WO2002060673A1

WO2002060673A1 - Method and device for the manufacture of corrugated plastic material

Info

Publication number: WO2002060673A1
Application number: PCT/SE2002/000180
Authority: WO
Inventors: Anders JÖNSSON; Mikael Petersson; Jacob Volckerts
Original assignee: Wellplast Ab
Priority date: 2001-02-02
Filing date: 2002-02-01
Publication date: 2002-08-08
Also published as: SE518497C2; SE0100340D0; SE0100340L

Abstract

Method and device for the manufacture of corrugated material, at least one first plane sheet (19; 20) and one second sheet (16) of plastic material arranged in wave shape being brought together for adhesion to each other and the wave-shaped sheet (16) running over core bars (17, 18). A portion of at least one sheet is heated at abutment against the core bars (17, 18) and the first sheet (19; 20) is brought to abutment against the second sheet (16) for welding the sheets (16; 19, 20) together. First members are arranged for feeding at least the first plane sheet and the second sheet of plastic material running over core bars 817, 18) and second members are arranged for binging together and adhering the sheets. Heating members (28) are arranged for transferring heat to a portion of at least one sheet (16; 19, 20) abutting against the core bars and including plastic material. Furthermore, guide members (17, 18; 21; 22) are arranged for bringing together the first and the second sheet in an abutment portion and for welding together in the portion heated by the heating members (28).

Description

Method and device for the manufacture of corrugated plastic material

FIELD OF THE INVENTION

The invention relates to a device for the manufacture of corrugated material, at least one first plane sheet and one second sheet of plastic material arranged in wave shape being brought together for adhesion to each other and the wave-shaped sheet running over core bars. First members are arranged for feeding at least one plane sheet and one second sheet of plastic material running over the core bars and second members are arranged for bringing together and adhering the sheets.

PRIOR ART

Since long, different forms of corrugated board have been manufactured and used, above all for packing and the like. Corrugated board has very good insulating and shock-absorbing properties, but it is also impaired by a plurality of disadvantages. The largest disadvantage is, perhaps, the bad moist-resistance thereof. When corrugated board becomes damp, it looses a large part of the supporting capacity and durability thereof.

It is also known, per se, to form sheets of plastic material to wave shape and connect such wave-shaped sheets with plane sheets of similar material. An example of this is shown and described in US-A-4897146. The sheet that is to be wave-shaped is heated by particular heating members, making the material plastic. The material is then formed to wave shape with core bars and one side of the wave-shaped material is pressed against a pre-heated plane sheet by a forming drum, which is made with recesses being ring-shaped and adapted to the core bars. An additional plane sheet is heated in the similar way and is pressed against the other side of the wave- shaped material by a second preferably cooled drum. The material that the sheets are made of is relatively stiff and the thickness of the sheets is such that at least the sheet that is to be wave- shaped has to be heated to such a high temperature that the material becomes plastic. When the joined sheets have cooled, the result is a corru- gated sheet material, which, e.g., may be used as roof or wall panels. The device and method according to US-A-4897146 works well for the intended purpose, but is less suitable for other purposes, e.g. for the manufacture of corrugated material for packing and the like.

THE INVENTION IN SUMMARY

An object of the invention is to provide a device for the manufacture of corrugated material of a plurality of material sheets, which are brought over core bars and heated and joined together in an effective way. These objects are attained by the invention having received the features mentioned in claim 1. At least one sheet is corrugated.

The device comprises heating members for the transfer of heat from the core bars to at least one sheet abutting against the core bars and comprising plastic material, and guide members for bringing together the first and the second sheet at the heating.

The heating members comprise a resistance wire connected to each core bar by a welded joint. Suitably, the resistance wire is welded to the end of the core bar and arranged in a recess of the core bar for abutment to at least a portion of the core bar in an abutment portion.

According to a first embodiment the core bars are individually arranged, each core bar having a welded resistance wire forming an electric circuit. In this embodiment the resistance wire and the core bar are connected with an electric power source, wherein current can be conducted through the resistance wire, the welded joint and back through the core bar, forming a closed electric circuit. A plurality of such core bar circuits may be connected to each other in series or in parallel. According to a second embodiment the core bars are arranged in pairs by means of a curve, wherein U-shaped core bar pairs are obtained. Each core bar pair is forming an electric circuit. The core bar pairs are designed with a bend in a vertical direction to avoid abutment of the corrugated sheet against the curves, to facilitate attachment of the core bars and to form closed electric circuits. Thus, a first core bar is connected with a second core bar through a curve. A first resistance wire is arranged at the first core bar by a first welded joint and a second resistance wire is arranged at the second core bar by a second welded joint. Further, an electric insulating layer is ar- ranged between the resistance wire and the core bar to avoid short-circuit in the abutment portion.

The resistance wires in the core bar pair are connected to an electric power source, wherein current can be conducted through the first resistance wire, the first welded joint, the first core bar, the curve, the second core bar, the second welded joint and, finally, the second resistance wire, forming a closed electric circuit. A plurality of core bar pairs may be connected to each other in series or in parallel. An electric insulating layer is arranged between each resistance wire to avoid short-circuit in the abutment portion.

According to one embodiment of the invention the core bars are made of steel to facilitate welding of the resistance wire, for example by laser welding. Suitably, the core bars are manufactured by cold drawing to obtain a suitable profile. The profile is cut to suitable length according to the application and is formed to separate core bars or U-shaped core bar pairs. The core bars are coated with an aluminium layer before the core bars are mounted in the device according to the invention. The aluminium layer is anodised, which is an oxidation process, wherein the outermost aluminium layer is transformed to aluminium oxide for effective electric insulation of the core bars. The aluminium oxide surface is removed in the end of the core bars where the resistance wires is to be welded to the core bars to allow welding of the resistance wires. For example, the aluminium oxide surface may be removed 5 mm from the ends of the core bars. The relatively rough aluminium oxide surface is then coated with a temperature resistant material, such as fluoroplastic, to reduce the friction between the core bars and sheets of material progressing over the core bars.

The resistance wires may be provided with a low-resistant material in a section of the resistance wires positioned before the portion of the resistance wires in which the sheets are welded together, whereby this portion of the resistance wires are kept cooler than the portion thereof having the welding function. Thus, the corrugated sheet slides easily over the cooler wire portion without sticking. In this section, the resistance wires may be coated with or entirely consist of the low-resistant material. The low-resistant material may be copper, silver, aluminium or the like.

Further, the resistance wires may be provided with a spring formation, keeping the resistance wires stretched also when they expand due to the heating of the wires or are pushed forward in the direction of the sheet due to the friction against the sheets.

The rolls for progression and applying of welding pressure against the core bars are provided with an elastic rubber covering. The flexibility of the rubber covering provides a longer welding distance. The welding distance is the distance the rubber covering of the rolls presses the outer sheets against the corrugated sheet. The increased welding distance provides an increased process rate.

Yet another advantage with elastic rolls, or elastic covering thereon, is that differences in pressure between the sheets and the rolls are levelled out, whereby the welding results between different welds is smoother than with inelastic rolls. Pressure differences may arise as a consequence of differences in tolerance regarding the common thickness of the core bars, coverings and resistance wires. Additionally, the rubber covering provides high friction against the sheets, preventing the sheets from sliding against the rolls. The elastic material may be rubber or elastomers, such as crude rub- ber, silicone rubber, buna-S (styrene rubber), polyurethane or any other suitable material. Consequently, each core bar presses against one roll with its wider base and against the opposite roll with its thinner resistance wire. Thus, an upper core bar presses against the upper roll with its wider base and against the lower roll with its resistance wire. A lower core bar presses against the lower roll with its wider base and against the upper roll with its thinner resistance wire. As the force against the upper and lower rolls, respectively, is the same from each core bar having a resistance wire, the pressure against the roll from the thinner resistance wire is substantially higher than the pressure from the wider base of the core bar against the opposite roll. Naturally, this is desirable from a welding point of view but contributes to the higher indentation of a roll having an elastic covering by the resistance wire than the core bar base. When a set of upper core bars and a set of lower core bars are used the upper roll will be subject to the higher pressure from the resistance wires and the lower pressure from the base of the core bars, alternately. The elastic roll is subject to indentation in a corresponding manner, wherein the roll is compressed further by the resistance wires than by the base of the core bars, every second indentation being deeper. The lower roll will be subject to alternating pressure in a corresponding manner as the upper roll. As a result the finished product of corrugated material is provided with irregular surfaces in the form of minor elevations across the material where each resistance wire has pressed.

By using rolls being more elastic in the peripheral direction and more inelastic in the axial direction this problem is avoided to a certain extent during maintenance of the longer welding distance. This can be obtained by us- ing rubber having different properties in different directions. Reinforced rubber may be used in the axial direction of the roll. Alternatively a two-layered rubber covering may be used. In the latter case, having a two-layered rubber, first the roll is covered with an inner layer of soft rubber, resulting in a high indentation distance in the peripheral direction or the production direc- tion. Then the inner rubber layer is covered with an outer layer of harder and non-compressible rubber allowing the indentation of the inner rubber layer but preventing any appreciable indentation difference in the rolls between one core bar base and the top of the next resistance wire. Consequently, an appreciable indentation difference is prevented in, for example, the upper roll between the base of an upper core bar and the resistance wire of the next lower core bar.

By the invention, the use of a plurality of different materials for the different layers in the corrugated product is enabled. Materials of different stiffness, toughness, friction and shock-absorption may, e.g., be chosen. The invention also enables a faster process start up without any extensive heat- ing of drums or the like. A manufacturing process may also be finished faster and be temporarily stopped.

The finished product has a plurality of the advantages of the corrugated cardboard, such as very good insulating and shock-absorbing properties, but has in addition higher moist-resistance and durability. Furthermore, packages having been manufactured of the finished product may be spray steam heated. Such packages neither emit dust particles. A material suitable in connection with the invention is blown polyethylene film with chalk as filler. For many applications, it is suitable with a film thickness in the range of 0,03- 0,4 mm.

Packages and other products that have been produced according to the invention are especially suitable in the food industry. Also in the pharmaceutical industry and the medical field, it may be advantageous to use the invention.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will now be described closer by embodiment examples, reference being made to the accompanying drawings, where Fig. 1 is a side view of an embodiment of the device according to the invention,

Fig. 2 is a cross-sectional view, which shows how a corrugated material according to a first embodiment is produced,

Fig. 3 is a cross-sectional view, which shows how a corrugated material according to a second embodiment is produced,

Fig. 4 is a cross-sectional view, which shows how a corrugated material according to a third embodiment is produced,

Fig. 5 is a cross-sectional view, which shows how a corrugated material according to a fourth embodiment is produced,

Fig. 6 is a side view of corrugation rolls for corrugation of the material according to one embodiment,

Fig. 7 is a cross-sectional view from the line B-B in Fig. 6 of the corrugation rolls in Fig. 6, Fig. 8 is a schematic view of the core bars according to a first embodiment,

Fig. 9 is a schematic view of the core bars according to a second embodiment,

Fig. 10 is a schematic view of the core bars in Fig. 9, illustrating the de- sign of the resistance wires,

Fig. 11 is a cross-sectional view of a core bar, and

Fig. 12 is a side view of one embodiment of the part B of the device according to Fig. 1, comprising an upper core bar having a resistance wire.

THE INVENTION

Fig. 1 shows in principal how a manufacturing line, which operates according to the invention, may be made. A first part A, which is shown with dash and dot lines, comprises a first roll 10, a second roll 11 and a third roll 12, all winded up with a suitable sheet material, as well as conventional sheet stretching members 13. The different sheet materials are brought together in a second part B and form different layers of the finished corrugated material. It should be observed that material thickness, mutual distance between different components and other geometrical relations in Fig. 1 as well as subsequent figures are not true to scale. A plurality of dimensions and distances have been changed in relation to real conditions in order to show features of the invention more clearly. The elements that are included in part A may all be made according to prior art. However, it is important to notice that various sheet materials, both thickness and the material as such, may be arranged on the different rolls. For most applications, it is suitable to use polyethylene (PE) and polypropylene (PP) with or without so called fillers. A suitable filler may be chalk.

Within the scope of the invention, entirely other materials may also be used. For instance, it is possible to use aluminum or other similar material in some layer in order to achieve high tightness against gas permeation. Materials that in itself cannot be heated together with the material of an adjacent sheet should be coated with or arranged next to a plastic layer.

Before a first sheet 16 that are to be corrugated or formed in wave shape is brought together with other sheets, it is suitably corrugated in a corrugation device. In the embodiment shown, the corrugation device comprises an upper corrugation roll 14 and a lower corrugation roll 15, which is described closer reference being made to Fig. 6 and Fig. 7. In such an em- bodiment, it may be suitable with pre-heating of the sheet 16 before the corrugation and/or heating of the corrugation rolls 14 and 15.

After the corrugation device, the corrugated sheet 16 is led in between at least one set of upper core bars 17 and one set of lower core bars 18. These are described further below. An upper sheet 19 from the first roll 10 and a lower sheet 20 from the third roll 12 are brought together with the corrugated sheet 16 at the core bars 17 and 18. The core bars 17 and 18 ex- tend in the common long direction V of the sheets, which is indicated at the corresponding arrow in Fig. 1. In certain applications, the corrugation device may be integrated with the core bars. The two sets of core bars 17 and 18 are suspended behind or outside the sheets in a way not shown closer. The sheets are heated by the core bars 17 and 18 and joined together to a corrugated sheet material through co-operation with an upper press roll 21 and a lower press roll 22, which also feeds the sheets forward. After the joining, the ready-formed sheet material is brought further in the direction of the arrow V in a conventional way by an advancing upper driving roll 23 and an advancing lower driving roll 24. The driving rolls 23 and 24 are included in a third part C, which in a conventional way may comprise at least one cutting mechanism 25, which is laterally adjustable to cut off the sheet material in suitable width, and one cutting mechanism to cut off the sheet material in suitable length. In the embodiment shown, the cutting mechanism for longi- tudinal cutting comprises an upper knife 26 and a lower knife 27 co-operating therewith. The knives 26 and 27 suitably move up and down and cut off the sheet material in sheets of suitable length. The size of the sheets is, to a large extent, dependent on the application for which they are intended. The third part C constitutes not in itself part of the invention and may be given another design depending on the application in question.

In Fig. 2, an example is shown of how the core bars may be arranged when a sheet material having three layers, one of which is corrugated, is to be produced. In this case, an upper line of core bars 17 is arranged with a certain mutual distance between adjacent core bars. A lower line of core bars 18 is arranged with the same mutual distance, but displaced in relation to the upper line, so that the space between the core bars is filled out with space for an intermediate sheet of material.

All core bars have, in this embodiment, triangular cross-section, but other shapes may be chosen depending on the application in question. The sheet 16 that is to be corrugated runs between the upper line of core bars 17 and the lower line of core bars 18. The upper sheet 19 runs exactly above the upper line of core bars 17 and will be pressed against the sheet 16 between the lower line of core bars 18 and the upper press roll 21. Correspondingly, the lower sheet 20 runs exactly below the lower line of core bars 18 and will be pressed against the sheet 16 between the upper line of core bars 17 and the lower press roll 22.

The pressure that is effected by the press rolls 21 and 22 may in an alternative embodiment be generated by reciprocating press plates. The press plates are quickly brought towards the core bars and press together a portion of the sheets in the way described above during the phase when the sheets are welded together. Next, the press plates are retracted, so that the sheets may be fed forwards and a new portion of the sheets comes in the correct position for welding together.

At least parts of the portions of the core bars 17 and 18 which abut against the sheet 16 and/or the upper sheet 19 and/or the lower sheet 20 are provided with heating members 28. By the heating members 28, heat is transferred to abutting and adjacent material sheets to such an extent that a joining of the sheets is achieved. The heating is local in smaller contact surfaces, which means that the desired temperature may be attained fast. In a preferred embodiment, the joining takes place in connection with the press rolls 21 and 22 driving the sheets forwards and, consequently, the sheets being in motion. In the other parts, the core bars 17 and 18 are not heated.

In a simple embodiment, the heating member 28 comprises electric heating conductors, which extend in the longitudinal direction of the core bars and which are supplied from conventional power supply units (not shown). It is also possible to transmit heat to abutting sheet portions in another way. The requisite energy may, e.g., be supplied to the contact surfaces through ultrasound, laser and other similar forms of energy permitting local or directed transmission of energy. The transmission of energy may also take place inductively or in a similar way and then be concentrated in the core bars 17 and 18, so that heating takes place locally. As mentioned above, there may be different material compositions in the different sheets. Aluminium foil or a similar material may be used in some layer. In certain applications, it is suitable to use an intermediate layer, e.g. - the sheet 16, with a lot of filler and two outer layers with less filler. Thereby, a sheet material is effected, which resists high load in the channel direction at the same time as the outer layers are very elastic. Such a sheet material is very suitable for use to packaging.

The material thickness may also vary in the different layers and according to the application in question. The sheet 16 that is to be corrugated may in that connection be made in a considerably thicker and stronger material than the other layers in order to obtain very good properties as for durability and impact resistance. In the same way, also other layers may be given desired properties as for, e.g., durability and impact resistance.

Fig. 3 shows an alternative embodiment with a third set of core bars 29 inserted between the upper line of core bars 17 and the lower line of core bars 18. The third line of core bars 29 has a cross-section adapted to other core bars and is provided with a second set of heating members 28' and 28". In the embodiment shown, the core bars 29 are made with square cross- section. Also the sheet which is to be corrugated is doubled in an upper cor- rugation sheet 16 and a lower corrugation sheet 16". The double heating members 28' and 28" enable, together with the heating members 28 of the upper core bars 17 and the lower core bars 18, the composition of a more complex sheet material. As is seen in Fig. 3, the result is two outer plane layers and two inner corrugated layers. Fig. 4 shows a simplified embodiment with only two material sheets. A lower sheet 20 is plane and a sheet 16 is corrugated in the same way as has been described above. In Fig. 5, an alternative embodiment is shown with a first corrugated sheet 16 and a second corrugated sheet 16'. In other respects, the embodiments according to Fig. 4 and Fig. 5 equal the embodi- ments described above. The side view in fig. 6 schematically shows how a device for corrugation of the sheet 16 may be made. The sheet 16, as well as the upper corrugation roll 14 and the lower corrugation roll 15 may be pre-heated at the corrugation. With reference also to Fig. 7 the upper corrugation roll 14 and the lower corrugation roll 15 are designed with grooves 45 and nodular tops 46. The tops 46 extend from the periphery of the corrugation rolls 14, 15 and are arranged mutually in parallel around the entire circumference of the corrugation rolls 14, 15. Then, the tops 46 of the upper corrugation roll 14 overlap the tops 46 of the lower corrugation roll 15. Thus, the tops 46 and grooves 45 of the upper corrugation roll 14 are horizontally displaced in relation to the tops 46 and grooves 45 of the lower corrugation roll 15. Further, the tops 46 of the upper corrugation roll 14 extend into the grooves 45 of the lower corrugation roll 15.

The shape of the tops and the grooves, respectively, are adapted to the shape of the core bars, wherein the sheet is corrugated for fitting between the upper and lower line of core bars 17, 18 before it reaches the core bars.

Preferably, the tops 46 are rounded for suitable stretching of the sheet in and around the portion of the sheet abutting against the tops 46. The tops 46 are thinner than the grooves 45 to obtain a more favourable corrugation of the sheet to be corrugated and to improve the properties of the corrugated sheet. In that way the corrugated sheet is provided with linear portions having thicker profile and more stretched, and thus thinner, folds. The sheet also runs clear in the space between the tops 46 and the grooves 45.

The design of the core bars 17, 18 is more evident from Fig. 8-12. It should be observed that material thickness, mutual distance between different components and other geometrical relations in the figures are not true to scale. For example, the core bars may be considerably more elongated in the longitudinal direction. With reference to Fig. 8 a first embodiment of the core bars 17, 18 is illustrated, wherein a first core bar 30 is arranged with a bend in the vertical direction. A core bar of the upper set of core bars 17 is arranged with a bend in the vertical direction, the rear portion of the core bar obtaining a higher position than the portion thereof designed for abutment against the sheets. A core bar of the lower set of core bars 18 is arranged with a bend in the verti- cal direction, the rear portion of the core bar obtaining a lower position than the portion thereof designed for abutment against the sheets. The bend in the vertical direction of the core bar facilitates attachment of the core bars by means of fastening means, which is not described further. Also, abutment of the rear portion of the core bars or the fastening means against the sheets, which consequently is stopping sheets incoming towards the core bars, is avoided.

Fig. 9 illustrates one embodiment of the core bars 17, 18, wherein a first core bar 30 and a second core bar 31 are arranged in parallel in pairs and mutually connected by means of a curve 32, forming a substantially horizontal U-shaped core bar pair 33. Further, such a core bar pair 33 is suitably designed with a bend in a vertical direction and the same design is used for upper as well as lower core bar pairs, wherein the curve 32 obtains a higher position than the ends for an upper core bar pair and a lower position for a lower core bar pair. The curve 32 is designed to facilitate the fas- tening of the core bars to a suitable holder, which is not described further. A plurality of core bar pairs 33 may be positioned in parallel with the ends extending in the long direction, forming the upper set and the lower set of core bars 17, 18. The extension of the ends in the long direction results in that the ends extend between the rolls 21 , 22. Preferably, the upper core bars 17 and the lower core bars 18 are mutually displaced in the horizontal direction across the long direction so that they overlap each other and each set of core bars can weld together the corrugated sheet 16 with the plane sheets 19, 20, respectively.

The core bars 17, 18 further comprise heating members 28 in the form of electric resistance wires for heating material sheets adjacent the core bars. According to one embodiment of the invention, which is particularly il- lustrated in Fig. 10 and 11 , a first resistance wire 34 is arranged in a recess along the surface of the first core bar 30 and a second resistance wire 35 is arranged in a recess along the surface of the second core bar 31. Thus, The resistance wires 34, 35 abut against the core bars 17, 18 in a contact por- tion. Fig. 11 further illustrates the material composition of the core bars according to one embodiment. The core 36 of the core bars is preferably made of steel or any other suitable material, and may consequently be manufactured by cold drawing, hot drawing or rolling, wherein the somewhat triangular profile is obtained. Other shapes may be chosen according to the current application. The profile may then be cut off and curved as desired. Further, the core bars may be coated with an aluminium layer 37, of which the outermost layer is transformed into aluminium oxide 38 by anodisation.

Anodisation is an oxidation process transforming the outermost aluminium layer to aluminium oxide 38. Aluminium oxide is an electric insulat- ing, durable and temperature resistant ceramic material and, consequently, suitable for insulation of the core bars to avoid short-circuit between the core bar and the resistance wire before these are welded together. Naturally, other suitable materials may be used. Further, the relatively rough surface of the aluminium oxide may be coated with a temperature resistant material, such as fluoroplastic or any other suitable material, to fill the pores of the aluminium oxide. Thus, the aluminium oxide 38 comprises a surface having low friction, which reduces the friction against the material sheets so that the sheets easily can cross the surface of the core bars also when the aluminium oxide is hot. Alternatively, the resistance wire may be insulated in a corresponding manner as the core bar to avoid short-circuit or an electric insulated conductor may be enclosed in the core bar.

According to one embodiment of the invention the aluminium oxide is removed from the ends of the core bars to enable welding of the resistance wires thereto. For example, the aluminium oxide may be removed a distance of about 5 mm from the ends of the core bars. One resistance wire is welded to each core bar and is arranged in the recess of the core bar so that the first resistance wire 34 is fastened to the end of the first core bar 30 by a first welded joint 39 and the second resistance wire 35 is fastened to the end of the second core bar 31 by a second welded joint 40. Thus, a closed electric circuit is obtained for each core bar pair 33, which can conduct current through the first resistance wire 34, the first welded joint 39, the first core bar 30, the curve 32, the second core bar 31 , the second welded joint 40 and, finally, the second resistance wire 35 when current is supplied to the free ends of the resistance wires from an electric power source. Due to that the core bars are overall electric insulated by the aluminium oxide coating, except in the welded joints, short-circuit of other surfaces of the resistance wires abutting against the core bars is avoided. The core bar pairs 33 may be connected in series or in parallel.

Preferably, the resistance wire is arranged in the recess of the core bar for abutment against the core bar along the portion of the upper and lower core bars interacting.

The welded joint between the resistance wire and the core bar is improved by the core of the core bar being of steel. The core may also be made in aluminium, even though steel is preferred. A core of steel also pro- vides a more temperature resistant core bar than a core bar entirely made of aluminium.

Preferably, the resistance wires are laser welded. This provides a welded joint having excellent electric conductivity, which hardly varies in resistance between different welds, which would result in different tempera- tures and, consequently, different welding results of the sheets at the different resistance wires/core bars. Further, strong holding of the wire in the longitudinal direction thereof is obtained. Other similar methods, such as spot welding and soldering, provides a similar result and may thus be used for welding the resistance wire to the core bar according to the invention. With reference to Fig. 12 the corrugated sheet 16 progresses across the core bars a section S before the tops of the corrugated sheet 16 are brought in contact with the plane sheet 19, and the rolls 21 , 22 is driving the sheets forward across the resistance wires and the core bars for welding together in a section X. In this embodiment of the invention the resistance wires are provided with a low-resistant material 41 in the section S and on to, or almost on to, the point in which the tops of the corrugated sheet 16 abut against the plane sheet 19. Thus, the low-resistant material 41 reduces the resistance and, hence, the temperature in this section of the resistance wire so that the corrugated sheet 16 not is stuck in the section S.

Preferably the resistance wires are arranged with a low-resistant ma- terial in portions of the resistance wires, which not are to be heated. Thus, the entire resistance wires apart from the section X may be provided with a low-resistant material.

To obtain a low-resistant portion of the resistance wires, these may be coated with a low-resistant material 41 , such as, copper, silver, aluminium or any other suitable material. The resistance wires may also be spliced with a low-resistant material or wires thereof. Wires for splicing may comprise low- resistant materials corresponding to those of the coating embodiment and alloys of different materials providing low resistance but also the possibility of splicing the materials.

Further, the free end of the resistance wire is provided with a spring formation 42 having a lock washer 43, wherein the resistance wire is kept in a stretched position during the welding. Thus, the advancing action acting upon the resistance wire by friction against the sheets and the rolls is counteracted and the resistance wire is kept stretched, even though the resis- tance wire is expanded due to heating. The spring formation 42 suitably acts upon the wire with a force which is greater than the friction against the sheets.

Claims

1. Device for the manufacture of corrugated material, first members being arranged for feeding at least one first plane sheet and one second sheet of plastic material arranged in wave shape and running over core bars (17, 18) and second members being arranged for bringing together and adhering the sheets, c h a ra c t e ri z e d in

that heating members (28) are arranged for transferring heat locally to contact surfaces between the core bars and at least one sheet (16; 19, 20) abutting against the core bars and including plastic material,

that the core bars (17, 18) extend in the longitudinal direction of the sheet (16; 19, 20), that . guide members (17, 18; 21 ; 22) are arranged for bringing together the first and the second sheet in an abutment portion and for welding together in the portion heated by the heating members (28), that the heating member (28) comprises an electric resistance wire

(34; 35) connected with a core bar, and that the electric resistance wire (34; 35) is electrically connected to an electric power source (44) in series with a core bar (17; 18, 30; 31 ) associated with the resistance wire (34; 35).

2. Device according to claim 1 , wherein an electric insulating layer is arranged between the electric resistance wire (34; 35) and the core bar (17; 18, 30; 31) associated with the resistance wire (34; 35).

3. Device according to claim 2, wherein the core bars comprise an aluminium oxide layer for electric insulation of the core bars.

4. Device according to claim 3, wherein the core bars comprise a core (36) of steel.

5. Device according to claim 4, wherein the core of the core bars is covered with an aluminium layer (37) having an outer layer transformed to aluminium oxide by anodisation.

6. Device according to claim 5, wherein the resistance wires are connected with the core bars by a welded joint (39, 40).

7. Device according to claim 6, wherein the welded joint is obtained by laser welding.

8. Device according to claim 6, wherein the welded joint is obtained by spot welding.

9. Device according to claim 4, wherein the core of the core bars has been manufactured by cold drawing, hot drawing or rolling.

10. Device according to claim 3, wherein the core bars are manufactured by extrusion.

11. Device according to claim 9, wherein the core bars are arranged in pairs connected with a curve (32), forming a substantially horizontal U- shaped core bar pair having a bend in vertical direction.

12. Device according to claim 11 , wherein an electric power source

(44) is connected with the resistance wires, the welded joints, the core bars and the curve for obtaining a closed electric circuit.

13. Device according to claim 3, wherein the core bars comprise an outer coating of temperature resistant material having low friction to reduce the friction of the core bars against the sheets.

14. Device according to claim 13, wherein the coating is a fluoroplas- tic.

15. Device according to claim 1 , wherein the resistance wires comprise a low-resistant material (41), such as copper, silver, aluminium or a suitable alloy, covering a section S.

16. Device according to claim 1 , wherein the resistance wire is provided with a spring formation (42) for stretching the resistance wire.

17. Device according to claim 9, wherein at least two core bars having resistance wires are connected in series or in parallel.

18. Device according to claim 11 , wherein the core bar pairs are connected in series or in parallel.