AN ARRANGEMENT FOR CONTINUOUS INDUCTION WELDING

Abstract: The invention relates to an arrangement for the continuous carrying out by means of induction heat of a continuous weld along a moving web of laminated packing material (8). The arrangement comprises two similar elongated metal plates (1) with two slots (2, 12). The metal plates (1) are arranged at some distance from one another and are connected electrically to one another and to an a.c. source (10). The laminated packing material (8) which is to be welded together is advanced between the two plates (1). Within the plates (1) pressure rollers (6) are arranged in a recess (5).

Description: The invention relates to a welding device in accordance with the opening part of claim 1 as described in SE-A-410 788. The welding of laminated packing material by means of induction heat is used in the packing industry. One example of such a laminated material comprises a carrier layer of paper and cardboard, aluminium foil and thermoplastic coatings along the inner and outer surfaces of the laminate. The technique of induction welding is based of the fact that the magnetic field around a conductor, flown-through by an alternating current, is capable of inducing a current in an adjoining electrically conducting material, which depending on the resistance of the material, brings about a more or less pronounced warming up of the material. Thus in induction welding an inductor loop or coil of the same configuration as that which the desired weld is to receive is fitted adjoining the laminate containing the aluminium foil, whereupon the laminate is pressed together with the material to which it is to be joined. The aluminium foil will now be heated in a pattern corresponding to the coil and by means of appropriately chosen current, frequency and processing period the material can be heated to the desired temperature, that is to say a temperature which is sufficiently high for adjoining layers of thermoplastic to be heated up to sealing temperature, causing the combined thermoplastic layers to fuse together, thus producing a tight and durable seal. A modern method for manufacturing liquid packages comprises a plane plastic-coated web which is formed to a tube in a filling machine and which, thereafter, is filled with contents and is sealed off along narrow zones located at a distance from one another, at right angles to the longitudinal direction of the tube. It is customary in this context to use induction welding for the transverse welds which divide the packing material tube 2 into individual packages. In the manufacture of a continuous pipe or tube from a plane packing material web, edge zones of the web are combined in an overlapping contact, whereafter the said overlapping edge zones are combined by heating and surface fusion. A customary method for forming continuous pipes or tubes from a plane web consists in carrying out the welding together of the overlapped edge zones, the so-called longitudinal joint weld, with the help of hot air, by means of which the combined thermoplastic layers are heated to sealing temperature, whereafter they are pressed together whilst being cooled at the same time. Very often one edge zone is provided with a narrow strip of thermoplastic which is welded first to one edge zone of the material web before the two edge zones are brought together for the purpose of forming a continuous tube. This is done so that a joint is obtained towards the inside of the package which is wholly covered by thermoplastic. Otherwise one edge of the joint is open towards the interior of the package, which means that the carrier layer of paper or cardboard of the packing laminate makes direct contact with the contents which then can penetrate into the carrier layer and dissolve the paper fibers. The welding on of the thermoplastic strip too is generally carried out with hot air in a manner similar to that of the longitudinal joint. This method,however, involves a relatively long warming up period for the filling machine , that is to say the time which is needed for heating up the hot-air element. Furthermore, it is necessary at short production stops in the filling machine to reheat the joint in order to obtain a continuous longitudinal joint without interruption. Such an arrangement to-day entails a great number of movable parts in order to make the process fully automatic, since the pressure roller which pressed together the joint has to be turned away and a separate heating element has to heat the area underneath the pressure roller. It is an object of the present invention to provide a welding device which enables a continous weld on a laminated packing material with a reduced heating up time. The invention is characterized in claim 1. Preferred embodiments thereof are claimed in subclaims. A preferred embodiment of the invention will now be described in greater detail with reference to the attached drawings, wherein Fig. 1 schematically shows the arrangement in a plan view Fig. 2 schematically shows the arrangement in a side view Fig. 3 shows how the arrangement is connected electrically and how the currents circulate in the arrangement Fig. 4 shows schematically an embodiment of the arrangement for the welding of a continous tube Fig. 5 shows the components included in Fig. 4 and how they are connected up electrically. In Fig. 1 and 2 is shown the arrangement comprising two elongated metal plates 1 of substantially identical shape, placed at some distance from one another. In the metal plates which constitute the inductor of the arrangement is incorporated a groove or a slot 2 which extends along the main part of the length of the metal plate 1. The groove 2 is placed asymmetrically on the width of the plate 1. The plates may be replaced by metal wires or metal strips placed densely adjoining one another or at some distance from one another and designed so as to cover a surface corresponding to the metal plates. In the preferred embodiment the groove 2 divides the plate 1 into a narrow part 4 and a wide part 3 where the wide part 3 is more than three times wider than the narrow part 4 = d. In the applications where it is not feasible to provide a part 3 which is three times as wide owing to lack of space, it is possible to make the wide part 3 approx. 2 times wider than the narrow part 4. In this case, however, an increased heating of the packing material 8 under the wide part 3 has to be reckoned with. The distance between the two inductor plates 1 should be chosen so that a good inductive coupling to the intermediate packing material 8 is achieved. In Fig. 2 the distance has been chosen so that it is half or less than half the width d. The length of the metal plates 1 is chosen appropriately so that they constitute at least one cutoff length, that is to say the distance between two transverse joints of the packing material 8. In this way a more uniform heating is obtained, since a relatively long plate 1 assists in evening out the differences in heating caused by differences in speed when the packing material 8 moves through the arrangement. In the farther end of the inductor plates 1 in relation to the direction of movement of the packing material 8 a wider recess 5 is incorporated. In this recess 5 are placed one or more pressure rollers 6 within the heating area of the inductor. The pressure rollers 6 are manufactured from some electrically insulating material and have concentrically inserted rings 7 of ferrite. In the plate 1 is incorporated a second slot 12, from the firstnamed slot 2 to one of the edges of the plate 1, so that the two sides which delimit the second slot 12 are electrically insulated from one another. Consequently a current can be conducted through the plate 1 by connecting a point 13 on one side of the second slot 12 to one outlet on an a.c. source 10 and connecting the second outlet of the a.c. source to a point 14 on the other side of the slot 12. Fig. 2 shows how a strip 9 of thermoplastic is introduced into the arrangement and is welded to the inductively heated packing material edge 8. The pressure rollers 6 ensure that good adhesion will result from the welding process. In Fig. 3 is shown how the two plates 1 are connected to an a.c. source, a so-called IH generator, from which a constant power can be obtained, which can be controlled for an optimum welding process. The heat loss in the metal plates 1 is relatively small so that the problem regarding heat emission to the environment is negligible. This means also that the whole arrangement can be placed in the filling machine without having to consider a diversion of the waste heat. The two inductor plates 1 are connected to one another so that they constitute two coils connected in series. As a result the same direction of current exists in both inductor plates 1, and an effectively induced current is obtained in the intermediate packing material 8, even if it were to "flutter", i.e. vary vertically during its movement through the arrangement. The current induced in the intermediate material 8 obtains approximately the same spread and density distribution as in the inductor plates 1. If the groove 2 of the plates 1 divides the plate 1 into a narrow part 4 d and a wide part 3 a·d, where a in accordance with the above preferred embodiment = 3, the following power factor can be calculated: We assume that the packing material 8 in the narrow part 4 has the resistance R. The resistance in the wide part 3 then will be R/a If the short sides are neglected in the calculation, the total power developed P tot in the packing material 8 will be P tot = I²(R+R/a). The development of power in the two parts can be compared by means of the calculations P narrow P tot ≈ I²R I²(R+R/a) = a a+1 if a = 3 3 4 P wide P tot ≈ I² R/a I²(R+R/a) = 1 a+1 if a = 3 1 4 With such a distribution on the inductor plates 1 the power obtained in the narrow part 4 thus is three times that in the wide part 3. The heating in the intermediate material 8 underneath the wide part 3 of the inductor plate 1 is calculated then in that the power ratio is equal to a and the wide branch 3 has a mass which is a times as great. The heating ratio becomes a², in the preferred embodiment 3² = 9. The heating in the narrow part 4 thus will be 9 times as great as that in the wide part 3. In practice this indicates that the material 8 which passes underneath the narrow part 4 of the inductor plates 1 is heated to such an extent that the thermoplastic melts and can be welded together with the other edge or with a thermoplastic strip 9. In the material 8 which passes underneath the wide part 3 a current is induced which is barely able to warm up the material 8. In the recess 5, wherein are arranged the pressure rollers 6, the current fed in follows the shape of the plate 1 and deviates consequently from the packing material edge 8. The current induced in the packing material 8 endeavours to follow the plate 1 as closely as possible, and an induced current is obtained therefore also underneath the pressure rollers 6 up to the far end of the groove 5. The pressure rollers 6 are made substantially of an electrically insulating material, with concentrically inserted rings 7 of ferrite, and where the electrically insulating material concentrically surrounds the ferrite rings 7. The ferrite ring 7 being axially displaced on the axle of the pressure roller 6 assists in a control of the induced current along the edge in spite of the plates 1 not being directly above and underneath the material edge 8. In Fig. 1 the ferrite ring 7 is placed so that it is located substantially in the extension the groove 2. When there is a need for any reason to make short production stops on the filling machine, the welding is carried out up to the point of contact 11 of the pressure rollers 6. Owing to induction welding furnishing a direct start-up without a preheating period and thanks to the material 8 being heated up right to underneath the pressure roller 6 when the machine is restarted, no part of the longitudinal joint will remain unsealed. In Fig. 4 as illustrated how an arrangement for the sealing of a continous tube by means of induction heat can be realized. The arrangement consists, as in Fig. 1, of two substantially elongated metal plates 1, slightly bent to facilitate the forming of the tube. The different legs for attachment 15 and current feeding 16 and 17 are insulated from one another e.g. by means of an insulating plate which at the same time constitutes a fixing means for the whole arrangement. For greater clarity, the insulating plate has not been displayed in Fig. 4. Only the legs 18 and 19 are electrically connected to one another, which is also illustrated in Fig. 5 The current feed legs 16 and 17 are connected to the IH generator 10 which furnishes alternating current to the arrangement. In a corresponding manner to that in the plane arrangement in Fig. 1-3, one or more pressure rollers 6 are arranged within the inductor plates 1 in a recess 5. The metal round the recess 5 in this embodiment has been formed so that it constitutes a coil with a number of turns in order to augment the induced current underneath the pressure roller 6, so that the ready-sealed longitudinal joint at no point can be weaker and thereby cause leakages in the finished packages. As is evident from the foregoing description, the present invention makes possible an arrangement for the continuous welding of a moving packing material web by means of induction heat. The arrangement requires no warming-up period and does not disturb the environment through waste heat. The arrangement also allows in case of short stops to obtain a continuous weld without having to adopt solutions comprising extra heating elements and mechanical parts for this and the movement of the pressure roller

Claims: Welding device for carrying out a continuous weld along a moving web of laminated packing material (8) comprising at least one metal layer coated by thermoplastic material, by means of induction heat from an electrically conducting coil connected to an alternating current source (10), the device comprising an inductor with at least one inductor plate (1) of electrically conducting material in which is incorporated a through slot (2) extending asymmetrically along the longitudinal direction of the inductor plate (1) and a second slot (12) extending from the firstmentioned slot (2) to one of the outer edges of the plate (1), the inductor plate (1) being electrically connectable to the alternating current source (10) on either side of said second slot (12), characterized in that the first slot (2) extends along the greater part of the length of the long inductor plate (1) thereby dividing it into a wide part (3) and a narrow part (4), and that the wide part (3) is at least two times wider than the width (d) of the narrow part (4). Welding device as claimed in claim 1, characterized in that said second slot (12) is located near one of the end edges of the inductor plate (1). Welding device as claimed in Claim 1 or 2 characterized in that the first slot (2) is placed so that the wide part (3) of the inductor plate (1) is at least three times wider than the narrow part (4). Welding device as claimed in one of the preceding claims, characterized in that two inductor plates (1) are arranged one above the other and that the distance between the two inductor plates (1) is no more than half the width (d) of the narrow part (4) of the inductor plates (1). Welding device as claimed in claim 4, characterized in that the packing material web (8) is adapted to move between the two inductor plates (1), a part of the material web (8) which is to be welded moving parallel with and along the narrow part (4) of the inductor plate (1). Welding device as claimed in one of the preceding claims, characterized in that one or more pressure rollers (6) are arranged within the inductor. Welding device as claimed in claim 6, characterized in that the pressure rollers (6) are manufactured substantially from an electrically insulating material with concentrically inserted ferrite rings (7) so that the electrically insulating material concentrically surrounds the ferrite rings (7). Welding device as claimed in claim 6 or 7, characterized in that the rollers (6) are disposed at least partially within recesses (5) situated near the end of said first slots (2) far from the second slot (12). Welding device as claimed in one of the preceding claims, characterized in that said inductor plates (1) are bent.

Inventors: HOLMSTROEM, GERT, Holmström, Gert
Applicants: TETRA ALFA HOLDINGS S.A.
Application year: 1990
Main classification: B65B51
IPC codes: B65B51, B29C65, H05B6, B23K13