Technical Field

The present invention relates to a method and a device at a longitudinal joint sealing with induction heat of a packaging blank or the like of polymer lined metal plate, wherein an inductor is applied against a joint, formed of an upper member and a lower member. Background of the Invention

A problem at such a longitudinal joint sealing of an overlapping joint with induction heat is that the heat is developped in the metal layer or member which is closest to the inductor, i e the upper member. This means that a for example about 5 mm wide burnt zone is formed on the upper member, where it otherwise would have been desireable to have a decoration, which should extend around the packaging without interruption. The Invention

The problem can according to the invention be solved in the way which is defined in the attended claims, namely in that the return current of the inductor is conducted via the lower member of the packaging blank. This means that the lower member becomes heated and that the upper member is heated indirectly from below, whereby the decoration on its upper side is not burnt. Brief Description of the Drawings

The invention shall be further described below under reference to the accompanying drawings, in which

Fig 1 in perspective illustrates the cooperation between a packaging blank with frusto-conical shape and an inductor beam for accomplishing a longitudinal joint in the packaging blank,

Fig 2 is a side view, namely from the right, of the arrangement shown in Fig 1 with an inductor according to the invention,

Fig 3 is a plan view of the device according to Fig 2,

Fig 4 is an end view from the left of the device according to Fig 3 ,

Fig 5 is a simplified section along the line V-V in Fig 3 , and Fig 6 is a plan view to a reduced scale of a mandrel for use together with the inductor beam shown in Figs 1 - 5.

Detailed Description of Preferred Embodiments

A frusto-conical packaging blank 1 shall - starting from a planar sheet - be given the conical shape shown in the drawings and be provided with a longitudinal joint in the way defined by the invention, whereupon it can be equipped with a bottom and a cover in a way not further shown or described here . The packaging blank 1 is preferably made of aluminium with a thickness of 50 - 300 μm, for example 85 μm. The aluminium surface is at least on one side lined with a sealable polymer, for example a polyester, but often on both sides. For accomplishing a longitudinal joint in the packa¬ ging blank 1 the technique with induction heat is utilized. For this purpose an inductor beam 2 is used and as dolly a mandrel 3 indicated in Fig 6 and for example made of fabric reinforced bakelite. Special problems are encountered at sealing compara¬ tively thick polymer lined aluminium with induction heat, above all due to the low resistance and high heat conduc¬ ting ability of the material. The current in the inductor and accompanying matching transformer becomes unacceptably high with normal inductors having only one turn.

The problem is further complicated at longitudinal joints, where the induced current shall follow the edge of the material as far as possible and then promptly turn. At the method according to the invention use is instead made of an inductor having several windings, prefe¬ rably an inductor with five turns . In the shown case one turn at the place where the longitudinal joint shall be accomplished is formed as an I-beam 4, whereas the other turns are constituted of litz wire 5. A litz wire contains a large number of conductors isolated from each other, in the present case about 400 conductors within a total diame¬ ter of 2 mm, and thus accomplishes an even distribution of the current in the wire.

The I-beam shape is chosen for enabling the applica- tion of the pressure on the joint necessary for the sea¬ ling, and the litz wires 5 are laid in the web of the I- beam for concentrating the current and accomplishing the heat necessary for rapid melting and sealing. The I-beam 4 with its litz wires 5 is laid into a ferritic mass for con- centrating the current. At a sealing of 85 μm aluminium sealing times of 30 - 300 ms can be accomplished.

All the return conductors of litz wire 5 (to the left in Figs 4 and 5) are laid next to each other under a copper plate 6 in the inner surface of the inductor beam for the purpose of distributing the current.

The inductor beam 2 can be provided with screw holes 2' for its mounting at the machine (not shown) for the sea¬ ling.

At overlapping joints, with which the present inven- tion is concerned, the heat is normally developped in the metal layer closest to the inductor. This means that a for example about 5 mm wide burnt zone can be formed on the surface of the packaging material. This zone must then be free from decoration in the same way as at steel cans and the like with welded longitudinal joint.

If the members of the longitudinal joint however are laid in the way which appears from Figs 4 and 5, i e if the member on which the return litz wires 5 and the copper plate 6 are applied is laid lowest in the joint, both the sealing current and the return current will pass through the lower member, which means that the heat will be con¬ centrated to the lower plate layer. The heating of the upper plate layer will hereby occur from below, and the upper printed decorated layer is not damaged. Where the current turns at the outer ends of the longitudinal joint, the current density becomes so great that the material in the packaging blank is damaged. This problem can be obviated in that a copper strip 7 (Fig 6) is arranged on the mandrel or dolly 3 at the back side of the packaging blank 1 in the vicinity of the place where the current turns. Such a strip 7 is in its longitudinal direc¬ tion displaced a few mm inside the end of the longitudinal joint. It will take care of the turning current and will relieve the joint. Such a strip 7 is normally arranged at each end of the longitudinal joint. It can however also occur that the packaging is provided with a cover before the longitudinal joint, and in such case it suffices with one strip 7 at the free end of the joint.

As a modification to the embodiment described above and shown in the drawings with an I-beam, it is also pos¬ sible to make use of other profiles, such as a U- or T- beam, and the number of litz wires can for example be in the area 3 - 7.

Another way of accomplishing the desired effect with current concentration and pressure application ability is to use an inductor in the form of a number, for example five, next to each other standing but from each other iso¬ lated copper rods, each with a height of 4 - 5 mm and a thickness of 0.8 mm. In the return conducting phase these

copper rods can be turned 90° and be placed at a certain distance from each other.