Abstract:
A method and packaging apparatus for packaging products starting from a continuous packaging web which moves uninterruptedly and wherein the web is formed into a package tube and, by the application of cross seals, is divided in separate packaged products. Prior to its folding, the film web is then stretched for obtaining, after application of the cross seals, a shrinkback effect. Due to this shrinkback effect, a tight-fitting package is obtained. The invention particularly relates to the manner in which the stretching of the film web is controlled, so that a good reproducible shrinking effect is obtained without the film web being subjected to undesired loads.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This nonprovisional application claims priority under 35 U.S.C. § 119( a ) on Patent Application No. 1019545 filed in THE NETHERLANDS on Dec. 12, 2001, which is herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a method for the use of a packaging apparatus. 
   The invention also relates to a packaging apparatus for practicing the method. 
   2. Description of Background Art 
   Such a method and apparatus are known from U.S. Pat. No. 3,589,091. In particular FIG. 14 and the passage in column 8, lines 27-63 thereof show and describe an apparatus with which products are packaged in a film which is supplied in a continuous web and which is stretched before the film is folded around the articles to be packaged. For the purpose of stretching the film, two pairs of drawing rollers 161, 162 are used, while one pair of drawing rollers located downstream are rotated at a higher speed than the pair of drawing rollers located upstream. What is thus achieved according to the US patent, is that the film is stretched. However, the US patent does not clarify in what manner the film web is supplied and in what manner the degree of stretching is determined. Without an accurate control of the transport of the film web downstream of the pair of drawing rollers and upstream of the pair of drawing rollers, with time, the film web will break or, conversely, sag upstream or downstream of, or between the pairs of drawing rollers. 
   Another drawback of the known apparatus and method is that the film shrinking back already occurs after the film has passed the downstream pair of drawing rollers 162. In particular when the film is supplied from below and is guided over a film guiding element for bringing the film in a horizontal plane, shrinking causes an irregular run, leading to tracking problems. Also when the apparatus is stopped, the film downstream of the downstream pair of drawing rollers 162 will have ample time to shrink back. What this leads to is that the first packages which are processed after restarting of the apparatus will not be tight-fitting packages, or that they can even be damaged. 
   The present invention contemplates a practical solution to these problems. 
   In practice, the packaging apparatuses marketed hitherto by applicant and the method utilized therewith yield packages in which the products are loosely enveloped by the film package. To the present day, with these known apparatuses, only one pair of drawing rollers was used for unrolling the film web from the film web roll. With the packaging apparatuses marketed by applicant, the loosely enveloping packages are the result of the fact that the cross sealing device is provided with sealing bars having a particular width, so that the products present in the package tube must be spaced at a particular distance from each other. Moreover, the cross seal has to be applied while the package tube moves in the transport direction at a speed of approximately 1.5 meters per second. That is why a space of almost 30 mm is required between two successive products plus the thickness of the product. All this results in two flaps at the end faces of the products, having a length of approximately 15 mm. Such flaps cause problems in the sorting machines of the postmen and, what is more, a non-tight-fitting package is less esthetic than a package in which the product is received in a tight-fitting manner. 
   To solve these problems, often, downstream of the packaging apparatus, a so-called shrink tunnel was utilized. In such a shrink tunnel, the packaged products are exposed to a high temperature under the influence of which the packaging film shrinks. Drawbacks of this known solution are that a shrink tunnel requires a high investment, that the use of the shrink tunnel demands a lot of energy and that, generally, shrink tunnels take up much space in a production facility, which is undesired. Moreover, special shrink film has to be utilized which is relatively expensive. 
   The apparatus known from the US patent forms a possible solution to the problems of too loose-fitting packages, that is, by stretching the film prior to folding. However, this known solution has the associated drawbacks already described. 
   SUMMARY OF THE INVENTION 
   An embodiment of the present invention contemplates a method and a packaging apparatus, in which each time, substantially the same shrinking effect is realized without the film web breaking or starting to sag or being undesirably loaded in a different manner, while, moreover, tracking problems due to irregular shrinkage behind the downstream pair of drawing rollers is prevented and wherein, also, the starting problems described are solved. 
   According to a first aspect of the method of the present invention, the packaging apparatus is provided with folding means for bringing the longitudinal edges of a continuous moving film web towards each other, the two longitudinal edges, brought towards each other, overlapping for forming a continuous package tube, while downstream of the folding means a cross sealing device is provided with the aid of which, transversely to the transport direction, cross seals are provided in the package tube, while upstream of the folding means with the aid of two pairs of drawing rollers the film is stretched such that after application of the cross seals, the film still shrinks in longitudinal direction in order that packages are obtained in which the cross seals substantially abut against the products packaged therein, so that the package extends relatively tight-fittingly around the products, wherein the film web is unwound from a film web roll which is placed on a roll holder of the packaging apparatus, while downstream of the two pairs of drawing rollers the film web is guided in a horizontal plane over a conveyor belt whose transport speed, in use, substantially corresponds to the speed of movement of the film in said horizontal plane, while above said conveyor belt, directly, or at a limited distance upstream from the cross sealing device, a pulling device is arranged which engages the package tube at the location of the overlapping edges, while also between the downstream pair of drawing rollers and the pulling device with the aid of the pulling device, the film is held in a stretched condition. 
   As the film is also kept in a stretched condition between the downstream pair of drawing rollers and the pulling device, and, therefore, a tensile force is applied to the film, the tracking problems in the path downstream of the film guiding element are solved. Moreover, stopping the apparatus does not lead, after restarting the apparatus, to a number packages being obtained which are not tight-fitting. The fact is that the products which lie on the film web extending between the film guiding element and the pulling device, lie on a stretched film web, which stretched film web can only shrink back after it has passed the pulling device. As the pulling device is arranged directly, or at a small distance, upstream of the cross sealing device, there is not any product which, during standstill of the apparatus, is present on a film web part which has the opportunity to shrink back. 
   Primarily, there is the tendency to, each time, obtain the same shrinking effect by applying an adjustable force to the film web roll or to the film web with the aid of a brake or drawing rollers. In doing so, there is the tendency to adjust the tensile force applied by means of a force measurement or the like. However, extensive experiments have shown that for a particular shrinkback length, each time, a different force is required to effect this shrinkback length. Of influence are inter alia, the film width, film thickness and the film type. Even within one run with the same film web, a different degree of shrinkback occurred at an equal application of force. Therefore, the conclusion of the experiments was that force measurement on a film web is not suitable for setting the desired degree of shrinkback. 
   According to a second aspect of the method of the present invention, the upstream pair of drawing rollers, the downstream trailing pair of rollers as well as the conveyor belt and the pulling device are driven by a controllable drive, which drives are mutually coupled by the control modules, which control modules control the mutual positions of the drives such that between the pairs of drawing rollers a desired stretch of the film is realized and that between the downstream pair of rollers and the pulling device a particular degree of stretch of the film is maintained. 
   With such a position control of the two pairs of drawing rollers, and of the pulling device with the conveyor relative to each other, an extremely accurately reproducible shrinking effect is achieved which will not or hardly vary during one run with a single film web. This in contrast to a control based on force measurement for determining the tensile stress in the film. 
   According to a third aspect of the method of the present invention, the film web is guided over a film guiding element for bringing this film into the horizontal plane of the conveyor belt, the film guiding element being designed as a tube provided with small holes, the tube being connected to a compressed air device such that compressed air flows out of the small holes and the friction between the film guiding element and the film web is minimized. It will be clear that, as a result of the method according to the invention, the friction experienced by the film web at the location of the film guiding element increases as a result of the tensile stress prevailing in the film web. With the film guiding element according to the third aspect of the present invention, the friction is reduced to a great extent. This not only leads to a lower energy consumption of the apparatus but, more importantly, to far fewer problems during start-up. The stick/slip problems usually occurring during start-up are substantially prevented with the film guiding element according to the third aspect of the present invention. 
   According to a fourth aspect of the method of the present invention, downstream of the film guiding element a sensor detects a register mark applied to the film web and, upon detection of the register mark, passes on a signal to a control, while the control controls the drives via the control modules such that the register mark is positioned relative to the products to be packaged in the desired manner. The advantage of positioning the sensor downstream of the film guiding element is that in that path, the film experiences no, or hardly any, stretch or shrink. There, the film is in a stable condition, so that drift of the register mark relative to the products present on the film in that path will not, or hardly, occur. 
   According to a fifth aspect of the method of the present invention, a control is provided with an input provision, wherein, via the input provision, an operator enters the desired package length before shrink-back and the degree of shrinkage and, optionally, a desired overstretch or understretch, or enters the packaging material, while this inputted package length, degree of shrinkage and optional overstretch or understretch is also an input signal of a control module. 
   With such an input screen, the operator can effect the desired shrinkage in a simple manner. 
   A practical method for adjusting the mutual positions of the drives according to the present invention includes, for determining the rotation of the first pair of drawing rollers, the second pair of drawing rollers as well as the conveyor belt and the pulling device, controlling the respective drives by the control modules with the aid of pulses, the ratio between the number of pulses given off to the various motors being always kept constant. 
   According to a further elaboration of the present invention, the lengthening of the film web relative to the original length can be in the range of 5% to 30%, preferably in the range of 5% to 15%. 
   A film which is eminently suited to the method according to the present invention is a film from Low Density Polyethylene. 
   Such a film does not exhibit undesirable effects under the influence of the lengthening but maintains its attractive transparency. 
   The packaging apparatus according to the present invention includes folding means for bringing the longitudinal edges of the film web towards each other, the two longitudinal edges overlapping for forming a continuous package tube, and provided with a cross sealing device located downstream of the folding means, with the aid of which cross seals extending transversely to the transport direction are provided in the package tube, a second pair of drawing rollers located upstream of the folding means and a first pair of drawing rollers located upstream of the second pair of drawing rollers for, viewed in the direction of movement of the film web before the folding means, extending the film in longitudinal direction, such that after application of the cross seals the film still shrinks in longitudinal direction in order that packages are obtained in which the cross seals substantially abut against the products packaged therein, so that the package extends relatively tight-fittingly around the products, characterized in that the apparatus is further provided with an unwinder onto which a film web roll can be placed, a conveyor belt whose transport speed, in use, substantially corresponds to the speed of movement of the film in the respective transport plane, wherein above said conveyor belt, directly or at a limited distance, upstream from the cross sealing device, a pulling device is arranged which engages the package tube at the location of the overlapping longitudinal edges, wherein the first pair of drawing rollers is provided with a first drive, wherein the second pair of drawing rollers is provided with a second drive, wherein the conveyor belt and/or the pulling device are provided with at least a third drive, wherein a control is provided with control modules arranged for controlling the mutual positions of the respective drives, such that not only between the pairs of drawing rollers a stretch is realized but also between the second pair of drawing rollers and the pulling device a particular degree of stretch is maintained. 
   With such a packaging apparatus, the advantages which are described for the method are obtained in an advantageous manner. 
   Due to the mutual dependency of the various drives, with relatively simple means the fact being that measuring wheels, encoders or means for measuring the web tension during stretching or the like are not necessary—a highly accurately reproducible shrinking effect is obtained. 
   The invention also relates to a packaging line comprising a packaging apparatus according to the invention, while upstream of the film guiding element a product-assembling device is provided, comprising a number of feeders and a product conveyor, the feeders being arranged for feeding subproducts to the product conveyor for forming multi-layered products often consisting of a number of subproducts, while upstream of the packaging apparatus and downstream of the product-assembling device, a product-positioning station is provided for positioning, in a controlled manner, the products fed by the product-assembling device on the film web in the packaging apparatus. 
   As such, the above-mentioned product-assembling devices and the product-positioning stations are known and, for a great many years already, marketed by applicant. Naturally, the use of the packaging apparatus according to the invention in such a line offers the great advantage that film-wrapped products are obtained whose film package fits closely around the products accommodated therein. Optionally, downstream of the packaging apparatus, a stacking station can be arranged for stacking the packaged products. The packaged products can, for instance, comprise graphical products such as magazines and newspapers. 
   Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further elaborations of the packaging apparatus are described in the subclaims. The method, the packaging apparatus and the packaging line will be further elucidated with reference to two exemplary embodiments, wherein: 
       FIG. 1  shows a side view of a packaging apparatus according to the state of the art; 
       FIG. 2  shows a top plan view of the packaging apparatus represented in  FIG. 1 ; 
       FIG. 3  shows a side view along line III—III of  FIG. 2 , wherein, on the unwinder, a stretching module has been placed; 
       FIG. 4  shows a front view from line IV−IV of  FIG. 2  of the unwinder with stretching module shown in  FIG. 3 ; 
       FIG. 5  shows a front view for a film guiding element destined for use in an apparatus according to the invention; 
       FIG. 6  shows a top plan view of the film guiding element shown in  FIG. 5 ; 
       FIG. 7  schematically shows a first exemplary embodiment of a possible control diagram of an apparatus according to the invention; 
       FIG. 8  schematically shows a second exemplary embodiment of a possible control diagram of an apparatus according to the invention; 
       FIG. 9  schematically shows a third exemplary embodiment of a possible control diagram of an apparatus according to the invention; 
       FIG. 10  shows a schematic side view of a second exemplary embodiment of the packaging apparatus; 
       FIG. 11  shows a side view of a packaging line; and 
       FIG. 12  shows a top plan view of the packaging line represented in FIG.  10 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now we described with reference to the accompanying drawings, wherein the same or similar elements have been identified by the same reference numeral throughout the several views. 
     FIGS. 1 and 2  show an example of a packaging apparatus according to the state of the art. The known apparatus is provided with an unwinder A onto which a film web roll can be placed. From the film web roll, a film web to be unwound is guided via guiding means to a film guiding element  7 . In the examples of the state of the art which are represented in  FIGS. 1 and 2 , these guiding means are not visible but they comprise in any case a pair of drawing rollers for unwinding the film web from the film web roll, a dancer roller for keeping the film web at a particular tension and one or more guiding rollers. In the apparatuses of  FIGS. 1-4 ,  7  and  8 , the axle  2  of the unwinder A extends parallel to the transport direction of the packaging apparatus  1  and the roll is arranged next to the packaging apparatus  1 . In the schematic exemplary embodiment of  FIG. 5 , the roll holder axle  2  extends perpendicularly to the transport direction of the packaging apparatus  1  and the unwinder A is arranged in alignment with the packaging apparatus  1 . 
   With the aid of the film guiding element  7 , the film web F is guided in a horizontal plane over a transport path or conveyor belt  8  whose transport speed, in use, substantially corresponds to the speed of movement of the film F in the horizontal plane mentioned. Optionally, the conveyor belt  8  can be provided with suction openings and a vacuum chamber can be disposed below the upper conveyor belt, so that the film F is sucked to the conveyor belt  8  so as to limit slip between the conveyor belt  8  and the film web F to a minimum. Further, the packaging apparatus is provided with folding means  9  with the aid of which the longitudinal edges of the film web F are brought towards each other until the brought together longitudinal edges overlap for forming a continuous package tube. In the present exemplary embodiment, the folding means comprise the rods  9  which are clearly visible in  FIGS. 1 ,  2 ,  7  and  8 . Above the conveyor belt  8 , at a short distance from or directly before a cross sealing device  11 , a pulling device  10  is arranged, which, in the present exemplary embodiments, is designed as a pulling belt. However, it is also possible that this pulling device is formed by one or more rollers. Directly downstream of the pulling device  10 , the earlier-mentioned cross sealing device  11  is arranged with the aid of which, transversely to the transport direction, cross seals are provided in the package tube between the successive products.  FIG. 1  further clearly shows an input provision  14 . 
   The appearance of the apparatus according to the invention deviates from the known apparatus shown in  FIGS. 1 and 2  in that on top of the unwinder A, a stretching module B is arranged in which a first pair of drawing rollers  22  and a second pair of drawing rollers  23  are included. Moreover, in the apparatus according to the invention, the dancer roller is left out because the web tension is controlled in a different manner. 
   As is clearly visible in  FIG. 3 , the film web F is guided from the film web roll over two guiding rollers  6   a ,  6   b  to then be guided between the first pair of drawing rollers  22 . Then, the film web F is guided over a third guiding roller  6   c  and between the second pair of drawing rollers  23 . From the second pair of drawing rollers  23 , via a fourth guiding roller  6   d , the film web is fed into the packaging apparatus, while the film web F passes a guiding roller  5  and, optionally, a fourth guiding roller  6   d , to be transported from that location to the film guiding element  7  (see FIG.  5 ). 
     FIG. 3  also clearly shows that the unwinder with stretching module has two positions  2 A,  2 B for placing a film web roll R. In that manner, rapidly, a new film web can be fed into the packaging apparatus when one of the rolls is empty. The fact is that, already, a second film web roll can be put ready before the first film web roll has completely unwound. In that manner, a considerable reduction of exchanging time is obtained. With the interrupted line, the path is indicated along which the film web F extends when it is unwound from the top film web roll.  FIG. 3  also clearly shows the servodrive  24  of the first pair of drawing rollers  22  as well as the servodrive  25  of the second pair of drawing rollers  23 . Also visible in  FIG. 3  are the brakes  12 A and  12 B, which prevent the rolls R from continuing to rotate when the packaging apparatus is stopped. 
     FIG. 4  further shows the unwinder with the stretching module disposed thereon in front view, while with the same reference numerals as in  FIG. 3 , corresponding parts are indicated. Also, in  FIG. 4 , a control housing  26  is clearly visible. 
   For obtaining a tight-fitting film package with an attractive appearance, for instance Low Density Polyethylene can be used as a film, while the lengthening with respect to the original length is in the range of 5% to 30%, preferably in the range of 5% to 15%. 
     FIG. 5  shows a front view of the film guiding element  7  which is used in the present exemplary embodiment. The film guiding element  7  is provided with a tube having two bent extremities  7   a ,  8   b . The tube is provided with holes  7   c . In mounted condition, the tube is connected to a compressed air device such that compressed air flows out via the small holes  7   c .  FIG. 6  shows a top plan view of the film guiding element  7  represented in FIG.  5 . In spite of the fact that the force with which the film web F is drawn against the film guiding element is great, with such a film guiding element  7 , the friction between the film web F and the film guiding element  7  is relatively small. In particular when starting the packaging process, this is of great importance because then, stick/slip problems hardly occur at the film guiding element  7 . 
     FIGS. 7-9  schematically show different variants of control of the drives  24 ,  25 ,  27  of the first pair of drawing rollers  22 , the second pair of drawing rollers  23  and the assembly of the transporter  8  and the pulling device  10 , respectively, which last two are driven by a common, third drive  27 . Highly schematically, the film web roll R, the brake  12  of the unwinder, the film guiding element  7  and the cross sealing device  11  are represented. Although the film web F in  FIGS. 7-9  is represented as a straight line, it should be pointed out that this has only been done for the purpose of the schematic representation; in practice, the film web F will run from a bottom side to the film guiding element  7  and be guided over the film guiding element  7  in the horizontal plane.  FIGS. 7-9  further show the longitudinal sealing device  30  which is also visible in FIG.  1  and which serves for interconnecting the longitudinal edges of the film web F. Further, a sensor  31  is indicated, provided directly downstream of the film guiding element  7 . The signals of the sensor  31  are guided to the control module  28  of the control  32 . The sensor serves for detecting a register mark applied to the film web F. On the basis of the register mark, the products are put on the film web. As the sensor  31  is arranged downstream of the film guiding element  7 , the risk of products still shifting relative to the register mark is reduced to a minimum. The fact is that the sensor  31  is located in an area of the film web F in which the film web F is in a substantially stable condition. 
   In the exemplary embodiment of  FIG. 7 , via the first control module  28 , the second drive  25  is coupled to the third drive  27 . The third drive  27  in its turn is coupled via the second control module  29  to the first drive  24 . In the exemplary embodiment of  FIG. 8 , the first drive  24  is coupled, via the first control module  28  to the second drive  25 . Moreover, the first drive  24  is coupled, by the second control module  29 , to the third drive  27 . In the exemplary embodiment of  FIG. 9 , the first drive  24  is coupled, via the first control module  28 , to the second drive  25 . This second drive  25  in its turn is coupled, via the second control module  29 , to the third drive  27 . In all these variants, the control modules  28 ,  29  ensure that the ratio between the number of pulses guided to the various drives  24 ,  25 ,  27  is always equal. What is thus effected is that the degree of stretch between the pairs of drawing rollers  22 ,  23  is constant and that the degree to which the stretch is maintained, reduced or increased in the path between the second pair of drawing rollers  23  and the pulling device  10 , is also constant. As the stretch can be set in two paths, the manner in which the total stretch is effected can be selected. For instance, between the two pairs of drawing rollers  22 ,  23 , first, some additional stretch can be effected, which additional stretch is slightly relaxed in the path between the second pair of drawing rollers  23  and the pulling device  10 . On the other hand, it is also possible that the stretch of the film in the path between the second pair of drawing rollers  23  and the pulling device  10  is slightly augmented. By varying these stretch ratios, the speed at which shrinkback takes place after the cross sealing device  11  as well as the degree of shrinkback can be influenced. A rapidly built-up stretch leads to a fairly rapid shrinkback, while a gradually built-up stretch leads to a somewhat slower shrinkback behavior. Moreover, the speed of stretching can also influence the degree of shrinkback of the film. This latter applies also for the duration in which the stretch is maintained. As a result of the presence of the two paths in which stretch can be effected and can be varied, the properties of the film can be taken into account such that the desired degree of shrinkage after the cross sealing device  11  occurs within a particular period of time. The fact is that generally, after the cross sealing device  11 , the products will be stacked and be tautened by a tautening strap, so that free shrinkback can no longer take place. Therefore, the shrinkback process must have taken place before the tautening strap is provided around a newly formed stack of products. 
   Via the input provision  14 , the degree of shrinkage, the package length and the optional overstretch can be made known to the control. However, it is also possible that the package length be derived from data known from the database as to the products to be packaged or from other settings of the apparatus relating to the package length, such as, for instance, the length of stroke of the cross sealing device. Generally, per product to be sent, the data base contains data about the annexes to include and therefore, potentially, has the length information of each product. Further, it is possible that the package length is measured with a sensor in the product-assembling path or directly behind the film guiding element, and is passed onto the control. Further, it is possible that the feeders in the product-assembling path are provided with sensors with the aid of which the length of a product present therein can be detected. Based on the package length, the desired shrinkage can be obtained. Optional overstretch or, conversely, understretch which is to take place in the first stretching path can be entered via the input provision. As these data are strongly dependent on the packaging material, the input could, for instance, comprise a description of the packaging material, from which the control itself then deducts to what degree overstretch or understretch is to take place in the first path. 
     FIG. 10  further shows a second exemplary embodiment of a packaging apparatus according to the invention, wherein the film web roll R is placed on an unwinder which is arranged in alignment with the packaging apparatus. Clearly visible in this Figure is the fixedly arranged guiding roll  5  which is not located in the unwinder but below the conveyor belt  8  in the packaging apparatus  1 . In the conventional, known apparatus, this was the location of the dancer roller. Further, the first and the second pair of drawing rollers  22 ,  23 , respectively, are clearly shown. Once again, it is emphasized that the representation in  FIG. 10  is of a schematic character. 
   Further,  FIGS. 11 and 12 , respectively, show a top plan view and a side view of a packaging line in which a packaging apparatus  1  according to the invention is included. The packaging line further comprises a product-assembling device  17  arranged upstream of the film guiding element  7  and which is provided with a number of feeders  18  and a product conveyor  19 . The feeders  18  are designed for feeding subproducts to the product conveyor  19  for forming multi layer products, often consisting of a number of subproducts. Upstream of the packaging apparatus  1  and downstream of the product-assembling device  17 , a product-positioning station  20  is provided for positioning, in a controlled manner, products dispensed by the product-assembling device  17  on the film web F in the packaging apparatus  1 . Downstream of the packaging apparatus  1 , a stacking station  21  for stacking the packaged products is provided. It will be clear that the packaging line and the packaging apparatus shown are particularly destined for packaging graphical products in film, such as magazines and newspapers, optionally while adding different types of products such as, for instance, CDs, cassettes and the like. 
   It will be clear that the invention is not limited to the exemplary embodiments described but that various modifications are possible within the framework of the invention and as defined by the claims.