| 摘要 |
PURPOSE:To make the birefringence of a board small and improve its C/N ratio by a method wherein the axial interval of a molding cavity is increased during a part of the time ranging from the start of injection to the dwelling process. CONSTITUTION:Two axially sliding members 20 and 30 are provided in order to axially increase a molding cavity. The axial interval T0 not shown of the molding cavity 4 at the start of injection process is set to the smaller then the desired axial dimension or thickness t1 of an optical disc, which is the final molded item to satisfy the formula: t1/10<t0<9t1/10. In operation, the dimension t0, which is set at the start of injection process, is increased continuously or stepwise during injection process or during the time ranging from the start of injection process to the initial stage of the dwelling process to the desired final dimension t1. Because the injection process lasts for about 0.5-2sec, the above-mentioned change of the axial interval is done by controlling the hydraulic pressures fed to a mold clamping mechanism (not shown) and to the axially sliding member 30 by means of a computer. |
