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

Abstract

There is provided a very thin light guide plate for a surface light source device and a method of manufacturing it without the use of high-precision molding machine. A surface light source device using this light guide plate generates outgoing light having high uniformity. According to the manufacturing method, a light guide plate free from a weld line or warp can be manufactured with good transferring characteristics. The light guide plate is thick on an incident surface 1 side and thin on a lower surface 4 side. At the central portion of an incident surface 1 in a longitudinal direction, a projecting portion obtained by cutting an overhang portion 7 at a position a distance D apart from the incident surface 1 is formed. The cut surface of the projecting portion is not made specular and is kept rough. In molding of the light guide plate, a molten material is supplied from the position of a gate mark 9. After molding, the overhang portion 7 is cut off by an ordinary cutter, thereby obtaining a light guide plate.

1. A light guide plate for a surface light source device, manufactured by injection molding, comprising:
 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 incident 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,  
 wherein said incident surface is partially or entirely formed as a cut surface which remains after cutting a protruding portion from the light guide plate, the protruding portion having a supplemental cavity around an injection molding gate.  
 
     
     
       2. A plate according to  claim 1 , wherein said cut surface is symmetrically formed with respect to an nearly central portion of said incident surface in the longitudinal direction. 
     
     
       3. A plate according to  claim 1  or  2 , wherein said cut surface is formed at a portion of said incident surface in a latitudinal direction. 
     
     
       4. A plate according to  claim 1 , wherein said cut surface projects from another plane of said incident surface. 
     
     
       5. A light guide plate according to  claim 2 , wherein said cut surface projects from another plane corresponding to the other portion of said incident surface. 
     
     
       6. A light guide plate according to  claim 3 , wherein said cut surface projects from another plane corresponding to the other portion of said incident surface. 
     
     
       7. A light guide plate according to  claim 4  or  5 , wherein said cut surface projects so that said cut surface is parallel to another plane corresponding to the other portion of said incident surface,
 the projecting distance of said projection being not greater than about 1 mm, and said cut surface having a surface roughness of not greater than about 50 μm in terms of 10-point average roughness (Rz).  
 
     
     
       8. A light guide plate according to  claim 6 , wherein said cut surface projects so that said cut surface is parallel to another plane corresponding to the other portion of said incident surface,
 the projecting distance of said projection being not greater than about 1 mm, and said cut surface having a surface roughness of not greater than about 50 μm in terms of 10-point average roughness (Rz).  
 
     
     
       9. A light guide plate according to claims  1 ,  2 ,  4  or  5 , wherein said light guide is provided with a protruding portion which has a location distant from said incident surface and functions for positioning said light guide plate when being assembled. 
     
     
       10. A light guide plate according to  claim 3 , wherein said light guide is provided with a protruding portion which has a location distant from said incident surface and functions for positioning said light guide plate when being assembled. 
     
     
       11. A light guide plate according to  claim 6 , wherein said light guide is provided with a protruding portion which has a location distinct from said incident surface and functions for positioning said light guide plate when being assembled. 
     
     
       12. 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.  
 
 
     
     
       13. A method of manufacturing a light guide plate according to  claim 12 , wherein said gate faces an extension plane of the emission surface or a plane parallel with the emission surface. 
     
     
       14. A light guide preform comprising:
 a guide plate portion and a protruding portion, the guide plate portion connecting to the protruding portion in a vicinity of a desired position of a light incident surface, the guide plate portion having a thickness direction and a light emission surface, the light emission surface being arranged perpendicular to the thickness direction, the guide plate portion having a thickness which decreases with increasing distance from the desired position of the incident surface, and  
 the protruding portion being separated from the desired position of the incident surface, in a direction extending away from the guide plate portion along a plane encompassing the light emission surface, the protruding portion have a thickness sufficient to allow uniform flow of a molten injection molding material therethrough.  
 
     
     
       15. A light guide plate according to  claim 1 , wherein the incident surface is formed from the cut surface without substantial intermediate processing. 
     
     
       16. A light guide plate according to  claim 1 , wherein the incident surface is formed without substantial additional processing after the cutting face is obtained. 
     
     
       17. A method of manufacturing a light guide plate for a surface light source device, the light guide plate having an incident surface for light input and an emission surface for light output, comprising:
   forming a preform using a mold which is provided with an injection molding gate located at a predetermined distance from an eventual location of the incident surface, the injection molding gate being located at a supplemental cavity to which a flow of molten resin is introduced through the injection molding gate, the molten resin flowing through the eventual location of the incident surface to expand generally symmetrically with respect to a direction perpendicular to the incident surface;        removing said preform from said mold; and        removing at least a portion of said supplemental cavity.     
     
     
       18. A method of manufacturing a light guide plate according to  claim 17 , wherein the molten resin turns after being introduced through said injection molding gate so as to flow through said supplemental cavity. 
     
     
       19. A method of manufacturing a light guide plate according to  claim 17 , wherein the light guide plate has a wedge shape with a thickness that decreases with increasing distance from the incident surface. 
     
     
       20. A method of manufacturing a light guide plate according to  claim 17 , wherein the light guide plate has a reflecting surface opposing the emission surface, the reflecting surface having an uneven shape. 
     
     
       21. A method of manufacturing a light guide plate according to  claim 17 , wherein the supplemental cavity has a longitudinal length less than a longitudinal length of the incident surface. 
     
     
       22. A method of manufacturing a light guide plate according to  claim 17 , wherein
   the emission surface extends in a plane, and        the injection molding gate faces the plate of the emission surface.     
     
     
       23. A method of manufacturing a light guide plate according to  claim 17 , wherein said injection molding gate is aligned longitudinally with a center portion of the incident surface. 
     
     
       24. A method of manufacturing a light guide plate according to  claim 23 , wherein the molten resin turns after being introduced through said injection molding gate so as to flow through said supplemental cavity. 
     
     
       25. A light guide plate for a surface light source device, having an incident surface for light input and an emission surface for light output, manufactured by injection molding which comprises:
   forming a preform using a mold which is provided with an injection molding gate located at a predetermined distance from an eventual location of the incident surface, the injection molding gate being located at a supplemental cavity to which a flow of molten resin is introduced through the injection molding gate, the molten resin flowing through the eventual location of the incident surface to expand generally symmetrically with respect to a direction perpendicular to the incident surface;        removing said preform from said mold; and        removing at least a portion of said supplemental cavity.     
     
     
       26. A light guide plate according to  claim 25 , wherein
   the light guide plate has a wedge shape with a thickness that decreases with increasing distance from the incident surface, and        a surface opposite the emission surface has an uneven configuration.     
     
     
       27. A light guide plate according to  claim 25 , wherein said injection molding gate is aligned longitudinally with a center portion of the incident surface. 
     
     
       28. A light guide plate according to  claim 25 , wherein the molten resin turns after being introduced through said injection molding gate so as to flow through said supplemental cavity. 
     
     
       29. A light guide plate according to  claim 25 , wherein the light guide plate has a reflecting surface opposing the emission surface, the reflecting surface having an uneven shape. 
     
     
       30. A light guide plate according to  claim 25 , wherein the supplemental cavity has a longitudinal length less than a longitudinal length of the incident surface. 
     
     
       31. A light guide plate according to  claim 25 , wherein
   the emission surface extends in a plane, and        the injection molding gate faces the plane of the emission surface.     
     
     
       32. A light guide plate for a surface light source device that has an incident surface for light input, an emission surface for light output and a wedge shape with a thickness which decreases with increasing distance from the incident surface, the light guide plate being manufactured by injection molding using a mold provided with a main cavity  shaped corresponding to a shape of said light guide plate and a supplemental cavity connected to said main cavity at a location corresponding to the incident surface, said injection molding comprising:    ( a )  introducing a molten resin through an injection molding gate so that the molten resin flows to extend through said supplemental cavity;      ( b )  allowing the molten resin to flow from said supplemental cavity through the eventual location of the incident surface to said main cavity; and      ( c )  filling said main cavity with the molten resin through a lateral extension of molten resin and through a forward transfer of molten resin from a thicker portion around the eventual location of the incident surface toward a thinner portion distanced from the eventual location of the incident surface.     
     
     
       33. A light guide plate for a surface light source device according to  claim 32 , wherein said injection molding gate is aligned longitudinally with a center portion of the incident surface. 
     
     
       34. A light guide plate for a surface light source device according to  claim 32 , wherein the molten resin turns after being introduced through said injection molding gate so as to flow through said supplemental cavity. 
     
     
       35. A light guide plate for a surface light source device according to  claim 32 , wherein the light guide plate has a reflecting surface opposing the emission surface, the reflecting surface having an uneven shape. 
     
     
       36. A light guide plate for a surface light source device according to  claim 32 , wherein supplemental cavity has a longitudinal length less than a longitudinal length of the incident surface. 
     
     
       37. A light guide plate for a surface light source device according to  claim 32 , wherein
   the emission surface extends in a plane, and        the injection gate faces the plane of the emission surface.