WO1987004314A1

WO1987004314A1 - A microwave applicator

Info

Publication number: WO1987004314A1
Application number: PCT/SE1986/000587
Authority: WO
Inventors: Sven Ekerot; Jan Svennebrink
Original assignee: STIFTELSEN INSTITUTET FÖR MIKROVA^oGSTEKNIK VID
Priority date: 1985-12-30
Filing date: 1986-12-29
Publication date: 1987-07-16
Also published as: SE8506159L; SE451228B; SE8506159D0; JPS63503338A

Abstract

A microwave applicator for heating bodies (2) which are substantially of elongate configuration and which incorporate dielectric material. The applicator forms a microwave cavity (3) through which bodies are intended to be transported or in which at least one body is intended to be present. The microwave applicator (1) is particularly characterized in that it is constructed for heating bodies (2) which contain a metallic core (2') and bodies which lack a metallic core, there being used mutually different oscillating modes for bodies which incorporate a metallic core and bodies which lack a metallic core.

Description

A MICROWAVE APPLICATOR

The present invention relates to a microwave applicator for heating primarily bodies which are substantially of elongated configuration and which incorporate dielectric material.

It is known industrially to heat elongated bodies with the aid of microwaves. Different kinds of applicator are used to this end, depending on whether the elongated body incorporates a metal core or not. If an elongated body incorporating a metallic core is fed into a cavity formed with the aid of an applicator specifically for heating bodies that do not incorporate a metallic core, the metal¬ lic core will short circuit the electric field and the cavity will obtain another resonance frequency and will no longer function as a heating device. The opposite case also applies.

Because of this there are used, under some circumstances, different applicators for different manufacturing stages, which complicates movement of material between said stages etc., and increases investment and maintenance costs.

The present invention affords a solution, inter alia, to the aforesaid problems and greatly increases the fields or areas in which microwaves can be suitably used for heating purposes.

Thus, the invention relates to a microwave applicator which is intended for heating primarily elongated bodies which incorporate a dielectric material and by means of which there is formed a microwave cavity through which such bodies are intended to pass, or within which at least one such body is intended to be present. The microwave applicator according to the invention is particularly characterized in that it is constructed so that both bodies which contain a metallic core and bodies which lack a metallic core can be heated therewith, there being used to this end mutually different oscillating modes according to the presence or absence of a metallic core.

The invention will now be described in more detail with reference to an exemplifying embodiment thereof and to the accompanying drawing, in which

Figure 1 is a schematic view of a microwave applicator seen at right angles to its main axis;

Figure 2 is a bottom view of the applicator illustrated in Figure 1 ;

Figure 3 is a schematic vertical, axial sectional view of a second embodiment of an applicator arrangement according to the invention; and

Figure 4 is a schematic vertical, axial sectional view of a third embodiment of an applicator arrangement according to the invention.

Figure 1 illustrates a microwave applicator 1 for heating elongated bodies 2 that incorporate dielectric material. There is formed with the aid of the applicator 1 a micro¬ wave cavity 3 which has a substantially circular cross- section in a plane at right angles to the longitudinal axis of said body 2. The applicator has two end portions 5 in which apertures 4 are provided. The reference 6 iden¬ tifies a waveguide which extends substantially radially into the cavity, for supplying microwaves thereto. The applicator 1 is constructed in a manner which enables both bodies that incorporate a metallic core, shown in broken lines 2' in Figure 1 , and bodies which have no metallic core to be heated, the length or axial extension of the cavity corresponding essentially to half the wave¬ length of the intended microwaves. In this regard, the cavity is designed to oscillate in a co-axial mode when heating a body that incorporates a metallic core, and oscillates in a cylindrical mode, preferably TM 010, the resonance frequency of which is independent of the length or axial extension of the cavity, while the diameter is determined by the resonance frequency. In this case both the diameter and the length are therefore determined by the two oscillating modes per se. The cylindrical mode JM 010 is sometimes written as TM_Q1Q.

The cavity 7 formed by the applicator 6 of the Figure 3 embodiment is intended for heating a so-called nozzle or die8 through which metal 9 is intended to run, the nozzle in this case forming the aforesaid elongated body.

The nozzle or die to be heated can be used for various purposes, as hereinafter described.

In Figure 3 there is illustrated a container vessel 10 having an opening 11 with which the nozzle 8 communicates, said nozzle being encased in and surrounded by a ceramic material 12, which preferably exhibits but a very small loss factor, e.g. Al₂0_, . It is preferred in many cases that parts of the cavity are formed by metal¬ lic sections 13, such as the sections 13 of a metallic net, for instance a network of platinum filaments or threads, embedded in or encased by the ceramic material surrounding the nozzle. The cavity of the illustrated embodiment has a downwardly widening metallic part 14. In many cases complimentary parts 15 of insulating brick or it

the like will be required in order to obtain a suitable arrangement. Microwaves are supplied to the cavity 7 with the aid of a waveguide 6* which extends substantial- ly radially to the cavity 7, said cavity, formed by sections 13 and 14, having a substantially cylindrical cross-sectional shape.

In the embodiment illustrated in Figure 4 the nozzle 8 has a freely outwardly extending part 16 which protrudes down from the opening 11 of a container vessel 10. The applicator 17 of this embodiment is arranged to form a cavity 18 of substantially cylindrical cross-section, the part 16 of the nozzle 8 extending into said cavity. The reference 17' identifies a waveguide corresponding to the waveguide 6 of the Figure 3 embodiment.

Both the applicator illustrated in Figure 3 and the ap¬ plicator illustrated in Figure 4 are constructed in the manner of the applicator illustrated in Figures 1 and 2, i.e. in a manner which enables the elongated body 2 to be heated irrespective of whether it incorporates a metallic core 2* or not.

Nozzles of the kind intended here are suitably made of a ceramic material, suitably a ceramic material such as zirconium dioxide Zr0₂, which exhibits substantial loss factors with regard to microwaves.

Nozzles intended to be heated with the aid of an appli¬ cator constructed in accordance with the invention can be used in a multiple of fields.

One field in this regard is that of heating a dielectric material such as a plastics material, which either incor- porates or lacks a metallic core, during transport of the material through the applicator. The material may com¬ prise, for example, a plastic coating on a ski-stick. A further field is one in which some part, either the di- electric or the metal core, remains stationary in the ap- plicator. This may apply in the manufacturing of tubing, piping, or some other hollow profile. The same appli¬ cator can be used herewith for manufacturing correspond¬ ing homogenous profiles. Another field is the direct casting of wire in which the nozzle or die is stationary in relation to the applicator, substantially in accordance with Swedish Patent (8003487-9), heating of the die preferably being effected both as a pre-heating mea¬ sure prior to introducing molten material thereinto and as a back-up heating measure during a molding or casting operation, therewith to control the nozzle or die temper¬ ature, for example in order to prevent blockages occurring in the nozzle. The reference 9' in Figure 3 identifies a stabilizing wire which extends through the nozzle. The invention can also be applied to heat a die or nozzle, both pre-heat and back-up heat, during more general cast¬ ing or molding processes, such as continuous casting pro¬ cesses.

In nozzle or die heating applications the material 12, 10 surrounding the nozzle or die is often preferred to ex- hibit extremely small loss factors, as opposed to the material from which the nozzle or die is made.

Another field in which the invention can be applied is the heating and heat-retention of nozzles or dies used as tools in, for instance, wire drawing processes.

Still another field is the heating of nozzles or dies and the material passing therethrough, this material, such as plastic, being pressed through the die or nozzle or caus¬ ed to flow therethrough in some other way. The manner in which the microwave applicator according to the invention operates will be understood in all essen¬ tials from the aforegoing. Thus, when suitably con- structed the applicator will form a cavity which oscil- lates in a given mode when used in conjunction with ma¬ terial having a metal core, and in a different given mode when used in conjunction with material that lacks a metal core, for example a coaxial mode in the former case and a cylindrical mode in the latter case.

It will be understood from the aforegoing that a micro¬ wave applicator constructed in accordance with the in¬ vention will afford important advantages over prior art applicators. For example, the applicator according to the invention is more flexible with regard to its use. Thus, one and the same applicator can be switched instanta¬ neously between pre-heating and back-up heating opera¬ tional modes.

The invention has been described in the aforegoing with reference to a number of exemplifying embodiments. It will be understood that other embodiments are conceivable and that minor modifications can be made without departing from the concept of the invention.

For example with regard to the aforesaid modes it will be obvious that there can be used a co-axial mode and another mode other than a cylindrical mode. It will also be understood that the length and diameter of a cavity ac¬ cording to Figure 1 need not be determined by the two oscillating modes per se. Frequency, length and diameter can be mutually adapted in a manner which will enable modes for heating both with and without a metallic core to be achieved with one and the same applicator.

In addition, other applicator embodiments than those illustrated are conceivable for use in heating or dies while retaining the overall purpose. Separate detail constructions are also conceivable. Both of the embodi- ments illustrated in Figures 1 , 2 and 4 have openings provided with flanges 19 for preventing microwave leakage,

Claims

1. A microwave applicator for heating primarily bodies which have substantially elongate configuration and which incorporate dielectric material, by means of which appli- cator there is formed a microwave cavity through which bodies are intended to be transported or in which at least one body is intended to be present, characterized in that the microwave applicator (1, 6, 17) is so con¬ structed that both bodies (2, 8) which incorporate a metallic core (2*, 9, 9') and bodies which lack a metallic core can be heated, the oscillating mode used when heating a body incorporating a metallic core being different to that used when heating a body that lacks a metallic core.

2. An applicator according to Claim 1, characterized in that the cavity (3, 7, 18) is constructed so that said cavity will oscillate in a coaxial mode when used to heat bodies (2, 8) which incorporate a metallic core (2', 9, 9') , and to oscillate in a different mode when used to heat bodies that lack a metal core.

3. An applicator according to Claim 1 or 2, characterized in that the cavity (3, 7, 18) has a substantially circular cross-section extending in a plane substantially perpen¬ dicular to the longitudinal axis of the body, and a length which corresponds substantially to half a wavelength of the intended microwaves.

4.. An applicator according to Claim 2 or 3, characterized in that the cavity (3, 7, 18) is constructed to oscillate in a cylindrical mode, preferably TM 010, the resonance frequency of which is independent of the length of the cavity while the diameter is determined by the resonance frequency, the diameter and the length being determined by the two oscillating modes per se.

5. An applicator according to Claim 1 or 2, characterized in that the cavity (3, 7, 18)" has a substantially circular cross-section extending in a plane substantially perpen- dicular to the longitudinal axis of said body, the fre- quency and length and diameter being mutaully adapted for heating both bodies that incorporate a metallic core and bodies which lack a metallic core.

6. An applicator according to Claim 1 , 2, 3, 4 or 5, characterized in that an elongated body (2) is intended to be transported through the applicator and heated there¬ with.

7. An applicator according to Claim 1 , 2, 3, 4 or 5, _^characterized in that the cavity (7, 18) is constructed for heating a so-called nozzle or die (8) , such as a nozzle or die through which molten metal (9) is intended to pass in conjunction with a casting or molding operation said nozzle or die constituting said elengated body.

8. An applicator according to Claim 7, characterized in that the nozzle or die comprises a ceramic ma.terial, such as Zr0₂, which exhibits substantial loss factors with re¬ gard to microwaves.

9. An applicator according to Claim 7 or 8, characterized in that at least a part of the nozzle or die (8) is em¬ bodied in and thus surrouded by a ceramic material (12) that preferably exhibits a very small loss factor, parts of said cavity (7) being formed with the aid of metallic parts (13) , such as parts (13) of a metallic net, embodied in, for example molded in, the ceramic material surround¬ ing the nozzle or die.

10. An applicator according to Claim 7 or 8, character¬ ized in that the nozzle or die (8) includes a freely out- wardly protruding mouth part (16) which is surrounded by a cavity (18) formed by the applicator.