FR2783665A1 - High frequency heating device for thermo hardening resin polymerization, has control of both magnetic and electrical fields - Google Patents

Abstract

The high frequency heater, is of the type having a electrical voltage generator and an application part fitted with two electrodes (1,2), connected to the generator, designed to be applied to a piece (3) between the electrodes, which is heated by the generated electrical field. The device has means for applying the voltage to a given part of the piece. The device includes means for spreading the electromagnetic field in a transverse way with respect to the sense of the piece (3), both for the magnetic and electric field. The device includes means for spreading the generated currents in a longitudinal way and means to ensure that the voltages along the perimeter of sections normal to the axis of propagation are in phase.

Description

High frequency heating device.

  The present invention relates to the field of dielectric heaters capable of providing energy to a material which is to be heated and possibly polymerized. Such devices are used in particular for the polymerization of thermosetting resins by placing a piece of resin between the electrodes of a capacitor and by applying a high frequency voltage,

of sufficient amplitude.

  There are known in the prior art high frequency generators whose power reaches several kW. These generators operate at frequencies authorized by regulations. For France, the 13.56 MHz and 27.12 MHz bands are generally used. The energy per unit of volume Pa supplied to the material or absorbed by it is proportional to the square of the electric field E to which it is subjected, to a coefficient ú "which measures dielectric losses and the frequency or the pulsation co of the electric field. So we have the following relation: Pa = ú ".E2. Co, E being defined in Volt / m. This energy Pa raises the temperature of the material. For a given time At, an element of density p and of heat capacity Cp, the temperature rise AT is given by the following relation:

AT = Pa.At.Cp-1. p-1.

  This relationship applies until the initiation of the polymerization reaction. As soon as the polymerization starts, the temperature rise accelerates, the polymerization being an exothermic chemical reaction. This acceleration of the rise in temperature is all the more important as the part is massive. If we want the part to polymerize in a homogeneous way and to avoid that the mechanical stresses which arise during polymerization are revealed after cooling as fragile points or points of initiation to their rupture with the effort, it is necessary to regulate the field electric which reigns in the material with care. This is all the more difficult as the part to be produced is large and its thickness varies since the electric field is basically expressed by the voltage between the electrodes of the capacitor divided by the distance between the electrodes

or the thickness of the part.

  In an ordinary device, whose electrodes match the shape of the part, the voltage between the electrodes is constant. If we want, for example, to polymerize pieces of considerable length compared to their thickness and whose thickness in the center is twice that of the ends, the electric field in the center of the piece is twice weaker than at the extremities. The local power which is proportional to the square of the electric field will therefore be four times lower in the center than at the ends. The ends of the part quickly reach the polymerization temperature. On the contrary, to obtain a good polymerization in the center of the part, it is necessary to heat the ends to a temperature which can exceed this

that the material can support.

  French patent 96 10 848 describes a heating device comprising an applicator element provided with two electrodes connected to a generator, and intended to apply to a part to be heated an electric field created by the electric voltage which is adapted to the geometric coordinates of said portion of the heated room. It is thus possible to vary the local voltage as a function of the thickness of the part, to obtain a simultaneous polymerization reaction of the whole part and to improve the mechanical characteristics of the part thus obtained. However, we realized that the results obtained did not always correspond to the modeling calculations, in particular to those concerning the electromagnetic field. This may result in a

  degradation of the quality of the polymerization of the part.

  The object of the invention is to remedy the drawbacks of the apparatuses of the prior art and to propose an apparatus capable

  perform constant and predetermined heating.

  The object of the present invention is to provide a high-frequency heating device providing an electromagnetic field

  suitably distributed in the room to be heated.

  The high-frequency heating device according to the invention is of the type comprising an electric voltage generator and an applicator element provided with two electrodes connected to the generator, intended to apply to a part placed between the electrodes and in front

  be heated, an electric field created by said electric voltage.

  The heating device comprises means for adapting the voltage applied to a given portion of the part at the coordinates

  of said portion of the part to be heated.

  The heating device comprises means for distributing the electromagnetic field transversely relative to the direction of the workpiece both for the electric field and for the magnetic field. In one embodiment of the invention, the device comprises means for distributing the currents longitudinally. In one embodiment of the invention, the device comprises means so that the tensions along the perimeter of the

  sections normal to the axis of propagation are in phase.

  Advantageously, the device comprises a power supply distributed in several coaxial lines in the same plane normal to propagation, said coaxial lines being connected to an electrode

  central. The feed can include four lines.

  The power supply may include a distributor with coupled lines. In one embodiment of the invention, the lines joining the distributor and the supply points of an electrode are

of equal length.

  In one embodiment of the invention, the lines are electrically and magnetically coupled to a main line

central in the distributor.

  The ends of the lines arranged in the distributor

  can be bypassed mutually.

  Advantageously, the device comprises at least one group of shunt impedances distributed in a plane normal to the axis

longitudinal of the part.

  The calculation of the configuration of the magnetic field supposes that the distribution of said magnetic field in the longitudinal direction of the part, is a transverse electromagnetic wave for both the electric and magnetic field. This structure of the electromagnetic field is disturbed by all the obstacles constituted by changes in the section of the part and the insertion of elements of

  fixing that the application element may include.

  Considering the dimensions of the part in the direction of the length, which are generally important compared to the wavelength and especially taking into account the perimeter which is also important compared to the wavelength, the disturbances of the electromagnetic field by obstacles are no longer located in the planes where obstacles are present, but affect large areas

in the cell.

  Indeed, when the length of the part is important compared to the wavelength, the Maxwell equations present numerous solutions susceptible of linear combinations between them. In the case where Maxwell's equations present only one solution, a perturbation remains localized and leads to return to said solution. On the contrary, when the Maxwell equations have many solutions, a disturbance risks leading to a distribution of the electromagnetic field resulting from a linear recombination different from the solutions to these Maxwell equations. It follows that the cell must have an electromagnetic field distribution

  transverse type despite the presence of these obstacles.

  For the same reasons, it is necessary to supply the application element while ensuring that the imposed currents flow according to

  a structure favoring the transverse propagation mode only.

  However, the transverse propagation in a coaxial structure of infinite length, requires that the currents in the walls are perfectly longitudinal and that the distribution of the tensions along the perimeter of the sections normal to the axis of propagation gives tensions in phase. These two criteria, longitudinal currents and equal phase of the voltages along the sections normal to the axis of

  propagation, are necessary and sufficient conditions.

  The present invention will be better understood on studying the

  detailed description of some embodiments taken as

  in no way limiting examples and illustrated by the appended drawings, in which: FIG. 1 is a diagrammatic view in transverse section of a part of the heating device, according to the prior art; Figure 2 is a longitudinal sectional view of the same heating device; Figure 3 is a schematic cross-sectional view of part of the heating device according to the invention; Figure 4 is a variant of Figure 3; Figure 5 is a schematic view in axial section of a distributor with coupled lines; Figure 6 is a schematic cross-sectional view of the distributor of Figure 5; and FIG. 7 is a curve showing the impedance as a function of the frequency according to calculation, and according to measurements carried out on

  installations constructed according to the prior art and according to the invention.

  As can be seen in Figures 1 and 2, a heater comprises an upper electrode 1 and a lower electrode 2, between which are arranged two parts 3 to be polymerized, for example of thermosetting resin. The parts 3 are of elongated shape and the upper 1 and lower 2 electrodes are of a shape adapted to the parts 3. The lower electrode 2 comprises three separators 4, made of insulating materials, making it possible to define in

  width of the space left for the rooms 3.

  The lower electrode 2 is supplied by a coaxial line

  5 connected in the middle of said lower electrode 2.

  In Figure 1, there is shown in dotted lines the upper electrode 1 in a raised position relative to the lower electrode 2 and used for demolding the parts 3. The upper electrode 1 is provided with an upper shell 6 and l the lower electrode 2 is provided with a lower shell 7. The shells 6 and 7 being provided to come and form an enclosure in which are

arranged electrodes 1 and 2.

  FIG. 3 shows a heating and polymerization device in accordance with the invention. This device includes a

  line supply coupled with four lines referenced 8 to 11.

  Of course, the apparatus comprises a voltage generator, not shown, capable of supplying a voltage at a frequency of several tens of MHz to the upper 1 and lower 2 electrodes.

to which it is connected.

  The lower electrode 2 is supported by a plurality of insulating columns 2a. The parts 3 which can be spring leaves have a section which varies lengthwise and it is important that the electromagnetic field applied corresponds to a voltage distribution as a function of the known length between

  the lower electrode 2 and the upper electrode 1.

  In Figure 3, the separators 4 are shown of equal width. However, one could provide a central separator of smaller width insofar as it is subjected to lesser stresses than the lateral separators. The same applies if the

  heating is provided for a number of rooms 3, greater than two.

  The tension profile as a function of the length can be calculated as described in French patent 96 10 848, the

  content is incorporated below by reference.

  However, the voltage profile is disturbed by the presence of the columns 2a and by the angular shape of the section of the electrodes 1 and

  2 and shells 6 and 7 which form a shield.

  It has been observed that if the apparatus is supplied by a single supply, as shown in FIGS. 1 and 2, the associated currents radiate almost circularly around the point A where the supply line 5 is connected to the electrode 2. As a result, all the points of electrode 2 located in a transverse plane containing point A are not at the same potential. The mode of propagation

  transverse is disturbed by this feeding.

  A purely transverse electromagnetic field distribution can be restored by dividing the supply of the central electrode into four points, A1, A2, A3 and A4, as illustrated in FIG. 3. We will make sure that the supply voltages

  at points A1, A2, A3 and A4, are well in phase.

  Figure 4 is a variant of Figure 3, where the voltage is distributed in the transverse plane by means of shunt impedances referenced 12 to 15, distributed at points Bl, B2, B3 and B4

  belonging to the same cross section of electrode 2.

  In FIGS. 5 and 6, the circular coaxial cable used to supply the heating appliance is shown. On a predetermined height h, relative to a plane P, the four lines 8 to 11 are arranged parallel to a main line 16, all the lines being short-circuited in the same plane P. The free ends of lines 8 to 11 are connected to supply points A1, A2, A3 and A4,

see figure 3.

  The electromagnetic energy which comes from the generator induces in lines 8 to 1 1 currents and equal voltages. Lines 8 to 11 each capture a share of this energy by electromagnetic coupling. For reasons of symmetry, the same amount of energy emerges from each of lines 8 to 11, if the impedances of the central electrode at points A1, A2, A3 and A4 are equal. The supply phase condition amounts to testing the value of the impedances measured at each of the points A1, A2, A3 and A4, so that we can then use

  four coaxial cables of identical length.

  The total impedance presented by this distributor can be pre-adapted with respect to the generator, that is to say present an impedance close to that of the generator so that there is little reflected energy. This can be achieved by varying the diameter of the main coaxial line 16 of the distributor, the end 17 of which is

  reduced diameter, or by any other known means.

  The embodiment disclosed here includes four supply lines. However, a heater could be provided with a different number of lines, for example two, six, eight, etc. In Figure 7, we can compare the input impedances of the device according to the type of power supply, in order to show

  the advantage offered by distributed power to four distributed lines.

  These measurements are carried out by a network analyzer in a fairly wide frequency domain. Complex impedance measurements at the input of an electrical circuit reflect the currents flowing in the circuit, because a complex impedance defines the ratio of the

  complex voltage on the complex current.

  For a tool 1.6 m in length, we measured the complex impedance when it is powered by a single central power supply as we usually would. The cell is in open circuit at its ends, this complex impedance varies according to the frequency. The imaginary part of the complex impedance has a pole at 20 MHz. The calculation of the latter as a function of the parameters of the equivalent electrical diagram of the apparatus determined by assuming that the cell operates in pure transverse mode, is indicated in FIG. 7 by the sign "+". We see that the

  measurements do not coincide with the calculation. The pole appears at 38 MHz.

  At the working frequency of 27.12 MHz, the calculated impedance is an inductance, while the complex impedance measured with a single supply is a capacitance. The actual currents in the cell are therefore not those that are calculated assuming a transverse propagation. If the cell is supplied with a supply distributed with two lateral branches, the measurement of the complex input impedance has an imaginary part which approaches the calculation points. If the cell is supplied with a power supply with four distributed branches, the imaginary part of the complex impedance is then comparable to the model. Thanks to the invention, there is a heating and polymerization device suitable for parts of large length and variable section and allowing fine modeling of the electromagnetic field and therefore the amount of heat provided to a

localized portion of the room.

  Of course, this device can be adapted to parts of very different shapes. This device makes it possible to significantly improve the quality of the parts thus treated.

Claims (10)

  1. High frequency heating device, of the type comprising an electric voltage generator and an applicator element provided with two electrodes (1, 2) connected to the generator, intended to apply to a part (3) disposed between the electrodes and in front being heated an electric field created by said electric voltage, the heating device comprising means for adapting the voltage applied to a given portion of the part to the geometric coordinates of said portion of the part to be heated, characterized in that it comprises means for distributing the electromagnetic field transversely to the direction of the   part for both the electric field and the magnetic field.   2. Device according to claim 1, characterized in that it comprises means for distributing the currents longitudinally. 3. Device according to claim 1 or 2, characterized in that it comprises means so that the tensions along the perimeter of the sections normal to the axis of propagation are in phase.   4. Device according to any one of the claims   above, characterized in that it comprises a supply distributed in several coaxial lines (8 to 11) in the same plane normal to propagation, said coaxial lines being connected to a central electrode (16).   5. Device according to claim 4, characterized in that   that the feed includes four lines.   6. Device according to claim 4 or 5, characterized by the   the power supply includes a splitter with coupled lines.   7. Device according to claim 6, characterized in that the lines joining the distributor and supply points of a electrode are of equal length.   8. Device according to any one of claims 6 or   7, characterized in that the lines are electrically coupled and   magnetically to a central main line in the distributor.   9. Device according to any one of claims 6 to 8,   characterized by the fact that the ends of the lines arranged in the   distributor are short-circuited mutually.   10. Device according to any one of the claims   above, characterized in that it comprises at least one group of shunt impedances distributed in a plane normal to the axis longitudinal of the part.