Optical disc and method for producing the optical disc

Abstract: An optical disc includes a disc base plate (21) of a light-transmitting synthetic material having a recess to mount a magnetic plate (26), and a holding part (29 a ), integral with the disc base plate (21), loosely to hold the magnetic plate (26) mounted in the recess with respect to the disc base plate (21). It is possible with the present optical disc to prevent distortion of the disc base plate (21) which would otherwise be caused by the difference in thermal expansion coefficients of the two components to inhibit double refraction of the disc base plate (21) caused by such distortion so as to maintain good recording/reproducing characteristics.

Description: The invention relates to a disc for recording information signals, such as an optical disc or a magneto-optical disc, referred to hereinafter as an optical disc, and to a method for producing such an optical disc. With previously proposed optical discs, information signals can be recorded to a high density and a disc of an extremely small size has been proposed. For example, a small-sized magneto-optical disc 64 mm or less in diameter has been proposed. Such a magneto-optical disc, which is small-sized and capable of recording to a high density, can be loaded onto a disc rotating and driving device to be rotated at high velocity. While the disc is rotated at high velocity, a light beam radiated from an optical pickup is directed to a recording track provided on a signal recording layer formed on a major surface of the disc, while an external magnetic field is applied by the magnetic head for recording desired information signals on the disc. For accurately radiating the light beam on the recording track of the magneto-optical disc rotated at an elevated velocity, it is necessary for the disc to be positively unified with a disc table of the disc rotating and driving device and to be positioned and loaded highly accurately with respect to the disc table. Meanwhile, for positively unifying the magneto-optical disc to the disc table and loading the disc accurately with respect to the disc table, a disc loading system has been proposed, for example in patent specifications US-A-4,787,009 and US-A-4,829,510, according to which, with the magneto-optical disc set on the disc table, a chucking magnetic plate of e.g. a magnetic metal material, placed at the centre of the magneto-optical disc, is magnetically attracted by a permanent magnet provided on the disc table, to load the magneto-optical disc on the disc table. In Figure 1 of the accompanying drawings, a magneto-optical disc 1 of previously proposed kind, is shown employed in a disc loading system taking advantage of magnetic attraction by a permanent magnet, a metal plate 3 as a magnetic plate is placed for closing a centre opening 2 formed in the disc 1. A disc rotating and driving device 4 rotationally to drive the magneto-optical disc 1 includes a spindle motor 5 having a rotary shaft 6 integrally carrying a disc table 7 and a permanent magnet 9 integrally mounted on the distal end of the rotary shaft 6 by means of a magnet holder 8. A centring member 11 is mounted at the centre of the disc table 7 by being housed within a housing recess 10 formed at the centre of the disc table, and is biased by a coil spring 12 so as to be moved axially of the rotary shaft 6. The magneto-optical disc 1 is set on the disc table 7, with the centring member 11 engaged in the centre opening 2 with the rim of the centring hole 2 supported by the disc table 7. The magneto-optical disc 1 is secured to the disc table 7 by the metal plate 3 being magnetically attracted by the magnet 9. When set and loaded on a surface 7 a of the disc table 7, the magneto-optical disc 1 has its centre of rotation aligned with the axis of the rotary shaft 6 by the centring member 11 which is introduced into engagement with the centring opening 2 while being moved axially of the rotary shaft 6. By thus attracting the metal plate 3 on the magneto-optical disc 1 by the magnet 9 of the disc table 7, and centring the disc by the centring member 11, when setting the magneto-optical disc 1 on the disc table 7, the disc 1 may be loaded on the disc table 7 as one with and with highly accurate positioning relative to the disc table 7. On the other hand, the chucking mechanism of positioning and integrally loading the magneto-optical disc 1 on the disc table may be simplified in structure. The magneto-optical disc 1, which is of small diameter and permits of high density recording of information signals, is housed in a disc cartridge 13, and loaded in this state onto the disc rotating and driving device 4. The previously proposed metal plate 3 mounted on the magneto-optical disc is bonded to a base plate or substrate of the magneto-optical disc which is formed of a synthetic resin, such as a transparent polycarbonate resin, with the aid of a UV curable adhesive or double-sided adhesive tape, in a manner to prevent positional deviation on the disc substrate. However, since the thermal expansion coefficient of the metal plate differs markedly from that of the disc base plate formed of synthetic resin, if the metal plate is joined to the disc base plate with the aid of the above-mentioned adhesive, the metal plate may be peeled off from the disc base plate after repeated expansion and contraction of the metal plate and the disc base plate due to changes in ambient temperatures. On the other hand, if the metal plate and the disc base plate are secured strongly to each other in a manner which will ensure that peeling after repeated expansion and contraction will not occur, distortion may be produced in the disc base plate with changes in the ambient temperature, so that double refraction is produced within the disc base plate and hence information signals cannot be recorded or reproduced with good recording/reproducing properties on or from the signal recording layer formed on the surface of the disc base plate. If the metal plate is bonded to the disc base plate with the aid of a UV curable adhesive, the risk is extremely high that double refraction will be produced within the disc base plate during curing of the adhesive. If UV curable adhesive is employed, it is also difficult to improve the productivity of the magneto-optical disc because of the curing time involved in curing the adhesive. On the other hand, since double-sided adhesive tape is lacking in bonding strength, sufficient reliability in bonding between the metal plate and the disc base plate cannot be assured with the use of such adhesive tape. Patent Specification US-A-4 733 388 which is considered to represent the closest prior art discloses several embodiments of optical discs with magnetic members mounted thereon. Mounting means for attaching the magnetic members to disc substrates include an annular L-shaped member, a plurality of L-shaped members spaced around the circumference of the magnetic member, a U-shaped member, an air sandwich structure with the magnetic member held between laminates and rivets extending through apertures in the magnetic member, through the disc substrate and through a backing member on an opposite side of the substrate to the magnetic member. According to one aspect of the invention there is provided an optical disc comprising: a disc base plate formed of a light-transmitting synthetic resin material and a magnetic member held on the middle of the disc base plate by holding means; characterised in that the magnetic member is received in a recess integrally moulded at the middle of one major surface of the disc base plate and is held therein by holding means integrally formed with the disc base plate and which hold the magnetic member in the recess with a predetermined gap between the magnetic member and the disc base plate. According to another aspect of the invention there is provided a method for producing an optical disc characterised by moulding a disc base plate with a circular recess integrally formed at the middle of one major face thereof, placing a magnetic member in the circular recess, the magnetic member having a diameter less than the inside diameter of the recess, and thermally deforming a rim of the recess of the disc base plate to mount the magnetic member with a predetermined gap between the recess and the magnetic member. Such an optical disc for recording information signals can have a chucking magnetic plate mounted on the disc base plate without employing an adhesive, such as a UV curable type adhesive. Double refraction will not be produced in the disc base plate thereby ensuring optimum recording and/or reproducing properties. The magnetic plate can be mounted easily on the disc base plate to ensure improved productivity. For holding the magnetic plate within the recess of the disc base plate, the end part of the upright wall formed around the rim of the recess is thermally deformed to form the projecting part projecting into the inside of the recess to prevent the magnetic plate from being detached from the recess of the magnetic plate. The present invention is diagrammatically illustrated by way of example in the accompanying drawings, in which: Figure 1 is a cross-sectional view showing a conventional magneto-optical disc loaded on a disc table of a disc rotating and driving device; Figure 2 is an exploded perspective view showing a magneto-optical disc according to a first embodiment of the invention; Figure 3 is an enlarged cross-sectional view, taken along line A-A of Figure 2 showing a disc base plate; Figure 4 is a perspective view showing the magneto-optical disc with a metal plate affixed to the disc base plate; Figure 5 is an enlarged cross-sectional view showing the magneto-optical disc with the metal plate affixed to the disc base plate; Figure 6 is an enlarged partial cross-sectional view showing a magneto-optical disc with a metal plate affixed to the disc base plate; Figure 7 is a perspective view showing a second embodiment of a disc base plate of a disc according to the first embodiment of the invention; Figure 8 is an exploded perspective view showing a magneto-optical disc according to the second embodiment of the invention; Figure 9 is an enlarged partial cross-sectional view of the magneto-optical disc shown in Figure 8; Figure 10 is an enlarged partial cross-sectional view showing the state in which a metal plate is attached to a disc base plate of the magneto-optical disc according to the second embodiment of the invention; Figure 11 is an enlarged partial cross-sectional view showing parts of the magneto-optical disc shown in Figure 10; and Figure 12 is an enlarged cross-sectional view showing a modified embodiment of a disc according to the invention. A magneto-optical disc 20, as shown in Figures 2 and 3, is provided with a disk-shaped disc base plate 21 produced by moulding a light-transmitting synthetic resin, such as polycarbonate resin. A recording layer for recording information signals is deposited on one major surface 21a of the disc base plate 21. Information signals can be recorded and reproduced by radiating a light beam on the signal recording surface from a signal write-read surface which is formed on the other major surface 21 b of the disc base plate 21 opposite to the major surface 21 a . A centring hole 22 to be engaged by a centring member 11 provided at the centre of a disc table 7 of the above-mentioned disc rotating and driving device 4 is formed at the centre of the disc base plate 21. A raised cylindrical engaging rim 23 is formed around the centring hole 22 on the major surface 21 b of the disc base plate 21. The engaging rim 23 is provided to increase the thickness of the centring hole 22 formed in the disc base plate 21 to ensure a sufficient engaging length of the centring member 11 engaged in the centring hole 22. On the major surface 21 b of the disc base plate 21, a position-regulating annular protuberance 24, to be engaged with a regulating projection around a disc table inserting opening in a disc cartridge on insertion of the magneto-optical disc 20 into the disc cartridge to regulate the housing position of the magneto-optical disc 20 in the disc cartridge, is provided to surround the engaging rim 23, as shown in Figure 3. An annular region between the engaging rim 23 and the position-regulating protuberance 24 presents a planar surface forming a setting reference surface 25 to be supported by a surface 7 a of the disc table 7 when the magneto-optical disc 21 is set on the disc table 7. At a mid-part of the major surface 21 a of the disc base plate 21, substantially concentric with the centring opening 22, there is formed a recess 27 to overly the centring opening 22. Within the recess 27, there is accommodated a metallic plate 26 which is formed of a magnetic metal material so as to be magnetically attracted by a permanent magnet 9 provided on the disc table 7. The recess 27 is of a diameter R 2 which is slightly larger than the outside diameter R 1 of the metal plate 26 so that a small gap is produced between the periphery of the recess and the metal plate 26 when the metal plate 26 is housed within the recess 27. An annular projecting rim 28 is formed around the recess 27 to ensure that the recess has a depth sufficient to accommodate the metal plate 26 of a predetermined thickness within the recess 27. A number of ribs 29 are formed to project from the end surface of the rim 28 to hold the metal plate 26 within the recess 27. In the present embodiment, six ribs 29 are formed at equal angular intervals, as shown in Figure 2. A step 30 is formed on the inner periphery on the bottom side of the recess 27. The step 30 acts as a clearance to sever a runner produced on injection moulding the disc base plate 21. That is, by provision of the step 30, the centring opening 22 or the recess 27 is not affected in moulding accuracy during severing of the runner. On application of ultrasonic waves, such as with ultrasonic honing, the ribs 29 are crushed towards the inside of the recess 27 to form end parts 29 a which are caulked against the rim of the metal plate 26 housed within the recess 27, as shown in Figures 4 and 5, to hold the metal plate 26 within the recess 27. Meanwhile, the caulking of the rib 29 may also be made by heating using a heated trowel instead of by heating by ultrasonic waves. That is, any suitable method may be employed for thermally caulking the ends of the ribs 29. It is to be noted that the ribs 29 are crushed by ultrasonic heating so that the ends 29 a of the ribs are not pressed against the metal plate 26 accommodated within the recess 27. That is, the ribs 29 are crushed so that a small gap D 1 , shown in Figure 6, remains between the metal plate 26 and the ends 29 a of the ribs 29 caulking the metal plate 26. Since the recess 27 has the diameter R 2 slightly larger than the outside diameter R 1 accommodated therein, as mentioned above, the metal plate 26 is loosely maintained within the recess 27 with the small gaps D 1 and D 2 along the thickness and the diameter thereof, respectively. The result is that, even although the disc base plate 21 and the metal plate 26 should undergo expansion and contraction in a repeated manner with changes in the ambient temperatures, any differential expansion or contraction caused by the difference in the thermal expansion coefficients may be absorbed by the gaps D 1 and D 2 without affecting each other to prevent distortions from being produced in the disc base plate 21, so that double diffraction is not produced in the disc base plate 21. The magneto-optical disc 20, with the metal plate 26 mounted on the disc base plate 21, as described hereinabove, is housed within the disc cartridge, similarly to the conventional magneto-optical disc 1, and is loaded in this state on the disc table 7 of the disc rotating and driving device 4. Although a plurality of ribs are provided in the above described embodiment to caulk and hold the metal plate 26 within the recess 27, an annular rib in the form of a continuous annular ring may instead be provided around the recess 27, as shown in Figure 7. The distal end of the annular rib 29 may be crushed to form a crushed end to hold the metal plate 26 accommodated within the recess 27. In the above-described first embodiment of the invention, the rib 29 is provided on the disc base plate 21 and is thermally caulked to hold the metal plate 26 against the disc base plate 26. However, in this case, the metal mould used for producing the disc base plate 21 tends to be complicated due to provision of the ribs 29 on the disc base plate 21. A second embodiment of a disc for recording information signals, designed to overcome this inconvenience, is hereinafter explained. It is noted that the disc for recording information signals according to the present second embodiment is again a magneto-optical disc, as in the preceding embodiment. Referring to Figure 8, a magneto-optical disc 101 is provided with a disc-shaped disc base plate 102 produced by moulding a light-transmitting synthetic resin, such as polycarbonate resin. A recording layer for recording information signals is deposited on one major surface 102 a of the disc base plate 102. With the magneto-optical disc 101, produced by using the disc base plate 102, information signals are recorded and reproduced by radiating a light beam on the signal recording surface from the side of a signal write-read surface which is formed on the other major surface 102 b of the disc base plate 102 opposite to the major surface 102 a . The disc base plate 102, constitutes a small-sized magneto-optical disc 101 having a diameter R of 64 mm and a thickness t of 1.2 mm. The radially outer region of the magneto-optical disc 101, excluding the radially inner region having a diameter equal to 32 mm, is a signal recording region D for recording information signals. The disc base plate 102 has a centre opening 103 engaged by a centring member 11 provided on a disc table 7 of a disc rotating and driving device 4, arranged as described previously. The centre opening 103 is formed through the disc base plate 102 and has its centre concentric with the centre of curvature of a recording track(s) formed concentrically or spirally on the signal recording layer. An annular projecting rim 104 is formed around the centre opening 103 on the other major surface 102 b of the disc base plate 102 within a non-recording region E other than the signal recording region D. The annular rim 104 plays the role of increasing the depth of the centring opening 103 formed in the thin disc base plate 102 to allow an increase in the amount of projection of the centring member 11 of the disc 7 into the centring opening 103. That is, the annular projecting rim 104 makes it possible to increase the height of the centring member 11 engaged with the centring opening 103. In this manner, the amount of relative movement between the magneto-optical disc 101 mounted on the disc table 7 and the centring member 11 on loading the magneto-optical disc 101 on the disc table 7 is increased to ensure a reliable centring operation of aligning the centre of rotation of the magneto-optical disc 101 with the centre of axis of the disc table 7. The projecting annular rim 104 has substantially the same height of projection h as the thickness t of the disc base plate 101 and has its planar surface as a loading reference surface 104 a of loading the magneto-optical disc on the disc setting surface 7 a of the disc table 7. An annular recess 106 to accommodate a magnetic metal plate 105 is formed on the major surface 102a of the disc base plate 102 substantially to overly the centring opening 103. The annular recess 106 is of a diameter less than the outside diameter r of the projecting annular rim 104 and of a depth substantially equal to the thickness t of the disc base plate 102. By forming the annular recess 106 of substantially the same depth as the thickness t of the disc base plate 102 substantially in alignment with the projecting annular rim 104, the projecting annular rim 104, having substantially the same height of projection h as the thickness t of the disc base plate 102, has a thickness w equal to the thickness t of the disc base plate 102, more precisely, the thickness of the signal recording region D of the disc base plate. Meanwhile, the inner rim of the bottom of a setting surface 107 of the annular recess 106 for the metal plate 105 is formed with a step 108. The function of the step 108 is to prevent burrs or the like from being produced on the setting surface 107 for the metal plate 105 so as to maintain the planar setting surface 107. The metal plate 105, accommodated in the recess 106, is formed by a disc-shaped thin magnetic metal sheet of e.g. a stainless steel plate SUS-430, with a thickness of the order of 0.4 mm, to facilitate machining and so as not to increase the weight of the magneto-optical disc 101. Referring to Figure 8, the metal plate 105 is formed in a disc shape of a size that can be accommodated within the annular recess 106. The metal plate 105 is provided with a centring hole 109 by means of which the metal plate 105 may be accommodated in the recess 106 with the centre of the metal plate 105 in alignment with the centre of the disc base plate 102. The positioning hole 109 may also be used for receiving a jig used for holding the metal plate 105 when introducing the metal plate 105 into the annular recess 106. If the metal plate 105 is simply formed as a planar disk, the centre opening 103 formed at the centre of the annular recess 106 could be of a shallow depth to accomodate the metal plate 105 having a depth approximately equal to the thickness t of the disc base plate 102 in the recess 106. However the metal plate 105 is of top-hat section with an upright intermediate section 110 which is of a height such that a central major surface 105 a thereof is flush with the major surface 102 a of the disc base plate 102 when the metal plate 105 is accommodated within the recess 106. An annular setting rim 111 for setting the metal plate 105 in the annular recess 106 is formed radially outwardly of the upright intermediate section 110 and parallel to the central major surface 105 a . That is, the metal plate 105 is in the form of a disk raised except for the rim portion, as shown in Figure 8. The upright section 110 may be formed by embossing or machining a metal material of an increased thickness, instead of by bending. The metal plate 105, formed as above, is arranged within the annular recess 106, as shown in Figure 9, with the annular setting rim 111 set in the annular recess 106 of the disc base plate 102. The metal plate 102 is accommodated in the recess 106 of the disc base plate 102 by a supporting projection 112 formed by thermally deforming a part of the rim of the recess 106 of the disc base plate 102 ultrasonically or using a heated trowel as in the preceding embodiment. Since the recess 106 is of an inside diameter slightly larger than the outside diameter of the metal plate 105, a gap D 4 is present between the inner wall of the recess 106 and the outer periphery of the metal plate 105, as shown in Figure 11. Besides, a gap D 3 is present between the surface of the supporting projection 112 facing the recess 106 and the rim 111 of the metal plate 105. The result is that differential expansion and contraction between the metal plate 105 and the disc base plate 102 due to the difference in thermal expansion coefficients of these parts is absorbed by the gaps D 3 , D 4 without producing distortion in the disc base plate 102 due to the inner stress otherwise caused by changes in the operating ambient temperatures of the magneto-optical disc. The metal plate in the first and the second embodiments is produced by punching or pressing a metal plate material, so that burrs tend to be produced on the rim of the metal plate in the punching direction. As a result thereof, the above-mentioned gaps between the metal plate and the recess of the disc base plate are diminished by these burrs of the metal plate so that there is the risk that the differential expansion or contraction caused between the metal plate and the disc base plate with changes in the operating ambient temperatures cannot be absorbed by these gaps. In such case, clearance means may be provided for clearing the burrs in the recess of the disc base plate. An arrangement for clearing the burrs is shown in Figure 12, in which the same parts or components as those of the preceding first embodiment are indicated by the same numerals and detailed description thereof is omitted for simplicity. The metal plate 26 is produced by punching or pressing a metal plate material, so that a burr 26 a is produced on the rim of the metal plate along the punching direction, as shown in Figure 12. An annular groove 21c for clearing the burr 26a of the metal plate 26 is formed on the bottom side outer periphery of the recess 27 of the disc base plate 21. The groove 21c is of a depth large enough to prevent the burr 26 a of the metal plate 26 from contacting the bottom of the recess 27 of the disc base plate 21. Thus, when the metal plate 26 is placed in the recess 22 of the disc base plate 21, the burr 26 a of the metal plate 26 faces the groove 21 c of the disc base plate 21, so that the metal plate 26 is not tilted within the recess 27 of the disc base plate 21 and a desired gap can be maintained between the metal plate 26 and the disc base plate 21. Although the invention has been explained in connection with the application thereof to a magneto-optical disc, the operation and result similar to those achieved with the magneto-optical disc may also be achieved with an optical disc in which a magnetic plate for magnetic attraction is provided on a disc base plate formed of synthetic resin.

Claims: An optical disc comprising: a disc base plate (21, 102) formed of a light-transmitting synthetic resin material and a magnetic member (26, 105) held on the middle of the disc base plate (21, 102) by holding means; characterised in that the magnetic member (26, 105) is received in a recess (27, 106) integrally moulded at the middle of one major surface of the disc base plate (21, 102) and is held therein by holding means (29 a , 112) integrally formed with the disc base plate and which hold the magnetic member (26, 105) in the recess (27, 106) with a predetermined gap (D 1 , D 2 , D 3 , D 4 ) between the magnetic member and the disc base plate (21, 102). An optical disc according to claim 1, wherein the recess (27, 106) in the disc base plate (21, 102) and the magnetic member (26, 105) are circular and the recess (27, 106) has an inside diameter (R 2 ) larger than the outside diameter (R 1 ) of the magnetic member (26, 105). An optical disc according to claim 2, wherein the holding means comprises stop means (29 a , 112) to limit movement of the magnetic member (26, 105) with respect to the disc base plate in the direction of its thickness. An optical disc according to claim 2, wherein the holding means comprises a projecting part (29 a ) projected from the major surface (21 a ) of the disc base plate (21) provided with the recess (27) towards the centre of the recess (27). An optical disc according to claim 4, wherein the holding means comprises the projecting part (29 a ) and a connecting part (29) connecting the projecting part to the major surface of the disc base plate provided with the recess. An optical disc according to claim 1, wherein a groove (21 c ) is formed towards an outer edge of the bottom of the recess to receive any burr on the outside edge of the magnetic member (26, 105). An optical disc according to claim 1, wherein the disc base plate (21, 102) has an annular projecting rim (23, 104) around a centre opening (22, 103) on one major surface (21 b , 102 b ) thereof and the recess (27, 106) is provided in the other major surface (21 a , 102 a ) thereof to accommodate the magnetic member (26, 105). A method for producing an optical disc characterised by moulding a disc base plate (21, 102) with a circular recess (27, 106) integrally formed at the middle of one major face thereof, placing a magnetic member (26, 105) in the circular recess (27, 106), the magnetic member (26, 105) having a diameter (R 1 ) less than the inside diameter (R 2 ) of the recess, and thermally deforming a rim of the recess (27, 106) of the disc base plate to mount the magnetic member (26, 105) with a predetermined gap (D 1 , D 2 , D 3 , D 4 ) between the recess (27, 106) and the magnetic member (26, 105). A method for producing an optical disc according to claim 8, wherein the recess is formed with an upright wall (29) around the rim thereof, and the end part (29 a ) of the upright wall (29) of the disc base plate is thermally deformed to form a projecting part (29 a ) at the end of the upright wall (29) to mount the magnetic member with the predetermined gap (D 1 , D 2 , D 3 , D 4 ) between the recess and the magnetic member.

Inventors: FUJIIE KAZUHIKO C O PATENTS DI, FUJISAWA HIROTOSHI C O PATENTS, MUKAWA HIROSHI C O PATENTS DIV, TAKAHASHI KENJI C O PATENT DIV, TSURUSHIMA KATSUAKI C O PATENT, YOSHIDA TADAO C O PATENTS DIVI, FUJIIE, KAZUHIKO C/O PATENTS DIVISION, FUJISAWA, HIROTOSHI C/O PATENTS DIV., MUKAWA, HIROSHI C/O PATENTS DIVISION, TAKAHASHI, KENJI C/O PATENT DIVISION, TSURUSHIMA, KATSUAKI C/O PATENT DIVISION, YOSHIDA, TADAO C/O PATENTS DIVISION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: G11B7
IPC codes: G11B23, G11B7