JPH0564570B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method of manufacturing a plastic substrate for recording media such as optical disks, optical cards, and optical tapes, and is particularly applicable to magneto-optical recording media. The present invention relates to a possible method of manufacturing a transparent plastic substrate.

(Prior Art) Birefringence of the transparent substrate poses a problem in optical high-density information recording media in which submicron-order information spots are recorded and reproduced using a laser beam through a transparent substrate. In particular, in recording media that read minute changes in the plane of polarization such as magneto-optical recording of 0.1 to 0.3 degrees, if the birefringence value is large, the CN
The ratio decreases and it is not practical. The above-mentioned transparent substrate is desirably made by injection molding polycarbonate from the viewpoint of cost and properties such as resistance to change due to water absorption, but polycarbonate resin has a drawback of high birefringence.

The applicant is Japanese Patent Application No. 59-12565 (Japanese Unexamined Patent Publication No. 60-
No. 155424) disclosed a method for greatly reducing the birefringence of injection-molded polycarbonate substrates by improving molding conditions, but subsequent research revealed that plastic substrates do not have flattened substrates, which had been previously thought. In addition to birefringence in the direction parallel to the surface,
We completed the present invention by discovering that there is birefringence in the direction perpendicular to the flat surface, and that the latter birefringence has a more significant effect on optical properties and therefore on the CN ratio. In other words, in the conventional birefringence measurement method, linearly polarized light was incident perpendicularly to the substrate surface, so birefringence in a direction perpendicular to the substrate surface was not observed.
However, for example, when the linearly polarized light is applied to the substrate surface,
When the incident light is tilted at 30 degrees, the transmitted light causes leakage light under crossed nicol conditions. This phenomenon cannot be explained by assuming that only birefringence exists parallel to the substrate surface, but can be explained by assuming that birefringence exists in a direction perpendicular to the substrate. When examined in more detail, the polycarbonate substrate has optical anisotropy with a refractive index nz in the direction perpendicular to the substrate surface and a refractive index nx, ny in the direction parallel to the substrate surface.
In general, |nx−ny|≒0. However | nz−nx
| and |nz−ny| are not zero but rather large values, for example 0.0005 to 0.0006, and if the thickness of the optical disc is 1.2 mm, the optical disc will have a value of 600 to
A retardation of 780 nm exists in the cross-sectional direction.

It is currently unclear why polycarbonate substrates have optical anisotropy similar to that of biaxial crystals, but the orientation of resin molecules in the molded cavity has a significant effect. is a fact. That is, in the behavior model of the molten resin in the molding cavity shown in FIG. is added. Therefore, a force for orienting the molten resin radially inward in the thickness direction of the molded cavity, a force for orienting it in the thickness direction, and a force for orienting it radially inward are simultaneously applied to the molten resin. In FIG. 1, the areas to which these forces are applied are indicated by A, B, and A, respectively. Although it is unclear which regions influence the three principal refractive indices nz, nx, and ny, it is thought that three regions with different orientation directions exist in the thickness direction of the substrate.

The present inventors hypothesized that it would be necessary to control the orientation in the above region B in order to reduce the high birefringence, which is one of the causes of the decrease in the CN ratio when using a polycarbonate resin substrate. As a result of various experiments based on the above, the present invention was completed. Conventional birefringence measurement methods, that is, methods in which linearly polarized light is incident perpendicularly to the substrate surface, cannot measure the influence of the refractive index nz in the direction perpendicular to the substrate surface. No disc substrate with this value existed prior to this application.

(Object of the Invention) Therefore, an object of the present invention is to provide a method for molding a transparent plastic substrate necessary for producing a recording medium with a high CN ratio used in an optical high-density information recording system.

(Structure of the Invention) The first feature of the present invention is that a transparent plastic substrate is used in an optical high-density information recording/reproducing system in which information is recorded and/or reproduced by making a laser beam incident through a flat transparent plastic substrate. In the injection molding method, a sheet-like or film-like plate member thinner than the substrate of the molded product is placed in advance in a molding cavity formed by a pair of split molds, and then molten resin is injected into the molding cavity. The method is characterized in that the plate-shaped body and the molten resin are integrated.

The above-mentioned optical high-density information recording/reproducing method itself is well known, and information is recorded and reproduced by focusing a laser beam to about 1 micron, and generally uses a disk-shaped recording medium. The above information may be recorded in the form of a prepit on one side of the transparent plastic substrate according to the invention during molding of the substrate, or on the surface of the plastic substrate with or without track grooves or preformat pits.
DRAW films such as Te-based, E- films such as Tb Fe Co-based, etc.
A DRAW film is attached and written by the user during use. In this case, the laser beam is incident through the transparent plastic substrate (so-called back reading method). The present invention can be applied not only to this back-side reading method but also to a so-called front-side reading method. In that case, the information is carried on a suitable support and the laser beam is incident through the transparent plastic substrate according to the invention, which is placed above this information. In either method, the birefringence of the transparent plastic substrate must be suppressed as much as possible.

In the present invention, the refractive index nz in the direction perpendicular to the surface of the plastic substrate is considered. As shown in FIG. 2, the transparent plastic substrate 5 has refractive indexes nx and ny that are parallel to the flat surfaces 6 and 7 of the substrate and orthogonal to each other, and a refractive index nz that is perpendicular to the flat surfaces 6 and 7. Assume. In the conventional birefringence measuring method, the linearly polarized light for observation was made incident on the flat surfaces 6 and 7 at right angles, so that the birefringence caused by nz could not be observed. The inventor observed the above nz by directly making the polarized light 8 incident on the flat surface 6 at an angle of incidence θ=30°, for example. This birefringence measurement method is the same as the conventional method except that the angle of incidence on the substrate is changed from 0° to 30°, so the details will be omitted. In short, it is sufficient to measure the transmitted light intensity under crossed Nicol conditions of linearly polarized light incident on the substrate at an incident angle of 30°.

According to the inventors' experiments, nx and ny are generally equal. However, the values of |nz−nx| and |nz−ny| are much larger than the conventionally thought birefringence, and these values are
0.0005 or more, and the CN ratio of a magneto-optical disk produced by forming a magneto-optical recording film on this substrate is about 48 dB.

On the other hand, according to the present invention, the CN ratio of a magneto-optical disk manufactured by forming the same magneto-optical recording film as above on a substrate in which the values of |nz-nx| and |nz-ny| are reduced to 0.004 or less. improved to 50dB. The reason why the CN ratio improves in this way is thought to be due to the increase in θk and the decrease in the noise level.

As the above-mentioned resin, all resins exhibiting refractive index anisotropy can be applied to the method of the present invention. In consideration of other properties, the present invention can be particularly effectively applied to polycarbonate resins. The dimensions of the molded cavity described above vary depending on the disc being molded, but the diameter is approximately 3
cm to about 30cm, thickness 1-2mm, generally 1.2mm
It is. The molding machine is appropriately selected depending on the size of the disc to be molded, and the molding conditions are the same as those used in normal disc molding, except for the special operations of the present invention described below. For polycarbonate resin, the injection cylinder temperature is generally 300-400
DEG C., the mold temperature is about 100 DEG C., and the flow rate of the resin into the cavity is from 10 to 50 ml/sec, which will of course vary depending on the disc size, and other conditions will be selected for other types. Regarding the injection conditions for optical disk substrates using polycarbonate resin, please refer to the above-mentioned Japanese Patent Application Laid-Open No. 155424/1983 by the present applicant.

The present invention is a type of so-called insert molding method, in which a transparent plastic plate in the form of a sheet or film is placed in advance in a molding cavity and then injection molded. The reason for placing the transparent plastic plate in advance in the molding cavity is to reduce the thickness of area B shown in FIG. That is, the thickness of the molding cavity into which the molten resin flows is reduced by the thickness of the plate-shaped body, and therefore the thickness of the region B is also reduced.

In principle, it is preferable that the resin constituting the plate-shaped body is substantially the same as the resin to be injected. By doing so, reflection and refraction of light at the interface between the plate-shaped body and the injected resin can be reduced to substantially zero. The above-mentioned plate-shaped body can be made by a well-known sheet molding method or film molding method, and is generally used for extrusion molding or casting molding. Naturally, this plate-shaped body must have low birefringence, and for this purpose, it is necessary to appropriately select the cooling rate during extrusion molding and/or the annealing temperature to remove cooling distortion. Naturally, the thickness of the plate-shaped body must be smaller than the final thickness of the substrate to be molded. Generally, the thickness of the plate is 80% or less of the final thickness of the substrate, preferably
Keep it below 50%. In the case of optical disk substrates,
Since the final thickness of the substrate is about 1.2 mm, the thickness of the plate is less than about 1 mm, preferably 0.6 mm.

The above-mentioned plate-like body may be cut into the same shape as the molding cavity and then set in the molding cavity, but it is also possible to supply it to the mold of an injection molding machine in the form of a long strip and use it in the mold during molding or during molding. It is preferable to cut the molded product later along the outer peripheral contour outside the mold. The above-mentioned strip can be fed intermittently to the mold using a feed roll and a take-up roll, but in order to ensure accurate feeding and positioning, holes are formed in advance at equal intervals along both edges of the strip. This hole may be fed by a sprocket.Furthermore, the positioning and release of the strip may be ensured by intermittently driving an endless belt having protrusions that engage with the hole through the mold. . In this case, the load on the center punch in the mold can be reduced by previously forming a hole in the strip that corresponds to the center hole of the disk. The above-mentioned feed roll, take-up roll, and endless belt are displaced in the axial direction of the mold in conjunction with the movements of the mold assembly such as mold closing and mold opening, making it easy to remove the strip from the mold. You can also.

In addition, in order to ensure the intermittent feeding of the strip, a portion of the strip that comes into contact with the mold, for example, the strip enters the mold, so that the strip is not easily deformed by the temperature of the mold. It is also possible to cool the mold at the point where it comes out and where it comes out.

It is not always necessary to completely prevent the occurrence of burrs during injection. That is, since the burr adheres to the plate-like body, it is sufficient to cut the burr at the same time when cutting the outline of the plate-like body.

In the case of optical disc molding, a center hole is required. This center hole is preferably formed within the mold using a center punch in order to keep the amount of eccentricity of the optical disk below a permissible value. When the thickness of the sheet or film according to the present invention becomes thicker, it becomes difficult to punch a hole in the sheet or film using the center punch of a conventional optical disc mold. For example, it is preferable to embed a heater to soften and melt the strip.

Hereinafter, an apparatus according to an embodiment of the present invention will be described using FIGS. 3 to 5.

FIG. 3 is a conceptual perspective view of a mold assembly when the present invention is applied to the mold assembly of a horizontal injection molding machine, and FIG. 4 is a sectional view taken along the - line. This mold assembly itself is basically the same as that for conventional optical disk substrate molding, and consists of a pair of split molds,
That is, it has a fixed side split mold 1 and a movable side split mold 2,
A molded cavity 3 is defined by these. In addition to the split molds described above, the mold assembly includes a center punch 4 and a plate group 6 that drives a molded product ejector (not shown), but these do not form part of the present invention. Therefore, the details will be omitted.

A stamper 8 having an information pit etc. is mounted on a stamper holder 9 on a platen 7 attached to at least one of the split molds, in the example shown, the fixed side split mold 1.
is held by.

A feature of the present invention is that a film-like or sheet-like plate member is inserted into the mold assembly and molded. In the illustrated embodiment, the plate-like body is supplied by intermittently feeding a rolled-up strip 10 to a mold. This band-shaped body 10
Equally spaced holes 11 are pre-formed on both side edges of the endless belt 12, and these holes engage with equally spaced protrusions 13 on the endless belt 12. Above endless belt 1
2 is intermittently driven when necessary in accordance with the molding cycle. Further, using a mechanism not shown, the endless belt 12, the feed roll 14 and the take-up roll 15 that guide it and transport the strip are moved in the axial direction, that is, in the direction perpendicular to the molding cavity 3, as necessary. It has also become possible to shift to

During operation, when the mold is in an open state, the endless belt and each roll 14, 15 are axially displaced by a certain amount in a direction away from the stationary mold, and then rotated to form a predetermined length of the strip 10. Saga Ryo split type 1,
Sent between 2. The above-mentioned feeding mechanism 12, 14,
When the mold 15 is axially returned to its original position, the split molds 1 and 2 are closed, and injection molding is then carried out according to conventional methods. That is, molten resin is injected from an injection cylinder (not shown) into the molding cavity 3 through the nozzle touch part 16, and the molten resin and the above-mentioned strip are integrated in the cavity. Next, a center hole is formed at the center of the integrated molded product by relative movement of the molds 1, 2 and the center punch 4 in the axial direction. In this case, in order to facilitate drilling of the strip, the center portion of the strip is locally heated using a heater (not shown) embedded in the punch tip and/or part of the mold adjacent to the center hole. is preferable.

After the resin has solidified, the split molds 1 and 2 are opened. In the present invention, since the molded disk is integrated with the strip 10, it is necessary to separate the disk from the strip 10. It is preferable to perform this separation operation outside the mold because the mold structure becomes simpler.
In this case, the outer periphery of the disk may be cut by cutting with an annular cutter using the center hole as a guide. On the other hand, when performing the above separation operation within a mold, it is necessary to provide a cutting mechanism in the mold. An example of this cutting mechanism is conceptually shown in FIGS. 4 and 5. In other words, this cutting mechanism is a moving ratio type 2
It consists of an annular cutter ring 20 embedded in the cutter ring 20, a sleeve 21 that holds the cutter ring 20, and a protruding rod 22 that drives the sleeve. If necessary, the cutting mechanism can be provided with heating means to heat the cutter ring 20. This cutter ring 20 is driven forward at an appropriate time to separate the disk from the strip 10. After the mold is opened, the disc of the separated molded product is removed from the mold using, for example, a robot. On the other hand, the strip from which the disks have been separated is fed by one step by the above-mentioned feeding mechanism for the next shot.

In the embodiment of FIG. 5, a movable sealing ring 30 is provided for sealing the cavity 3 of the mold assembly according to the invention. The role of the seal ring 30 is to prevent sealing failure of the cavity 3 due to non-uniform thickness of the band-shaped body. This movable seal ring 30 is slidably housed in a hydraulic chamber 31 formed in the split mold 1 or its platen 7, and is moved forward to come into contact with the strip when the mold is clamped. Degassing can be performed by adjusting the pressing force of this movable ring 30. In the present invention, as mentioned above, the occurrence of burrs does not have to be a serious problem, so the accuracy of the parting line of the split mold does not need to be controlled so strictly.

Further, in the mold of the embodiment shown in FIG. 5, an annular resin reservoir 32 is provided outside the stamper holder 9 in the radial direction. This resin reservoir 32 is effective in preventing an increase in birefringence at the outer periphery of the disk due to turbulence in the flow of the resin around the outer periphery of the disk and non-uniform cooling.

What has been illustrated and described above is merely one embodiment of the present invention, and the present invention is not limited thereto, and various modifications can be made. For example, using a vertical injection molding machine,
Adding a position detector to the feeding of the strip, interlocking control of the forming machine and disk peripheral cutting machine, pre-charging the strip in order to hold the strip uniformly in the cavity, or Instead, it is also possible to perform insert molding by punching out a disc-shaped plate with the same dimensions as the inner and outer diameters of the disc in advance, holding it in a vacuum absorption means provided in the mold, and then injecting it. be.

[Brief explanation of the drawing]

FIG. 1 is a model diagram showing the behavior of molten resin in a conventional molding cavity. Fig. 2 is an explanatory diagram of refractive indices nx, ny, and nz for explaining the present invention, Fig. 3 is a conceptual perspective view of an injection mold assembly for carrying out the method of the present invention, and Fig. 4 is the above-mentioned FIG. 5 is a conceptual diagram of a mold assembly, a longitudinal cross-sectional view taken along the line - in FIG. 3, and FIG. 5 is an enlarged cross-sectional view of a portion of FIG. 4. Symbols in the figure, 1, 2... split mold, 3... molding cavity, 4... center punch, 10... band-like body, 12... endless belt, 14, 15... roll, 20... cutter ring, 30...Movable seal ring.

Claims (1)

[Scope of Claims] 1. In a method for manufacturing a plastic substrate for high-density recording media by injecting molten resin into a molding cavity formed by a pair of split molds, a sheet-like or film-like transparent plastic plate is prepared in advance. A method comprising injecting molten resin into the mold cavity after the body has been placed in the mold cavity. 2. The method according to claim 1, wherein the resin is a polycarbonate resin.