Patent Publication Number: US-2006011030-A1

Title: Rotary die cutter for forming a non-linear line of perforations in a strip of material

Description:
This application is a divisional of application Ser. No. 10/394,360 entitled “A ROTARY DIE CUTTER FOR FORMING A NON-LINEAR LINE OF PERFORATIONS IN A STRIP OF MATERIAL” and filed in the U.S. Patent and Trademark Office on Mar. 21, 2003, the entirety of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      Today, there exist many articles and packages that include a strip of material that has at least one tearable line of perforations formed therein. The tearable line of perforations can be torn open so as to open the article or to open the package in which the article is contained. Various packaging materials use a tear strip or a tearable line of perforations to enable the end user to easily open the package. Some disposable absorbent articles, such as infant diapers, child training pants, adult incontinence pants, feminine menstrual pants, etc. employ one or two lines of perforations to allow the wearer to open, inspect and even change the product without having to remove other articles of clothing. Many refastenable, disposable absorbent garments utilize tearable lines of perforations and refastenable attachment members which cooperate together to allow the garment to be opened and closed more than once. Such refastenable articles also permit the user to adjust the snugness of the garment relative to their body anatomy.  
      Up until now, most of such lines of perforations have been linear in configuration. The primary reason for forming a linear line of perforations is that it is easy to accomplish using a straight flex knife. The tooling is relatively cheap and can be quickly manufactured. However, one drawback with using a linear line of perforations is that such a design may not be the most advantageous configuration for the article or package it is to be used in or on. Many absorbent articles which are intended to be worn about the torso of a human body to absorb urine and/or feces might function better and/or appear more aesthetically pleasing if the lines of perforations were non-linear in configuration. A curved or arcuately shaped design for each line of perforations could provide the wearer of the article with extra material located adjacent to the point where the line of perforations is to be grasped so as to enable it to be easily torn open. This feature could be especially beneficial to older adults who may be suffering from arthritis. Another problem with refastenable, disposable pant-like garments is that the attachment members tend to cover up a major portion of the lines of perforations and makes them hard to be seen. This is especially true for incontinence garments being worn by elderly adults who may suffer from impaired vision. By utilizing non-linear lines of perforations, such as a concave or convex configuration, a greater portion of each of the lines of perforations is visually present. The ability of the wearer of the article to visually see and recognize the location of each tearable line of perforations is a consumer preference.  
      Up until now, manufacturers have shied away from having to create a non-linear line of perforations in their products and/or packages because the cost of the tooling required to make such a line of perforations is expensive and the engineering needed to make the tooling work at high speeds is difficult.  
      Now a rotary die cutter has been invented for forming at least one non-linear line of perforations in a strip of material in a cost effective and efficient manner.  
     SUMMARY OF THE INVENTION  
      Briefly, this invention relates to a rotary die cutter for forming at least one non-linear line of perforations in a strip of material. The rotary die cutter includes a rotatable anvil roll having a first end, a second end, and a hardened peripheral surface located between the first and second ends. The rotary die cutter also includes a rotatable knife roll having a first end, a second end, a width extending from the first end to the second end, and a peripheral surface located between the first and second ends. The knife roll has at least one knife positioned on the peripheral surface that has a non-linear configuration. The knife extends across at least about half of the width of the knife roll and has a plurality of land areas each separated by a notch. The knife roll is coaxially aligned with the anvil roll to form a nip therebetween through which the strip of material can pass. For each rotation of the die cutter, the knife will pass through the strip of material and be brought into direct contact with the hardened peripheral surface of the anvil roll and form a non-linear line of perforations in the strip of material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of a rotary die cutter having a rotatable knife roll with at least one knife positioned thereon and a rotatable anvil roll that cooperates with the knife roll to form a non-linear line of perforations in a strip of material.  
       FIG. 2  is an end view of  FIG. 1  showing a strip of material traveling through the nip created by the interaction of the knife roll and the anvil roll.  
       FIG. 3  is a top view of a strip of material depicting two non-linear lines of perforations that can be later used to form a front waist panel of a refastenable, disposable absorbent garment.  
       FIG. 4  is an end view of a solid knife roll having an outer peripheral surface with three knives secured thereto and spaced 120 degrees apart.  
       FIG. 5  is an end view of a solid knife roll having an outer peripheral surface with four knives secured thereto and spaced 90 degrees apart.  
       FIG. 6  is an end view depicting an alternative way of securing a pair of the knives to the knife roll.  
       FIG. 7  is an end view depicting a pair of knives separated by a distance of less than 180 degrees.  
       FIG. 8  is an enlarged end view of the knife shown in  FIG. 2 .  
       FIG. 9  is an enlarged front view of three of the teeth shown in  FIG. 1 .  
       FIG. 10  is a top view of a portion of a non-linear knife having a plurality of land areas each separated by a notch wherein each land area has a pair of spaced apart side edges that are positioned on an arc A-A and are radially aligned to the direction of travel of the strip of material that is being perforated.  
       FIG. 11  is a top view of a portion of a non-linear knife having a plurality of land areas each separated by a notch wherein each land area has a pair of spaced apart side edges that are positioned on an arc A-A and are aligned parallel to the direction of travel of the strip of material that is being perforated. 
    
    
     DETAILED DESCRIPTION  
      Referring to  FIGS. 1 and 2 , a rotary die cutter  10  is shown for forming at least one non-linear line of perforations in a strip of material  12 . The rotary die cutter  10  includes a rotatable anvil roll  14  and a rotatable knife roll  16 . The anvil roll  14  is cylindrical in shape and has a first end  18 , a second end  20  spaced apart from said first end  18 , and a longitudinal central axis X 1 -X 1 . The anvil roll  14  can be a solid roll that has a hardened peripheral surface  22  located between the first and second ends,  18  and  20  respectively. The anvil roll  14  can be formed from ferrous metal, steel, a steel alloy or from some other material known to those skilled in the art. Desirably, the peripheral surface  22  is smooth and free from irregularities, roughness or projections. Most desirably, the peripheral surface  22  has an even consistency of smoothness throughout. The anvil roll  14  has a diameter d 1  that can be of almost any desired dimension. The exact diameter d 1  of the anvil roll  14  should be sized to handle the length, width and thickness of the strip of material  12  that will pass over its peripheral surface  22 . Desirably, the diameter d 1  of the anvil roll  14  will range from between about 2 inches (about 5 centimeters (cm)) to about 20 inches (about 51 cm). More desirably, the diameter d 1  of the anvil roll  14  will range from between about 5 inches (about 13 cm) to about 15 inches (about 38 cm). Most desirably, the diameter d 1  of the anvil roll  14  will range from between about 8 inches (about 20 cm) to about 12 inches (about 30 cm).  
      The anvil roll  14  also has a face width w 1  that extends from the first end  18  to the second end  20 . The face width w 1  is measured parallel to the longitudinal central axis X 1 -X 1 . The face width w 1  can be of almost any desired dimension. Desirably, the face width w 1  will range from between about 4 inches (about 10 cm) to about 50 inches (about 127 cm). More desirably, the face width w 1  will range from between about 6 inches (about 15 cm) to about 20 inches (about 51 cm). Most desirably, the face width w 1  will range from between about 10 inches (about 25 cm) to about 14 inches (about 36 cm).  
      Still referring to  FIGS. 1 and 2 , the knife roll  16  is also cylindrical in shape and has a first end  24 , a second end  26  spaced apart from the first end  24 , and a longitudinal central axis X 2 -X 2 . The knife roll  16  can be either a solid roll or a hollow roll. Desirably, the knife roll  16  is a solid roll. The knife roll  16  has a peripheral surface  28  located between the first and second ends,  24  and  26  respectively. A collar or sleeve  30  can be positioned on, secured to or integrally formed onto the peripheral surface  28  of the knife roll  16 . The collar or sleeve  30  can be snuggly fitted over the peripheral surface  28 , for example by being shrink fitted in place. Other means known to those skilled in the art for securing the collar or sleeve  30  to the peripheral surface  28  can be used. The collar or sleeve  30  functions as one way of securing or attaching at least one knife  32  indirectly to the outer peripheral surface  28  of the knife roll  16 . Alternatively, one or more knives  32  can be directly secured to the peripheral surface  28 .  
      The knife roll  16  can be formed from ferrous metal, steel, a metal alloy or from some other material known to those skilled in the art. The knife roll  16  has a diameter d 2  that can be of almost any desired dimension. The diameter d 2  is the outer diameter of the knife roll  16  and would also include the thickness of any collar or sleeve  30  that may be present. The exact diameter d 2  of the knife roll  16  should be sized to handle the length, width and thickness of the strip of material  12  that will pass over its outer surface. The diameter d 2  of the knife roll  16  can be smaller than, equal to or larger than the diameter d 1  of the anvil roll  14 . In order to extend the life of the knife roll  16 , it is advantageous to size the diameter d 2  of the knife roll  16  to be different from the diameter d 1  of the anvil roll  14  to ensure that the knife or knives  32  do not contact the anvil roll  14  at the same location on each revolution. Desirably, diameter d 2  of the knife roll  16  is either smaller than or larger than the diameter d 1  of the anvil roll  14 . Desirably, the diameter d 2  of the knife roll  16  will range from between about 2 inches (about 5 cm) to about 20 inches (about 51 cm). More desirably, the diameter d 2  of the knife roll  16  will range from between about 5 inches (about 13 cm) to about 15 inches (about 38 cm). Most desirably, the diameter d 2  of the knife roll  16  will range from between about 8 inches (about 20 cm) to about 12 inches (about 30 cm).  
      The knife roll  16  also has a face width w 2  that extends from the first end  24  to the second end  26 . The face width w 2  is measured parallel to the longitudinal central axis X 2 -X 2 . The face width w 2  can be of almost any desired dimension. Desirably, the face width w 2  will range from between about 4 inches (about 10 cm) to about 40 inches (about 102 cm). More desirably, the face width w 2  will range from between about 6 inches (about 15 cm) to about 20 inches (about 51 cm). Most desirably, the length will range from between about 10 inches (about 25 cm) to about 14 inches (about 36 cm).  
      Referring now to  FIGS. 1-7 , different ways of securing or attaching one or more knives  32  to the peripheral surface  28  of the knife roll  16  is depicted. The knife roll  16  can have a single knife  32  secured or attached to its peripheral surface  28  which protrudes radially outward from knife roll  16 . However, many times, it is more desirable to secure two or more knives  32  to the peripheral surface  28 . The numbers of knives  32  used will depend on how many lines of perforations  34 , see  FIG. 3 , one wishes to form in the strip of material  12 . The spacing of the lines of perforations  34  in a given length of the strip of material  12  will also impact on the number of knives  32  that are secured to the knife roll  16 . In  FIG. 3 , the strip of material  12  has two sections  35  and  37 , each of which will be used to form the front panel on a disposable absorbent garment. In each section,  35  and  37 , a pair of non-linear lines of perforations  34  is formed by the die cutter  10 . The shape of the lines of perforations  34  and their location on each section of material will be determined by one&#39;s particular needs.  
      Still referring to  FIGS. 1-7 , the knife roll  16  will desirably have two or more knives  32  formed into or positioned on the peripheral surface  28 , see  FIGS. 1, 2  and  4 - 7 . By “formed into” is meant that the knife or knives  32  can be integrally formed on or in the peripheral surface  28 , such as by casting, welding or by some other means known to those skilled in the art. In  FIGS. 4 and 5 , the knives  32  are shown as an integral part of the knife rolls,  16 ′ and  16 ″ respectively. The knives  32  can be uniformly or randomly spaced about the peripheral surface  28  of the knife rolls  16 ,  16 ′ or  16 ″. In  FIG. 4 , three knives  32  are formed into the peripheral surface  28  and each knife  32  is uniformly spaced apart by an angle α. The angle α is 120 degrees. In  FIG. 5 , four knives  32  are formed into the peripheral surface  28  and each knife  32  is uniformly spaced apart by an angle α. The angle αis 90 degrees in  FIG. 5 .  
      By “positioned on” is meant that the knife or knives  32  can be physically secured to the peripheral surface  28  via an intermediate attachment member. The intermediate attachment can be by way of the collar or sleeve  30  or by a flange, bracket or some other uniquely shaped member.  
      In  FIGS. 1 and 2 , one way of attaching one or more knives  32  indirectly to the knife roll  16  is depicted. In  FIGS. 1 and 2 , two knives  32  are secured or attached to the collar or sleeve  30  which is then tightly fitted or permanently attached to the peripheral surface  28  of the knife roll  16 . The collar or sleeve  30  can have a cylindrical shape with a relatively thin thickness. The collar or sleeve  30  can be formed from the same material as the knife roll  16  or be formed from a different material. The collar or sleeve  30  can be hardened, if desired. The two knives  32  formed on the collar or sleeve  30  are uniformly spaced 180 degrees apart.  
      In  FIG. 6 , an alternative way of attaching one or more knives  32  indirectly to the peripheral surface  28  of the knife roll  16  is depicted. Instead of a 360 degree collar  30 , a pair of flanges  36  is utilized. It should be noted that a single flange  36  could be used, if desired. In  FIG. 6 , each flange  36  is an arcuate member that spans a predetermined angle on the peripheral surface  28  of the knife roll  16 . Each flange  36  can range from between about 1 inch (about 2.54 cm) to a dimension that spans roughly about half the circumference of the peripheral surface  28 . Each flange  36  contains a knife  32  and each flange  36  is secured to the peripheral surface  28  of the knife roll  16  by two or more screws  38 . Desirably, four or more screws  38  are used to secure each of the flanges  36  to the outer periphery  28 . Other attachment mechanisms can also be employed, such as bolts and nuts, roll pins, a slot and key mechanism, a weld joint, etc. The attachment can be a mechanical, electromechanical or chemical bond, i.e. an adhesive bond. Such means for attaching two members together are known to those skilled in the art. It should be noted that each flange  36  can be sized and shaped to fit one&#39;s particular needs. Each flange  36  can be in the form of an arcuate member, as shown in  FIG. 6 , or be in the shape of a rib, an L-shaped bracket, a T-shaped bracket, a portion of a rim or some other unique configuration.  
      It should be noted that it may be beneficial to use a pair of flanges  36  which are offset from one another so as to provide a counter balance. It has been found that a balanced knife roll  16  performs better over an extended period of time since less vibration and instability is present in the die cutter  10 .  
      Referring to  FIG. 7 , another way of attaching one or more knives  32  indirectly to the peripheral surface  28  of the knife roll  16  is shown. In  FIG. 7 , one will notice that a flange  40  is secured to the peripheral surface  28  by two or more screws  36 . The flange  40  contains two knives  32  which protrude radially outward from the flange  40  and are spaced apart by an angle Θ. The angle Θ is less than about 180 degrees, desirably less than about 120 degrees, and more desirably, less than about 90 degrees. To offset or compensate for the weight of the flange  40 , a counter weight flange  42  is secured to the opposite side of the peripheral surface  28  of the knife roll  16 . The counter weight flange  42  does not have any knives  32  secured to it. Because no knives  32  are present, the counter weight flange  42  can be made thicker, longer or from a heavier material in order to more evenly balance the knife roll  16 .  
      It should be noted that the flanges  36 , shown in  FIG. 6 , and the flanges  40  and  42 , shown in  FIG. 7 , can be shaped and sized to contact or abut against one another, if desired. When a pair of flanges are utilized on the knife roll  16 , each flange can be of a different arcuate dimension yet together they can encompass 360 degrees around the circumference of the knife roll  16 . When the pair of flanges  36  or the two flanges  40  and  42  do contact or abut against one another, a structure similar to the collar or sleeve  30  can be obtained.  
      Returning to  FIGS. 1 and 2 , each knife  32  rises above the outer or topmost boundary of the peripheral surface  28  and protrudes radially outward from the center point of the knife roll  16 . Each knife  32  also has a non-linear configuration. “Non-linear” is defined herein as meaning not a straight line. A line that deviates from a straight line, such as a curved or arcuate line, a concave line, a convex line, two or more straight lines that are attached at an angle or aligned in close proximity to one another to form a non-linear shape, such as a V-shaped line, a W-shaped line, a saw tooth line, etc. are considered non-linear. Other non-linear configurations known to those skilled in the art can also be used. The non-linear configuration of the knife  32  is viewed as extending across the face width w 2  of the knife roll  16 . Each knife  32  has a length I 3  that extends across at least about half of the face width w 2  of the knife roll  16 . Desirably, each knife  32  has a length I 3  that extends across at least about 70% of the face width w 2  of the knife roll  16 . More desirably, each knife  32  has a length I 3  that extends across at least about 90% of the face width w 2  of the knife roll  16 . Most desirably, each knife  32  has a length I 3  that extends completely across the face width w 2  of the knife roll  16 .  
      Referring to  FIG. 8 , each knife  32  has a base  44 , an apex  46 , and a pair of side walls  48  and  50 . Each knife  32  also has a height h 3  and a thickness t 3 . The height h 3  is the dimension from the base  44  to the apex  46 . The collar or sleeve  30  has an outer peripheral surface  52  and the base  44  is located on this surface. In  FIG. 6 , each of the pair of flange  36  has an outer peripheral surface  53  and the base  44  is located on this surface. In  FIG. 7 , the flange  40  has an outer peripheral surface  55  and the base  44  is located on this surface. The height h 3  can range from between about 0.3 cm to about 2.5 cm. Desirably, the height h 3  can range from between about 0.5 cm to about 2.1 cm. More desirably, the height h 3  can range from between about 0.7 cm to about 1.8 cm. Most desirably, the height h 3  can range from between about 0.9 cm to about 1.5 cm. The thickness t 3  Of the knife  32  is the dimension between the pair of side walls  48  and  50 . As shown in  FIG. 8 , the knife  32  has a triangular shape since the side walls  48  and  50  taper to a point or rounded edge at the apex  46 . Therefore, the maximum thickness t 3 -occurs approximately at the base  44 . The thickness t 3  can range from between about 0.2 cm to about 1 cm at the base  44  and will taper to a cutting edge approximate the apex  46 . It should be noted that other geometrical shapes for the knife  32  can be utilized if desired. The various shapes of a knife or cutting blade  32  are known to those skilled in the art.  
      Referring now to  FIG. 9 , the knife  32  also has two or more land areas  54 , preferably a plurality of land areas  54 , each separated by a notch  56 . The land areas  54  and the notches  56  cooperate to provide the knife  32  with a serrated or tooth like appearance which is capable of forming one or more lines of perforations  34  in the strip of material  12 , see  FIG. 3 . Each land area  54  has a pair of spaced apart side edges  58  and  60 . The side edges  58  and  60  can be tapered or angled relative to one another or they can be aligned parallel to one another. Normally, the side edges  58  and  60  taper inward from the peripheral surface  52  to the apex  46 . However, the side edges  58  and  60  could taper outward from the peripheral surface  52  to the apex  46 , if desired. The distance or dimension between the side edges  58  and  60  creates a width w 4  in each of the land areas  54  measured at the apex  46 . Each of the notches  56  also has a width w 5  which is the dimension between adjacent land areas  54  measured at the plane of the apex  46 . The width w 4  of each of the land areas  54  can be less than, equal to or greater than the width w 5  of each of the notches  56 . The land areas  54  will correspond to the length of slits or cuts  62  formed in the strip of material  12  and the notches  56  will correspond to the unbroken areas  64  located between the slits or cuts, see  FIG. 3 . The land areas  54  and the notches  56  can be sized to any desired dimension so as to produce the predetermined spacing in the line of perforations  34  one wishes to obtain in the strip of material  12 . It has been found that the kind of material the line of perforations  34  is formed into, the thickness of the material  12 , the shape of the knife  32 , the sharpness of the knife  32 , the speed of the anvil roll  14  and the speed of the knife roll  16 , as well as other characteristics of the die cutter  10 , can all effect the appearance and shape of the line of perforations  34 .  
      When forming one or more lines of perforations  34  in a woven or nonwoven material, such as an elastic, an elastic laminate, a thermoplastic film, a spunbond web, a bonded carded web, a stretch bonded laminate, etc., wherein the material has a thickness of less than about 1 cm, good results can be obtained when the land areas  54  are sized to have a width w 4  that is greater than the width w 5  of the adjacent notch  56  when measured at the plane of the apex  46 . Desirably, the width w 4  of the land areas  54  is at least two times as large as the width w 5  of an adjacent notch  56  when measured at the plane of the apex  46 . More desirably, the width w 4  of the land areas  54  is at least three times as large as the width w 5  of an adjacent notch  56  when measured at the plane of the apex  46 . Most desirably, the width w 4  of the land areas  54  is at least four times as large as the width w 5  of an adjacent notch  56  when measured at the plane of the apex  46 . An example of a specific width w 4  for each of the land areas  54  is 0.6 cm and a width w 5  for each of the notches  56  is 0.15 cm.  
      Referring again to  FIG. 1 , the die cutter  10  is assembled so that the anvil roll  14  and knife roll  16  are coaxially aligned and are spaced apart to form a nip  66  therebetween. The size of the nip  66  can be adjusted to accommodate the thickness of the strip of material  12  that will pass therethrough. The nip  66  can be any desired dimension but should not be larger than the height h 3  of the knife  32 . The strip of material  12  should not be compressed when passing through the nip  66  unless one desires compaction to take place. The strip of material  12  will move or travel in a machine direction, designated (MD), through the die cutter  10 . Desirably, the anvil roll  14  and the knife roll  16  are aligned perpendicular to the machine direction so as to enable the knife  32  to cut across the width w 3  of the strip of material  12 . It should be noted that the strip of material  12  has a width w 3  and the length I 3  of each knife  32  should be greater than this width w 3  Desirably, the length I 3  of each knife  32  is at least about 1.5 cm greater than the width w 3  of the strip of material  12 . More desirably, the length I 3  of each knife  32  is at least about 2.5 cm greater than the width w 3  of the strip of material  12 . This relationship is important because it will assure that as the strip of material  12  passes through the nip  66  and the line of perforations  34  can be formed regardless of any transverse movement or weave that the material  12  may experience.  
      The knife roll  16  can rotate in a clockwise direction while the anvil roll  14  rotates in a counter clockwise direction. As depicted in  FIG. 1 , this will cause the strip of material  12  to move from right to left through the die cutter  10 . It is possible to reverse the rotational directions of both the anvil roll  14  and the knife roll  16 , if desired. For each rotation of the die cutter  10 , the knife  32  will pass through the strip of material  12  and be brought into direct contact with the hardened peripheral surface  22  of the anvil roll  14 . This action will cause the serrated knife  32  to form a non-linear line of perforations  34  in the strip of material  12 . The number of lines of perforations  34  and the distance the lines of perforations  34  are spaced apart from one another will depend on the number of knives  32  present on the knife roll  16 , the speed of the anvil and knife rolls,  14  and  16  respectively, and the speed at which the strip of material  12  is traveling through the nip  66 .  
      It should be noted that the surface speed of the strip of material  12  can be matched to the surface speed of the rotary die cutter  10  within plus or minus 10%. The anvil roll  14  and the knife roll  16  should be capable of operating at a surface speed of at least about 100 feet per minute, desirably at least about 1,000 feet per minute, and more desirably, at least 1,500 feet per minute. Die cutters  10  can also be constructed that are capable of even faster speeds.  
      The knife roll  16  can be controlled to rotate at a slower speed, the same speed or at a faster speed than the anvil roll  14 . Desirably, the knife roll  16  rotates at either a faster or a slower speed than the anvil roll  14 . This will help assure that for each revolution of the knife roll  16 , the knife  32  does not contact the same location on the anvil roll  14 . By allowing the knife  32  to contact a different location of the anvil roll  14  on subsequent revolutions, the life of the knife  32  can be extended. This decreases maintenance cost and leads to a more cost efficient operation.  
      Lastly, referring to  FIGS. 10 and 11 , two different embodiments are shown for the arrangement of the land areas  54  of the knife  32  relative to the machine direction (MD). In  FIG. 10 , the land areas  54  are aligned on an arc A-A and a majority of the side edges  58  and  60  of each of the land areas  54  are aligned at an angle to the machine direction.  
      The arc A-A has a center point and a radius (r). In this arrangement, a majority of the side edges  58  and  60  are radially aligned at an acute angle to the machine direction when measured from the center point of the arc A-A. The knife  32  is centered on the arc A-A such that the apex  46  of the knife  32  is coterminous with the arc A-A. It should be noted that the distance between adjacent land areas  54  can be of the same dimension, as shown, or be of a different dimension to suit one&#39;s particular needs. In  FIG. 11 , the land areas  54  are aligned on an arc A-A and the side edges  58  and  60  of each of the land areas  54  are aligned parallel to the machine direction. The arc A-A has a center point and a radius (r). In this arrangement, each of the side edges  58  and  60  are aligned parallel to one another and the knife  32  is centered on the arc A-A such that the apex  46  of the knife  32  is coterminous with the arc A-A. It should be noted that the distance between adjacent land areas  54  can be of the same dimension, as shown, or be of a different dimension to suit one&#39;s particular needs. The serrated knife pattern shown in  FIG. 11  may be easier to manufacture than the pattern shown in  FIG. 10 . With the knife pattern depicted in  FIG. 11 , the strip of material  12  will contain a plurality of slits  62  separated by non-cut regions  64 . The slits  62  will be about equal to the width w 4  of each of the land area  54  and the non-cut regions  64  will correspond in size to the distance between two adjacent land areas  54 .  FIG. 3  shows the slits  62  and the non-cut regions  64  after the strip of material  12  has been perforated. The slits  62  will be approximately uniform in length when formed in the strip of material  12  when using the patterns depicted in  FIGS. 10 and 11 .  
      While the invention has been described in conjunction with several specific embodiments, it is to be understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope of the appended claims.