Method of obtaining bend lines on packaging material

Abstract: Bend lines on packaging material are usually obtained by folding, which results in fold lines which are raised on one side of the material. According to the invention the raised parts of the fold lines are removed by mechanical processing, e.g. milling, which gives a considerable increase in the fold lines' weakening effect and facilitates subsequent bending of the material.

Description: The present invention concerns a method of obtaining bend lines on packaging material according to the preamble of claim 1 (e.g. US-A-4 468 212). Consumer packagings for drinks such as juice and milk are often manufactured from a flexible packaging material which, through cutting, bending and sealing, is formed into a filled and closed packaging container of the desired shape. The packaging material is usually a laminate which contains a bearer layer of fibrous material, e.g. paper, which is coated at least on one side, facing the contents, with a liquid-tight, thermoplastic material. The packaging laminate can also contain another layer of plastic or metal foil to ensure a better lightproof quality, gas barrier or tolerance of liquids. While the packaging material is still in strip or sheet form it is usually also provided with a pattern of bend lines or fold lines which weaken the material linearly and facilitate the forming of the same into packaging containers of the desired shape by bending. The bend or fold lines are obtained in the conventional manner, that is to say through the packaging material in strip form being passed between rollers with male and female tools, which press the desired pattern of fold lines into the material. The fold line pattern obtained thus exhibits a positive and a negative side, i.e. the linear deformation of the material caused by the fold tool results in raised fold lines on the one, positive side of the material and corresponding linear depressions on the opposite, negative side of the material. This weakening of the material obtained through the fold processing is in other words caused by the fact that the fibre layer of the material is distorted so that the fibres in the region of the fold line are are displaced in a wave-like manner, but not broken off or cut off. No real reduction in the thickness of the material in the fold line takes place, but only a displacement from the negative side of the material to the positive side of the material. Conventional fold lines thus give a linear weakening of the material, but the flexibility of the material is to a large extent retained, since no reduction in the material thickness in the fold lines and no cutting off of fibres takes place. The fold lines certainly achieve thereby a simpler and more precise process of bending, but the material retains its elasticity and in the absence of external forces it attempts to straighten out again to the original, mainly flat position. Conventional, folded bend lines do offer a sufficient weakening of the material for the majority of purposes, but when extra high demands are made for accurate shaping of the packaging or sharp, straight bends, a further weakening of the material is required, which not only facilitates the bending but also "kills" the flexibility of the material so that the material to the greatest possible extent remains in the folded position in the absence of external forces. In packaging laminate which contains a central bearer layer of fibrous, liquid-absorbent material it is a well known practice to ensure a folding over of the longitudinal edge of the strip of packaging material which, after shaping to a packaging container, is coated inside the same and thus in contact with the contents in liquid form. The folding over of the edge of the strip of packaging material is done in several stages. First a milling or grinding of the edge is carried out with the aim of reducing the material thickness so that the thickness of the edge after folding over is mainly the same as the total thickness of the material. Then the area of reduced thickness is provided with a longitudinal fold line, mainly extending centrally in this area, after which the edge is doubled over and sealed within the area of reduced thickness. With certain types of material and material thicknesses it has proved difficult with conventional folding to ensure a sufficiently great weakening to make sure that the edge folded over remains in the doubled over position until the sealing has been completed. This has entailed the width of the folded area being successively reduced so that finally the edge remains unfolded, which, if it is not detected, causes suction of the edge and leakage in the finished packaging container. Bend lines which weaken the material to a greater extent than conventional fold lines can be achieved with the aid of a well known method, which is also used for reduction of thickness of larger material parts, e.g. in joints overlapping each other. In this method one side of the strip of packaging material is subjected to a milling or grinding process at the same time as the strip of material passes through a master tool, i.e. a tool which is provided with a pattern of the raised areas which serve as a holder in the milling and grinding process. For rational use several master tools must be applied on a counter roll and the master tools must be given individual shape for each type of weakening pattern that must be ground on the material, which is shown to be unreasonably expensive in manufacturing the packaging material for a large number of different types or sizes of packaging containers. In addition the method gives a line with worse definition, i.e. the transition from the ground area to the adjacent, unground area of the packaging material is gradual and not distinct. A further manner of ensuring weakened or thinned areas of material is to use a conventional grinding or milling which results in a pattern of recessed grooves in one side of the material. These grooves certainly serve as indications for folding, but they are very badly defined with sliding transfer to the unground material and therefore give badly defined, crooked bend lines. The method further entails a considerably greater removal of the fibre material, which gives a lot of shavings that have to be handled. There is thus a wish within the packaging material industry to ensure a method which can at a reasonable cost provide a packaging material with well defined bend lines in the desired pattern. An aim of the present invention is to ensure a method of achieving bend lines on packaging material, with this method not being affected by the disadvantages of the abovementioned, previously known methods but providing shaping of well defined bend lines in the desired pattern on different types of packaging material and laminate. A further aim of the present invention is to ensure a method by means of which it is possible to ensure fold or weakening lines which not only weaken the material so that the folding is facilitated but also to a certain extent cutting of the fibre layer of the material so that the tendency of the material after folding to return to its original position is reduced. A further aim of the present invention is to ensure a method of achieving fold lines on the packaging material, with this method being simple and rational to carry out and adapt to different types of material and packagings and in addition being cheap and uncomplicated. The abovementioned and other aims have been achieved according to the invention through the fact that a method of achieving fold lines on packaging material is given the characteristic that in a first stage the material is provided with the desired pattern of raised parts on one side of the material, after which in a second stage the material is subjected to a mechanical processing which wholly or partly removes the raised parts. Preferred embodiments of the method according to the invention have further been given the characteristics that can be seen from the sub-claims. A preferred embodiment of the method according to the invention will be described more closely below with particular reference to the enclosed drawing, which only shows the details indispensable for understanding the invention. Fig. 1 shows a part of the packaging material which, in accordance with the method according to the invention, has been provided with a raised part. Fig. 2 shows the packaging material according to fig. 1 which has been mechanically processed in accordance with the method according to the invention. Fig. 3 shows the packaging material according to fig. 2 after folding along a fold line achieved according to the invention. Fig. 4 shows a fold line achieved in accordance with the method according to the invention in another type of packaging material. Fig. 5 shows the packaging material according to fig. 4 after folding over of a longitudinal edge. The method according to the invention is, as previously mentioned, intended to be used in order to achieve fold lines on packaging material of different sorts, e.g. packaging material of the sort used for production of consumer packagings for contents in liquid form, e.g. milk packagings. Even though the method according to the invention can of course be used with many different types of material it is illustrated in the figures as used in its preferred embodiment. Figs. 1, 2 and 3 thus show part of a packaging material (1) on the one hand in strip or sheet form (figs. 1, 2), on the other during shaping into a packaging container(fig.3). The packaging material(1) contains a relatively thick (c. 0.5 millimetres) bearer layer (2) of fibrous material, e.g. paper. The packaging material (1) is, as already mentioned, designed to be used for manufacture of packaging containers for contents in liqid form and must therefore in its final form contain further layers of liquid-tight material. In the stage of manufacture in which the material finds itself in fig. 1 it can be provided with an outer layer (3) of thermoplastic material, which is laminated to one side of the bearer layer (2). Usually this layer is made of polythene, but other types of thermoplastic can also be envisaged. The liquid-tight layer (3) can alternatively be applied to one side of the bearer layer at a later stage, possibly at the same time as a further layer (4) is applied to the opposite side of the bearer layer (2). The layer (4) can also consist of suitable thermoplastic material and be applied through extrusion coating in a hot state or in the form of a prefabricated plastic film. In order to facilitate the shaping of the packaging material (1) in sheet or strip form into finished, e.g. parallelipiped-shaped packaging containers the packaging material is provided with a pattern of fold lines (5) which weaken the material linearly and facilitate or make possible rectilinear folding of the material so as to form the edges and corners of the packaging container. The fold lines (5) are preferably linear, but can also have the form of surfaces or regions of larger area, e.g. where the fold lines meet or cross each other, in corner parts, sealing regions etc. Preferably the lines are obtained through conventional folding, i.e. the packaging material is passed through a male and a female tool which press the material between them so that it is given a positive side, on which the fold lines (5) exhibit raised parts (6), and also a negative side, on which the fold line has the form of a depression (7). This distortion or deformation of the packaging material entails that the fibres in the bearer layer (2) are given a corresponding deformation and thus run partly up through the raised part (6) of the fold line (5), which is illustrated in fig. 1. The raised part (6) of the fold line (5) is thus obtained preferably by rolling the packaging material between fold rollers, but other types of pressing procedures can also be envisaged. When the packaging material according to the first stage of the invention is provided with the raised parts (6) in the desired pattern, preferably linear, a second stage is carried out in accordance with the invention in which the material (1) is subjected to a mechanical processing which wholly or partly removes the raised parts (6). The processing, which in the first embodiment of the invention, which is illustrated in figs. 1-3, is preferably a face milling of the positive side of the material (1), principally removes altogether the raised parts (6) and is terminated when these are reduced to an equal height with the adjacent, mainly flat surface of the material. The positive side of the material thereby becomes completely flat and can, as illustrated in fig. 2, subsequently be coated with the second, liquid-tight layer (4) of thermoplastic material, e.g. polythene. Through the mechanical, chip-removing processing of the positive side of the packaging material those parts of the bearer layer's fibres are removed which extend upwards in the raised part (6), and the packaging material therewith obtains fold lines in which the fibre layer (2) is not only deformed and weakened but also reduced in thickness, which reduces the flexibility of the material so that it acquires after folding an increased tendency to remain in the folded position. This is illustrated in fig. 3, where the packaging material according to fig. 2 is folded ninety degrees along the fold line (5) during shaping into a packaging container. The forming of the fold lines in accordance with the method according to the invention results in well defined and distinct fold lines which considerably reduce the flexibility and elasticity of the material and thereby make it possible to achieve rectilinear folds along the edges of the packaging container with greater accuracy, which gives the packaging container a more even, cleaner appearance and in consequence of this a better hndling rigidity. Another embodiment of the method according to the invention is illustrated in figs. 4 and 5. A packaging material (8) in strip or sheet form, which can be of the same type as the packaging material (1), must in accordance with the method according to the invention be provided with a fold line (10) extending along a longitudinal edge (9) in order to enable folding over of the edge region with the aim of preventing the fibrous bearer layer (11) of the packaging material from absorbing the contents along the longitudinal edge (9), which, after shaping of the packaging material into the packaging container, is situated inside the same in contact with the contents in liquid form. With folding over of the narrow edge regions of the strips of packaging material what is called flaking technology is usually employed, i.e. the present thickness of the edge region is mainly reduced to half in order to avoid its becoming thicker after folding over than the original thickness of the packaging material. The reduction of the thickness is done through grinding or milling or by means of a rotating knife edge which the material is made to pass in conjunction with the manufacture. In accordance with the method according to the invention the packaging material (8) is provided with the fold line (10) before the mechanical, thickness-reducing processing. The fold line (10), which is preferably formed with the aid of two rollers provided with male and female bending tools respectively and working in conjunction, is placed so that the positive, raised side is situated on the side of the packaging material (8) which is to be processed. The subsequent, thickness-reducing processing therefore removes both the raised part of the fold line (10) and adjacent parts of the packaging material (8) so that the thickness of the edge region in its entirety amounts mainly to half the original thickness. The packaging material (8) can at this stage of manufacture already be coated in the known and previously described manner with external layers (12), (13) of thermoplastic material but it is also possible to apply one or both of these layers at a later stage. As soon as the mechanical, thickness-reducing processing of the edge region of the packaging material (8) has taken place a 180 degrees folding of the edge region along the fold line (10) is carried out in a manner which is in itself well known, after which the folded part is sealed in the folded position with the aid of a suitable binding agent (hot melt). The folding must be done in a straight line and parallel along the longitudinal edge (9), which is considerably facilitated by the well defined fold line achieved according to the invention, which has a high material-weakening effect and which thanks to the removal of the raised part and the simultaneous cutting off of material fibres reduces the flexibility of the material to such a high degree that the folded edge region remains in this position until its gluing has been completed. Through this an even, folded-round edge of material is ensured which gives a satisfactory watertightness, enables a perfectly good sealing of the material after the shaping of the same into a packaging container and guarantees a seal against contents penetrating into the fibre layer (11). The method according to the invention can in its various embodiments be used to obtain well defined fold lines in different types of packaging material for various purposes. Compared with earlier, conventional bending technology a considerably stronger and more permanent weakening of the material is achieved. Compared with earlier grinding technology in combination with master tools a more well-defined fold region is obtained, at the same time as the cost is considerably reduced, since no master tool is necessary. In comparison with conventional grinding without master tools a considerably better definition of the weakening lines is achieved according to the invention without the technology requiring significantly more work or costs.

Claims: Method of achieving fold lines (5) on packaging material (1), by providing (8), the material (1), (8) in a first stage with the desired pattern of raised parts (6) on one side of the material and corresponding depressions (7) on the opposite side of the material, characterised by the fact that in a second stage the material (1), (8) is subjected to a mechanical processing which completely or partly removes the raised parts. Method according to the patent claim 1, characterised by the fact that the raised parts (6) are linear. Method according to the patent claim 2, characterised by the fact that the raised lines are achieved through folding. Method according to the patent claim 1, 2 or 3, characterised by the fact that the processing is of chip-removal type. Method according to one or more of the foregoing claims, characterised by the fact that the processsing is terminated when the raised parts (6) have been processed to the same height as the adjacent surface of the material (1). Method according to one or more of the patent claims 1 to 4 incl., characterised by the fact that the processing reduces the material thickness both in the raised parts and in adjacent parts of the material (8). Method according to one or more of the foregoing patent claims, characterised by the fact that the processed side of the material (1), (8) is coated with a layer (4), (13) of plastic material.

Inventors: BRUNLID JOHN-ERIK, BRUNLID, JOHN-ERIK
Applicants: TETRA LAVAL HOLDINGS & FINANCE, TETRA LAVAL HOLDINGS & FINANCE S.A.
Application year: 1991
Main classification: B31B1
IPC codes: B65D5, B31B1