Optical disc having signal recording layer on each side and method for producing same.

This invention relates to a double-sided optical disc having a recording surface for information signals on each side thereof and, more particularly, to a double-sided optical disc composed of a pair of optical discs each having a signal recording surface on one side thereof and bonded to each other by an adhesive.

There has been proposed an optical disc in which data signals in the form of pits are formed by resin molding using a stamper and subsequently read out by an optical pickup.

The optical disc is constituted by a disc-shaped base material formed of a synthetic resin, such as polycarbonate, acrylic resin or epoxy resin, or glass, and a signal recording layer of, for example, a magnetic material, formed on the major surface of the disc shaped base material.

There has also been proposed a double-sided optical disc 102 comprising a pair of optical disc halves 101a, 101b bonded to each other as shown in Fig. 1 of the accompanying drawings to enable data signals to be read and/or written from or on both major surfaces of the disc. In this construction the rear sides of the optical disc halves 101a, 101b of the double-sided optical disc 102 are bonded together so that the front (signal recording) sides face outwards. The optical disc halves 101a, 101b are bonded together by applying an adhesive 103 of, for example, a rubber material, over the back sides thereof and pressing them together.

Meanwhile, when the adhesive 103 is applied to the optical disc halves 101a, 101b to produce the double-sided optical disc 102, the surface of the adhesive 103 tends to have irregularities. This is because the adhesive 103 is applied by roll coating in which the adhesive is applied to the outer periphery of a rotating applicator roll and by bringing the applicator roll into rolling contact with the optical discs 101a, 101b.

If the surface of the adhesive 103 has irregularities, the bonded optical disc halves 101a, 101b tend to deviate from optimum smoothness due to the surface irregularities of the adhesive 103. Unless the smoothness of the optical discs 101a, 101b is maintained, the signal recording layer is less flat and this renders it difficult to optimally write and/or read data signals on or from the signal recording layer.

JP-A-61-242355 and JP-A-61-267948 both disclose double-sided optical discs formed by gluing two halves together.

It is therefore an object of the present invention to provide an improved double-sided disc and method of producing it by applying an adhesive by so-called roll coating on a pair of optical disc halves and bonding the disc halves together under pressure.

In accordance with the present invention, there is provided a process for producing a double-sided optical disc comprising the steps of:    applying an adhesive to at least a pair of disc halves, and    bonding the two disc halves together; characterised by the step of:    rotating one of the said optical disc halves through a predetermined angle relative to the other optical disc half in a plane parallel to the surface of the optical disc half so as to counteract surface irregularities in the adhesive applied to said optical disc halves.

A further aspect of the invention provides a double-sided optical disc formed by bonding a pair of optical disc halves together by transferring adhesive from the outer peripheral surface of an adhesive coated rotating applicator roller by rolling contact of the optical disc halves therewith with said applicator roll extending in a rolling contact direction over the disc half, characterised in that:    the respective optical disc halves are bonded together a predetermined angle relative to each other that the respective rolling contact directions defined on the optical disc halves cross each other at said predetermined angle.

With the double sided optical disc formed by a pair of disc halves bonded together by an adhesive, since the disc halves are bonded together at a relative angle such that the direction in which the applicator roll had rolling contact with one of the disc halves intersects, i.e. is not substantially parallel to, that in which the application roll has rolling contact with the other disc half, it becomes possible to counterbalance surface irregularities on the adhesive layers applied to the disc halves to render the signal recording layer on the optical disc flat. The disc halves may be joined with an angle of 60° to 90° between the respective rolling contact directions, or in one embodiment substantially at right angles.

Thus in accordance with the invention the method can reliably produce a double-sided optical disc having superior flatness and can also provide a double-sided optical disc which exhibits flatness and satisfactory recording and/or reproducing characteristics for data signals.

The invention will be further described by way example with reference to the accompanying drawings, in which:-

Fig. 1 is a perspective view showing the construction of a double-side optical disc.

Fig. 2 is a perspective view showing the construction of a movable table for transporting a pair of optical disc halves for producing the double-side optical disc according to the present invention.

Fig. 3 is a side elevational view showing the construction of an adhesive applicator unit employed for producing the double-side optical disc.

Fig. 4 is a perspective view showing a pair of optical disc halves to which the adhesive has been applied by the adhesive applicator unit.

Fig. 5 is a partial perspective view illustrating the state of the adhesive applied to the optical disc.

Fig. 6 is a side elevational view, shown partially in cross-section, and showing a base block and a deforming/retaining device employed for producing the double-side optical disc.

Fig. 7 is a side elevational view, shown partially in cross-section, and showing the state in which the base block approaches the deforming/retaining device.

Fig. 8 is a plan view of a double-side optical disc constituted by bonding the disc halves together.

Fig. 9 is a side elevational view, shown partially in cross-section, and showing the construction of the rotating/retaining device and the cutting tools employed for producing the double-side optical disc.

Fig. 10A is a plan view showing the construction of the rotating/retaining device and the cutting tools.

Fig. 10B is a plan view showing the state of the rotating/retaining device and the cutting tools when the rotary cams for advancing the cutting tools have been rotated by 90 degrees.

Fig. 10C is a plan view similar to Fig. 10B when the rotary cams for advancing the cutting tools have been rotated further by 90 degrees from the state shown in Fig. 10B.

Fig. 10D is a plan view similar to Figs. 10B and 10C when the rotary cams for advancing the cutting tools have been rotated further by 90 degrees from the state shown in Fig. 10C.

By referring to the drawings, a preferred embodiment of the present invention will be explained in detail.

The double-side optical disc according to the present invention is prepared by bonding a pair of optical disc halves 101a, 101b as shown in Fig. 1 to each other so that data signals may be written on and/or read from both major surfaces of the disc halves. The optical disc halves 101a, 101b of the double-side optical disc 102 are bonded at the back surfaces thereof with the adhesive 103 so that the front surfaces, that is the signal recording surfaces, will face outwards.

Each of the disc halves 101a, 101b is a disc on which data signals may be recorded, and which is constituted by a disc base material which is formed into a disc shape from synthetic resin such as polycarbonate, acrylic resin or epoxy resin or glass, and a signal recording layer which is formed on the major surface of the disc base material and which is formed of a magnetic material. The signal recording layers are formed on the back surfaces of the optical discs 101a, 101b. That is, the data signals are written on and/or read from the signal recording layers by a laser light radiated from the front surfaces of the disc halves 101a, 101b and transmitted through the disc base materials of the discs halves 101a, 101b.

Disc hubs 104, 104 are secured to the disc base materials of the optical disc halves 101a, 101b. These disc hubs 104, 104 are adapted to be chucked to a disc drive device used for writing and/or reading data signals on or from the double-side optical disc 102, and are mounted at the center on the front surfaces of the base material sides of the disc. The disc hubs 104, 104 are formed of a rigid material, such as material, in the shape of a disc, and provided with a centering hole 104a used for chucking the double-side disc by the disc driving device.

The detailed structure of the double-side optical disc according to the present invention and the step-by-step process for producing the double-sided optical disc will be hereinafter explained.

For producing the double-side optical disc, a pair of optical disc halves 101a, 101b are placed side-by-side on a movable table, as shown in Fig. 2. These disc halves 101a, 101b are placed on the movable table 1 so that the back surfaces thereof bearing the signal recording layers will face upwards. The movable table 1 is provided with a plurality of suction openings 1a in an area thereof supporting the disc halves 101a, 101b. These suction openings 1a communicate with an air suction hole of a vacuum pump, not shown. Thus, when the vacuum pump is in operation to draw air thereinto by means of the suction openings 1a, the optical discs 101a, 101b are sucked onto the movable table 1.

Referring to Fig. 2, the movable table 1 is moved past the lower end of a roll-coat type adhesive applicator device 2 in the horizontal direction as indicated by an arrow A in Fig. 2. The applicator device 2 includes an adhesive reservoir 3 and an applicator roll 4 mounted within a roll mounting aperture 3a formed at the bottom side of the reservoir 3 so that the roll 4 will face both the inside of the reservoir 3 and the outside below the mounting aperture 3a. An adhesive 103 of a rubbery material, such as a so-called hot melt, heated to a workable temperature, is stored in the reservoir 3. The applicator roll 4 is adapted to be rotated as indicated by an arrow B in Fig. 3 by a rotating device, not shown, and is mounted with its rotational axis parallel to the upper surface of the movable table 1. As the applicator roll 4 is rotated, the adhesive 103 stored in the reservoir 3 is applied on the outer periphery of the roll 4.

The optical disc halves 101a, 101b, placed on the movable table 1, are transported by the movable table 1, as it is contacted with the coating roll 4 coated with the adhesive 103. Since the roll 4 is kept rotating, it is slidingly contacted with the disc halves 101a, 101b. The adhesive 103, applied on the outer periphery of the roll 4, is transferred to the back surfaces of the optical disc halves 101a, 101b. Thus the adhesive 103 is applied to the overall surface of the optical disc halves 101a,101b, which emerge from the lower end of the applicator device 2, as shown in Fig. 4.

The surface of the adhesive 103, thus applied to the optical discs 101a, 101b, has irregularities, as shown in Fig. 4, formed by projections 103a and recesses 103b extending at substantially right angles to the direction in which the applicator roll 4 had the rolling contact with the optical disc halves 101a, 101b, (that is the direction of relative movement between the roll 4 and the optical disc halves 101a, 101b, so shown by an arrow C in Fig. 5) caused by the adhesive 103 stretching between the roll 4 and the optical disc halves 101a, 101b while it is coated because of the viscosity of the adhesive 103.

One 101a of the optical disc halves 101a, 101b is then placed on the base block 5, as shown in Fig. 6, with the back surface thereof facing upwards, as shown in Fig. 6. A positioning pin 6 is implanted on the base block 5. The optical disc half 101a is mounted in position on the base block 5 with the positioning pin 6 passed through the centering hole 104a.

The other 101b of the optical disc halves 101a, 101b, is held in position above the block 5 by a deforming/retaining device 7, as shown in Fig. 6, so that the back side of the optical disc half 101b faces the optical disc half 101a placed on the base block 5. The deforming/retaining device 7 has a peripheral suction hole 7a communicating with a vacuum pump and adapted for holding the peripheral area of the optical disc half 101b and a central pressor member 7b for pressing and supporting the central area of the optical disc half 101b. Thus the optical disc 101b is supported in the deformed state by the deforming/retaining device 7 as it has its peripheral region retained by the device 7 under suction and is pressed downwards at its center so that the middle section on the back surface of the optical disc half 101b is bowed downwards.

The deforming/retaining device 7 is then rotated by, for example, 90 degrees about the central vertical axis of the optical disc half 101b, so that the direction along which the optical disc half 101a was contacted by the applicator roll 4 is at a (non-zero) angle to that along which other optical disc half 101b was contacted by the applicator roll 4.

The deforming/retaining device 7 is then moved in a direction towards the block 5, as shown by an arrow D in Fig. 6, so that the optical disc halves 101a, 101b are stacked one upon the other, with the back surfaces thereof facing each other with the adhesives 103 in between. At this time, since the optical disc half 101b, held by the deforming/retaining device 7, has been deformed so that the middle portion thereof is swollen downwards, the adhesive 103 applied at the middle portion of the optical disc half 101a and the adhesive 103 applied at the middle portion of the optical disc half 101b are first contacted with each other, as shown in Fig. 7, with the contact area then proceeding gradually toward, the outer peripheries of the optical disc halves 101a, 101b. With the optical disc halves 101a, 101b being overlapped in this manner with each other, air bubbles in the adhesive 103 applied on the optical disc halves 101a, 101b are expelled towards the outer peripheries of the optical disc halves 101a, 101b.

The optical disc halves 101a, 101b are then bonded together under a pressure of, for example, 1 ton applied from a press, not shown. At this time, the optical disc halves 101a, 101b are placed relative to each other with the direction of rolling contact of the optical disc half 101a by the roll 4 intersecting that of rolling contact of the optical disc half 101b by the roll 4, that is, with the direction of the projections 103a of the adhesive 103 on the optical disc half 101a intersecting that of the projections 103a of the adhesive 103 on the optical disc half 101b, as shown by dotted and chain dotted lines in Fig. 8. Thus the irregularities on the surface of the adhesive 103 applied on the optical disc half 101a are counterbalanced by those on the surface of the adhesive 103 applied on the optical disc half 101b to prevent the surface irregularities on the surface of the adhesive 103 from affecting the flatness of the optical disc halves 101a and 101b.

The double-side optical disc 102, thus formed by bonding the optical disc halves 101a and 101b to each other, is machined on its outer periphery. To this end, the disc 102 is rotated about its axis as it is held by a rotating/retaining device 8, as shown in Fig. 9, and has its outer periphery machined by first and second cutting tools 9 and 10 kept in contact with the outer periphery of the rotating disc 102.

The rotating/retaining device 8 includes a rotary table section 11 adapted for supporting the double-side optical disc 102 thereon and rotatably supported by a pair of bearings 11c. The rotary table section 11 includes a circular rest section 11a for supporting the outer periphery of the double-side optical disc 102. The rest section 11a is formed of an elastic material, such as silicon rubber. The rest section 11a is provided with a suction bore 11b communicating with a suction port of a vacuum pump 12. Thus, when the vacuum pump 12 is in operation to draw in air by means of the suction bore 11b, the double-side optical disc 102 is sucked onto the rest section 11a. The rotary table section 11 is adapted to be rotated by a driving electric motor 13 by means of an endless driving belt 14 at a constant velocity of say 900 to 1200 revolutions per minute.

The rotating/retaining device 8 also includes a presser member 15 facing the rotary table section 11 and adapted to be supported by and pressed against the rest section 11a of the rotary table section 11. The presser member 15 has a circular presser section 15a in register with the rest section 11a. Thus the double-side optical disc 102 is rotated in the rotating/retaining device 8 as it has its outer peripheral region clamped and held by the rest section 11a and the presser section 15a.

The first cutting tool 9 is of a flat type and is supported for movement towards and away from the double-side optical disc 102 with a flat edge section 9a facing the outer periphery of the double-side optical disc 102 held by the rotating/retaining device 8. The first cutting tool 9 is normally biased by a first biasing spring 16 in a direction away from the optical disc 102 sown by an arrow E in Figs. 9 and 10.

The second cutting tool 10 is of a vee type and is supported for movement towards and away from the double-side optical disc 102 with a vee edge section 10 facing the outer periphery of the disc 102 held by the rotating/retaining device 8. The second cutting tool 10 is biased by a second biasing spring 17 in a direction away from the double-sided optical disc 102 as shown by an arrow F in Figs. 9 and 10.

The first and second cutting tools 9 and 10 are moved with a time difference relative to each other towards the double-side optical disc 102 at the rate of several millimeters per second, against the bias of the biasing springs 16 and 17, by first and second rotary cams 18 and 19 rotated unidirectionally at a constant velocity as shown by arrows G and H in Fig. 10.

Thus, when the disc 102 is rotated by the rotating/retaining unit 8 and the first and second rotary cams 18 and 19 have been rotated by about 90 degrees, the first cutting tool 9 is moved by the first rotary cam 18 towards the double-side optical disc 102 as shown at B in Fig. 10. Thus the edge section 9a of the first cutting 9 is brought into contact with the outer periphery of the double-side optical disc 102 to cut the outer periphery of the disc 102 to a cylindrical contour.

When the first and second rotary cams 18 and 19 have been rotated further by 90 degrees, the cutting by the first cutting tool 9 has nearly been completed, as shown in at C Fig. 10, and by means of the second rotary cam 19, the second cutting tool 10 is moved towards the double-side optical disc 102. Thus the edge section 10a of the second cutting tool 10 is brought into contact with the outer periphery of the double-side optical disc 102 to chamber the upper and lower edges on the outer periphery of the double-side optical disc 102.

When the first and second rotary cams 18 and 19 have been rotated further by 90 degrees, the first cutting tool 9 is reset to the initial state, while the cutting by the second cutting tool 10 has nearly been completed, as shown at D in Fig. 10. When the rotary cams 18 and 19 have been rotated further by 90 degrees, the second cutting tool 10 is reset to the initial state shown at A in Fig. 10, while the cutting of the outer periphery of the optical disc 102 is completed.

The double-side optical disc 102, having its outer periphery retained by the rotating/retaining device 8 and cut by the first and second cutting tools 9 and 10, may be machined to a predetermined outside diameter within the tolerance of say ±0.2 mm.