JPH0579011B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to recording media such as optical disks, optical cards, and optical tapes, and methods for manufacturing the same, and in particular to transparent plastic substrates that support information layers. This invention relates to improvements in the molding method. The present invention particularly relates to an injection molding method for transparent plastic substrates 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, 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 780.
A retardation of mm 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 C, respectively. Although it is unclear which region affects the three principal refractive indices n _Z , n _x , and n _y mentioned above, it is thought that there are three regions with different orientation directions in the thickness direction of the substrate. It will be done.

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 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 a method for manufacturing a transparent plastic resin substrate molded by injection molding for a recording medium with a high C/N ratio used in an optical high-density information recording system. There is a particular thing.

(Structure of the Invention) The first feature of the recording medium provided by the present invention is that it uses 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 for transparent plastic substrates used, the resin is injected into a flat disk-shaped molding cavity formed by a pair of split molds, and the resin is molded while being subjected to ultrasonic waves.

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 (applicable not only to the so-called back reading method, but also to the so-called front reading method. In that case, the information is carried on a suitable support and the laser The beam is incident through a transparent plastic substrate according to the invention which is placed above this information.In both systems, 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} | was lowered to 0.0004 or less was improved to 51 dB. in this way
The reason for the improvement in the CN ratio is considered to be the increase in θ _k and the decrease in the nozzle 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 operation of the present invention in the pressure holding step described below. In the case of polycarbonate resin, the injection cylinder temperature is generally 300-400℃, the mold temperature is about 100℃, and the flow rate of resin into the cavity is 10-500ml/sec, which of course varies depending on the disk size. , other conditions are selected for other types. The injection conditions for optical disk substrates using polycarbonate resin are described in the above-mentioned Japanese Patent Application Laid-Open No. 1989-1999 by the present applicant.
Please refer to No. 155424.

A feature of the injection molding direction of the present invention is that molding is performed while applying ultrasonic vibration to the resin in the molding cavity. Generally, when ultrasonic vibrations are applied to a polymer substance in a molten state, that is, at a temperature above the glass transition point (Tg), molecular movements such as sliding diffusion of molecular chains, segmental movement, and dispersion of side chains are induced. The activation energy required for these molecular relaxations is generally 1~
The energy is 30 Kcal/mol, and if this energy can be applied by ultrasonic vibration, it is possible to relax the orientation of molecules that occur when they flow into the flat cavity at ultrahigh speed during injection. In the present invention, the orientation of the resin in the direction perpendicular to the surface shown in FIG. 1 is relaxed or dispersed by applying ultrasonic waves to the resin in the molded cavity. 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 −
It becomes possible to reduce n _x | and |n _z −n _y | to 4×10 ⁻⁴ or less.

The above-mentioned ultrasonic vibration needs to be performed from the time when the resin starts flowing into the molding cavity until the end of the pressure holding process. In fact, it may be carried out immediately after the molten resin is completely filled into the molding cavity by the injection process for 1 to 2 seconds and before the mold opening process starts. 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, it is preferable to carry out the process when the area B in FIG. 1 is at the final solidification stage. The frequency and power of the ultrasonic vibration can be appropriately selected by those skilled in the art depending on the dimensions of the molded product, the type of resin, and the injection conditions.

Although ultrasonic vibrations according to the present invention can be applied to the entire mold assembly, it is generally preferred to apply them to a portion of the mold assembly. Further, in some cases, the vibrator of the ultrasonic oscillator may be directly inserted into the resin filled in the molded cavity. The direction of vibration may be any direction as long as it relaxes or disperses the orientation of the molecules, but it is generally preferable to apply the vibration in the direction along the flat surface. Preferably, this vibration is a reciprocating vibration or a torsional vibration. These vibrations can be generated using a single or multiple vibrators.

Hereinafter, the structure of a mold assembly for carrying out the method of the present invention will be explained using FIGS. 3 and 4.

Figures 3 and 4 are conceptual longitudinal and cross-sectional views of an injection mold assembly to which the method of the present invention is applied;
Items not directly related to the present invention are omitted.

The above mold assembly consists of a pair of split molds that approach and separate from each other via tie bars 3, that is, a fixed side split mold 1.
and a movable split mold 2, each split mold has platens 4 and 5.
A temperature control flow path 6 is formed in each platen.

In the illustrated embodiment, a movable platen 7 is rotatably held on one of the platens, in the case of the figure a stationary platen 4 via bearings 8 and 9 provided on the inner and outer peripheries. A stamper 10 having information pits or guide tracks for optical discs is held by stamper holders 11, 12 on at least one of the platen or movable platen. A molding cavity 13 is defined between the stamper 10 and the platen 5.

The molten resin flows from an injection cylinder (not shown) into the molding cavity 13 through the nozzle touch part 14, and a hole is formed in the center of the molded disk by relative movement between the mold assembly and the center punch 15. be done.

Ultrasonic vibration, which is a feature of the present invention, can be applied to one split mold,
In the illustrated embodiment, the ultrasonic waves are applied to the movable platen 7 by ultrasonic oscillators 20, 30 attached to the stationary split mold 1. The ultrasonic oscillators 20, 30 are held on the split mold 1 by fixtures 22, 33. Vibration horn 2 fixed to ultrasonic oscillators 20, 30
1 and 31 are in contact with protrusions 23 and 33 protruding from both ends of the movable platen 7 in the diametrical direction. The protrusions 23, 33 are urged in the direction of the vibrating horn by springs 24, 34 housed in recesses formed in the platen 4. As can be seen from Figure 4,
In this embodiment, vibration is applied tangentially to both diametrical ends, so that the movable platen torsionally vibrates.

The mold assembly is further provided with means for centering the movable platen 7. This means includes a tapered annular groove 40 formed in the movable platen, a cylindrical sleeve 41 having opposing tapered surfaces that can be inserted into the groove 40, and fixed to the circular sleeve and arranged at equal intervals in the circumferential direction. It consists of a plurality of connecting rods 42 and a ring 43 that integrates the connecting rods 42 with each other outside the mold. This ring 43 is driven by drive means, not shown, and is pushed into the mold if centering is required.

In operation, the ultrasonic generators 20, 30 are activated immediately after the resin is injected into the mold cavity 13 , and are deactivated when the resin has substantially solidified. The centering cylindrical sleeve 41 is the ultrasonic oscillator 2
During the operation of 0 and 30, it is moved backward, and after the oscillation stops, it is moved forward to perform centering. All these operations can be performed using computer control.

The above embodiments are merely illustrative, and the present invention is not limited thereto, and various modifications are possible. For example, the ultrasonic vibration may be applied parallel or perpendicular to the flat surface of the molded cavity instead of torsional vibration. When applying vibration in a direction perpendicular to the flat plane, an ultrasonic oscillator can be attached directly to the centering means, and the same member used for vibration and centering can be used. Additionally, means for reducing friction, such as air bearings, may be provided between the platen and the movable platen. Also,
Of course, the movable platen, the ultrasonic oscillator, and the centering means can also be provided on the movable side mold.

[Brief explanation of drawings]

Figure 1 is a conceptual diagram of a model showing the behavior of molten resin in the molding cavity, and Figure 2 is a conceptual diagram of the model showing the behavior _of molten resin in the molded cavity.
_FIG . ₃ is a conceptual vertical cross-sectional view of a mold assembly including the present invention, and FIG. 4 is a conceptual cross-sectional view taken along the - line in FIG. 3. Symbols in the figure, 7...Movable platen, 20, 30...
... Ultrasonic transmitter, 21, 31 ... Vibration horn, 2
3, 33... protrusion, 40, 41, 42, 43...
Centering means.

Claims (1)

[Claims] 1. A transparent flat disc-shaped light molded by injecting molten resin into a flat disc-shaped molding cavity formed by a pair of split molds for molding an optical disk substrate. In an injection molding method for a disk substrate, an ultrasonic transmitter that vibrates tangentially is brought into contact with at least one split mold, and the resin in the molding cavity is injected into the flat disk-shaped molding cavity before the resin is substantially solidified. A method characterized by applying ultrasonic waves that vibrate in an oblique plane.