Patent Application
20060254213
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.
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.
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.
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'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.
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.
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.
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.
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.
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.
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.
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.
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.
In the embodiment of the invention depicted in
The path of material 160 from a coil 165 to be pre-stretched is shown in
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.
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
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.
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.
After the material 160 exits the pre-stretching device 100 through the exit point 180, it enters the material accumulator 200 shown in
In one embodiment of the invention, the accumulator wheel assembly 265 is positionally adjustable. For example and as shown in
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
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.
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.
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.
| Number | Date | Country | |
|---|---|---|---|
| 60673029 | Apr 2005 | US |
