Electric heating – Inductive heating – With heat exchange
Patent
1998-07-13
1999-10-19
Walberg, Teresa
Electric heating
Inductive heating
With heat exchange
1562722, H05B 610
Patent
active
059683996
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an inductor for use at sealing or heating of a thermoplastic or a thermoplastic coated electrically conducting packaging material, the inductor including a current conductor to be connected to a source for high-frequency current.
BACKGROUND OF THE INVENTION
The induction technology can advantageously be used at sealing of thermoplastic coated packaging material, including at least one layer of metal foil, preferably aluminium foil. Hereby an inductor as described above is used, which in the metal foil creates heat, melting the thermoplastic. In order to obtain a sealing the inductor is used together with a dolly.
The sealing is performed in such a way that the packaging material, which is to form a packaging, is fed stepwise or continuously, depending on the design of the apparatus. With the material lying immovable over the dolly a short while the inductor is applied against the material and is energized, so that the thermoplastic is melted. By a mechanical pressure between the inductor and the dolly with the thermoplastic molten a sealing is provided between two packaging material layers.
In accordance with previously known techniques, for example, as disclosed in applicant's U.S. Pat. No. 4,704,509, issued Nov. 3, 1987, (which patent is hereby incorporated by reference), conductor is for example constituted of a copper conductor cast into plastic material containing ferrite. As disclosed in applicant's above-identified U.S. Patent, the high frequencies normally employed in such sealing processes are normally between 2 MHz and 100 MHz.
A critical property of the inductor is the possibility to maintain the current conductor at a decently low working temperature in spite of a high-frequency on the sealing operations, i.e. to cool the current conductor. In a previously known inductor this is obtained in that the current conductor is a copper tube directly cooled with water. This design is very sensitive to mechanical loads, and the life span is short due to collapsed copper tubes/current conductors.
For this reason known conductors have worked against comparatively soft dollies, and their current conductors may have been provided with ridges for obtaining sufficient mechanical clamping force.
At sealing of new packaging materials, for example thermoplastic coated plate material, and filling of products, which are difficult to handle, for example highly viscous products, it is called for stronger inductors with improved possibilities to handle mechanical loads. These demands can hardly be fulfilled with inductors of conventional types.
THE INVENTION
In order to solve the above mentioned and other problems and in order to achieve an inductor with improved properties and with greater field of use than previously known inductors an inductor according to the invention is characterized in that the current conductor is integrated in a body of a powder metal material, whose granules are provided with an electrically isolating surface layer.
The powder material is preferably constituted of granules of an iron material with a surface layer of iron oxide. In the material there is also a small amount, for example 0.5%, of a plastic based binder, such as a phenol based binder.
In pressed condition this material has excellent mechanical properties, high permeability at used frequencies, small magnetic remagnetizing losses, good heat conductivity and high resistance to temperatures.
The current conductor in the inductor according to the invention does not demand any direct cooling, for example with water, which means that it can be made in a homogeneous rod, usually of copper, which per se increases the ability of the inductor to withstand mechanical loads. The cooling is instead accomplished indirectly in that the heat is conducted away effectively by the powder material. If, however, extra cooling under special circumstances is needed, cooling canals can be provided in the body of the inductor and be made wider, so that also the risk for blocking is increased.
The
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Pwu Jeffrey
Tetra Laval Holdings & Finance S.A.
Walberg Teresa
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