Abstract:
In order to reduce the percentage of leaks in sealed packages, the sealing jaws contain one or more raised regions that correspond to problem areas for leaks, i.e., adjacent to back seals and folds in the package design. The raised regions are raised approximately 4-20 mils above the regions that seal the normal two layers of material.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to packaging of a product in a heat-sealable pouch, and more particularly to producing a hermetic seal in portions of a heat-sealable pouch where adjoining areas have a variable number of layers of material. 
     2. Description of Related Art 
     POUCH PACKAGING 
     Many snack foods, like chips, pretzels, etc., are packaged in pouches formed of very thin packaging films. These pouches can be manufactured on vertical form, fill, and seal packaging machines that, as their name implies, forms a package, fills it with a product, and seals the filled package. 
     One such packaging machine is seen diagrammatically in FIG.  1 . Packaging film  110  is taken from a roll  112  of film and passed through tensioners  114  that keep it taut. The film then passes over a former  116 , which directs the firm into a vertical tube around a product delivery cylinder  118 . As the tube is pulled downward by drive belts  120 , the vertical tube of film is sealed along its length by a vertical sealer  122 , forming a back seal  124 . The machine then applies a pair of heat-sealing jaws  126  against the tube to form a transverse seal  128 . This transverse seal  128  acts as the top seal on the bag  130  below the sealing jaws  126  and the bottom seal on the bag  132  being filled and formed above the jaws  126 . After the transverse seal  128  has been formed, a cut is made across the sealed area to separate the finished bag  130  below the seal  128  from the partially completed bag  132  above the seal. The film tube is then pushed downward to draw out another package length. Before the sealing jaws form each transverse seal, the product to be packaged is dropped through the product delivery cylinder  118  and is held within the tube above the transverse seal  128 . 
       FIG. 1   c  shows a sealing carriage on a prior art form, fill, and seal packaging machine.  FIG. 1   b  is a cross-section of a prior art sealing assembly installed in the carriage. Opposed transverse seal facings  102 ,  104  are shown attached to opposed bridge assemblies  106 ,  108 . Behind each seal facing  102 ,  104  is a heat probe  128 ,  130  capable of heating the exposed surface of the seal facings up to 450° F. during operation. Placed between the seal facings  102 ,  104  and their respective bridge assemblies  106 ,  108  is a heat separator  136 ,  138 . The heat separators  136 ,  138  provide a thermal barrier between the seal facings  102 ,  104  and the bridge assembly  106 ,  108  in order to minimize heat transfer to the bridge assembly  106 ,  108  and other components attached thereto. A thermocouple or thermistor  132 ,  134  is placed between the heat separator  136 ,  138  and the heat probe  128 ,  130  in order to monitor the seal facing  102 ,  104  temperature. As previously described, a transverse seal is formed when a tube of film placed between the seal facings  102 ,  104  is flattened as the seal facings  102 ,  104  are brought in close proximity to each other. To accomplish this, both bridge assemblies  106 ,  108  move towards each other on stationary rods  110 ,  112 . 
     Also shown attached to the bridge assemblies  106 ,  108  is a pair of opposed crumb plates  114 ,  116 . When the bridge assemblies  106 ,  108  are brought together, the crumb plates  114 ,  116  slightly overlap, thereby forming a temporary “S” shaped constriction in the flattened film tube. This “S” shaped constriction provides a means for keeping product out of the transverse seal area while the tube of film above the constriction is filled with product. Two stripper bars  118 ,  120  are also shown attached to the bridge assemblies  106 ,  108 . The stripper bars  118 ,  120  are typically spring loaded and provide the stripping function previously described. 
     Both of the seal facings  102 ,  104  are constructed with a central channel  141 ,  143  to accommodate a knife or cutting tool  122 , best seen in  FIG. 1   c . This knife  122  is recessed within the channel  141  of one of the seal facings  102  and held in place by two knife brackets  124 ,  126  attached to the bridge assemblies  106 ,  108 . After the transverse seal has been formed on the flattened film tube, the knife brackets  124 ,  126  are rotated slightly, thereby exposing the knife  122  to the transverse seal. This action cuts the transverse seal, thereby completing the process of forming, filling, and sealing a single package. 
     The prior art carriage described above is designed such that the seal facings  102 ,  104  can be removed in order to install another set of seal facings that provide for different vertical widths of the resultant transverse seals. The facing assembly shown in  FIGS. 1   b  and  1   c  is aligned at the factory, and it is not recommended that the operator remove more than the seal facings  102 ,  104 , crumb plates  114 ,  116 , and stripper bars  118 ,  120  from the facing assembly. An enlarged perspective of one of the facings by itself can be seen in  FIG. 3   b.    
     There can be variations to the specific art described above. For instance, some machines do not require crumb plates and stripper bars, although other machines do. Additionally, in many packaging machines, the facing  102 / 104  is not a separate, replaceable part of the jaws, but is integral with the jaws. 
     There are three main parameters of the sealing mechanism that are typically changed to correct improper sealing of a bag: temperature, pressure, and dwell time (the time the seal jaws are closed to form the seal). The materials used generally seal within a given range of temperatures, such as 375-425°, although this range can vary, depending on the accompanying pressure and dwell time. Of these three variables, the pressure is generally set at the factory by a mechanic, and is not easily changeable. A typical pressure would be about 300 pounds of pressure across the entire facing, with the pressure generally fairly evenly distributed across the entire facing. Thus, for an eight-inch wide bag, there can be approximately eight square inches of packaging contacted when making the top/bottom seal, or a pressure of about 37.5 pounds per square inch for a seal that is ½ inch wide. 
     In contrast to the factory-set pressure, the temperature and dwell time are operator decisions at the time the product is packaged. The operator will generally be familiar with the specific materials being used for a package and can vary the time and temperature parameters as needed to obtain an effective seal, within the constraints of the situation. One such constraint is that increasing the temperature past a given range for a material can result in burning, or melting a hole through the material. An additional constraint is the effective throughput of a machine, which can be affected by the dwell time. For instance, if a seal formed at a given temperature and pressure is not holding after {fraction (1/10)} of a second, increasing the dwell time of the sealing mechanism to ⅕ second, or even ½ second, may significantly improve the seal, but it may also mean that the machine can only package a fraction of the product it can handle at a lower dwell time. A dwell time that requires additional machines to meet a production schedule is not an economic solution. 
     A typical film used for packaging snack foods is seen in FIG.  2 . The outermost layer  202  is an OPP, short for oriented polypropylene, while the innermost layer  206  is a metalized oriented polypropylene. An oriented polymer material has been specially treated so that the molecules tend to align in a given direction, causing the material to tend to preferentially tear in that direction. Sandwiched between the two OPP layers is a layer  204  of polyethylene that contains any printing on its outer surface. The innermost, metallic layer  206  can itself be a layered laminate and contains a sealant layer  208  on what will be the inside of the package. This sealant layer is composed of a ter-polymer, composed of ethylene, propylene, and butylene. The bag is sealed by bringing together two sections of the metallic layer, with their sealant layers together. When heat and pressure are applied through the jaws, the adjacent sealant layers melt together and form a seal. Other materials used in packaging are polyester, paper, polyolefin extrusions, adhesive laminates, and other such materials, or a layered combination of the above. 
     Ideally, every seal on every package would be hermetic, or leak-proof, even under pressure changes. This is especially important with snack foods, so that flavor and freshness are preserved. Areas where the package has a back seal, folds, or gussets provide extra layers of material in the seal, but this problem becomes more acute with thicker packaging materials, additional folds in the package design, and smaller packages.  FIG. 3   a  shows a cross-section along the length of a pair of prior art crimperjaws  300  having a bag  350  that is about to be sealed between the jaws  300 . In this drawing, the areas near the back seal and the gusset are enlarged to form  FIGS. 3   c  and  3   e  respectively. In  FIGS. 3   d  and  3   f , each of these locations is then shown again after the seal has been made, with an arrow pointing to the small area where triangular capillary leaks tend to occur. As can be seen in these enlargements, the immediate areas where the number of layers changes is the most likely location for a leak. While it is probably impossible to totally eliminate leakers in the production line, the goal is always to achieve a vanishingly small number of them. 
     Co-pending patent application Ser. No. 10/061,133, filed Jan. 31, 2002, which is owned by the assignee of this application and has overlapping inventorship, deals with the problem of sealing a package having a large fold and a relatively thick packaging material, although in this application, a hermetic seal was not a primary concern. In this co-pending application, the sealing jaws were modified to provide relief in the areas of the bag having the thicker layers. An exemplary embodiment from this application is seen in cross-section in FIG.  4 . In this drawing, the facings  400  have been relieved in sections A, C, and E of the bag  450 , where the extra layers of the bag are present, so that a more even pressure is applied across the whole length of the seal. In the embodiment shown, the given areas of each facing were relieved by an amount equal to one thickness of the packaging material used. The edges of the relief areas do not form sharp corners, but are beveled approximately 45 degrees to allow for some slippage in the alignment of the material in the machine. 
     U.S. Pat. No. 6,230,781 to Smith addresses a hermetic seal in a different manner. A cross-section of the crimpers of Smith is seen in FIG.  5 . These crimpers  500  are set up to provide a rocking motion as they seal, but otherwise do the same job and have many of the same problems as the crimpers discussed above. In order to address the varying number of layers of material to be sealed, Smith&#39;s crimpers incorporate an insert  510  of a resilient material on one side of the sealing area. The insert  510  has the form of a thick piece of rubberized material, stiffened with a core of metal  515 . The resilient material, in contrast to an all-steel crimper, can provide some “give” to allow for extra layers of material. 
     It has long been considered desirable to reduce the number of leaking packages produced in the production line without increasing dwell time, even when the design of the package makes this more difficult. 
     SUMMARY OF THE INVENTION 
     In the disclosed crimpers, the areas that produce the leaks are given extra pressure by providing slightly raised areas on the surface of the facing. These raised areas target approximately ⅛ to ¼ inch to either side of problem areas. The raised areas are slight, approximately 1 to 30 mil, and more preferably 4 to 20 mil (i.e., 0.004-0.020 inches), for a film thickness of 2 to 4 mils (i.e., 0.002 to 0.004 inches) and can almost escape visual detection, but are sufficient to provide the necessary hermetic seal. The innovative crimpers provide the advantage of a simple, economical method of sealing areas that have previously been recalcitrant. The raised regions of the sealers can be the same material as the jaws or facings, i.e., steel, or they can be formed of a resilient material that provides some “give” during sealing. These regions can be formed integrally with the facings or they can be manufactured as inserts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will be best understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1   a  is a diagrammatic view of a form, fill, and seal machine, known in the prior art. 
         FIG. 1   b  is a cross-section and  FIG. 1   c  is a perspective drawing, both of a sealing assembly from a form, fill, and seal machine. 
         FIG. 2  shows the layers in a typical packaging material for snack foods. 
         FIG. 3   b  shows a perspective of a prior art facing.  FIG. 3   b  shows the top of a bag having both a fin seal and a cross-seal, captured between a pair of facings such as are shown in  FIG. 3   a  and demonstrates the problems areas when sealing.  FIGS. 3   c - 3   f  demonstrate the areas on such a bag where pinhole leaks tend to occur. 
         FIG. 4  shows sealing jaws, from a co-pending patent application, which address the problem of sealing across a varying number of layers of packaging material. 
         FIG. 5  shows a cross-section of the crimpers of Smith in U.S. Pat. No. 6,230,781. 
         FIG. 6   a  shows a cross-section of a pair of crimpers designed to work with the bag shown, according to an embodiment of the present invention.  FIG. 6   b  shows a perspective of the modified crimper of  FIG. 6   a.    
         FIG. 7  shows a cross-section of a pair of crimpers designed to work with another bag, according to an alternate embodiment of the present invention. 
         FIG. 8   a  shows a cross-section of an alternate pair of crimpers.  FIG. 8   b  shows a perspective of the modified crimper of  FIG. 8   a.    
         FIG. 9  shows the impression made on a sheet of pressure sensitive paper using the crimpers of FIG.  7 . 
     
    
    
     DETAILED DESCRIPTION 
     Several embodiments of the innovative invention will now be described with reference to  FIGS. 6-8 . 
     In a first embodiment, the bag to be formed has only a back lap seal, seen in  FIG. 6   a . A cross-section of one embodiment of the innovative jaws are shown positioned on either side of the bag  650 . Here, the jaws  600  are formed entirely of steel, with the targeted area around the back seal being formed as an insert  610  to the rest of the facing. The targeted area on the facing has a thickness that is only 4 mils (i.e., 4 thousandths of an inch) thicker than the rest of the facing (note that these drawings are not to scale). This thickness is barely perceptible to the naked eye, but can be enough to provide a hermetic seal. With only one layer difference in the thickness across the seal, the use of more pressure over the back seal would not be necessary in all situations, but would be especially helpful when the packaging materials are thick. A perspective of this facing is seen in  FIG. 6   b.    
     In a second embodiment,  FIG. 7  shows a set of crimpers designed to use with the bag  750  shown. In this embodiment, the bag  750  to be formed contains a fin seal, as well as deep pleats at each end of the bag, so that there are three areas A, C, E having four layers of material, while other areas B, D have only two layers. The lower crimper  700  has raised areas that correspond to the junction between regions A and B, between regions C and D, and between regions D and E. This embodiment of the crimpers  700  contains inserts  710  in the targeted areas, each insert having a surface layer  715  of a resilient material. The resilient material is composed of rubber, synthetic rubber, or similar material, has a thickness in the range of ⅛ to {fraction (1/16)} inch, and is fastened to the rest of the insert by gluing. 
     Given that the bag in  FIG. 7  has large areas with four layers of material to be sealed, it can be desirable to provide an alternate embodiment, shown in  FIG. 8   a , especially if the packaging material is fairly thick. In this alternate embodiment, the surface of the crimper  800  occupies at least three levels. If we consider the sections C, E of the crimpers  800  that will contain only two layers to be the “normal” height for a bag of this thickness, the three sections B, D, F are raised to provide extra sealing power at the areas most likely to cause problems, i.e., where the number of layers changes. The areas A, C that contain four layers of material are relieved from the “normal” height in order that there be no bridging across the sections C, E having only two layers. To put it another way, sections  805  of the crimpers  800  are the farthest away from the upper crimper  800 , section  820  is slightly closer to the opposite crimper, and sections  810  are the closest to the opposite crimpers  800 . In this embodiment, the entire sealing surface is formed as a single piece having the raised and relieved portions integral with the other portions of the facing. A perspective of one of the crimpers  800  is shown in  FIG. 8   b.    
       FIG. 9  shows a tracing made by clamping a piece of pressure-sensitive paper in the jaws of FIG.  7 . As seen in this figure, the areas of the paper  902 ,  904 ,  906  that correspond to the raised sections of the facing received a greater percentage of the available pressure. Adjacent regions  910 ,  912 ,  914 ,  916  will receive somewhat less than they would receive using a prior art facing, but still sufficient to provide consistent sealing in these non-critical areas. In testing using the inventive sealing jaws, this configuration has been found to improve sealing in the problem areas. The exact amount of improvement would depend on the design of the bag, the materials used in its manufacture, and the specific embodiment of the innovative facings.