Patent Publication Number: US-2004047523-A1

Title: Block bottom bag

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates to multiwall bags of the type commonly referred to as “block bottom bags,” and more specifically, to such a bag designed so that the entire interior surface area of the bag that is exposed to product contained in the bag is covered with a plastic liner.  
       BACKGROUND  
       [0002] Block bottom bags, which are also commonly called “satchel bottom” bags are very useful for holding bulk quantities of material and as such are used ubiquitously in numerous industries. While there are numerous names that are used to describe the bag, they are characterized as having multiple paper plies and a plastic liner, typically polyethylene, and a folded block bottom that is flat when the bag is filled. The open top end of such bags may be closed in several ways, for example by folding it over and “pinching” it shut to seal the bag. When the open end of such bags is closed with a pinched seal the bag is called a “pinch block bottom bag.” 
       [0003] Block bottom bags are manufactured from roll stock paper—typically a standard kraft paper, and roll stock polyethylene sheet material. Although there are several well-known processes according to which the bags may be made, briefly described, the bags are fabricated by laminating or gluing a polyethylene layer to an innermost layer of paper. Several additional layers of paper, some of which have been cut and scored by knife blades, are then bonded (as by gluing) to the inner paper layer. Each layer of paper and the plastic layer are “stepped” or laterally offset relative to the next adjacent layer as they are laid down to produce a continuous sheet of a flattened material that typically has three or more layers of paper, and a layer of plastic. Rotating knife blades cut perforations in the blank prior to forming the blank into a tube. The blades are registered relative to one another to produce a stepped pattern for each of the layers in the longitudinal direction. The tube is then bottomed, which is the step in the process where the block bottom is formed and the bottom of the individual bags are closed and sealed. The bottoming process involves tucking or folding the sides of the bag inwardly and closing and gluing the bottom flaps over the folded-in sides. The opposite end of the individual bags are left open, to be closed and sealed later by, for example, the customer after filling. For ease of reference herein, block bottom bags are referred to as BB bags.  
       [0004] After the block bottom is formed, the open end of BB bags may be sealed according to several known processes, for example by folding over the top layers and heat-sealing the plastic liner to the outer ply of paper. Other process steps may often be used as well, for example, printing the outer surface of the outer layer of paper with various information such as the identity of the product that will be held in the bag, the manufacturer of the product, and the net weight of the bag.  
       [0005] BB bags offer numerous advantages. These include that the empty bags are flat and many empty bags may be stacked into a relatively small space. This reduces the costs of shipping many empty bags from the manufacturer to the user. The bags are well suited to holding bulk dry materials such as granulated products. The bags are easily filled, and once filled are easily sealed. Once sealed, the bags are strong and typically avoid sifting or leakage, although many conventional BB bags advantageously allow for some migration of air into and out of the sealed bag. Moreover, filled bags have a squared bottom and flat non-gusseted sides that makes the bags particularly suitable for stacking when filled. As such, the filled bags are easily layered in stable stacks on pallets. And in addition to the advantages just described, BB bags are quite economical to manufacture and use compared to other known bulk bags.  
       [0006] But despite the many advantages of BB bags, there are also several problems. One complaint is that conventional BB bags have sharp corners in the bottom area that may be prone to puncture and other damage caused by impact. One solution to this problem is offered in U.S. Pat. No. 5,553,943, which discloses a BB bag that includes forming a sealed bottom end of the plastic liner that is freely detachable from the paper plies. This construction provides a bag that is air-tight when filled and sealed, and is said to avoid problems caused by the sharp corners found in standard BB bags. However, while the bag disclosed in the &#39;943 patent is useful in many instances, it is not appropriate for use in all situations where bulk materials need to be packaged. For example, in some situations an airtight bag is not desired. Also, the inner plastic liner in a bag such as that described in the &#39;943 patent may not conform to the shape of the block bottom when the bag is formed and filled. This can result in the bag not standing upright during processing between the filling station and the next operation. Since most filling operations are highly automated, such bag handling failure can be a significant problem. Moreover, the bag of the &#39;943 patent may be more expensive to manufacture and may require specialized equipment compared to conventional BB bags.  
       [0007] Another, more significant problem inherent in conventional BB bags has to do with paper that remains exposed to the interior of the bags, and thus to the product contained in the bag. Because as noted BB bags are manufactured with the paper laid down in stepped fashion, when the bottom of conventional bags is formed, there are necessarily two relatively small tabs of paper that remain exposed on the interior of the bag. Stated in another way, owing to manufacturing constraints in making a conventional stepped bag, there are two small tabs of paper on the interior bottom of the bag that are not covered with the polyethylene liner, thus exposing paper to the interior of the bag.  
       [0008] This structural characteristic of conventional BB bags is known to cause several problems. First, when the bags are used for agricultural commodities, exposure of the commodity to paper is a concern with regard to contamination. The polyethylene sheet material that is used to line BB bags is less prone to contamination, such as bacterial contamination than paper. It is undesirable to have an agricultural product exposed to paper that typically is not sterilized.  
       [0009] Further, paper can absorb oils and other fluid components that might be present in the dry bulk products, while polyethylene typically will not absorb oils and other fluids. Absorption of oils and the like from the product into the paper can result in damage to the bag and damage to the product.  
       [0010] Accordingly, in view of the shortcomings inherent in conventional BB bags, there is an opportunity to supply a bag that has all of the advantages of conventional bags, yet avoids the shortcomings.  
       [0011] The illustrated embodiment of the present invention is just such a bag. Most significantly, the bag of is constructed in a manner that it eliminates the paper tabs exposed to the interior of the bag that are notorious in conventional BB bags. Product contained in a bag constructed according to the present invention is exposed to only the polyethylene liner and does not touch the paper plies. This provides substantial advantages from a product quality perspective and may improve sanitation of the product contained in the bag. The bag is equally as economical to manufacture as prior conventional BB bags, and may be manufactured with standard equipment used to make conventional bags, with appropriate modifications.  
       [0012] The bag of the present invention is constructed with multiple plies of paper such as kraft paper that are glued together and stepped relative to one another. The layer of kraft that is adjacent what will become the interior of the bag has a layer of plastic laminated onto it.  
       [0013] The bag is formed from a planar, multi-layer sheet of blank material. Each layer in the blank is stepped in the lateral or cross-bag direction. The layers are also stepped in some portions of the bag in the longitudinal direction. The knives that cut the continuous multi-layer material into individual planar blanks from which the individual bags are formed have been modified so that the knife registry creates specific offset or step patterns in the longitudinal direction. As a result of the offset or stepped pattern in the blanks, when the block bottom is formed in the bag, only plastic liner is exposed to the interior. A special step formed in the bag prevents leakage or “sifting.” 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014] The invention will be better understood and its numerous objects and advantages will be apparent by reference to the following detailed description of the invention when taken in conjunction with the following drawings.  
     [0015]FIGS. 1, 2 and  3  show a conventional BB bag constructed and formed according to the prior art.  
     [0016]FIG. 1 is a perspective view of a prior art BB with the block bottom formed and the upper portions of the bag removed to expose the interior bottom of the bag, and illustrating the paper tabs that are exposed in prior art bags.  
     [0017]FIG. 2 illustrates a prior art BB bag folding in a flat condition, and in which the bottom has been fully formed and the flaps glued in place.  
     [0018]FIG. 3 is a plan view of an unformed multi-wall BB blank formed according to the prior art, before the blank is formed into a tube.  
     [0019]FIGS. 4 through 13 illustrate the BB bag of the present invention.  
     [0020]FIG. 4 is a plan view of an unformed multi-wall BB blank formed in accordance with the present invention, illustrating the blank before it has been formed into a tube and before the block bottom has been formed.  
     [0021]FIG. 5 is a view of a bag according to the present invention, formed from the blank illustrated in FIG. 4, and folded in a flat condition.  
     [0022]FIG. 6 is a cross sectional view taken along the line  6 - 6  of FIG. 5, illustrating the layers of the bag in exaggerated thickness and showing the overlapping layers forming seams.  
     [0023]FIGS. 7 through 12 are a sequential series of front elevational views that show the steps in the formation of the block bottom.  
     [0024]FIG. 7 shows a formed tube that is open at both ends and is ready for having the block bottom formed.  
     [0025]FIG. 8 is a view of the formed tube of FIG. 7, illustrating the main cross-bag score or fold line and two diagonal fold lines formed in the tube.  
     [0026]FIG. 9 is the next sequential step and illustrates the plies at one side of the tube folded upwardly toward the top of the bag at the main cross bag fold line, defining the upper and lower flaps of the block bottom, the bag corners tucked inwardly, and exposing the four diagonal fold lines and two secondary fold lines;  
     [0027]FIG. 10 illustrates the bag of the present invention in the condition shown in FIG. 9, adding to that view the adhesive applied to various portions of the flaps (adhesive shown in stippling).  
     [0028]FIG. 11 shows the lower flap folded over onto itself at the secondary fold line and adhered to the bottom of the bag.  
     [0029]FIG. 12 is the final step in the sequence, illustrating the upper flap folded over onto itself at the secondary fold line and adhered to the bottom of the bag to complete the formation of the block bottom.  
     [0030]FIG. 13 is a perspective view of a bag according to the present invention with the block bottom formed and the upper portions of the bag removed to expose the interior bottom of the bag, and illustrating the absence of exposed paper. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0031] The BB bag of the present invention is fabricated from multiple layers of kraft paper, the innermost layer of which (when the bag is formed) is laminated to the adjacent layer of plastic that lines the interior of the bag. In some of the drawing figures, the edges of the various paper plies are shown with line styles that are intended to differentiate the layers. Thus, with reference for example to FIG. 3, the edges of the outermost or external layer of kraft (“outermost” meaning the outer or external layer in the finished bag) are shown with a solid line. The edges of the middle ply of kraft are illustrated with a regular dashed line. And, the edges of the innermost ply are shown with a line comprising dashes separated by two dots. In a drawing that uses this line convention, where the edges of two or more layers are coextensive, the line is shown as solid. To aid in clarity, reference numbers in the 100 series are used to identify structural elements and the like associated with the outermost kraft ply. Reference numbers in the 200 series are used to identify structural elements in the next adjacent ply, and numbers in the 300 series are used to identify structural elements in the third, or innermost kraft layer. Finally, also for the purposes of clarity, like reference numerals are used throughout the specification to identify the same element in various figures.  
     [0032] Although the bag of the present invention is disclosed in terms of a preferred embodiment in which the bag has four plies (three plies of paper and an inner ply of plastic), the principals of the invention as claimed are applicable to BB bags having both greater and fewer plies. As such, the number of plies described herein is not a limitation of the invention.  
     PRIOR ART  
     [0033] A conventional prior art BB bag  10  is shown in FIGS. 1 through 3. With reference to FIG. 1, the outermost layer, also called a ply, (typically kraft paper)  100  is glued to an adjacent middle ply  200 , again typically kraft. Middle ply  200  is likewise glued to an inner ply  300  (kraft). Inner ply  300  has a sheet of plastic  400  glued thereto. Plastic ply  400  is typically a polyethylene material, but those skilled in the art recognize that there are many materials available suitable as liner material, and the selection of the material used for the liner will depend upon numerous variables, including for example the nature of the material to be held in the bag, the environmental conditions the bag will be exposed to, and the like.  
     [0034] During formation of the blank from which the finished bag will be formed, each layer or ply in bag  10  is laterally stepped or offset relative to the next adjacent layer, as best illustrated the cross sectional illustration of FIG. 6, so that when a tube is formed from the blank the overlapping edges of each layer form a longitudinal seam that is laterally offset from the seam in the next adjacent layer. It should be noted that the bag according to the present invention is identical to the prior art bag with respect to the cross sectional configuration shown in FIG. 6. Therefore, referring specifically to FIG. 6, it may be seen that outer lateral edge  102  of outer layer  100  overlaps with the opposite outer lateral edge  104  of the same layer. In the same way, the outer lateral edge  202  of middle layer  200  overlaps with the opposite outer lateral edge  204 . Outer lateral edge  302  of inner layer  300  overlaps with outer lateral edge  304 , and the opposite lateral edges  402  and  404  of plastic ply  400  overlap and are sealed to one another. The opposite edges of each layer are overlapped and a strip of suitable adhesive such as hot melt glue is applied to the overlapped area. As detailed below, as the blank is formed the plastic ply is glued onto the inner facing surface of inner ply  300  with plural lines of adhesive such as hot melt applied between these two plies. As such, plastic ply  400  is laminated to inner layer  300  over the entire surface of the plastic ply that contacts layer  300 . Plastic ply  400  is laterally offset from inner ply  300  in the same manner described above with respect to the other layers as shown in FIG. 6.  
     [0035] Referring to FIG. 3, a single blank  12  formed according to the prior art is illustrated in a planar condition prior to its being formed into a tube. For reference purposes, the bag axis is defined as the axis extending in the direction of arrow A—that is, the axis that extends along the longitudinal axis of the formed bag. The cross-bag axis is transverse to the bag axis, and is illustrated with arrow B. It will be appreciated that continuous roll stock material is used to form a continuous sheet of multi-layer blank in which the kraft layers are glued to one another and the plastic ply is laminated to the inner layer of kraft as described above. As the roll stock is laid down, each ply is shifted along the longitudinal or bag axis relative to the adjacent layer or layers, creating a stepped pattern in along the cross-bag axis. The layers of kraft are glued to one another such that they are laterally offset or stepped relative to the next adjacent ply. As shown in FIG. 3, the opposite lateral edges  102  and  104  of outer ply  100  are laterally offset from the opposite lateral edges  202  and  204  of the adjacent kraft ply, middle ply  200 , and so on. Plastic ply  400  is not shown in FIG. 3 so that the other layers may be shown more clearly. Nonetheless, as noted, it is laterally offset from the inner layer of paper  300 .  
     [0036] The layers comprising bag  10  are also longitudinally offset relative to the next adjacent layers along the bag axis. Rotating knives that cut the various layers form these offsets, however, rather than physically shifting or staggering of the layers as they are laid down as described above with respect to the lateral offsets. The knives also cut various cut out segments when forming the blanks. The knives cut each individual layer separately prior to the layers being glued to one another. Again referring to FIG. 3, inner ply  300  is seen as being cut into a regular rectangle having linear edges, that is, opposite lateral edges  302  and  304  are straight and parallel, and top end edge  306  and bottom end edge  308  are straight and parallel. Although not shown in FIG. 3, plastic ply  400  is laminated to the inner facing surface of inner layer  300  with the top and bottom edges of the plastic cut coextensive with top and bottom edges  306  and  308 , but with the lateral edges  402  and  404  laterally offset from the lateral edges of adjacent layer  300 . Middle ply  200  is cut so that it is offset from inner ply  300  along the bag axis, so that the top edge  206  and bottom edge  208  are stepped from the respective top and bottom edges  306  and  308  of inner layer  300 . Moreover, the top end edge  206  of middle ply  200  includes a cut out segment  210  that extends between longitudinally extending fold lines  14  and  16 , each of which extends along the length of blank  12  along the bag axis. It will be appreciated that because the cut out segment  210  is cut with a rotating knife as the blank material is conveyed past the knife, forming the cut out segment  210  necessarily forms a complementary extended section  212  that extends between fold lines  14  and  16  at the opposite or bottom end of the blank. Finally, outer ply  100  is cut such that it is offset along the bag axis from middle ply  200 . Thus, top end edge  106  is offset from edge  206 , and bottom edge  108  is offset from bottom edge  208 . As with middle ply  200 , outer ply  100  has a cut out segment  110  that extends between fold lines  14  and  16 , and a complementary extended section  112  that extends across the same distance. In the prior art bag shown in FIG. 3, the top end edges  106 ,  206  and  306 , and the bottom end edges  108 ,  208  and  308  are longitudinally offset from one another across the entire width of the bag, including in the portion of the bag between fold lines  14  and  16 .  
     [0037] Fold lines  14  and  16  define the front panel of bag  10 , which in FIG. 3 is identified with reference number  18 , and the back panel of the bag, labeled with reference number  20 . It will be noted that the back panel is divided into two sections, one on either side of front panel  18 , and that the stepped pattern in the various plies is the same on all parts of the back panel so that when the blank is folded along fold lines  14  and  16  to form a tube, the back panel sections at opposite sides of the blank are overlapped and sealed.  
     [0038] Prior to forming blank  12  into a tube, slits  22 a are cut through outer ply  100  on the front panel  18  of the blank. Parallel slits  24   a  are cut through middle ply  200  outwardly of slits  22   a  on the front panel. Slits  24   a  are slightly shorter than slits  22 , and there is no slit in the inner ply  300  or in plastic ply  400 .  
     [0039] Similarly, slits  22   b  are cut through outer ply  100  on back panel  20  (which as noted above is divided into two sections, one on either side of the front panel), and parallel slits  24   b  are cut through middle ply  200  on the back panel. Slits  24   b  are slightly longer than slits  22   b.  All of the slits are cut through the adjacent bottom edge of the respective ply, as illustrated.  
     [0040] The purpose of the slits  22  and  24  is detailed below in respect of the preferred embodiment. It will be noted that the slits in each preceding overlying ply are sequentially shorter than the slits in the adjacent ply.  
     [0041] Blank  12  is formed into a tube and the block bottom is formed (in a manner detailed below) to form an empty bag  10  as shown in FIG. 2. To form the blank into a blank tube, it will be appreciated that the flat or planar blank  12  is folded along fold lines  14  and  16 , which form the lateral side edges of bag  12  in FIG. 2 and the outer lateral edges of the two portions of back panel  20  (i.e., located on opposite sides of front panel  18  in the flat blank shown in FIG. 3) are overlapped and glued as described above. As a result of the cut out segments  110  and  210 , the upper edges of the outer, middle and inner plies  106 ,  206  and  306 , respectively, define an upwardly extending flap  28  on the back side  20  of the bag. Flap  28  may be folded over the upper edges  106 ,  206  and  306  on the front panel  18  of the bag after it is filled to seal the upper open mouth of the bag. The flap is typically heat sealed to close the bag, although other types of seals are commonly used.  
     [0042] Referring now to FIG. 1, the bottom portion of a prior art bag  10  that has been formed from a blank  12  as shown in FIG. 3 is illustrated. Importantly, owing to the manner in which blank  12  is formed, when the block bottom of bag  10  is formed and the bag bottom is folded over and sealed, two strips of paper  26  are exposed to the interior of the bag. Said in another way, with the prior art bag shown in FIG. 1, plastic ply  400  does not cover the two strips identified with number  26 , and as such, these strips are uncoated kraft paper. For the reasons described above, these uncoated paper strips are undesirable.  
     PREFERRED EMBODIMENT OF THE INVENTION  
     [0043] The preferred embodiment of the present invention will now be described with reference to FIGS. 4 through 12. A blank  12  similar to the blank  12  that is described above with respect to FIG. 3, but embodying the present invention, is shown in FIG. 4. Blank  12  is a planar sheet that comprises three layers or plies of paper and an innermost layer of plastic. The plastic layer is laminated onto the layer of paper that will become the innermost layer in the formed bag. The plastic is glued to the innermost layer of paper over substantially the entire surface of the plastic layer and the plastic layer is therefore non-detachable from the innermost paper layer. This is preferably accomplished by laying down plural strips of adhesive along the bag axis on the innermost paper ply and adhering the plastic thereto. As noted above, the plastic (which is not shown in FIG. 4) is laterally stepped relative to the next adjacent layer of paper so that when the tubular bag is formed the opposite lateral edges of the plastic are overlapped and sealed to form a seam (FIG. 6). Because the plastic is adhered to the paper over the entire surface of the plastic, the plastic is inseparable from the inner layer of paper so that when the block bottom is formed, the plastic layer conforms to the shape and folds of the paper layers.  
     [0044] Referring to FIG. 4, a single blank  12  formed according to the present invention is illustrated prior to its being formed into a tube. The bag axis is the axis extending in the direction of arrow A—that is, the axis that extends along the longitudinal axis of the formed bag. The cross-bag axis is transverse to the bag axis, and is illustrated with arrow B.  
     [0045] Roll stock material is used to form a continuous sheet of multi-layer blank in which the layers of paper are glued to one another and the plastic ply is laminated to the inner layer of kraft as described above. The layers of kraft are glued to one another such that they are laterally offset from one another along the cross-bag axis, as shown. The opposite lateral edges  102  and  104  of outer ply  100  are laterally offset from the opposite lateral edges  202  and  204  of the adjacent kraft ply, middle ply  200 , and so on.  
     [0046] The blank  12  and thus bag  10  according to the present invention differs from the blank and bag formed according to the prior art in the manner that rotating knives that cut the continuous sheets of planar roll stock paper that form the blank are registered with respect to one another, and thus the manner in which the blank and block bottom are formed.  
     [0047] With continuing reference to FIG. 4, longitudinal fold lines  14  and  16  extend along the length of the bag along the bag axis and define a front panel  18  and a rear panel  20 . The top edge  30  of blank  12  on rear panel  20  is cut with rotating knives so that thee three layers of paper ( 100 ,  200  and  300 ) are stepped relative to one another along the bag axis as shown. The identical stepping pattern is formed on rear panel  20  on the opposite side of fold line  16  at top edge  30 . The top edge  32  of front panel  18  (that is, the top edge of the blank between fold lines  14  and  16 ) is cut such that all three layers of paper and the plastic are coextensive—that is, the edges are co-linear, except at a cut out step segment  34  that is cut from the outer ply  300 . The function of step segment  34  is detailed below.  
     [0048] Turning to the bottom edge of the blank  12  of FIG. 4, the three layers of paper are stepped relative to one anther at bottom edge  36  of rear panel  20  in a complementary manner to the top edge  30 . Bottom edges  108 ,  208  and  308  are thus stepped in the longitudinal direction along the bag axis as illustrated. The paper layers at the bottom edge  38  of front panel  18  are, however, cut such that they are coextensive and the edges are co-linear, except for the extended step segment  40  of outer ply  100  that is complementary to and a result of step segment  34  cut into ply  100  at top edge  32 .  
     [0049] As with the blank shown in FIG. 3, in the blank  12  of the present invention, ply  300  is seen as being cut into a regular rectangle having linear edges, that is, opposite lateral edges  302  and  304  are straight and parallel, and top end edge  306  and bottom end edge  308  are straight and parallel. The plastic ply  400  (which is not shown in FIG. 4) is laminated to the inner facing surface of inner layer  300  with the top and bottom edges of the plastic cut coextensive with top and bottom end edges  306  and  308 , but with the lateral edges offset from the adjacent layer.  
     [0050] Prior to forming the flattened blank  12  into a tube, slits are cut in the blank in the manner described above with respect to FIG. 3. Specifically, slits  22   a  are cut through outer ply  100  on the front panel  18 , and parallel slits  24   a  are cut through middle ply  200  outwardly of slits  22   a  on the front panel. Slits  24   a  are slightly shorter than slits  22 , and there is no slit in the inner ply  300  or in plastic ply  400 . Slits  22   b  are cut through outer ply  100  on back panel  20  and parallel slits  24   b  are cut through middle ply  200  on the back panel. Slits  24   b  are slightly longer than slits  22   b.    
     [0051] The next step in forming a finished bag is forming a tubular blank that has open top and bottom ends from the planar blank shown in FIG. 4. This is done in a standard manner by folding blank  12  along fold lines  14  and  16  such that the opposite lateral edges overlap as illustrated in FIG. 6 and as described above. The overlapping lateral edges are sealed as described above to form the tubular blank.  
     [0052] The closed block bottom is formed next, and is explained herein with reference to the sequence of illustrations beginning with FIG. 7 and continuing through FIG. 12. With reference to FIG. 7, the tubular blank is shown in a flat condition with front panel  18  facing the viewer and with fold lines  14  and  16  shown as defining the opposite lateral edges of the flattened tube. When the tube is formed as shown, slits  22  and  24  in the respective front and back panels ( 18  and  20 ) are aligned with one another. Diagonal fold lines  50  and  52  are formed by creasing the tube to facilitate folding. The fold lines extend from the respective lateral edges of the flattened tube and extend to the stepped segment  34  along bottom edge  38 . A primary cross-bag fold line  54  is next formed laterally across the tube, transverse to the bag axis, and extending between the points where diagonal fold lines  50  and  52  intersect the respective lateral edges of the tube (FIG. 8). Fold lines  50 ,  52  and  54  are formed in all plies of the tube. The lower lateral corners of the flattened tube shown in FIG. 8 are referred to with reference numbers  53  and  55 , respectively.  
     [0053] Turning to FIG. 9, formation of the block bottom of the bag is begun by folding the plies defining the front panel  18  upwardly along fold line  54  and by tucking the sides inwardly along diagonal fold lines  50  and  52 . The corners  53  and  55  are thus folded or tucked inwardly toward the longitudinal centerline of the bag, as shown in FIG. 9. The bag thus folded lies essentially flat, and diagonal fold lines  50  and  52  define a front flap in front panel  18 , denoted generally with reference number  56 , and a back flap in back panel  20 , denoted generally with reference number  58 —the two flaps  56  and  58  separated by cross-bag fold line  54 . Since the flattened blank is tubular, folding the tube in the manner just described leaves an opening denoted with reference number  59  in the bottom of the tube. The opening  59  is defined by the lateral edges  60  of front flap  56 , lateral edges  62  of back flap  58 , and by upper edge  68  and lower edge  70  of flaps  56  and  58 , respectively, and the perimeter of the opening as just noted defines a regular rectangle.  
     [0054] Owing to the manner in which bottom edges  36  and  38  of blank  12  were formed (detailed above with reference to FIG. 4) and the manner in which slits  22  and  24  were cut into the plies (also detailed above), the edges of each layer of material that define lateral edges  60  of front flap  56  are aligned or coextensive with one another. The end edges of the plies of back flap  58  are, in contrast, stepped relative to one another as shown so that plies  100 ,  200  and  300  are stepped relative to one another. Similarly, and again owing to the manner of forming blank  12  detailed above, the ply edges at upward edge  68  of front flap  56  are coextensive except for the stepped segment  40 , whereas the ply edges  108 ,  208  and  308  at lower edge  70  of back flap  58  are offset from one another.  
     [0055] Secondary cross-bag fold lines  64  and  66  are formed along the cross-bag axis across front flap  56  and back flap  58 , respectively. Secondary fold lines  64  and  66  are parallel to one another and to fold line  54 , and equally spaced on opposite sides thereof.  
     [0056] The next step in the formation of the block bottom is gluing the various flaps to one another to close opening  59  in such a manner that the interior of the bag thus formed is entirely lined with polyethylene layer  400 . Turning to FIG. 10, adhesive  72  (such as a paste glue or a hot melt glue) is shown by stippled shading. Adhesive  72  is applied to the exposed surfaces of upper and lower flaps  56  and  58  as shown. Specifically, adhesive  72  is applied to front flap  56  along a zone that laterally borders opening  59 , and along a complementary zone that borders opening  59  at back flap  58 . However, adhesive is not applied along the lower edge  70  of back flap  58 . With the adhesive applied as indicated, back flap  58  is folded upwardly along secondary fold line  66  and adhered to the now-facing surface, as illustrated in FIG. 11. The lateral side edges  62  of opening  59  in back flap  58  align with the lateral side edges  60  of front flap  56  in the longitudinal direction (arrow A in FIG. 4) and the lower edge  70  extends just beyond fold line  54 . Next, front flap  56  is folded downwardly along secondary fold line  64  over the already-folded-over back flap  58  and is glued to the outer-facing surface of the back flap. This closes opening  59  and seals the opening. Step segment  40  ensures that the seal is complete and leak free. Because the side edges  60  and  62  of the respective flaps are aligned along the longitudinal axis when the flaps  56  and  58  are folded over onto themselves as described, the entire inner-facing surface of the now-formed bag is lined with polyethylene, and there is no exposed paper.  
     [0057] The bag thus formed has an open upper end and is ready for shipment to the end user for filing. The empty bags lie flat and may be efficiently stacked onto pallets and the like for shipping.  
     [0058] Referring now to FIG. 13, it may be seen that the finished bag of the present invention comprises a tubular body having an open end and a closed end defined by the satchel-fold, also known as satchel bottom, described in detail above. The interior space defined in the bag is free from exposed paper—that is, the entire interior surface of the bag to which bulk material contained in the bag is exposed, is covered with plastic ply  400 . This avoids the problems inherent with known BB bags relating to exposed paper in the bag interior.  
     [0059] When a bulk material has been filed into the bag through the open end, the flap  28  (FIG. 5) on back panel  20  is folded over front flap  18  and sealed in place to entirely close and seal the bag.  
     [0060] While the present invention has been described in terms of a preferred embodiment, it will be appreciated by one of ordinary skill that the spirit and scope of the invention is not limited to those embodiments, but extend to the various modifications and equivalents as defined in the appended claims.