JPH03162920A - Injection molding mold assembly for high density information recording and storing plastic disc - Google Patents

Abstract

PURPOSE:To enable injection molding of a high density information recording and storing plastic disc which has the uniform wall thickness distribution in the radial direction and also has excellent configurational stability by enlarging the axially directional clearance dimension of an annular molding cavity successively as being separated from the injection gate to the radial direction. CONSTITUTION:The axially directional clearance dimension of an annular molding cavity 3 is enlarged successively as being separated from the injection gate 5 to the radial direction. Namely, the cavity formed by being defined by split molds 1, 2 are the same to the conventional ones; however, the axial wall thickness of a stamper 4 is tapered so that it becomes thin as separated from the gate 5; in other words, it is separated from the virtually central surface 6 of the molding cavity. The dimension of the cavity formed by being defined in this mold assembly is thin in its central part and thick in its peripheral part Thereby a high density information recording and storing plastic disc can be injection-molded which has the uniform wall thickness distribution in the radial direction and also has excellent configurational stability.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to improvements in a method for molding substrates for high-density information recording carriers such as video disks and optical disks, particularly plastic substrates. The present invention relates to an improved mold assembly for use in injection molding disks.

(Prior art) In general, plastic substrates used for video discs, optical discs, etc. are extremely thin and flat, with a thickness of about 1.2 to 1.5 am and a maximum diameter of about 300 mm, and the material has poor fluidity. Even today, it is difficult to mold completely satisfactory plastic substrates, since acrylic and polycarbonate resins are used. Among the various performances required for this type of disk base, the dimensional stability and shape stability of the molded product are important, and the thickness of the disk must be, for example, 1.2 ± 0.1 mm. , the molded product shall not have any warpage or twisting. Such ultra-precise plastic discs are difficult to mold even with the best precision injection molding machines developed today.

The biggest reason for this is that when the molten resin is uniformly injected into the extremely thin and large diameter molding cavity as mentioned above,
There is a difference in resin assimilation speed and filling pressure between the vicinity of the gate and the location far from the gate, resulting in uneven wall thickness of the molded product and residual strain that may cause the molded product to warp or twist. be.

To eliminate these drawbacks, PMMA or PC with a low molecular weight and good fluidity can be used as the material resin, but in order to improve the durability, heat resistance, and shape stability of the disc, it is necessary to use a high molecular weight resin with poor fluidity. It is necessary to use PMMA or PC, and if such a high molecular weight material is used, not only the dimensional stability and shape stability described above will deteriorate, but also the transferability from the stamper will deteriorate.

A method for manufacturing a substrate for a high-density information recording carrier, which is the object of the present invention, by injection molding is described, for example, in Japanese Patent Application Laid-open No. 56-139940 and Japanese Patent Application Laid-open No. 57-203517. The molding cavity of the mold assembly used in this injection molding method is made to have substantially the same dimensions as the molded article, taking into account the desired shrinkage rate of the molded article. However, the wall thickness of the plastic disc actually made using this mold is not the same as the dimensions of the molding cavity, and the center part is relatively thicker than the outer periphery, and there are sink marks. Therefore, the shape stability of the molded product is also poor.

<Problems to be Solved by the Invention> An object of the present invention is to provide a metal that enables injection molding of a plastic disk for a high-density information recording carrier having a uniform wall thickness distribution in the radial direction and excellent shape stability. The purpose of the present invention is to provide a mold assembly.

(Means for Solving the Problems) A mold assembly provided by the present invention includes a first split mold, a second split mold, and a stamper supported by either one of these two split molds. A flat annular molding cavity is defined by the stamper and the surface of one of the two mold parts, and an injection gate provided in at least one of the two mold parts is inserted into the annular mold cavity. A mold assembly used for injection molding a plastic disk for a high-density information storage carrier by injecting molten resin, the axial gap dimension of the annular molding cavity increasing radially away from the injection gate. It is characterized by a continuous increase in size.

In the mold assembly of one embodiment of the present invention, the first split mold and the second split mold are arranged such that the axial gap dimension of the annular molding cavity continuously increases as the distance from the injection gate in the radial direction increases. Either or both of the mutually opposing surfaces of the stamper are inclined with respect to an imaginary central plane of the annular molding cavity, so that the axial wall thickness of the stamper is substantially uniform in the radial direction.

In the mold string solid body according to another embodiment of the present invention, the axial wall thickness of the stamper is made continuous such that the axial gap dimension of the annular molding cavity continuously increases as it moves away from the injection gate in the radial direction. The surfaces of the first split mold and the second split mold are made parallel to each other.

The injection gate is preferably provided near the center of the annular molding cavity, but it can also be a flush gate near the radial outer periphery of the annular molding cavity.

(Function) The present invention continuously increases the axial dimension of the molding cavity as it radially moves away from the gate through which molten resin is injected into the molding cavity, that is, the axial dimension of the molding cavity has an enlarged taper. This is based on the experimental fact that the above objective can be achieved by adding .

Variations in the axial gap size distribution of the molding maneuverability according to the present invention can be achieved by varying the radial thickness distribution of the stamper.

More generally, this can be done by one of the following methods: (1) The axially opposing surfaces that define the molding cavity are parallel to the virtual center plane of the molding cavity, and the stamper is (2) A method in which the thickness of the stamper is made uniform and a slope is applied toward the axially opposing surface of the molding cavity; (3) A combination of (1) and (2) above. method.

The injection direction of the molten resin may be radially outward or inward of the molding cavity, but generally the molten resin is injected into the center of the flat molding cavity and expanded radially outward. Regarding this type of injection molding machine for plastic disks for high-density information recording carriers,
No. 9940 and Japanese Patent Application Laid-open No. 58-115900 by the present applicant.
Please refer to the issue. On the contrary, the present invention can also be applied to a shape in which the molding material is injected radially inward from the outer periphery of a flat molding cavity. In this case, please refer to Japanese Patent Application No. 126183/1983 filed by the present applicant.

The surface of a stamper used for molding a plastic disk for a high-density information recording carrier, which is the object of the present invention, generally has submicron-order irregularities for control signals or tracking.

The maximum value of the axial wall thickness of the molding cavity according to the present invention (
The difference (d) between D1) and the minimum value (D2) may be 100μ or less when the radius (L) of the plastic disk to be molded is 1500mm and the thickness is 1.2mm, and preferably this d=10~ 50μ.The value of d generally decreases as the radius of the plastic disk decreases.

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

First, an outline of a conventionally known mold assembly for molding a plastic disk for a high-density information recording carrier will be explained using FIG. Molten resin such as PMMA, PCSPVC, etc. is injected from an injection molding machine (not shown) into a molding cavity 3 defined between split molds 1 and 2 through a central gate 5.

Generally, a stamper 4 having concavo-convex signal information is fixed on an axially opposing surface of the molding cavity 3 by a fixture (not shown). The dimensions of the molding cavity 3 are, for example, an axial thickness of 1.2 mm and a diameter of 30 On++.
n, and is extremely thin with L (radius)/D (thickness) = 125. This radius is made to be substantially the same as the dimensions of the disk, which is the most imposing item.

However, in reality, the dimensions of the plastic disk molded in the molding cavity of this conventional method are not the same as the dimensions of the molding cavity, and the part near the center gate 5 is thicker, and the part of the cavity farther from the center gate 5 is thicker. The outer periphery becomes thinner. The reason for this is that the L/D of the molding cavity 3 is 125, which is extremely flat and thin, resulting in a difference in the solidification speed of the injected molten resin between the center and the outer periphery of the cavity. This is because solidification has already begun, and the molten resin supplied in the final injection process fills only the vicinity of the gate.

In order to eliminate this drawback, a radial temperature distribution of the mold is provided, but this alone is not sufficient.

In order to eliminate this drawback of the conventional product, the present invention increases the axial dimension of the molding cavity as it moves away from the gate. Embodiments of the present invention will be described below with reference to FIGS. 2 to 4. 2 and 3 are conceptual diagrams showing the principle of the present invention, and the relative dimensions of each member are exaggerated in order to clearly show the effects of the present invention, and those that are not related to the essence of the present invention are Not shown. For detailed mold structures of plastic disks for high-density information recording carriers, please refer to the above-mentioned Japanese Patent Application Laid-Open Nos. 56-139940 and 1982-115900.
Please refer to the issue.

Fig. 2 is a conceptual diagram of a mold assembly showing an embodiment of the present invention. The -4° axial wall thickness is tapered such that it becomes thinner as it moves away from the gate 5, in other words, away from the virtual central plane 6 of the molding cavity. As shown in FIG. 3, the molding cavity 3' defined by this mold assembly is thin at the center and thick at the periphery. The axial thickness D2 (minimum value) of the molding cavity near the center gate 5 and the axial thickness D2 near the outer periphery. (maximum value) difference d
(1D+D2) varies depending on the injection molding conditions, but as an example, L/D. =0.008 then d
<100μ, preferably d-10 to 50μ. In short, the thickness distribution in the radial direction of the final shaped product is selected to be uniform under given injection molding conditions.

The above-mentioned tapered stub bar 4'' can be manufactured by the method described in Japanese Patent Application No. 121690/1982 filed on July 6, 1980 by the present applicant.

In FIG. 2, the stamper is provided only on the cavity forming surface of the split mold 2, but it can also be provided on the split mold 1, or on both the split molds 1 and 2, respectively.

FIG. 4 shows another embodiment of the present invention, in which the axial gap dimension variation of the molding cavity 3'' is provided by the cavity-shaped face of the split mold itself. The thickness of the stamper 4'' is uniform, and the molding cavity forming surface of each split mold 1, 2 has a center gate 5''.
It tapers away from the imaginary center plane 6 of the molding cavity as it moves away from the molding cavity. In Fig. 4, both split molds 1 and 2 are tapered, but it is also possible to tape only one of them, and the stamper 4
” can also be attached to the split mold 1.

In the embodiments described above, the present invention is applied to injection molding using a center supply method in which molten resin is provided with an injection gate 5 at the center of the molding cavity. The present invention can also be applied to peripheral supply systems. Regarding this type of injection mold, please refer to Japanese Patent Application No. 126183/1983 filed by the present applicant.

In the case of this peripheral supply method, the taper is naturally applied in the opposite direction.

Furthermore, although the embodiment shown in FIGS. 2 and 4 uses a stamper, the present invention can also be applied to the case where a flat disk is molded only using the molding cavity forming surface of the split mold 12 without using a stamper. can be applied. In that case, molding may be carried out using a mold l12 shown in FIG. 4 with the stamper 4'' removed.

Next, examples of the present invention will be shown.

Example: Patent application No. 121, 1982, dated July 6, 1982, filed by the present applicant.
, 690 (title of the invention "Method and apparatus for electroforming a stamper for manufacturing a high-density information recording carrier"),
A stamper with the radial thickness distribution shown in the lower part of the figure was manufactured. This stamper was manufactured so that the difference between the wall thickness near the center and the wall thickness near the periphery was 30 μm.

This stamper was set in a mold, and injection molding was carried out using an injection molding machine (DM-200 manufactured by Meiki Manufacturing Co., Ltd.) by supplying methylmethacrylate resin (MFR: 2 g/10 mm) from the center. In this case, the injection cylinder temperature was 300°C and the mold temperature was 55°C.

As shown in the upper part of FIG. 4, the thickness distribution in the radial direction of the obtained plastic disk had a difference of only 15 μm between the thickness near the center and the thickness near the outer periphery. This level of thickness is a value that is sufficiently practical as a disk for high-density information recording carrier. Furthermore, the sink mark on the outer periphery of the disk made with this stamper was also much smaller than that of the comparative example.

(Comparative Example) A stamper having the radial thickness distribution shown in the lower part of FIG. 5 was manufactured by the same method as in the above-mentioned example of the present invention. That is, this stamper has a wall thickness difference of only 8 μm between the center portion and the outer peripheral portion, which is close to conventionally used stampers of uniform thickness.

The thickness distribution in the radial direction of a disk molded using this stamper under the same molding conditions as in the example of the present invention is as shown in the upper part of FIG.
It was big. In addition, the sink mark on the outer periphery of the disk 15 was also much larger than that of the present invention.

As is clear from the above description, it is clear that a molded article of uniform thickness can be obtained by providing the stamper with an axial thickness distribution that decreases away from the gate.

It will be understood that the invention is not limited to the specific embodiments described above, but can be modified in various ways.

[Brief explanation of the drawing]

FIG. 1 is a conceptual sectional view of a conventionally known mold. FIG. 2 is a conceptual sectional view showing one embodiment of the present invention.
Components corresponding to the figures are given the same reference numerals as in FIG. 1. FIG. 3 is an explanatory diagram showing the shape of the molding cavity in FIG. 2. FIG. 4 is a conceptual cross-sectional view showing another embodiment of the present invention. FIG. 5 is a diagram showing the axial wall thickness (μ) of the stamper and the l 6 molded product according to an embodiment of the present invention versus the distance (mm) from the center. FIG. 6 is a diagram similar to FIG. 4 of a comparative example showing the axial wall thickness distribution of a stamper and a molded product according to the conventional method. (Symbols in the figure) 1, 2: Split mold, 3: Molding cavity 4: Stamper, 5: Injection gate, 6: Virtual central plane

Claims (5)

[Claims] (1) Consisting of a first split mold, a second split mold, and a stamper supported by one of these two split molds, the surface of the stamper and one of the two split molds is A flat annular molding cavity is defined, and a molten resin is injected into this annular molding cavity through an injection gate provided in at least one of the two split molds to inject a plastic disk for a high-density information recording carrier. In the mold assembly used for molding, the axial gap dimension of the annular molding cavity (3) (
A mold assembly characterized in that D) continuously increases in size as it moves away from the injection gate (5) in the radial direction. (2) The first split mold (1) and the second split mold (2) are arranged such that the axial gap dimension of the annular molding cavity increases continuously as it moves away from the injection gate in the radial direction.
one or both of the mutually opposing surfaces of the stamper (4) are inclined with respect to the imaginary central plane (6) of the annular molding cavity, and the axial wall thickness of the stamper (4) is substantially uniform in the radial direction. A mold assembly as claimed in claim 1. (3) The thickness of the stamper (4) in the axial direction is continuously thinned so that the axial gap dimension of the annular molding cavity increases continuously as it moves away from the injection gate in the radial direction; 2. A mold assembly according to claim 1, wherein opposing surfaces of the split mold and the second split mold are parallel to each other. (4) The mold assembly according to any one of claims 1 to 3, wherein the injection gate is located near the center of the annular molding cavity. (5) The mold assembly according to any one of claims 1 to 3, wherein the injection gate is a flush gate near the radial outer periphery of the annular molding cavity.