Patent Publication Number: US-2011052865-A1

Title: Structured perforated plastic sheet

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. application Ser. No. 12/050,262, filed on Mar. 18, 2008. 
    
    
     TECHNICAL FIELD 
     This invention relates to a structured perforated plastic sheet, more particularly to a structured perforated plastic sheet having a tri-layer configuration. 
     BACKGROUND OF THE INVENTION 
     U.S. Patent Application Publication No. 20040043189 discloses a structured perforated plastic sheet that can be used as a wearer-contacting top sheet of an absorptive device, such as a diaper or bandage, a water-retaining material for agricultural applications, a sanitary napkin, or a packaging or a decorative material. The structured perforated plastic sheet has a bi-layer configuration including a top layer of a net-like structure and a bottom layer of a net-like structure. The net-like structure of the top layer defines a plurality of top holes. The net-like structure of the bottom layer defines a plurality of bottom holes. Each of the top holes has an area much larger than that of each of the bottom holes, and overlies plural ones of the bottom holes. The net-like structure of the top layer can serve to break a liquid body into smaller portions and distribute them among the top holes, thereby enhancing distribution of the liquid among the bottom holes. 
     Referring to  FIG. 1 , U.S. Pat. No. 5,514,105 discloses a structured perforated plastic sheet. The structured perforated plastic sheet has a plurality of uppermost capillary openings  810 , each of which is defined by a polygonal opening-defining wall  81 . The polygonal opening-defining wall  81  of each of the uppermost capillary openings  810  is defined by a multiplicity of intersecting primary fiber-like elements  811 . Each of the uppermost capillary openings  810  is subdivided into smaller middle capillary openings  820  by a secondary fiber-like element  821 . One of the middle capillary openings  820  is further subdivided into smaller lower capillary openings  830  by a third fiber-like element  831 . The lowest ends of the primary fiber-like elements  811  are coplanar with the lowest end of the secondary fiber-like element  821  and the lowest end of the third fiber-like element  831 . The polygonal opening-defining wall  81  has four side walls  815 . Each of the four side walls  815  has an inclined portion  8151  and a straight vertical portion  8152  extending downwardly and bent from the inclined portion  8151 . The third fiber-like element  831  meets the straight vertical portion  8152  of one of the four side walls  815  of the polygonal opening-defining wall  81 . The straight vertical portion  8152  of each of the four side walls  815  extends from the inclined portion  8151  to a lowest end of the side wall  815  along a linear direction without any deflection or bending except at the interfaces where it meets the second fiber-like element  821  and the third fiber-like element  831 . 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a structured perforated plastic sheet that can further enhance distribution of a liquid body and thereby promote transport of the liquid body and that can exhibit a tri-layered configuration. 
     Accordingly, a structured perforated plastic sheet comprises a plastic sheet body including a network of interconnected cells. The network has a bottom layer, a middle layer higher than the bottom layer, and a top layer higher than the middle layer. The top layer includes a plurality of top ribs that are interconnected to form a loop and to define a top hole. Each of the top ribs has a top wall and two side walls extending downwardly from two opposite sides of the top wall. The side wall of each of the top ribs has a lowest end. The middle layer includes a plurality of middle ribs disposed below the top walls of the top ribs within the top hole and meet respectively the side walls of the top ribs to divide the top hole into a plurality of middle holes. Each of the middle ribs has a top wall and two opposite side walls extending downwardly from two opposite sides of the top wall of the middle ribs and has lowest ends, respectively. The bottom layer includes a plurality of bottom ribs that are formed within and divide each of the middle holes into a plurality of bottom holes. Each of the bottom ribs has a top wall, and two opposite side walls that extend downwardly and respectively from two opposite sides of the top wall of the bottom rib. Each of the side walls of the top and middle ribs starts to deflect in a plane of the top walls of the bottom ribs, and having a bottom rib-connecting part that is sidewisely connected to the bottom ribs, that extends from the plane of the top walls of the bottom ribs to a plane of the lowest ends of the top and middle ribs, and that bends from a remaining part of a corresponding one of the side walls of the top and middle ribs. The side walls of the top and middle ribs have bending lines that cooperatively form a plurality of loop-shaped bending lines each extending around one of the middle holes in the plane of the top walls of the bottom ribs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In drawings which illustrate embodiment of the invention: 
         FIG. 1  is a fragmentary perspective view of a conventional structured perforated plastic sheet; 
         FIG. 2  is a fragmentary schematic top view of the first preferred embodiment of a structured perforated plastic sheet according to this invention; 
         FIG. 3  is a fragmentary perspective view of the first preferred embodiment; 
         FIG. 4  is a fragmentary perspective view of the second preferred embodiment of a structured perforated plastic sheet according to this invention; 
         FIG. 5  is a fragmentary sectional view of the second preferred embodiment; 
         FIG. 6  is a photo image showing the tri-layer configuration of the third preferred embodiment of the structured perforated plastic sheet according to this invention; 
         FIG. 7  is a photo image showing the tri-layer configuration of the fourth preferred embodiment of the structured perforated plastic sheet according to this invention; 
         FIG. 8  is a schematic view illustrating a stacked structure of an embossing mold that can be used to make the second preferred embodiment of this invention; 
         FIG. 9  is a schematic view illustrating how an outward deflection occurs in a side wall of a top rib of a top layer of a structured plastic sheet due to the stacked structure of the embossing mold of  FIG. 8 ; and 
         FIG. 10  is a schematic view illustrating how an inward deflection occurs in a side wall of a top rib of the structured plastic sheet due to the stacked structure of the embossing mold of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIGS. 2 and 3  illustrate the first preferred embodiment of a structured perforated plastic sheet according to this invention. The structured perforated plastic sheet can be used as a wearer-contacting top sheet of an absorptive device, such as a diaper and bandage, or as a water-retaining material for agricultural applications, a sanitary napkin, a packaging or decorative material. The structured perforated plastic sheet includes a plastic sheet body  100  having a tri-layer configuration that includes a network of interconnected cells  6 . The network includes a planar top layer  5 , a planar middle layer  4 , and a planar bottom layer  2  that is disposed below the middle layer  4 . The top layer  5  is formed with a pattern of top holes  52 , each of which is defined by a plurality of top ribs  51  that are interconnected to form a polygonal loop and to define one of the top holes  52 . The middle layer  4  includes a plurality of middle ribs  41 . The bottom layer  2  is formed with a pattern of bottom holes  22 , each of which is defined by a plurality of bottom ribs  21  to form a polygonal loop and to define one of the bottom holes  22 . Each of the middle ribs  41  meets adjacent ones of the top ribs  51 . The tri-layer configuration of the plastic sheet body  100  is capable of breaking a liquid body (not shown) into smaller portions through the top, middle and bottom layers  5 ,  4 ,  2 , thereby enhancing distribution and transport of the liquid body from the top layer  5  to the bottom layer  2 . 
       FIGS. 4 and 5  illustrated the second preferred embodiment of a structured perforated plastic sheet according to this invention. The structured perforated plastic sheet comprises a plastic sheet body  100  including a network of interconnected cells  6 , which has a bottom layer  2 , a middle layer  4  superimposed on and integrally formed with the bottom layer  2 , and a top layer  5  superimposed on and integrally formed with the bottom layer  2 . The top layer  5  includes a plurality of top ribs  51  that are interconnected to form a loop and to define a top hole  52  for each cell  6 . Each of the top ribs  51  has a top wall  511  and two side walls  512  extending downwardly from the top wall  511 . Each side wall  512  of each top rib  51  has a lowest end  5121 . The middle layer  4  includes a plurality of middle ribs  41 . The middle ribs  41  for each cell  6  are disposed below the top walls  512  of the top ribs  51  within the top hole  51  and meet respectively the side walls  512  of the top ribs  51  to divide the top hole  52  into a plurality of middle holes  42 . Each of the middle ribs  41  has a top wall  411  and two opposite side walls  412  extending downwardly from two opposite sides of the top wall  411  of the middle ribs  41  and having lowest ends  4121 , respectively. The bottom layer  2  includes a plurality of bottom ribs  21  that have lowest ends  2121 . The bottom ribs  21  for each cell  6  divide each of the middle holes  42  into a plurality of bottom holes  22 . Each of the bottom ribs  21  has a top wall  211  and two opposite side walls  212  extending downwardly and respectively from two opposite sides of the top wall  211  of the bottom rib  21 . 
     Each side wall  512  of the top rib  51  starts to deflect in a plane (P 1 ) (see  FIG. 5 ) of the top walls  211  of the bottom ribs  21 , and thus has a bottom rib-connecting part  23  that is connected to the bottom ribs  21 , that extends from the plane (P 1 ) of the top walls  211  of the bottom ribs  21  to the plane (P 2 ) of the lowest ends  5121 ,  4121 ,  2121  of the top, middle and bottom ribs  51 ,  41 ,  21  and that bends from a remaining part of the corresponding side wall  512 . Each side wall  412  of the middle rib also starts to deflect in the plane (P 1 ) of the top walls  211  of the bottom ribs  21  and thus has a bottom rib-connecting part  23  that is connected to the bottom ribs  21 , that extends from the plane (P 1 ) of the top walls  211  of the bottom ribs  21  to the plane (P 2 ) of the lowest ends  5121 ,  4121 ,  2121  of the top, middle and bottom ribs  51 ,  41 ,  21  and that bends from the remaining part of the corresponding side wall  412 . Therefore, the side walls  512 ,  412  of the top and middle ribs  51 ,  41  respectively have bending lines  61 ,  62  which cooperatively form a plurality of looped bending lines  600  each extending around one of the middle holes  42  in the plane (P 1 ) of the top walls of the bottom ribs  21 . How the side walls  512 ,  412  of the top and middle ribs  51 ,  41  deflect will be explained hereinafter. 
     The number of the bottom holes  22  in each of the cells  6  is greater than 3 and less than 15 so as to achieve a higher fluid distribution and flow as well as a sufficient mechanical strength to support the cell structure of each cell  6 . A hole area rate is defined hereinafter as the total area of the holes in a unit area of the plastic sheet divided by the unit area of the plastic sheet and multiplied by 100%. Preferably, the hole area rate of the bottom holes  22  ranges from 20% to 40%, while the hole area rate of the top holes  52  ranges from 65% to 85%. 
     Each of the top ribs  51  defines a depth (D 1 ) from the top wall  511  of the top rib  51  to the plane (P 1 ) of the top walls  211  of the bottom ribs  21 . The depth (D 1 ) is preferably greater than 0.3 mm and less than 1.5 mm. Each of the bottom ribs  21  defines a second depth (D 2 ) from the top wall  211  of the bottom rib  21  to the lowest end  2121  thereof. The ratio of the first depth (D 1 ) to the second depth (D 2 ) preferably ranges from 1:1 to 1:3. 
     The two side walls  512  of each of the top ribs  51  define a width (W t ) that is preferably greater than 0.15 mm and less than 2.0 mm. 
     The side walls  212  of each of the bottom ribs  21  define a width (W b ) that is preferably greater than 0.15 mm and less than 1.0 mm. 
     Each of the top holes  52  preferably has a minimum width (W th ) greater than 2 mm and less than 30 mm. 
     Each of the bottom holes  22  preferably has a minimum width (W bh ) greater than 0.1 mm and less than 10 mm. 
       FIGS. 6 and 7  are photo images to illustrate the tri-layer configurations of the third and fourth preferred embodiments of the structured perforated plastic sheet of this invention, respectively. 
     A process of making the structured perforated plastic sheet according to the present invention includes: providing an embossing mold; and embossing and perforating a plastic sheet by using the embossing mold to emboss the plastic sheet and by applying heat and vacuum pressure to the plastic sheet while the plastic sheet is being embossed. 
       FIG. 8  illustrates a preferred embodiment of an embossing mold  70  for forming the structured perforated plastic sheet of the second preferred embodiment. The embossing mold  70  includes first, second and third screen molds  71 ,  72 ,  73  that are stacked one above the other for forming the tri-layer configuration of the structured perforated plastic sheet. The first, second and third screen molds  71 ,  72 ,  73  have different hole patterns. 
     The first screen mold  71  has a plurality of interconnected first looped members  711  respectively bounding first screen holes  712 . The second screen mold  72  has a plurality of interconnected second looped members  721  respectively bounding second screen holes  722  and disposed below the first screen holes  712 . The third screen mold  73  has a plurality of interconnected third looped members  731  respectively bounding third screen holes  732  and disposed below the second screen holes  722 . Each second looped member  721  spans over a group of the third looped members  731 . Each of the first screen holes  712  overlaps a group of the second screen holes  722  such that the second looped members  721  are out of alignment with the first looped members  711 . The second screen mold  72  has no looped member that is aligned with the first looped member  711  and the third looped member  731  and that corresponds in shape and position to the first and third looped members  711 ,  731 . The third screen mold  73  also has no looped member that is aligned with the first looped member  711  and the second looped member  721  and that corresponds in shape and position to the first and second looped members  711 ,  721 . 
     In use, the assembly of the first, second and third screen molds  71 ,  72 ,  73  is mounted on a vacuum forming drum (not shown) which is heated and is vacuumed inside to provide a suction force to the assembly of the first, second and third screen molds  71 ,  72 ,  73 . When the plastic sheet (not shown) is advanced to the vacuum forming drum, the plastic sheet is softened and embossed under vacuum pressure so that the plastic sheet is formed into the top, middle and bottom ribs  51 ,  41 ,  21  together with the top, middle and bottom holes  52 ,  42  and  22 . 
     Referring to  FIGS. 9 and 10  in combination with  FIGS. 4 and 8 , the top rib  51  in a region of the structured perforated plastic sheet is formed by the first looped member  711  of the first screen mold  71  and the third looped member  731  of the third screen mold  73 . Because the third looped member  731  is not aligned with the first looped member  711 , when a suction pressure (S F ) is applied in a downward direction as indicated by arrows in  FIGS. 9 and 10 , the side wall  512  of the top rib  51  starts to deflect in the plane (P 1 ) ( FIG. 5 ) of the top walls  211  of the bottom ribs  21 . The deflection may be outward ( FIG. 9 ) or inward ( FIG. 10 ). In a likewise manner, deflections also occur in the middle ribs  41  because the second looped members  721  are out of alignment with the third looped members  731 . 
     With the multi-layer configuration of the structured perforated plastic sheet of this invention, distribution and transport of a liquid body can be enhanced. 
     With the invention thus explained, it is apparent that various modifications and variations can be made without departing from the spirit of the present invention. It is therefore intended that the invention be limited only as recited in the appended claims.