FIELD OF APPLICATION OF THE INVENTION

The present invention relates to a method and an apparatus for welding at least two sheetings of hot-weldable material to each other, such as two plastics film sheetings, whereby the two sheetings are caused to overlap each other in the area where they are to be welded to each other and are subsequently advanced in an advancement direction in contact with an essentially cylindrical surface of a roller, said roller rotating about an axis-of rotation, and whereby, during the advancement of the two sheetings, a welding seam of a given shape is provided and which extends essentially longitudinally of the advancement direction of the sheetings and welds the two sheetings to each other, and whereby the two sheetings are pressed against each other to the effect that at least a portion of the overlapping area on the two sheetings is pressed against a welding face on the roller, which welding face has the same shape as the welding seam, extends essentially all the way around the roller and is heated to a temperature close to or higher than the melting temperature of the hot-weldable material, to the effect that, during the procedure of welding the sheetings to each other, heat energy is transferred from the welding face on the roller to the sheetings.

STATE OF THE ART

Today, several different methods are known for welding sheetings of hot- weldable materials to each other and which are suitable for joining very long sheetings by welding, which sheetings are initially provided with welding seams extending primarily longitudinally of the longitudinal direction of the sheetings, and, not until after that, they are punched from the welded sheeting and are optionally provided with transverse welding seams for forming individual products.

Such production methods are often used in connection with mass production of eg bags and other products, and it is a constant challenge in the context of such production processes to increase the production rate without ensuing consequences with regard to a deterioration of the product quality.

Thus, a known method consists in that the sheetings of the hot-weldable material are pulled across a stationary welding bulb, which entails, however, much wear on the welding bulb at high^production rates and, maybe, reduced longevity of the plastics material in the area of the welding seam, since hereby undesired pull effects are brought about in the plastics material. In other known methods of the kind set forth above, and whereby those undesired pull effects are considerably reduced, the heat required for the welding is provided by means of a laser, thermal radiation or hot air. However, by those methods it may be difficult to accomplish an accurately configured welding seam. Moreover, it may be a problem that thin film material may have a propensity to wrinkle at the welding seam.

DE 197 34 578 A1 describes a method for welding film material, whereby the film material is conveyed across a welding drum with embedded welding electrodes extending in the circumferential direction of the drum and in the axis direction of the drum, respectively. Such electrodes are in the form of a number of welding teeth that are in connection with and are heated by an internal heat layer configured centrally in the drum. The welding drum is displaceable between a welding position, wherein the drum abuts on the film to be welded, and a withdrawn position in which the drum is not in contact with the film and thus does not melt it apart or unintentionally fuses it at production onset or in case of production stoppages. However, it may be a drawback that it is necessary to pull the drum away from the film when welding is not to be performed, eg at production onset or in case of production stoppages, in order to avoid unintentional melting of the film. OBJECT OF THE INVENTION

Based on this, it is the object of the present invention to provide a method and an apparatus for forming welding seams longitudinally of a sheeting welded thereto and consisting of two or more layers of hot-weldable material, such as thermoplastics, whereby the manufacturing process is simplified and/or proceeds more quickly, especially at production onset or- in case of production stoppages.

This is accomplished by a method according to the invention in that the transfer of heat energy from the welding face on the roller to the sheetings is reduced in such a way at production onset or in case of production stoppages, while simultaneously the sheetings remain in contact with the cylindrical surface of the roller, that the risk of fusing the sheetings is minimised. Hereby it can be obviated that, in order to avoid unintentional fusing of the sheetings, the roller must be shifted to a position away from the sheetings to be welded in such situations when the sheetings are not advanced as in case of normal production rates, as is the case eg at production onset or in case of production stoppages. The manufacturing process may be simplified and proceed more quickly, and moreover the apparatus for exercising the method can be simplified.

By longitudinally extending welding seam as set forth above is intended a broken or unbroken welding to each other of two materials whose expanse in the longitudinal direction is substantially longer than transversally of that direction. According to an embodiment of the invention, the transfer of heat energy from the welding face of the roller to the sheetings at production onset or in case of production stoppages is reduced by interruption or reduction of the current through an electrically conductive wire or an electrically conductive band that extends around the roller in the surface of same, and the welding face is constituted wholly or partially by a surface on the electrically conductive wire or the electrically conductive band, the roller comprises an outer tube on which the substantially cylindrical surface of the roller is configured, and the outer tube has a given material thickness measured transversally of the surface, and the electrically conductive wire or the electrically conductive band-substantially has a material thickness which is smaller than the material thickness of the tube measured in the same direction. Hereby a very quick cooling of the welding face and hence of the sheeting to be welded can be obtained in a simple manner, it being possible that the total heat energy stored in the electrically conductive wire or the electrically conductive band can be minimised due to the reduced material thickness of the wire or the band. The transfer of heat energy from the welding face on the roller to the sheetings can hereby be reduced significantly and very quickly. This enables that the risk of fusing of the plastics material at production onset or in case of production stoppages can be minimised.

According to an alternative embodiment of the invention, the roller comprises an outer tube on which the essentially cylindrical surface of the roller is configured, and the welding face is divided into small welding faces, each of which is arranged on three or more separate segments, and those segments are movably arranged in the roller to the effect that they may occupy a first position in which the welding face is essentially flush with the cylindrical surface on the roller, and a second position in which the welding face is withdrawn into the roller and underneath the cylindrical surface, and the transfer of heat energy from the welding face on the roller to the sheetings is reduced at production onset or in case of production stoppages by displacement of said three or more separate segments from said first position to said second position. The transfer of heat energy from the welding face on the roller to the sheetings can hereby be reduced significantly and very quickly in a simple manner.

According to a preferred embodiment of the invention, the two sheetings are pressed against the welding face by means of a band that extends across the welding face and abuts on the overlapping portion of the two sheetings on the side thereof which is opposite to the roller.

In that the band, the sheetings as well as the roller's surface are advanced in the advancement direction of the two sheetings at substantially the same rate in the area where the two sheetings abut on the roller, it is accomplished, in a preferred embodiment, that unintentional pull effects in the material sheeting are further minimised.

The invention also relates to an apparatus for welding at least two overlapping sheetings of hot-weldable material to each other, such as two plastics film sheetings, said apparatus comprising a roller which is rotatably journalled about an axis of rotation and has an essentially cylindrical surface, and means for pressing at least the overlapping portion of the two sheetings against the roller's surface, and a conveyor means configured with a view to advancing the two sheetings in an advancement direction which is substantially perpendicular to the axis of rotation of the roller, and said apparatus being configured for forming a desired welding seam on the overlapping portion of the two sheetings, said welding seam extending essentially in the advancement direction, and wherein a welding face is configured on the roller, which welding face has the same shape as the welding seam it is desired to provide on the two sheetings and which extends essentially all the way around the roller on the surface thereof, and wherein the apparatus is further configured for local heating of the welding face to a temperature close to or higher than the melting temperature of the hot- weldable material for transfer of heat energy from the welding face on the roller to the sheetings during the welding to each other of the sheetings.

The apparatus is characterised by being configured for such reduction of the transfer of heat energy from the welding face on the roller to the sheetings, while said sheetings simultaneously remain in contact with the cylindrical surface of the roller, that the risk of fusing the sheetings can be minimised. Hereby the above-referenced properties can be accomplished.

Advantageously, the welding face may be thermally insulated relative to the remaining part of the roller by means of a material having poorer heat conductivity than the material from which the welding face is configured.

The means for pressing at least the overlapping portion of the two sheetings against the surface of the roller may advantageously comprise a welding pressure band which, on a part of the roller's surface, extends at least across the welding face and longitudinally thereof, and in such a manner that the welding pressure band is pressed inwards towards the roller, whereby the two sheetings can be introduced between the welding pressure band and the roller, whereby the overlapping portion of the two sheetings is pressed inwards against the welding face. In this context, the welding pressure band is, according to a preferred embodiment of the invention, configured from a bendable material, such as a band made of a Teflon-based plastics material having a melting temperature which is higher than the temperature to which the welding face is heated. According to a preferred embodiment of the invention, the means for advancing both the two sheetings and the welding pressure band and the surface of the roller are configured for advancing them at substantially the same speed in the advancement direction of the two sheetings.

According to a preferred embodiment of the invention, the roller may comprise an outer tube on which the essentially cylindrical surface of the roller is configured and wherein the outer tube has a given material thickness measured from the surface and in the direction towards the axis of rotation of the roller, and wherein the welding face is completely or partially constituted by a surface on an electrically conductive wire or an electrically conductive band extending around the roller in that surface, and substantially has a material thickness which is smaller than the material thickness of the tube measured in the same direction, and wherein the electrically conductive wire or the electrically conductive band is connected to a power source for electrically heating the wire or the band. Hereby the properties described above can be obtained.

In this context, it is a further advantage if the tube is made primarily of metal, such as aluminium, and if the electrically conductive wire or the electrically conductive band is electrically insulated from the remainder of the tube.

According to one embodiment, the electrically conductive wire or the electrically conductive band is biased by means of a bias device configured in a recess of the roller. According to one embodiment, the bias device is configured such that an area of the heat band is conveyed into the recess and kept biased in a direction into the recess by means of a pin around which the heat band is conveyed, and which is biased eg by means of a spring in a direction into the recess. Hereby it can be obtained that the band is capable of expanding and contracting in operation as a consequence of changes in temperature without the band becoming slack and maybe falling off the roller. According to an alternative, preferred embodiment of the invention, the roller comprises an outer tube on which the substantially cylindrical surface of the roller is configured, and the welding face is divided into small welding faces, each of which is arranged on three or more separate segments, and wherein those segments are movably arranged in the roller to the effect that they may occupy a first position in which the welding face is substantially flush with the cylindrical surface on the roller, and a second position in which the welding face is withdrawn into the roller and underneath the cylindrical surface. Hereby the welding face can be pulled away from the roller's surface and into the roller to the effect that the risk of fusing the plastics material at production onset-or in case of production stoppages is minimised. Hereby the -above- described properties can thus be obtained.

According to a further preferred embodiment, the apparatus comprises two or more rollers, each of which has a welding face, and the welding pressure band is configured such that the one side of the welding pressure band abuts on the welding face of one of the rollers, and the other side abuts on the welding face of another one of the rollers. This is particularly advantageous in that production of products consisting of four or more material sheetings is hereby enabled, each of which is welded in pairs of two at one or more welding seams that are longitudinal relative to the material sheeting. Thus, the same welding pressure band will, in a simple manner, function to the effect that it ensures against all four material sheetings being unintentionally welded to each other. It is particularly advantageous, in the context of this embodiment, that the rollers are not to be displaced away from the material sheetings, eg at production onset or in case of production stoppages, where the welding of the sheetings is to be interrupted temporarily. Such displacement of eg two rollers rolling on each their side of the material sheetings will cause problems in terms of maintaining the correct tensioning of the material sheetings. As an example of a product of this type can be mentioned bags that are, at their opening, provided with a reinforcing layer of material or which are provided with a drawstring hem for receiving a drawstring for drawing in the opening of the bag. Such bags often consist of a total of four material layers that are joined by welding in pairs of two at the opening of the bag.

Moreover, the invention relates to an apparatus for welding together at least two overlapping sheetings of hot-weldable material, such as two plastics film sheetings, said apparatus comprising a roller which is rotatably journalled about an axis of rotation and has a substantially cylindrical surface, and means for pressing at least the-overlapping portion of the two sheetings against the roller surface, and a conveyor configured with a view to advancing the two sheetings in an advancement direction which is essentially perpendicular to the axis of rotation of the roller and which apparatus is configured for forming a desired welding seam on the overlapping portion of the two sheetings, said welding seam extending substantially in the direction of advancement, and wherein a welding face is configured on the roller, said welding face having the same shape as the welding seam it is desired to provide on the two sheetings and which extends substantially all the way around the roller on the surface thereof, and wherein the apparatus is furthermore configured for locally heating the welding face to a temperature close to or higher than the melting temperature of the hot-weldable material, wherein the roller comprises an outer tube on which the essentially cylindrical surface of the roller is configured; and wherein the outer tube has a given material thickness measured transversally of the surface; and wherein the welding face is completely or partially constituted of a surface on an electrically conductive wire or an electrically conductive band which extends around the roller in the surface thereof, and substantially has a material thickness which is smaller than the material thickness of the tube measured in the same direction; and wherein the electrically conductive wire or the electrically conductive band are connected to a power source for electrically heating the wire or the band.

Moreover, the invention relates to an apparatus for welding to each other at least two overlapping sheetings of hot-weldable material, such as two plastics film sheetings, said apparatus comprising a roller which is rotatably journalled about an axis of rotation and has a substantially cylindrical surface, and means for pressing at least the overlapping part of the two sheetings against the roller surface, and a conveyor configured with a view to advancing the two sheetings in an advancement direction which is essentially perpendicular to the axis of rotation of the roller, and said apparatus being configured for forming a desired welding seam on the overlapping part of the two sheetings, said welding seam extending essentially in the advancement direction, and wherein a welding face is configured on the roller, said welding face having the same shape as the welding seam it is desired to provide on the two sheetings, and which extends substantially all the way around the roller on the surface thereof, and wherein the apparatus is further configured for locally heating the welding face to a temperature close to or higher than the melting temperature of the hot-weldable material, wherein the roller comprises an outer tube on which the substantially cylindrical surface of the roller is configured, and wherein the welding face is divided into small welding faces, each of which is located on three or more separate segments, and wherein those segments are movably arranged in the roller to the effect that they are capable of occupying a first position in which the welding face is substantially flush with the cylindrical surface of the roller, and a second position in which the welding face is withdrawn into the roller and below the cylindrical surface.

LIST OF FIGURES

Figure 1 shows a roller according to the invention, see from the front. Figure 2 is a sectional view of the roller shown in figure 1 , in a section along the shaft of rotation of the roller.

Figure 3 is a sectional view of the roller shown in figure 1 , seen from one side in a sectional view through the heat band on the roller.

Figure 4 is an explanatory sketch showing an alternative embodiment of a welding face of a roller according to the present invention, seen from the side.

Figure 5 is an explanatory sketch showing a construction of a roller arrangement in an apparatus according to the present invention, seen from the side. Figure 6 is an end view of a roller according to the invention.

Figure 7 is a sectional view along the line VII-VII through the roller shown in figure 6. Figure 8 is an enlarged sectional view of the roller shown in figure 7.

Figure 9 is a perspective view of the roller shown in figures 6 through 8, seen from the end shown in figure 6. EMBODIMENT OF THE INVENTION

Thus, figures 1 , 2, and 3 show an embodiment of a roller for use in an apparatus according to the present invention, in the following designated a welding roller 1 , which welding roller is configured as a tube 2 with a cylindrical surface 3, and wherein the tube 2 is rotatably journalled on a stationary shaft 4 by means of bearings 28. The shown tube 2 is made of metal, and on the tube 2 a recess 5 is configured that extends all the way around the tube 2. In that recess 5, an electrically conductive band 6 is embedded, in the following designated a heat band 6, which heat band forms a welding face 7 extending almost all the way around the pipe 2; figure 3, however, showing that the heat band 6 is, to each side of the welding roller 1 , conveyed into the tube 2 and is secured there to the effect that the heat band 6 is kept in place on the surface 3 of the welding roller 1. The heat band 6 is electrically and thermally insulated from the remainder of the welding roller 1 , and the recess 5 being filled with a thermally and electrically insulating material 8, such as thermosetting plastic, and current being, in a commonly known manner, applied to the heat band 6, the heat band may be heated to a temperature which is sufficient for welding together a material sheeting consisting of a hot-weldable material abutting on the heat band 6. Conversely, the heat thereby produced by the heat band 6 will substantially not be distributed into the remainder of the welding roller 1 , due to the thermally and electrically insulating material 8 arranged in the recess 5. It is a major advantage of this embodiment that it is possible to heat and cool the heat band 6 very quickly, whereby quicker onset and running-in of a welding process is accomplished, but more importantly that the risk of fusing the material sheeting ting be welded is minimized, it being possible to quickly cool down the heat band in case of production stoppages, etc, due to the very low amount of thermal energy contained in the heat band 6.

Figures 6 through 9 show more clearly how the electrically conductive wire or the electrically conductive band 6, also designated the heat band 6, can be mounted in a biased way to the roller 1 . It is an advantage that the heat band 6 is biased since the band may expand and contract in operation due to changes in temperature, and it can thus be prevented that the band becomes slack and maybe falls off the roller 1 . Reference being made in particular to figure 9, it will appear that the heat band 6 has two ends 24 that are introduced into a recess 30 in an end plate 31 of the roller 1 and are connected to each its terminal 25 situated in the recess 30, from where the heat band 6 extends in the recess 5 round along the surface 3 of the welding roller. The respective terminals 25 are, by means of not shown cables, connected to each their slip shoe 26 gliding on a stationary slip ring 27, whereby the heating band 6 is provided with current from the above- mentioned, not shown power source. In the shown embodiment, the electrically conductive wire or the electrically conductive band 6 is biased by means of -a -bias device 29 arranged on the opposite side of the roller 1 relative to the terminals 25. The bias device 29 is configured in a recess 32 in the end plate 31 of the roller 1. In the shown embodiment, the recess 32 has the same configuration as the recess 30, ie a triangular shape with an apex facing towards the surface 3 of the roller 1 . A central area of the heat band 6 is conveyed into the recess 32 and is kept biased in a direction into the recess 32 by means of a pin 33, around which the heat band 6 is conveyed and which is biased by means of a spring 34 in a direction into the recess 32. In the shown embodiment, the pin 33 is arranged in such a way that it is secured at the end of a splice 35 which is rotatably journalled about a slide bearing 36. Obviously, the heat band 6 may be biased in many other suitable ways. In the shown embodiment, the band may advantageously be mounted and biased, respectively, at opposite sides of the roller 1 , it being hereby possible to secure the band better in the recess 5. Alternatively, however, the band may be biased at terminals 25, it being possible that they are biased in a direction into the recess 30, and hereby the bias device 29 may optionally be omitted. However, it may also be advantageous to provide two or more bias devices. In case of the above-described embodiment, the welding seam provided on the material sheetings will be interrupted where the heat band 6 is conveyed into the welding roller 1 . If necessary, such interruption of the welding seam may, however, be reduced to a very small gap, it being possible to convey the wire or the heat band 6 into the welding roller 1 with a very small gap. Albeit one could imagine alternative solutions to this, such as optionally using several heat bands 6 next to each other that are displaced to the effect that they extend all the way around the welding roller 1 , it is a solution which is difficult to configure in practice in a manner which is stable and durable, while simultaneously enabling the making of a relatively narrow welding seam. Figure 4 shows an explanatory sketch wherein the welding face 7 is arranged on several separate segments 10,--1 that can all be shifted into the roller. From that embodiment, a total of four separate segments 10, 1 1 will appear, but it will be obvious to the person skilled in the art that more and optionally fewer can be used.

The shown segments that each carry a portion of the welding face 7 are configured such that the two of the segments 10 can be shifted freely in a direction towards each other as indicated by arrows 12, whereas the other two of the segments 1 1 can be shifted towards each other along the arrows 13 only if the two first-mentioned segments 10 have already been shifted in a direction towards each other. In this manner, it can be enabled that the four segments jointly form a welding face that extends all the way around the roller 1 in the surface 3 thereof. Moreover the risk of fusing the plastics material at production onset or in case of production stoppages can hereby be minimised, it being possible to shift the segments inwards in the roller 1 and away from the sheetings to be joined by welding as described above.

Now, figure 5 shows an explanatory sketch showing an arrangement consisting of two welding rollers 1 a and 1 b, each of which is rotatably journalled about an axis of rotation 14a and 14b, and which may each be configured as shown in figures 1 , 2, and 3, or alternatively in accordance with the principle shown in figure 4.

Moreover, a section of a welding pressure band 15 is shown, said band being conveniently configured from a Teflon-based material whereby it tolerates a certain heat without melting and such that it is not susceptible to welding to other plastics materials. The welding pressure band 15 extends across a portion of the surface 3a, 3b on both welding rollers 1 a and 1 b and is pressed against them by use of known means (not shown). On both sides of the welding pressure band, a range of a-total of four sheetings of hot-weldable material 16, 17, 18, 19 is shown, such as sheetings of thermosetting plastic, wherein the two first sheetings 16, 17 are arranged on the one side of the welding pressure band 15, and the two other sheetings 18, 19 are arranged on the opposite side of the welding pressure band 15.

If the welding rollers 1 a, 1 b are rotated in the direction of the arrows 20, then the first material sheetings 16, 17 and the other material sheetings 18, 19, along with the welding pressure band 15 that separate the first material sheetings 16, 17 and the second material sheetings 18, 19 from each other, and wherein the welding pressure band, at least according to the invention, extends across the welding face 7 on the welding rollers 1 a, 1 b, then the two former material sheetings 16, 17 will be welded to each other by a longitudinally extending welding seam when they travel in abutment on the first welding roller 1 a, and the other material sheetings will also be welded to each other when they travel by the second welding roller 1 b.

Thereby, when the material sheetings 16, 17, 18, 19 have travelled past both welding rollers 1 a, 1 b, a first welded sheeting 21 of the two first material sheetings 16, 17, will have been formed, and so will a second welded sheeting 22 of the two other material sheetings 18, 19. However, due to the welding pressure band 15, the two welded sheetings 21 , 22, will not be welded to each other unintentionally.