Magnetic encoder manufacturing method

A magnetic encoder manufacturing method for manufacturing, using an injection molding mold, a magnetic encoder which includes an annular support member formed from a steel plate, and an annular plastic magnet, and has no adhesive layer between the annular support member and the annular plastic magnet,

• the annular plastic magnet contains magnetic powder and a thermoplastic resin,
• the thermoplastic resin is polyamide or polyphenylene sulfide,
• the annular plastic magnet being formed in a shape having, on an outer circumferential side thereof, a turning molded portion covering an outer circumferential part of the annular support member so as to turn around toward an inner diameter side of the annular support member,
• a gate of the injection molding mold being a disk gate located on an inner diameter side of the annular plastic magnet,
• a length L of the disk gate in an axial direction being in a range of 0.2 mm≤L≤0.6 mm,
• mechanical characteristics of a material of the annular plastic magnet being set such that a tensile strength is 65 MPa or more, and a Young's modulus is 4000 MPa or more and 15000 MPa or less,
• the method comprising:
• opening the injection molding mold and placing the annular support member as an insert object in the injection molding mold;
• and
• closing the injection molding mold and injecting the material of the annular plastic magnet that is melted, into a cavity in the injection molding mold through the disk gate, to perform injection molding of the annular plastic magnet having the shape.