JP2000321441A - Light guide plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent large warpage or deformation after a plate is formed, by controlling the stress on the surfaces of two principal planes facing each other to a specified value or smaller. SOLUTION: In the light guide plate 2, the stress on the surface of two principal planes facing each other is specified to <=200 kg/cm2. As for the means to control the stress on the surface of the light guide plate 2 to <=200 kg/cm2, the following methods for the production are used. For example, the light transmission plate is produced by an injection molding method by low pressure molding to reduce the feeding pressure, the plate is produced by an injection compressive molding method, or the plate is produced by a normal injection molding method and then subjected to annealing treatment. The synthetic resin used for injection molding is not especially limited as far as it is a transparent thermoplastic resin, and for example, a polymethylmethacrylate, a polycarbonate, a polystyrene, a thermoplastic elastomer, or copolymers of these can be used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light guide plate of a lighting device used for a backlight of a liquid crystal display device or the like.

[0002]

2. Description of the Related Art FIG. 1 is a schematic diagram showing the structure of a lighting device used for a backlight of a liquid crystal display device. As shown in FIG. 1, the lighting device includes a light source 1 such as a cold-cathode tube, a light guide plate 2 arranged such that an incident end face 2 a is located near the light source 1, and a light guide plate 2 arranged on a surface of the light guide plate 2. Diffused sheet 3
And the reflection sheet 4 disposed on the side of the light guide plate 2 opposite to the diffusion sheet 3. In the lighting device having such a configuration, light from the light source 1 enters the light guide plate 2 from the incident end face 2a, and the light incident into the light guide plate 2
Incident surface 2a while being reflected by the surface of
Transmitted in the opposite direction. In the meantime, part of the light exits the light guide plate 2 from the surface of the light guide plate 2 and passes through the diffusion sheet 3,
The illumination light having uniform brightness can be obtained by exiting the illumination device as diffused light.

Conventionally, in a lighting device having the above configuration,
In order to obtain a uniform diffused light, a dot-like pattern whose density is changed according to the distance from the light source is printed on the back surface of the light guide plate (the surface on the side of the reflection sheet 4), or the unevenness is processed. It is textured or provided with a prism-shaped protrusion.

When a light guide plate is manufactured by an injection molding method, a pattern or the like having a dot-like dense and dense distribution is generally formed by using a mold in which a concave and convex pattern reverse to a desired concave and convex pattern is formed in a predetermined region. Processed. In the light guide plate injection molding method, there is a pressure-holding step. In the pressure-holding step, the molten resin is injected and filled into the mold cavity, and the gate is cooled and solidified through the sprue or runner molten resin until the gate is cooled and solidified. The shape of the mold is transferred by applying pressure to the mold. After the gate is solidified, the resin in the mold is cooled and solidified to obtain a molded product (light guide plate).

[0005]

When the molten resin injected and filled into the cavity by the injection molding method comes into contact with the cavity surface, the molten resin is rapidly cooled, and the molten resin is filled into the cavity while forming a cooled solidified layer. At the time of filling the molten resin, a pressure distribution is generated in the cavity, which becomes a residual stress of the molded product, which causes warpage and deformation after molding. In addition, during injection molding of the light guide plate, a holding pressure is applied to correct molding shrinkage, which also causes residual stress in the molded product.

[0006] In order to reduce the generation of the cooling solidified layer which causes the residual stress, it is necessary to cope with molding conditions such as increasing the temperature of the molten resin, increasing the mold temperature, and increasing the filling speed. Are being considered. Warpage of the light guide plate, it is necessary to avoid that residual stress that causes deformation, etc. remains in the light guide plate after injection molding.However, if the residual stress is such that warpage, deformation, etc. do not occur. No inconvenience occurs even if it remains on the light guide plate after molding. However, conventionally, no study has been made on the reference value of the residual stress remaining in the light guide plate after molding, and each time the molding conditions are changed, a long-term reliability test is performed to determine whether the molding conditions are acceptable. I had to decide.

The light guide plate of the present invention has been made to solve the above problems, and has as its object to provide a reference value of residual stress for a light guide plate that does not cause large warpage or deformation after molding.

[0008]

The light guide plate of the present invention for solving the above-mentioned problems is characterized in that the stress at the surface portions of the two opposing main surfaces is 200 kg / cm ² or less. In this light guide plate, it is preferable that the stress at the surface portions of the two opposing main surfaces is 200 kg / cm ² or less, and that the stress difference between the two main surfaces is within 20%. Here, the “principal surfaces” are two opposing surfaces having a large area corresponding to the light-emitting surface and the back surface of the light guide plate. The stress at the surface portion of the main surface is measured by a method described in the following examples or a method similar thereto.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The light guide plate of the present invention is manufactured by an injection molding method or the like. The stress at the surface of the light guide plate is 200
Means for reducing the pressure to kg / cm ² or less include: manufacturing a light guide plate by an injection molding method using low pressure molding that reduces a filling pressure; manufacturing a light guide plate by an injection compression molding method; and a guide manufactured by a normal injection molding method. An annealing process (post-processing) is performed on the optical plate. The synthetic resin used in the injection molding is not particularly limited as long as it is a transparent thermoplastic resin, and examples thereof include polymethyl methacrylate, polycarbonate, polystyrene, a thermoplastic elastomer, and a copolymer thereof. The light guide plate of the present invention may have a tapered shape. In this case, the ratio of the thickness of the thick side to the thickness of the thin side (thickness of the thick side / thickness of the thin side plate) Thickness) is usually 1.5 or more. The aspect ratio of the light guide plate is generally about 4/3 as a ratio (vertical / horizontal) between a vertical dimension and a horizontal dimension in a use state.

[0010]

The present invention will be described below in detail with reference to examples. In addition, the light guide plate of an Example is manufactured by the injection molding method, and the apparatus used for the manufacture is as follows.

(A) Injection molding machine IS-350GS manufactured by Toshiba Machine Co., Ltd. was used.

(B) Dies Three types of dies are used, and the sizes and the like of the dies are as follows. (Mold No.1) Length 200mm x Width 270mm
And has a plate-shaped cavity having a thickness of 2.0 mm. As shown in FIG. 2, the shape of the light guide plate manufactured by using this mold has a width of 25 mm at the center of the side.
A gate part having a size of 1.8 mm and a thickness of 1.8 mm is provided. The gate portion of the light guide plate is cut with a heat nipper or a saw blade. (Mold No.2) Length 200mm × Width 270mm
And a tapered cavity with a maximum thickness of 2.0 mm and a minimum thickness of 0.8 mm. As shown in FIG. 3, a light guide plate having an uneven thickness manufactured using this mold has a width of 25 mm and a thickness of 1 mm at a position 30 mm from the thick side end surface of the vertical side. A gate portion having a size of 0.8 mm is provided. (Mold No. 3) 210 mm vertical dimension × 290 mm horizontal dimension
And a tapered cavity with a maximum thickness of 2.0 mm and a minimum thickness of 0.8 mm. As shown in FIG. 4, a light guide plate having an uneven thickness manufactured using this mold has a width of 25 mm and a thickness of 1 mm at a position 30 mm from the thick side end surface of the vertical side. A gate portion having a size of 0.8 mm is provided.

(C) Molding material Parapet GH-1000S manufactured by Kuraray Co., Ltd., which is a methacrylic resin molding material, was used.

(D) Method for measuring the stress on the surface of a molded product low speed saw (manufactured by Bühler: ISO)
MET) from each part in the surface of the molded product to a width of about 0.
A 6 mm strip was cut out, and both cut surfaces were polished using a No. 2000 sandpaper and a metal abrasive to obtain a test piece having a mirror surface. About the obtained test piece, a Babinet corrector-type precision strain meter (manufactured by Toshiba Glass Co., Ltd .: SVP-30)
In II), the retardation R of the strain was measured under the condition of a magnification of 5 using a Na light source, and the stress was calculated by the following equation. Strain (stress) = R / (3.8 × T) (where R is the retardation and T is the thickness (cm) of the test piece in the retardation measuring section.
3.8 is the photoelastic constant ((nm / cm) / (kg / cm ² )) of polymethyl methacrylate. )

(E) Method of Measuring Deformation As shown in FIGS. 5A and 5B, the measuring positions of the deformation of the light guide plate 5 are at each corner and center of the molded product. Specifically, the light guide plate 5 was left standing on the surface plate 6, and the gap L between the surface plate 6 and the light guide plate 5 was measured with a thickness gage to determine an error between the shape of the mold and the shape of the light guide plate. .

(F) Constant Temperature / Humidity Test The reliability test of the light guide plate was carried out using a constant temperature / humidity bath manufactured by Tabai Co., Ltd. The test conditions are 300 hours at 50 ° C. and 80% RH, which is a standard evaluation method for liquid crystal display devices.

(Examples 1 to 3 and Comparative Examples 1 to 3)
A light guide plate was manufactured using the injection molding machine of (a) and the mold of (b), and the surface layer stress of the obtained light guide plate was measured by the measurement method of (d). Then, in the thermo-hygrostat of (f), 50
The light guide plate was left in an environment of 80 ° C. and 80% RH, and the amount of deformation after 300 hours was measured by the measuring method (e). Table 1 shows the results. The measurement positions of stress and deformation shown in Table 1 are the measurement positions A to D at the four corners of the light guide plate, and the measurement positions E
Is the center position (see FIG. 5; the same applies to Table 2 described later).

[0018]

[Table 1]

As shown in Table 1, the stress of the surface layer was 200
In the light guide plates of Examples 1 to 3 which are equal to or less than kg / cm ² , the deformation amount after the constant temperature and high humidity test is small, whereas the stress of the surface layer portion of Comparative Examples 1 to 3 exceeds 200 kg / cm ^2. The light plate has a large deformation after the constant temperature and high humidity test. Example 1
The light guide plates of Nos. To 3 are tested at 70 ° C. for 300 hours,
A test of leaving at −20 ° C. for 300 hours and a test of alternately leaving at a low temperature of −20 ° C. and a high temperature of 70 ° C.
The amount of deformation was also small in each of the repeated tests.

(Examples 4 to 9) As in Examples 1 to 3,
Using the injection molding machine (a) and the mold (b), the molding conditions were changed to produce a light guide plate. The stress of the surface layer of the obtained light guide plate was measured by the measuring method (d). afterwards,
The light guide plate was allowed to stand in an environment of 50 ° C. and 80% RH in the constant temperature and humidity chamber of (f), and the amount of deformation after 300 hours was measured by the measuring method of (e). Table 2 shows the results.

[0021]

[Table 2]

As shown in Table 2, the stress of the surface layer was 200
In the light guide plates of Examples 4 to 6 in which the stress difference between the front surface and the back surface is not more than 20% at kg / cm ² or less, the deformation amount after the constant temperature and humidity test is extremely small, while the stress of the surface layer portion is small. Is 20
0 kg / cm ² or less, 20% stress difference between front and back
In Examples 7 to 9 that exceed the above, the deformation amount after the constant temperature / humidity test was slightly large.

[0023]

The light guide plate conforming to the reference value of the present invention with respect to the residual stress of the surface portion does not cause large warping or deformation after molding.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a lighting device using a light guide plate.

FIG. 2 shows the mold No. used in the embodiment. FIG. 2 is a view showing the shape of a molded product according to No. 1.

FIG. 3 shows the mold No. used in the embodiment. FIG. 3 is a view showing a shape of a molded product according to No. 2;

FIG. 4 shows the mold No. used in the embodiment. FIG. 3 is a view showing a shape of a molded product according to No. 3;

FIG. 5 is a diagram for explaining a measurement position and a measurement method of a deformation amount of the light guide plate.

[Explanation of symbols]

 1: light source 2: light guide plate 2a: incident end face of the light guide plate 3: diffusion sheet 4: reflection sheet 5: light guide plate 6: surface plate

Claims (3)

[Claims] 1. A light guide plate wherein the stress at the surface portions of two opposing main surfaces is 200 kg / cm ² or less. 2. The stress on the surface portions of the two main surfaces facing each other is 200 kg / cm ² or less.
The light guide plate according to claim 1, wherein a stress difference between the two main surfaces is within 20%. 3. The light guide plate according to claim 1, wherein the light guide plate is manufactured by an injection molding method.