Abstract:
A mushroom-type hook strap for mechanical fasteners has a series of hooks integrally formed on the strap, with each hook formed by a polygonal shape situated on top of an axis of a single stem, thereby forming a mushroom type hook having a polygonal head portion on the axis of the single stem, with protruding portions in substantially every direction for engaging fabric loops of a loop strap.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application is a divisional application of U.S. application Ser. No. 12/288,789 filed Oct. 23, 2008 and now U.S. Pat. No. 8,168,103 issued May 1, 2012, the contents of which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a method for making a hook strap for mechanical fasteners, and more specifically to a method for making a mushroom-type hook strap for mechanical fasteners and a mushroom-type hook strap made thereby. 
     BACKGROUND OF THE INVENTION 
     A hook-and-loop fastener is a well know fastener type and wildly adapted in a variety of fields that need to fasten two articles together, such as garment, hats/caps, sporting goods, etc. A hook-and-loop fastener mainly consists of a hook strap on which a plurality of plastic hooks are formed and a loop strap on which a plurality of fabric loops are formed and by engaging/disengaging the hooks on the hook strap and the loops on the loop strap, a fastening/unfastening function is provided. 
       FIGS. 1   a  and  1   b  illustrate a method for making a hook strap which generally includes: providing thermoplastic resin  1  into a plastic extruder  2 ; extruding melted thermoplastic resin  1  from the extruder  2  into a plurality of die cavities  4  formed in an outer periphery of a molding roller  3 ; cooling the molding roller  3  to form a hook strap  7  on which a plurality of upstanding stems  6  are molded; striping the hook strap off the molding roller  3  by a stripping roller  5 ; and feeding the hook strap through a nip formed between a pair of shaping rollers and the shaping roller that contacts the stems heats the free end of the stems to a temperature above a flow point of the thermoplastic reason to shape the stems into hooks with desired shape. 
       FIGS. 2 and 3  illustrate the structure of two existing mushroom-type hook strap respectively, wherein  FIG. 2  shows an array of circular-headed mushroom-type hooks ( FIG. 2   b ) that are formed from cylindrical stems ( FIG. 2   a ); and  FIG. 3  shows an array of elliptic-headed mushroom-type hooks ( FIG. 3   b ) that are formed from tetragon stems ( FIG. 3   a ). Although these two mushroom types of hooks both may provide certain fastening/unfastening function, they both have their own deficiencies. For instance, when engaging the circular-headed mushroom-type hooks illustrated in  FIG. 2  with the fabric loops on the loop strap, the effective engaging percentage is relative low because the smooth periphery of the circular head of the hooks does not provide protruding portions that can engage with the fabric loops, and thus has a drawback of unable to provide sufficient fastening strength. Additionally, circular-headed mushroom-type hooks can provide effective engaging only when the head of the hooks fully inserted into the fabric loops forming a lockup type engagement. However, although this lockup type of engagement may provide a firm engagement, the fabric loops usually will be destroyed by the hooks during a detaching operation between the hook strap and the loop strap, and after several times of engaging/detaching operations the fastening capability will drop sharply or even loosing the fastening capability because the percentage of the loops been destroyed increases as the engaging/detaching operation number increases. As a result, the effective lifetime of this circular-headed mushroom-type hooks is relative short. 
     Although the ends of the major axis of the elliptic-headed mushroom-type hooks illustrated in  FIG. 3  can form effective engagement with the fabric loops and thus can provide higher engagement percentage and larger fastening strength than above-mentioned circular-headed mushroom-type hooks, however, because this type of hooks form effective engagement with the fabric loops only at the ends of the major axis of the elliptic head, the effective fastening strength is provided only in a direction parallel the major axis of the elliptic head. The effective engagement with the loops in a direction perpendicular to the major axis of the elliptic head is much less than in the parallel direction. In other words, the fastening function provided by this type of hook-and-loop fastener is not an all-directional uniform fastening function, but a directional fastening function. Therefore, when the engagement between this type of elliptic-headed mushroom-type hooks and the loops is subjected to an external impact force in a specific direction (such as in a direction perpendicular to the major axis of the elliptic head), an unintentional disengaging may occur and pose a insufficient reliability problem to this type of hooks. 
     In view of the above-mentioned disadvantages/problems associated with the existing mushroom-type hook strap, there exists a need for a method for making a mushroom-type hook strap which has a higher engaging percentage with the loop strap, has a longer lifetime, and is capable of providing a reliable and all-directional uniform fastening function. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to provide a method for making a mushroom-type hook strap which has a higher engaging percentage with the loop strap, has a longer lifetime, and is capable of providing a reliable and all-directional uniform fastening function. This object may be achieved by a method for making a mushroom-type hook strap for mechanical fasteners. This method includes the steps of: 
     providing a rotational molding roller, the molding roller having a plurality of substantially Y-shaped molding cavities formed in an outer periphery surface thereof and a cooling device provided therein; 
     providing an extruder, the extruder having a extruding die disposed imminently close to the outer periphery surface of the rotational molding roller with a gap being formed therebetween; 
     feeding a thermoplastic material to the extruder and melting the thermoplastic material in the extruder so as to extrude the melted thermoplastic material into the plurality of substantially Y-shaped molding cavities and onto the outer periphery surface of the rotational molding roller through the extruding die; 
     cooling the thermoplastic material on the outer periphery surface of the rotational molding roller and within the substantially Y-shaped molding cavities with the cooling device so as to form a hook strap on which an plurality of hook preforms are molded, each preform having a substantially V-shaped free end; 
     peeling off the hook strap from the molding roller; and 
     feeding the hook strap through a nip formed between a pair of calender rolls, wherein the calendar roll that contacts the V-shaped free end of the hook preforms is heated to a temperature above a plastic deforming temperature of the plastic material so as to deform the V-shaped free end of each hook preform into a polygonal head shape and thereby forming a mushroom type hook having a polygonal head portion. 
     According to one embodiment of the present invention, the plan containing the two intersected arms of the V-shaped free end of the hook preform is parallel to the direction that the hook strap is fed through the calendar rolls. 
     According to another aspect of the present invention, a mushroom-type hook strap is produced by the method described above. 
     Features and objects of the present invention other than the above will become clear by reading the description of the present specification with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein: 
         FIGS. 1   a  and  1   b  are schematic views illustrating an apparatus form making traditional hook straps; and 
         FIGS. 2   a  and  2   b  are enlarged schematic perspective views showing the structure of one of existing mushroom-type hooks; 
         FIGS. 3   a  and  3   b  are enlarged schematic perspective views showing the structure of another existing mushroom-type hooks; 
         FIG. 4  is a schematic view illustrating an apparatus for making the inventive mushroom-type hook strap according to one embodiment of the present invention; and 
         FIG. 5  is an enlarged schematic perspective view showing the structure of the hook preforms before and after being fed through the calendar rolls. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS. 4 and 5  in which a preferred embodiment of the method of the present invention and a mushroom-type hook strap made thereby is illustrated to describe the present invention. 
       FIG. 4  is a schematic view illustrating an apparatus  10  for making the inventive mushroom-type hook strap according to one embodiment of the present invention. The apparatus  10  mainly includes a rotational molding roller  20 , an extruder  30 , a stripping roll  40  and a pair of calendar rollers  50  and  52 . The molding roller  20  has a plurality of substantially Y-shaped molding cavities  22  formed in an outer periphery surface thereof and a cooling device (not shown), such as a cooling water circulating system, provided therein. The extruder  30  includes a melting chamber  32  for melting the plastic material, and the melting chamber  32  is surrounded by heating coils  33 . The extruder  30  further includes a feed tank  34  disposed at one end of the melting chamber  32  and an extruding die  36  operationally coupled to the other end of the melting chamber  32 . The extruding die  36  is disposed imminently close to the outer periphery surface of the rotational molding roller  20  with a gap being formed therebetween. This gap may be varied by moving the extruding die  36  relative to the outer periphery surface of the rotational molding roller  20 . 
     When executing the inventive method of the present invention, a thermoplastic material (for example, polyethylene) is firstly supplied to the feed tank  34  and through which the thermoplastic material is fed into the extruder  30 . The thermoplastic material is melted in the melting chamber  32  by the heating coils  33  (for example, the temperature in the melting chamber is heated up to 235° C.). The melted thermoplastic material is then ejected through the extruding die  36  onto the outer periphery surface of the rotational molding roller  20  and into the plurality of substantially Y-shaped molding cavities  22 . After the thermoplastic material that on the outer periphery surface of the rotational molding roller  20  and within the plurality of substantially Y-shaped molding cavities  22  is cooled and solidified by the cooling device of the molding roller  20 , it is peeled off from the molding roller  20  by a stripping roll  40  to formed a hook strap  60  and the hook strap  60  includes a backing  62  and a plurality of hook preforms  64  integrally molded on the backing  62 . The hook strap  60  is then fed through a nip formed between the calendar rolls  50  and  52 . 
     In the embodiment shown in  FIG. 5 , each of the preforms  64  is molded into a polygonal peg (such as a tetragon peg as shown in the drawing) and includes a stem portion  66  and a substantially V-shaped free end  68 . In addition, in the embodiment shown in  FIG. 5 , the plane that contains the intersected arms of the V-shaped free end of the hook perform  64  is parallel to a direction D that the hook strap  60  is fed through the calendar rolls  50  and  52 . 
     The calendar roll  50  that contacts the V-shaped free ends of the hook preforms  64  is heated to and maintained at a plastic deforming temperature of the thermoplastic material. For example, in the case that polyethylene is used as the thermoplastic material the calendar roll  50  is heated to and maintained at 130° C. Additionally, a nip is formed between the calendar rolls  50  and  52  and this nip can be adjusted such that the nip is smaller than a sum of the thickness of the backing  62  and the height of the preform  64  and substantially equal to the height of the stem portion  66  of the hook preform  64 . Thereby, while being fed through the calendar rolls  50  and  52 , the V-shaped free end  68  of the preform  64  will be plastically deformed as a polygonal head portion  68 ′, as shown in  FIG. 5 . 
     Because the hooks of the hook strap produced by the method of the present invention has a polygonal head portion  68 ′, a number of protruding portions are provided by the head portion  68 ′ to form effective engagement with the loops of a loop strap and a higher engaging percentage can thus be provided. In addition, because the hooks of the hook strap of the present invention engage the loops of a loop strap with the protruding portions of polygonal head portion, no loops will be destroyed during a detaching operation and thus a longer lifetime for the hook strap may be provided. Moreover, the polygonal head portion of the hooks of the hook strap of the present invention provides protruding portions in substantially every direction for engaging the fabric loops of a loop strap and thus the hook strap of the present invention can provide a uniform engaging function in substantially all direction. 
     As illustrated in  FIG. 5 , the polygonal head portion  68 ′ has a flat top in a direction normal to an axis of the stem  66  which is tetragonal in shape and has concave-shaped surfaces on two opposing sides. As illustrated, the protruding portions protrude in eight different directions normal to the stem  66  axis. 
     Although the present invention has been described above according to the preferred embodiment thereof, this does not mean that the scope of the present invention is limited to specific structure and operation steps described above. In fact, there exist various modifications and variations under the principle and spirit disclosed above. For instance, an auxiliary cooling device, such as a cooling fan, may be disposed relative to the molding roller  20  at a position diametrically opposite to the extruding die  36  to provide extra cooling effect to the hook strap on the outer periphery surface of the molding roller  20 . 
     It will be apparent to people skilled in this art that many modifications can be made to the disclosed structures/steps without departing from the scope of the invention. Therefore, it is the intent of the appended claims to cover all such variations and modifications as come within the sprit and scope of this invention.