Method and unit for heating tear-off zones of a strip of packaging material for producing sealed packages of pourable food products

Abstract: A method of heating tear-off zones (2) of a strip (3) of packaging material, the method including the step of successively heating the tear-off zones (2) by means of a heating element (15) located along a path (A) of the strip (3); the heating step being performed as the strip (3) is advanced, and including the step of activating the heating element (15) for successive time intervals, each of a duration (T) selectable as a function of the travelling speed of the strip (3), so as to enable the tear-off zones (2) to be heated completely regardless of the size of the tear-off zones (2) in the direction of the path (A).

Description: The present invention relates to a method and unit for heating tear-off zones of a strip of packaging material for producing sealed packages of pourable food products. As is known, many pourable food products, such as fruit juice, UHT (ultra-high-temperature processed) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material. A typical example of such a package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is formed by folding and sealing laminated strip packaging material. The packaging material has a multilayer structure comprising a layer of fibrous material, e.g. paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene, and, in the case of aseptic packages for long-storage products, such as UHT milk, also comprises a layer of oxygen-barrier material defined, for example, by an aluminium film, which is superimposed on a layer of heat-seal plastic material and is in turn covered with another layer of heat-seal plastic material eventually defining the inner face of the package contacting the food product. As is known, such packages are made on fully automatic packaging machines, on which a continuous tube is formed from the packaging material supplied in strip form; the strip of packaging material is sterilized on the packaging machine itself, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, after sterilization, is removed, e.g. vaporized by heating, from the surfaces of the packaging material; and the strip of packaging material so sterilized is maintained in a closed sterile environment, and is folded and sealed longitudinally to form a vertical tube. The tube is filled with the sterilized or sterile-processed food product, and is sealed and cut at equally spaced cross sections to form pillow packs, which are then folded mechanically to form the finished, e.g. substantially parallelepiped-shaped, packages. Alternatively, the packaging material may be cut into blanks, which are formed into packages on forming spindles, and the resulting packages are filled with the food product and sealed. One example of such a package is the so-called "gable-top" package commonly known by the trade name Tetra Rex (registered trademark). To open such packages, various solutions have been proposed, one of which consists in forming, at a corner portion of a flap on the package, a tear-off zone normally defined by a preferential tear line defined by a succession of perforations extending through the outer layers of the packaging material down to the layer of barrier material; and the package is opened by lifting the flap and tearing or cutting off the corner portion along the perforations. Packages of this type, however, cannot, obviously, be closed once opened, and must therefore be handled with care to prevent spillage of the remaining food product inside the package. By way of a solution to the problem, packages of the type described above have been provided with closable opening devices, which substantially comprise a frame defining an opening and fitted over a tear-off zone, e.g. a pierceable or pull-off portion, in a wall of the package; and a cap hinged to the frame. The cap is normally molded integrally with the frame, and is initially sealed to the frame, along a peripheral edge surrounding the opening, by a thin breakable annular connecting portion. Once unsealed, the cap is movable between a closed position cooperating in fluidtight manner with the frame, and an open position. Alternatively, threaded caps are also used, which are separate from and initially screwed to the frame. The tear-off zones for receiving the opening devices of the packages may be formed, for example, by forming a series of through holes in the fibrous layer of the strip packaging material before the fibrous layer is laminated with the thermoplastic and barrier layers, which, at the end of the laminating process, therefore cover the holes. The tear-off zone can be detached from the rest of the package material in various ways, e.g. by means of a cutting member activated by unscrewing the cap, or, if the cap is fixed directly to a respective tear-off zone of the package, by simply detaching the cap from the frame. Whichever the case, to enable the packages to be opened easily, the tear-off zones are heated locally before the strip packaging material is folded longitudinally into a vertical tube. The heating operation is performed using a heating element, e.g. an inductor, located along the path of the strip packaging material and interacting with the strip at the tear-off zones. The heating operation provides for local decrystallization or melting of the polymers forming the zone of packaging material affected, and which subsequently resolidify in amorphous form, so that the locally heated zone of the packaging material remains as aseptic and as compatible with the food products as before, while at the same time being more fragile and, therefore, easier to tear. The heating operation is performed by arresting the strip of packaging material, as each tear-off zone moves past the heating element, long enough for the polymers forming the tear-off zone to decrystallize, so that the feed rate of the strip depends on the decrystallization time, thus impairing the output rate of the packaging machine. Moreover, specially designed heating elements must be used, depending on the shape and/or size of the tear-off zone for heating, thus reducing the versatility of the heating elements. US 3,909,582 discloses a method of forming a line of weakness in at least one but not all layers of a multilayer laminate, as defined in the preamble of claim 1. WO 98/14317 discloses a method of and an apparatus for treating a laminate, wherein, by way of a groove provided through a covering of the laminate, a limited zone of a polymer layer substantially aligned with the groove is heated to become more readily breakable. US 5,470,300 refers to a system and method in the manufacture of plastic bags for accurately registering a web having graphic material printed repetitively thereon with respect to a transverse cutting and sealing device. US 4,265,616 refers to a process for forming a continuous longitudinal heat-weakened strip in a cross-lapped oriented polyolefin film. US 5,000,321 discloses different opening aids for a package made of multilayer compound foil material, including a notch or groove formed by a laser beam in order to define a more readily breakable zone. It is an object of the present invention to provide a method of heating tear-off zones of a strip of packaging material for producing sealed packages of pourable food products, designed to eliminate the aforementioned drawbacks. According to the present invention, there is provided a method of heating tear-off zones of a strip of packaging material for producing sealed packages of pourable food products, as claimed in claim 1. The present invention also relates to a unit for heating tear-off zones of a strip of packaging material for producing sealed packages of pourable food products, as claimed in claim 6. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows, schematically, a unit in accordance with the present invention, for heating tear-off zones of a strip of packaging material for producing sealed packages of pourable food products; Figure 2 shows a top plan view of a portion of the strip of packaging material featuring one of said tear-off zones; Figure 3 shows a larger-scale cross section, indicating the component layers, of the Figure 2 strip portion; Figure 4 shows a larger-scale view in perspective of an induction heating element of the Figure 1 unit. Number 1 in Figure 1 indicates as a whole a unit in accordance with the present invention for heating tear-off zones 2 of a strip 3 of packaging material for producing known aseptic sealed packages (not shown) of pourable food products, such as pasteurized or UHT milk, fruit juice, wine, etc. Strip 3 is fed along a path A and has, on one face, a number of optical reference codes C (the borders between different coloured areas of which are indicated schematically by dash lines in Figure 2) defined, in the example shown, by respective bar codes equally spaced along strip 3, in the direction of path A, with the same spacing as that of tear-off zones 2. More specifically, each reference code C has a predetermined relationship with the respective tear-off zone 2. That is, a line C1 of each reference code C identifies, with reference to path A, the downstream end of the respective tear-off zone 2. Unit 1 may be incorporated in a packaging machine (not shown) for continuously producing the aforementioned packages from strip 3 of packaging material. More specifically, strip 3 is folded and sealed longitudinally in known manner to form a vertical tube, which is filled with the sterilized or sterile-processed food product for packaging, is sealed along equally spaced cross sections, and is subjected to successive mechanical folding operations to form the finished packages. Unit 1 is located along path A of strip 3, upstream from a station for forming said vertical tube. With particular reference to Figures 2 and 3, strip 3 has a multilayer structure, and comprises a main sheet 4 having a number of through openings 5 - circular in the example shown - and defined by a layer of fibrous material 6, e.g. paper, covered, on the side eventually forming the outer face of the package, with a film 7 of heat-seal plastic material - in the example shown, polyethylene. Main sheet 4 is also covered, on the side eventually forming the inner face of the package, with a laminated sheet 8 covering openings 5 and defining, at openings 5, said tear-off zones 2. Laminated sheet 8 comprises a layer 9 of electrically conductive barrier material defined, for example, by an aluminium film, and which is covered on both sides with respective layers 10, 11 of heat-seal plastic material, e.g. polyethylene. In the example shown, layer 11 is covered, on the opposite side to that contacting layer 9 of barrier material, with a further layer 12 of heat-seal plastic material, normally polyethylene. Zones 2 may be covered externally with closable opening devices (not shown) substantially comprising a frame applied to the packaging material about a respective opening 5 in main sheet 4, and a cap hinged or screwed to the frame, and which permits withdrawal of the food product from the package and safeguards the food product from contact with external agents once the package has been opened by tearing respective zone 2. Tear-off zones 2 may also be defined by tear-off zones of strip 3 defining, for example, corner portions of respective opening flaps of the finished packages. An important aspect of the present invention (Figure 1) lies in unit 1 comprising an induction heating element 15 located along path A and activated selectively to locally heat zones 2 of strip 3 by inducing electrical current in barrier material layer 9 of zones 2; and an activating device 16 for activating the heating element as strip 3 advances and for successive time intervals, each of a duration T selected as a function of the traveling speed of strip 3, so as to enable complete heating of tear-off zones 2, regardless of the size of the tear-off zones in the direction of path A. With reference to Figures 1 and 4, heating element 15 substantially comprises a substantially parallelepiped-shaped body 17 made of electrically insulating material, small in height as compared with the other dimensions, and positioned with a base face 18 parallel to strip 3; and an inductor 19 carried by body 17 and defined, in the example shown, by a conductive copper plate having a rectangular-strip-shaped operating portion 20 housed in a complementary groove 21 formed in face 18 of body 17. Assuming strip 3 were stationary, inductor 19 is designed to heat an area E1 of strip 3 having the same profile as operating portion 20. Inductor 19 also comprises two terminals 22 projecting outwards from body 17 and connected to a highfrequency electrical current source 23 of activating device 16. To prevent heating element 15 from overheating, body 17 is fitted through with two cooling conduits 24 supplied with cooling fluid. As shown in Figure 1, unit 1 also comprises a guide-slide assembly 25 for adjusting the position of heating element 15 with respect to strip 3 in two directions B, D perpendicular to each other and parallel to the portion of strip 3 travelling past heating element 15. More specifically, direction B is parallel to the traveling direction of strip 3 close to heating element 15. Assembly 25 comprises a first supporting member 26 mounted to slide in direction D along two fixed guides 27; and a second supporting member 28 connected integrally to body 17 of heating element 15 and mounted to slide in direction B along a guide 29 formed on supporting member 26 on the opposite side to guide 27. With reference to Figure 1, activating device 16 comprises a position sensor 30, e.g. a photocell, located upstream from heating element 15 along path A, positioned facing strip 3, and generating a pulse presence signal S1 indicating the passage of a reference code C of strip 3 past sensor 30; and a control device 31, which receives presence signal S1 and supplies source 23 with a pulse control signal S2, the length of the pulses of which is related to the desired duration T of the time intervals in which heating element 15 is activated. More specifically, presence signal S1 assumes a high logic level during the time period in which a reference code C of strip 3 travels past sensor 30, and a low logic level in any other condition. On receiving a presence signal S1 pulse, control device 31 generates in control signal S2 a corresponding pulse with a delay R related to the speed of strip 3 and to the distance, along path A, between sensor 30 and heating element 15, so that, for each activation of heating element 15, strip 3 is heated locally at the tear-off zone 2 associated with the detected reference code C. The amplitude (and therefore power), length and delay of the control signal S2 pulses are adjustable manually by means of respective adjusting elements 32, 33, 34 shown schematically in Figure 1. Source 23 receives control signal S2 and generates a current i, which is supplied to inductor 19 of heating element 15 via a known impedance-matching transformer 35. The length of time current i is supplied to inductor 19, and therefore the desired duration T of the time intervals in which heating element 15 is activated, is related to the length of the control signal S2 pulses, whereas the amplitude, and therefore the power, of current i is related to the amplitude of the control signal S2 pulses. More specifically, the amplitude of current i is such as to induce a current in barrier material layer 9 of the portion of strip 3 facing the operating portion 20 of inductor 19 when control signal S2 assumes a high logic level, and is zero when control signal S2 assumes a low logic level. In actual use, as strip 3 travels along path A, the passage of a reference code C past sensor 30 switches presence signal S1 from a low to a high logic level. In response, control device 31 generates a corresponding pulse in control signal S2, which, in turn, supplies current i to inductor 19 of heating element 15. The pulse in control signal S2 is generated by control device 31 with such a delay R with respect to the switch in presence signal S1 that inductor 19 is supplied as the zone 2 of strip 3 associated with the reference code C detected by sensor 30 travels past heating element 15. Upon inductor 19 being supplied with current i, an electrical current is induced in barrier material layer 9 of the zone 2 of strip 3 facing inductor 19, so that laminated sheet 8 covering a corresponding opening 5 in main sheet 4 is heated locally. More specifically, on account of the geometry of operating portion 20 and thanks to the travelling of strip 3, inductor 19 heats a rectangular area E2 of strip 3 containing a respective tear-off zone 2 and greater than area E1, which inductor 19 would heat in the case of strip 3 was arrested. The size of area E2 in a direction perpendicular to the travelling direction of strip 3, i.e. in direction D, is determined by the longitudinal extension of strip-shaped operating portion 20 of inductor 19, and, in the example shown, is greater than the diameter of openings 5 in main sheet 4 of strip 3; whereas, on the basis of what was stated above, the size of area E2 in the travelling direction of strip 3 depends on the speed of strip 3 and on the duration T of the time intervals in which heating element 15 is activated, i.e. on the set pulse length of control signal S2 generated by control device 31. The heating operation produces local decrystallization or melting of the polymers forming the affected area E2 of strip 3, and which subsequently resolidify in amorphous form, so that the locally heated zone of the packaging material remains as aseptic and as compatible with the food products as before, while at the same time being more fragile and, therefore, easier to tear. The advantages of unit 1 and the method according to the present invention will be clear from the foregoing description. In particular, by performing the heating operation as strip 3 advances, and by varying the length of the control signal S2 pulses, i.e. the duration T of the time intervals in which heating element 15 is activated, it is possible, using an inductor (19) of a predetermined configuration, to heat tear-off zones 2 of strip 3 of different sizes in the travelling direction of strip 3, thus greatly increasing the versatility of the inductors. Moreover, performing the heating operation as strip 3 advances, strip 3 may be fed continuously or in steps, at a rate independent of the time taken to perform the heating operation. Clearly, changes may be made to unit 1 and to the method as described and illustrated herein without, however, departing from the scope of the accompanying Claims. In particular, each reference code C may be associated with two or more tear-off zones 2 of strip 3 aligned in a direction perpendicular to the travelling direction of strip 3.

Claims: A method of heating tear-off zones (2) of a strip (3) of packaging material for producing sealed packages of pourable food products and fed along a given path (A); said tear-off zones (2) of said strip (3) eventually being at least partly detached from adjacent portions of the packaging material to define respective pour openings of said packages; said method comprising the step of successively heating said tear-off zones (2) by means of a heating element (15) located along said path (A); said heating step being performed as said strip (3) is advanced, and comprising the step of activating said heating element (15) for successive time intervals, each of a duration (T) selectable as a function of the travelling speed of the strip (3), so as to enable said tear-off zones (2) to be heated completely regardless of the size of the tear-off zones in the direction of said path (A); the method being characterized by comprising the step of identifying the tear-off zone (2) for heating at a detecting station (30) located upstream from said heating element (15) along said path (A); said step of activating said heating element (15) being performed after said identifying step and with a predetermined delay (R) related to the travelling speed of said strip (3) and to the distance, along said path (A), between said detecting station (30) and said heating element (15), so that, for each activation of the heating element (15), said strip (3) is heated locally at the identified said tear-off zone (2). A method as claimed in Claim 1, characterized in that said strip (3) comprises a number of reference codes (C), each associated with at least one respective said tear-off zone (2); and in that said identifying step comprises the step of detecting the passage of a said reference code (C) of said strip (3) past said detecting station (30). A method as claimed in Claim 2, characterized by comprising the step of adjusting the length of said delay (R) between said step of detecting said reference codes (C) and said step of activating said heating element (15) as a function of the travelling speed of said strip (3) and of the distance, along said path (A), between said detecting station (30) and said heating element (15). A method as claimed in any one of the foregoing Claims, characterized by comprising the step of regulating the duration (T) of said time intervals in which said heating element (15) is activated as a function of the travelling speed of said strip (3) and of the geometry of said tear-off zones (2) for heating. A method as claimed in any one of the foregoing Claims, characterized in that said tear-off zones (2) of said strip (3) comprise at least one layer (9) of electrically conductive material; and in that said step of activating said heating element (15) is performed by inducing an electrical current in said layer (9) of electrically conductive material of said tear-off zone (2) located at the heating element (15). A unit (1) for heating tear-off zones (2) of a strip (3) of packaging material for producing sealed packages of pourable food products and fed along a given path (A); said tear-off zones (2) of said strip (3) eventually being at least partly detached from adjacent portions of the packaging material to define respective pour openings of said packages; said unit (1) comprising at least one heating element (15) located along said path (A) and activated selectively to successively heat said tear-off zones (2), and activating means (16) for activating said heating element (15) as said strip (3) is advanced, and for successive time intervals, each of a duration (T) selectable as a function of the travelling speed of the strip (3), so as to enable said tear-off zones (2) to be heated completely regardless of the size of the tear-off zones (2) in the direction of said path (A); characterized by comprising identifying means (30) located upstream from said heating element (15) along said path (A) to successively identify said tear-off zones (2) for heating; said activating means (16) comprising drive means (23, 31) for activating said heating element (15), following identification of said tear-off zones (2) for heating, with a predetermined delay (R) related to the travelling speed of said strip (3) and to the distance, along said path (A), between said identifying means (30) and said heating element (15). A unit as claimed in Claim 6, characterized in that said strip (3) comprises a number of reference codes (C), each associated with at least one respective said tear-off zone (2); and in that said identifying means comprise sensor means (30) generating a presence signal (S1) indicating the passage of a said reference code (C) of said strip (3) past the sensor means (30). A unit as claimed in Claim 7, characterized in that said drive means (23, 31) comprise control means (31) receiving said presence signal (S1) and generating a control signal (S2) to activate said heating element (15). A unit as claimed in Claim 8, characterized in that said presence signal and said control signal (S1, S2) are pulse signals; and in that the pulses of said control signal (S2) are generated with said delay (R) with respect to the corresponding pulses of said presence signal (S1). A unit as claimed in Claim 9, characterized in that the pulses of said control signal (S2) are of a length related to the desired duration (T) of said time intervals in which said heating element is activated. A unit as claimed in any one of Claims 6 to 10, characterized in that said tear-off zones (2) of said strip (3) comprise at least one layer (9) of electrically conductive material; and in that said heating element (15) comprises an inductor (19) facing said strip (3) to induce a current in said layer (9) of electrically conductive material of said tear-off zone (2) located at the heating element (15). A unit as claimed in Claim 11, characterized in that said drive means (23, 31) comprise a current source (23) receiving said control signal (S2) and supplying said inductor (19) with a current related to the control signal (S2). A unit as claimed in any one of Claims 9 to 12, characterized in that said control means (31) comprise first adjusting means (33) for adjusting the length of said pulses of said control signal (S2) as a function of the travelling speed of said strip (3) and of the geometry of said tear-off zones (2) for heating. A unit as claimed in any one of Claims 9 to 13, characterized in that said control means (31) comprise second adjusting means (34) for adjusting the length of said delay (R) between corresponding pulses of said presence signal and said control signal (S1, S2) as a function of the travelling speed of said strip (3) and of the distance, along said path (A), between said sensor means (30) and said heating element (15).

Inventors: JOENSSON ROGER, LARSSON BERNT, JOENSSON, ROGER, LARSSON, BERNT, Jönsson, Roger
Applicants: TETRA LAVAL HOLDINGS & FINANCE, TETRA LAVAL HOLDINGS & FINANCE SA
Application year: 2000
Main classification: B29C59
IPC codes: B29C71, B65B61, B29C35, B29C59, B65H37