Abstract:
A method of manufacturing a roller includes: forming a hollow cylindrical body that includes a fully solidified outer peripheral surface and a semi-solidified interior, with an inner peripheral surface swelling inwardly; and pressing the swollen portion of the inner peripheral surface outwardly by pulling an insert die from the semi-solidified cylindrical body, to thereby allow the outer periphery of the cylindrical body to swell outwardly in a barrel-like manner. A primary molded product including a through hole along a central axis is thereby formed, and a secondary resin is injected into the through hole of the primary molded product.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method of manufacturing a roller for guiding a tape. More particularly, the invention relates to a method of manufacturing a barrel-shaped roller including an outer periphery that swells at an intermediate portion thereof.  
           [0003]    2. Description of the Related Art  
           [0004]    Generally, in audio and video apparatus, a magnetic tape is often used to record and play back information. The magnetic tape is wound around a pair of reels and conveyably retained within a cassette case.  
           [0005]    For example, as shown in FIG. 8, a magnetic tape cassette  100  employed for business use at a broadcasting station includes an upper half  102  and a lower half  104  that form a case. A pair of reels  109  is rotatably supported inside the case. Each reel compirses a lower flange  105 , a hub  106  that is integrally fixed to an upper portion of the lower flange  105 . Each reel comprises, and an upper flange  108  that is mounted to an upper portion of the hub  106 . A magnetic tape T is wound around the pair of the reels  109 .  
           [0006]    A guide roller  114  for guiding the magnetic tape T when the magnetic tape T is played or rewound is provided in the vicinity of an opening at the front side of the magnetic tape cassette  100 . The guide roller  114  is rotatably supported by a shaft  110  that is generally supported from below by the lower half  104 .  
           [0007]    The guide roller  114  includes an outer periphery whose intermediate portion swells like a barrel in order to ensure traveling stability of and efficiently guide the magnetic tape T.  
           [0008]    The guide roller  114  is manufactured in accordance with the procedures described below in order to prevent a parting line (PL) from being formed on the surface of the guide roller  114 .  
           [0009]    First, as shown in FIGS. 9A and 9B, a cylindrical primary molded product  118  is formed by injection-molding.  
           [0010]    Next, as shown in FIG. 10, molten resin M for secondary molding is injected into the interior of the primary molded product  118 . As shown in FIG. 11, the primary molded product  118  swells due to pressure exerted by the molten resin M, whereby the primary molded product  118  is formed into a barrel shape. The injected molten resin M and the primary molded product  118  are molded integrally with each other to form a secondary molded product. Consequently, a barrel-shaped guide roller  114  is formed (see FIG. 8).  
           [0011]    However, setting the thickness, diameter, and molding temperature of the primary molded product  118  is difficult when the primary molded product  118  is swollen by molten resin M being injected therein. There has thus been a strong demand for an easy and efficient method of manufacturing barrel-shaped rollers.  
           [0012]    This demand is not limited to barrel-shaped rollers used for magnetic tape cassettes, but includes general-use rollers that are formed into a barrel shape and used for guiding a tape.  
         SUMMARY OF THE INVENTION  
         [0013]    In view of the aforementioned facts, an object of the present invention is to provide a method by which an excellent barrel-shaped roller can be easily manufactured.  
           [0014]    A first aspect of the present invention is a method of manufacturing a roller including an outer periphery that swells in a barrel-like manner, comprising the steps of: (a) forming a hollow cylindrical body that includes a uniform outer peripheral surface and an inner peripheral surface that swells inwardly; and (b) pressing the swollen portion of the inner peripheral surface outwardly.  
           [0015]    Accordingly, a parting line is not formed on a roller surface, whereby an excellent barrel-shaped roller can be easily manufactured.  
           [0016]    In the method of the first aspect, the cylindrical body is molded with an insert die, which includes a central portion having formed thereat a curved recess, and an outer mold, which includes an inner peripheral surface that defines an outer periphery of a cavity formed by joining the outer mold with the insert die, the inner peripheral surface of the outer mold being uniformly flat.  
           [0017]    When the roller is formed by injection-molding, the aforementioned cylindrical body is molded with an insert die, which includes a central portion having formed thereat a curved recess, and an outer mold, which includes an inner peripheral surface that defines an outer periphery of a cavity formed by joining the outer mold with the insert die, the inner peripheral surface of the outer mold being uniformly flat.  
           [0018]    Namely, resin is injected in a cavity defined by the insert die and the outer mold. Then, the outer mold is released in a state in which the cylindrical body is semi-solidified. Further, the cylindrical body is pulled from the insert die to press the swelling portion outwardly towards the outer periphery of the cylindrical body. Therefore, the inwardly swelling portion is pressed to thereby swell the outer periphery of the semi-solidified roller equal to the volume of the resin remaining at the inwardly swelling portion.  
           [0019]    The timing at which the semi-solidified roller is pulled from the insert die is determined by considering the material of the resin to be injected, and the entire dimension and thickness of the roller obtained by molding. However, the timing is generally determined by pulling the cylindrical body from the insert die when a skin layer of solidified injected resin is formed. Consequently, it may be easy to form the outer periphery of the semi-solidified roller in a barrel-like manner.  
           [0020]    Further, in the method of the first aspect, further comprises injecting resin into the hollow portion of the cylindrical body after step (b).  
           [0021]    After molding the primary molded product by swelling the outer periphery of the cylindrical body into a barrel-like manner, resin can be injected into the hollow portion of the primary molded product to mold the secondary molded product. Accordingly, the interior of the through hole of the primary molded product can be formed into a desired shape.  
           [0022]    A second aspect of the present invention is a method of manufacturing a roller including an outer periphery that swells in a barrel-like manner, the method using an insert die and an outer mold, the outer mold including an inner peripheral surface that defines an outer periphery of a cavity formed by joining the outer mold with the insert die, with the inner peripheral surface of the outer mold being cylindrical and uniformly flat, the method comprising the steps of: (a) injecting resin material into the cavity and disposing the resin material along an outer peripheral surface of the insert die to thereby form a hollow cylindrical body that includes a uniform outer peripheral surface and an inner peripheral surface including a portion that swells inwardly; (b) releasing the outer mold; and (c) pressing the swollen portion of the inner peripheral surface outwardly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a developed perspective view of a magnetic tape cassette according to an embodiment of the present invention;  
         [0024]    [0024]FIG. 2 is a side cross-sectional view of a mold used for manufacturing a barrel-shaped roller;  
         [0025]    [0025]FIG. 3 is a side cross-sectional view showing resin injected into a cavity of the mold;  
         [0026]    [0026]FIG. 4 is a side cross-sectional view of a semi-solidified roller from which a fixing mold has been released;  
         [0027]    [0027]FIG. 5 is a side cross-sectional view of a movable mold that has been pulled from the state shown in FIG. 4;  
         [0028]    [0028]FIG. 6 is a side cross-sectional view showing manufacture of a secondary molded product by injecting secondary resin into a primary molded product;  
         [0029]    [0029]FIG. 7 is a cross-sectional perspective view of a barrel-shaped roller manufactured by injecting the secondary resin;  
         [0030]    [0030]FIG. 8 is a developed perspective view of a conventional magnetic tape cassette;  
         [0031]    [0031]FIG. 9A is a side cross-sectional view of a primary molded product that is formed during manufacture of a conventional barrel-shaped roller;  
         [0032]    [0032]FIG. 9B is a plan view of the primary molded product of FIG. 9A;  
         [0033]    [0033]FIG. 10 is a side cross-sectional view showing secondary resin being injected into the primary molded product of FIGS. 9A and 9B; and  
         [0034]    [0034]FIG. 11 is a side cross-sectional view of the conventional barrel-shaped roller that is formed by swelling of the primary molded product into a barrel shape due to pressure exerted by injecting the secondary resin. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0035]    Description of an embodiment of the present invention will now be given. As shown in FIG. 1, a magnetic tape cassette (betacam L cassette)  10  that is used for business use by a broadcasting station includes an upper half  12  and a lower half  14  that form a case.  
         [0036]    A pair of reels  20  is rotatably supported inside the case. Each of the pair of the reels  20  has a lower flange  24 , a hub  26  that is integrally fixed to an upper portion of the lower flange  24 , and an upper flange  28  that is mounted to an upper portion of the hub  26 . A magnetic tape T is wound around the reels  20 . A window  13  made by a clear plate is disposed at the upper half  12 .  
         [0037]    A pair of compression coil springs  30 A and  30 B is interposed between the upper half  12  and the pair of the reels  20 . The compression coil springs  30 A and  30 B urge the corresponding reels  20 A and  20 B toward the lower half  14 .  
         [0038]    A pair of guide rollers  44  is disposed in the vicinity of an opening  16  at the front side of the magnetic tape cassette  10  to guide the magnetic tape T when it is played or rewound.  
         [0039]    The magnetic tape cassette  10  includes tape pads  46  that slidably contact and press an undersurface of the magnetic tape T with an appropriate urging force to thereby prevent magnetic tape T from slackening.  
         [0040]    The magnetic tape cassette  10  includes a pair of shafts  50  that are typically fixed to and project from an inner wall surface  14 U of the lower half  14 . Each of the shafts  50  penetrates a through hole  44 H that is formed along the central axis of each guide roller  44 . The guide rollers  44  are axially and rotatably supported around the shafts  50 .  
         [0041]    The guide rollers  44  are formed by resin such as polyacetal (POM) that is slick and have comparatively high mechanical strength.  
         [0042]    Each guide roller  44  is injection-molded so that its outer periphery is swollen (see FIG. 7) in order to ensure traveling stability of and efficiently guide the magnetic tape T.  
         [0043]    [0043]FIG. 2 shows a metal mold  60  for injection-molding the guide rollers  44 . The mold  60  includes a fixed mold  62 , which has a flat cylindrical inner peripheral surface  63 , and a movable mold  64  that can move vertically.  
         [0044]    A cylindrical insert die  70  projects perpendicularly from the movable mold  64  and includes an outer peripheral surface  71 . The outer peripheral surface  71  includes a central portion having formed thereat a curved recess  72 . When the insert die  70  is seen in cross section, the recess  72  appears as a substantial arc that gradually tapers inward (see FIG. 2).  
         [0045]    A pin-like die  69  that is used at the time of secondary molding is disposed on the insert die  70  so as to protrude vertically from the central axis of the insert die  70 .  
         [0046]    When manufacturing the barrel-shaped guide roller  44  with the mold  60 , the movable mold  64  is set in accordance with the fixed mold  62  at a position at which resin is injected (see FIG. 2). Then, molten resin is injected from a gate (not shown) into a cavity  80  formed between the fixed mold  62  and the movable mold  64 .  
         [0047]    After the molten resin has been injected (see FIG. 3), a surface of the injected resin solidifies to form a skin layer. A cylindrical semi-solidified roller  82  (see FIG. 4), whose interior has not yet fully solidified, is thereby formed. The fixed mold  62 , which is the outer mold is released to thereby free the outer periphery of the semi-solidified roller  82 .  
         [0048]    The lower end of the freed semi-solidified roller  82  is supported by an ejector plate  68 , which is a component of the movable mold  64 . Further, an inwardly swelling portion  84  is formed by resin remaining in a space formed by the recess  72 .  
         [0049]    As shown in FIG. 5, when the ejector plate  68  is lifted up, while the inwardly swelling portion  84  is pressed outwardly by an upper portion  70 U, which is disposed above the recess  72  of the insert die  70 , and the semi-solidified roller  82  is forcibly ejected from the insert die  70 , which is a so-called “die removal by force”. As a result, an outwardly swelling portion  86  is formed around the outer periphery of the semi-solidified roller  82 , and the semi-solidified roller  82  is formed into a barrel-shape. A through hole  78 , whose inner diameter is substantially the same as that of the upper portion  70 U, is formed in the roller. Consequently, a primary molded product  79  is finished.  
         [0050]    Thereafter, as shown in FIG. 6, molten resin for secondary molding is injected into the through hole  78  to form a secondary resin portion  88 . The secondary resin portion  88  solidifies and is integrated with the primary molded product  79 , whereby a barrel-shaped guide roller  44  is formed (see FIG. 7).  
         [0051]    It should be noted that the guide roller  44  can be molded by preparing an exclusive mold for molding the secondary molded product and using the exclusive mold to inject secondary resin into the through hole  78  of the primary molded product  79 . In this case, since the primary molding process and the secondary molding process are respectively carried out with mutually different molds, a secondary molded product can be formed during the molding of the primary molded product, whereby manufacturing efficiency can be improved. Further, since the pin-like die  69  does not need to be disposed at the insert die  70 , the metal mold can be structured more simply.  
         [0052]    The size of the cavity  80  is determined on the basis of the desired size of the outwardly swelling portion  86 . With reference to FIG. 2, an example of the cavity  80  will be explained. A length L2 of the recess  72  is 7.5 mm. A distance DC between the deepest portion of the recess  72  and the inner peripheral surface  63  of the fixed mold  62  is 0.5 mm. A length L1 from the uppermost end of the insert die  70  to the uppermost end of the recess  72  is 5 mm. A distance DU between the upper portion  70 U of the insert die  70  and the inner peripheral surface  63  is 0.4 mm. A length L3 from the lowermost end of the recess  72  and the ejector plate  68  is the same as L1 (i.e., 5 mm). A distance DL between the insert die  70  and the inner peripheral surface  63  is the same as DU (i.e.,  0 . 4  mm).  
         [0053]    As described above, in accordance with the present embodiment, an excellent barrel-shaped roller can be formed efficiently and quickly in a more simple manner.  
         [0054]    An embodiment of the invention has been described above. However, the invention is not limited to this embodiment, and can be modified without departing from the spirit and the scope of the invention. For example, methods of manufacturing barrel-shaped rollers for tape cassettes other than magnetic tape cassettes can be included in the scope of the invention.