Unit for heat sealing a sealing strip to a strip of packaging material for pourable food products

Abstract: A heat-sealing unit (1) wherein a strip (2) of packaging material for pourable food products and a sealing strip (3) are fed respectively along a first and a second path (A, B) having at least one common portion (C) along which the sealing strip (3) is heat sealed under pressure to a longitudinal edge of the strip (2); the unit (1) having a first and a second nozzle (13, 14) located upstream from the common portion (C) along the first and second path (A, B) respectively, and which respectively provide for preheating the strip (2) and the sealing strip (3).

Description: The present invention relates to a unit for heat sealing a sealing strip to a strip of packaging material for pourable food products. Many pourable food products, such as soft drinks, fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material. Typical examples of such a packages are the parallelepiped packaging containers for liquid or pourable food products known by the name of Tetra Brik® and Tetra Brik Aseptic (registered trademark), which is formed by folding and sealing a strip-rolled packaging material. The packaging material comprises layers of fibrous material, e.g. paper, covered on both sides with thermoplastic material, e.g. polyethylene. This material is used for manufacturing non-aseptic packages containing pasteurized milk, pasteurized fruit juice, etc.. In aseptic packages, the side of the packaging material eventually contacting the food product in the package - hereinafter referred to simply as the inner plastic layer - also has a sheet of barrier material, e.g. aluminium, which is also coated with thermoplastic material. As is known, such packages are produced on fully automatic packaging machines, on which a continuous tube is formed from the strip-fed packaging material; the strip of packaging material for manufacturing aseptic packages is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution; following sterilization, the sterilizing agent is removed, e.g. vaporized by heating, from the surfaces of the packaging material; and the strip of packaging material so sterilized is kept in a closed sterile environment, and is folded and sealed longitudinally to form a continuous, vertically-fed tube which is subsequently filled with the food product. Beforehand, a longitudinal edge of the inner plastic layer of the strip of packaging material is applied with a sealing strip of thermoplastic material, to which the packaging material is subsequently heat sealed to form the vertical tube. The tube is then filled with the sterilized or sterile-processed food product, and is sealed and cut at equally spaced cross sections into pillow-pack packages, which are subsequently folded mechanically to form parallelepiped packages. Packaging machines of the above type are known in which the sealing strip is normally applied to the inner plastic layer of the strip of packaging material at a pressing station to which the strip of packaging material, heated beforehand, and the sealing strip are fed along different paths, and where the sealing strip is pressed on to a longitudinal edge of the strip of packaging material. The strip of packaging material is normally preheated by a hot-air nozzle located upstream from the pressing station and having a row of air supply orifices facing the strip of packaging material. The pressing station normally comprises a pair of rollers, between which the preheated strip of packaging material and the sealing strip are rolled simultaneously. One such pressure station is known from EP-A-703063. As it is fed between the rollers, the strip of packaging material transfers part of its own heat to, and so softens, the sealing strip, so that the pressure exerted by the rollers seals the sealing strip to the strip of packaging material. Knonw from EP-A-816052 is a heat-sealing unit as defined in the preamble of claim 1. The inner plastic layer of the packaging material to which the sealing strip is sealed must be of at least a given minimum thickness, to prevent the layer from overheating and so resulting in leakage of the product at the sealing strip. It is an object of the present invention to provide a unit for heat sealing a sealing strip to a strip of packaging material for pourable food products, which provides, in a straightforward, low-cost manner, for employing packaging materials with an inner plastic layer of less than said minimum thickness. According to the present invention, there is provided a heat-sealing unit as defined in claim 1. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawing, which shows a section of a heat-sealing unit in accordance with the present invention. Number 1 in the accompanying drawing indicates as a whole a unit for heat sealing a sealing strip 3 to a strip 2 of packaging material for pourable food products. Unit 1 may be incorporated in a packaging machine (not shown) for continuously forming, from a tube of said packaging material, aseptic sealed packages (not shown) containing a pourable food product, such as pasteurized or UHT milk, fruit juice, wine, etc. More specifically, the tube is formed in known manner downstream from unit 1 by folding and sealing strip 2 of packaging material along sealing strip 3; is filled with the sterilized or sterile-processed food product for packaging; and is then sealed and cut into packages along equally spaced cross sections. The packaging material comprises layers of fibrous material, e.g. paper, covered on both sides with thermoplastic material, e.g. polyethylene; and the side of the packaging material eventually contacting the contents of the package - hereinafter referred to simply as the inner plastic layer - also has a sheet of barrier material, e.g. aluminium, which is also coated with thermoplastic material. Sealing strip 3, on the other hand, is made of thermoplastic material. By means of known devices (not shown), strip 2 and sealing strip 3 are fed respectively along a first and a second path A, B having a common portion C along which sealing strip 3 is heat sealed under pressure to a longitudinal edge of the inner plastic layer of strip 2. More specifically, whereas path A is a straight horizontal path, path B comprises, in addition to portion C in common with path A, a straight upstream portion B 1 parallel to path A, and an oblique portion B 2 connecting portion B 1 to portion C. Unit 1 comprises a pressing station 4 for pressing sealing strip 3 on to strip 2, and defining the common portion C of paths A and B; and a heat-treating device 5 for preheating strip 2 and sealing strip 3 along respective paths A and B upstream from pressing station 4. Pressing station 4 comprises a pair of pressure rollers 7, 8 having respective parallel axes, rotating in opposite directions, and located adjacent to each other to define a small-section passage 9 through which strip 2 and sealing strip 3 are rolled simultaneously. More specifically, in the illustrated example, one (7) of said pressure rollers is much larger in diameter than the other (8) though it should be noted that rollers having the same diameter may also be used. In the example shown, sealing strip 3 extends about the larger-diameter pressure roller 7, and about a guide roller 10 having an axis parallel to the axes of rollers 7, 8, and located at the junction of portions B 1 and B 2 of path B. Heat-treating device 5 comprises a first and a second nozzle 13, 14 located upstream from portion C along respective paths A and B, and for respectively directing hot air on to strip 2 and sealing strip 3 to preheat both prior to rolling at pressure rollers 7, 8. More specifically, nozzle 14 is branch-connected to nozzle 13. Nozzle 13 is formed on an end portion of a hot-air supply conduit 15, and comprises, facing strip 2, a substantially flat bottom wall 16 parallel to strip 2 and in turn comprising a number of through hot-air supply orifices 18. Facing pressure roller 7, nozzle 13 is also defined by a substantially flat lateral wall 19 sloping with respect to wall 16, defining a connection for nozzle 14, and having a through hole 20 enabling air flow from nozzle 13 to nozzle 14. Nozzle 14 comprises, integrally, a box portion 21 located along portion B 2 of path B, between guide roller 10 and pressure roller 7, and facing a respective portion of sealing strip 3; and a tubular end portion 22 extending from box portion 21 towards nozzle 13, and connected integrally to wall 19 by a fastening member 23. More specifically, fastening member 23 comprises a tubular main portion 24 to which tubular portion 22 of nozzle 14 is interference fitted coaxially; and a peripheral end flange 25 fitted to wall 19 by means of a number of screws 26. Box portion 21 of nozzle 14 is substantially parallelepiped, and comprises a lateral wall 27 closed on opposite sides by two base walls 28, 29, a first (28) of which is parallel to and faces sealing strip 3, and has a row of through air supply orifices 30, and the second (29) of which has a through hole 31 from the peripheral edge of which tubular portion 22 extends integrally. In the event the packaging machine is arrested, heat-treating device 5 is moved, by known actuating means not shown, into a rest position in which nozzles 13, 14 are located to the side of and clear of strip 2 and sealing strip 3. Unit 1 also comprises a further nozzle 32 for directing cold air between wall 28 of nozzle 14 and sealing strip 3, and which is activated during startup supply of sealing strip 3, until the steady operating speed is reached, to prevent sealing strip 3 from being damaged by overheating. The advantages of unit 1 according to the present invention will be clear from the foregoing description. In particular, by preheating sealing strip 3, strip 2 of packaging material need no longer transfer such a large amount of heat to sealing strip 3, and may therefore be heated to a temperature below the preheat temperature required of known heat-sealing units. This therefore enables the use of packaging materials with an inner plastic layer thinner than that of normally used packaging materials, and, in view of the large number of packages (several billion) produced yearly, provides for a significant reduction in the consumption of plastic material, and for advantages in terms of both cost and environment. Clearly, changes may be made to unit 1 as described and illustrated herein without, however, departing from the scope of the accompanying Claims.

Claims: A heat-sealing unit (1) for an automatic packaging machine wherein a strip (2) of packaging material for pourable food products and a sealing strip (3) are fed respectively along a first and a second path (A, B) having at least one common portion (C) along which said sealing strip (3) is heat sealed under pressure to a longitudinal edge of said strip (2) of packaging material; said unit (1) comprising first heating means located along said first path (A), upstream from said common portion (C), to preheat said strip (2) of packaging material, and second heating means located along said second path (B), upstream from said common portion (C), characterized in that said second heating means are arranged to preheat said sealing strip (3), in that said first heating means and said second heating means are connected in series with each other, and in that it further comprises a nozzle (32) for directing cold air between said second heating means (14) and said sealing strip (3), said nozzle (32) being selectively activated during startup supply of said sealing strip (3) along said second path (B). A unit as claimed in Claim 1, characterized in that said first heating means comprise a first nozzle (13) located at an end portion of an hot air supply conduit (15), and in that said second heating means comprise a second nozzle (14) branch-connected to said first nozzle (13). A unit as claimed in Claim 2, characterized in that said first nozzle (13) comprises a bottom wall (16) provided with hot air supply orifices (18) facing said strip (2) of packaging material, said second nozzle (14) comprising a first portion (21) having air supply orifices (30) facing said sealing strip (3) and a second portion (22) extending from said first portion (21) and branch-connected to said first nozzle (13).

Inventors: ROSEBERG BENGT, ROSEBERG, BENGT
Applicants: TETRA LAVAL HOLDINGS & FINANCE, TETRA LAVAL HOLDINGS & FINANCE SA
Application year: 1998
Main classification: B29C65
IPC codes: B29C65