JPH10283681A - Manufacture of optical disk substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To mold a substrate while utilizing the deformation repulsive force of a viscoelastic body and to uniformize transfer property, double refractivity and thickness by providing the viscoelastic body whose thickness is changed in accordance with the position of a cavity on the back surface of a cavity core. SOLUTION: A viscoelastic plastic material 13 is provided on the back surface of the cavity core of a movable side shape 3 of a molding metallic mold and the thickness is varied so that it becomes thicker as it moves from the center section of the substrate to the outer peripheral section. Let t1 and t0 be the thicknesses of the material 13 at the inner and outer peripheral section, Δδbe the difference in the substrate thickness at the inner and the outer peripheral sections, Δp be the molding pressure difference of the inner and outer peripheral sections, E is the elastic modulus of the material 13 and t0 is obtained from t0 =ti +Δδ.E/Δp where ti satisfies ti <=S.(E/Δp).tc and a molding shrinkage ratio S in the substrate thickness direction is set approximately 1% and tc is the thickness of the cavity. Thus, the thickness of the substrate is molded with an approximately uniform thickness, the pressure value is reduced and the substrate double refractivity is kept at a low level.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing an optical disk substrate.

[0002]

FIG. 1 shows a typical molding die 1 used for injection molding of an optical disk substrate according to the prior art. As shown in FIG. 1, the substrate is formed by mounting a stamper 4 made of nickel for forming and transferring information pits and laser guide grooves 11 in a mold cavity and filling molten resin from a gate provided at the center of the substrate. After forming the center hole of the substrate, cooling and solidifying, the mold is released to complete the molding. (FIG. 1 shows the state before the substrate is released.) In the formation of an optical disk substrate, precise transfer of information pits and laser guide grooves on the order of submicrons, suppression of substrate birefringence, and warpage of the substrate Is required to be small, and furthermore, the accuracy of the substrate thickness is required to be improved. In particular, since the thickness of a substrate is being reduced in order to increase the density, it has become important to improve the uniformity of the thickness.

In the conventional injection molding, the filled resin is different in temperature and pressure with respect to the cavity position due to a difference in filling time, and is inferior in transferability, birefringence, and uniformity of substrate thickness.
Substrate deformation is also large.

[0004] As one method for solving such a problem, an injection compression molding method has been developed. This molding method applies a compressive force across the entire surface of the substrate in the direction perpendicular to the substrate thickness after injection filling of the molten resin. Can be improved.

[0005] However, these methods apply the same pressure to the entire surface of the substrate at the same time at the same time even though the temperature and pressure of the filling resin are different with respect to the cavity position. It is impossible to completely eliminate variations such as birefringence and unevenness in substrate thickness.

A conventional method of applying a compressive force in injection compression molding includes a hydraulic mechanism in which a pressure source is provided in a mold,
This is an active loading method using a mold clamping device of a molding machine, a hydraulic mechanism provided on a platen or the like of the molding machine, and the like. In other words, it is basically provided with a pressure load mechanism and supplies pressure load energy for each molding shot, so that the equipment cost of a molding machine and a mold and the running cost due to energy supply are higher than injection molding, and the response Sex is a problem.

[0007]

As described above, in the prior art, it is difficult to obtain uniform transferability, birefringence, and substrate thickness, and there are problems in cost. An object of the present invention is to solve these problems.

[0008]

In order to solve the above problems, the present invention provides a viscoelastic body on the back surface of a cavity core,
In response to the resin pressure during molding, it realizes a passive pressure loading method by the deformation reaction force of the viscoelastic body. By using a viscoelastic body,
The condition of the compression load is changed according to the state of the filling resin at the position in the cavity.

[0009]

Embodiments of the present invention will be described below.

Embodiment 1 FIG. 2 shows an embodiment in which a plastic material 13 is used as a viscoelastic body and the thickness of the plastic material is changed from the center of the substrate toward the outer periphery.
FIG. 2 shows an example in which a plastic material 13 is provided on the back surface of the cavity core 5 of the movable die 3 of the molding die 1.

The difference between the inner peripheral portion and the outer peripheral portion of the thickness of a substrate having a thickness of 0.6 mm and a diameter of 120 mm formed by ordinary injection molding is 0.025 mm. Incidentally, in the injection compression molding method according to the prior art, the thickness difference between the inner and outer peripheral portions is about half, 0.013 mm.

Therefore, in order to eliminate the uneven thickness, the outer peripheral portion of the substrate is 0.1 mm thicker than the inner peripheral portion by molding pressure.
The thickness of the plastic material was determined by the following equation so that the cavity thickness increased by about 025 mm.

[0013]

T ₀ = t _i + △ δ · E / △ p (Equation 1) where t _i is the thickness of the plastic material at the inner peripheral portion of the substrate, t ₀ is the thickness of the plastic material at the outer peripheral portion of the substrate, △ δ is the thickness difference between the inner and outer peripheral portions of the substrate, Δp is the molding pressure difference between the inner and outer peripheral portions, and E is the elastic modulus of the plastic material. However, in order to prevent the plastic material from being significantly compressed and deformed by the molding pressure to form a substrate thicker than the thickness of the cavity, the thickness t _i of the plastic material on the inner peripheral portion of the substrate satisfies the following expression.

[0014]

T _i ≦ S · (E / △ p) · t _c (Equation 2) where S is the molding shrinkage in the substrate thickness direction (about 1%), t _c
Is the cavity thickness.

In this embodiment, nylon 66 containing 30 wt% of glass fiber having high heat resistance and high elastic modulus is used as a plastic material.
Was used. The dimensions are shown in Table 1.

[0016]

[Table 1]

Accordingly, when the resin is filled, the cavity thickness at the outer peripheral portion of the substrate becomes larger by about 0.025 mm than that at the inner peripheral portion. Therefore, the formed substrate has a substantially uniform thickness.

On the other hand, as shown in FIG. 3, the time variation of the residual pressure of the resin after the gate seal decreases with the cooling shrinkage of the molding resin in the conventional metal mold injection molding (A to B). ), After B, the pressure applied to the substrate becomes zero. In the case of the mold provided with the viscoelastic body according to the present invention, the residual pressure changes as A to C, and the pressure value becomes lower than in the case of the conventional metal mold, so that the substrate birefringence can be suppressed low.

(Embodiment 2) In Embodiment 1 of FIG. 2, the thickness is changed with one kind of plastic material.
Now, using multiple plastic materials with different elastic moduli,
Plastic materials 13 ′, 13 ″, 13 ″ ″ concentrically from the outer circumference to the inner circumference of the cavity in order of decreasing elastic modulus.
Was placed. In this example, as shown in Table 2, three types of plastic materials were used in forming a substrate having a diameter of 120 mm. The thickness of each plastic material is determined based on the difference between the molding pressure and the elastic modulus of each plastic material by the difference between the inner and outer peripheral thicknesses of the substrate.
6.7 mm, which can eliminate 025 mm. That is, when the resin is filled, the cavity thickness of the outer peripheral portion of the substrate becomes larger than the inner peripheral portion by about 0.025 mm, and the molded substrate has a substantially uniform thickness.

[0020]

[Table 2]

[0021]

According to the present invention, since the viscoelastic body is provided on the back of the movable core, the same effect can be obtained even if it is provided on the back of the fixed core.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a conventional optical disc substrate molding die.

FIG. 2 is a sectional view of a movable core portion of the optical disk substrate forming die according to the present invention.

FIG. 3 is a characteristic diagram showing a pressure behavior during molding (after completion of pressure holding) according to the present invention.

FIG. 4 is a sectional view of a movable core portion of the optical disc substrate forming die according to the present invention.

[Explanation of symbols]

3 movable type, 4 stamper, 7 stamper inner peripheral holder,
9: punch for forming a center hole, 10: ejector, 12: cooling circuit, 13: plastic material.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Shigehisa Suzuki 1-88 Ushitora, Ibaraki-shi, Osaka Hitachi Maxell, Ltd.

Claims (2)

[Claims] 1. A method for molding an optical disk substrate, comprising: mounting a viscoelastic body on the back surface of a cavity core; and molding the viscoelastic body with a molding die having a thickness changed according to the location of the cavity. Production method. 2. A method for manufacturing an optical disk substrate, comprising: forming a plurality of viscoelastic bodies having different viscoelastic characteristics on a back surface of a cavity core by using a molding die.