PURPOSE: To realize the uniform and smooth injection of a resin into a cavity, and to prevent the generation of non-filling and a shrinkage void by tapering a dam block in approximately parallel with a gate angle.

CONSTITUTION: A gate 14 in gate-port depth of 0.2-0.6mm and gate width of 0.4-1.2mm is shaped to a bottom force 12. The angle of the gate is inclined at 10-15° to a mold mating surface L. The surface of the dam block 16 of a top force 11 oppositely faced to the gate 14 is tapered at 10-15° in approximately parallel with the angle of the gate. Since the taper of the dam block is made parallel with the angle of the gate, a resin injected from the gate to a cavity gradually flows into a top-force cavity and a bottom-force cavity uniformly. Consequently, molding pressure is applied equally to the resin in the top force and the bottom force, the periphery of a frame is also filled uniformly, and molding pressure simultaneously reaches to the position 18 of a gas bent and a gas is discharged easily. Resistance in a gate port is lowered and the entrainment of air is also prevented, thus reducing the generation of non-filling and a shrinkage void.