Apparatus for stretching a continuous plastics film in a packaging machine

Abstract: An apparatus for stretching a continuous plastics film in a packaging machine (11), in which products (12) fed one after the other are wrapped and sealed within said continuous film, comprising a first conveyor belt (13), a transverse welding apparatus (14) and a second conveyor belt (18), in which the transverse welding apparatus consists of a horizontally traversing carriage (15) which supports a pair of superposed welding bars (16,17) and a pressing belt (46) facing the second conveyor belt. The conveyor belts are driven by a speed variator (21) via a pair of transmissions, the first of which drives an end roller of the first conveyor belt and the second drives an end roller of the second conveyor belt by way of a linkage which varies the speed of the two conveyor belts, there being also provided upstream of said transverse welding apparatus, both above the first conveyor belt and operationally linked to the vertical movement of the upper welding bar (16), elements for clamping (51) the film onto the products (12) wrapped in it.

Description: This invention relates to a packaging machine. In machines for packaging products fed one after another, using continuous plastics film of heat-shrinkable type or not, it is known to weld the film, when wrapped around the product, in a direction transverse to the product and film feed direction, to form a package which is at least partly closed. A conventional packaging machine of said type is known from EP-A-0 304 978 on which the preamble of claim 1 is based. It should be noted that the package obtained is always larger than the product which it contains, resulting in a wastage of plastics material, and a package in which the film does not perfectly and completely adhere to the contained product. The film excess within the package is the result of the fact that it is normally closed by a hot bar welder which compresses the film between itself and a counter-welder within the separation space between one product and the next. Any excess film visible in the finished package is removed by known heat-shrinkage which causes the film to adhere to the product. This means firstly that the products to be packaged have to be fed spaced apart, and secondly that further equipment is required for the subsequent heat-shrinkage. It is therefore the main object of the present invention to solve said problems in such a manner as to obtain packages in which the film which wraps the products adheres perfectly to them when the transverse weld has been made. A further object strictly related to the latter object is to dispense with the need for further equipment (such as a heat-shrinking device) representing both additional investment and operating cost. These objects are attained according to the present invention by an apparatus for axially stretching a continuous plastics film in a packaging machine, said packaging machine being defined by the features of claim 1. The machine according to the invention advantageously enables finished packages to be obtained in which the plastics film is perfectly wrapped around, and adhering to the products. In this manner the packages are suitable for immediate subsequent handling or further treatment by other machines, or for immediate use. The structural and operational characteristics and advantages of the machine according to the present invention will be more apparent from the description relating to the Figures in which: Figure 1 is a perspective view of an end portion of a packaging machine provided with a apparatus according to the present invention; and Figures 2, 3 and 4 are schematic elevational views of that part of the packaging machine comprising the apparatus of the invention in different operating positions. With reference to Figure 1, a stretching apparatus for a continuous plastics film according to the present invention is incorporated by way of example into a partly illustrated packaging machine indicated overall by 11. The packaging machine 11 is of known type and enables products 12, arranged stacked in an entry portion of the machine, to be fed one at a time by a pusher conveyor (not shown). A first conveyor belt 13, shown only partly, receives the products 12 and feeds them towards a transverse welding apparatus, indicated overall by 14. The transverse welding apparatus 14 comprises a carriage 15 traversable horizontally relative to the frame of the machine 11 and carrying upper 16 and lower 17 welding bars movable vertically within the carriage 15. Downstream of the welding apparatus 14 there is provided a second conveyor belt 18 for removing the packaged products 12. The frame of the machine 11 also supports in known manner, not shown, at least one roll of plastics film material for wrapping the products 12. A central drive, shown partially as a portion of a transmission shaft 19, drives via a right-angle drive 20 a speed variator 21 connected to the belts of the two conveyors 13 and 18 and to a control unit 22 for the carriage 15 and for the relative welding bars 16 and 17. Specifically, the first conveyor belt 13 is driven, via its roller 23, fixed onto the shaft 24, by a transmission 25 extending from one side of the speed variator 21. A further transmission portion 26 (Figure 2), also deriving its motion from the shaft 24, is arranged between said roller 23 and a first roller 27 of a pressing belt 28 which extends as a flattened endless loop and is situated upstream of the transverse welding apparatus 14. The pressing belt 28 moves synchronously with the underlying conveyor belt 13 and lies a certain distance above and facing it, in order to embrace the passing feed products 12. The other side of the speed variator 21 drives a second transmission consisting of two portions 29 and 30 and driving the second conveyor belt 18. The first portion 29 of the second transmission rotates a pulley 31 fixed onto a shaft 32 supported freely and rotatably within a roller 33 about which the first conveyor belt 13 passes. A further pulley 34, also fixed onto the shaft 32, rotates to rotate the second transmission portion 30, which passes about a unit definable as a linkage for varying the speed of the second conveyor belt 18 relative to the first 13. This unit consists of a pair of idle pinions 35 supported on a slide 36 moved with reciprocating vertical movement. The slide 36 is driven by a transmission linkage comprising a lever 37 positioned between the slide 36 and a position-adjustable eccentric element 38. The eccentric element 38 is rotated by a further transmission 39 extending from the right-angle drive 20. Besides passing about the idle pinions 35 and pulley 34, the transmission portion 30 passes about further pinions 40 to define a path consisting of two adjacent loop portions joined at a central zone formed by the slide 36. In this manner, as the slide is vertically mobile the joining point between the two loop portions is also vertically mobile. By means of a shaft 41 on which it is fixed, one of said pinions 40 rotates a further roller 42 which drives the second conveyor belt 18. A further transmission portion 43 deriving its motion from a further roller 44 about which the second conveyor belt 18 passes rotates a roller 45 of a second pressing belt 46 facing the upper part of the second conveyor belt 18. The second pressing belt 46, also in the form of a flat endless loop, is supported via its end rollers 45 and 47 on a block 48 fixed to the end of the rod 49 of a cylinder 50. The cylinder 50 is secured to the carriage 15 and by the movement of its rod 49 causes the second pressing belt 46 to move vertically towards or away from the second conveyor belt 18. It should be noted that means for clamping the film onto the products wrapped in it are provided rigid with the upper welding bar 16. These clamping means comprise a pair of brackets 51 supporting a roller 52 via vertical lateral rods 57, with vertical damper elements 53 therebetween. The roller 52 acts as a stop roller for the pre-stretched film 54, being rotatable only in the direction of advancement of the products 12. The movement of the carriage 15 carrying the rollers about which parts of the conveyor belts 13 and 18 pass and slide and the welding bars 16 and 17 is derived, as stated, from a control unit 22 which derives its movement laterally from the right-angle drive 20 at 55. The control unit 22 is not described specifically herein as it is already known. On the carriage, in addition to the deviation roller 44 there are provided a further two rollers 56 about which there pass facing portions of both the conveyor belts 13 and 18 and which are separated from each other to define an aperture for the passage of the welding bars 16 and 17. The rollers 42, 33 with the relative pinions 34 and 40 about which the second transmission portion 30 passes are instead supported rotatably on the frame of the machine 11. The operation of a stretching apparatus in a packaging machine according to the present invention is as follows. As stated, the products 12 are fed one after another onto the first conveyor belt 13 by way of an interposed continuous plastics film which is arranged to wrap them completely by known means, not shown. In this respect, usually the longitudinal edges of the film are superposed onto each other to define almost a continuous tube of plastics material containing the products spaced apart. The conveyor belt 13 then feeds the products wrapped in this manner to the transverse welding apparatus 14, which transversely welds the leading and trailing ends of each product to form the package. In the packaging machine provided with an apparatus according to the present invention, the products wrapped in this manner upstream of the transverse welding apparatus pass between the first conveyor belt 13 and the pressing belt 28, which maintains the film clamped onto the product. The products are then fed between the welding bars 16 and 17 and inserted between the second conveyor belt 18 and the second pressing belt 46. During this stage, as shown in Figure 2, the speed variation linkage about which the second transmission portion 30 driving the second conveyor belt 18 passes acts to effect a pre-stretching of the film in the zone between the two pressing belts 28 and 46. In this respect, the slide 36 operated by the lever 37 moves upwards so that the speed of the second conveyor belt 18 is higher than that of the first conveyor belt 13. As both the pressing rollers 28 and 46 are connected by rigid transmissions 26 and 43 to the relative conveyor belts 13 and 18, these also move at different speeds, so that during their movement they clamp between them the film portions wrapped about the products to consequently pre-stretch the film within said zone. During this stage the carriage 15 withdraws from the pressing belt 28. Then the eccentric element 38, continuing to rotate (Figure 3), reverses the vertical movement of the slide 36, which descends to reduce the speed of rotation of the second conveyor belt 18 with respect to the first conveyor belt 13. During this stage the control unit 22 causes the welding bars 16 and 17 to approach the continuous film 54 in the zone between the two pressing belts 28 and 46. In addition the roller 52, supported by the upper welding bar 16, moves downwards to rest on the next product 12 wrapped in film, so as to secure the film to the product on the upstream side of the roller 52 and prevent it from stretching. During this stage the film tends to slacken only within the welding zone between the pressing rollers 28 and 46, and the carriage 15 approaches the pressing belt 28. As can be seen in Figure 4, the eccentric element 38 in continuing to rotate causes the slide 36 to descend further, and further reduce the speed of advancement of the conveyor belt 18, so that the film zone between the two pressing belts further reduces. When welding has taken place, once the welding bars disengage from the film, the leading and trailing welded film portions on finally being released arrange themselves so that they completely and correctly wrap on one side the trailing end of the completed package, and on the other side the leading end of the package still to be completed. When this cycle is complete the entire operation of pre-stretching and subsequent welding of the plastics film is repeated, to complete the commenced package by welding its trailing end and forming a new leading end weld on the next product, and so on continuously. With an apparatus of the present invention it is no longer necessary to feed the products spaced apart from each other in order to form the leading and trailing end welds on the package. In this respect, said spacing is achieved by simply using the elasticity of the plastics material, with a consequent material saving. A further advantage is that in the packages wrapped in the plastics film, the film perfectly adheres to the product, with the result that these are suitable for easy and direct handling by further machines. A further advantage is that the usual subsequent heat-shrinkage treatment required to make the film of heat-shrinkable material adhere to the product is no longer necessary. This dispenses with further machinery such as heat-shrinking ovens, resulting in a saving of investment, production and maintenance costs. The provision of a vertically movable block 48 carrying the second pressing belt means that this latter can be disengaged from the product-containing film for maintenance of the apparatus, or during operation if the packaging production cycle is operating incorrectly.

Claims: A packaging machine (11), in which products (12) are fed sequentially, are wrapped in a continuous tube of a plastic film, and are sealed within said continuous tube, said machine comprising in sequence, a first conveyor belt (13), a transverse welding apparatus (14), and a second conveyor belt (18), all driven by a central drive unit, said transverse welding apparatus (14) comprising a carriage (15) which traverses horizontally with respect to the frame of the packaging machine (11), said carriage supporting a pair of superposed welding bars (16,17), an upper (16) and a lower (17) bar, said bars being movable vertically between a position in which they are separated from the continuous tube containing the products (12) and a position in which they engage said tube to perform the transverse welding operation, there being further provided a first pressing belt (46) upperly facing said second conveyor belt (18), characterised in that said first and second conveyor belts (13,18) are driven by a speed variator (21) via a pair of transmissions (25; 29,30), a first transmission (25) directly driving an end roller (23) of said first conveyor belt (13) and the second transmission (29,30) driving an end roller (42) of said second conveyor belt (18) by way of a linkage (35,36,37,38,40), thus providing a speed variation with respect to the speed of said first conveyor belt (13), there being also provided upstream of said transverse welding apparatus (14), both above said first conveyor belt (13) and operationally linked to the vertical movement of said upper welding bar (16), means (51,52) for clamping said film onto a product (12) wrapped in it in order to perform an axial stretching operation of said tube, wherein said speed variation linkage comprises a pair of idle pinions (35) supported on a slide (36) which is movable vertically with reciprocating movement and is operated by a lever (37) controlled by an eccentric element (38) connected to a central drive (19,20) of said packaging machine (11), and said second transmission comprises a first (25) and a second portion (30) arranged in succession, said second portion (30) extending through a path consisting of two adjacent loop portions joined together at a central zone defined by said slide (36), the connection point between said two loop portions also being vertically movable, there being further provided a control unit (22) causing said welding bar (16,17) to approach said continuous film in a zone between said first pressing belt (46) and a second pressing belt (28) facing said first conveyor belt (13). The machine as defined in claim 1, characterised in that said clamping means (51,52) are rigidly connected to said upper welding bar (16), and comprises a pair of brackets (51) supporting a roller (52) via vertically mounted lateral rods (57) with vertical damper elements (53) interposed, said roller (52) being rotatable only in the product advancement direction. The machine of claim 1, characterised in that said first pressing belt (46) upperly facing said second conveyor belt (18) is supported on a block (48) connected to the end of the rod (49) of a cylinder (50) which is itself connected to said carriage (15).

Inventors: BALLESTRAZZI ARIS, TASSI LAMBERTO, BALLESTRAZZI, ARIS, TASSI, LAMBERTO
Applicants: SITMA SPA, SITMA S.P.A.
Application year: 1992
Main classification: B65B9
IPC codes: B65B57, B65B9, B65B51