1.  Field of the Invention
The present invention relates to a synthetic resin intake manifold used in an internal combustion engine for a motor vehicle, and more particularly to a resin intake manifold manufactured by connecting protrusions of weld portions of at least two separated bodies to each other in accordance with a vibration welding or the like. 
2.  Description of Related Art
Conventionally, a resin intake manifold is manufactured by vibration welding protrusions in welded portions of a plurality of separated bodies to each other, for example, as described in Japanese Unexamined Patent Publication No.  9-177624. 
However, there is a case that set reference surfaces in the weld portions of the separated bodies are arranged in a curved shape or an inclined state, in accordance with a die cutting direction at a time of molding the separated bodies and a connecting direction between the separated bodies.  Further, in some vibrating directions of the vibration welding, a burr (a weld burr) B is generated more than a predetermined amount at a time of welding protrusions 3 and 7 of weld portions 2 and 6 in separated bodies 1 and 5 to each other, as shown in FIG.  1A, and there is a case that the burr B protrudes over a cover wall 4 arranged in the periphery of the protrusions 3 and 7. 
The burr B getting over the cover wall 4 is hard to be viewed if it appears in an inner side (an inner peripheral side) of the intake manifold.  Accordingly, it takes a lot of trouble with checking out the burr.  If the intake manifold is mounted to a vehicle in a state in which the burr B is left in the inner peripheral side, and is used, and the burr B is peeled away together with passing of an intake fluid, there is a risk that the burr flows to an engine side so as to be attached to a valve or the like, thereby generating an obstacle in a valve closing operation, so that this phenomenon is not preferable. 
Further, if the burr B appears in an outer peripheral side of the intake manifold, an outer appearance design of the intake manifold is lowered, and this phenomenon is neither preferable. 
Accordingly, it is necessary to check out presence of the burr B. Further, if the burr B is generated, it is necessary to carry out an extra finishing work such as a grinding work or the like for removing the burr B, thereby increasing a manufacturing man-hour and cost of the resin manifold.  Therefore, it has been desired to easily prevent the burr generated at a time of vibration welding from being exposed to the inner peripheral side or the outer peripheral side of the product. 
On the other hand, in the conventional resin intake manifold, it is desirable in view of a manufacturing man-hour and cost to integrally form a tank chamber such as a resonator or the like, in comparison with the case that the tank chamber is formed as an independent body. 
However, in the case that the tank chamber has a large capacity, for example, it is necessary to form the tank chamber by adding the other pieces to two main pieces constituting the intake manifold, it is hard to secure a strength of each of the weld portions unless the weld portions of at least three pieces are properly arranged.  In this case, it is reversely necessary to use a slide jig or the like so as to secure a proper welding strength, so that the manufacturing man-hour and cost are increased. 
In accordance with the structure mentioned above, for example, as shown in FIG.  1B, there is exemplified a structure in which a first piece 101 and a second piece 105 are welded, an intake flow passage is formed at a position (not shown), the first piece 101 is provided with a tubular peripheral wall portion 104 forming a tank chamber 107 capable of structuring a resonator, and a third piece 106 is arranged as a cover member closing an end portion of the peripheral wall portion 104.  In accordance with the structure mentioned above, in the first piece 101, a collar portion 102 for welding to the second piece 105, and a collar portion 103 for welding to the third piece 106 are arranged in both end portions of the tubular peripheral wall portion 104. 
However, in the structure mentioned above, among pressurizing jigs 108 and 109 for pressurizing and supporting at a time of vibration welding the first piece 101 and the third piece 106, the pressurizing jigs 109 and 109 for supporting the collar portion 103 are structured such as to slide in an orthogonal direction to an axis of the peripheral wall portion 104 for preventing the pressurizing jigs 109 and 109 from being interfered with the collar portion 102, so that the structure of the pressurizing jigs 109 and 109 is complex, thereby increasing the welding cost.  Further, since the pressurizing jig 109 supports in a cantilever state, it is hard to obtain a sufficient welding strength, it is necessary to make a welding time long to devise a countermeasure, and a lot of welding man-hour is required. 
Further, since the conventional resin intake manifold is formed in accordance with a synthetic resin injection molding using a molding die at a time of manufacturing the resin intake manifold, the resin intake manifold is formed by being separated into a plurality of pieces, as described in Japanese Unexamined Patent Publication No.  8-252864.  In particular, a portion running into an independent intake passage from a surge tank via a funnel portion is formed in a state of being separated as another piece.  Further, in the case that the resin intake manifold is formed in a state of being separated into a plurality of pieces, the pieces are welded by weld lines corresponding to bonded portions in accordance with a vibration welding after being molded, whereby an entire resin intake manifold is manufactured. 
However, in this kind of resin intake manifold, in the case that the weld line is formed so as to cross over in the portion running into the independent intake passage from the surge tank via the funnel portion, a step portion tends to be generated in the weld portion, and in the case that the step portion is formed so as to cross over a communication passage of the funnel portion perpendicularly, a turbulence is generated in an intake air flow in the portion, and a ventilation resistance of the intake air is increased. 
Further, in the case that the separated pieces are welded in the weld line and the intake manifold is integrally formed, if the weld line is formed two-dimensionally or flat, the weld line is formed so as to be positioned within one flat surface, so that in the case that a vibration from the engine and a vibration pressure caused by an intake pulsation are applied at a time of being attached to the engine and used, a pressure resisting strength and a vibration resisting strength in the weld line tend to be lowered.