JPH0579009B2 - - 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 is related to the improvement of. The present invention particularly relates to a transparent plastic substrate applicable to magneto-optical recording media and a method for manufacturing the same.

(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
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 _z , n _x , and n _y are influenced, 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 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, 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, at least part of the time from the time when the molten resin starts flowing into the molding cavity for molding the transparent plastic substrate until the time when the molten resin substantially solidifies,
The point is that the volume of a portion of the molded cavity is changed. This volume change can be done mechanically or hydraulically.

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} | was lowered to 0.0004 or less was 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 mm.
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 ordinary disc molding, except for the special operation of the present invention in the holding pressure 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 portion of the molding surface constituting the molding cavity, which is a feature of the present invention, can generally be constructed by a movable member movably held by the split mold. This moving member is arranged in a portion other than the recording area of the optical recording medium, generally in the periphery of the mold cavity. The moving direction of this moving member is not particularly limited, but in consideration of the structure and strength of the mold, it is generally preferable to move the moving member in a direction perpendicular or oblique to the flat surface of the molded product. This moving member is reciprocated by a drive body provided outside the mold via an ejector rod.

In the case of a flat disc-shaped molding cavity for molding a substrate of an optical disk, the moving member can be arranged at the inner and/or outer periphery of the annular molding cavity. The moving member can be constituted by a ring, which is housed in a groove formed in the mold and connected to a plurality of ejectors so that a portion thereof can protrude into the mold cavity. The rings on the inner and outer peripheries of the mold cavity can be driven independently or in conjunction with each other. For example, it is possible to first inject one of the inner and outer rings with the ring retracted and the other protruded, and after filling the mold cavity, the one ring can be advanced and the other retracted. Note that a gate between the sprue and the molded cavity can also be defined by the tip of the ring on the inner circumference.

The volume change of the molded cavity, which is a feature of the present invention, can also be performed by fluid pressure. In this case, a fluid supply path is provided in the outer ring or the stamper holder, and after the resin is filled into the molded cavity, pressurized fluid is supplied from the fluid supply path to the outer periphery of the resin in a radially inward direction. Add force.

It is preferable to provide a heat resistant filter or a heat resistant bladder at the open end of the fluid supply path.

The above-mentioned moving member that mechanically changes the volume and the above-mentioned pressurizing means using fluid pressure can also be used in combination. Which method is used may be selected as appropriate, and in either case, 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 volume change described above be carried out at least in part from the start of the injection process to 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 while the area B in FIG. 1 is still unsolidified. By using the above method of the present invention, a part of molding strain and cooling strain is alleviated, and a disk substrate with small values of birefringence |n _z −n _x | and |n _z −n _y | is molded. can.

Hereinafter, the present invention will be explained using the drawings.

A mold assembly for molding an optical disk substrate according to an embodiment of the present invention shown in FIG. 3 includes a pair of split molds 1 and 2;
A tie rod 3 connects these, and platens 4 and 5 each having a temperature control path 6 fixed to each split mold.
and a stamper 9 held on the platen by stamper holders 10 and 11.
Molten resin is injected from an injection cylinder (not shown) through a nozzle touch 7 into a molding cavity 8. The center hole of the disk is formed by relatively moving the split molds 1 and 2 and the punch 12, and after the resin is cooled and solidified, the molds 1 and 2 are opened and the molded product is taken out.

In order to change the volume of the molding cavity, which is a feature of the present invention, in the illustrated embodiment, the inner and outer annular grooves 13 and 14 are formed in the inner and outer peripheral parts of the platen 5 of the movable split mold 2. A ring 15 and an outer ring 16 are slidably housed.
The width of these rings, that is, the radial dimension corresponds to the amount of movement of the molten resin, and is actually about 0.1 to 5 mm.
Preferably it is about 0.1 to 2 mm. These rings 15, 16 are fixed to ejector rods 17, 18 passing through the split mold 2 via connecting screws 19, 20. A plurality of ejector rods 17, 18 are uniformly distributed in the circumferential direction, and their outer ends are fixed to drive rings 21, 22 outside the mold.
The drive rings 21 and 22 are driven at appropriate timing.

During operation, for example, resin is injected with the circumferential ring 17 protruding, and after the molding cavity is filled with the resin, the circumferential ring 17 is retracted and the outer ring 18 is protruded. The timing of these ejections varies depending on the type of resin and the dimensions of the mold, but it must be performed before the resin completely solidifies.

In a variant, the circumferential ring 17 could be left in the retracted position (or the circumferential ring 17 could be omitted) and the outer ring 18 could be pushed out after filling the mold cavity. Furthermore, both rings can be pushed out or retracted at the same time.

FIG. 4 shows a variant for carrying out the method according to the invention, in which the change in mold cavity volume is effected by means of a fluid. In this figure, a fluid supply path 31 is formed in the opposing ring 30 of the thrust-type stamper holder assembly 10, 30. It is connected to 32. A filter 33 made of porous ceramic or the like can be fixed to the opening of the fluid supply path 31. Instead of this filter 33, a heat-resistant elastic bladder or a mechanical expansion member may be used.

During operation, for example, resin is injected and filled into the cavity 8 with the circumferential ring 17 protruding, and immediately after that, pressurized fluid, such as air, is supplied via the fluid supply path 31 and the inner circumferential ring 17 is make them retreat.

The present invention is not limited to the above-mentioned specific examples, and various modifications are possible. For example, the mechanically movable member is constituted by a slidable inflatable ring incorporated in the opposing ring 30, and the inflatable ring is caused to protrude radially inwardly by the pressurized air. It's okay.

[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 longitudinal sectional view of an injection mold assembly used to carry out the method of the present invention. FIG. 4 is an enlarged partial sectional view of a pressurized fluid supply means used in another mold assembly for carrying out the method of the present invention. Symbols in the figure: 1, 2... split mold, 4, 5... platen, 8... molding cavity, 9... stamper, 10... stamper holder, 15, 16...
...Ring, 17, 18... Ejector, 21,
22...Driving ring.

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 the injection molding method of the invention, during at least a portion of the time between the time when the molten resin begins to flow into the mold cavity and the time when the molten resin substantially solidifies, the inside of the molding surface constituting the surface of the molding cavity is Characterized by changing the volume of only a portion of the inner periphery and/or outer periphery of the molding cavity by causing only a portion of the periphery and/or the outer periphery to protrude into or retreat from the molding cavity. injection molding method. 2. The method according to claim 1, wherein a portion of the inner circumference and/or outer circumference of the molding surface is constituted by a movable member movably held by at least one split mold. 3. Claim 2, in which the moving member moves in a direction perpendicular to the flat surface of the molded cavity.
The method described in section.