JPH0579010B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for manufacturing recording media such as optical disks, optical cards, and optical tapes, and in particular to a method for manufacturing a transparent plastic substrate that supports an information layer. It is about improvement. The present invention particularly relates to a method of manufacturing a transparent plastic substrate applicable to magneto-optical recording media.

(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, a large birefringence value causes 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 present applicant is Japanese Patent Application No. 59-12565
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, polycarbonate substrates have optical anisotropy, with a refractive index n _z in the direction perpendicular to the substrate surface and refractive indices n _x , n _y in the parallel directions to the substrate surface. |n _x −n _y |≒0. However, |n _z −n _x |
and |n _z −n _y | are not zero, but rather large values, such as 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 in which region the three principal refractive indices n _x , n _z , and n _y are influenced, it is thought that there are three regions with different orientation directions in the thickness direction of the substrate. .

The present inventors hypothesized that it would be necessary to control the orientation in the region B above in order to reduce the high birefringence, which is one of the causes of a 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 measuring methods, that is, methods in which linearly polarized light is incident perpendicularly to the substrate surface, cannot measure the influence of the refractive index n _z in the direction perpendicular to the substrate surface. No disk substrates with refractive values existed prior to this application.

(Object of the Invention) Therefore, the object of the present invention is to provide an optical disk substrate used in an optical high-density information recording system, and in particular, to provide a method for manufacturing a polycarbonate resin substrate molded by injection molding. Our goal is to provide the following.

(Structure of the Invention) The first feature of the molding method provided by the present invention is that it is an optical high-density information recording and reproducing method in which information is recorded and/or reproduced by entering a laser beam through a flat transparent plastic substrate. In the injection molding method used for transparent plastic substrates, the axial spacing of the molding cavity formed by a pair of split molds for molding the plastic substrate is determined by the time from the start of injection to the pressure holding step. At least in part, it is about increasing from small to large.

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 n _z 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 indices n _x and n _y that are parallel to the flat surfaces 6 and 7 of the substrate and perpendicular to each other, and refractive index n _z that is perpendicular to the flat surfaces 6 and 7. Assume that you have. In the conventional birefringence measurement method, the linearly polarized light for observation was incident on the flat surfaces 6 and 7 at right angles, so the birefringence caused by the above n _z could not be observed. The inventor observed the above n _z by making the linearly 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, n _x and n _y are generally equal. However, the values of |n _z −n _x | and |n _z −n _y | 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 above |n _z −n _x | and |
The CN ratio of a magneto-optical disk produced by forming the same magneto-optical recording film as above on a substrate in which the value of n _{z −ny} | is reduced to 0.0004 or less is improved to 50 dB. in this way
The reason for the improvement in the CN ratio is considered to be 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 500 ml/sec, which will of course vary depending on the disk 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.

Increasing the axial distance between mold cavities from the injection process to the pressure holding process, which is a feature of the present invention, is generally done by separating one of a pair of split molds from the other. It can be carried out. The purpose is to relax or disperse the orientation of the resin in the direction perpendicular to the surface shown in FIG. It is important that the above-mentioned increase in the axial spacing is carried out from the start of the injection process to at least part of the holding pressure process. In practice, the injection may be performed for 0.3 to 2 seconds until the molten resin is completely filled into the molding cavity. Generally, this is done at an appropriate timing taking transferability issues into account, but the surface of the molten resin filled in the cavity begins to solidify due to the mold temperature, and the cooling temperature is transmitted to the inside. Do it before. In other words, the region B in FIG. 1 is in the final solidification stage. Due to the above described increase in the axial spacing of mold cavities according to the present invention, area B in FIG. 1 becomes substantially smaller. That is, the proportion of the A region formed by the cooling effect from the mold surface increases relatively. As a result, the absolute value of the difference between the refractive index N _z in the direction perpendicular to the flat surface of the transparent plastic substrate and the refractive index n _x and n _y in the direction parallel to the flat surface: |n _z −n _x | and |n _z −n _y | can be set to 4×10 ⁻⁴ or less.

The amount of increase in the axial distance varies depending on the type and dimensions of the molded product, especially the external dimensions and thickness, injection speed, etc. For example, in the case of an optical disk having a thickness of 1 to 2 mm, the above-mentioned axial spacing at the start of injection is preferably 9/10 or less and 1/10 or more of the desired axial dimension of the desired final molded product. If this initial interval dimension is less than 1/10, excessive shearing force is applied to the resin, which is undesirable, and if it is more than 9/10, there is no effect. In the case of optical disk substrates, the thickness of the final molded disk is 1.2 mm, so the initial spacing above is 0.12 ~
Preferably it is 1.08mm. Moreover, the final value of the above-mentioned interval dimension when increasing does not necessarily have to be the same as the desired dimension of the final molded product. Generally, in order to improve transferability, it is preferable to make the axial spacing dimension slightly larger than the desired dimension at the time of the increase, and then decrease it again to maintain the desired dimension until the mold opens.

In order to carry out the method of the present invention, it is necessary to use a mold assembly in which the axial spacing of the molding cavities can be varied and the cavities can be sealed. A general mold for molding an optical disk cannot be used in the present invention because the dimensions of the molding cavity formed when the split mold is closed are equal to the desired dimensions of the final molded disk. A mold assembly for carrying out the method of the present invention must be such that the axial or thickness dimension of the molding cavity can be reduced to less than the desired final dimension of the plastic substrate of the final molded product. At the same time, the mold assembly must sufficiently seal the outer periphery of the molding cavity to prevent burrs from forming.

The prototype of the mold assembly that satisfies the above requirements is disclosed in Japanese Patent Application No. 1987-245678 filed by the applicant on November 1, 1985 under the title "Mold for Injection Molding of Optical Disk" ( JP-A-62-105616). In this application, an axial sliding member is provided on the outer periphery of the cavity in order to suppress the occurrence of burrs.
In the present invention, an application of this can be used. In other words, the axial spacing of the molding cavities formed when the pair of split molds are completely closed is smaller than the desired axial dimension of the plastic substrate of the final molded product, and a sliding member is provided on the outer periphery of the cavity. A mold assembly can be used to carry out the method of the invention.
The axial spacing dimension when the above split molds are completely closed is 1/10 of the desired axial dimension of the final molded product, as mentioned above.
~9/10.

The present invention will be explained below using the drawings.

FIG. 3 is a conceptual cross-sectional view of a mold assembly for carrying out the method of the present invention in an optical disk molding method. It has a split mold 2, and both split molds approach and move away from each other only in the axial direction by means of tie bars 3. The movable split mold 1 is driven by a hydraulic cylinder or the like (not shown). A stamper 5 is held by stamper holders 7 and 8 on the flat surface of the molding cavity 4 formed by the split molds 1 and 2. The stamper 5 can be attached to one or both of the split molds. The molten resin is injected into the molding cavity from an injection cylinder (not shown) through the nozzle touch part 9, and a hole is made in the center of the molded product by relative movement between the split molds 1 and 2 and the center punch 10. .

In order to carry out the axial enlargement of the mold cavity, which is a feature of the method according to the invention, two axial sliding members 20, 30 are provided in the embodiment of FIG.
These sliding members 20 and 30 are connected to each chamber 31 formed in one of the split molds, in the figure, the fixed side split mold 2,
It is composed of a ring integrated with a piston that slides in the axial direction in the flow path 2.
It is moved in the axial direction by the fluid pressure supplied through 2 and 32.

In the method of the present invention, when the split molds 1 and 2 are completely closed, that is, at the start of the injection process, the axial distance t ₀ of the molding cavity 4 is the desired axial dimension of the final molded optical disk, ie, the thickness t. It is smaller than ₁ .

During operation, the dimension t ₀ defined at the start of the injection process is increased continuously or stepwise during the injection process or at the beginning of the holding process to the final desired dimension t ₁ . In some cases, in order to improve transferability, the axial dimension is once increased to t ₂ larger than t ₁ and then finally decreased again to t ₁ .
Generally, the injection process is performed in about 0.5 to 2 seconds, so the above-mentioned change in the axial distance dimension is achieved by using a computer to supply hydraulic pressure to the mold clamping mechanism (not shown) and the axial sliding member 30. Control.

The axial sliding member 20 prevents the occurrence of burrs that may occur due to an increase in the axial dimension of the molded cavity 4 during the injection process, as described in the aforementioned Japanese Patent Application No. 60-245678 filed by the present applicant. In order to do this, as the movable split mold 1 moves, it is urged in the axial direction by hydraulic pressure supplied to the chamber 21 so that its tip always comes into contact with the stamper surface.

Meanwhile, the outer axial sliding member 30 serves as an auxiliary cylinder for accurately controlling the axial spacing. That is, this auxiliary cylinder is used when the movable split mold 1 separates from the stationary split mold 2 due to the deenergization of the clamping cylinder, that is,
When the axial spacing of the molding cavities according to the present invention is increased, the movable split mold 1 is pushed out by the hydraulic pressure supplied through the flow path 32 .

The above-mentioned auxiliary cylinder 30 is merely an example, and other means capable of accurately controlling the interval between the two split molds 1 and 2 may be used, for example, by attaching a threaded rod connected to a rotational drive device to one of the split molds, and It is also possible to attach a nut member to the mold to be threaded therewith, and control the rotation of the threaded rod to adjust the spacing between the two split molds. Furthermore, the axial sliding member 30 can also be attached to the outer end surface of the split mold.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram of a model showing the behavior of molten resin in a molding cavity. Figure 2 shows the refractive index n _z ,
Diagram for explaining n _x and n _y . FIG. 3 is a conceptual cross-sectional view of a mold assembly for carrying out the method of the present invention. Symbols in the figure: 1, 2...split mold, 4...molding cavity, 20, 30...axial sliding member.

Claims (1)

[Claims] 1. A transparent flat disk-shaped optical disk substrate molded by injecting molten resin into a molding cavity formed by a pair of split molds for molding the optical disk substrate. In this injection molding method, the axial gap dimension of the molding cavity is reduced by separating one split mold from the other split mold during at least a part of the time from the start of injection to the holding pressure step. In a method comprising increasing the axial gap size from small to large and then decreasing the axial gap size again to the desired size, two sliding members 20, 30 are provided near the outer periphery of the molded cavity and are slidable in the axial direction; A method characterized in that the above operation is performed by independently driving these sliding members 20 and 30.