Abstract:
A system and method are disclosed for pre-stretching plastic material that is later used in wrapping a plurality of objects. Plastic material is continuously or non-continuously provided to a pre-stretching device including a plurality of rollers. The plastic material is passed around the plurality of rollers within the pre-stretching device and is then fed through an accumulator to a strapping device. The passing of the plastic material around the plurality of rollers causes the plastic material to stretch by a predetermined amount before entering the strapping device.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional Patent Application Ser. No. 60/673,029, filed Apr. 20, 2005 and herein incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to the manufacture and use of strapping products. More particularly, the present invention relates to the preparation of strapping materials for use in securing items such as plastic containers.  
       BACKGROUND OF THE INVENTION  
       [0003]     The plastic container industry has utilized strapping systems to unitize their products for over thirty years. In the past twenty years, this industry has migrated to plastic strapping systems to unitize their products.  
         [0004]     In recent years, the plastic container industry has undergone its own transformation. Plastic containers have become lighter and more cost efficient to manufacture. Additionally, the appearance, shape and design of containers has become a marketing tool. These changes to the plastic containers have created packaging issues for the unitization of plastic bottles.  
         [0005]     Historically, the amount of initial strap tension applied to a plastic container skid/unit was great enough to allow for the tension decay typical of plastic strapping, as well as the tension drop resulting from the shifting and shrinkage of individual plastic containers, while maintaining adequate package containment pressure to ensure proper unitization. New plastic container designs and increasing market pressure to produce containers at lower prices (meaning less resin per container), however, have created new challenges for the strapping process. In today&#39;s market, new container designs often do not allow for initial strapping tension levels above about 65 lbs. For this reason, the tension levels that are used fall to about 40 lbs. At this level, strapping is unsuitable for maintaining adequate package containment pressure to ensure proper unitization of the plastic containers.  
         [0006]     Without an acceptable level of performance from strapping products, additional packaging products have been introduced to assist the packaging of plastic containers. In particular, the use of plastic stretch film has been introduced to aid in packaging. With the introduction of stretch film, the industry found an improved process that provided the level of unitization required to transport their plastic containers to their customer. However, the introduction of stretch film to the unitization process for plastic containers also increased the cost of unitization by a multiple of about 2.5. Therefore, it would be desirable to develop a system and method for unitizing bundles of plastic containers that possess the benefits of stretch film while also lowering the unitization cost.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides for an improved system and method of pre-stretching material for use in unitizing groups of plastic bottles. The present invention involves the use of a novel pre-stretching machine having a plurality of rollers. The rollers are used to pre-stretch the plastic strapping, which is then transferred to an accumulator before being used to package the respective containers.  
         [0008]     The pre-stretch system and method of the present invention provides for the elimination of stretch film while producing higher retained strap tension that provides for a secure load of plastic containers. The present invention effectively eliminates the need for high tension of the package while providing a strap that is within the dynamic working range for retained tension. The plastic strapping can be pre-stretched without destroying the properties and characteristic of the plastic strapping, instead only temporarily losing stability and rigidity in the strapping.  
         [0009]     These and other objects, advantages and features of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the several drawings described below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a graph illustrating the relationship between strap load vs elongation over time;  
         [0011]      FIG. 2  is a side view of a pre-stretching device, a material accumulator and a packaging machine according to one embodiment of the present invention;  
         [0012]      FIG. 3  is a front view of pre-stretching device of  FIG. 2 , along with a representation of a portion of the material accumulator with the position of the accumulator wheel assembly, both according to one embodiment of the invention;  
         [0013]      FIG. 4  is a side view of the pre-stretching device of  FIG. 3 ;  
         [0014]      FIG. 5  is a top view of the pre-stretching device and material accumulator portion of  FIG. 3 ;  
         [0015]      FIG. 6  is a graph illustrating the effect of pre-stretching on retained tension; and  
         [0016]      FIG. 7  is a graph illustrating the effect of pre-stretching on stiffness over time. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     Plastic strapping is produced using a stretching process that is similar to that employed in the production of fibers. This process results in the preferential orientation of polymer molecules (or chains) along the strap length and a several-fold increase in tensile strength.  
         [0018]     Oriented plastic exhibits viscoelastic-plastic behavior in response to mechanical loading. Elastic deformation and recovery is partly immediate and partly time-dependent. Yielding occurs over a broad range rather than at a well defined point, and the yield deformation itself is time dependent and only partly permanent (or “plastic”), the rest being recoverable over time. It is believed that the yield corresponds to plastic shearing between polymer chains corresponding to the disruption of secondary inter-chain bonds, for example as discussed in Northolt, M. G. et al., Polyer Vol. 36, No. 18, pp. 3485-3492, 1995, herein incorporated by reference, which provides a good description of the material mechanics and propose relevant theoretical models.  
         [0019]      FIG. 1  illustrates how upon loading, unloading and almost immediate reloading, the yield “knee” in the load-elongation curve is no longer apparent. However, with a sufficient delay prior to reloading, the original yield characteristics return as the material seeks an equilibrium and secondary inter-chain bonds are re-established.  
         [0020]     Being viscoelastic, plastic strapping is susceptible to time dependent relaxation upon loading. Plastic strap that is loaded and held at a constant elongation will undergo a load decay (stress relaxation) while plastic strap that is under constant load will continue to elongate over time (creep). This behavior contrasts with that of steel, for example, where creep and stress relaxation are negligible.  
         [0021]     As previously mentioned, stress relaxation can compromise package unitization. It has been recognized that fibers which exhibit viscoelastic effects such as stress relaxation could be diminished by mechanical conditioning consisting of pre-loading the fibers to a load level that is higher than that used in the second loading. This is discussed, for example, in Leaderman, H., Elastic and Creep Properties of Filamentous Materials and Other High Polymers, 1943, herein incorporated by reference. The present invention makes use of this technique, hereafter referred to as “pre-stretching”, to reduce the strap tension losses that result from stress relaxation and package settling.  
         [0022]      FIGS. 2-5  show a pre-stretching device  100  and a material accumulator  200  constructed according to one embodiment of the present invention. As shown in  FIGS. 2-5 , the pre-stretching device  100  includes a bevel gear motor  110  that is used to provide power to a plurality of rollers  120 . Although a bevel gear motor  110  is depicted in  FIGS. 2-5 , other types of motors may be used. A pair of polychains  130  are used to transfer the energy generated by the bevel gear motor  110  to the plurality of rollers  120 .  
         [0023]     In the embodiment of the invention depicted in  FIGS. 2-5 , ten of the rollers  120  are included in the pre-stretching device  100 , with the rollers  120  separated into a first roller group  140  and a second roller group  150 .  
         [0024]     The path of material  160  from a coil  165  to be pre-stretched is shown in  FIGS. 2 and 3 . The material  160 , which can comprise polyester or polypropylene strapping in various embodiments of the invention, is fed from the coil  165  through an entrance point  170  on the pre-stretching device  100 . In one embodiment of the invention, the material  160  passes over a preliminary tower  185  before reaching the entrance point  170 . After passing through the entrance point  170 , the material  160  passes around each of the rollers  120  in the first roller group  140 . Once the material  160  has passed around the last of the rollers  120  in the first roller group  140 , it travels to the second roller group  150 , where it passes around each of the remaining rollers  120  before exiting the pre-stretching device  100  through an exit point  180 . In one embodiment of the invention, the first roller group  140  and the second roller group  150  each comprise five of the rollers  120 .  
         [0025]     Generally, the material  160  can be pre-stretched to any elongation level that exceeds that which is applied to the package, but that is less than the break elongation of the strap. In one embodiment, for polyester strapping, the pre-stretch elongation level should be at least about 5% in order to realize a benefit to unitization and it should be no greater than about 10% so as to minimize the risk of the strap fracturing during pre-stretching. Within this range, higher pre-stretching levels offer greater potential benefits to unitization, but at the cost of greater reductions to flexural rigidity and the resulting challenges to consistent feeding of the strap through large arches.  
         [0026]      FIG. 7  illustrates the effect of pre-stretching on stiffness of the strap. As can be seen from  FIG. 7 , pre-stretching results in a lower bending load being necessary to achieve a particular deflection angle. The impact of the prestreching on stiffness dissipates over time, as can be seen from the increase in stiffness from the 0.5 minute sample through to the 19 hour sample. In addition, a greater amount of pre-stretching will result in a greater reduction in stiffness as can be seen in the 10% pre-stretching sample in comparison to the non-stretched and the 7% pre-stretch samples.  
         [0027]     In one embodiment of the invention, the ratio of the roll surface speed of the second roller group  150  to that of the first roller group  140  is fixed at about 1.07:1. This causes the material  160  to be stretched by up to about seven percent over its original length. Importantly, this design maintains the original properties and characteristics of the material  160  and maintains the integrity of the material  160  due to the recovery of the pre-stretched strapping, ultimately creating a tighter fit around the objects to be strapped during the unitization process. This embodiment of the present invention may be used in conjunction with a commercial strapping system to achieve improvement tension retention on PET bottle loads, as shown in  FIG. 3 . It may be seen that one week following strap application, the tension loss in the pre-stretched straps was two thirds that of the straps that were not pre-stretched.  
         [0028]     In one embodiment of the invention, each of the plurality of rollers  120  is preferably coated with urethane rubber. The urethane rubber coating provides a high friction surface which prevents the material  160  from slipping on the rollers  120 . It will be evident to those skilled in the art that slippage could also be prevented by using other types of surface finish, or by increasing the wrap angle or the number of rollers, or perhaps by pre-stretching in a plurality of stages.  
         [0029]     The material  160  can be fed through the pre-stretching device  100  at a variety of speeds. In one embodiment of the invention, the material  160  is routed through the pre-stretching device  100  at about five feet per second. In another embodiment, the material  160  is routed through the pre-stretching device  100  at about four feet per second. At this speed, the material  160  is stretched by up to about eight percent over its original length. Importantly, this design maintains the original properties and characteristics of the material  160  and maintains the integrity of the material  160  due to the recovery of the pre stretch strapping, ultimately creating a tighter fit around the objects to be strapped during the unitization process.  
         [0030]     After the material  160  exits the pre-stretching device  100  through the exit point  180 , it enters the material accumulator  200  shown in  FIGS. 2, 3  and  5 . The material accumulator  200  includes a housing  210 . The material accumulator  200  includes an accumulator entrance region  230  for the material  160 . Substantially opposite the entrance region  230  is an exit region  240 . The exit region  240  includes an accumulator wheel assembly  265  which, in one embodiment of the invention, includes a wheel bracket  250  for mounting a flat accumulator wheel  260  and a flanged accumulator wheel  270 . The material  160  passes through the accumulator entrance region  230 , across the material accumulator  200 , and through the accumulator exit region  240  through the accumulator wheel assembly  265 , particularly the flat accumulator wheel  260  and the flanged accumulator wheel  270 . The flanges on the flanged accumulator wheel  270  act as a guide for the material  160 .  
         [0031]     In one embodiment of the invention, the accumulator wheel assembly  265  is positionally adjustable. For example and as shown in  FIG. 2 , the accumulator wheel assembly  265  is located at the top of the material accumulator  200 . In  FIG. 3 , on the other hand, the accumulator wheel assembly  265  is positioned closer to the bottom of the material accumulator  200 . The relative height of the accumulator wheel assembly  265  has a direct effect on the speed at which the material  160  passes through the pre-stretching device  100  and the material accumulator  200 . In particular, and in one embodiment of the invention, the material  160  moves at a maximum speed when the accumulator wheel assembly  265  is located near the bottom of the material accumulator  200 , with the accumulator wheel assembly  265  moving at progressively lower speeds as the accumulator wheel assembly  265  is raised.  
         [0032]     After exiting the material accumulator  200  though the accumulator exit region  240 , the material  160  is routed to a conventional strapping system  300 , as shown in  FIG. 2 .  
         [0033]     In another embodiment of the invention (not shown), a non-continuous strap feed can be used. This embodiment allows for the momentary control of pre-stretch material  160  using an accumulator to “hold” the material  160  as it waits to move into active feeding around a skid of objects to be unitized.  
         [0034]     The foregoing description of embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments and with various modifications as are suited to the particular use contemplated.