Tapered light guide plate for surface light source device and method of making by injection molding via supplementary cavity

A method of manufacturing a light guide plate for a surface light source device which comprises an incident surface in a longitudinal direction of a slender light source and an emission surface for emitting incident light received at said incident surface in a direction perpendicular to the longitudinal direction of said incidence surface,

• said light guide plate being large in thickness at a position near said incident surface and decreasing in thickness with increasing distance from the incident surface, the method comprising the steps of:
• (a) providing a mold with a gate arranged at a position separated from a desired position of the incident surface, the gate being separated along a plane of the emission surface, the mold being further provided with an additional cavity to guide molten material from said gate to the desired position of the incident surface;
• (b) injecting molten material into the mold through the gate to form a guide plate portion and a protruding portion, the guide plate portion connecting to the protruding portion in a vicinity of the desired position of the incident surface;
• and
• (c) cutting the guide plate portion from the protruding portion in the vicinity of the desired position of the incident surface, the cutting operation providing a cut surface on the guide plate portion, the cut surface serving as at least a portion of the light incident surface.