GB2171976A - A plastics sleeve stretching and dispensing device - Google Patents

Abstract

A stretching and dispensing device for a sleeve of plastic film for use in applying sleeves to palletted loads comprises a rectangular support frame 5 at the respective corners of which there are four drive heads 6 each connected to an upstanding film support 11. The film supports are diagonally movable by the drive heads to stretch out the sleeve of plastic from a size smaller than an object to be wrapped in the plastic film to a size larger than the object. Each film support includes a top stripping bar 13 which projects radially outwardly beyond its associated film support and includes two orthogonally disposed arms 2 each having a curved extension portion 15 profiled to substantially match the profile induced in the plastic film as it is stripped from the stripping bars onto the object to avoid line contact at the ends of the arm and hence localised high film stresses thereat. As shown, each portion 15 is constituted by an intermediate portion joining a depending inclined extension 14 to the arm 2. In a modification, (Fig 5) curved profiled end elements are fitted to the arms. <IMAGE>

Description

SPECIFICATION A stretching and dispensing device for a sleeve of plastic film This invention relates to a stretching and stripping frame for use in a wrapping method in which a sleeve of elastically stretchable plastic film is provided as a puckered band on the frame and is subsequently stripped onto an object, such as a load on a pallet, to provide and elastically embracing sheath for the load. This method and equipment for the above purpose are described in European Patent Application Number 83301929.2.

In the earlier developments there is provided a rectangular frame which.is continuous and is telescopically expandible for the purpose of supporting a puckered band of plastic whilst it is stretched and whilst it is removed in a wrapping operation. A frame of the above type is expensive to manufacture. As sometimes happens in use the telescopic frame can be damaged and cost of restoring the frame to freely operating telescopic condition is substantial.

Simplification of equipment is always desirable if this can be achieved without sacrificing the effectiveness of the equipment. One proposed simplification was to provide a support system for the band of puckered plastic which was not a telescopically expandible frame but was comprised only of four corner assemblies which were relatively movable to so as to expand a sleeve of plastic film mounted as a puckered band around the corner assemblies. Each corner assembly included a stripping bar which could be likened to what would be left if a rectangular frame (of the telescopic type previously used) with curved corners had large sections of the four sides of the frame (which included the telescopic interconnections) removed.

Such an arrangement appears logical but attempts to provide such a support arrangement have been accompanied by one particular problem. Specifically, the plastic film would frequently tear as it was stripped from the corner assemblies over the stripping bars.

Close observation during stripping operations has revealed thatthefilm under expansion pressure is stretched taught and straight between the corner assemblies and is tight about the object being wrapped. In the tranition zone between the object (which is usually substantially rectangular in shape and is smaller in size than the rectangular form of the stretched puckered band) and the corner supports the plastic adopts a complex curved profile. It has been observed that the tears always occur at the ends of the stripping bars. It was alos noted that the tears follow"lines" of high stress which develop in the plastic at the ends of the stripping bars, which it was concluded, was because the plastic bore very firmly on the edges of those ends during the plastic stripping operation.

Ball type end fittings for the stripping bars were tried but these also produced lines because of the point contact between the periphery of the ball and the plastic. It was then found that if ends for the stripping bars were provided which had a profile substantially the same as the curve developed in the plastic in the transition zone between the corner assemblies and the load being wrapped lines of high stress were not produced and tears were substantially eliminated.

Accordingly, this invention provides a stretching and dispensing device for a sleeve of plastic film when mounted as a puckered band around said device, said device comprising a rectangular ring frame, a drive head fixed to each corner of the ring frame, an elongated member mounted in each drive head with an end which projects substantially diagonally into the ring frame, the elongated members being in a common plane parallel to the plane of the ring frame, power input means on the drive heads connected to the elongated members to allow them to be moved longitudinally in either direction relative to the drive heads, each of said ends of said elongated members is fixed to an upstanding pressure leg of a supportfor plastic film and each pressure leg is coupled to an upstanding film support leg spaced from its associated pressure leg, the film support leg has a slot therethrough to allow a pressure wheel of a plastic film puckering apparatus with which said stretching and dispensing device is adapted to co-operate to engage the pressure leg, a stripping member fixed to the top of each film leg, each stripping member comprising a retaining portion having a pair of orthogonally arranged arms of substantially the same length and a curved interconnection between first ends of the arms, the retaining portion is mounted on the plastic support leg so as to project beyond the film support leg into the space between the film support leg and the associated pressure leg, and an arm extension at the other end of each of said arms which includes a profiled portion which is substantially the same profile as the curve that would be induced during a wrapping operation in plastic sheet material in the transition zone between said retaining portions and the load being wrapped.

A presently preferred embodiment ofthe invention will now be desribed with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a stretching and dispensing device produced in the development of the present invention, Figure 2 is a fragmentary side view of portion of the arrangement of Figure 1 when looking in the direction of the arrow A, Figure 3 is a diagrammatic perspective view of a stretching and dispensing device according to the invention, Figure 4 is a view similar to Figure 2 but utilising the device of Figure 3, and Figure 5 is a an alternate form of extension for the stripping members of the Figure 3 device.

Referring to Figure 1, there are four corner elements 1 each including a stripping member comprised of two orthogonally disposed arms 2 with a curved portion 3joining the arms 2. There is a sleeve of plastic film material in the form of a stretched puckered band indicated 4 around the the corner elements 1. There is a load L to be wrapped and it will be noted that it is smallerin cross-section than the expanded size of the puckered band.

Curves C indicate the appearance from above of the transition zone in the plastic sleeve between the puckered band and the load, about which the plastic forms an elastically embracing sheath. Figure 2 illustrates the appearance in the direction of the arrow A, Figure 1.

From Figure 1 it is clear that there is a very severe and localised pressure applied to the plastic as it passes over the edges E of the ends of the arms 2. It is this localised load that causes stress lines of high stretch in the plastic which weakens it and leads to the tearing of the plastic, as discussed above.

Referring nowto Figure 3, this represents a presently preferred form of stretching and dispensing device for plastic. As illustrated there-is a generally rectangular ring frame 5 with a drive head 6fixed at each corner. Extending upwardly into each drive head 6 there is a drive shaft 7 to provide power to the drive head. The shafts 7 are adapted to be driven by a puckering apparatus, when the stretching and dispensing device is engaged with the puckering apparatus as described in the patent application 83301929.2 Mounted in the drive heads 6 there are shafts 8 which move substantially diagonally relative to the ring frame. The shaft 8 are connected within the drive heads to the shafts 7 so that the shafts 8 move longitudinally in one direction or the other dependent upon which direction the shafts 7 are rotated.

Fixed to the inner end of each of the shafts 8 there is an upstanding pressure leg 9 connected at 10 to another upstanding leg 11. The legs 11 have slots 12 therethrough to allow puckering wheels of the puckering apparatus referred to above to engage against the pressure legs 9, as disclosed in the patent application 83301929.2.

Fixed to the tops of the film engagable legs 11 there are stripping members 13. These members are important to the operation of the device. They project slightly over the band of plastic film when engaged with the legs 11, see Figure 2 where the band of puckered plastic film is passing overthe stripping member arms 2. It is a natural feature of plastic film that when stretched it will try to resume its unstretched form. If the stripping arms 2were in the same plane as the legs 11 about which the film is tightly engaged and then the film was drawn off the legs 11 in a stripping operation there would be a natural tendancy for the film to first creep and then accelerate in the stripping direction in an uncontrolled manner.The success of the wrapping process using the equipment as set out in the wrapping process using the equipment as set out in the patent application 12311/83 is to have the plastic drawn off at a rate controlled by the passage of the ring frame over the load. To ensurethatthere is a controlled removal of the plastic the arms 2 project beyond the plastic on the legs 11 so that the plastic must first stretch move and enlarge from the band size before it can pass over the arms 2. As the plastic has a natural tendancy to resist stretching it is held trapped on the legs 11 below the arms 1 until pulled thereover in the stripping operation. For this reason it is necessary to have a stripping member associated with each corner assembly and to have it positioned as described.

It will be seen that each stripping member comprises two orthogonally arranged arms 2 with a corner 3 of large radius joining the arms 2, as shown in Figure.1. At each end of the arms 2 there is an extension 14; As illustrated the extensions 14 are connected through a curved portion 15 to the arms 2.

The curvature of the portion 15 approximates the curvature that occurs in the plastic as it leaves the arms 12 of the stripping member and develops in the transition zone between the stripping members and the load being wrapped, see Figure 1. Because the curve 15 and the curve in the plastic are very similar, even if not exactly the same, there is no local "line" contactofthetypethatwill cause localised high stress and a weakness which could lead to a tear developing in the plastic.

The portions 14 which are linear extensions of the curves 15 and extend downwardly divergently away from the arms 2 are not of primary importance in the elimination of high stress zones in the plastic but towards the end of a stripping sequence, when the plastic is being pulled from the lower ends of the arms 11 adjacent the joiners 10,the plastic will pass firmly over the portions 14 and so provide a smooth approach for the plastic to the curved portions. 15.

Figure 4 is a view very similar to Figure 1 but shows the apparatus of Figure 3 in use. It is to be noted that the plastic does not have a distinct change of direction where it passes over the ends of the arms 2 (see the condition E in Figure 1) but progresses as a curve into the transition zone, this can be readilyvisualised when the shape of the stripping bars as shown in Figure 3 is considered.

Figure 5 is an alternative arrangement for the avoidance of high stress iine contact between the ends of the arms 2 and the plastic. Ends 16 of the predetermined external profile are provided and are inserted into the ends of the arms 2, which are usually made of tubular material.

Claims (5)

1. Astretching and dispensing device for a sleeve of plastic film when mounted as a puckered band around said device, said device comprising a rectangular ring frame, a drive head fixed to each corner of the ring frame, an elongated member mounted in each drive head with an end which projects substantially diagonally into the ring frame, the elongated members being in a common plane parallel to the plane of the ring frame, power input means on the drive heads connected to the elongated members to allow them to be moved longitudinally in either direction relative to the drive heads, each of said ends of said elongated members is fixed to an upstanding pressure leg of a support for plastic film and each pressure leg is coupledto an upstanding film support leg spaced from its associated pressure leg, the film support leg has a slot therethrough to allow a pressure wheel of a plastic film puckering apparatus with which said stretching and dispensing device is adapted to co-operate to en gage the pressure leg, a stripping member fixed to the top of each film support leg, each stripping member comprising a retaining portion having a pair of orthogonally arranged arms of substantially the same length and a curved interconnection between first ends of the arms, the retaining portion is mounted on the plastic support leg so as to project beyond the film support leg into the space between the film support leg and the associated pressure leg, and an arm extension at the other end of each of said arms which includes a profiled portion which is substantially the same profile as the curve that would be induced during a wrapping operation in plastic sheet material in the transition zone between said retaining portions and the load being wrapped.

2. A stretching and dispensing device as claimed in claim 1 wherein the stripping members are unitary with the retaining portions and the extensions formed by bending elongated members to the desired shape.

3. A stretching and dispensing device as claimed in claim 1 wherein the extensions each include an externally profiled member fixed to the ends of the arms of the stripping members.

4. A stretching and dispensing device as claimed in claim 2 wherein the extensions of each stripping member include linear portions which extend downwardly and divergently away from said arms.

5. A stretching and dispensing device for a sleeve of plastic film when mounted as a puckered band around said device as hereinbefore described with reference to the accompanying drawings.