TORKKEL, Olli (Jalavatie 12, Parola, FI-13720, FI)
HOVIKORPI, Jari (Seiskarintie 5, Kotka, FI-48310, FI)
VÄHÄVIHU, Pekka (Kihtersuonkatu 5, Hämeenlinna, FI-13210, FI)
TORKKEL, Olli (Jalavatie 12, Parola, FI-13720, FI)
HOVIKORPI, Jari (Seiskarintie 5, Kotka, FI-48310, FI)
Claims
1. A method for manufacturing conical paperboard cups from a curvilinear side wall blank (16) and a circular bottom blank (14) which are of polymer-coated pa- perboard, in which method at least one of two side seam areas (17, 19) of the side wall blank (16) is heated for softening the polymeric coating, the side seam areas (17, 19) are laid on top of each other and the side seam areas (17, 19) are pressed together for forming a side seam of the cup, characterised in that the side seam area (17) is heated by a laser beam (28, 40, 128) which is guided to move over the side seam area (17) as the blank (16) remains stationary.
2. A method according to claim 1, characterised in that both of the side seam areas (17, 19) of the side wall blank (16) are heated by the laser beam before pressing them together.
3. A method according to claim 1 or 2, characterised in that a bottom seam area (18) of the side wall blank (16) is heated by a laser beam (28, 41) which is guided to move over the bottom seam area (18) as the blank (16) remains stationary, after which the bottom blank (14) is brought into contact with the bottom seam area (18).
4. A method according to any one of preceding claims, characterised in that a laser beam (40, 41) is guided to move over the seam area (17, 18) being heated by means of a mirror (44, 46) or a prism (48) which is arranged to turn or rotate around an axis (45, 47, 50) which is in a perpendicular plane in relation to the incoming direction of the laser beam (40, 41).
5. A method according to any one of claims 1-3, characterised in that the laser beam (40) is directed at the seam area (17) being heated by means of a mirror (42) which is moved over the seam area (17) being heated.
6. A method according to claim 1 or 2, characterised in that the bottom seam area (18) of the side wall blank (16) is heated by means of the stationary laser beam (41) as the blank (16) moves past the laser beam (41), after which the bottom blank (14) is brought into contact with the bottom seam area (18).
7. A method according to claim 3 or 6, characterised in that, after the bottom blank (14) has come into contact with the bottom seam area (18), an inner surface (34) of the cup bottom area is heated by means of a laser beam (33), the lower part of the side wall blank (16) is folded on top of an edge flange of the bottom blank (14) and the edge flange and the lower part of the side wall blank (16) are pressed together for forming the bottom seam of the cup.
8. A method according to claim 7, characterised in that the laser beam (33) is directed at the inner surface (34) of the cup bottom area by means of a mirror (31) or equivalent which distributes the radiation evenly in the area (34) being heated.
9. A method according to any one of preceding claims, characterised in that a collimated laser beam is used.
10. A method according to any one of preceding claims, characterised in that the laser beam is produced as pulses which have a defined duration and power.
11. A method according to any one of preceding claims, characterised in that the power of the laser beam or pulse is adjusted so that a different heating power is applied to the different parts of the seam area being heated.
12. A method according to any one of preceding claims, characterised in that the surface temperature of the polymeric layer heated with the laser beam is measured and the power of the laser beam is adjusted on the basis of the measured surface temperature.
13. An apparatus for manufacturing conical paperboard cups from a curvilinear side wall blank (16) and a circular bottom blank (14) which are of polymer-coated paperboard, which apparatus comprises heating means for heating at least one of two side seam areas (17, 19) of the side wall blank (16) for softening the poly- meric coating and means for laying the side seam areas (17, 19) on top of each other and pressing together for forming a side seam of the cup, characterised in that the heating means comprise means (34) for producing a laser beam and means for directing the laser beam (28, 40, 128) at the side seam area (17, 19) of the side wall blank (16) so that the laser beam (28, 40, 128) scans over the side seam area (17, 19) being heated as the blank (16) remains stationary.
14. An apparatus according to claim 13, characterised in that the means for directing the laser beam (26) at the side seam area (17) comprise a mirror (42) which is arranged to move over the side seam area (17).
15. An apparatus according to claim 13, characterised in that the means for directing the laser beam (26) at the side seam area (17) comprise a mirror (44) which is arranged to rotate around its axis (45) between two extreme positions which axis (45) is in a perpendicular plane in relation to the incoming direction of the laser beam (40).
16. An apparatus according to claim 13, characterised in that the means for directing the laser beam (40) at the side seam area (17) comprise a prism (48) which is arranged to rotate around its axis (50) which axis (50) is in a perpendicular plane in relation to the incoming direction of the laser beam (40).
17. An apparatus according to any one of claims 13-16, characterised in that it comprises first means (42, 44, 48) for directing the first laser beam (40) at the first side seam area (17) of the side wall blank (16) and second means for directing the second laser beam at the second side seam area (19) of the side wall blank (16) which is located on the opposite side of the side wall blank (16) at its opposite edge.
18. An apparatus according to any one of claims 13-17, characterised in that it further comprises means (46) for directing the laser beam (41) at the bottom seam area (18) of the side wall blank (16) so that the laser beam (41) scans over the bottom seam area (18) being heated as the blank (16) remains stationary.
19. An apparatus according to any one of claims 13-17, characterised in that it further comprises means (43) for directing the laser beam (41) at the bottom seam area (18) of the side wall blank (16) so that the laser beam (41) remains stationary as the side wall blank (16) moves in relation to the laser beam (41).
20. An apparatus according to any one of claims 13-19, characterised in that it further comprises means for guiding a laser beam (33) into the bottom area of a cup (21) being manufactured, which means comprise a guide ring (29) in which is joined an arm (30) at one end of which there is a mirror (31), which arm (30) is arranged to rotate along an annular track defined by the guide ring (29) and which mirror (31) is located so that in each position of the arm (30) the laser beam hits the mirror (31) and reflects through it into the cup bottom area (34) being heated.
21. An apparatus according to any one of claims 13-20, characterised in that the means for producing the laser beam (34) comprise a carbon dioxide laser.
22. An apparatus according to any one of claims 13-21, characterised in that the means for producing the laser beam (34) are arranged to produce a collimated laser beam.
23. An apparatus according to any one of claims 13-22, characterised in that the means for producing the laser beam (34) are arranged to produce recurring laser pulses which have a defined duration and power.
24. An apparatus according to claim 23, characterised in that the means for producing the laser beam (34) are arranged to produce laser pulses the power of which during the pulse is adjustable so that different heating powers can be ap- plied to different points of the seam area being heated.
25. An apparatus according to any one of claims 13-24, characterised in that it comprises means for measuring the surface temperature of the area heated with the laser beam and means for adjusting the power of the laser beam on the basis of the measured surface temperature. |
Method and apparatus for manufacturing paperboard cups
The invention relates to a method for manufacturing conical paperboard cups from a curvilinear side wall blank and a circular bottom blank which are of polymer-coated paperboard. For manufacturing a cup, at least one of two side seam areas of the side wall blank is heated for softening the polymeric coating, the side seam areas are laid on top of each other and the side seam areas are pressed together for forming the side seam of the cup.
The invention also relates to an apparatus for manufacturing conical paperboard cups from a curvilinear side wall blank and a circular bottom blank which are of polymer-coated paperboard. The apparatus comprises heating means for heating at least one of two side seam areas of the side wall blank for softening the polymeric coating and means for laying the side seam areas on top of each other and pressing them together for forming the side seam of the cup.
The types of paperboard cups described are used, inter alia, when consuming cold and hot beverages.
In cup manufacturing, the side seam of the cup is formed by laying two opposite edges of the side wall blank on top of each other and by sealing them together. The bottom seam of the cup is formed by arranging the bottom blank provided with an edge flange inside a cone formed by the side wall blank, by bending the lower edge of the side wall blank on top of the edge flange of the bottom blank and by sealing the parts together. It is possible to do the sealing by means of glue, but most often it is implemented by heat sealing. For heat sealing, at least one of the seam areas to be joined is heated for melting the polymeric layer on the surface of the paperboard, after which the heated areas are laid on top of each other and pressed together. As the polymeric layer cools down, the surfaces adhere to each other providing a durable and liquid-proof seam.
In heating surfaces being sealed, inter alia, hot air blowing, ultrasound, combustion gases produced by a gas burner, and heated contact elements have been used. Most frequently, hot air blowing has been used for melting the polymeric layer. Specification US 4490130 describes a method in which hot air is produced by guiding compressed air past electric resistors, after which the air is guided into areas being heated via nozzle holes in movable nozzle parts. From specification US 6852071 is known the use of gas-fired heating elements. Methods according to prior art include many problems. The use of compressed air is expensive and it brings about considerable noise problems. The energy requirement of electric resistors is great. Heat is brought onto the surface spotwise, whereby it is distributed unevenly in the area being heated. Heat can also spread in an unnecessarily large area. In the case of a production break, overheating, which can cause a risk of fire and energy losses, occurs easily. The device is not immediately ready for use, but the resistors have to be heated for some time in order to achieve an even working temperature. The use of gas burners involves a risk of fire.
The use of a laser beam is known as such in the heat sealing of packing materials, but no one has earlier suggested using it in manufacturing conical paperboard cups. From specification EP 0147833 is known a method for forming the side seam of a tubular article by utilising laser heating. From specification EP 0237192 is known a method for heat-sealing plastic-coated surfaces by laser heating, especially for forming the side seam of a container. In both known sealing methods, a material web or blanks on the conveyor are linearly conveyed past a laser beam which remains stationary. These methods are well applicable to form the linear side seams of articles having the shape of a cylinder or a rectangular prism, but they do not allow sealing conical cups the side wall blank of which is curvilinear instead of rectangular. Furthermore, the annular bottom seam of the cup cannot be produced with known methods.
The object of the invention is to solve problems related to prior art.
A method according to the invention is characterised by what is presented in the characterising part of claim 1.
Again, an apparatus according to the invention is characterised by what is presented in the characterising part of claim 13.
In the method according to the invention, the side seam areas of the side wall blank or at least one of them are heated with a laser beam which is guided to move over the side seam area as the blank remains stationary, after which the side seam areas are laid on top of each other and pressed together for forming the side seam of the cup.
It is also possible to heat the bottom seam area of the side wall blank with a laser beam which is guided to move over the bottom seam area being heated as the blank remains stationary, after which the bottom blank is brought into contact with the bottom seam area.
It is possible to make the laser beam scan over the seam area by guiding the travel of the laser beam by means of mirrors or by making the mirror guiding the laser beam to travel over the seam area e.g. by means of a servo motor. The operation of the laser beam is advantageously synchronised with the motions of the paper- board blank in the cup making machine so that the laser beam scans over the seam area as the blank remains stationary and, as the blank is moving, the laser beam is switched off.
The manufacture of a paperboard cup in known cup making machines consists of repeated transition stages and stop stages. It is advantageous to heat the side seam areas of the cup during a stop stage. The bottom seam area can be heated either during a stop stage or during a transition stage. In the latter case, the blank moves past the stationary laser beam.
Advantageously, the laser beam is also used in finishing the bottom seam of the cup. Then, the laser beam is directed at the inner. surface of the cup bottom area by means of a mirror or a prism which distributes radiation evenly into the whole area being heated. After this, the lower edge of the side wall of the cup is folded on top of the edge flange of the bottom part and the parts are pressed together for providing a strong and durable bottom seam.
The apparatus for manufacturing paperboard cups comprises heating means which include means for producing a laser beam and means for directing the laser beam at the side seam area of the side wall blank so that the laser beam scans over the side seam area being heated as the blank remains stationary.
The means for directing the laser beam at the side seam area can comprise a mir- ror or a prism which is arranged to turn or rotate around an axis which is in a perpendicular plane in relation to the incoming direction of the laser beam.
Alternatively, the means for directing the laser beam at the side seam area can comprise a mirror and means for moving it over the seam area.
Advantageously, the apparatus for manufacturing paperboard cups also comprises means for guiding the laser beam into the bottom seam area of the side wall blank before joining the side wall blank and the bottom blank together. The laser beam can be guided into the bottom seam area by means of a mirror or a prism which turns or rotates around its axis. Alternatively, the mirror can be guided to move over the bottom seam area being heated. A third alternative is to use a stationary mirror which directs the laser beam at the bottom seam area in the stage in which the side wall blank moves past the mirror.
Furthermore, the apparatus for manufacturing paperboard cups can comprise means for guiding the laser beam into the bottom area of the cup before finishing the bottom seam of the cup.
A laser beam suitable for heating" a thin polymeric coating is advantageously produced by means of a carbon dioxide laser. No focusing lenses are used in guiding the laser beam. The laser beam is collimated i.e. made parallel, whereby it maintains its width substantially constant even though the distance to the point to be heated varies.
The laser beam can be produced as recurring pulses which have a defined duration and power. Then, each laser pulse produced with the laser and moved by means of the mirror is arranged to heat one of the seam areas of the blank. After this, a next blank is transferred into the same position in the corresponding seam area at which a next movable laser pulse is directed. Thus, no power losses occur during the time when the blank is being transferred from one position to another. If the bottom seam area is heated by a stationary laser beam during the transfer of the blank, the duration of the laser pulse corresponds to the time which is spent to the transfer of the blank past the laser beam from one position to another.
Laser heating is a quick, accurate and effective way to soften the polymeric coating of the seam areas, because of which the novel heat sealing method is well applicable to the serial production of paperboard cups at current speeds which can be up to 300 cups/minute. The advantages of laser heating are the accurate control of temperature and heating time and the precise outline of the area being heated. Only those areas are heated in which adhesion is required. Operating costs are low and need for maintenance small compared e.g. with hot air blowing. A relatively extensive temperature range can be used. The consumption of compressed air is substantially decreased, which also diminishes the noise level. Savings in energy are considerable. A typical cup machine requires an electric power of around 40 kW solely for heating compressed air, whereas the power requirement of the laser
unit is estimated to be 2 kW at maximum. The temperature profile is even, whereby there is no risk of forming burn marks on the cups. The seams of the cup become tight, whereby leakings and pinhole problems in the coating are avoided.
The invention will now be described with reference to the figures of the accompanying drawings, to the details of which the invention is by no means intended to be narrowly defined.
Fig. 1 shows the principle of manufacturing paperboard cups.
Fig. 2 illustrates the heating of the seam areas of a side wall blank as a laser head moves over an area being heated.
Fig. 3 illustrates the heating of the seam areas of a side wall blank as a laser beam is guided by means of mirrors over the area being heated.
Fig. 4 shows an apparatus for heating the inner surface of a cup bottom by a laser beam.
Fig. 5 shows the heating of the inner surface of a bottom by a laser beam which is guided to travel over the area being heated.
Fig. 6 shows an arrangement corresponding to Fig. 2 in which the mirror guiding the travel of the laser beam is arranged to move over the area being heated.
Fig. 7 shows an arrangement corresponding to Fig. 3 in which the laser beam is guided by means of a rotating mirror into the area being heated.
Fig. 8 illustrates the use of a prism for guiding the laser beam into the area being heated.
Fig. 1 schematically shows a part of a paperboard cup making machine. The apparatus comprises a cam turret 10 and a mandrel turret 11 both of which rotate step by step around their axes in the direction indicated by the arrow. The mandrel turret 11 includes a group of radially extending mandrels 12 a head 13 of which can be pushed outwards and pulled inwards. The mandrels 12 move along from one position to another as the mandrel turret 11 rotates step by step around its axis. Each position of the mandrel 12 forms one work station in which one work stage of cup manufacturing is performed. The work stages are designated with reference numbers S1-S8. By such a cup machine, it is possible to manufacture up to 300 cups a minute, whereby one work stage with its following transfer takes around 0.2 seconds.
Into the first work station Sl is brought a bottom blank 14 of a cup which is fastened in the mandrel head 13 by means of suction. The mandrel head 13 moves to the position pulled inwards. In the second work station S2, an edge flange 15 folding outwards from the plane surface of the bottom blank 14 is stiffened.
Simultaneously, the cam turret 10 receives a side wall blank 16 cut of a paper- board web or taken from a storage magazine and transfers it to a heating stage in which its side seam areas 17, 19 and bottom seam area 18 are heated. After this, the cam turret 10 transfers the side wall blank 16 at the point of the work station S3 where the blank 16 is wrapped around the conical mandrel 12 so that the heated side seam areas 17, 19 of the side wall blank 16 go on top of each other. The bottom blank 14 is pushed by means of the mandrel head 13 outwards so that the edge flange 15 comes into contact with the heated bottom seam area 18 of the side wall blank 16. The side seam areas 17, 19 are pressed together.
In the fourth work station S4, the cup bottom is heated by means of a heating device 20 shown only as reference. In the fifth work station S5, the lower edge of the side wall 16 is folded against the edge flange 15 of the bottom blank 14. In the sixth work station S6, the bottom seam of a cup 21 is finished by pressing the
seam areas of the side wall and the bottom together. In the seventh work station S7, the cup 21 is detached from the mandrel 12 and transferred to the following work stages. In the eighth work station S8, the mandrel 12 is ready to start a new cycle for manufacturing the next cup.
The present invention relates to sealing paperboard cups and especially to heating areas to be sealed before joining them together. Paperboard material is heated at two points: for the first time before manufacturing the side seam of the cup as the side wall blank 16 is on the cam turret 10 and for the second time before manufac- turing the bottom seam of the cup as the cup is in the work station S4.
The side wall blank 16 shown in Figs. 2 and 3 comprises a curvilinear lower edge 22, a curvilinear upper edge 23 and two side edges 24 and 25 the mutual distance of which increases from the lower edge 22 of the blank towards its upper edge 23. The blank 16 is manufactured from paperboard material at least one and advantageously both surfaces of which are coated with polymer which is typically polyethylene (e.g. PELD or PEHD). The polymeric coating makes the cup material heat-sealable and impermeable to liquid. Additives may be added to the polymeric material, e.g. pigment, for ensuring that the coating will heat up and soften owing to the effect of the laser beam.
The apparatus shown in Fig. 2 for heating the seam areas 17, 18 of the side wall blank 16 comprises two laser heads 26 and 27. The first laser head 26 is arranged to heat the first side seam area 17 which is located next to the first side edge 24 of the side wall blank 16. The laser head 26 is connected by means of a radiation conductor 35 to a laser 34 and arranged to move over the side seam area 17, whereby a laser beam 28 discharging from the laser head 26 heats up the seam area 17. Reference numbers 26 and 26' show two extreme positions of the laser head 26. The quick and accurate to-and-fro motion of the laser head 26 is pro- vided e.g. by means of a servo motor.
The second laser head 27 heats up the bottom seam area 18 which is located next to the lower ^ edge 22 of the blank 16. In Fig. 2, the second laser head 27 is stationary, whereby the bottom seam area 18 is heated as the side wall blank 16 moves transferred by the cam turret 10 a step forward. In this arrangement, first the side seam area 17 is heated by means of the moving laser head 26 as the blank 16 remains stationary, and after that the bottom seam area 18 is heated as the blank 16 moves past the stationary laser head 27. Below the blank 16, there is advantageously a third laser head which is not shown in Fig. 2. This third laser head heats up the side seam area 19 (Fig. 1) on the opposite i.e. lower surface of the blank 16 and parallel to the second side edge 25 which area is joined together with the first side seam area 17 when forming the cup.
In an alternative arrangement, also the second laser head 27 is installed movable, whereby the bottom seam area 18 can be heated at the same time as the side seam area 17 as the blank 16 remains stationary and only the laser head 26, 27 moves. Then also such an embodiment is possible in which the one and the same laser head heats up both the seam areas 17 and 18 on the upper surface of the side wall blank 16.
Fig. 3 shows an alternative arrangement for heating the side seam area 17 and the bottom seam area 18 of the side wall blank 16. In this case, both laser heads 126 and 127 are stationary and they are provided with an optical reflection system (not shown in the figure) by means of which a laser beam 128 is guided to scan over the whole side seam area 17 and correspondingly over the bottom seam area 18. Reference numbers 128 and 128' show two extreme positions of the laser beam 128. The optical reflection system comprises turning and/or rotating mirrors which guide the travel of the laser beam according to commands given by a computer.
Fig. 6 shows an improved method for heating the side seam area 17 and the bottom seam area 18 of the side wall blank 16 by means of two laser beams 40 and
41 produced by lasers 34. The first laser beam 40 is guided by means ' of a first mirror 42 into the side seam area 17. The mirror 42 is installed at a fixed angle in relation to the incoming direction of the laser beam 40 and it is arranged to move between two extreme positions in the direction shown by the arrow. The quick and even to-and-fro motion of the mirror 42 can be provided e.g. by means of a servo motor. The second laser beam 41 is guided by means of a second mirror 43 into the bottom seam area 18 the shape of which follows the curvilinear shape of the lower edge 22 of the blank 16. The second mirror 43 guiding the travel of the laser beam 41 is so located in a fixed position. Thus, the bottom seam area 18 is heated by the laser beam 41 in the stage in which the side wall blank 16 moves transferred by the cam turret 10 (Fig. 1) a step forward. In this way, a curvilinear shape is effortlessly provided for the bottom seam area 18. Below the blank, it is further possible to arrange a third laser beam (not shown in the figure) to heat the second side wall area 19 on the opposite surface of the blank 16.
Fig. 7 shows a second improved arrangement for heating the side seam area 17 and the bottom seam area 18 of the side wall blank 16 by means of two laser beams 40 and 41 produced by lasers 34. The travel of the first laser beam 40 is guided by a first mirror 44 which is fastened on an axis 45 which is in a perpen- dicular plane in relation to the incoming direction of the laser beam 40. The mirror 44 rotates around its axis 45 between two extreme positions so that, when rotating, it guides the laser beam 40 to scan over the whole side seam area 17. The travel of the second laser beam 41 is guided by a second mirror 46 which is ro- tatably fastened on an axis 47. The mirror 46 rotates around its axis 47 between two extreme positions guiding the laser beam 41 to scan over the whole bottom seam area 18. Simultaneously, the mirror 46 can make a small controlled curvilinear motion by means of which is provided the curvilinear shape of the bottom seam area 18.
Alternatively, a reflecting prism 48 shown in Fig. 8 can be used for heating the side seam area 17 which prism is arranged to rotate around its axis 50 in the direc-
tion shown by the arrow. When the prism 48 rotates, its each face 49 in its turn reflects the laser beam 40 coming from the laser 34 into the side seam area 17 so that each point of the side seam area 17 becomes heated. The rotation axis 50 of the prism 48 is in a perpendicular plane in relation to the incoming direction of the laser beam 40. The prism is advantageously synchronised to rotate at the pace of the cup-manufacturing machine. The rotational motion of the prism may be continuous or there may be pauses during the blanks transferring stages.
Fig. 4 shows an example of an apparatus which can be used for heating the bottom of the cup 21 before finishing the bottom seam and Fig. 5 shows the same device in operation. The heating apparatus comprises a laser head 20 which is provided with means for guiding a laser beam 33 into an area 34 being heated which annu- larly circles the bottom 14 of the cup 21. The laser beam guiding means comprise a guide ring 29 in which connects an arm 30 at the end of which there is a mirror 31. The guide ring 29 is installed into connection with the laser head 20 so that an imaginary central axis 32a of the guide ring 29 passes through an outlet 32 of the laser beam. The mirror 31 is installed oblique so that the laser beam 33 hits the mirror. The arm 30 and the mirror 31 as its extension are made to rotate around the central axis 32a of the quide ring 29, whereby the mirror 31, when rotating, guides the travel of the laser beam 33 so that the laser beam 33 scans over the whole area 34 being heated. The rotational motion of the arm 30 and the mirror 31 is provided either by bringing the whole guide ring 29 to a rotational motion or by guiding the arm to slide along an annular track formed by the guide ring 29.
When the area 34 surrounding the inner surface of the bottom has been heated, the bottom seam is finished by pressing the lower edge of the side wall 16 and the edge flange of the bottom 14 tightly together.
The laser beam is advantageously produced by means of a carbon dioxide laser. Advantageously, use is made of a laser beam that is collimated, i.e. made parallel, the width of which does not change even though the travel of the laser beam to the
target varies slightly. The width of the seam area heated by means of the laser beam is advantageously 6-8 mm. The laser can be guided to produce successive laser pulses so that each pulse is adequate to heat one seam area. During a pulse, the power of the laser beam can be adjusted so that a different heating power is applied to different parts of the seam area being heated.
In an advantageous embodiment, the apparatus is provided with the measurement of the surface temperature of the heated seam area, on the basis of which the power of the laser is controlled as feedback.
The radiation energy can be optimised so that the polymeric material softens exactly for the correct amount but the paperboard layer below does not burn. Modern computer-controlled lasers make possible the quick and accurate heating of a defined area, because of which the use of laser is well applicable in the serial pro- duction of paperboard cups. As described above, the minimum of one and advantageously even four or more laser beams can be used in cup manufacturing of which beams each replaces one hot air blowing or equivalent heating method used earlier at the corresponding point.
The claims will now be presented and, within the inventive idea defined by the claims, many different variations of the invention are possible.
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