WO2000035250A1

WO2000035250A1 - Unit for induction heating and assembly having such a unit

Info

Publication number: WO2000035250A1
Application number: PCT/SE1999/002200
Authority: WO
Inventors: Göran Langstedt; Tord Cedell
Original assignee: Linlan Induction AB
Current assignee: Linlan Induction AB
Priority date: 1998-11-26
Filing date: 1999-11-26
Publication date: 2000-06-15
Anticipated expiration: 2001-05-26
Also published as: AU2013900A; SE513132C2; SE9804058D0; SE9804058L

Abstract

A unit (2) for induction heating of a workpiece comprises a disc-shaped base plate (5). At least one pole (6) is connected to the base plate (5) and arranged to abut against one side of the workpiece or a tool containing the workpiece. Moreover the unit (2) comprises a coil arranged round each pole (6) and designed to be connected to a voltage source for generating a magnetic field in and round said at least one pole (6). The unit (2) is designed so that two identical, juxtaposed units have an essentially constant distance between poles neighbouring each other, both between said units and, if the unit has more than one pole (6), within the respective units. A heating assembly comprises at least two such units (2) which are designed and arranged side by side in such manner that each pole (6) of the heating assembly has an essentially constant distance to the neighbouring pole or poles (6).

Description

UNIT FOR INDUCTION HEATING AND ASSEMBLY HAVING

SUCH A UNIT

The present invention relates generally to treatment of materials by electromagnetic heating thereof. More specifically, the invention relates to an induction heating unit and a heating assembly having at least two such units. The invention also relates to a press provided with such heating, use of such a unit or such an assembly, and methods for controlling the same.

When manufacturing products wholly or partially of plastic or composite material, use is made of a press, whose press tool must be heated. This normally takes place by removing the press tool from the press, placing it in a preheating device, such as a furnace, and after heating again mounting it in the press. A preheating device of this type is known from e.g. US-A-5 , 023 , 419. The handling of the press tool is very time-consuming, which means that the number of finished products per hour will be small .

With a view to reducing the time consumed, it is known to heat the tool in situ in the press. This can be carried out, for example, by the press being provided with hot plates, by means of which conduction heat is transferred to the tool. This process, however, is still far too ime-consuming for economy in large-scale manufacture of components which are wholly or partially made of composite material.

The above problem has been solved by means of an induction heating device as disclosed in WO 97/26776. This device is intended for heating a workpiece in a press and consists of a core of electric sheet steel enclosing the workpiece, a coil arrangement placed round the core and a voltage source. The voltage source is connected to the coil arrangement to generate a magnetic field in and round the core. The core has two poles which between them receive a press tool which encloses the workpiece to be treated. The poles are displaceable relative to each other to apply a press force to and conduct the generated magnetic field into the tool . This device enables rapid heating of the tool/work- piece but still has a number of drawbacks . The core and opposite poles of the device must in terms of size be adjusted to the tool which is being treated. Therefore the construction yields in a fairly poor geometric flex- ibility in that it is difficult to treat tools of different size in one and the same device. It has also been found that there is a risk of local overheating of the tool/workpiece . A further drawback arises when the penetration depth of the magnetic field in the tool/work- piece has to be very small, such as in the treatment of thin tools/workpieces . The penetration depth certainly decreases with an increase of the frequency of the applied voltage, but an increased frequency at the same time causes undesirably great losses in the core, i.e. a low degree of efficiency of the heating device.

An object of the present invention is to obviate the above problems, i.e. to provide a more uniform and more controlled heating and an increased geometric flexibility. A further object is to provide induction heating with a high degree of efficiency independently of the thickness of the workpiece/tool . It is also desirable to be able to provide different temperatures in different parts of the tool/workpiece in one and the same process cycle . According to the invention, these and other objects that will be evident from the following specification are now wholly or partially achieved by means of a heating unit, a heating assembly and a press according to appended claims 1, 25 and 37. Preferred embodiments are defined in the dependent claims. The objects are also achieved wholly or partially by methods according to appended claims 40 and 41. The invention is applicable to heat treatment of workpieces enclosed in a press tool. A non-limiting example of such workpieces is raw materials which wholly or partially consist of plastic or composite material. The invention is also applicable to direct heat treatment of workpieces, such as components comprising both metallic and polymeric materials, with a view to achieving a separation of these materials.

In contrast to conventional constructions for induc- tion heating, the present invention starts from separate units, or modules, which are designed to be arranged against the tool/workpiece. Since the unit has no core in the usual meaning, i.e. a core enclosing the tool/ workpiece, one or more units can be arranged essentially arbitrarily over the tool . This yields great geometric flexibility since the number of units can be selected and positioned so that heating of desired parts of the tool is achieved.

Since the unit is designed in such manner that two identical, juxtaposed units together have an essentially constant distance between poles nearest or neighbouring each other, both within a unit and between the juxtaposed units, a uniform distribution of the magnetic field over the tool is made possible when several units are combined to a large assembly. By the distance between two poles is meant the minimum distance between the outer circumferences of the poles. Since the strength of the generated magnetic field decreases in proportion to the square of the distance from the respective poles, a constant pole distance within such an assembly results in a more uniform heating of the tool/workpiece arranged under the units .

According to a preferred embodiment, the outer contour of the base plate is formed with at least one abut- ment portion which is designed to abut against a corresponding portion of another, identical unit. Consequently, two units can be made to abut against one another and, thus, without any mutual spacing cover a portion of the tool/workpiece.

According to a particularly preferred embodiment, the outer contour of the base plate is completely com- posed of rectilinear abutment portions. Thus, many units can be arranged with their base plates edge-to-edge over the tool/workpiece.

In a preferred embodiment, each unit has at least two poles, which are arranged in a two-dimensional pat- tern over the base plate. This enables a still more uniform distribution of the generated magnetic field over the workpiece/tool and, thus, a more uniform heating of the same. Moreover, when using the inventive unit in a press, the press forces will be more uniformly distri- buted over the tool/workpiece.

Preferably, the pattern of poles within a unit is rotationally symmetrical since this results in a uniform distribution of the magnetic field as well as any press forces over the tool . To reduce the risk of local overheating, the poles are in cross-section preferably formed without sharp or right-angled edges.

According to a preferred embodiment, the coils are connected to the voltage source in such manner that the currents in the coils of poles neighbouring each other are phase-shifted relative to each other. This means that the magnetic flux will alternatingly be directed away from and to each pole. Seen over a longer period of time, the magnetic flux will be essentially homogeneously dis- tributed over the workpiece/tool, thereby achieving good uniformi-y in heating.

In a preferred embodiment of the unit, the phase difference between the coils of poles neighbouring each other is an integer multiple of 60 degrees. This means that both two-phase and three-phase systems, and also other systems, can be used and that a uniform line load can be υrovided. Equivalent advantages apply to the inventive heating assembly.

In a particularly preferred embodiment of the heating assembly, units are arranged on each side of the tool/workpiece. By controlling the phase difference between opposite poles, the penetration depth of the magnetic field through the tool/workpiece can thus be controlled. For instance, the penetration depth is at its maximum when two opposite poles are driven in opposition, i.e. with a phase difference of 180 degrees, and at its minimum when the phase difference is 0 degrees. Thus, the penetration depth may vary while maintaining a high degree of efficiency. Besides, the assembly can be used to heat a workpiece of uneven thickness since each pair of opposite poles can be controlled individually to generate a magnetic field having a desired penetration depth. Thus the assembly can be used to provide a uniform temperature distribution in the workpiece independently of its form. Alternatively, different temperatures can be provided in different parts of the workpiece in one and the same process cycle.

The inventive method comprises at least one of the steps of : a) determining the frequency at which a maximum active power transfer occurs from each unit to the work- piece or tool to be heated; b) tuning a resonance circuit so that a maximum active power transfer occurs to the workpiece/tool; c) locking the phase of the voltage supplied to the respective coils for providing a uniform heating in a plane extending parallel to the poles of the unit; and d) controlling, by pulse width modulation, the duty cycle of the voltage supplied to the respective coils so chat the desired power transfer is obtained. In the preferred embodiment, each of the above steps makes ic possible to influence both the power transferred co the cool and the uniformity in heating. In the heating assembly, each unit is preferably controlled individually for optimum control of the heating, i.e. the temperature in the tool/workpiece.

Moreover, the above method enables experimental recording of a suitable period of time for current and voltage supply to achieve the desired heating in a large number of different applications. Thus, a number of different "heating schedules" can be recorded, which can subsequently be run regularly. The invention will now be described for the purpose of exemplification with reference to the accompanying drawings, which schematically and for the purpose of exemplification illustrate currently preferred embodiments . Fig. 1 is a sectional view of a heating assembly comprising a number of inventive units for induction heating of a workpiece.

Fig. 2a is an exploded view of an inventive unit, and Fig. 2b is a top plan view of a heating assembly with corresponding units.

Figs 3a-3f are top plan views of different embodiments of inventive units, which are made to abut directly against each other to form a heating assembly. For the sake of clarity, only the poles and base plates of the units are shown.

Fig. 4 is a top plan view of three heating units which are arranged side by side and which each have a centrally arranged pole of circular cross-section. For the sake of clarity, only the poles and base plates of the units are shown.

Fig. 5a is a diagram of the time variation of the phases in a three-phase system, and Figs 5b-5c illustrate schematically the magnetic flux between the poles within a unit at two different points of time. Fig. 6 is a sectional view of two units which are arranged on each side of a workpiece. Description of Preferred Embodiments

Fig. 1 illustrates an assembly for induction heating of a tool . The assembly comprises two separate bodies 1 each composed of a plurality of separate heating units or modules 2. The bodies 1 are arranged on each side of a tool 3 which contains a workpiece 4 to be heated.

Each heating unit or module 2 comprises in the shown embodiment a disc-shaped base plate 5, three poles 6 and a coil 7 arranged round each pole 6. The poles 6 are fixedly connected to the base plate 5 and are arranged to abut against the tool 3. Each coil 7 is connected to a voltage source 8 and a control unit 9. For the sake of clarity, only the connection of two poles to the voltage source 8 and the control unit 9 is shown, which are adapted to generate via the coils 7 a magnetic field in and round each pole 6. The magnetic field heats the tool 3 and the workpiece 4, as will be described in more detail below. The illustrated heating assembly can be mounted in, for example, a press so that opposite press forces are applied to the sides of the bodies 1 facing away from the tool 3.

Fig. 2a shows in more detail an inventive module 2. For better clarity, the parts of the module 2 are separated from each other. The disc-shaped base plate 5 of the module 2 consists of a bottom part 5' and a top part 5". In the embodiment shown, the bottom part 5' has milled grooves 10, in which a cooling medium can pass while flowing from an inlet tube 11 to an outlet tube 12. The cooling medium is adapted to carry off heat which is pos- sibly conducted into the module 2 in the heating of the tool 3. The cooling medium can be water, air or some other fluid suitable for the purpose. Three poles 6 projecting perpendicularly from the base plate 5 are formed on the top part 5" of the base plate 5. A coil 7 is arranged round each pole 6. The coils 7 are via lines

13 connected to the voltage source 8 and the control unit 9 (not shown) . In the embodiment shown in Fig. 2a, the cross-section of the poles 6 is essentially circular. The cross- section of the poles 6 can alternatively be elliptic or polygonal, e.g. hexagonal. However, it is preferred for the cross-section of the poles 6 to have no sharp or right-angled corners since the generated magnetic field tends to be concentrated in such corners and cause local overheating of subjacent portions of the workpiece 4. The poles 6 are arranged in a two-dimensional pattern on the base plate 5 of the module 2 to provide as uniform heating as possible of the workpiece 4. For the same reason, the poles 6 are arranged in a constant spaced-apart relationship on the base plate 5. Moreover, the cross-sectional area of the poles 6 is maximised so that as large a contact surface as possible is provided to conduct the generated magnetic field into the tool. For good controllability of the temperature distribution in the workpiece 4, the poles 6 should cover a total of at least about 20% of the surface of the base plate 5. The modules 2 are preferably so designed that two identical, juxtaposed modules 2 have an essentially constant distance between each pole 6 and its neighbouring poles 6 both within a module 2, if each module has at least two poles, and between adjoining modules 2. Fig. 2b shows a body 1 which comprises a plurality of separate, juxtaposed modules 2. For the sake of clarity, the coils of the modules are omitted. The modules 2 are arranged in and mutually fixed by a frame structure 1 ' . The poles 6 of the modules 2 are in the shown example arranged in a two-dimensional pattern at an essentially constant distance to each other. In this case, the maximum geometric flexibility will be achieved when each unit is formed with a single, central pole (not shown) . It will be appreciated that the base plates 5 of the modules 2 may have an essentially arbitrary outer contour.

However, it is preferred for the base plate 5 of each module 2 to have such an outer contour that a por- tion of a module 2 can be brought to plane, direct abutment against a corresponding portion of another, identical module 2, so that these modules 2 without any mutual spacing cover a portion of the tool 3. This embodiment allows a simpler design of the heating assembly 1 since the frame structure 1 ' can be omitted or at least simplified. The absence of air gaps between the modules 2 also increases the heating efficiency of the assembly 1. Examples of particularly preferred embodiments are given in the top plan views in Figs 3a-3e. In Figs 3a and 3b, each base plate 5 has an outer contour in the form of a triangle. More specifically, the outer contour of the base plate 5 is equilateral in Fig. 3a and isosceles in Fig. 3b. The outer contour of the base plate 5 can alter- natively have portions which essentially have the form of an arc tan curve, as illustrated in Fig. 3c. In this case, the outer contour consists of two mutually symmetrical arc tan portions and two rectilinear portions, which at right angles connect the two arc tan portions with one another. This construction allows modules to abut against each other without any mutual spacing and also enables both an optimal positioning of the poles and a maximising of the cross-sectional surface of the poles. In fact it is desirable to avoid strong magnetic fields in the joint between two juxtaposed modules 2 since this may cause great losses. Thus, each pole 6 should have a maximum distance to the outer contour of the base plate 5 while at the same time the poles 6 should cover as much as possible of the surface of the base plate 5. A good compromise between these contradictory requirements is achieved by means of the module shown in Fig. 3c. For reasons of manufacture, it is, however, preferred for the outer contour of the base plate 5 to be made up of rectilinear portions. An example of a "rectilinear" arc tan curve is given in Fig. 3d. Further examples are given in Figs 3e and 3f, where the outer contour of the base plate 5 has the form of a regular, i.e. equilateral, hexagon or a "honeycomb structure" .

A particularly preferred embodiment of the inventive module is shown in Fig. 4, which illustrates three such juxtaposed modules 2. Each base plate 5 has the form of a regular hexagon and has only one centrally arranged pole 6 which is circular in cross-section. This outer contour and arrangement of the poles are easy to accomplish and guarantee that each pole 6 has a constant distance to adjoining poles 6 when the modules 2 abut against each other. Moreover, in this embodiment it is possible to provide both an adequate distance from the pole 6 to the outer contour of the base plate 5 and good coverage of the surface of the base plate 5. Below follows a description of a module and an assembly according to the invention. It will first be described how the magnetic field changes over time within a module with three poles. Then the interaction between two such opposite modules will be discussed. Finally, a method for automatic control of the heating will be described.

Each module is individually controlled in the preferred embodiment. This enables local control of the magnetic field that is being generated and, thus, heating of different parts of the workpiece or tool to different degrees. In the embodiment according to Fig. 2a each coil 7 is preferably connected to an associated phase of a voltage source 8 with three phases. The poles 6 abut against the tool 3 and thus form a magnetically closed circuit when the coils 7 are supplied with a voltage. The magnetic field which is generated when a current flows through the coils 7 causes different losses in the tool 3, which result in the tool 3 being heated. The loss mechanisms which cause the heating are hysteresis loss, eddy current loss and anomalous loss, also called micro- eddy current loss. Control of the phases makes it possible to affect the magnetic field and, thus, the heat- ing, both along the surface and in depth, as will be described below.

Fig. 5a shows how the three phases of the voltage source, which are designated R, S and T, vary over time, and Figs 5b and 5c illustrate the time variation of the magnetic flux between the poles of a module with three poles, as is illustrated in e.g. Figs 3a-3f, or an assembly of three modules abutting against each other and each having one pole, as shown in e.g. Fig. 4. Each pole 6 is via a coil connected to an associated phase of the voltage source 8. At the point of time ti (Fig. 5a) the R phase has its maximum current intensity, and the magnetic flux flows from the R-phase-connected pole R, in equal parts, to the poles S, T that are connected to the S and T phases, respectively, of the voltage source. To facilitate understanding, a vectorial description of the flux is conceivable, where two vectors of the same value point from the R pole at the S and T pole, respectively. The resulting vector will then be directed straight downwards in the Figure. At a later point of time t₂ (Fig. 5a), the T phase has its minimum current intensity, and the magnetic flux flows in equal parts from the R and S poles of the modules to the T pole. The resulting vector will at this point of time instead be directed obliquely down- wards to the right in the Figure. Thus, the resulting flux vector has been turned between the points of time ti and t₂. This discussion can be continued in the same way and leads to the conclusion that the resulting flux vector has turned counterclockwise through a full revolution when a full period has been passed and the R phase is once more at its maximum. This results in a distribution of the flux between the different poles R, S, T which is very uniform over time and, thus, also a very uniform heating along the surface of the workpiece. If also a number of modules are placed quite close to each other to form an assembly, a synergistic effect is achieved between the poles 6 on different modules 2 and not only within the individual modules, especially when each pole 6 has a constant distance to and is connected to another phase than the neighbouring poles 6, both within and between the modules. In a further, not shown embodiment of the invention, the module has two poles, one central pole and one concentrically arranged, annular pole. The poles are surrounded by at least one coil each. In this concentric arrangement, the coil of the annular pole is supplied with a voltage which is phase- inverted relative to the voltage supplied to the coil of the central pole. Thus, the magnetic flux moves back and forth in the radial direction between the two poles so as to provide uniform heating . In all cases described above, the poles associated with the respective phases have essentially the same cross-sectional area since this results in uniform load of the voltage source and good uniformity in heating. By placing modules on each side of the tool/work- piece, the heating thereof can be controlled very accurately since a magnetic field can be generated and conducted through the tool/workpiece. Fig. 6 shows schematically a first module 2a, which with its poles 6 is arranged on a first side of a workpiece 4, and a second module 2b which is arranged on a second, opposite side of the workpiece 4. Thus the poles are facing each other and aligned in pairs. Each module is individually controllable .

First assume, as indicated in Fig. 6, that the modules are arranged so that the R pole of the first module 2a is positioned in front of the R pole of the second module 2b, that the S pole of the first module 2a is positioned in front of the S pole of the second module 2b, and that the T pole of the first module 2a is posi- tioned in front of the T pole of the second module 2b.

If the cwo modules 2a, 2b are driven by the same voltage, two opposite magnetic fluxes are generated. This results in the fluxes deflecting before having time to penetrate far into the workpiece 4. Consequently, only surface heating of the workpiece 4 is provided. If instead the voltage supply to the second module 2b is phase-inverted by 180 degrees compared with the first module 2a, the opposite condition is achieved, viz. that the magnetic flux is driven straight through the workpiece 4, which causes uniform heating of the entire workpiece 4, not only its surface. By a variable phase difference existing between the modules 2a, 2b, which is between 0 and 180°, different penetration depth in the workpiece 4 can thus be achieved. This can be performed, for example, by a simple phase locking loop.

The heating of the workpiece 4 can also be con- trolled by physically turning the opposed modules 2a,

2b relative to each other, for example so that the poles opposing each other are R-S, S-T and T-R, respectively. The poles need not necessarily be positioned exactly in front of each other, but can also be partially displaced in the lateral direction relative to each other.

In the currently preferred embodiment of the invention, the heating is automatically controlled. The tool 3 is equipped with a number of temperature sensors, which are of a type known per se and whose output signals func- tion as input signals of the control unit 9 which controls the voltage supply to the heating modules.

The control unit 9 can be described as a circuit which can be affected on at least one of essentially three levels. On the outermost level, where the slowest control is performed, the supplied voltage is controlled by a simple type of regulator, for example an on/off regulator or a P/D regulator. By changing, depending on the measured temperature, the duty cycle of the voltage supply, i.e. by pulse width modulation controlling the length of the periods with voltage supply to the module, the heating can be controlled. The energy supplied to the tool in fact depends on, inter alia, the length of the periods with voltage supply to the coils.

On the next lower level there are components for controlling the active power transfer to the tool. This results in more rapid control than the one that can be carried out by means of the temperature sensors. These components comprise a resonance circuit, which comprises a capacitor having a variable capacitance or alternatively an inductor having a variable inductance . The active or real power transfer to the tool is calculated by measuring current, voltage and phase angle. Subsequently, the capacitance of the capacitor or alternatively the inductance of the inductor is controlled until the power factor { cosφ) , and thus also the active power transfer to the tool, reaches its maximum. In ideal terms, this means that the driving stage is only affected by a resistive load, i.e. cosφ = 1.

On the third and lowest level, there are components which, if possible, control the frequency until electric resonance has been obtained. This means that the capacitor is tuned with the coil and that the power factor is at its maximum. Consequently, also the active power transfer to the tool is maximised, and the reactive (or "useless") effect is minimised. This means that the heat- ing of the tool is maximised. This is the quickest and most basic control method.

Only a few possible embodiments of the invention have been described above. A person skilled in the art can, on the basis of the description, provide a number of variants which are suitable for the application in question and which are within the scope of protection of the appended claims. For example, the design of the control unit may be varied to achieve the maximum power transfer to the tool. Also other pole patterns and base plates than those described above can be provided. For instance, each base plate can have more than three poles. Also in this case, it is preferred for neighbouring poles to be connected to different phases of the voltage source. Moreover the poles should be arranged in a rota- tionally symmetrical pattern on the base plate, so that a turning of the module through 120 degrees maximum trans- fers the pole pattern to itself. In the case of six poles on the base plate and three phases of the voltage source, the pattern should be rotationally symmetrical for a turning of the module through 60 degrees.

It should be emphasised that a heating assembly according to the invention may comprise pairs of opposite units, or a body which is arranged on one side of the tool/workpiece and is formed of a plurality of units, or a combination of these. If units are only arranged on one side of the tool/workpiece, a magnetically conductive abutment, however, should be arranged on the opposite side of the tool/workpiece. Without such an abutment, the generated magnetic field will essentially extend directly between the poles of the units, which makes the penetration depth small. With an opposite abutment, the magnetic field is to a greater extent driven through the intermediate tool/workpiece.

Preferably the poles of the modules are made of a material other than electric sheet steel since it may be difficult and costly to arrange poles in electric sheet steel in a suitable pattern and with a suitable cross- section, i.e. without sharp or right-angled corners. The poles are preferably made by compacting a metal powder, preferably of pure iron or an iron alloy containing silicon or nickel, and a binder, such as epoxy or phenolic resin. This method of production is per se known in other fields, see e.g. US-A-2 , 937 , 964. Before compaction, each metal powder particle is preferably coated with a thin surface layer of an insulating material, such as an oxide. Thus the metal powder particles are isolated from each other. When the powder is compacted at high pressure, a material forms, which has magnetic properties that are convenient in the context and a strong struc- ture that can be worked to the desired shape by machining. According to a preferred embodiment, the base plate and the poles are made in one piece of this material. Alternatively, the poles can be manufactured separately and then fixed to the base plate by gluing, bolt joints or some other equivalent technique. The base plate can be made of this material, but also other materials are conceivable, for instance electric sheet steel. The base plate can also be composed of a number of separate parts.

Claims

1. A unit for induction heating of a workpiece (4), said unit comprising a disc-shaped base plate (5) , at least one pole (6) connected to the base plate (5) and arranged to abut against one side of the workpiece (4) or a tool (3) containing the workpiece (4), a coil (7) which is arranged round each pole (6) and which is designed to be connected to a voltage source (8) for generating a magnetic field in and round said at least one pole (6) , the unit being designed so that two identical, juxtaposed units have an essentially constant distance between poles (6) neighbouring each other, both between said units and, if the unit has more than one pole (6), within the respective units.

2. A unit as claimed in claim 1, wherein the base plate (5) has an outer contour comprising at least one abutment portion which is designed to abut against a cor- responding portion of another, identical unit.

3. A unit as claimed in claim 2, wherein at least one abutment portion is rectilinear.

4. A unit as claimed in claim 2 or 3 , wherein at least one abutment portion essentially has the form of an arc tan curve.

5. A unit as claimed in any one of the preceding claims, wherein the base plate (5) has an outer contour comprising at least three rectilinear portions.

6. A unit as claimed in any one of the preceding claims, wherein the base plate (5) has an outer contour comprising rectilinear portions only.

7. A unit as claimed in any one of the preceding claims, wherein the outer contour of the base plate (5) has the form of a triangle. 8. A unit as claimed in claim 7, wherein said triangle is equilateral .

9. A unit as claimed in claim 7, wherein said triangle is isosceles.

10. A unit as claimed in any one of the preceding claims, wherein the outer contour of the base plate (5) has the form of a hexagon, preferably a honeycomb structure .

11. A unit as claimed in any one of the preceding claims, wherein all poles (6) cover a total of at least about 20% of the surface of the base plate (5) . 12. A unit as claimed in any one of che preceding claims, wherein each pole (6) in a section along the base plate (5) has no sharp or right-angled corners.

13. A unit as claimed in any one of the preceding claims, wherein each pole (6) is essentially elliptic in cross-section.

14. A unit as claimed in any one of the preceding claims, wherein each pole (6) is essentially circular in cross-section.

15. A unit as claimed in any one of the preceding claims, comprising at least two poles (6) which are arranged on the base plate (5) in a two-dimensional pattern for abutment against one side of the workpiece (4) or the tool (3) containing the workpiece (4) .

16. A unit as claimed in claim 15, wherein said pat- tern is such that a turning of the base plate (5) through an angle of 120 degrees maximum essentially transfers the pattern to itself.

17. A unit as claimed in claim 15 or 16, wherein said pattern has the form of a regular polygon. 18. A unit as claimed in claim 15 or 16, wherein the poles (16) are arranged in a concentric pattern.

19. A unit as claimed in any one of claims 15-18, wherein there is a mutual phase difference between the currents flowing through the coils (7) of poles (6) neighbouring each other on the unit.

20. A unit as claimed in claim 19, wherein said phase difference is an integer multiple of 60 degrees.

21. A unit as claimed in any one of claims 15-20, wherein the coils (7) of poles (6) neighbouring each other are connected to different phases of the voltage source (8) . 22. A unit as claimed in claim 21, wherein poles (6) are allocated to the voltage source (8) so that the poles of each phase have the same total cross-sectional area.

23. A unit as claimed in any one of the preceding claims, wherein cooling coils (10) are formed in the base plate (5) to be passed by a cooling medium.

24. A unit as claimed in any one of the preceding claims, wherein each pole (6) is made by compacting a metal powder and a binder.

25. A heating assembly for induction heating of a workpiece (4), comprising at least two units (2) each comprising a disc-shaped base plate (5) , at least one pole (6) connected to the base plate (5) and a coil (7) arranged round the associated pole (6) , and a voltage source (8) connected to each coil (7) to generate a magnetic field in and round said poles (6) , which are designed to abut against a first side of the workpiece

(4) or a tool (3) containing the workpiece (4), the units (2) being designed and arranged side by side in such manner that the heating assembly has an essentially constant distance between poles neighbouring each other (6) , both between said units (2) and, if each unit (2) comprises more than one pole (6), within the respective units (2) .

26. A heating assembly as claimed in claim 25, wherein the base plate (5) of each unit (2) has an outer contour comprising at least one abutment portion abutting against a corresponding portion of another unit (2) .

27. A heating assembly as claimed in claim 25 or 26, wherein each pole (6) in a section along the base plate

(5) has no sharp or right-angled corners. 28. A heating assembly as claimed in any one of claim 25-27, whose poles (6) are arranged in a two-dimensional pattern.

29. A heating assembly as claimed in claim 28, which has an essentially constant distance between the poles

(6) .

30. A heating assembly as claimed in any one of claims 25-29, wherein there is a mutual phase difference between the currents flowing through the coils (7) of poles (6) neighbouring each other.

31. A heating assembly as claimed in any one of claims 25-30, comprising at least one magnetically con- ductive abutment which is arranged on a second, opposite side of the workpiece/tool.

32. A heating assembly as claimed in any one of claims 25-31, wherein at least one corresponding unit (2) is arranged on a second, opposite side of the workpiece/ tool.

33. A heating assembly as claimed in claim 32, wherein the poles (6) of the opposite units (2) arranged on each side of the workpiece (4) are arranged in front of each other. 3 . A heating assembly as claimed in claim 32 or 33, comprising a control device (9) which is adapted to electrically control a phase difference between the currents flowing through the coils (7) of said opposite poles (6) for the purpose of varying the penetration depth of the generated magnetic field in the workpiece/tool.

35. A heating assembly as claimed in claim 32 or 33, comprising a control device (9) which is adapted to mechanically perform a mutual turning of said opposite heating units (2) for the purpose of varying the pene- tration depth of the generated magnetic field in the workpiece/tool .

36. A heating assembly as claimed in claim 34 or 35, comprising a temperature sensor for measuring the temperature in the tool (3) , the output signal of the temperature sensor being an input signal of the control device (9) .

37. A press for manufacturing products wholly or partially of plastic or composite material, comprising a unit as claimed in any one of claims 1-24 or an assembly as claimed in any one claims 25-36. 38. Use of a unit as claimed in any one of claims

1-24 or an assembly as claimed in any one of claims 25-36 for heat treatment of components comprising both metallic and polymeric materials for the purpose of separating the metallic and polymeric materials from each other. 39. Use of a unit as claimed in any one of claims

1-24 or an assembly as claimed in any one of claims 25-36 for heating a press tool which is mounted in a press for manufacturing products wholly or partially of plastic or composite material . 40. A method for controlling a unit as claimed in any one of claims 1-24 or an assembly as claimed in any one of claims 25-36 comprising at least one of the steps of: a) determining the frequency at which a maximum active power transfer occurs from each unit (2) to the workpiece (4) ; b) tuning a resonance circuit so that a maximum active power transfer occurs place to the workpiece (4); c) locking the phase of the voltage supplied to the respective coils (7) for providing a uniform heating in a geometric plane extending parallel to the poles (6) of the unit (2) ; and d) controlling, by pulse width modulation, the duty cycle of the voltage supplied to the respective coils (7) so that the desired power transfer is obtained.

41. A method for controlling a unit as claimed in any one of claims 1-24 or an assembly as claimed in any one of claims 25-36, wherein each unit (2) is controlled individually.