Abstract:
An apparatus for sealing a thermoplastic-coated packaging material, the material having at least one layer of a metal foil, includes an inductor having at least three substantially parallel, spaced apart conductors. A method for forming sealed, filled packages and a method for sealing packaging material are also provided.

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for sealing a thermoplastic-coated material, with at least one layer of a metal foil. 
     BACKGROUND ART 
     It has long been known in the packaging industry to employ packaging materials comprising outer layers of thermoplastic, these outer layers being capable of being sealed together by fusion after heating and compression for providing tight and durable sealing joints or seams. For example, such a material may be fed in web form through a filling machine and formed into a tube which is longitudinally sealed by means of a lap joint. This material tube is filled with its contemplated contents, whereafter the tube is transversely sealed and severed to form individual packages which, in a later stage, are finally formed, for example into parallelepipedic packages or cartons. 
     A packaging material including a metal foil, preferably aluminium foil, is ideally employed when the transverse seals are made by so-called induction sealing. An apparatus for such induction sealing is disclosed and described in Swedish Patent Specification SE 451 973. In this instance, an induction current is induced in the aluminium layer of the packaging material, this current heating the thermoplastic layer disposed adjacent the aluminium layer so that the thermoplastic layer can be sealed to a corresponding thermoplastic layer. 
     The above-described method of transversely sealing packaging blanks as described in the above-disclosed patent specification has, however, proved to be unsuitable for highly viscous products and products containing particles such as kernels, fruit seeds or fibres. According to the above-described method, two substantially parallel transverse seals are made between which the packaging tube is severed to form individual packaging blanks. In this instance, sealing flaps or fins are created which lie outside the transverse seals and where product residues may accumulate. These product residues can, on storage of the package, take on a highly disagreeable and unhealthy appearance. 
     OBJECT OF THE INVENTION 
     One object of the present invention is to provide an apparatus for induction sealing which is suitable for use together with products which are highly viscous or which contain particles such as kernels, fruit seeds or fibres and in which the transverse seal of the package is made such that no sealing flaps or fins are created where product residues may gather. 
     SOLUTION 
     This and other objects have been attained according to the present invention in that the apparatus of the type described by way of introduction includes three conductors, these being disposed such that the current density in the conductors is approximately equal. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING 
     One preferred embodiment of the present invention will now be described in greater detail hereinbelow, with particular reference to the accompanying Drawing, in which: 
     FIG. 1 is a cross sectional through an inductor according to an embodiment of the present invention; and 
     FIG. 2 is a partially cross-sectional view of an apparatus according to an embodiment of the present invention, showing how packaging material is sealed using an inductor according to the present invention. 
    
    
     The Drawings show only those details essential to an understanding of the present invention, the arrangement of the inductor in a filling machine (which is well-known to a person skilled in the art) having been omitted. 
     DESCRIPTION OF PREFERRED EMBODIMENT 
     An inductor 1 according to the invention is shown in FIG. 1. The inductor 1 displays three substantially parallel conductors 2, 3 which are disposed in an electrically insulating carrier layer 4 which, in its interior, houses a core 5 of a ferrite material. The core 5 of ferrite material is insulated from the three conductors 2, 3. Both of the outer conductors 2 display a gently rounded cross section or alternatively a square or rectangular cross section. Both of the outer conductors 2 accommodate cooling ducts 6 in their interior, these being intended for coolant. The central conductor 3 is of substantially U-shaped appearance. Each conductor displays, on its upper surface 7, a bead or ridge 14 which projects above the surrounding surface 7 a distance of approx. 0.3 mm. 
     The three conductors 2 and 3 are preferably connected in parallel to a high frequency alternating current source (not shown). In such instance, a considerably higher current force will be obtained in the central conductor 3, but because of the configuration of the central conductor 3 (with the upwardly open U), an inductor 1 will be obtained in which conductors 2, 3 have approximately the same current density on the upper surface 7. 
     An inductor 1 according to the present invention may also be connected in series or inductively by means of a capacitor. 
     The inductor 1 is reinforced by a steel body 8 and the entire inductor 1 is disposed in a sealing jaw 9 to which may be imparted the desired sealing movement and which may exert the desired sealing pressure. The sealing jaw 9 is operative to move towards a likewise movable counter jaw 10. The packaging material 11 which is intended to be transversely sealed is advanced between the sealing jaw 9 and the counter jaw 10. The packaging material 11 is disposed between the two jaws 9 and 10 such that two thermoplastic layers 12 which are intended to be fused or sealed together are oriented towards one another. 
     When the two sealing jaws 9 and 10 move towards one another and there entrap between them the packaging material 11 which is to be transversely sealed, a seal will be created in three substantially parallel sealing lines wherein the centremost is of largest extent. When the two sealing jaws 9 and 10 are urged towards one another, the ridges 14 disposed on the conductors will locally exert a slightly greater compressive force and will contribute in creating a more reliable and tighter sealing of the packaging material 11. The packaging material 11 is intended to be severed in the centremost sealing joint or seam, in that a knife 13 disposed in the movable counter jaw 10 moves down through the material 11 into the U-shaped groove of the central conductor 3. 
     After transverse sealing employing an inductor 1 according to the present invention, any possible product residues will be enclosed between the transverse seal of the outer conductors 2 and the transverse seal of the central conductor 3. In such instance any possible product residues will be enclosed in a material which is just as aseptic as the remainder of the package and will not constitute any obstacle to the filling of products containing, for example, fibres, fruit seeds or kernels into the containers. The package severed in the central sealing joint or seam will not display any sealing fin where product residues may gather. 
     As has been apparent from the foregoing description, the present invention realizes an apparatus for transverse sealing of packaging blanks in which no sealing fin or flap is created on severing of the packaging blanks to form individual packages. In this instance, a transverse seal will here be obtained which is well suited in the filling of contents of high viscosity or contents which contain particles such as fruit seeds, kernels or fibres. 
     The present invention should not be considered as restricted to that described above and shown on the Drawing, many modifications being conceivable without departing from the spirit and scope of the appended claims.