Method for producing shutter for disc cartridge

A method for producing a shutter for a disc cartridge including a cartridge case, the shutter including a shutter window which is substantially asymmetric with respect to a center line of the shutter, the shutter slidably opening and closing an opening portion formed on the cartridge case so as to allow at least a part of a disc-shaped recording medium contained within the cartridge case to be exposed to the outside through the opening portion and the shutter window, said method comprising the steps of:

• providing a molding cavity shaped to define a plastic molded shutter as having an upper main plate, a lower main plate, and a coupling plate connecting the upper main plate to the lower main plate in an integrally molded state, the cavity having a U-shaped cross-sectional contour;
• injecting molten synthetic resin into the molding cavity to form said plastic shutter, said injecting performed from an injection gate positioned in an effective opening region adjacent to an edge of the shutter at the coupling plate, a distance from the injection gate to points on right halves of the upper main plate and the lower main plate that are farthest from the injection gate being substantially equal to a distance from the injection gate to points on left halves of the upper main plate and the lower main plate that are farthest from the injection gate, said injecting step forming a raised gate mark on said plastic shutter;
• determining a linear region along said coupling plate, wherein said opening portion has a width dimension parallel to a length dimension of said coupling plate so that when said width dimension is orthogonally projected onto said coupling plate it defines said linear region along said length dimension of said coupling plate;
• and
• positioning said injection gate within said linear region of said coupling plate such that said raised gate mark formed during said injecting step remains opposite said opening portion as said plastic shutter slidably opens and closes said opening portion, said linear region defining said effective opening region.