Manufacturing magnetic recoding discs

Abstract: In manufacture of a stretched surface recording disc (3), a laminate is formed by superposing magnetic sheets (2A, 2B) on upper and lower surfaces of a base (1) with a bonding agent interposed between the regions to be bonded. An outer annular portion and an inner annular portion of the resulting laminate are both clamped by clamp members (25, 27). The clamped inner annular portion including both magnetic sheets (2A, 2B) and the base (1) is displaced relative to the clamped outer annular portion in a direction perpendicular to the surface of the base, thereby stretching the laminate and bonding the magnetic sheets (2A, 2B) to the base (1) while the annular portions are so displaced.

Description: This invention relates to manufacturing magnetic recording discs from a laminate of magnetic sheets on a base. Tension type magnetic discs have previously been proposed wherein magnetic sheets are joined under tension to upper and lower surfaces of a relatively rigid base in such a manner that a narrow gap is provided between the magnetic recording region of each sheet and the base. Such magnetic discs and a method of producing the discs are disclosed, for example, in European Patent Application No. EP-A-O 186 427 wherein an annular portion of recording medium film is provided on each side of an annular support having raised spaced annular ridges and is stretched and adhered to attachment surfaces of the annular support near the inside and outside diameters respectively by a beam cured adhesive. Stretching is effected by forming two assemblies, each comprising a film stretched and clamped in a temporary holder formed by two rings and pressing the two assemblies together with the annular support sandwiched therebetween and clamped by centre clamping plates. The method described therein, however, presents a number of problems. For example, it is not suitable for continuous manufacture since the magnetic sheets on the two surfaces must be tensioned by temporary holders, and only a small tension adjustment range is possible for the two magnetic sheets. Furthermore, an annealing step is necessary to release the stress caused in the magnetic sheets due to the tensioning operation, and the annealing step must be performed independently of the bonding step. Therefore, the previously-proposed process provides operational difficulties and problems with respect to product quality. Other types of stretched surface recording discs and methods for their manufacture are disclosed, for example, in Patent Specifications US-A-4 365 257, US-A-4 464 693, US-A-4 543 619, US-A-4 573 096, US-A-4 573 097, US-A-4 581 667, US-A-4 623 570, US-A-4 625 384, US-A-4 631 609, and US-A-4 670 072. According to one aspect of the invention there is provided a method of producing a magnetic disc comprising a base having a circular periphery interposed between two magnetic sheets, each magnetic sheet being bonded to the base along two annular concentric regions, the method comprising: forming a laminate of the base sandwiched between the two magnetic sheets by superposing the magnetic sheets on upper and lower surfaces of the base with a bonding agent being interposed between regions to be bonded; clamping an inner annular portion of each of the sheet onto a portion of the base and clamping an outer annular portion of the sheets located radially outward of the inner annular portion and outward of the periphery of the base; characterised by displacing the clamped inner annular portion including both the magnetic sheets and the base relative to the clamped outer annular portion in a direction perpendicular to the surfaces of the base so that the two sheets between the clamped inner portion and the periphery of the base are radially stretched and the two sheets are each applied to the base along the annular concentric regions; and bonding the magnetic sheets to be base at the annular concentric regions while the annular portions are so displaced to bond the stretched sheets onto the base. According to another aspect of the invention there is provided apparatus for manufacturing a stretched surface recording disc comprising a base sandwiched between two magnetic sheets which are secured at annular concentric regions to the base, the apparatus being characterised by: an anchor table having a central recess; a shift member positioned in the central recess of the anchor table; means to engage the shift member for vertical displacement relative to the anchor table; means coacting with the shift member to clamp an inner annular portion of each of the magnetic sheets and the base to be laminated to the shift member to move therewith; means to clamp an outer annular portion of each of the magnetic sheets at a position outside of a circular periphery of the base so that when the shift member and the clamped inner portion are shifted relative to the means for clamping the outer portions, each of the sheets is stretched radially on the base and is applied to the annular concentric regions and can be bonded thereto. A preferred embodiment of the present invention, to be described in more detail hereinafter, provides a method for manufacturing a magnetic recording disc in which stretched magnetic sheets are bonded to both sides of a disc shaped base simultaneously. The method is such that it lends itself to mass production techniques which provide improved productivity. A magnetic disc is produced by first forming a laminate of a core or base and two magnetic sheets superposed on the upper and lower surfaces of the base with a bonding agent being interposed between predetermined regions of the sheets and the base. The outer annular portion and the inner annular portion of the laminate are clamped and then the clamped inner portion of the laminate is displaced relative to the clamped outer portion in a direction perpendicular to the surface of the base. This causes each of the two magnetic sheets to be tensioned with the required tension and the magnetic sheets to be applied to the base along the annular concentric regions. While under tension and so applied, the bonding to the base is effected by subjecting the laminate to an elevated temperature, whereby the two magnetic sheets are joined to the base simultaneously with the required tension being maintained. The bonding may be accompanied by annealing of the sheets under tension so as to relieve some of the stress present, such that it is reduced to a predetermined value. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a cross-sectional view illustrating an initial stage in the formation of a magnetic disc according to an embodiment of the present invention; Figure 2 is a view similar to Figure 1 but showing the formation condition during a tensioning stage; Figure 3 is a cross-sectional view of the finished magnetic disc; and Figure 4 is an exploded view of the assembly shown in both Figures 1 and 2. As shown in Figure 1, the preferred embodiment of the present invention involves forming a laminate by superposing magnetic sheets 2A and 2B on upper and lower surfaces of a relatively rigid base 1 while interposing a bonding agent between the regions which are to be mutually bonded. The method involves clamping an inner portion and an outer portion of the laminate and lifting the clamped inner portion of the laminate relative to the clamped outer portion in a direction perpendicular to the surface of the base 1, as shown in Figure 2, while keeping the magnetic sheets 2A and 2B in contact with the outer portion of the base 1. Then, bonding is accomplished while the sheets are under tension due to the displacement between the outer and inner annular portions. As shown in Figure 1, the two magnetic sheets 2A and 2B are clamped annularly at the outer portions thereof beyond the periphery of the base 1. The inner portions of the two magnetic sheets 2A and 2B are supported annularly and pressed fixedly against the surfaces of the central portion of the base 1 in such a manner as to be movable relative to the clamped outer portion in a direction perpendicular to the surface of the base1. According to the present method, the two magnetic sheets 2A and 2B are simultaneously stretched outwardly, that is radially, and are kept taut with a required tension. By heating and hardening the bonding agent, which is typically a thermosetting resin or the like, interposed between the regions of the base 1 and the two magnetic sheets 2A and 2B to be bonded, there is produced, as illustrated in Figure 3, a magnetic disc 3 in which the magnetic sheets 2A and 2B are adhered to the opposed surfaces of the base 1. The tension on each of the magnetic sheets 2A and 2B can be adjusted over a wide range by controlling the amount of relative lifting of the clamped inner portion with respect to the clamped outer portion. In Figures 3 and 4, the base 1 is shown shaped as a circular disc with a centre hole 4, and annular grooves 5A and 5B each having a predetermined width W formed in the upper and lower surfaces of the base 1 concentrically in at least the recording regions of the magnetic disc 3. Annular outer reference planes 6A1,6B1 and annular inner reference planes 6A2,6B2 perpendicular to the axis of the base 1 are respectively formed at the outer edge and the inner edge of the annular grooves 5A and 5B. One flat surface is defined by the planes 6A1 and 6A2, and another flat surface is defined by the planes 6B1 and 6B2, respectively. Outer bevels 7A1 and 7B1 are formed in the peripheries of the outer reference planes 6A1 and 6B1 so that the base 1 becomes gradually thinner towards the circumference from the reference planes 6A1 and 6B1. Inner bevels 7A2 and 7B2 are formed inside the inner reference planes 6A2 and 6B2 closer to the central portion of the base 1 so that the base 1 becomes gradually thinner toward the centre. If necessary, annular grooves 8A1,8A2 and 8B1,8B2 for receiving excess bonding agent may be formed in the bevels 7A1,7A2 and 7B1,7B2, respectively. The base 1 may be composed of a metal plate of aluminium, an aluminium alloy, or the like, or may be moulded out of thermosetting or thermoplastic resin. Typically, the base 1 can be produced by injection moulding a polymer material with or without a filler mixed therein. The polymer material may be a resin such as a polysulphone, a polyphenylene, a polyester, or a polyetherimide. Non-magnetic inorganic particles of glass, glass beads, calcium carbonate, or the like may be included as a filler. Each of the magnetic sheets 2A and 2B is composed of a non-magnetic plastics sheet of polyethylene terephthalate film or the like with a magnetic layer formed thereon. The magnetic layer can be made by coating with a magnetic paint composed of magnetic particles in a binder, or a magnetic thin film of a ferromagnetic metal such as cobalt, nickel, iron, or alloys thereof can be deposited directly on the plastics sheet by sputtering, vacuum evaporation, ion plating or liquid phase plating. The magnetic sheets 2A and 2B are positioned so that the non-magnetic plastics surfaces thereof are kept in contact with the outer and inner reference planes 6A1 and 6A2, 6B1 and 6B2, respectively, on the upper and lower surfaces of the base 1, and the inner and outer portions are bonded respectively to the bevels 7A1 and 7A2, 7B1 and 7B2. The bonding agent is applied annularly on to the regions of the base 1 for bonding the magnetic sheets 2A and 2B thereto, that is, on the bevels 7A1 and 7A2, 7B1 and 7B2 and/or on to the regions of the magnetic sheets 2A and 2B corresponding to the bevels 7A1 and 7A2, 7B1 and 7B2. The glass transition temperature of the bonding agent should be substantially higher than room temperature, for example, more than 120 . x . C. It is desirable that the bonding agent should be of the thermosetting type such as an epoxy adhesive wherein an epoxy resin and a hardening agent are reacted upon heating. A typical hardening agent suitable for this use is, for example, dicyandiamide which exhibits almost no reaction at room temperature or at customary storage temperatures, but is rendered active when heated to 150 . x . C or so and reacts with the epoxy radical in the epoxy resin. One such epoxy bonding agent available is known as "XNR3505" which is a product of Ciba-Geigy Ltd. The joining operation and a jig to be used therefore will now be described. As shown in the exploded view of Figure 4, the jig includes a spacer ring 9 having an inner diameter greater than the outer diameter of the base 1, and having a thickness corresponding to the thickness t between the reference planes 7A1 and 7B1 of the base 1 and also between the reference planes 7A2 and 7B2. The assembly includes an anchor table 12 having a circular shallow recess 10 with an inner diameter greater than the outer diameter of the spacer ring 9 and also a circular deep recess 11 formed at its centre. In the recess 11, a shaft 13 is rotatably supported along its axis. A shaft support member 14 such as a ball bearing provides support for the shaft 13. At the inner end of the shaft 13 there is disposed a screw rod 15 which is held so as to be rotatable with the shaft 13, and has a screw thread about its peripheral surface. In the bottom portion of the recess 11, guide pins 16 are fixed upright about the axis of the recess 11. In the recess 11 there is provided a shift member 17 which is movable vertically relative to the anchor table 12 along the axis of the recess 11. The shift member 17 has through-holes 18 for the insertion of guide pins 16 and also has, in its central portion, a lead screw 19 engageable with the screw rod 15 so that when the screw rod 15 is rotated together with the shaft 13, the lead screw 19 is moved up or down along the screw rod consequently to cause a vertical motion of the shift member 17. On the top surface of the shift member 17 there is integrally formed an inner tilt ring 20 which has in its upper end face a slope formed corresponding to the inner bevel 7B2 on the lower surface of the base 1. In the outer portion, there is integrally formed an outer tilt ring 21 which has, on its upper end face, a slope formed corresponding to the outer bevel 7B1 on the lower surface of the base 1. In the central portion of the shift member 17 there is formed a shaft 23 which has a centre lead screw 22 which is insertable into the centre hole 4 of the base 1. If necessary, an O-ring 24 may be disposed in the outer bottom portion of the shallow recess 10. There is further provided an inner clamp member 25 for clamping the inner portions of the magnetic sheets 2A and 2B in co-operation with the shift member 17. The clamp member 25 is equipped on its lower end face with an inclined ring 251 which has a bevel corresponding to the inner bevel 7A2 of the base 1. The clamp 25 can be formed by a columnar body having along its central axis a clamping screw rod 26 engaged with the centre lead screw 22 of the shaft 23 on the shift member 17. An outer clamp member 27 is provided for clamping the outer portions of the magnetic sheets 2A and 2B jointly against the anchor table 12. The outer clamp member 27 is shaped in the form of a ring and has on its lower surface an annular projection 28 which is inserted into the shallow recess 10 of the anchor table 12. The clamp member 27 further has, at its periphery, a flange 29 abutting against the top end face of the outer portion of the anchor table 12, the flange 29 being furnished with set screws 30 which are driven into screw holes 31 formed in the top end face of the outer portion of the anchor table 12. With the structure as described above, the magnetic sheet 2B is placed in the shallow recess 10 of the anchor table 12 as illustrated in Figure 1. The magnetic layer of the magnetic sheet 2B faces the anchor table. The sheet 2B is formed so as to have an outer shape greater in diameter than the base 1 and corresponding to the inner shape of the recess 10. It has, in its central portion, a centre hole 32B which is smaller in diameter than the inner tilt ring 20 and through which the shaft 23 extends. The base 1 is set on the magnetic sheet 2B with a shaft 23 of the shift member 17 inserted into the centre hole 4, and the spacer ring 9 is disposed around the base 1. Subsequently the second magnetic sheet 2A is placed in the hollow recess 10 of the anchor table 12 astride the base 1 and the spacer ring 9. The non-magnetic base side of the sheet 2A faces the base 1. The sheet 2A is formed so as to have an outer shape greater in diameter than the base 1 and corresponding to the inner shape of the recess 10. It has, in its central portion, a centre hole 32A which is smaller in diameter than the tilt ring 251 of the inner clamp member 25 and through which the shaft 23 of the shift member 17 extends. Then the screw rod 26 of the inner clamp member 25 is engaged with the centre lead screw 22 of the shaft 23, and the inner clamp member 25 is tightened to the state where the two magnetic sheets 2A,2B and the base 1 are held between the two tilt rings 20 and 251. (In the state shown in Figure 1, the clamp member 25 is not tightened completely and therefore sufficiently firm clamping is not yet achieved). When the outer clamp member 27 is tightened on the anchor table 12 by driving the set screws 30 into the screw holes 31, the outer portions of the two magnetic sheets 2A and 2B are clamped while being held between the bottom of the recess or the O-ring 24 and the projection 28 of the clamp member27. In this state, the shaft 13 is rotated to move the shift member 17 upwardly so that, as illustrated in Figure 2, the clamped inner portion of the laminate of the two magnetic sheets 2A,2B and the base 1 interposed therebetween is lifted relative to the clamped outer portion in a direction perpendicular to the surface of the base 1. Since the two magnetic sheets 2A and 2B are clamped at the inner and outer portions thereof, the two sheets are stretched radially and are thereby rendered taut with tension existing between the two clamped portions. With the laminate of the base 1 and the two magnetic sheets 2A and 2B held in the anchor table by the inner clamp member 25 and the outer clamp member 27, the complete assembly is heated in a tunnel furnace or the like to a temperature within a range of about 100 to 180 . x . C for a predetermined time, such as a temperature of 150 . x . C for thirty minutes. The bonding agent is hardened to bond the magnetic sheets 2A and 2B to the bevels 7A1, 7A2, 7B1 and 7B2 of the base 1. At this stage, due to the existence of the grooves 8A1, 8A2, 8B1 and 8B2, any surface bonding agent is deposited into the grooves and is thereby prevented from flowing towards and into the annular grooves 5A and 5B in the base 1. In the same step as the heat treatment for hardening the bonding agent, it becomes possible to perform annealing for elimination of the stress caused in the magnetic sheets 2A and 2B as a result of stretching. The tension applied in joining the magnetic sheets 2A and 2B to the base can be adjusted by properly selecting the amount of lift of the clamped inner portion from the clamped outer portion in the above-described jig and the thickness of the spacer ring 9. In order finally to obtain a desired tension after annealing the magnetic sheets 2A and 2B, the tension to be applied in the above step is adjusted so as to be two to three times the final tension. When the magnetic sheets 2A and 2B are bonded to the upper and lower surfaces of the base 1, the peripheral regions of the magnetic sheets 2A and 2B projecting beyond the periphery of the base are severed and removed. Then, as illustrated in Figure 3, the magnetic sheets 2A and 2B positioned between the upper reference planes 6A1 and 6A2 and between the lower reference planes 6B1 and 6B2 of the base 1 can be stretched while flat with a predetermined tension so that a gap corresponding to the depth of the grooves 5A and 5B is maintained over the width W between the base 1 and each of the magnetic sheets 2A and 2B on the annular grooves 5A and 5B, whereby a desired tension type magnetic disc 3 is obtained. It is to be understood that the aforementioned jig can be modified in a variety of ways. As for the bonding agent, epoxy adhesive is convenient for handling, but other adhesives are usable as well. The bonding agent is not limited to thermosetting materials alone, and may be composed of a radiation curable type resin or other suitable material. According to the preferred embodiment of the present invention as described above, the magnetic sheets 2A and 2B positioned on the two surfaces of the base 1 are clamped at both the inner and outer portions thereof, and the clamped portions are displaced relatively from each other so that the two magnetic sheets 2A and 2B are stretched simultaneously with the required tension. Thus, in comparison with the previously-proposed process of stretching the magnetic sheets individually, the operation can be simplified considerably to bring about a substantial improvement. The jig used for performing the clamping and tensioning steps can be further utilised for hardening the bonding agent in a tunnel furnace and also for annealing the magnetic sheets 2A and 2B to reduce the stress caused in the stretching stage. Consequently, the process can be carried out as a continuous operation, leading to high mass productivity. The method can also result in magnetic discs of remarkably uniform quality.

Claims: A method of producing a magnetic disc (3) comprising a base (1) having a circular periphery interposed between two magnetic sheets (2A, 2B), each magnetic sheet (2A, 2B) being bonded to the base along two annular concentric regions (7A1, 7B1, 7A2, 7B2), the method comprising: forming a laminate of the base (1) sandwiched between the two magnetic sheets (2A, 2B) by superposing the magnetic sheets (2A, 2B) on upper and lower surfaces of the base (1) with a bonding agent being interposed between regions to be bonded; and clamping an inner annular portion of each of the sheet (2A, 2B) onto a portion of the base (1) and clamping an outer annular portion of the sheets (2A, 2B) located radially outward of the inner annular portion and outward of the periphery of the base (1); characterised by displacing the clamped inner annular portion including both the magnetic sheets (2A, 2B) and the base (1) relative to the clamped outer annular portion in a direction perpendicular to the surfaces of the base (1) so that the two sheets (2A, 2B) between the clamped inner portion and the periphery of the base (1) are radially stretched and the two sheets are each applied to the base along the annular concentric regions (7A1, 7B1, 7A2, 7B2); and bonding the magnetic sheets (2A, 2B) to the base (1) at the annular concentric regions (7A1, 7B1, 7A2, 7B2) while the annular portions are so displaced to bond the stretched sheets (2A, 2B) onto the base (1). A method according to claim 1, wherein the bonding is effected at an elevated temperature. A method according to claim 2, wherein the temperature is sufficient to anneal the magnetic sheets (2A, 2B) and partially relieve stresses formed therein. A method according to claim 3, wherein the stretching of the magnetic sheets (2A, 2B) during the displacing produces a tension substantially in excess of a desired tension, and the annealing reduces the tension to the desired tension. A magnetic disc (3) produced by a method according to any one of claims 1 to 4. Apparatus for manufacturing a stretched surface recording disc (3) comprising a base (1) sandwiched between two magnetic sheets (2A, 2B) which are secured at annular concentric regions (7A1, 7B1, 7A2, 7B2) to the base (1), the apparatus being characterised by: an anchor table (12) having a central recess (11); a shift member (17) positioned in the central recess (11) of the anchor table (12); means (13 to 15) to engage the shift member (17) for vertical displacement relative to the anchor table (12); means coacting with the shift member (17) to clamp an inner annular portion (7A2, 7B2) of each of the magnetic sheets (2A, 2B) and the base (1) to be laminated to the shift member (17) to move therewith; means (27) to clamp an outer annular portion of each of the magnetic sheets (2A, 2B) at a position outside of a circular periphery of the base so that when the shift member (17) and the clamped inner portion are shifted relative to the means for clamping the outer portions, each of the sheets is stretched radially on the base and is applied to the annular concentric regions (7A1, 7B1, 7A2, 7B2) and can be bonded thereto.

Inventors: AIZAWA TOSHIO C O SONY MAGNETI, AIZAWA, TOSHIO C/O SONY MAGNETIC
Applicants: SONY CORP, SONY CORPORATION
Application year: 1988
Main classification: G11B5
IPC codes: G11B5