Stretch Film Wrapping Device

1. Field of the Invention:

The present invention relates to a stretch film wrapping device, and, more particularly, to a device for rotatably supporting a roll of plastic film for tension stretch wrapping of objects, containers and the like.

2. Background of the Invention:

In shipping containerized goods, the general practice is to stack a plurality of the containers on a pallet and to utilize material handling equipment such as fork trucks to load and unload the palletized containers. To prevent containers from shifting or from falling off of the palletized load, it is generally required to band or wrap the palletized load. For example, metal or plastic straps or bands in conjunction with clips or buckles have been used for many years for this purpose. In recent years, there has been an increasing use of plastic stretch film to wrap and secure a palletized load of containers. The advantages of the use of plastic stretch film are enumerous. The stretch film produces a tight wrap and provides greater protection to the containers from damage due to harsh weather, abrasion, punctures and the like. The plastic stretch film is of relatively low cost and does not require the use of adhesives, clips or other fasteners.

While there are thus significant advantages to the use of plastic stretch film for wrapping palletized loads of containers, these advantages are not realized unless the wrapping procedure is properly executed. The two most important features of a proper wrap are the uniformity of the wrap and the proper tension of the plastic as it is applied. If the proper tension is not applied to the plastic there will be inadequate stretching of the plastic resulting in a loose wrap with poor holding qualities. In addition, the less the stretch of the plastic, the more the quantity of plastic which will be required, thereby resulting in a higher cost. On the other hand, if the tension on the plastic is too high, the plastic will rip and the advantages of a continuous wrap are lost. Random variations in the plastic tension during application can result in both ripped plastic and a loose wrap. Thus, the goal is to maximize the stretch without risk of tearing, while maintaining a constant predetermined tension during the wrapping procedure.

Although a variety of complex machines have been developed, their high cost, maintenance problems and lack of versatility have rendered them less than satisfactory in most instances. Because of this, a variety of hand held dispensers have been developed. The low cost, simplicity and portability of hand held dispensers would make them the ideal selection in most operations. However, heretofore, none of the hand held dispensers has been entirely satisfactory. To be satisfactory, a hand held dispenser must be durable, low cost and easy to use. Rolls of film must be easily and quickly replaceable. Most important, however, is that the tension adjustment for optimizing plastic stretch must be precise and consistent and produce uniform predetermined tension across the full width of the film for a uniform wrap. While various tension control devices have been developed, none has proven to be satisfactory. Tapered core plugs and other internal core friction devices such as expandable rubber gripping devices have proven to rely too much on the "feel" of the operator resulting in significant variations in the friction settings from operator to operator or from day to day with the same operator. In addition, precise and repeatable predetermined tension settings are not possible with these devices.

Other devices have utilized a pair of opposing core holders and adjusting the clamping force between the core holders on the plastic roll core ends to vary the tension. One such device utilizes a pair of rotatable core holders with the distance between the core holders being adjusted by means of a threaded rod and corresponding internally threaded tube interconnecting the core holders. Although once properly set, this type of device yields relatively uniform tension on the plastic, precise and consistently reproducable predetermined tension settings have not been possible.

Thus, there remains a current need for a hand held stretch film wrapping device which permits precise tension adjustments and consistent reproducable predetermined tension settings.

According to the present invention there is provided an improved hand held stretch film wrapping device for rotatably supporting a roll of plastic film having a tubular rigid central core. The wrapping device is durable, easy to use and permits quick and simple roll replacement. A unique built-in tension control system optimizes the plastic stretch with precision tension adjustability and permits consistent reproduceable predetermined tension settings. The optimized and consistent plastic stretch yields tighter more secure palletized loads, prevents plastic tearing and minimizes the amount of plastic used with the consequential cost savings.

The wrapping device comprises a base member with a bottom core holder for rotatable engagement by one end of a film roll core. A rotatable top core holder opposes the bottom core holder for engagement by the other end of the film roll core. A length adjustable axle member interconnects the core holders to effect clamping of the film roll between the core holders, with the top core holder being rotatably mounted on the axle member and axially shiftable upwardly on the axle member. In the preferred embodiment, the axle member includes a lower section and an upper section threadably engaged with each other whereby the length adjustment of the axle is achieved by varying the degree of threaded engagement of the axle sections. It is especially preferred that the lower axle section be rigidly affixed to the base member and be internally threaded at its upper end and that the upper axle section have external threads at its lower end to threadably engage the lower axle section.

A top tension control assembly is provided above the top core holder at the top end of the axle member for adjusting the clamping force on the film roll. The tension control assembly includes spring biasing means engaging the top core holder to resist upward axial shifting of the top core holder when the axle member is adjusted to reduce the distance between the core holders to effect clamping of the film roll between the core holders. In the preferred embodiment, the top tension control assembly includes an adjustment knob at the top of the upper axle section which is rigidly affixed thereto, and the spring biasing means is interposed between the knob and the top core holder and is rotatable with respect to the upper axle section. When the knob is rotated to vary the length of the axle by changing the degree of threaded engagement of the axle sections, the clamping force between the core holders on the film roll will vary in direct proportion to the force of the spring biasing means and resist upward axial movement of the top core holder with respect to the upper axle section. The preferred spring biasing means is a disc spring washer, and it is especially preferred to provide a needle bearing washer rotatably interposed between the knob and the disc spring washer.

Also in the preferred embodiment, a resilient pad is interposed between the bottom core holder and the base member, and a handle grip is rigidly affixed to the base member.

FIG. 1 is a perspective view of an operator using the stretch film wrapping device of the present invention to stretch wrap a pallet load of containers;

FIG. 2 is a bottom, perspective view of the stretched film wrapping device;

FIG. 3 is a side elevational view of the wrapping device with the roll of plastic film removed;

FIG. 4 is a side, cross-sectional view of the wrapping device;

FIG. 5 is an exploded perspective view of the wrapping device;

FIG. 6 is a plan view of the wrapping device; and

FIG. 7 is a bottom view of the wrapping device.

Referring to the drawings in greater detail, FIG. 1 illustrates the use of the stretched film wrapping device 20 to stretch wrap a pallet load of containers 10 with plastic stretch wrap 11. The details of wrapping device 20 are shown in FIGS. 2 through 7. In FIGS. 3 and 5, the roll of plastic stretch film has been removed to more clearly illustrate the interior structure of wrapping device 20.

Wrapping device 20 includes base plate 21. While base plate 21 can be of a variety of configurations and materials, the configuration shown (FIGS. 5 and 7) provides stability when the device is set down by resisting tip-over which can nick and tear the plastic film. In addition, a die cast zink web design is preferred for superior strength, accuracy and flatness, while minimizing the overall weight of the device. Handle grip 22 is rigidly affixed to the narrow end of base plate 21. The upper portion of handle grip 22 is covered with a cushion foam wrap 23 for comfort and to prevent slipping from the operator's hand. The upper portion of handle 22 is also angled inwardly to make it easier to wrap the lower rows of containers on a pallet, while also locating the grip nearer the center of gravity for less worker fatigue. To minimize weight, handle 22 is preferably made of metal tubing with a protective end cap 24 inserted in the upper end. Handle 22 can be rigidly attached to base plate 21 by insertion into an aperture 25 and locked in place by means of a pin 26 passed through apertures 27 in base plate 21 and corresponding apertures 28 in the lower end of handle 22. For increased structural stability, it is preferred that the lower end of handle 22 be press fit into aperture 25 as well as being pinned in place.

At the enlarged end of base plate 21, a bottom core holder 29 is mounted, preferably non-rotatably. As shown, bottom core holder 29 is a cylindrical member with two stepped down diameters. This is to permit accomodating plastic film rolls of different core diameters. In addition, while bottom core holder 29 can be permanently attached to base plate 21, it is preferred that it be removable for repair or replacement or for interchangeability with bottom core holders sized to accomodate roll cores of differing diameters. When bottom core holder 29 is removable, an anti-rotation pin 30 is press fit into the bottom of holder 29 and inserted through aperture 31 in base plate 21 when bottom core holder 29 is installed. With the bottom end of lower axle section 32 passing through central aperture 33 in bottom core holder 29 and being press fit in base plate aperture 34 and locked in place by pin 35, bottom core holder 29 remains securely in an aligned position during the wrapping operation. A snug fitting O-ring 36 may be provided around axle section 32 on top of bottom core holder 29 to keep bottom holder 29 seated on base plate 20 when changing plastic rolls or when no plastic roll is installed.

Preferably, a resilient pad 37, e.g., rubber or the like, is interposed between bottom core holder 29 and base plate 21. The purpose of pad 37 is to provide a degree of automatic vertical alignment to compensate for uneven plastic roll core ends to prevent skewing of the plastic roll when installed. Since the bottom end of the plastic roll core rotates on the surface of bottom core holder 29 during the wrapping operation, it is important that the bearing surfaces of bottom core holder 29 be even and smooth. To optimize the smoothness of the roll rotation, it is preferred that the bearing surfaces of bottom core holder 29 be coated with a slippery but dry and durable coating such as Teflon.

Opposing bottom core holder 29 is top core holder 38 which has corresponding step down diameter bearing surfaces to engage the top end of the plastic roll core. Core holders 29 and 38 are interconnected by a length adjustable axle member which includes lower axle section 32 and upper axle section 39. Upper axle section 39 is a threaded rod which threadably engages internal threads 40 of tubular lower axle section 32. Adjusting the length of the axle member by increasing or decreasing the threadable engagement of upper axle section 39 with lower axle section 32 will produce a corresponding increase or decrease in the distance between upper core holder 38 and bottom core holder 29 to thereby vary the clamping force on the film roll core ends.

An important aspect of the present invention is the unique precision tension control assembly incorporated in the upper portion of the device. Adjustment knob 41 is rigidly affixed to the top of upper axle section 39. Adjustment knob 41 is rotated to increase or decrease the threaded engagement of upper axle section 39 into lower axle section 40 to increase or decrease the clamping force of the core holders. Since upper core holder 38 is designed to rotate with the film roll as it rotates during the wrapping operation, upper core holder 38 is mounted rotatably with respect to upper axle section 39. To enhance the smoothness of the rotation of upper core holder 38, a bronze bearing 42 is installed in the central aperture of upper core holder 38 which is rotatable about upper axle section 39 and within the central aperture of upper core holder 38. Although not required for proper operation of the device during the wrapping procedure, to facilitate roll changing without loose parts, washer 43 and lock nut 44 are installed on upper axle section 39 below upper core holder 38 to provide a unitized construction.

The precision tension adjustment results from upper core holder 38 being axially shiftable upwardly on upper axle section 39 toward the base of adjustment knob 41. A spring biasing means 45 is interposed between the lower end of adjustment knob 41 and the top of core holder 38 which resists the upward axial movement of core holder 38. To optimize the precision of the tension adjustment and the consistent reproducability of predetermined tension settings, spring biasing means 45 should have a spring load curve which will provide a consistent gradual increase in pressure as the spring means is compressed. The preferred spring means has been found to be a disc or belleville spring washer. The load curves of disc spring washers are determined by the ratio of the overall spring height to the spring thickness. Optimum height to thickness ratios have been found to be between about 0.4 and 1.0. Height to thickness ratios substantially below or above this range do not exhibit consistent gradual increases in pressure applied throughout the deflection of the washer. An especially preferred height to thickness ratio is about 0.7.

In order to optimize the smoothness of the rotation of upper core holder 38 and to prevent wear and galling, it is preferred to interpose a needle bearing sandwiched between a pair of thrust washers between the top of disc spring washer 45 and the bottom surface of adjustment knob 41. Similarly, washer 48 is interposed between the bottom of disc spring washer 45 and the top of upper core holder 38.

To load a roll of plastic film in the wrapping device of the present invention, adjustment knob 41 is rotated counter-clockwise until upper axle section 39 is disengaged from lower axle section 32. With the upper sub-assembly removed, a roll of plastic film having a tubular rigid central core is placed over lower axle section 32 until the lower end of the core is seated on bottom core holder 29. The upper sub-assembly is then replaced by rotating adjustment knob 41 in a clockwise direction. The rotation of adjustment knob 41 is continued until upper core holder 38 engages the upper end of the film roll core. Since there will be no upward force exerted on top core holder 38 up to this point, disc spring washer 45 remains in a relaxed condition. Once top core holder 38 engages the top end of the film core, a substanial resistance will be felt by the operator in continuing to rotate adjustment knob 41 in a clockwise direction. At the point of this substantial increase in resistance to rotation of adjustment knob 41, the radial position of indicator arrow 49 should be noted. Upon further rotation of adjustment knob 41 in a clockwise direction, the resistance of the engagement of top core holder 38 with the top end of the film roll core will urge top core holder 38 toward adjustment knob 41 with the axial shift being resisted only by the spring force of disc spring washer 45. Thus, at this point until disc spring washer 45 is completely deflected, the increase in clamping force between the core holders is completely a function of the spring force of disc spring washer 45 instead of the degree of threading advance of upper axle section 39 into lower axle section 32. Since disc spring washer 45 provides a consistent gradual increase in applied pressure throughout its deflection, a direct corresponding consistent gradual increase in the clamping force between the core holders is achieved.

For a given kind and thickness of plastic stretch film a trial is conducted to determine the requisite clamping force to yield the optimum film stretch during the wrapping operation. At this desired level of clamping force, the new radial location of indicator 49 with respect to its initial location at the start of the deflection of disc spring washer 45 is noted, e.g., 1/4 turn, 1/2 turn, full turn, etc. Once this initial determination of the optimum clamping force has been determined, as new rolls of the same kind and thickness of plastic stretch film are installed, once the initial resistance is felt, adjustment knob 41 is simply and quickly rotated until indicator 49 traverses the predetermined radial arc, at which point the predetermined optimum clamping force will automatically be reproduced. Once the clamping force is thus set, it will remain constant throughout use of the entire roll of stretch film. During use, unwanted changes of the clamping force cannot occur through loosening of axle section 39 with respect to its threaded engagement with axle section 32 because of vibrations or the like since the force of disc spring washer 45 produces a friction thread locking effect.

The wrapping device of the present invention is suitable for use with virtually any kind of plastic stretch film which is rolled on a tubular rigid central core. For example, stretchable films such as nylon, polypropylene, polyvinylchloride, polybutylene, polyethylene, copolomers or blends thereof and the like may be used. In addition, various alternatives such as plastic netting can also be used. Thus, as used herein, "film" is meant to include variations such as "netting". In the embodiments illustrated in FIG. 4, the roll of plastic film has a tubular rigid cardboard central core 12. However, any tubular rigid central core can be used satisfactorily as long as the core diameter corresponds with the core holder diameters for proper allignment and engagement therewith.

While the preferred embodiments of the present invention have been described and illustrated, it will be obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit of the present invention. Accordingly, the scope of the present invention is deemed to be limited only by the appended claims.