Method of Manufacturing an Optical Recording/Reproducing Disc

This invention relates to a method of manufacturing an optical recording/reproducing disc, and more particularly an optical recording/reproducing disc which is difficult to injure, and which is provided at the outer peripheral surface with a disc protecting layer to thereby protect a region for recording/reproducing signals.

Conventionaly, an optical recording disc has been formed by a round plastic plate having a recording layer capable of optically recording/reproducing and thereafter another plastic plate placed over the recording layer for protecting it. The protective plastic plate for the recording layer, however, has a problem in that the surface of the recording disc, when placed on a flat surface, is injured by contact with dust or foreign objects during manufacture. This has been a problem also with an optical recording/reproducing disc using a laser beam. In other words, a problem has been created in that the flaws on the protective plate will appear as noise during recording and/or reproducing of signals.

An optical recording/reproducing disc according to the invention comprises a round plastic plast, a recording layer provided on the plastic plate and enabling optical recording and/or reproducing, and a round protective layer of plastic material provided on the recording layer and having at the outer peripheral end a projection higher by 10 to 200.mu. than the inner peripheral surface at the effective recording/reproducing surface, thereby to protect the recording layer, so that when the disc is placed on a flat surface the projection of the protective layer forms a space between the recording layer and the flat surface, thereby preventing flaws from being produced by contact with dust or a foreign object.

FIG. 1 is a sectional side view of an embodiment of an optical recording/reproducing disc manufactured according to the method of the invention, and

FIGS. 2, 3 and 4 are partially sectional views of outer peripheral end portions of the embodiment of FIG. 1 and of modified embodiments manufactured according to the method of the invention.

The optical recording/reproducing disc base member of the invention, as shown in FIG. 1, mainly comprises three layers. Namely, a recording layer 2 necessary for optical recording and/or reproducing is interposed between two plastic plates 1 of acrylic resin each having a center bore and a diameter of 210 mm. In further detail, the recording layer 2 utilizes the principle that irradiation by a laser beam causes a phase transition to change the reflectivity of the layer.

FIGS. 2 and 3 are partially enlarged sectional views of a portion designated by A in FIG. 1, and each shows the disc in FIG. 1 coated with a protective layer 3. The disc in FIG. 2 is chamfered as at 4 at the corners of the outer peripheral end portion of disc, and the disc in FIG. 3 is rounded-chamfered at the same portion. A projection 6 is provided at the outermost periphery of each disc. In FIG. 4, the projection 6 of the protective layer 3 provided at the outermost peripheral end of the disc comprising the plastic plates 1 and recording layer 2 preferably has a height m as large as possible. However, when height m becomes too large, the area of projection 6 and width l thereof are increased, resulting in a poor appearance. Thus, the projection 6 preferably has a height of about 200.mu. or less from the viewpoint of the positional relation with the laser head. However, a height of 10.mu. or less has not been effective in prevention of flaws caused by dust or the like. The embodiment in FIG. 4 is obtained by a coating operation wherein a coating liquid is filled in a dip tank, and the coating material is applied simultaneously to both surfaces of the resin disc by rotating the disc by a rotary shaft, thereby simultaneously forming the protective layer 3 and projection 6.

The coating apparatus and method used for this embodiment provide for a constant number of rotations during dipping: 50 rpm, during spinning: 2000 rpm, and during irradiation by ultraviolet rays: 750 rpm.

In addition, the coating material comprises 100 parts by weight of a composition consisting of 40 parts by weight of dipentaerythritol-hexacrylate, 10 parts by weight of epoxyacrylate, 60 parts by weight of trimethylol-propanetriacrylate, and 10 parts by weight of hydroxyethylmethacrylate, and added with 3 parts by weight of benzoinisobutyl ether, which composition is mixed with isopropyl alcohol and methyl isobutyl ketone to form a solvent of viscosity of 16 cps (at 25.degree. C.).

After experiments under the above conditions and with an angle .alpha. of chamfer and radius R of roundness of chamfer, the results in Tables 1 and 2 were obtained.

When the angle .alpha. was larger than 60.degree., the width l of the thick portion of film increased, and in a case of no chamfering, a width l of over 1 cm appeared. Hence, the angle .alpha. of chamfer preferably is 60.degree. or less, further preferably 5.degree. to 45.degree.. The angle of 5.degree., not to say absolutely from the relation to the recording region, should be defined by a chamfer surface not reaching the recording layer 2. Also, in FIG. 2, the chamfered portion may be round.

In FIG. 3, the end portion of disc is chamfered semicircularly, in which case radius R is 1/2 to 4/5, preferably 1/2 to 3/5, as large as the disc thickness.

In FIG. 4, the angle of chamfer is changed from 10.degree. to 80.degree. and the coating solvent viscosity from 5 to 30 cps (by B type viscometer at 20.degree. C.), so that the disc was obtained which of the desired dimensions shown in Table 3.

The effect of preventing flaws was discriminated in such a manner that sand of particle size of 300.mu. or less was scattered on a base on which the disc was reciprocated 10 times under its weight, and resultant flaws formed on the disc were inspected by a depth gage and visual observation. The discrimination or classification of the flaws is as follows (A, B and C practicable):

A: non flaw,

B: several slight flaws (not affecting signals),

C: several deep flaws (affecting signals according to the location of flaw),

D: innumerable slight flaws,

E: innumerable deep flaws, and

F: other defects (such as appearance).

From the above results, a projection of 10 to 200.mu. in height is provided at the outermost periphery of the disc, thereby making it possible to provide a practical disc having recording and reproducing regions which are difficult to injure.

As seen from the above, the optical recording/reproducing disc of the invention, even when placed on the flat plane, is difficult to injure, due to the projection formed in the protective layer at the outermost peripheral end of the disc, thereby being not only convenient to operate but also available for storing signals.