GB2097361A - Bag - Google Patents

Abstract

A bag is formed from a stack of sheets of polyethylene film. The outermost sheet has free edge portions folded inwardly to embrace the free edge portions of the remaining sheets, and the inwardly directed edge portions 20 thermally welded to the adjacent sheet. The stack is then folded so that the inwardly directed portions are adjacent one another and the side edges of the stack are then welded to form a bag. The bag has a greater strength than a corresponding bag made from a single sheet of plastics material having the same overall thickness. The outermost sheet may be of low density, and the remaining sheets of high density, polyethylene. Metallic foam or net-like sheets may be incorporated. <IMAGE>

Description

SPECIFICATION Improvements in or relating to a bag The present invention relates to a bag and more particularly relates to a bag made of a plastics material.

It has been known to make bags of a plastics material by taking a single sheet of material, folding the sheet so that parts of the sheet are superimposed, and heat welding superimposed portions of the sheet to form a bag. It is also known to make a plastics bag, such as a plastics carrier bag, by folding in the terminal edge regions of a sheet of plastics material and welding them to the main body of the sheet, then folding the sheet axially, so that said folded-in regions are superimposed, and then welding portions of the sheet to form a bag. The folded-in regions form the mouth of the bag, and holes may be punched in these folded-in regions to form handles for the bag.

Such bags are frequently made of polyethylene. It has been proposed to make such bags of low density polyethylene film, and whilst such bags have a pleasing shiny external appearance, the bags do not have significant strength unless the bags are made of a very thick film of low density polyethylene.

This, of course, utilises a substantial quantity of raw material making the overall price of the bag high.

It has also been proposed to manufacture bags by a similar process from high density polyethylene film. When such a film is utilised it is possible to make a bag having the same strength as a bag made from the low density film with a much thinner high density film. However, such bags do not present a pleasing external appearance, and also if a bag of substantial strength is to be prepared again a thicker film must be utilised, again utilising more raw material.

The present invention seeks to provide a bag of plastics material, and a method of making such a bag, in which the bag has a greater strength than a prior bag made from the same quantity of raw material.

According to one aspect of this invention there is provided a bag formed from a plastics material, said bag comprising a plurality of sheets of plastics material film that are superimposed, the sheets being connected together to form a bag, each side wall of the bag thus comprising a plurality of separate thicknesses of plastics material film.

According to another aspect of this invention there is provided a method of forming a bag from a plastics material, said method comprising superimposing a plurality of sheets of plastics material and connecting the sheets together to form a bag, each side wall of the bag thus comprising a plurality of separate thicknesses of plastics material.

Preferably the bag is formed from a web comprising a stack of sheets of plastics material, the stack being folded so that portions thereof are superimposed, the folded stack having portions thereof connected to each other to form a bag. Said portions of the folded stack may be thermally welded to one another.

Advantageously one sheet of the stack may have protruding free edges thereof folded around the free edges of the remaining sheets of the stack, said inwardly folded portions defining the mouth of the bag.

The inwardly folded portions may be connected to the remaining sheets of the stack for example by means of thermal welding.

Conveniently the sheet that defines the exterior surface of the bag may be formed of low density polyethylene, and also the sheets other than the sheet forming the exterior of the bag may be formed of high density polyethylene.

The bag may comprise between two and twenty, preferably between five and ten, superimposed sheets of plastics material film, and said plastics material film may be between four and one hundred microns, conveniently between four and twenty microns and preferably substantially eleven microns, thick. It is to be understood that any appropriate number of superimposed sheets may be used.

A bag according to the invention may be fabricated from a web comprising a plurality of interleaved centre fold sheets. Also one or more of said thicknesses of plastics film may be black, or may be thermally reflective.

In one embodiment of the invention at least two of said separate thicknesses are separated by a thermally insulating barrier, which may be constituted by a foam interlayer, or by an air gap formed by a net, or means which act to space the thicknesses of film apart.

A method according to the invention may comprise the step of interleaving a plurality of centre fold sheets by passing a first such sheet along a path, introducing a second such sheet to an interleaved position within the first such sheet by passing the second sheet over the leading edge of a guide means which is inserted between the two thicknesses of the first sheet, said leading edge being inclined relative to the movement of the first sheet, and optionally interleaving further centre fold sheets in a corresponding way.

The invention also relates to an apparatus for performing such a method comprising means defining a path to be followed by the first sheet, a plurality of guide means in the form of guide plates, and means for supplying centre fold sheets to each guide plate to be folded over the leading edge thereof.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described by way of example with reference to the accompanying drawings in which: FIGURE 1 is a perspective view of part of a stack of sheets in a plastics material film illustrating an initial step in the manufacture of a bag in accordance with the invention; FIGURE 2 is a view corresponding to Figure 1 showing a subsequent step in the manufacture of a bag; FIGURE 3 is a perspective view showing yet a further step in the manufacture of a bag; FIGURE 4 is a side perspective view showing yet a further step in the manufacture of a bag; FIGURE 5 is a front perspective view of a bag made in accordance with the invention, with parts thereof cut away to show the structure of the bag;; FIGURE 7 is a view showing a larger part of the apparatus shown in Figure 6.

It is to be appreciated that, at the present time, plastics bags are manufactured, by machines, from a plastics material web supplied on a roll. The plastics material web is usually extruded from an extruding apparatus which initially produces a tube of extruded plastics material. The tube of material is subsequently cooled and then flattened, and wound onto a reel. The flattened tube of material may be cut at one folded edge of the tube before it is wound onto the reel, and thus a web of two thicknesses joined at one side edge, and called "centre fold sheet" is wound onto the reel. Alternatively both of the folded side edges of the tube may be before it is wound onto the reel and cut the resultant two thicknesses of web are wound together onto the reel as "double wound sheet".

Machines which are utilised to manufacture plastics bags from the above described webs are well known, and as a web is passed through such machine in the manufacture of a bag the direction of the movement of the web can change from horizontal to vertical and back to horizontal. However, for the sake of simplicity of explanation, and to facilitate a ready understanding of the invention, the accompanying drawings illustrate the various processes in the manufacture of plastics bags from a web only schematically, the web being shown as travelling always in a horizontal plane. However, it is to be noted that the web need not merely travel in a horizontal plane during the various process steps hereinafter described, but may travel vertically.Also it is understood that a man skilled in the art will be familiar with machines that fold in and weld edge portions of a web, and devices which fold elongate webs in half axially, and thus such devices are not described in detail herein.

Referring initially to Figure 1 of the accompanying drawings a bag of plastics material in accordance with the present invention is made from a plurality of relatively thin sheets of plastics material film. As shown in Figure 1 the sheets are initially superimposed one upon the other to form a web 1 and in this particular example the web comprises four upper sheets 2 to 5, and one lower sheet 6, the lower sheet being wider than the sheets 2 to 5 so that two side edge portions 7, 8 of the upper sheet protrude beyond the side edges of the lower sheets 2 to 5. The sheets may be stored initially on separate reels (although some of the sheets may be stored on reels in the form of doubie wired sheet), and may be drawn off from the reels by being passed together through the nip of a pair of drive rolls.

In this particular embodiment the sheets 2 to 5 are all made of high density polyethylene film, and the lower most sheet 6 is formed of low density polyethylene film. In each case the sheets are 11 microns thick.

Referring now to Figure 2, in the next stages of the manufacture of a bag the protruding side edge regions 7, 8 of the lower sheet are folded upwardly and inwardly so that these regions 7, 8 embrace the side edges of the sheets 2 to 5. At least the free edges of each of the inwardly folded portions 7, 8 are then heat welded to the surface of the next adjacent sheet 2 with a lateral weld, as schematically illustrated by the arrows 9, 10. Whilst heated elements may be utilised for performing this welding process a heated backing member may also be utilised, the action of such a heated backing member being schematically indicated by the arrows 11, 1 2 and then a portion of the sheet 6 that is adjacent the lowest most sheet 5 of the stack will be firmly welded to that sheet 5.Indeed the welding pressure and temperature may be such that all the sheets of the stack may be fused together. During this process the web is travelling continuously to the right as indicated in Figure 2 by the arrow 1 3.

As can be seen, in Figure 2, the web is folded in half about an axial centre line 15, and the two halves 1 6, 1 7 are moved towards one another as illustrated schematically by the arrows 18, 1 9. This can, in fact, be accomplished before, during or after the above described welding process.

It can be seen, in Figure 2, that there is an elongate weld line 20, caused by the welding processes 9, 10 connecting the inwardly directed free edge portions 7 and 8 of the sheet 6 to the exposed surface of the sheet 2. Turning now to Figure 3 the web 1 can be seen in a fully folded position, and in this condition the elongate weld 21 formed by the welding process 12 can be seen.

Referring now to Figure 4 the elongate web is moved past a welding and cutting head. The welding head comprises two elongate members extending transversely across the web relative to its direction of movement, which is again indicated by the arrow 1 3. The head comprises two members 22, 23 which are heated and which have spaced faces 24, 25 which engage the web 1 from the opposite sides thereof, welding the portions of the web trapped between the opposed faces together.

These welded portions of the web form the side edges of the resultant bags as will be explained hereinafter. A knife 26 is located between the two heated surfaces 25 of the upper member 23, and when the heated members have welded the said portions of the web together the knife moves downwardly severing the thus completed bag 27 from the rest of the web.

As can be seen, in Figure 5, the bag 27 has an open mouth 28, and has two welded side edges 29, 30, the welded side edges being formed by the action of the above described heated members 22, 23. As can be seen in Figure 5 part of the bag has been cut away to show the structure of the bag.

The rear wall of the bag, that comprises the portion 16 of the stack as shown in Figure 2, and the inwardly directed free edge portions 7 of the sheet 6 and the associated weld line 20 can be clearly seen. Also the inwardly directed portion 8 of the sheet 6 forming part of the portion 1 7 can be seen and the weld line 20 connecting that portion to the sheet 2 can also be seen. The remaining sheets can also be identified, and the weld line 21 connecting part of the sheet 6 that is now on the exterior of the bag to the adjacent sheet 5 can also be seen.

It will be appreciated that a bag fabricated as described above, has, on its external surface, a low density polyethylene film, and thus the bag has a pleasing external appearance. Printing of a high quality can be provided on this external surface if desired. This external film is tucked inwardly at the mouth of the bag and is then sealed to the innermost surface of the plurality of layers of thin film which, together with the outer layer, define the bag.

It has been found that by utilising a plurality of thin films of polyethylene a bag can be fabricated which has a far greater strength than a bag formed from a single film having a thickness equal to the sum of the thickness of the separate thin films. This is because the thickness to strength ratio of a plastics material film is greatest for thin films. It has been found that, in utilising the present invention, to fabricate a bag having a strength corresponding to the strength of a bag formed from a single film of polyethylene of a predetermined thickness it is possible to utilise, for example, eight films of polyethylene each of such a thickness that the total thickness of the eight films is less than one half of the overall thickness of the single film.Thus it will be appreciated that by utilising the present invention a bag can be fabricated having the same strength as a prior proposed bag, but which utilises less than one-half of the raw material in its manufacture. Also it is to be appreciated that the bag described above has the advantage of a low density external polyethylene film, which provides a pleasing appearance, and on which printing can be provided, and the bag also has the advantage of high density inner films, which provide strength.

Whilst the invention has been described above, by way of example, with reference to a bag made from a stack of five sheets it is envisaged that any number of sheets, for example between 2 and 20 sheets may be used. In certain circumstances it is preferred that the uppermost sheet will be of low density polyethylene and that the remaining sheets will all be smaller than the uppermost sheet and will be of high density polyethylene. Alternatively, however, all the sheets in the stack may be made of the same material, and in some cases all the sheets may be of the same size.

Two bags made by a method as described above were fabricated from polyethylene film having a thickness of eleven microns, one bag having six thicknesses of film and the other bag having eight thicknesses of film. These bags were compared with a bag made by prior art techniques from a single thickness of polyethylene, this bag having an overall thickness of two hundred and fifty microns. Thus the two hundred and fifty micron thick prior art bag was compared with bags in accordance with the present invention having overall thicknesses of sixty six microns and eighty eight microns respectively.

The bags were compared by subjecting the bags to a free-falling dart impact test in accordance with B.S. 2782 1965 method 306F, A.S.T.M.D. 1 709-62 T method A and P.F.M.SJl,the voluntary code of practice of the Packaging Film Manufacturers Association. The results of these drop dart tests are shown below in Tables 1 and 2, Table 1 being the results relating to the prior art bag and Table 2 being the results relating to the bags in accordance with the present invention.

TABLE 1 Weight in Grammes % Failure 750 0 800 20 850 50 900 60 950 80 1000 90 1050 100 400 400 F50=1050-50 ( 0.5) 100 =1050-50 x3.5 = 1050175 F50 = 875 Grammes TABLE 2 Six Thicknesses of Film Eight Thicknesses of Film Weight in Grammes % Failure Weight in Grammes % Failure 700 0 950 0 725 10 975 10 750 30 1000 40 775 40 1025 40 800 60 1050 70 825 90 1075 80 850 100 1100 100 330 340 330 340 F50=850-25 ( -0.5) F50=1100-25 ( -0.5) 100 100 =850-25 x 2.8 =1100-25 x 2.9 =850-70 = 100-72.50 F50 = 780 Grammes F50 = 1027.5 Grammes more reflective sheets of plastics material formed from a suitable masterbatch of plastics material.Also means may be provided which constitute a thermal barrier between two of the sheets of plastics material. This may comprise an interlayer of foam material, or means to space the two sheets apart to provide an air space between the two sheets. These means may comprise one or two net-like interlayers, or an interlayer of granulated foam, or zones of granulated foam adhered or bonded to selected spaced zones of one, or both, of the two sheets. Thus blobs of adhesive could be deposited on one sheet of the web, and granulated foam applied to the web, with the excess granulated form being blown away, leaving "islands" of granulated foam on the sheet, before the composite web is formed.

Alternatively the bag may incorporate, or subsequently be provided with, a sheet of plastics material incorporating air bubbles. Of course, where a high degree of thermal insulation is required two or more such thermal barriers can be provided.

Claims (39)

1. A bag formed from a plastics material, said bag comprising a plurality of sheets of plastics material film that are superimposed, the sheets being connected together to form a bag, each side wall of the bag thus comprising a plurality of separate thicknesses of plastics material film.

2. A bag according to claim 1 wherein the bag is formed from a web comprising a stack of sheets of plastics material, the stack being folded so that portions thereof are superimposed, the folded stack having portions thereof connected to each other to form a bag.

3. A bag according to claim 2 wherein said portions of the folded stack are thermally welded to one another.

4. A bag according to claim 2 or 3 wherein one sheet of the stack has protruding free edges thereof folded around the free edges of the remaining sheets of the stack, said inwardly folded portions defining the mouth of the bag.

5. A bag according to claim 4 wherein the inwardly folded portions are connected to the remaining sheets of the stack.

6. A bag according to claim 5 wherein the inwardly folded portions are connected to the remaining sheets of the stack by means of thermal welding.

7. A bag according to any one of the preceding claims wherein the sheet that defines the exterior surface of the bag is formed of low density polyethylene.

8. A bag according to any one of the preceding claims wherein the sheets other than the sheet forming the exterior of the bag are formed of high density polyethylene.

9. A bag according to any one of the preceding claims comprising between two and twenty superimposed sheets of plastics material film.

10. A bag according to claim 9 comprising between five and ten superimposed sheets of plastics material film.

11. A bag according to any one of the preceding claims wherein said plastics material film is between four and one hundred microns thick.

12. A bag according to any one of the preceding claims wherein said plastics material film is between four and twenty microns thick.

13. A bag according to claim 11 wherein said plastics material film is substantially eleven microns thick.

14. A bag according to any of the preceding claims fabricated from a web comprising a plurality of interleaved centre fold sheets.

1 5. A bag according to any one of the preceding claims wherein one or more of said thicknesses of plastics film is black.

1 6. A bag according to any one of the preceding claims wherein one or more of said thicknesses of plastics film is thermally reflective or provided with a thermally reflective coating.

1 7. A bag according to any one of the preceding claims wherein at least two of said separate thicknesses are separated by a thermally insulating barrier.

1 8. A bag according to claim 1 7 wherein said thermally insulating barrier is constituted by a foam interlayer.

1 9. A bag according to claim 1 7 wherein said thermally insulating barrier is constituted by an air gap.

20. A bag according to claim 19 wherein the air gap is formed by a net, or means which act to space the thicknesses of film apart.

21. A method of forming a bag from a plastics material, said method comprising superimposing a plurality of sheets of plastics material and connecting the sheets together to form a bag, each side wall of the bag thus comprising a plurality of separate thicknesses of plastics material.

22. A method according to claim 21 wherein the bag is formed from a web comprising a stack of sheets of plastics material, the stack being folded so that portions thereof are superimposed, the folded stack having portions thereof connected to each other to form a bag.

23. A method according to claim 22 wherein portions of the folded stack are thermally welded to one another.

24. A method according to claim 22 or 23 wherein one sheet of the stack has protruding free Whilst the invention has been described above with reference to a process which utilises, as an initial starting material, a plurality of sheets of flat web, when it is desired to manufacture bags of substantial size, such as potato sacks or sacks for agricultural fertilizers, or for other powdered or granular chemicals, it is necessary to utilise, as an initial material, centre fold sheet, since only such a material is readily available in a sufficiently large size.It will be appreciated that it is not practicable merely to superimpose several webs of centre fold sheet to form a composite web suitable as the initial supply to a bag making machine, and thus, if centre fold sheet is to be utilised, it is necessary to interleave the centre fold sheet so that a composite web having a configuration similar to that illustrated above with reference to Figure 3 is obtained. This is accomplished by utilising angled guide blades, and the principle of operation of the interleaving apparatus will be described with reference to Figure 6.

Turning now to Figure 6 a first web 31, which comprises a centre fold sheet is illustrated as travelling in a straight horizontal path as illustrated by the arrow 32. As the web approaches an interleaving station 33 the upper and lower portions of the web are separated slightiy, so that the two portions of the web pass above and below a guide plate 34 which is inserted in position so that the leading edge 35 of the guide plate is inclined at an angle of 450 to the axis of movement of the web 31.

The inner most portion 36 of the guide plate 34 is located immediately adjacent the fold in the centre fold sheet 31. The guide plate is a flat plate of metal, of any convenient size, and the leading edge of the plate is rounded. A second web, 37, which is also a centre fold sheet which is to be interleaved with the web 31 is suppiied from a reel, not shown, and this web initially travels towards the open side of the web 31, that is to say the side defined by the two free edges of the sheet forming the web 31, in a direction indicated by the arrow 38. The web 37 passes over the upper surface of the guide plate 34 and is then folded around the inclined rounded leading edge 35 of that plate, and thus the direction of movement of the web 37 is turned, when considered in the horizontal plane through an angle of 900.It can be seen that the web 37 thus moves away from the guide plate 35 in the direction indicated by the arrow 39, and the open side of the web 37, defined by the free edges of the sheet forming the web 37, is substantially aligned with the free edge of the sheets defining the web 31.

It will be appreciated that by utilising the above described guide plate the webs 31 and 37 have been interleaved.

Turning now to Figure 7 an apparatus for interleaving a plurality of centre fold webs is illustrated.

The first web 31 is initially stored on a reel 40. A conventional centering device 41 is provided which centres the reel 40 in response to signals derived from a sensor 42, which senses the actual position of the web leaving the reel. Thus the web leaving the reel is maintained to have a fixed position relative to the sensor 42. The web 31 passes over an initial guide plate 34, as described above, and thus the direction of movement of the web is effectively turned through 900, and the web 31 then proceeds towards the right as illustrated in Figure 7. The web then passes the interleaving station 33 that has been described above, and the web 37, which is initially mounted on a reel 40A associated with a centering device 41 A and a sensor 42A passes over the guide plate 34 and is interleaved with the web 31.The two interleaved webs then proceed towards a second interleaving station 43 where a further web 44, which is a centre fold sheet, stored on a reel 40B associated with a centering device 41 B and a sensor 42B is interleaved into the centre of the combined webs 31 and 37.

It will be appreciated that by utilising an appropriate number of guide plates and associated reels as described above any number of centre fold sheets can be interleaved. Only two interleaving stations have been illustrated for the sake of simplicity. The combined web is drawn towards a bag making machine by rollers schematically illustrated as drive rollers 45, and thus the various webs that form the composite web are all drawn towards the bag making at a constant speed. It will be appreciated that the composite web drawn past the rollers 45 will have a form substantially identical with that illustrated in Figure 3, so that the welds 20, 21 will not be provided. However, the composite web can then be passed through an appropriate welding machine that will effect the welds 20, 21 as necessary, and then the further steps in the bag making process described above can be repeated.

It will be appreciated that bags made by the method described above may take many forms. For example, carrier bags may be made, with holes punched in them to form handles, or with separate handles secured thereto, such as injection moulded handles. Bags with draw-string closures can be provided, and bags of all sizes can be produced. Large bags may be used, for example, to hold potatoes in 25 kg quantities. Such bags may be perforated with punched out holes, or other such means, such as micro-perforations, to enable the contents of the bag to "breathe".

If a bag in accordance with the invention is to be used to contain a substance which should not be subjected to light, e.g. a compost material on which fungi or the like may grow if the compost is subjected to light, then the innermost layer of the web forming the bag may be of black polyethylene.

Equally, if it is desired that the contents of the bag should be visible, then the bag may be made transparent.

Bags in accordance with the invention may be made to have excellent thermal insulating properties, and may then be used with advantage to transport cold items, such as frozen foodstuffs purchased from a supermarket or freezer centre, or to transport hot items such as hot take-away food, e.g. fish and chips, or Chinese food. Such thermally insulated bags may incorporate one or more heat reflective layers, such as aluminized surface on one or more of the sheets of plastics material, or one or edges thereof folded around the free edges of the remaining sheets of the stack, said inwardly folded portions defining the mouth of the bag.

25. A method according to claim 24 wherein the inwardly folded portions are connected to the remaining sheets of the stack.

26. A method according to claim 25 wherein the inwardly folded portions are connected to the remaining sheets of the stack by means of thermal welding.

27. A method according to any one of claims 21 to 26 wherein the sheet that defines the exterior surface of the bag is formed of low density polyethylene.

28. A method according to any one of claims 21 to 26 wherein the sheets other than the sheet forming the exterior of the bag are formed of high density polyethylene.

29. A method according to any one of claims 21 to 28 comprising the step of superimposing between two and twenty sheets of plastics material.

30. A method according to claim 29 comprising the step of superimposing between five and ten sheets of plastics material.

31. A method according to any one of claims 21 to 30 wherein said plastics material film is between four and one hundred microns thick.

32. A method according to any one of claims 21 to 30 wherein said plastics material film is between four and twenty microns thick.

33. A method according to claim 32 wherein said plastics material film is substantially eleven microns thick.

34. A method according to any of claims 21 to 33 comprising the step of interleaving a plurality of centre fold sheets by passing a first such sheet along a path, introducing a second such sheet to an interleaved position within the first such sheet by passing the second sheet over the leading edge of a guide means which is inserted between the two thicknesses of the first sheet, said leading edge being inclined relative to the movement of the first sheet, and optionally interleaving further centre fold sheets in a corresponding way.

35. An apparatus for performing the method of claim 34 comprising means defining a path to be followed by the first sheet, a plurality of guide means in the form of guide plates, and means for supplying centre fold sheets to each guide plate to be folded over the leading edge thereof.

36. A bag substantially as herein described with reference to and as shown in the accompanying drawings.

37. A method of making a bag substantially as herein described with reference to the accompanying drawings.

38. An apparatus according to claim 35 and substantially as herein described with reference to the accompanying drawings.

39. Any novel feature or combination of features disclosed herein.