JPH0579008B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an injection molding method for transparent plastic substrates used in optical high-density information recording systems such as optical disks and optical cards, and particularly relates to a method for injection molding transparent plastic substrates used in optical high-density information recording systems such as optical disks and optical cards. The present invention relates to a method of molding a transparent plastic substrate that can be applied to 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 _z , n _y in the direction parallel to the substrate surface, and generally |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 there are three regions with different orientation directions in the thickness direction of the base. .

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 for providing a recording medium with a high CN ratio used in an optical high-density information recording system.

(Structure of the Invention) The injection molding method according to the present invention is characterized by molding a transparent plastic used in an optical high-density information recording/reproducing system, which is molded by injecting molten resin into a molding cavity formed by a pair of split molds. In the method for injection molding a substrate, at least one of the split molds is molded for at least a portion of the time from the time the molten resin starts flowing into the mold cavity until the time the molten resin substantially solidifies. The point is to cause relative displacement along the contact surface of the split mold.

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 simultaneously forming a magneto-optical recording film as described 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
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.

A feature of the present invention is that a flat transparent plastic substrate is molded by injecting molten resin into a flat molding cavity formed by a pair of split mold parts, and a flat transparent plastic substrate is molded through the molded transparent plastic substrate. Recording used by an optical high-density information recording and reproducing system, which is constructed by arranging an information layer recorded and/or reproduced by a laser beam incident on at least one side of the transparent plastic substrate. In a method for injection molding a transparent plastic substrate as a medium, the split mold is heated at least part of the time from when the molten resin begins to flow into the mold cavity until the molten resin substantially solidifies. The point is that at least one side is relatively displaced in the direction along the abutting surfaces of the bisegmented molds.

It is appropriately selected depending on the shape of the plastic substrate to be molded in the direction of relative displacement of the split molds. That is, in the case of a disc-shaped disk substrate, the fixed type and/or the movable type are rotated relative to each other. This rotation angle varies depending on the thickness of the disk substrate, but for a disk with a thickness of 1.2 mm, it ranges from about 0.1° to about 45°.
Preferably it is about 0.5° to about 10°. The rotation direction may be one direction, but it may also be rotated alternately in both directions. The timing of when to perform this rotation varies depending on the structure of the injection molding machine, injection conditions, resin used, and disk dimensions, but generally it is performed after injection and filling of the molten resin into the molding cavity is completed. preferable. Specifically, after the injection process is completed for about 0.5 to 2 seconds, the mold clamping force is rapidly released, and as the resin solidifies, the split molds are rotated relative to each other, and then the mold clamping force is increased again to transfer. It is preferable to improve the properties.

In the case of molding a substrate for a rectangular card-like optical card, the two split dies are displaced parallel to each other in one direction parallel to the card surface, or alternately displaced in both directions.

In either case, the purpose is to relax or disperse the orientation perpendicular to the surface shown in FIG.

The above relative displacement needs to be performed during at least a part of the pressure holding process. In fact, the injection process may be carried out immediately after the molten resin is completely filled into the molding cavity by the injection process for about 0.5 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, B in Figure 1
It is preferable to carry out this process while the area is still unsolidified.

In fact, in order to perform the above relative movement, it is necessary to rapidly reduce the mold clamping force that closes the split mold to a large extent, in some cases to nearly zero, after injection is completed. At the stage when the mold clamping force has decreased or become substantially zero, at least one of the split molds, preferably a portion of the split molds, is relatively displaced by a suitable drive mechanism. The drive mechanism may be mechanical, such as a rack-and-pinion assembly, or hydraulic. Suitable friction-reducing means, such as mechanical bearings, air bearings, etc., may also be used to facilitate the movement.

By using the above method of the present invention, some of the molding distortion and cooling distortion are alleviated, and the above-mentioned birefringence |
A disk substrate with small values of n _z −n _x | and |n _z −n _y | can be formed.

Hereinafter, an injection mold assembly for carrying out the method of the present invention will be briefly described using FIGS. 3 and 4.

FIG. 3 is a conceptual sectional view of a mold assembly when the method of the present invention is applied to injection molding of an optical disk substrate. In this figure, all mechanisms not directly related to the present invention are omitted. As is well known, the mold assembly for injection molding of optical disk substrates consists of a pair of split molds 1 and 2.
That is, it has a fixed side split mold 1 and a movable side split mold 2, and displacement of both split molds in directions other than the axial direction is regulated by tie bars 3.

Platens 4 and 5 having temperature adjusting grooves 6 are fixed to each of the split molds 1 and 2, respectively. The molten resin is injected from an injection cylinder (not shown) into the molding cavity 8 through the nozzle touch part 7,
After opening the center opening for the optical disk by relative movement between the punch 9 and the mold assembly, the mold is cooled and solidified, and then the mold is opened and taken out as a molded product. The surface of the molding cavity is defined by the platen surface or stamper 10. In the figure, the stamper 10 is placed on the fixed side split die 1 with the holder 1.
1, but it can of course also be attached to the movable split mold 2.

A feature of the mold assembly for carrying out the method of the present invention is the movable platen 20 and its driving means. The movable platen 20 is mounted on one side of the split mold, in the case of the figure, the platen 4 of the fixed side split mold 1, and a ball bearing 2
1 and 22 so as to be rotatable. The platen 4 is provided with a spiral groove for supplying fluid, for example, air, to the contact surface between the platen 4 and the movable platen 20 in order to overcome the rotational resistance caused by the friction of the contact surface in the radial direction between the platen 4 and the movable platen 20. 23 is formed, and compressed air is supplied to this spiral groove 23 via a line 24 during rotation. Note that, instead of the above-mentioned fluid bearing, a mechanical separation means similar to an ejector having a mechanical bearing such as a ball bearing at the tip may be used to overcome the above-mentioned resistance force. Naturally, these friction reducing means are energized only when the movable platen 20 rotates, and are normally inactive.

In order to rotate the movable platen 20, driving means using fluid pressure is used in the illustrated embodiment. This drive means is comprised of a cylinder-piston assembly as shown in FIG. That is, a locking portion 27 formed on the movable platen 20 is pushed by the free end of a piston rod 26 fixed to a piston sliding in a cylinder 25 fixed to the platen 4. The locking portions 27 are provided at both ends of the movable platen 20 in the diametrical direction and are housed in housing portions 28 formed in the platen 4. The housing portion 28 and the locking portion 27
A return spring 29 for returning the movable platen 20 to the initial position when the cylinder 25 is deenergized is housed between the cylinder 25 and the cylinder 25. The circumferential length of the accommodating portion 28 is determined by the amount of rotation of the movable platen 20.

In order to hold the movable platen 20 rotatably relative to the platen 4, a certain gap is required in the radial direction between the two. This gap makes it difficult to center signal surfaces such as tracks on stamper 10 with respect to the mold assembly. That is, there is a drawback that the stamper 10 moves in the radial direction with each shot. To overcome this drawback, a centering mechanism is provided in the illustrated embodiment. This centering mechanism is composed of a centering ring 30 having a tapered portion at its tip, and an annular groove 31 having an opposing tapered surface corresponding to the taper formed on the surface of the movable platen 20. The centering ring 30 is retracted when the movable platen 20 is not in operation, that is, when the movable platen 20 is rotating, and moves forward to center and fix the movable platen 20 when the movable platen 20 is not rotating.

In use, after the molten resin is filled into the cavity 8, preferably immediately after the filling is completed, the clamping force that closes the split molds 1 and 2 is rapidly reduced, preferably to near zero, to reduce the above-mentioned friction. After the reducing means is energized, the driving means 25 is energized to rotate the movable platen 20. Thereafter, the mold clamping force is increased again to maintain the pressure holding process for a certain period of time, and after the resin has solidified, the molded disk substrate is opened and taken out.

It is clear that the above specific examples are merely illustrative and that various changes can be made without departing from the spirit of the invention. For example, in the above specific example, the platen 4 and the movable platen 20 are separate bodies,
It is also possible to rotate the platen 4 itself. Alternatively, instead of the centering ring 30 described above, a plurality of centering rods distributed at equal intervals in the circumferential direction may be used, and these centering rods may be simultaneously driven by a drive ring provided outside the mold assembly. can. Further, the drive means for the movable platen is composed of an arcuate rack formed on the outer periphery of the locking portion 27 and a worm that passes through the platen and is screwed into the platen, and the worm is rotated in forward and reverse directions by a motor. The movable platen may also be rotated.

[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 cross-sectional view of a mold assembly for carrying out the method of the present invention, and is a cross-sectional view taken along the line - in FIG. FIG. 4 is a cross-sectional view taken along the - line in FIG. 3. Code in the figure: 20...Movable platen, 21, 22
... Bearing, 23 ... Air bearing groove, 25 ...
...Cylinder, 26...Piston rod, 27...
...Locking portion, 28... Accommodating portion, 30... Centering ring, 31... Annular groove.

Claims (1)

[Scope of Claims] 1. A transparent optical disk substrate molded by injecting molten resin into a flat disc-shaped cavity formed by a pair of split molds for molding the optical disk substrate. In an injection molding method, at least one of the split molds is brought into contact with the split molds for at least a portion of the time from the time when the molten resin starts flowing into the mold cavity until the time when the molten resin substantially solidifies. An injection molding method characterized by relative displacement along a surface by an angle of 45° or less.