Abstract:
In manufacturing a magnetic tape, a long web of non-magnetic base material is coated with a magnetic layer and, thereafter, subjected to a smoothing process, wherein the surface of the magnetic layer is abraded, ground, and then wiped out. For grinding, sapphire blades are kept in contact with the surface of the magnetic layer with their edges oriented in a transverse direction to the magnetic tape, and are moved back and forth in the transverse direction to the magnetic tape, while the magnetic tape is being conveyed in a lengthwise direction of the magnetic tape. Of the sapphire blades, adjacent two are moved in opposite directions to each other.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a magnetic recording medium and a method of smoothing a surface of the magnetic recording medium.  
           [0003]    2. Background Arts  
           [0004]    A magnetic tape is widely used as a magnetic recording medium for recording audio signals and/or video signals. To manufacture the magnetic tape, a surface of a long web of non-magnetic base film is coated with a magnetic layer that is made by dispersing a magnetic material and a binder material in a solvent, wherein the binder material is composed of a resin and a curing agent. Thereafter, the magnetic layer is subjected to treatments for orienting the magnetic fields and the like. After drying the magnetic layer, the long web coated with the magnetic layer is slit into a predetermined width, and is coiled into a roll.  
           [0005]    Because the magnetic tape manufactured in this way has many granular components, like insufficiently-fixed ferromagnetic powers, on the surface of the magnetic layer. These insufficiently-fixed granular components can drop off the magnetic tape as the tape is moved by a magnetic head, and stain the surface of the magnetic head, damaging the quality of the recorded or reproduced signal. As the ferromagnetic powers removes off the magnetic layer, the electromagnetic conversion characteristics of the magnetic recording medium deteriorates, which appears as an output power reduction.  
           [0006]    To avoid this problem, a process of smoothing the surface of the magnetic layer is carried out prior to slitting the magnetic tape. In the smoothing process, the surface of the magnetic layer is abraded by means of an abrasive tape, and then ground by means of a grinding tool that is made of a very hard material, like artificial sapphire or ruby, or carbide alloy, thereby to remove the insufficiently-fixed granular components or extraneous substances deposited on the magnetic layer. Thereafter, because shavings produced by the abrading and grinding of the magnetic layer may be deposited on the surface of the magnetic layer, the magnetic tape is subjected to a cleaning process for cleaning the shavings off the magnetic layer.  
           [0007]    The grinding tool used in the smoothing process for the magnetic layer has conventionally been fixed in position, so the contact position of the grinding tool with the magnetic layer is maintained unchanged. As a result, the grinding tool is worn off earlier around the contact position, which results a stepped portion on the edge of the grinding tool. The stepped portion provides scratches or scuff marks on the magnetic layer.  
         SUMMARY OF THE INVENTION  
         [0008]    In view of the foregoing, an object of the present invention is to provide a magnetic recording medium free from scuff marks and scratches on its surface.  
           [0009]    Another object of the present invention is to provide a method of smoothing a surface of a magnetic recording medium.  
           [0010]    In a magnetic recording medium manufactured by coating a long web of non-magnetic base material with a magnetic layer, according to the present invention, a surface of the magnetic layer of the magnetic recording medium is ground by at least a grinding tool that is moved back and forth in a transverse direction to the long web, while the magnetic recording medium is being conveyed in a lengthwise direction of the long web.  
           [0011]    Since the grinding tool is moved back and forth in the transverse direction, the grinding tool would not be kept contact in the same portion with the magnetic recording medium, but the portion of the grinding tool that is in contact with the magnetic recording medium and is thus grinding the surface of the magnetic layer changes continually. Accordingly, the problem of partial abrasive wear of the grinding tool would not arise, so the grinding tool is prevented from having a stepped portion on its edge. Thus, the magnetic recording medium of the present invention is free from the scuff marks or the scratches that could otherwise be caused by the stepped portion.  
           [0012]    According to another aspect of the present invention, a method of smoothing a surface of a magnetic layer of a magnetic recording medium after manufacturing the magnetic recording medium by coating a long web of non-magnetic base material with the magnetic layer, comprising the steps of putting at least a grinding tool on the surface of the magnetic layer with an edge of the grinding tool oriented in a transverse direction to the long web; and moving the grinding tool back and forth in the transverse direction while conveying the magnetic recording medium in a lengthwise direction of the long web.  
           [0013]    According to a preferred embodiment, a plurality of the grinding tools are arranged at regular intervals along the conveying direction of the magnetic recording medium, and adjacent two of the grinding tools are moved in opposite directions to each other. Thereby, the magnetic recording medium is prevented from fluctuating in the widthwise direction during the grinding.  
           [0014]    It is preferable to move the grinding tool at a high speed in initial and final stages of one stroke, and at a low speed in a middle stage of one stroke. This configuration enforces the effect of preventing the partial abrasive wear of the edge of the grinding tool. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The above and other objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when read in association with the accompanying drawings, which are given by way of illustration only and thus are not limiting the present invention. In the drawings, like reference numerals designate like or corresponding parts throughout the several views, and wherein:  
         [0016]    [0016]FIG. 1 schematically shows a smoothing apparatus for smoothing the surface of a magnetic tape, embodying the method of the present invention;  
         [0017]    [0017]FIG. 2 shows a perspective view of a grinding section of the smoothing apparatus; and  
         [0018]    [0018]FIG. 3 shows a diagram illustrating a relationship between reciprocation of sapphire blades of the grinding section and moving speed. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0019]    A smoothing apparatus  10  shown in FIG. 1 consists of an abrading section  11 , a grinding section  12  and a wiping section  13 , to carry out abrasion, grinding and wiping of the surface of a magnetic layer of a magnetic tape  15  successively. The magnetic tape  15  is wound into a roll  15   a,  and is supplied from the roll  15   a  in a direction shown by an arrow “A”, and is conveyed by a plurality of feed rollers  17  through from the abrading section  11 , the grinding section  12  to the wiping section  13 , and is rewound into a take-up roll  15   b.    
         [0020]    Tension rollers  18  are disposed in the abrading section  11 , the grinding section  12  to the wiping section  13 . The tension rollers  18  rotate following to the conveying movement of the magnetic tape  15 , and are connected to a tension mechanism  19 . The tension mechanism is controlled such that the magnetic tape  15  is brought into contact with an abrasive tape  22 , sapphire blades  25 ,  26 ,  27  and  28  and a non-woven tape  42  with a proper tension. The proper tension is preferably 30 g to 300 g per one tape, while the magnetic tape  15  is preferably conveyed at a speed of 7 m/sec to 10 m/sec.  
         [0021]    The abrading section  11  is constituted of a supply roll  23   a  and a take-up roll  23   b  of the abrasive tape  22  and a pad or post  21  for the abrasive tape  22 . While the magnetic tape  15  is being conveyed, the supply roll  23   a  and the take-up roll  23   b  are rotating to feed the abrasive tape  22  in an opposite direction to the conveying direction “A” of the magnetic tape  15 , and the pad  21  presses the abrasive tape  22  onto the magnetic layer of the magnetic tape  15 . Thus the abrasive tape  22  abrades the surface of the magnetic layer. The abrasive tape  22  is manufactured by coating a base material with an abrasive coat that is composed of an abrasive agent dispersed in a binder, and then slitting the base material into a width after drying the abrasive coat. The base material may be a synthetic resin film or sheet, e.g. a polyethylene terephthalate film, or a non-magnetic metal foil, e.g. an aluminum foil, or a metal foil, e.g. a stainless steel foil, or paper, ceramic sheet or the like. The abrading agent may be composed of a combination of materials having a Mohs hardness of 5 to 9, e.g. diamond, SiO 2 , Cr 2 O 3 , ZnO 2 .  
         [0022]    The grinding section is constituted of the sapphire blades  25  to  28 , holders  29  for holding the sapphire blades  25  to  28 , and a driving mechanism  30  for driving the holders  29  to move back and forth in a transverse direction to the magnetic tape  15 , i.e. a widthwise direction of the magnetic tape  15 . The sapphire blades  25  to  28  are ordinary sapphire blades having a shape of a triangular prism each, and are aligned along the conveying direction or lengthwise direction of the magnetic tape  15  at predetermined intervals. The edge of each sapphire blade  25  to  28  is oriented transversely to the magnetic tape  15 , i.e. across the width of the magnetic tape  15 , and is kept in contact with the surface of the magnetic layer of the magnetic tape  15  by the tension rollers  18 .  
         [0023]    As shown in FIG. 2, the holders  29  are mounted in a base frame  31  with their guide grooves  29   a  engaged with guide rails  31   a  of the base frame  31 . The guide rails  31   a  extend perpendicularly to the conveying direction “A” of the magnetic tape  15 , such that the holders  29  are movable along the guide rails  31   a  back and forth in the transverse direction to the magnetic tape  15 , as indicated by arrows “B” and “C”. A coiled spring  32  is disposed between a top side of each holder  29  and a top wall  31   b  of the base frame  31 , so as to urge the holders  29  to move in one direction shown by the arrow “B”, i.e. downward in the drawings. A drive pin  29   b  is provided on the bottom of each holder  29 . Since the holders  29  are urged downward, the drive pins  29   b  are put down through holes  31   d  of a middle wall  31   c  of the base frame  31 , and are kept in contact with peripheral surfaces of cam members  33   a,    33   b,    33   c  and  33   d.    
         [0024]    The cam members  33   a  to  33   d  are coupled to each other through a rotary shaft  34 , and rotate together with the rotary shaft  34 . The rotary shaft  34  is pivotally held between a pair of holding arms  35  that are formed on a bottom wall  31   e  of the base frame  31 . A transmission gear  34   a  is securely mounted on one end of the rotary shaft  34 . The transmission gear  34   a  is coupled to a drive gear  36   a  of a motor  36  through a not-shown transmission mechanism, so the rotary shaft  34  and thus the cam members  33   a  to  33   d  are turned by the driving force of the motor  36 . The drive pins  29   b,  the cam members  33   a  to  33   d,  the rotary shaft  34 , the motor  36  and the gears  34   a  and  36   a  constitute the driving mechanism  30 . As the cam members  33   a  to  33   d  rotate, the drive pins  29   b  move up and down along the peripheral surfaces of the cam members  33   a  to  33   d,  so the holders  29  and thus the sapphire blades  25  to  28  move back and forth in the transverse direction to the conveying direction “A” of the magnetic tape  15 , as shown by the arrows “B” and “C”.  
         [0025]    Accordingly, the contact position of the sapphire blades  25  to  28  with the magnetic tape  15  changes continually, so the edges of the sapphire blades  25  to  28  will not be partly worn out, or a stepped portion will not easily be formed on the edge of the sapphire blade. Consequently, the magnetic tape  15  will not be scratched or scuffed by the stepped portions on the edges of the sapphire blades  25  to  28 . In addition, the sapphire blades  25  to  28  last longer in comparison with those mounted stationary.  
         [0026]    The cam members  33   a  and  33   c  for the sapphire blades  25  and  27  are mounted on the rotary shaft  34  at an angle that is staggered 180 degrees from an angular position of other two cam members  33   b  and  33   d  for the sapphire blades  26  and  28 . According to this configuration, the sapphire blades  25  and  27  always move in the opposite direction to the sapphire blades  26  and  28 . That is, the sapphire blades  25  and  27  move in the direction “B” as the sapphire blades  26  and  28  move in the direction “C”. On the other hand, the sapphire blades  25  and  27  move in the direction “C” as the sapphire blades  26  and  28  move in the direction “B”. This configuration is effective to prevent the magnetic tape  15  from being fluctuated by the movement of the sapphire blades  25  to  28 , in the widthwise direction of the magnetic tape  15 , i.e. in the up-down direction in the illustrated embodiment.  
         [0027]    Furthermore, the contour of the peripheral surface of each cam member  33   a  to  33   d  is defined such that each sapphire blade  25  to  28  changes its speed during one stroke. FIG. 3 shows the change in speed of the sapphire blades  25  to  28  during the strokes, wherein a characteristic curve  38  shown by a solid line represents the moving speed of the sapphire blades  25  and  27 , and a characteristic curve  39  shown by chain-dotted lines represents the moving speed of the sapphire blades  26  and  28 . As apparent from these curves in FIG. 3, each sapphire blade  25  to  28  is moved at a high speed in initial and final stages of one stroke, and is moved at a low speed in a middle stage of one stroke. Changing the moving speed of the sapphire blades  25  to  28  in this way during one stroke more effectively prevents the abrasive wear in the same portion of the edge of the sapphire blade  25  to  28 , and thus prevents formation of steps in the edge.  
         [0028]    One stroke Ls of each sapphire blade is preferably 1 mm to 2 mm in amplitude, and the time T taken for one reciprocation of each sapphire blade is preferably 10 seconds to 3 minutes, and more preferably 10 seconds to 1 minute.  
         [0029]    Reference is made again to FIG. 1. In the wiping section  13 , the non-woven tape  42  is fed in the opposite direction to the conveying direction “A” of the magnetic tape  15 , by rotating a supply roll  43   a  and a take-up roll  43   b.  As the non-woven tape  42  is fed from the supply roll  43   a  to the take-up roll  43   b,  a pad  41  presses the non-woven tape  42  onto the surface of the magnetic layer of the magnetic tape  15 , thereby to wipe out extraneous substances, like shavings produced by the abrading and grinding in the abrading section  11  and the grinding section  12 . The non-woven tape  42  is made of a single-layered suede-like non-woven fabric that is formed from densely interlocked bundles of polyester fibers and does not substantially contain a binder, like polyurethane, or may be made of a non-woven fabric that is formed by bonding polyester fibers with a binder, like polyurethane.  
         [0030]    Now the operation of the smoothing apparatus  10  will be briefly described.  
         [0031]    In response to a start command entered in the smoothing apparatus  10 , the magnetic tape  15  is fed out from the supply roll  15   a  and is conveyed through the feed rollers  17 . The tension mechanism  19  applies a predetermined tension to the magnetic tape  15  through the tension rollers  18 . The magnetic tape  15  first goes through the abrading section  11 . In the abrading section  11 , the abrasive tape  22  is brought into contact with the surface of the magnetic layer of the magnetic tape  15 , and is moved in the opposite direction to the conveying direction “A” of the magnetic tape  15 , thereby to abrade the surface of the magnetic layer.  
         [0032]    Thereafter, the magnetic tape  15  goes through the grinding section  12 . In the grinding section  12 , the driving mechanism  30  drives the holders  29  of the sapphire blades  25  to  28  such that the sapphire blades  25  to  28  move back and forth in the transverse direction to the conveying direction “A” of the magnetic tape  15 , each individually in the opposite direction to the adjacent sapphire blades, while the edges of the sapphire blades  25  to  28  are being kept in contact with the surface of the magnetic layer of the magnetic tape  15  by the tension rollers  18 . The surface of the magnetic layer is thus ground by the sapphire blades  25  to  28 . Through the abrasion and the grinding, granular components like insufficiently-fixed ferromagnetic powders, are removed from the surface of the magnetic layer of the magnetic tape  15 .  
         [0033]    Finally, the magnetic tape  15  goes through the wiping section  13 . In the wiping section  13 , the non-woven tape  42  is brought into contact with the surface of the magnetic layer of the magnetic tape  15 , and is moved in the opposite direction to the conveying direction “A” of the magnetic tape  15 , thereby to wipe out the extraneous substances, like shavings produced by the abrading and grinding of the magnetic layer. After passing through the abrading, grinding and wiping sections  11  to  13 , the magnetic tape  15  is rewound into the take-up roll  15   b.    
         [0034]    In the illustrated embodiment, the sapphire blades are moved back and forth in the direction rectangular to the lengthwise direction of the magnetic tape, it is possible to move the sapphire blades obliquely to the lengthwise direction of the magnetic tape.  
         [0035]    Although the sapphire blades are used as the grinding tools in the above embodiment, the grinding tools may be blades made of alumina, cermet, zirconia, silicone nitride, silicone carbide, diamond, carbide alloy and the like. Although the wiping is done only on the surface of the magnetic layer of the magnetic tape in the above embodiment, it is possible to do the wiping on the reverse surface of the magnetic tape.  
         [0036]    Thus, the present invention is not to be limited to the above embodiment but, on the contrary, various modifications will be possible to those skilled in the art without departing from the scope of claims appended hereto.