Method for manufacturing resin gear with core metal

A method for manufacturing a resin gear with a core metal composed of a metallic sleeve having opposing surfaces extending radially to a circumferential outer periphery of the sleeve and of a synthetic resin annular tooth part covering the outer periphery of the metallic sleeve, the method comprising:

• a primary molding step of forming an annular inner member by injection molding a first molten resin in a first cavity molding die with the metallic sleeve as an insert, thereby forming a primary molded article in which the first resin contacts each of the opposing surfaces and outer periphery of the metallic sleeve;
• and
• a secondary molding step of putting the primary molded article into a second cavity molding die and forming an annular outer member by injection molding a second molten resin that is a different material than the first molten resin, wherein:
• the resin gear with the core metal is configured as a worm wheel of a worm gear;
• the first cavity molding die and the second cavity molding die each have gates for injecting the respective first and second molten resins, the gates of each of the first and second cavity molding dies being pin gates, tunnel gates, or side gates;
• the number of the gates of the first cavity molding die and the number of the gates of the second cavity molding die are plural and identical to one another and the gates of each of the first and second cavity molding dies are arranged circumferentially about the first cavity molding die and about the second cavity molding die, respectively;
• the primary molded article has opposing surfaces extending radially to an outer circumference defining an outer peripheral surface of the primary molded article to have tooth-shaped projections configured to engage with the annular outer member, has gate marks corresponding to the position of the gates of the first cavity molding die during the primary molding step, and has radially extending weld lines circumferentially intermediate between adjacent ones of the gate marks of the primary molded article;
• and
• in the secondary molding step, the second resin is charged between the tooth-shaped projections and into contact with each of the weld lines and opposing and outer peripheral surfaces of the primary molded article, and the positions of the gates of the second cavity molding die are radially outward from the gate marks of the primary molded article and circumferentially intermediate or nearly circumferentially intermediate between the gate marks and the respective weld lines adjacent to the gate marks of the primary molded article.