GB2089286A - Manufacture of thermoplastic bags - Google Patents

Abstract

A machine for producing thermoplastic bags from a web forms web segments (WS), each defining two bags, and stacks these segments on one of a plurality of interconnected and spaced accumulators (88 or 188) indexed at a stacking station. As the segments are stacked they are engaged by means which ensures formation of a registered stack. In one embodiment said means comprises stacking posts (90) on which the segments are registered by means of holes punched in the segments upstream of the stacking station. In an alternative embodiment pneumatically actuated fingers (218), provided at the stacking station, operate to retain the segments on the accumulator (188) during stacking and a clamp (198) holds the stack in register during indexing from the stacking station. When a predetermined number of segments have been stacked on the accumulator, the accumulator moves the stack to a station where the segments are cut, or cut and joined, along a central line of the stack to produce two individual stacks of bags. <IMAGE>

Description

SPECIFICATION The manufacture of thermoplastic bags This invention relates to a method of and apparatus for producing thermoplastic bags.

Machines for producing bags from thermoplastic web material come in a variety of configurations depending upon the size and style of bags to be produced and are adaptable to process material of various thicknesses and chemical compositions.

Certain bag machines are dedicated to produce one style of bag (side weld or bottom weld) or their design is of a universal character such that by replacement of certain components side weld or bottom weld bags can be made.

Directly or indirectly the object of bag machine developments is to decrease the time to produce a bag. The most important consideration in reducing machine cycle time is substantially wholly dependant upon whether the output of the bag machine can be organized such that successive bags are accumulated in neatly registered stacks with each stack containing a predetermined number of bags.

One approach currently practiced to produce registered counted stacks is to utilize a bag stack accumulating conveyor, known as a wicketer. Basically, the accumulating conveyor includes a plurality of sets of pins which are successively positioned at a stacking station and remain there until a predetermined number of bags have been deposited on the pins. By using this procedure each individual bag is provided with a pair of holes whose pitch matches the spacing of the wicket pins. After a predetermined number of bags have been accumulated, advance of the web is arrested and the conveyor carrying the set of pins is indexed to transport a completed bag stack away from the stacking station and to position a succeeding pair of pins at the stacking station.

Machines utilizing a wicket stacker are in very wide use because stacks of several hundred bags of low gauge high density polyethylene can be quickly and accurately stacked since windage problems have a very minimal effect on proper stacking.

According to the present invention in one aspect, there is provided a method of producing thermoplastic bags characterized by the steps of producing segments each defining two bags, transferring the segments successively on one stacking device of an indexing conveyor having a plurality of said stacking devices spaced along its length and engaging the segments on the stacking device at at least one point whereby to retain the segments as a registered stack, and severing the segments of the stack after indexing from the location at which the segments are received on any given stacking device.

According to the present invention in a second aspect, there is provided apparatus for producing thermoplastic bags characterized by means for producing segments each defining two bags, means for transferring the segments successively on to one stacking device of an indexing conveyor having a plurality of said stacking devices spaced along its length and means for engaging the segments when on any given one of said stacking devices at at least one point whereby to retain the segments as a registered stack, and means for severing the segments of the stack after indexing from the location at which the segments are received on any given stacking device.

When it is desired to produce bags from tubular film, certain considerable advantages result in printing the web on one or both sides. More particularly the pattern achievable by the printing press can be duplicated on either side of the longitudinal median of the web, because processing by the bag machinery constructed in accordance with the principles of this invention allows creation of two bags from a web segment.

The rate at which bags made from thermoplastic web material can be produced by both aspects of the invention is double the rate of machine cycle time and the physical characteristics of the material, such as its lack of stiffness, is no longer a factor limiting accurate stacking and stack maintenance when accumulated stacks are transferred to further processing apparatus. More particularly, the process and apparatus of the present invention allow rapid controiled transfer and the creation of stacks containing several hundred web segments when processing the thinnest available low density polyethylene.

Apparatus embodying the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figures 1A and lB, considered together, is a side elevation of a web processing machine; Figure 2 is also a side elevation to a reduced scale but incorporating mechanisms for producing a different style of bag; Figure 3 is a perspective view, to an enlarged scale, illustrating slitting knives for removing a narrow strip of web from tubularfilm; Figure 4 is a perspective view, to an enlarged scale, illustrating gusset-forming devices; Figure 5 is a perspective view, to an enlarged scale, illustrating devices for forming spaced lines of perforations in the web tube; Figure 6 is a perspective view of punching devices for forming a pattern of holes in a medial band of the web;; Figure 7 is a perspective view of a seal bar and seal roll at the sealing and severing station intended to produce a web segment having a pattern of four holes formed therein; Figure 8 is a perspective view illustrating placement of a web segment on a stacking device located at the stacking station; Figures 9 and 10 illustrate, respectively, a side and end view, to an enlarged scale, of a cutting mechanism for separating the web segments; Figure 11 is a perspective view illustrating a stacking device at the cutting or separating station; Figure 12 is another perspective view showing the web segments after they have been severed and being retained by upwardly projecting pins on the stacking device; Figure 13 is an end view of Figure 12; Figures 14 and 15 illustrate two styles of bags that can be made by apparatus and the method in accordance with the present invention;; Figure 16 is a modified form of the transfer mechanism adapted to transfer web segments of greater transverse dimension; Figure 17 is a side elevation, with parts cut away to show working details of a second embodiment of a machine in accordance with the invention; Figure iSis a plan view of the machine of Figure 17; Figure 19 illustrates, to an enlarged scale, a part of the machine of Figures 17 and 18; Figure 20 is a plan view of the detail of Figure 19; Figures 21 and 22 are cross-sections, respectively, on the lines 21-21 and 22-22 of Figure 19; and Figure 23 is a cross-section simiiar to that of Figure 22, but showing a modification.

Considering Figures 1 A and 1 B jointly, a webprocessing machine in accordance with the invention is generally indicated by 20. The main components of the machine include an unwind stand 22 rotatably supporting, on a shaft 24, a roll of thermoplastic material, hereinafter sometimes referred as a web roll WR, and a plurality of idler rolls 26 directing the thermoplastics film TF to a tension control mechanism 28 mounted in a frame structure 30. The film is threaded around a plurality of stationary idler rolls 32 and a plurality of idler rolls 34 mounted for rotation in a vertically-movable crosshead 36. The crosshead is guided for vertical movement by tubular rods 38 in which is disposed a rope or cable 40 having its ends connected to a weight 42 and, by virtue of pulleys 44, travelling in a generally rectangular loop.The combined weight of the rollers 34 and the crosshead 36 is greater than the weight 42 so that the differential weight allows the crosshead to move downwardly in order to apply tension to the thermoplastic film being looped between the idler rollers 32 and the idler rollers 34. The thermoplastic film is then directed, respectively, around idler rolls 46,48 and 50 and thereafter the web is directed to a perforating unit 52 including perforating wheels 54 and web drive rolls 56, one of which functions as a platen roll for the perforating wheel 54. The film is thereafter threaded through a dancer unit 58 serving to maintain a predetermined tension on the film portion downstream of the drive rolls 56. Mounted between the idler rolls 46 and 48 and between the idler rolls 48 and 50 are gusset-forming means 60 operating to inwardly fold the opposite edges of the thermoplastic film.

The film web leading from the dancer unit 58 passes over an idler roll 62 and between draw rolls 64 which are adjacent a sealing and severing station 66 which includes a seal roll 68 and a vertically reciprocating seal bar 70 heated to effect sealing and severing of the film to produce web segments of uniform dimension. Between the sealing and severing station and the idler roll 62 and mounted on an accessory section 72 of the bag machine 74, a punching unit 76 is disposed. The punching unit, as will be explained in greater detail hereinafter, is activated during periods of web repose to produce a pattern of four holes in a medial band of the film web.

The portion of film projected beyond the sealing and severing station 66 is supported by a series of laterally-spaced belts 78 travelling in a generally triangular orbit defined by idler pulleys 80 and a drive pulley 82. After the web portion projecting beyond the seal bar 70 is severed and sealed, it is immediately grasped by raidally-extending arms 84 of a rotary transfer means 86 whose construction and mode of operation are conventional in the art.

The arms 84 are provided with a series of holes or orifices connected to a source of vacuum and on coming into contact with the web portion or web segment grasping is effected and the web segment is rotated through an angle of substantially 180 and deposited on a stacking device 88 supporting upwardly-extending pins 90 which are spaced in a pattern substantially identical to the pattern created by the punching unit 76. It will be observed by inspection of Figure 1 B that a plurality of such stacking devices 88 is mounted on a conveyor 92 which is driven by an indexing drive unit 94 and travels in a path determined by idler sprockets 96 and a drive sprocket 98. Along the upper reach of a conveyor 92 and mounted on a frame structure 100, a cutting or a cutting combination unit 102 for dividing a stack of web segments into bags is provided.

After a predetermined number of web segments have been stacked on a stacking device, located at a stacking station SS, advance of the thermoplastic film upstream of the sealing and severing station 66 is momentarily arrested so that a successive stacking device 88 is located at the stacking station SS.

During creation of stack of web segments on one of the stacking devices 88, the web segments, on a downstream one of the stacking devices located immediately below the unit 102, are divided by a knife 104 moved downwardly by concurrently operating actuators 106.

Although it is a common practice to convert plastics film to produce web segments which are stacked and thereafter divided such that each web segment produces two bags, the present invention, by providing a positive transfer and a stacking station constraining each web segment and the completed stack, allows the accumulation of several hundred web segments which are maintained in registry as the completed stack is indexed to a cutting or a cutting and combining mechanism which will hereinafter sometimes be referred to as a cutting and/or blocking unit. The advantages resulting from such a process performed by the disclosed exemplary apparatus, permits the processing of the thinnest available, low-density, polyethylene film and producing lipped bags wherein the lip portion can be very narrow, for example, 25 millimeters or approximately one inch. In the event it is desired to produce lipped bags, (this refers to bags wherein the corresponding edges defining the mouth are uneven or spaced from each other) a slitting device 108 is mounted on the frame structure 30. The slitting device 108 is shown in Figure 3 and comprises a support rod 110 carried by the frame 30 and a block 112 adjustablyfixed to the support rod. The block 112 mounts laterally spaced knives 114which are orientated to cut and accordingly remove a film strip 116 from one layer of the tubular film and thereby create a gap 118 in one panel of the initially tubular thermoplastic film. Conventional means (not shown) such as a tube connected to a source of vacuum may be provided for disposing of the waste film strip 116.

Figure 4 illustrates the action of the gusset forming means 60 in which the illustrated form comprises wheels 120 secured to short stub shafts 122 which are freely rotatably mounted on the frame structure 30. Wheels 120 are provided with a bevelled edgeto facilitate inward folding of the film and twill be noted that one wheel engages one edge of the film as it approaches the idler roll 48 while the other wheel engages the film as it progresses away from the idler roll 48. If desired, the gusset forming means may consist of two spaced, stationary, flat plates with a wheel or other flat plate accommodated therebetween.

In producing another style of bag wherein the edges of the opposed panels forming the mouth are even, the slitting mechanism 108 is either rendered inoperative or removed and perforating mechanism (Figure 5) is rendered operative to produce spaced lines of perforations 124 on both plies of the gusseted thermoplastic film TF. The central band of the film between the lines of perforations 124 is the zone in which web segment mounting holes are made, as illustrated in Figure 6, where it will be seen that each actuator 125 has its output rod connected to a plate 126 carrying two hole punches 128.

Conventional controls energize the actuators 125 to plunge the hole punches through the thermoplastic film creating four holes 130 in the gap 118 through the thermoplastic film. Punching occurs during the time when the web is momentarily arrested to effect sealing and severing.

When the gusseted thermoplastic film is fed by the draw roll 64 between the seal roll 68 and the seal bar 70 (Figure 7) the increment of thermoplastic material projected towards the rotary transfer means 86 will define the longitudinal dimension of the web segment WS and it will be seen to include the pattern of four holes 130, gusseted lateral edges 132 and leading and trailing seals 134. The thermoplastic film illustrated in Figures 6 and 7 includes the gap 118 created by removing a film strip 116 by the knives 114. If the apparatus is set up to produce even-edged bags, the thermoplastic film will include the lines of perforation 124 and the four holes 130 in each web segment.

As hereinbefore mentioned, the portion of the thermoplastic film projected beyond the seal roll 68 and the seal bar 70 is supported by a series of laterally spaced belts 78 and upon being severed and seaied, the web segment is grasped by one pair of laterally spaced arms 88 that are downwardly adjacent but moving in a counterclockwise direction (as viewed in Figures 1 B and 2) to engage and hold, by virtue of the fact that the arms 88 are hollow and are connected to a hollow hub 136 connected to a source of vacuum and mounted on a shaft 137. As each pair of arms approaches the transfer station TS, a web segment is grasped and transferred onto the stacking device 88 located at the stacking station SS as shown in Figure 8. Collection of web segments on a stacking device continues until a predetermined number of segments have been accumulated.The predetermined number of web segments is determined by conventional counter mechanisms that condition the web feeding draw raws 64 to stop their intermittent rotation for a sufficient period of time established by the operator to allow the indexing drive unit 94 to move the stacking devices 88 of the upper reach of the conveyor 92 toward the cutting or cutting and combining unit 102. Concurrently, a succeeding, empty stacking device is positioned at the stacking station SS.

The creation of stacks of web segments on each of the stacking devices 88 continues with an interruption occurring when a full complement of web segments have been accumulated on a stacking device. When a stacking device with a full complement of web segments is positioned directly under the unit 102, the web segments are parted or separated along a line CP (Figures 14 and 15) located centrally of the edges 132. Stacking on a succeeding device 88 continues during this separation.

Figures 9 and 10 illustrate the construction of the cutting or cutting and combining unit 102 while Figures 11, 12 and 13 illustrate the mode of operation. It will be seen that the frame 100 includes a plate 138 mounting the pneumatic actuators 106, such that actuator rods 140 project through apertures in the plate 138. To resist bending moments imparted to the plate 138, square section hollow beams 142 are rigidly (as by welding) secured to the plate 138. Fastened to the lower ends of the rods 140, by means of a threaded fastener 144, is a carrier plate 146 having affixed thereto, by means of fasteners 148 and spacers 150, a holder 152, including a clamp bar 154, retaining the knife 104, formed with a sharp, serrated, edge 158.Disposed on opposite sides and closely adjacent to the holder 152 are clamping bars 160, each of which is rigidly connected to guide rods 162 biased downwardly by springs 164. Each guide rod is slidably mounted in bushings 166 secured to the plate 146 and they are retained within the bushings by threaded nuts 168.

Reference to Figure 10 will show that the clamping bars serve to press the stack of web segments downwardly while the knife 104 cuts through the stack of web segments to thereby produce individual bag stacks which are, in addition, retained on the stacking device 88 by the pins 90a, 90b.

To facilitate cutting or cutting and joining a stack of web segments, heat is supplied to the holder 152 and conducted to the knife 104 carried thereby. Heat is supplied by an elongate, tubular, resistance heater 170 associated with a temperature controller (not shown) in order to adjust the temperature of the knife 104. Cutting without combining or blocking of the severed edges is greatly facilitated if the knife is raised to a temperature of 93"C to 1 05 C. This level of temperature has been found to reduce the penetration forces required for the knife to cut through a stack of web segments.When it is desired to cut and join the severed edges the knife temperature is raised to approximately 298do. While these temperatures have been found to produce the desired results with certain types of thermoplastic film, skilled operators can determine the temperature levels best suited for the particular requirements of the film used.

Suitable conventional controls are provided for controlling the flow of pressure air to the actuators 106 to effect extension and retraction of the rods 140 and accordingly, accomplish cutting.

To preserve the sharp condition of the knife 104 as it passes through the stack of web segments, the stacking device 88, which comprises a flat rectangu lar plate 172, also includes a block of wood 174 secured to the plate 172. It has been found that the block of wood 174 provides a very durable reaction surface which allows penetration of the knife to ensure cutting of all web segments and yet prolong the life of the edge of the knife.

Creation of a stack of web segments on the stacking device 88 by impaling successive web segments on the pins, collectively identified by the numeral 90, is reliably ensured by making the pair of pins 90a shorter than the pair of pins 90b (Figure 8).

The differential elevation of the sets of pins 90a and 90b is such that a plane containing their ends is substantially co-planar with a plane containing the transfer arms 84. Accordingly, as a web segment is being impaled on the pins 90, penetration of the pins through the holes 130 occurs substantially simultaneously.

Figures 11, 12 and 13 illustrate the operation of the cutting or the cutting and sealing device which, as mentioned hereinabove, is dependant upon the temperature of the knife 104. When one of the stacking devices 88 is positioned directly below the unit 102, the web segments carried by the pins 90 are divided along the line CP designating the cutting plane. If the temperature of the knife 104 is at approximately 105"C, the web segments are divided (Figure 12) and the adjacent plies are not joined.

Thus, on being divided, two bag stacks BS are created and are maintained on the stacking device by the pins 90. The bag stacks as illustrated in Figure 12 are formed with a lip produced when the cutting knives 114 remove the strip of material 116.

When it is desired to combine or block the divided web segments, the knife temperature is maintained at approximately 298"C so that as the knife progresses through the stack of web segments, the edges 176 fuse or weld all of the resulting edges of each bag stack together. When the stacking device carrying two bag stacks progresses beyond the cutting unit 102, unloading of the stacks by the operator can be effected, but the upper reach of the conveyor 92 is such that instantaneous removal is not essential.

Figures 14 and 15 show the configuration of two styles of bags produced by the process and apparatus in accordance with the hereinbefore described embodiment.

Figure 14illustrates lipped bags wherein the upper margins defining the bag mouth are uneven, resulting from the removal of the strip 116 to thereby produce the gap 118.

Figure 15 illustrates the configuration of the web segment when the perforator 52 is rendered operable to produce perforations 124. Afterthe web segments have been divided into individual bags along the line CP, the edges of the bags are even since the lines of perforation 124 are formed in both panels.

The modification of Figure 16 illustrates a con struction providing the transfer mechanism 86 with the ability to handle web segments of greater transverse dimension and yet provide stability and control to the segment as it is transferred. Control is accomplished by providing four hubs 136 and associated radially-extending arms 84 mounted on the shaft 137. On engaging and transferring web segments WS the arms 84 grasp the segment at substantially equally spaced lateral zones to firmly hold the wider web segment.

If it is desired to process a roll of flat single ply material, conventional folding apparatus operating to fold the opposed edge portions toward the longitudinal median of the web to define a gap 118 of desired width, may be used. Using a roll of flat web, folded so that the edges are brought in contact or slightly overlap and subsequently perforating both plies to produce even edged bags has signifi cant advantages in printing since the graphic pattern would exist on both panels of the bags and yet simplify the printing process. To print both panels of bags derived from tubular stock requires a special printing procedure involving passing the web through the press twice.

The second embodiment of apparatus, forming a part only of the bag making machine as a whole, serves only to stack and sever the bag segments, the formation of the bag segments being effected on a machine as described with reference to Figure 1A or, indeed on any other machine suitable for producing bag segments as hereinbefore described.

Parts similarto those of the first described embodiment will be given the same reference numerals and will not be further described. The main difference in relation to the first embodiment is the employment of clamping means to hold the bag segments in position immediately on completion of transfer of each bag segment from the wheels 86 on to the conveyor 92. The conveyor 78 can be seen in Figure 18 to have gaps 79 to facilitate lifting the bag segments by the arms 84 of the wheels 86.

In place of the stacking devices 88 of the first embodiment, stacking devices 188 are used which are shown in outline in Figures 17 and 18 and in detail in Figures 19 and 20. Each stacking device 188 includes securing means 190 serving to secure a plate 192 rigid with a bent-up end stop or guide 194 and having a downwardly inclined edge portion 196 at the forward end of the plate (considered in the direction of motion of the conveyor). The parts of the bag segments lying on the edge portion 196 can co-operate with a clamp 198 pivoted slightly beyond the edge and being in the form of a U-shaped piece of wire. The clamp 198 is rigid with a rod 200 of J-shape with the longer limb connected to the free ends of the U-shaped clamp. A helical spring 202 acts on and encircles the rod 200 which in turn biases the clamp towards the edge portion 196. The clamp can be withdrawn from the edge portion 106 by a pneumatic or hydraulic actuator 204 through the intermediary of a transverse plate 206 mounted on the free end of the piston rod 208 of the actuator.

As each stacking device 188 reaches the stacking station SS, it is disposed to co-operate with a pair of pneumatic actuators 210, the cylinders 212 of which are rigidly mounted on part of the framework 214 of the apparatus. The piston rod 216 of each actuator has an end portion 218 made of or coated with a friction material to assist in gripping the topmost bag segment of the stack. The actuators operate alternately and hence in the side view of Figure 19 the end portions of both actuators 212 are visible.

The actuators 212 are subject to the action of control means (not shown) which may energize the actuators in dependance upon the angular location of the rotary transfer means 86. Cams, fast for rotation with the transfer means 86 may be used.

In operation, with one of the stacking devices 188 located at a stacking location SS, bag segments 220 (chain lines) are successively delivered by the arms of the wheels 86 with the actuators 212 both in a position at which no clamping or interference with a bag segment being deposited, is caused. Immediately on completion of deposition with one edge of the segment accurately located against the stop or guide 194, one of the friction members 218 will descend and hold the bag segment in position. As the next segment is deposited the first actuator will withdraw the corresponding friction member 218 and the other actuator will descend to clamp the just deposited bag and so on. Alternatively, the actuators may operate synchronously. On completion of the formation of a predetermined number of segments in the stack, the clamp 198 will be allowed to operate by retraction of the actuator 204.The conveyor 92 is then indexed forwardly and the completed stack of segments 220 is removed manually. Owing to the clamp 198 accurate registration will be maintained.

In the meanwhile the bag making machine will have been interrupted for an appropriate number of cycles to enable the conveyor indexing to be completed. The whole cycle of stacking is then resumed, the clamp 198 being withdrawn from the end portion 196 of the plate 192.

The secure and accurately balanced location of the bag segments 220 on the stacking device 188 and the presence of the clamp 198 facilitates the separation of the bag segments to form individual bags and apparatus to effect this may be used as in the first embodiment. The conditions to effect separation and block formation will be the same. If such separation is to be carried out it will be desirable to provide hollow stacking pins or wickets on each stacking device as shown at 222 in Figure 23. A rotary knife, not shown, preferably heated will form a cut (and seals) at 222.

In the modification of Figure 23, each stacking device 188 may incorporate two, opposed, hinged side plates 226, which can be raised after separation to facilitate removal of the completed stacks of bags.

Operation of the side plates may be controlled by linear cams.

Claims (27)

1. A method of producing thermoplastic bags comprising the steps of forming segments from a web ofthermoplasticfilm material each segment defining two bags, transferring the segments successively to an indexible stacking station forming one of a plurality of interconnected, spaced, stacking stations, engaging the segments successively at the stacking station at at least one point on the last received segment whereby to retain the segments as a registered stack, and severing the segments of successive stacks after indexing from the location at which the segments are received on any given said stacking station to form stacks of individual bags.

2. A method according to claim 1 comprising the further steps of forming at least two holes in each segment spaced so that each eventual bag of the segment will include at least one such hole, stacking the successive segments on upstanding elongate members, each receiving one ofthe holes of each segment whereby the segments are engaged at at least two points.

3. A method according to claim 1 or claim 2 wherein the segments are severed to form bags when the stacked segments lie in draped condition on either side of said at least one point of engagement whereby to apply a tension force produced by gravity at the severing zone.

4. A method according to claim 2 wherein each hole is adjacent to the longitudinal centre of the web and the holes spaced at regular intervals such that each segment contains at least four holes.

5. A method according to any one of the preceding claims wherein the segments are so produced that the lateral edges of the web are equally spaced from the longitudinal median line of the web, and each segment is produced by transversely severing and sealing the web at equally spaced intervals longitudinally of the web, each segment being engaged and gripped on opposite sides of the median line as it is produced and being transferred to a said stacking station where it is engaged at at least said one point, and wherein said severing includes cutting or cutting and combining the web segments by mechanical cutting assisted by heat in a plane containing the longitudinal median line of the web segments whereby to divide the stack of web segments into two bag stacks.

6. A method according to any one of the preceding claims wherein a central strip is removed from one ply of the web to produce bags having uneven edges bounding the bag opening.

7. A method according to any one of claims 1 to 5 wherein at a central zone of the web, continuous lines of perforations are formed through both plies to produce even edge bags in each bag stack, the bags of each stack being detachably secured at the perforations to a respective part of a central strip.

8. A method according to claim 5, claim 6 or claim 7, wherein said cutting or cutting and combining is effected at approximately 105 C, to effect cutting without joining the adjacent plies of web.

9. A method according to claim 5, claim 6 or claim 7 wherein said cutting or cutting and combining is effected at approximately 298"C to effect combining of the adjacent plies of web to produce two blocks of interconnected bagas from the stacked segments.

10. A method according to claim 1, or any one of claims 3 to 9, wherein the point of engagement of each segment is effected pneumatically.

11. A method according to claim 10 wherein on completion of a stackthe pneumatically generated force is withdrawn and stack registration is maintained during indexing of the stacking station by mechanical clamping.

12. A method of manufacturing thermoplastic bags from a web of thermoplastic material comprising forming segments of the material each of which segments defines two bags, forming four holes in each segment spaced so that each eventual bag will include two such holes, stacking the successive segments on upstanding elongate members, each receiving one of the holes of each segment and severing the stacked segments whereby to form two bags from each segment.

13. A method according to claim 12 wherein the elongate members are mounted on a conveyor which can be indexed on completion of a stack having a predetermined number of segments, succeeding elongate members serving to accumulate subsequent stacks.

14. A method according to claim 12 wherein the segments are severed to form bags when the stacked segments iie in draped condition on either side of said elongate members whereby to apply a tension force produced by gravity at the severing zone.

15. A method of stacking and cutting web segments produced from an elongate web unwound from a roll of thermoplastic material so that the lateral edges of the web are equally spaced from the longitudinal median line of the web and providing, closely adjacent to and on either side of the longitudinal median line, at least one aperture in each web segment which segment is produced by transversely severing and sealing the web at equally spaced intervals longitudinally of the web, engaging and gripping each web segment on opposite sides of the apertures as it is produced and transferring it to a stacking station where it is impaled on upstanding elongate members correspondingly spaced to the spacing of the apertures and mounted on support plate means positioned at the stacking station, having a length at least equal to the width of the segments and a width substantially less than the length of the segments, translating the support from said stacking station when a desired number of web segments have been impaled on said elongate members to a cutting or cutting and combining station, and cutting or cutting and combining the web segments at said last-mentioned station by a heated knife located in a plane containing the longitudinal median line of the web segments wherey to divide the stack of web segments into two bag stacks.

16. In apparatus for producing thermoplastic bags, means for producing segments each defining two bags, an indexing conveyor, meansfortransfer- ring the segments successively on to one stacking device mounted on the indexing conveyor which has a plurality of said stacking devices spaced along its length, means for engaging the segments when on any given one of said stacking devices at at least one point whereby to retain the segments as a registered stack, and means for severing the segments of the stack after indexing from the location at which the segments are received on any given stacking device to form stacks of individual bags.

17. Apparatus according to claim 16 wherein the indexing conveyor is a wicket conveyor, and the engaging means are wicket pins.

18. Apparatus according to claim 16 or claim 17 comprising means for forming at least one aperture on either side of the longitudinal median line of the web during periods of web repose, said apertures enabling the segments to be engaged by the wicket pins.

19. Apparatus according to claim 16, claim 17 or claim 18 comprising means for removing a strip from the central portion of one web ply.

20. Apparatus according to any one of claims 17 to 19 wherein said aperture-forming means comprises two reciprocable pneumatic actuators and two punches each mounted on a respective actuator, said actuators being spaced longitudinally to produce a pattern of four apertures in each web segment.

21. Apparatus according to any one of claims 16 to 20 wherein said severing means comprises a reciprocable knife holder, an elongate serrated knife mounted on the holder, means for electrically heating said holder and said knife to a desired temperature, and means substantially coextensive with and located on opposite sides of said knife for compressing the accumulated web segments before and while cutting occurs.

22. Apparatus according to claim 16, comprising at least one pneumatic actuator the piston rod of which serves as the means for making point engagement with the segments.

23. Apparatus according to claim 22 comprising clamping means operative to maintain registration of a stack on completion of a predetermined number of segments in the stack and during indexing of the conveyor to the severing means.

24. A thermoplastic bag-making apparatus comprising means for producing from a web of thermoplastic film a succession of segments each adapted to form two bags, a wicket conveyor, means for transferring the segments from the segment producing means to the wicket conveyor and means for severing the stacked segments while the stacks are carried on the wicket conveyor so that each segment forms two bags.

25. Apparatus for producing bags from thermoplastic web material comprising means for intermittently advancing equal increments of the web, means extending transversely to the path of web advance for severing and sealing the leading portion of the web and thereby define a web segment, means for forming at least one aperture on either side of the longitudinal median line of the web during period of web repose and before severing and sealing occurs, means for accumulating a predetermined number of web segments on one of a plurality of stacking devices mounted on an indexable conveyor operable to locate successive stacking devices at a stacking station, said conveyor being operable, on accumulating a predetermined number of web segments on one of said stacking devices located at a stacking station, to index said conveyor and position a successive one of said stacking devices at the stacking station, means for transferring each web segment at it is produced on one of said stacking devices located at the stacking station, mounting posts on each stacking device on which each web segment is impaled, said posts being positioned to correspond to the apertures formed in each web segment, and means in the path of said conveyor and spaced from said stacking station for cutting said web segments along the longitudinal median line whereby to form two stacks of bags.

26. A method of producing bags substantially as hereinbefore described with reference to the accompanying drawings.

27. Apparatus for producing bags substantially as herein before described with reference to the accompanying drawings.