage forming member  120  is the oil passage forming member  120  which includes the recessed portion  121  recessed to the opposite side from the baffle plate  110  and which is fixed to the one face of the baffle plate  110  to thereby form the oil passage  130  with the recessed portion  121  and the baffle plate  110 . 
     With this structure, it is possible to make only the welding portion  124  (portion to be welded) of the oil passage forming member  120  likely to melt in the laser welding. 
     The method of manufacturing the welded body according to the embodiment includes the oil passage forming member preparing step (absorption-side synthetic resin part preparing step) of preparing the laser light absorbing oil passage forming member  120  (absorption-side synthetic resin part), the baffle plate preparing step (transmission-side synthetic resin part preparing step) of preparing the laser light transmissive baffle plate  110  (transmission-side synthetic resin part), the disposing step of laying the oil passage forming member  120  and the baffle plate  110  over each other, and the laser welding step of radiating the laser light from the side of the baffle plate  110  to melt the oil passage forming member  120  to thereby weld the oil passage forming member  120  and the baffle plate  110  to each other to obtain the welded body. The baffle plate  110  is formed so that the welded area  111  which is laser-welded to the oil passage forming member  120  is higher in laser transmittance than the non-welded area  112  which is the other area. 
     With this structure, it is possible to make only the welding portion  124  (portion to be welded) of the oil passage forming member  120  likely to melt in the laser welding. 
     In the baffle plate  110  in the method of manufacturing the welded body according to the embodiment, the welded area  111  is formed to be smaller in thickness than the non-welded area  112  so that the welded area  111  is higher in laser transmittance than the non-welded area  112 . 
     With this structure, it is possible to make only the welding portion  124  (portion to be welded) of the oil passage forming member  120  likely to melt in the laser welding. 
     In the method of manufacturing the welded body according to the embodiment, the baffle plate  110  is the baffle plate  110  which is disposed on the cylinder head cover  20  and the oil passage forming member  120  is the oil passage forming member  120  which includes the recessed portion  121  recessed to the opposite side from the baffle plate  110  and which is fixed to the one face of the baffle plate  110  to thereby form the oil passage  130  with the recessed portion  121  and the baffle plate  110 . 
     With this structure, it is possible to make only the welding portion  124  (portion to be welded) of the oil passage forming member  120  likely to melt in the laser welding. 
     Although the embodiment of the invention has been described above, the invention is not restricted to the above-described structure and can be changed in various ways without departing from the scope of the invention described in the claims. 
     For example, though the synthetic resin welded body formed by the baffle plate  110  and the oil passage forming member  120  and the method of manufacturing the same have been described in the embodiment, the invention is not restricted to them. The invention may be applied to any synthetic resin welded body formed by an absorption-side synthetic resin part having absorbance of laser light and a transmission-side synthetic resin part having transmittance of the laser light. 
     Although the type of the engine for which the baffle plate  110  and the oil passage forming member  120  are used is not specified in the embodiment, the baffle plate  110  and the oil passage forming member  120  according to the embodiment can be applied to any type of engine. 
     Although the entire baffle plate  110  is made of the synthetic resin having the transmittance of the laser light and the entire oil passage forming member  120  is made of the synthetic resin having the absorbance of the laser light in the embodiment, the invention is not restricted to it. It is essential only that the portions related to the welding (the welding portion  124  and the recess-shaped portion  111   a ) of the baffle plate  110  and the oil passage forming member  120  be respectively made of the above-described materials. 
     Although the recess-shaped portion  111   a  is formed in the lower face of the baffle plate  110  (see  FIG. 5 ) in the embodiment, the invention is not restricted to it. For example, as in a second embodiment shown in  FIG. 9 , a recess-shaped portion  111   a  may be formed in an upper face of a baffle plate  110 . In the second embodiment, a welding portion  124  of an oil passage forming member  120  is not housed in the recess-shaped portion  111   a  and comes in contact with a lower face of the baffle plate  110 . Alternatively, recess-shaped portions  111   a  may be formed respectively in a lower face and an upper face of a baffle plate  110 . 
     Although the welded area  111  is formed to be smaller in thickness than the non-welded area  112  so that the welded area  111  is higher in laser transmittance than the non-welded area  112  in the embodiment, the structure for making the laser transmittance of the welded area  111  higher than that of the non-welded area  112  is not restricted to it. Other embodiments of the welded area  111  and the non-welded area  112  according to the invention will be described below. 
     A baffle plate  140  according to the third embodiment shown in  FIG. 10  is different from the baffle plate  110  according to the first embodiment (see  FIG. 5 ) in that the baffle plate  140  includes a colored portion  141  in place of the recess-shaped portion  111   a . Therefore, structures of the baffle plate  140  which are the same as those of the baffle plate  110  according to the first embodiment will be provided with the same reference signs and will not be described below. 
     The colored portion  141  is a colored portion of an upper face of the baffle plate  140  in a non-welded area  112 . The colored portion  141  is formed by coating the upper face of the baffle plate  140  with coloring material. To put it concretely, the colored portion  141  is formed by applying colored paint to the non-welded area  112  on the upper face of the baffle plate  140 . 
     Because the colored portion  141  is formed in the non-welded area  112  in this manner, the welded area  111  is formed to be higher in laser transmittance than the non-welded area  112 . Therefore, it is possible to suppress melting of a portion of an oil passage forming member  120  around a welding portion  124  in radiating laser. As a result, it is possible to make only the welding portion  124  likely to melt. 
     A baffle plate  150  according to the fourth embodiment shown in  FIG. 11  is different from the baffle plate  140  (see  FIG. 10 ) according to the third embodiment in that the baffle plate  150  includes a colored portion  151  in place of the colored portion  141 . Therefore, structures of the baffle plate  150  which are the same as those of the baffle plate  140  according to the third embodiment will be provided with the same reference signs and will not be described below. 
     The colored portion  151  is a portion of the baffle plate  150  which is colored from an upper face to a lower face in a non-welded area  112 . The colored portion  151  is formed by using colored synthetic resin including coloring material in advance as material of the non-welded area  112 . At this time, transparent synthetic resin is used as material of a welded area  111 . The baffle plate  150  according to the fourth embodiment is formed by two-color molding. 
     Because the colored portion  151  is formed in the non-welded area  112  in this manner, the welded area  111  is formed to be smaller in thickness than the non-welded area  112 . Therefore, it is possible to suppress melting of a portion of an oil passage forming member  120  around a welding portion  124  in radiating laser. As a result, it is possible to make only the welding portion  124  likely to melt. 
     As described above, in each of the baffle plates  140  and  150  in welded bodies according to the third and fourth embodiments, the non-welded area  112  is colored so that the welded area  111  is higher in laser transmittance than the non-welded area  112 . 
     With this structure, it is possible to make only the welding portion  124  (portion to be welded) of the oil passage forming member  120  likely to melt in the laser welding. 
     In each of the baffle plates  140  and  150  in methods of manufacturing the welded bodies according to the third and fourth embodiments, the non-welded area  112  is colored so that the welded area  111  is higher in laser transmittance than the non-welded area  112 . 
     With this structure, it is possible to make only the welding portion  124  (portion to be welded) of the oil passage forming member  120  likely to melt in laser welding. 
     Although the third and fourth embodiments of the invention have been described above, the invention is not restricted to the above-described structures and can be changed in various ways without departing from the scope of the invention described in the claims. 
     For example, although the colored portion  141  is formed on the upper face of the baffle plate  140  in the non-welded area  112  in the third embodiment, the invention is not restricted to it. A colored portion  141  may be formed on a lower face of a baffle plate  140  in a non-welded area  112 . 
     Although the colored portion  141  is formed by applying colored paint to the non-welded area  112  in the third embodiment, the invention is not restricted to it. A colored portion  141  may be formed by sticking a colored film to a non-welded area  112 . 
     Although each of the colored portions  141  and  151  in the third and fourth embodiments is formed by coloring only the non-welded area  112 , the invention is not restricted to it. Each of the colored portions  141  and  151  may be formed by coloring a welded area  111  with a color which is likely to transmit laser light and coloring a non-welded area  112  with a color which is less likely to transmit (more likely to absorb) the laser light than the color on the welded area  111 . Alternatively, each of the colored portions  141  and  151  may be formed by coloring a welded area  111  with a lighter color (causing the welded area  111  to include less coloring material) and coloring a non-welded area  112  with a darker color (causing the non-welded area  112  to include more coloring material). 
     Although the baffle plate  150  is formed by the two-color molding in the fourth embodiment, the invention is not restricted to it. A baffle plate  150  may be formed by molding a welded area  111  and a non-welded area  112  separately and joining them together later. 
     A baffle plate  160  according to the fifth embodiment shown in  FIG. 12  is different from the baffle plate  150  (see  FIG. 11 ) according to the fourth embodiment in that the baffle plate  160  includes a low-transmittance material portion  161  in place of the colored portion  151 . Therefore, structures of the baffle plate  160  which are the same as those of the baffle plate  150  according to the fourth embodiment will be provided with the same reference signs and will not be described below. 
     The low-transmittance material portion  161  is a portion of the baffle plate  160  which is from an upper face to a lower face of the baffle plate  160  in a non-welded area  112  and made of material which is higher in laser transmittance than material for a welded area  111 . As the material of the low-transmittance material portion  161 , synthetic resin having lower laser transmittance than synthetic resin used for the welded area  111  is used. The low-transmittance material portion  161  is formed by two-color molding. 
     Because the low-transmittance material portion  161  is formed in the non-welded area  112  in this manner, the welded area  111  is formed to be higher in laser transmittance than the non-welded area  112 . Therefore, it is possible to suppress melting of a portion of an oil passage forming member  120  around a welding portion  124  in radiating laser. As a result, it is possible to make only the welding portion  124  likely to melt. 
     As described above, in the baffle plate  160  in the welded body according to the fifth embodiment, the welded area  111  is made of the material which is higher in laser transmittance than material for the non-welded area  112  so that the welded area  111  is higher in laser transmittance than the non-welded area  112 . 
     With this structure, it is possible to make only the welding portion  124  (portion to be welded) of the oil passage forming member  120  likely to melt in laser welding. 
     In the baffle plate  160  in a method of manufacturing a welded body according to the fifth embodiment, the welded area  111  is made of the material which is higher in laser transmittance than material for the non-welded area  112  so that the welded area  111  is higher in laser transmittance than the non-welded area  112 . 
     With this structure, it is possible to make only the welding portion  124  (portion to be welded) of the oil passage forming member  120  likely to melt in laser welding. 
     Although the fifth embodiment of the invention has been described above, the invention is not restricted to the above-described structure and can be changed in various ways without departing from the scope of the invention described in the claims. 
     For example, although the baffle plate  160  is formed by the two-color molding in the fifth embodiment, the invention is not restricted to it. A baffle plate  160  may be formed by molding a welded area  111  and a non-welded area  112  separately and joining them together later. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be applied to a synthetic resin welded body formed by welding synthetic resin parts to each other and a method of manufacturing the same. 
     REFERENCE SIGNS LIST 
     
         
         
           
               110 : Baffle plate (Transmission-side synthetic resin part) 
               111 : Welded area 
               111   a : Recess-shaped portion 
               112 : Non-welded area 
               120 : Oil passage forming member (Absorption-side synthetic resin part) 
               121 : Recessed portion 
               130 : Oil passage