JPH10149587A - Forming method of optical disk substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain good transfer property in both of the inner and outer peripheral parts by forming a heat insulating layer of a specified thickness range on the lower surface of a stamper or forming a heat insulating layer having the thickness changing from the center part of the substrate to the outer peripheral part. SOLUTION: As for a ceramic material 13 to form a heat insulating layer, zirconia is used and is formed to 0.2 to 1.0mm thickness range. When the forming process is carried out at 380 deg.C as the upper limit temp. for a resin, the center part of the substrate where a gate is disposed reaches 380 deg.C, however because the temp. in the peripheral part of the substrate far from the gate is lower by about 30 deg.C, the resin temp. in the peripheral part is 350 deg.C. From previous experiments, it is necessary that the temp. on the interface between the surface of the stamper 4 and the resin is >=130 deg.C which is higher than the thermal deformation temp. (126 deg.C) in order to obtain enough transfer amt. Therefore, it is necessary that the zirconia layer is >=0.2mm thick. On the other hand, when an heat insulating layer is formed, generally the cooling rate decreases and the growing cycle increases.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for forming an optical disk substrate.

[0002]

2. Description of the Related Art FIG. 1 shows a molding die used for injection molding of an optical disk substrate. As shown in FIG. 1, a method for forming a substrate is to mount a nickel stamper for forming and transferring information pits and laser guide grooves in a mold cavity, and fill a molten resin through a gate provided at the center of the substrate. Molding is completed by releasing after cooling and solidification. In forming such an optical disk substrate, the most important issue is the precise transfer of information pits and laser guide grooves on the order of submicrons. In particular, when the substrate thickness is reduced to half of the conventional thickness of 0.6 mm for the purpose of higher density, the conventional technology of the optical disc substrate using the polycarbonate resin hardly increases the groove depth of the stamper as shown in FIG. Since the resin has not been transferred and the molten resin is cooled by the mold during filling (under the molding conditions described below, the temperature decreases by about 20 to 30 ° C.), the transferability tends to decrease as the distance from the gate increases.

[0003] Techniques for improving transferability include:
(1) High-speed filling of molten resin, (2) Molding at a high mold temperature, and the like. In the high-speed filling molding method of the above-mentioned prior art (1), as shown in FIG. 6, although transferability is improved to some extent, complete molding transfer cannot be obtained. In addition, the problem that the transferability decreases as the distance from the gate decreases is not solved. By increasing the output of the molding machine, higher-speed filling than in the example shown in FIG. 6 is possible, but a lot of technical development is required to increase the speed due to generation of burrs and the like.

In the prior art (2), the mold temperature is at most close to the thermal deformation temperature of the molding resin (less than the thermal deformation temperature: the thermal deformation temperature of the polycarbonate resin is 126.degree. 6), the transferability is not satisfactory as shown in FIG. 6 as described above, and the problem that the transferability decreases with distance from the gate remains as in the prior art (1). Up to.

[0005] As a method for preventing the decrease in transferability,
As disclosed in Japanese Patent Application Laid-Open No. 1-278322, a method has been proposed in which molding is performed by increasing the mold temperature (cavity temperature) in a portion far from the gate. However, in this method, even when the substrate is released from the mold, the mold temperature is high (set at a temperature equal to or higher than the thermal deformation temperature in order to improve transferability). Not satisfied. In addition, a plurality of mold temperature control mechanisms are required in terms of equipment. As one method for solving this problem, a mold temperature cooling / heating cycle method has been developed in which the mold temperature is set to be equal to or higher than the heat deformation temperature when the resin is filled, and the mold temperature is set to be equal to or lower than the heat deformation temperature when releasing the mold. However, in this method, there is a new problem that the equipment becomes more complicated and the molding cycle becomes longer.

[0006]

As described above, in the prior art, a plurality of mold temperature control mechanisms are required to obtain uniform transferability, and the obtained molded product is increased in deformation due to mold release and removal. Forced to. An object of the present invention is to solve these problems.

[0007]

In order to solve the above-mentioned problems, means for keeping the temperature of the molten resin on the surface of the stamper higher than the temperature at which the transferability can be improved and eliminating the temperature difference between the central portion and the outer peripheral portion of the substrate. A heat insulating layer having a thickness within a certain value range on the lower surface of the stamper or a heat insulating layer having a thickness changed from the central portion to the outer peripheral portion of the substrate is provided.

[0008]

Embodiments of the present invention will be described below with reference to the drawings.

Embodiment 1 FIG. 2 shows an embodiment according to the present invention in which zirconia is used as the ceramic material 13 for forming the heat insulating layer and the thickness is set to a certain value or more.

How the temperature at the interface between the filled resin and the stamper 4, which is a major factor determining the mold transferability, changes with the thickness of the heat insulating material was simulated in advance. Table 1 shows the specifications used in the simulation, and the results are shown in FIG.

[0011]

[Table 1]

The results in FIG. 3 show that the initial mold temperature is 110 ° C. at which no deformation of the substrate occurs at the time of mold release, and the maximum temperature at the boundary surface between the resin and the stamper (about 0.2 s after filling the resin).

When molding is performed at 380 ° C., which is the upper limit temperature of the resin temperature, 380 ° C.
Since there is a drop of about 30 ° C from the gate to the outer periphery of the substrate, the resin temperature at the outer periphery is 350 ° C. From the experimental results so far, in order to obtain a sufficient transfer amount, the interface temperature between the resin and the surface of the stamper 4 is higher than the thermal deformation temperature (126 ° C.) 1
FIG. 3 shows that the thickness of the zirconia layer is required to be 0.2 mm or more because the temperature is required to be 30 ° C. or more.

On the other hand, in general, when a heat insulating layer is provided, the cooling rate becomes slow and the molding cycle becomes long. Therefore, as a temperature condition at which the substrate can be released, the boundary temperature between the resin and the stamper is 120 ° C.
And, also consider the time to reach the temperature,
The result is shown in FIG. From FIG. 4, from the viewpoint of productivity,
In order to keep the cooling time under 2 seconds, the thickness of zirconia must be
It must be less than 1.0mm. Based on these results, the thickness of the zirconia heat insulating layer was set to 0.2 m over the entire substrate.
m uniform.

The zirconia heat-insulating layer 13 can be formed by bonding a pre-sintered material to the surface of the movable core 5 or by directly forming it on the surface of the movable core 5 by sputtering. There are drawbacks such as difficulty in obtaining a tie and the latter having an extremely slow forming speed. Therefore, in the present embodiment, a method of forming by a thermal spraying method and then polishing the surface to finish the flatness to 0.01 mm or less and the surface roughness Ra to 0.3 μm is adopted.

As a result, even when the temperature of the molten resin immediately after filling is lower than the inner peripheral portion of the substrate near the gate, the boundary surface temperature with the stamper immediately after filling the resin exceeds 130 ° C., which is higher than the thermal deformation temperature of the resin. As shown in 6, good transferability can be obtained in both the inner and outer peripheral portions. In addition, since the cooling rate of the resin is low in this molding die, the internal stress (birefringence) of the molded product can be suppressed to a small value.

In the above example, zirconia (thermal conductivity: 3.7 W / mK) was used as the heat insulating ceramic. However, similar to zirconia, a titania-based material having a low thermal conductivity (thermal conductivity: 3.7 W / mK) was used.
2.9-4.1W / mK). By the way, resin material is generally used for the heat insulation layer, but the elasticity and hardness of the resin material are much smaller than those of the ceramic material. And cannot be applied to the molding of the optical disk substrate.

Embodiment 2 FIG. 5 shows an embodiment according to the present invention in which zirconia is used as the ceramic material 13 forming the heat insulating layer and the thickness is changed according to the distance from the gate.

When the resin temperature is set to the upper limit of 380 ° C., the molten resin temperature at the center of the substrate where the gate is provided is 380 ° C.
It can be estimated to be 350 ° C because of the temperature drop of about 30 ° C in the outer periphery of the substrate farther from the gate. Therefore, from FIG.
The thickness of the zirconia layer is 0.13 mm or more at the center of the substrate and 0.2 mm or more at the outer periphery of the substrate. As described above, the thickness of the zirconia heat insulating layer was increased as the distance from the gate was increased, and was set to 0.13 mm at the inner periphery of the substrate and 02 mm at the outer periphery.

In order to form the zirconia heat insulating layer having a thickness gradient as described above, the surface of the movable core 5 which has been subjected to the inclined surface processing is performed by the above-described thermal spraying method, and then the surface is polished. To a flatness of 0.01 mm or less and a surface roughness Ra of 0.3 μm.

[0021]

According to the present invention, even if the molten resin temperature immediately after filling is lower than the inner peripheral portion of the substrate near the gate,
The boundary surface temperature with the stamper immediately after filling the resin is 130 ° C., which exceeds the thermal deformation temperature of the resin, and is the same at the inner and outer peripheral portions of the substrate. As shown in FIG. Transferability can be obtained.

[Brief description of the drawings]

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

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

FIG. 3 is a diagram showing the relationship between the thickness of a heat insulating material that forms the basis of the present invention and the maximum temperature of the surface of a stamper.

FIG. 4 is a diagram showing the relationship between the thickness of a heat insulating material that forms the basis of the present invention and the time required for the surface temperature of the stamper to reach 120 ° C.

FIG. 5 is a sectional view of a movable core portion of the optical disk substrate forming die according to the present invention.

FIG. 6 is a view showing the molding transferability of an optical disk substrate according to the related art and the present invention.

[Explanation of symbols]

1: optical disk substrate molding die, 2: fixed die, 3: movable die,
4 stamper, 5 movable core, 6 optical disk substrate, 7 stamper inner peripheral holder, 8 stamper outer peripheral holder, 9 center punch forming punch, 10 ejector, 11 ...
Information pit and laser light guide groove, 12 cooling circuit, 13
... Zirconia insulation layer,

Claims (2)

[Claims] 1. An optical disc substrate forming method, wherein a heat insulating layer made of ceramic is formed between a stamper and a mold core, and the thickness of the ceramic layer is set in a range of 0.2 mm to 1.0 mm. Molding method. 2. A method for molding an optical disk substrate, wherein a heat insulating layer made of ceramic is formed between a stamper and a mold core, and the thickness of the ceramic heat insulating layer is increased from the center of the substrate toward the outer periphery. Optical disc substrate molding method.