Injection moulding tool for video recording discs - avoids eccentric or birefringence interference with reflected laser signals

摘要

An injection moulding tool for producing mouldings with a precisely located central hole is provided with an integral punch cutter and ejector pin system for simultaneously cutting the hole and isolating the sprue and annular web gate from the moulding before the moulding is ejected. Esp. for mfr. of plastic discs for recording information such as television by laser etching of a spiral track at 0.5-1 mu m pitch on the disc surface, where the spiral and the disc must be concentric to within 50-100 mu m, the centre of gravity of the disc must be within 0.25 mm of the geometric centre, and the signals reflected from the disc surface must not be subject to distortion due to birefringence effects from variations in material orientation.