JPH06103547B2 - Manufacturing method of plastic substrate for optical disk - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Industrial Application> The present invention records and reproduces information by a laser beam or the like.
The present invention relates to a method for manufacturing a plastic substrate used for an optical disc for erasing, and more particularly to a method for manufacturing a plastic substrate having a uniform birefringence distribution in the radial direction.

<Prior Art> A so-called optical disc that records information in the information storage layer or reproduces the information stored in the information storage layer by irradiating the information storage layer including a recording layer or a light reflection layer with a laser beam. A glass or plastic disk is generally used as the substrate. Among them, the plastic substrate has the advantages that it is lighter in weight than the glass substrate, can be easily molded, has less risk of being damaged during handling, and can be manufactured at a lower cost than the glass substrate.

<Problems to be Solved by the Invention> However, the plastic substrate has a problem that the C / N as a disk system cannot be increased because the birefringence becomes large mainly due to the molecular orientation strain at the time of molding.
For this reason, efforts have been made to lower the molecular weight of the resin and improve the fluidity to reduce the birefringence during injection molding.

Attempts have also been made to make the radial birefringence distribution uniform by adjusting injection conditions (mold temperature, injection speed, holding pressure, mold clamping force, etc.) during injection molding (11th plastic optical parts (Lens, Disc) Applied Technology Development Lecture, Proceedings of Lecture, pp. 3-1 to 3-14, '85 .8.
29-30).

However, it has been found that even if the birefringence is reduced or the distribution is made uniform during injection molding, the value of the birefringence and its distribution greatly change due to the subsequent thermal history. That is,
The manufacturing process of an optical disk includes a heating process such as drying of a plastic substrate and sputtering of a recording layer after injection molding, and the value and distribution of birefringence may greatly change due to the temperature rise of the plastic substrate during this heating process. I understand.

Therefore, the present invention has a uniform birefringence distribution characteristic and is excellent in
An object of the present invention is to provide a method for manufacturing a plastic substrate for an optical disc that can achieve C / N.

<Means for Solving Problems> The method for manufacturing a plastic substrate for an optical disk according to the present invention has been made in view of the above points, and the radial birefringence distribution of the plastic substrate becomes uniform after the heating step. Thus, a birefringence distribution is given in the radial direction during injection molding.

<Operation> During injection molding, the birefringence distribution in the radial direction is given in advance in anticipation of the birefringence distribution in the radial direction that fluctuates in the heating step after the injection molding. A plastic substrate having a uniform refractive distribution can be obtained.

<Examples> Examples of the present invention will be described below with reference to the drawings.

The manufacturing method of the present invention, when a plastic substrate for an optical disk is manufactured by injection molding, has a plurality of radial direction fluctuations caused by a heating process (hereinafter referred to as annealing) such as drying of the plastic substrate performed after the injection molding and sputtering of the recording layer. In consideration of the refraction distribution, the birefringence distribution is given in advance in the radial direction.

The birefringence distribution is given in the radial direction in advance during injection molding by adjusting the injection molding conditions (mold temperature, injection speed, holding pressure, mold clamping force, etc.).

Materials having excellent transparency such as polymethylmethacrylate (PMMA) and polycarbonate (PC) are used as plastic substrates for optical disks.

Here, the term "annealing" refers to a heat treatment in which an injection-molded plastic substrate is left in a predetermined temperature environment for a predetermined time and then cooled to room temperature. For example, when using polycarbonate (PC) to make a substrate, 100
That is, the heat treatment is performed continuously at 2 ° C. for 2 hours. The amount of change in birefringence depends on the annealing temperature and is not significantly affected by the annealing time. That is, the birefringence distribution fluctuates largely about 30 minutes after the plastic substrate reaches a predetermined temperature, hardly changes thereafter, and even if it changes, the amount of change is very small. Further, after the heat treatment is once performed, the birefringence does not fluctuate much even if it is reheated if the temperature is equal to or lower than the first temperature.

(Experimental example) Below, the experimental example is shown and the effect of this invention is clarified.

FIG. 1 shows measurement results of birefringence distribution before and after annealing of a plastic substrate having a birefringence distribution in the radial direction produced by the manufacturing method of the present invention. In this experimental example, the birefringence distribution in the radial direction is given at 20 nm or more during injection molding, and the birefringence distribution in the radial direction is changed by annealing the plastic substrate thus manufactured at 100 ° C. for 2 hours continuously. It is about 10 nm.

On the other hand, in order to compare with this experimental result, the measurement result of the radial birefringence distribution of the plastic substrate manufactured without giving the birefringence distribution in the radial direction (that is, manufactured by the conventional manufacturing method) is shown in FIG. Shown in. From the figure, even if a plastic substrate having a radial birefringence distribution of 3 nm or less is manufactured at the time of injection molding, the distribution becomes 25 nm or more by annealing at 100 ° C. for 2 hours continuously.

As described above, the birefringence distribution fluctuates due to annealing, but it is necessary to give the birefringence distribution in consideration of not only the fluctuation amount but also the direction of fluctuation during injection molding. That is, when the birefringence changes from the minus side to the plus side due to annealing, it is necessary to make the birefringence on the inner peripheral side of the plastic substrate negative.

The above-described manufacturing method of the present invention can be applied by giving a birefringence distribution in the radial direction at the time of injection molding in anticipation thereof even when heat treatment is performed separately from drying, sputtering and the like. It is a thing.

<Effects of the Invention> As described above, according to the method for manufacturing a plastic substrate for an optical disk of the present invention, a plastic substrate having a uniform birefringence distribution in the radial direction can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a radial birefringence distribution immediately after injection molding (before annealing) and a radial birefringence distribution after annealing of a plastic substrate for an optical disk manufactured by the manufacturing method of the present invention, and FIG. FIG. 6 is a diagram showing an example of a radial birefringence distribution immediately after injection molding (before annealing) and a radial birefringence distribution after annealing of a plastic substrate for an optical disk manufactured by the manufacturing method of FIG.

Claims (1)

[Claims] 1. A method of manufacturing a plastic substrate for an optical disk, which is produced by injection molding, wherein the birefringence distribution in the radial direction during injection molding is such that the birefringence distribution in the radial direction of the plastic substrate becomes uniform after the heating step. A method of manufacturing a plastic substrate for an optical disc, characterized by: