Joining structure, joining method, and method of manufacturing resin member for joining structure

A method for manufacturing a resin member, the method comprising:

• providing a mold with a cylindrical opening defined by a cylindrical inner surface and having a central axis, the mold including a plurality of convex portions extending from the cylindrical inner surface radially inward toward the central axis such that each of the convex portions has a proximal end and a distal end, the convex portions being symmetrical relative to the central axis, at least two of the convex portions extending less than 180 degrees around the central axis and being aligned with one another such that an imaginary plane extending orthogonally to the central axis passes through the at least two convex portions;
• providing an insert along the central axis to form a mold cavity between the insert and the cylindrical inner surface of the mold, the distal ends of the convex portions not extending to the insert such that the mold cavity includes an open space between the distal end of the convex portions and the insert;
• pouring a liquid resin into the mold cavity;
• and
• solidifying the liquid resin to form a resin member having a central axis corresponding to the central axis of the mold and a cylindrical opening formed by the insert, the resin member having walls whose thickness is defined between an outer surface of the resin member and the cylindrical opening of the resin member, the resin member having a plurality of concave portions formed by the convex portions of the mold and being symmetrical relative to the central axis of the resin member, the concave portions not extending into the cylindrical opening and causing the thickness of the walls of the resin member to be reduced at the locations of the concave portions.