U.S. patent number RE40,146 [Application Number 09/977,241] was granted by the patent office on 2008-03-11 for tapered light guide plate for surface light source device and method of making by injection molding via supplementary cavity.
This patent grant is currently assigned to Enplas Corporation. Invention is credited to Tsuyoshi Ishikawa, Hiroshi Yamazaki.
United States Patent |
RE40,146 |
Ishikawa , et al. |
March 11, 2008 |
**Please see images for:
( Certificate of Correction ) ** |
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.
Inventors: |
Ishikawa; Tsuyoshi (Tokyo,
JP), Yamazaki; Hiroshi (Shiraoka, JP) |
Assignee: |
Enplas Corporation (Saitama,
JP)
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Family
ID: |
16477131 |
Appl.
No.: |
09/977,241 |
Filed: |
October 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
08520648 |
Aug 29, 1995 |
05967637 |
Oct 19, 1999 |
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Foreign Application Priority Data
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Aug 29, 1994 [JP] |
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203624/1994 |
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Current U.S.
Class: |
362/615;
362/23.16; 385/146 |
Current CPC
Class: |
B29C
45/0025 (20130101); B29D 11/00663 (20130101); G02B
6/0046 (20130101); B29C 2045/0027 (20130101); B29C
2045/2695 (20130101); B29K 2995/0026 (20130101); B29L
2011/0075 (20130101); G02B 6/0001 (20130101); G02B
6/0065 (20130101); Y10S 385/901 (20130101) |
Current International
Class: |
F21V
7/04 (20060101); G02B 6/10 (20060101) |
Field of
Search: |
;362/31,26,311,330,23,29,551,257,558-561,296,297,317,326,327,341
;385/146,901 ;264/1.1,1.24,2.7,5 ;262/239 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3-59526 |
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Mar 1991 |
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JP |
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3-104906 |
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Oct 1991 |
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JP |
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5-75738 |
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Oct 1993 |
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JP |
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5-75739 |
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Oct 1993 |
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JP |
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Primary Examiner: Lee; Jong-Suk (James)
Assistant Examiner: Negron; Ismael
Attorney, Agent or Firm: Staas & Halsey LLP
Claims
What is claimed is:
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 .mu.m 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 .mu.m 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.
.Iadd.15. A light guide plate according to claim 1, wherein the
incident surface is formed from the cut surface without substantial
intermediate processing..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
.Iadd.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..Iaddend.
Description
.Iadd.More than one reissue application has been filed for the
reissue of U.S. Pat. No. 5,967,637. The reissue applications are
U.S. application Ser. No. 09/977,241 (the instant application) and
U.S. application Ser. No. 11/165,241, all of which are divisional
reissues of U.S. Pat. No. 5,967,637..Iaddend.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light guide plate for surface
light source device and a method of manufacturing it and, more
particularly, a light guide plate for a surface light source device
used as a backlight for a liquid crystal display for OA equipment,
a television set, a measuring instrument, a watch, or the like, and
a method of the light guide plate by injection molding.
2. Description of Related Art
A surface light source device is well known. A small surface light
source device is used as a backlight for a liquid crystal display
wristwatch, and a large surface light source device is used as a
backlight for an advertising display panel or an illumination
device for a show window. An LED is used as the light source for
the small device, and a fluorescent tube is used as the light
source for the large device. As a light guide plate, an acrylic
plate cut so as to have a proper dimension is used.
Since a surface light source device is required to have a small
thickness and to cause a predetermined plane to emit uniform light,
a light source is generally arranged at a lateral position of the
light guide plate. For this reason, the light guide plate has been
subject to various processings which include shaping to a special
shape, surface roughening with sandpaper, special tool or apparatus
and polishing of incident surface to making it specular.
In recent years, with epochmaking progress of a liquid crystal
display technique and development of OA equipment,
electronic-communication equipment, or the like, demand for a
surface light source device used in a liquid crystal display device
having a size of about 10 inches steeply increases. As a light
source arranged at a lateral position of a light guide plate, a
long-life extremely slender fluorescent tube having a diameter of 4
mm or less is developed. Light guide plates have come to be
manufactured by injection molding which has a small number of steps
and provides mass-production of light guide plates with stable
quality.
A light guide plate used in a surface light source device is
required to be variously improved to make it possible that a plane
having a designed area emits source light guided in a latitudinal
direction as uniform plane light. Various proposals associated with
the improvements have been made. Almost all the various proposals
use one of the following techniques: a technique in which a surface
opposing the emission surface, i.e., a reflecting surface, is
processed (formation of an uneven surface, coating, or printing) by
some means to modify its reflectance distribution; a technique in
which a reflecting surface is arranged not parallel to an emission
surface, but formed by various planes and curves; and a technique
obtained by combining both the above techniques to each other.
In these techniques, a light guide plate having a reflecting
surface not parallel to the emission surface is thick at the
incident surface for source light or near the incident surface and
thin at positions distant from the incident surface. Such light
guide plates are disclosed in Japanese Patent Laid-open No.
3-59526, Japanese Utility Model Laid-open No. 3-104906, Japanese
Utility Model Laid-open No. 5-75738 , or Japanese Utility Model
Laid-open No. 5-75739, and in FIG. 7, FIG. 8. FIGS. 7 and 8 are
side views showing light guide plates. Referring to FIGS. 7 and 8,
each upper surface is the incident surface, and each left surface
is the emission surface. The present invention relates to a light
guide plate having a shape as shown in FIG. 7 or 8 and to an
improvement of a method of manufacturing the light guide plate.
A conventional method of molding a light guide plate is described
here with reference to a light guide plate having a typical space
shown in FIGS. 9 and 10. FIG. 9 is a plan view showing the light
guide plate when viewed from the emission surface side, and FIG. 10
is a right-side view of FIG. 9. Therefore, referring to FIGS. 9 and
10, an upper surface is the incident surface 1 which is
perpendicular to the emission surface 2 in FIG. 10. Since a
fluorescent tube is arranged in the longitudinal direction of the
incident surface at a position outside and near the incident
surface, the thickness of the light guide plate is defined in
consideration of the diameter of the fluorescent tube.
A reflecting surface 3 is obliquely formed with respect to the
emission surface 2 so that the reflecting surface 3 can directly
reflect incident light from the incident surface 1. Thus, the
thickness of the guide plate gradually decreases downward in FIGS.
9 and 10.
When the light guide plate is to be formed by injection molding, a
specific position on a specific surface for a gate (opening through
which molten material is injected into a mold cavity) of the mold
must be determined. Since the light guide plate is used in a
surface light source device, the gate position is determined in
consideration of the utilization efficiency of light, uniform
emission from a large emission surface, profitability and so on.
Judging from this point of view, it is conventionally assumed that
the incident surface should be specular to efficiently receive
source light. In addition, all the surface should be specular to
obtain uniform outgoing light.
According to conventional understanding, a gate arranged on the
incident surface provides an increase in cost because the incident
surface must be made specular by high-accuracy polishing or buff
finish serving as post-processing. Therefore, such arrangement of
gate on the incident surface has not been employed.
The emission and reflecting surfaces generally must have effective
areas as large as possible. In addition, in order to obtain uniform
outgoing light, an uneven surface having a variety of shapes such
as a net-like pattern is often formed on the reflecting surface,
otherwise coating or printing is often applied to the reflecting
surface. Therefore, it has been avoided to arrange a gate on these
surfaces.
Referring to FIGS. 9 and 10, a gate may be arranged on the lower
surface 4. However, the light guide plate has a very small
thickness at lower surface 4. For example, an ultra-thin light
guide plate for office Automation ("OA") equipment having a size of
about 10 inches used, the thickness at the lower surface is about 1
to 2 mm, and a gate can hardly be arranged on the lower surface.
Even if a gate is arranged on the lower surface, a sufficient
injection pressure cannot be obtained by a general molding machine,
an acrylic molten material cannot be preferably injected into the
cavity. Thus, transferring characteristics are considerably
degraded.
In order to increase the injection pressure, or increase the
temperature of the mold to make the flow of the material easy, a
highly expensive molding machine of high-accuracy control is
required. For this reason, the cost of the light guide plate
inevitably increases. Furthermore, the number of gates may be
increased. In this case, however, the cost of the mold increases
and a so-called weld line is inevitably formed, leading to a fatal
problem for uniform outgoing light. Therefore, it is not practical
to arrange a gate on the lower surface.
For these reasons, a gate is conventionally arranged at a position
on one of side surfaces 5 and 6 in FIG. 9, in particular, in the
thick portion near the incident surface. Similarly, the above
arrangement is employed in molding for a two-light light guide
plate in which the inclination direction of the reflecting surface
3 with respect to the emission surface 2 is reversed at the central
portion of the reflecting surface 3, the lower surface 4 has a
thickness almost equal to that of the incident surface 1 so that
another fluorescent tube is arranged outside and near the lower
surface 4, as shown in FIG. 11.
However, according to a conventional molding method, as shown in
FIG. 9, if a gate G is arranged on the side surface 5, the material
quickly flows in a flow path A, but slowly flows in a flow path B
at injection. Thus, the material does not uniformly flow from the
incident surface 1 to the lower surface 4 and the flow varies
depending on the position. In addition, the flow on the side
surface 5 is considerably different from the flow on the side
surface 6. For this reason, the pressure difference and temperature
difference between regions fall into disorder.
As a result, in particular, when an uneven surface having a variety
of shapes is formed on the reflecting surface, the shape of the
uneven surface cannot be desirably transferred. In addition,
problems such as formation of an weld line and warp after molding
arise easily. In the ultra-thin light guide plate as shown in FIGS.
9 and 10, since a material cannot completely filled in a cavity by
molding machine and an ordinary molding method, a troublesome
examination must be performed for conditions set for molding
processing at every slight change in shape of the light guide
plate.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a light guide
plate for a surface light source device, which can be manufactured
easily by using a ordinary molding machine on the basis of ordinary
conditions and a method of manufacturing it.
In order to achieve the above object, in a light guide plate which
is manufactured by injection molding and has an incident surface in
the longitudinal direction of a slender beam, a large thickness
near the incident surface, a thickness decreasing as distant
therefrom, and an emission surface for emitting incident light from
the incident surface in a direction perpendicular to the
longitudinal direction of the incident surface, the incident
surface is partially or wholly formed by a cut surface.
In the light guide plate for the surface light source device
according to the present invention, preferably, the cut surface is
nearly symmetrically formed in the longitudinal direction with
respect to an almost central portion of the incident portion in its
longitudinal direction, and the cut portion is formed on a
projecting portion parallel to the other portion of the incident
surface. The dimension of the projection is set to be about 1 mm or
less, preferably, 0.5 mm or less, and the surface roughness of the
projection is set to be 50 .mu.m or less, preferably, 20 .mu.m or
less in a unit of 10-point average roughness (Rz).
In addition, according to the method of manufacturing the light
guide plate for the surface light source device according to the
present invention, a gate is arranged at a designed position which
is the nearly central position of the incident surface and
overhangs or protrudes from the position parallel to the emission
surface in the molding mold, and a cavity in which a molten
material injected from the gate nearly symmetrically flows in the
longitudinal direction of the incident surface is supplementarily
formed, wherein the molten material is molded by said mold, and
then the overhang portion (also referred to as protruding portion)
molded by the supplementary cavity is cut to form a cut
surface.
In the above manufacturing method, the gate is preferably formed as
facing an extension of the emission surface or a surface parallel
to the emission surface.
A light guide plate for the surface light source device according
to the present invention is preferably manufactured by the
following method. In a mold to be used, at a nearly central
position of the incident surface of the light guide plate to be
manufactured and distant from the incident surface by a designed
dimension, the gate is arranged toward the same plane as that of
the emission surface of the light guide plate or a plane parallel
thereto and a supplementary cavity portion is formed such that a
material injected from the gate symmetrically flows in the
longitudinal direction of the incident surface.
Therefore, the material injected from the gate advances and spreads
in the supplementary cavity as curving at 90.degree. and the
material flows into the cavity portion forming a thick portion near
the incident surface at a nearly uniform pressure from an area
larger than that of the gate. Thereafter, the material flows
parallel from the thick portion to a distal thin portion to fill
the cavity. For this reason, approximately even and regular flow
and nearly uniform pressure can be obtained in the regions of the
cavity. Thus incomplete filling and the formation of a weld line
can be prevented, with the result that the shape of the rough
reflection surface can be accurately transferred to the
product.
After molding, the unnecessary portion molded by the supplementary
cavity is cut off to manufacture a light guide plate. The cut
surface is used without changing its state and need not be
subjected to specular finish.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view showing the semi-finish state of a light
guide plate according to an embodiment of the present
invention;
FIG. 2 is a right-side view showing the semi-finish state of the
light guide plate shown in FIG. 1;
FIG. 3 is a right-side view showing the finish state of the light
guide plate shown in FIG. 1;
FIG. 4 is a graph showing the relationship between the surface
roughness of the cut surface of the light guide plate and the
luminance of an emission surface;
FIG. 5 is a right-side view showing the finish state of the light
guide plate in FIG. 3 in which an overhang portion 7 in FIG. 1 is
completely cut off;
FIG. 6 is a right-side view similar to FIG. 3 showing the finish
state of the light guide plate in which the shape of a projecting
portion 8 is modified;
FIG. 7 is a side view showing a light guide plate having a shape
different from that of the light guide plate according to the
embodiment shown in FIG. 1 and so on;
FIG. 8 is a side view showing a light guide plate having a shape
further different from the light guide plate according to the
embodiment shown in FIG. 1 and the like and the light guide plate
shown in FIG. 7;
FIG. 9 is a plan view for explaining a prior art with respect to a
case wherein an ultra-thin light guide plate similar to that of the
embodiment shown in FIG. 1 and the like is molded;
FIG. 10 is a right-side view for explaining a prior art with
respect to a case wherein an ultra-thin light guide plate similar
to that of the embodiment shown in FIG. 1 and the like is molded,
similarly as in FIG. 9; and
FIG. 11 is a side view showing an ultra-thin light guide plate
having the shape illustrated in FIG. 9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A preferred embodiment of the present invention will be described
below with reference to FIGS. 1 to 4. FIG. 1 is a plan view showing
a light guide plate in a state in which the material of the runner
portion is removed after molding; FIG. 2 is a right-side view of
FIG. 1; and FIG. 3 is a right-side view showing the light guide
plate in a finish state in which the unnecessary portion is cut
off. FIG. 4 is a graph showing the relationship between the surface
roughness of the cut surface and luminance. In all of the drawings,
the same reference numerals denote the same parts in.
The light guide plate for the surface light source device according
to this embodiment is similar to the ultra-thin light guide plate
as described in FIGS. 9 and 10. This light guide plate has the
following shape. That is, referring to FIG. 1, an overhang portion
7 is cut at a position distant from an incident surface 1 by a
designed dimension D, and this cut surface is also used as an
incident surface for source light. As is apparent from FIG. 3, the
remaining portion of the overhang portion 7 after the cutting is
formed as a projecting portion 8 having a thickness equal to that
of the thick portion, and the cut surface is formed as a surface
parallel to the other portion of the incident surface.
Referring to FIGS. 2 and 3, although a reflecting surface 3 is
shown as a flat surface, the reflecting surface 3 really has a fine
uneven shape. The reflecting surface 3 is designed to reflect
source light incident from the incident surface in many directions
and to cause the emission surface 2 to finally emit uniform light.
Although various improved shapes are opposed as the uneven shape,
descriptions thereof are omitted here. Referring to FIG. 1, a gate
mark 9 is formed in the overhang portion 7. As is apparent from
this, a gate G in the mold is arranged as to face one surface of
the overhang portion 7, i.e., the same plane as that of the
emission surface 2.
Therefore, the material supplied from a runner R is injected from
the gate G into a cavity and then the course of the material is
bent at about 90.degree..
As described above, an ultra-thin light guide plate is used in this
embodiment and the size of the light guide plate will be described
below. Referring to FIG. 1, the lengths of the incident surface 1
and the lower surface 4 are 180 mm respectively, and the lengths of
side surfaces 5 and 6 are 143 mm respectively. Referring to FIG. 3,
the thick portion has a thickness of 3.5 mm, and the thin portion
has a thickness of 1.5 mm. The projecting dimension D of the
projecting portion 8 is 0.3 mm. A distance between the center of
the gate mark 9 and the incident surface 1 is 18 mm.
A method of manufacturing the light guide plate by injection
molding will be described below. The manufacturing method is to be
described with reference to a mold structure, as a matter of
course. However, a complex drawing is omitted here and the
semi-finish state of the light guide plate shown in FIGS. 1 and 2
will be compared to a molding cavity for the sake of descriptive
convenience. Therefore, the cavity for forming the overhang portion
7 is called a supplementary cavity because the overhang portion 7
is cut off after molding as described above.
In molding, referring to FIG. 2, the acrylic-resin-based molten
material injected from the gate G through the runner R advances in
the supplementary cavity as curving 90.degree.. Since the
supplementary cavity has a width which increases along the
direction of the advance, the material flows into the cavity near
the incident surface from an area larger than that of the gate G at
a uniform pressure. Thereafter, although the molten material
laterally spreads in the form of a fan, the molten material, as a
whole, flows from the thick portion on the incident surface side to
the thin portion on the lower side portion. Although the direction
of the flow is not exactly parallel, the molten material flows at
the degree of parllelization considerably higher than that of the
prior art described in FIG. 9, at a uniform pressure.
In this manner, according to this embodiment, although the
ultra-thin light guide plate is made, the material regularly flows
in the regions in the cavity by an ordinary molding machine under
ordinary pressure control and temperature control to fill the
cavity and corners thereof completely. In addition, the uneven
shape formed on the reflecting surface can be well transferred, no
weld line is formed and warping is avoided in spite of the
ultra-thin light guide plate.
In this embodiment, attention must be paid to that the flowing
portion from the supplementary cavity to the thick portion has a
length which is about 1/7 the length of the incident surface 1. In
order to obtain a further well material flow, the ratio of the
length of the flowing portion to the length of the incident surface
1 may be increased, and the supplementary cavity may be formed as
indicated exemplary by a two-dot chain line in FIG. 1. In this
case, as in another case described later, the uniformity of light
emitted from the emission surface had no abnormality.
After the semi-finish structure is molded, the overhang portion 7
is cut. This cutting may be performed by using various cutters
including a diamond cutter. In this embodiment, the cutting was
performed by a conventional cutter having a rotary blade for
cutting an acrylic material. Referring to FIG. 5, it is ideal that
this cut surface is on the same place as that of the incident
surface 1. However, in consideration of mass production, the
cutting is performed at a position distant from the incident
surface 1 by 0.3 mm.
In the light guide plate manufactured as described above, the
incident surface portion, other than the cut surface, is formed as
a specular surface. Therefore, it must be examined whether the
presence of the projecting portion 8 and the roughness of the cut
surface prevent the emission surface 2 from emitting light having a
uniform distribution. Thus, the examination result is described
below.
A light guide plate having the above size and a cut surface having
a surface roughness of 10 .mu.m in unit of 10-point average
roughness (Rz) and a light guide plate having the above size and a
cut surface having a surface roughness of 60 .mu.m in the unit were
prepared. The luminance (cd/m.sup.2) of light emitted from emission
surfaces were measured, and the luminance are compared with a
measurement value for the specular surface obtained by polishing
the cut surface. The results are shown in the following table. In
this measurement, a cold cathode tube having a diameter of 3 mm and
a tube surface luminance of 13,700 cd/m.sup.2 was arranged 1 mm
distant from the incident surface 1 in the longitudinal direction
of the incident surface 1. A lamp holder having an inner surface of
a silver reflecting side opposite to the incident surface opposing
the surface with respect to the cold cathode tube. In addition, a
sheet having a front surface on which a silver reflecting surface
was formed was arranged along the rear side of the reflecting
surface 3 of the light guide plate such that the silver reflecting
surface was almost parallel to the reflecting surface 3 of the
light guide plate. Three positions located near the central portion
of the incident surface and distant from the incident surface
toward the lower surface 4 were set, respectively.
TABLE-US-00001 TABLE Surface Roughness Measurement Position (Rz) 10
mm 71 mm 132 mm 10 .mu.m 691 972 681 60 .mu.m 748 633 593 Specular
696 699 672 Surface
According to the above measurement results, it was clarified that
luminance uniformity was considerably degraded at a surface
roughness of 60 .mu.m, and that the luminance uniformity at a
surface roughness of 10 .mu.m was almost equal to the luminance
uniformity obtained by performing specular finish. It was also
clarified that the measurement result obtained by performing the
specular finish was not influenced by the presence of the
projecting portion.
Since the above results were obtained, cut surfaces having four
surface roughnesses of 20 .mu.m, 30 .mu.m, 40 .mu.m, and 50 .mu.m
were prepared to measure luminance at the position of 10 mm where
abnormal issue occurred at the surface roughness of 60 .mu.m. FIG.
4 is a graph on which measurement results obtained by a total of 7
cut surfaces are plotted.
As a result, it was clarified that the surface roughness of the cut
surface was set to be 50 .mu.m without any problem in the light
guide plate having the size of this embodiment. When measurement
was performed such that the projecting dimension (D) of the
projecting portion 8 in the light guide plate having a cut surface
having a surface roughness of 50 .mu.m was set to be 1 mm, the
almost same result as described above could be obtained. For this
reason, it was estimated that the projecting dimension could be
further increased without any problem. However, when the projecting
dimension was excessively large, the fluorescent tube should be
away from the incident surface by a distance corresponding to the
excessive dimension, thereby increasing a surface light source
device in size. Therefore, in consideration of manufacturing
efficiency, the projecting dimension is preferably set to be about
1 mm or less.
In the above embodiment, as shown in FIG. 2, it should be noted
that the gate G faces the place extending from the emission surface
2. One reason why this arrangement is employed is that the
following point is considered. That is, as is apparent from the
size of the gate mark 9 shown in FIG. 1, the size of the gate G of
the ultra-thin light guide plate can be determined regardless of
the thickness of the light guide plate. Another reason is that,
when a material widely flows from the supplementary cavity to the
thick portion, the pressure acting on the material can be
advantageously averaged to some extent. However, the present
invention is not limited to this arrangement, and the gate G may be
or arranged downward in FIG. 2.
In addition, the thickness of the projecting portion 8 (i.e., the
thickness of the overhang portion 7) need not be equal to the
thickness of the thick portion as shown in FIG. 3. As shown in FIG.
6, the thickness may be decreased and the cut surface may be formed
on a portion of the incident surface with respect to the
latitudinal direction. In addition, the whole incident surface may
be formed as a cut surface. In this case, the luminance slightly
decreases as a whole, and material loss caused by cutting off the
overhang portion increases. For this reason, in the light guide
plate having the size described in the above embodiment, the
thickness of the projecting portion 8 is preferably set to be 25 to
35 mm with respect to the longitudinal dimension of the incident
surface, i.e., 180 mm.
The position of the cut surface or the gate position is ideally
located just at the central portion of the light guide plate in the
lateral direction as shown in FIG. 1. However, if the position is
laterally moved to some extent, the uniformity of outgoing light is
rarely influenced.
In addition, in the present invention, as shown in FIG. 1, overhang
portions 5a and 6a each having a proper thickness may be arranged
on the side surfaces 5 and 6, respectively. The overhang portions
5a and 6a are generally used to position the light guide plate when
the light guide plate is incorporated in the surface light source
device. In the light guide plate manufactured by the conventional
method shown in FIG. 9, there is trouble in filling the material
into the thin portion. For this reason, when the overhang portions
5a and 6a are to be arranged near the thin portion, the shape of
the thin portion is not desirably transferred, with the result that
the distribution of outgoing light in the thin portion cannot be
made uniform.
However, according to the present invention, the above problem can
be solved. In this case, the reason why the overhang portions 5a
and 6a are desirably arranged near the thin portion of the light
guide plate is described below with reference to experiment
data.
The following table shows results obtained by measuring luminance
distribution on the emission surfaces of an 8-inch (133.6
mm.times.175.5 mm) light guide plate having thicknesses of 3 mm on
the incident surface and 1 mm on the lower surface and a 9-inch
(160 mm.times.220 mm) light guide plate having thicknesses of 3 mm
on the incident surface and 1 mm on the lower surface.
A case wherein the overhang portions 5a and 6a are arranged at the
upper end positions of the side surfaces 5 and 6 is represented by
A; another case wherein the overhang portions 5a and 6a are
arranged at positions which is 1/3 the distance between the
incident surface 1 and the lower surface 4 apart from the incident
surface 1 is represented by B, and the other case wherein the
overhang portions 5a and 6a are arranged at the lower end positions
is represented by C. In addition, measurement positions are defined
as follows.
On the emission surface 2, [1], [2] and [3] are laterally aligned
at a position 10 mm distant from the incident surface 1, [4], [5]
and [6] are laterally aligned at an intermediate position between
the incident surface 1 and the lower surface 4, and [7], [8] and
[9] are laterally aligned at a position 10 mm distant from the
lower surface 4. At the same time, the following conditions are
satisfied. That is, [1], [4] and [7] are vertically aligned at a
position 10 mm distant from the side surface 5, [2], [5] and [8]
are vertically aligned at an intermediate position between the side
surfaces 5 and 6, and [3], [6] and [9] are vertically aligned at a
position 10 mm distant from the side surface 6.
In this case, the surface roughness of the cut surface of the
overhang portion 7 is 10 .mu.m in a unit of 10-point average
roughness (Rz). The tube surface luminance of the cold cathode tube
is 19,500 cd/m.sup.2 when the 8-inch light guide plate is used, and
is 23,900 cd/m.sup.2 when the 9-inch light guide plate is used.
Other experiment conditions are the same as those described in the
above example.
TABLE-US-00002 TABLE In Case of 8 Inches (cd/m.sup.2) Position of
Over- hang Measurement Position Portion [1] [2] [3] [4] [5] [6] [7]
[5] [9] A 964 1078 980 1060 1100 1065 1090 1080 1085 B 995 1678
1020 1055 1100 1058 1060 1080 1065 C 1040 1078 1050 1050 1100 1055
1038 1080 1035
TABLE-US-00003 TABLE In Case of 9 Inches (cd/m.sup.2) Position of
Over- hang Measurement Position Portion [1] [2] [3] [4] [5] [6] [7]
[5] [9] A 1030 1127 1035 1150 1150 1148 1165 1130 1170 B 1060 1127
1075 1142 1150 1145 1100 1130 1110 C 1110 1127 1116 1135 1150 1138
1115 1130 1120
As is apparent from the above two tables, when the overhang
portions 5a and 6a are closer to the incident surface 1, the
luminance at positions near both the end ([1] and [3]) near the
incident surface 1 much decrease. Therefore, in consideration of
the uniformity of the luminance distribution on the emission
surface 2, the overhang portions 5a and 6a are preferably arranged
at positions distant from the incident surface 1 and located on the
side of lower surface 4 with respect to at least the intermediate
position between the incident surface 1 and the lower surface 4,
i.e., positions near the thin portion.
According to the present invention, assume that a mold having a
structure in which a molten material injected from the gate flows
from the incident surface 1 to the lower surface 4 is used. In this
case, when the overhang portions 5a and 6a are arranged on the side
of lower surface 4 with respect to the intermediate position
between the incident surface 1 and the lower surface 4, the
overhang portions 5a and 6a having good shape transferring
characteristic and a high shape accuracy can be easily formed. When
the shape accuracy of the overhang portions 5a and 6a is high as
described above, an accuracy in positioning the light guide plate
can be increased.
The relationships between portions (A, B, C) of the overhang
portions 5a and 6a with respect to the incident surface 1 and the
decrease in luminance near both the ends near the incident surface
1 are the same as those obtained for a light guide plate having a
uniform thickness from the incident surface 1 to the lower surface
4. For this reason, also in this case, the mold preferably has the
structure in which the molten material injected from the gate flows
from the incident surface 1 to the lower surface 4 and the overhang
portions 5a, 6a are desirably arranged on the side of lower surface
4 with respect to the intermediate position between the incident
surface 1 and the lower surface 4. Although the overhang portions
5a and 6a are arranged on the side surfaces 5 and 6 in FIG. 1, the
overhang portions 5a and 6a may be arranged on the emission surface
2, the reflecting surface 3, or the lower surface 4 depending on
the situation.
In addition, the shape of a light guide plate to which the present
invention can be applied is not limited to the shape of the light
guide plate described in this embodiment as a matter of course.
More specifically, the present invention can be applied to the
light guide plates having the shapes described in the
above-mentioned known publications and the light guide plates
having the shapes shown in FIGS. 7, 8 or 11. When the light guide
plate having the shape shown in FIG. 11 is used, it should be noted
that two upper and lower incident surfaces are formed.
In this case, when a mold structure is designed such that the
molten material flows from one incident surface to the other
incident surface, the material flows almost in the same manner as
the embodiment shown in FIG. 1 while a gradual changing from a fan
shape flow pattern with some extent into a parallel flow pattern is
caused until it reaches the central position in the flow direction,
i.e. the thin portion. Passing through the central position, a
fan-like flow pattern tends to appear again because of the
increasing thickness.
However, in general, this tendency is very weak, with the result
that filling is completely realized substantially in the same
manner as the embodiment shown in FIG. 1, with the parallel flow
pattern kept.
It will be understood easily from the above detailed description
that the present invention provides light guide plates for a
surface light source device, including ones of ultra-thin types,
which are not subject to incomplete filling in the cavity of the
used mold, incomplete transferring of unevenness of various shapes
and generation of weld line or warping, and further which enable
uniform light emission.
* * * * *