The invention relates to a process and a plant for the manufacture of solid castings from an essentially liquid reactive medium and to an oven for heating an essentially liquid medium in accordance with the respective independent patent claim.

The manufacture of solid castings is today being used for a very wide variety of purposes. In particular, castings of the kind that include as a part of the casting a component or some other body that has been cast inside d e casting are also being produced. One area in which such a procedure is very common is the electrical engineering components field in which components or bodies are encapsulated, for example for the purpose of protection against environmental effects or for insulation.

Such processes for the manufacture of solid castings from reactive liquid media as the casting material are known, for example, from DE-A-2028 873. In the process described therein, the casting material used is a highly reactive epoxy resin material which is distin¬ guished especially by the fact that, when its gelation temperature is exceeded, a reaction takes place in which thermal energy is released, and the thermal energy released during die reaction ensures that the reaction, once triggered, tiien proceeds, as it were, by itself, and the casting material becomes solid. To produce d e casting, the casting material is introduced into a casting mould of which the inner wall has been heated to a temperature that lies above the gelation temperature of the casting material. The casting material is introduced into the mould from the base. In that operation, the casting material is supplied at a temperature which lies distinctly below the gelation temperature so that the reaction does not under any circumstances commence before the casting material has been introduced into the mould.

Ald ough the above process has proved very successful, certain improvements are still possible. For example, the duration of a cycle, that is to say the period for which the mould is occupied in order to produce a single casting, is comparatively long, since the temperature of the casting material lies distinctly below d e gelation temperature when it

is introduced into the mould and the casting material has to be heated at least to the gelation temperature, at which the reaction then commences. There is consequently still room for improvement as regards employing the moulds to capacity. On the other hand, however, care must also be taken that the castings do not contain any bubbles or cracks. Cracks may occur in the casting especially if either the reactivity of the casting material is markedly increased, so that the energy peak during the reaction of the epoxy resin becomes too high, or if the inner wall of the mould is heated to a temperature too far above the gelation temperature of the epoxy resin. As a consequence the thermal stresses in the casting may be too high, which may result in the formation of cracks.

An aim of the invention is therefore to reduce d e duration of a cycle, tiiat is to say the period for which the mould is occupied to produce a single casting, and at die same time to manufacture castings that are free from bubbles and cracks.

That aim is achieved in accordance with the process by substantially preheating the casting material comprising an essentially liquid medium direcdy before it enters the casting mould of which the inner wall has been heated to a temperature above the gelation temperature of the casting material. That measure, that is to say substantially heating the casting material direcdy before it enters the mould, prevents the reaction of die casting material from commencing before the material has been fed into die mould. Heating of the casting material in the mould therefore does not have to commence at the otherwise customary lower temperature. As a result, the time taken for the casting material to be heated to die gelation temperature in the mould and for the reaction of die casting material to commence is considerably reduced. The duration of a cycle is consequendy shortened considerably. At die same time it is possible to produce in tiiat manner castings that are free from bubbles and cracks.

In one example embodiment of d e process according to die invention, d e casting material used is a mixture of hexahydrophthalic acid diglycidyl ester, hexahydrophthalic acid anhydride, benzyldimethylamine and a silanised quartz powder, in a ratio of approx¬ imately 100 parts by weight of hexahydrophdialic acid diglycidyl ester : approximately 90 parts by weight of hexahydrophdialic acid anhydride : approximately 0.5 parts by weight of benzyldimethylamine : approximately 285 parts by weight of silanised quartz powder. The temperature of die casting material in die supply tank is in tiiat case approximately from 30°C to 60°C, preferably approximately from 40°C to 50°C. During die preheating operation direcdy before introduction into die mould, die casting material is heated to a

temperature of approximately from 90°C to 110°C, preferably approximately from 95 °C to 100°C. The preheated casting material is then fed to the casting mould, which has a temperature of approximately from 130°C to 150°C, preferably approximately from 140°C to 145°C. Using that casting material and those temperatures it is possible to produce castings of especially good quality.

The plant proposed in accordance with the invention in order to achieve the aim comprises a supply tank for providing die essentially liquid reactive medium. In tiiat supply tank the temperature of die medium, mat is of the casting material, is below the gelation tempera¬ ture. The plant also comprises a feeding means for feeding die medium into a casting mould. The casting mould is heated to a temperature that lies above the gelation tempera¬ ture of die casting material. Connected direcdy upstream of the casting mould is an oven dirough which the casting material flows and is tiiereby preheated. It is d erefore not necessary for the casting material to be heated in die mould starting from the otherwise customary lower temperature. As a result, die time taken for the casting material to be heated to the gelation temperature in the mould and for the reaction of d e casting material to commence is considerably reduced. On die otiier hand, die measure of connecting the oven direcdy upstream of die casting mould and thus substantially heating the casting material shortly before it enters die mould prevents the reaction of the casting material from commencing before the casting material has been fed into die mould. It is thus possible for the duration of a cycle, tiiat is to say the period for which the mould is occupied in order to produce a single casting, to be shortened considerably. At the same time it is possible to produce in tiiat manner castings that are free from bubbles and cracks, that is to say that suffer no loss in quality.

In one example embodiment of die plant according to die invention, tiiere is in addition connected between die supply tank and die oven a separate pressure vessel from which the casting material is taken and fed dirough d e oven into die casting mould. Such a pressure vessel is advantageous especially in view of the fact that die casting material can be stored tiierein, as it were, ready for the casting operation. Depending on die nature of the casting material.used (for example in the case of epoxy resins), die material is first of all subjected to a degassing operation by being stirred in a large supply tank fitted with a stirrer before it is ready for the casting operation.

In a further development of die example embodiment just described, d e pressure vessel is arranged on a set of scales. The weight of die filled pressure vessel is ascertained by

weighing and die scales are then, for example, reset to zero before the mould is filled. Resetting of d e scales to zero is not, however, essential. When an amount which is to be introduced into the mould is taken from the pressure vessel, then when the desired amount of casting material has been taken from the pressure vessel the scales send a signal to a control means. The pressure under which die casting material is conveyed to the casting mould is tiien increased.

The plant according to the invention is an important field of use for the oven according to die invention which is connected direcdy upstream of the casting mould. The oven is so designed tiiat the medium to be heated, especially a reactive casting material, passes through an inlet into a channel. The medium flows through the oven in that channel. As it flows through the channel the medium is heated by means of electromagnetic radiation, which is effected by means of several separate heating units arranged along the channel. Each heating unit comprises an electromagnetic radiator having a waveguide tiiat guides die electromagnetic radiation to die channel through which the medium to be heated flows and tiiat couples the radiation into that channel.

In an advantageous example embodiment of die oven, die waveguides of die individual radiators are so arranged along die channel that they couple the radiation into the channel transversely to die direction of flow of the medium to be heated. Adjacent radiators can be substantially decoupled from one another. Decoupling of die individual radiators from one another ensures that tiiey have a better working area and tiius ensures tiiat die medium flowing through is heated well.

An oven radiator may be so designed as to comprise a high frequency generator (HF generator) and, connected thereto, a waveguide which opens into the channel. That wave¬ guide guides die radiation generated by d e HF generator to the channel and couples it into tiiat channel. The cross-section of die waveguide is preferably rectangular, which is important from the point of view of the excitation of die modes necessary for the proposed heating process and capable of propagation which provide a uniformly good heating of the medium flowing through die channel.

To decouple adjacent radiators there may additionally be provided in die channel at least one, as a rule metallic, decoupling diaphragm tiiat is arranged essentially at right angles to the direction of flow and between the radiators viewed in the direction of flow. That diaphragm may serve not only to decouple adjacent radiators, however, as will be

explained in die following.

In a further development, there is arranged in the channel, the geometric shape and dimensions of which can be determined with a view to the necessary minimisation of die space requirement and taking into consideration the casting material that is to be heated, a separate pipe through which the medium to be heated flows. Minimisation of the space requirement renders possible any desired integration in other machines or plants or parts of machines or plants. The pipe is passed dirough a passage in the decoupling diaphragm and preferably consists of a material of which the dielectric losses are negligible in die operative wavelength range. The decoupling diaphragm in tiiat arrangement acts in addition as a support for the pipe.

The diaphragm may be designed to lead in the form of a funnel towards its passage for the pipe, die generatrix of the funnel in the longitudinal direction viewed in the plane of longi¬ tudinal section complying with an exponential function having a negative exponent The exponential function with which the generatrix of the funnel of die diaphragm in the longi¬ tudinal direction complies may be described especially by

a(z) = a ₂ x exp -(3.13 x 10 ^' Φ x k x z x (1-f _c f) ² )

wherein z denotes the coordinate on die longitudinal axis of the channel, a(z) denotes d e distance of d e respective point of the generatrix of die funnel from the longitudinal axis of the channel, aj denotes die distance from die longitudinal axis of the channel at the beginning of the funnel, i.e. when z = 0, k denotes die wave number, φ is the attenuation in dB of d e return component of d e wave compared with d e forward component, f _c denotes die minimum possible frequency, that is d e lower limiting frequency, and f denotes die actual frequency of the wave. Witii such diaphragms it is possible, even in die case of small diaphragm lengths in the direction of d e longitudinal axis, nevertheless to achieve a good attenuation effect (diaphragm lengtii in die direction of die longitudinal axis ≤ 20 mm).

Preferably, there is always a decoupling diaphragm arranged between two adjacent heating units so tiiat decoupling of die individual units, especially the generators of tiiose units, is ensured and thus stable operation of the generators is assured.

As has already been mentioned, die radiators may comprise an HF generator with a wave-

guide connected thereto as radiation guide, which guides die radiation generated by die HF generator to the channel and couples it into the latter. Adjacent radiators should preferably.be decoupled from one anotiier in that arrangement For that purpose the radiators may be so arranged along the channel that the coupling of the high-frequency radiation into the channel occurs with a different direction of polarisation in each case. That can be achieved, for example, by offsetting adjacent radiators by a certain angle with respect to one anotiier, preferably by an angle of approximately 90°, and/or by using suitable polarisation filters between the individual radiators. In addition, the offsetting, preferably by an angle of approximately 90°, reduces locally unfavourable superposition of the field components of the electromagnetic field produced, as a result of which a more homogeneous temperature distribution can be achieved in the area of space in which the high-frequency radiation is acting.

It is possible to arrange the HF generators direcdy, tiiat is to say without waveguides, along the channel. So that tiiere is no interference between the HF generators, decoupling of the HF generators is possible, for example by offsetting them with respect to one another and/or by using polarisation filters between them.

A further aspect of the oven according to die invention relates to the possibility of repeatedly passing the medium to be heated dirough the oven and exposing it to die heating electromagnetic radiation. By that means better use is made of die interior of the oven, especially the channel, and also of die radiation fed into the channel.

There are especially provided in die channel a forward pipe and a return pipe, the medium to be heated flowing first through the forward pipe and tiien through the return pipe. The longitudinal axes of the two pipes are arranged at a distance from the longitudinal axis of the channel that is so selected that the electrical field component of the radiation has a maximum on the longitudinal axis of the two pipes. In that manner the radiation coupled into the channel is used twice. That also simplifies the control and regulation of the output of the magnetrons. If, for example, the temperature of the resin at the outlet of the return pipe is measured and it is established tiiat it is too high or too low, then the output of the magnetrons has to be regulated by a smaller amount than would be necessary if it were to be used only once, thus rendering possible more rapid regulation of die output In addition, die homogeneity of die temperature distribution in the casting material is thereby increased.

A further aspect of the invention relates to the possibility of the oven being of modular design with each individual module comprising an electromagnetic radiator having a waveguide which opens into a channel portion bounded at each end by closing walls, thus defining a resonance chamber. The electromagnetic radiation is coupled into that chamber. By means of passages dirough the closing walls there is guided through the resonance chamber at least one separate pipe dirough which the medium to be heated flows. That is of advantage especially if relatively large amounts of a casting material have to be heated within a short period, and consequendy more energy in the form of microwave radiation has to be supplied since also a larger amount of casting material has to be heated. The modular construction is very advantageous since die individual modules are simple to assemble and consequendy it is possible for ovens of even greater output also to be constructed using the same modules simply by connecting several individual modules one after another.

The channel portion of such a module is, for example, in the form of a hollow cylinder and has an internal diameter selected to be approximately n x λ _g 1.236, n being a natural number and λ _g die wavelengtii of the radiation in the waveguide. Furthermore, die length of d e channel portion is smaller than half die wavelength and is approximately in die region of half the internal diameter of die channel portion, it being possible for that length to vary by a constant that depends on die frequency of the radiation and on die medium flowing through the pipe. That constant A is inversely proportional to the frequency employed and to the dielectric constant of the casting material. The length of the channel portion is so selected tiiat the electrical field component of the radiation has a minimum at the passage through the closing wall. In principle it is then possible to do without separate decoupling measures, but even so the closing walls naturally have to be provided between the individual units in order to define die resonance chamber for the wave propagating therein. The closing wall does not, however, have to have a funnel with an exponential curve.

The distance between the longitudinal axes of the forward and return pipes may in partic¬ ular be approximately half the internal diameter of die channel portion for pipe diameters mat range from a quarter of the internal diameter of die channel portion to half the internal diameter of die channel portion. The distance between d e longitudinal axes of the forward and retum pipes for pipe diameters at are in the region of less than a quarter of the internal diameter of the channel may be approximatley half the internal diameter of die channel plus an amount obtained by multiplying a factor with the pipe diameter, tiiat

factor ranging from 0.5 to 1.2

A further aspect of the oven according to die invention relates to the possibility of convey¬ ing the medium to be heated along a helical line through the oven. As a result, a longer interaction between the casting material and the electromagnetic field can be achieved, that measure serving to achieve a higher efficiency of the oven since the path along which the casting material is transported in the oven is longer.

According to a further aspect of the invention, there is provided in each waveguide a displaceable tuning screw which can be so displaced tiiat it represents an open circuit for the wave going towards die channel and a short circuit for the wave returning from the channel. The tuning screw is displaceable in a slot so tiiat when there are different high- frequency ratios in the transition plane between the waveguide and die channel it is possible to effect optimal adaptation of die output. It can also be displaced in the direction into and out of the waveguide and can consequendy always be optimally adjusted for different frequencies.

In the following, the invention is explained in detail with reference to the drawings, in which at least partly in section or in diagrammatic form:

Fig. 1 shows a general plan of an example embodiment of a plant according to the invention,

Fig. 2 is a cut-away portion of an example embodiment of an oven of the invention,

Fig. 3 shows the funnel of a variant of the decoupling diaphragm,

Fig.4 is a view along the line IV-IV in Fig.2,

Fig. 5 shows a variant of a helical course of a pipe through which the casting material flows in the oven,

Fig. 6 is a further example embodiment of an oven according to the invention,

Fig.7 is a section along the line Nil- VH of Fig. 6,

Fig. 8 shows a heating unit of the example embodiment of die oven according to Fig. 6,

Fig. 9 is a side view of a closing wall of the heating unit,

Fig.10 is a further side view of the closing wall of Fig. 9

and

Fig.11 is a variant of the oven according to die invention which comprises two modules each having three heating units.

In the example embodiment of die plant according to die invention shown in Fig. 1, there is a supply tank in the form of a degassing mixer 1 in which the casting material is degassed by stirring. The oudet of die degassing mixer 1 can be closed by means of a valve VI. Degassed casting material can be fed into a pressure vessel 2 through a supply line which can be closed by a valve V2. If the casting material from the degassing mixer 1 is not to go into the pressure vessel 2, for example if the pressure vessel 2 is being repaired, a by-pass line BP which can be closed by means of a valve V3 is provided dirough which the casting material can flow out of the degassing mixer 1. Normally, however, when the casting material is removed from die degassing mixer 1 it is fed into the pressure vessel 2.

The pressure vessel 2 is arranged on scales 3. Casting material can be removed from the pressure vessel 2 by means of the pressure. In a case where the casting material flows through the by-pass line BP, the pump PI takes over the function of die pressure in the branch in which the pressure vessel is arranged. The casting material taken from the pressure vessel 2 (or the casting material conveyed by d e pump PI) flows through a microwave oven 4 and then passes through a supply line, which can be closed by means of a valve V5, into the casting mould 5 of which the inner wall has been heated to a temper¬ ature that lies above the gelation temperature of the casting material. The casting material is fed into the mould from the base, as described in DE-A-202873 already mentioned at die beginning. The casting is moulded and produced in the casting mould 5.

The example embodiment of the plant according to die invention shown in Fig. 1 further-

more comprises, in addition, a drain valve N4, which connects a discharge line to a collecting vessel 6 and is closable. All the valves VI, V2, V3, V4 and V5, the pressure vessel 2, the pump PI and the scales 3 are connected to a control means 7, the mode of operation of which is explained in d e following in connection with the description of the operation of the plant.

When the plant is started, at first only the degassing mixer 1 is full of the casting material, e.g. the mixture mentioned hereinbefore. The temperature of the casting material in the supply tank is approximately from 30°C to 60°C, preferably approximately from 40°C to 50°C, and is tiius distinctly below the gelation temperature of the mixture, which is unreactive at that temperature. After the casting material (mixture) has been degassed, die control means 7 opens the valves VI and V2, valve V3 remaining closed. The casting material coming from the degassing mixer 1 thus passes into the pressure vessel 2 and die latter is filled. Once the pressure vessel 2 is full, valves VI and V2 are closed again. The casting material is now located in the pressure vessel 2 ready for the casting operation.

The control means 7 then first of all opens the valve V4 and casting material is conveyed until the air has been removed from die lines and die oven 4, that is to say until the casting material has reached die collecting vessel 6. The valve V4 is then closed again and valve V5 is opened until the casting material has flowed out of die casting head. The valve V5 is then also closed again and d e plant is then completely free of air and is thus ready to be used for the casting operation.

The scales 3 are reset to zero, and die mould halves 51 and 52 with the heating plates 510 and 520 are closed. Valve V5 is then opened so tiiat the casting material can be fed into the casting mould 5. The temperature of the casting mould 5 is approximately from 130°C to 150°C, preferably approximately from 140°C to 145°C, that is to say above the gelation temperature of the casting material. The scales, which monitor the weight of die pressure vessel 2 with d e casting material contained therein, send a signal to the control means 7 when a particular adjustable amount of casting material has been taken from the pressure vessel 2, and die control means 7 increases the pressure under which casting material is supplied to d e casting mould 5. The adjustable amount of casting material depends in each case on the casting material used and on d e geometry of the casting to be produced. The amount of casting material which passes into the casting mould 5 at elevated pressure is only that amount which is necessary to compensate for the shrinkage in volume during the reaction of the mixture in die casting mould 5 in order to produce a bubble-free

casting. The elevated pressure is, however, maintained. Once the volume shrinkage of the casting material in the mould has been compensated, then in spite of the fact that pressure is still being applied no more casting material passes through the microwave oven 4 into the casting mould.

When the casting has been moulded and produced to the extent that it can be removed from the casting mould 5, the valve V5 is closed and die mould halves 51 and 52 are opened again. The mould is tiien cleaned, the mould halves 51 and 52 are closed again and die valve V5 is opened again so that a new casting can be produced in the same manner.

In the interval between the manufacture of two castings, the lines commencing at the pressure vessel 2 and passing through the microwave oven 4 to die casting head 50 remain filled with casting material. If that interval exceeds a particular duration, the control means 7 opens the valve V4 so that die reactive casting material located in die lines can pass into the collecting vessel 6, since otherwise reaction of the casting material might occur and die solidified casting material might block the lines, especially in the micro¬ wave oven 4, as a result of which the operation of the entire plant would dien have to be interrupted.

In the microwave oven 4, which has already been mentioned several times and which is connected direcdy upstream of die casting mould 5, die casting material is heated by means of electromagnetic radiation (by means of microwave radiation) to a temperature just below its gelation temperature, that is to say, for example, to a temperature of approx¬ imately from 90°C to 110°C, preferably to approximately from 95°C to 100°C, for die mixture mentioned above, before it is conveyed to die casting mould 5. Witii the plant according to die invention, therefore, the casting material fed into the casting mould does not have to be heated starting from the otherwise customary lower temperature of approx¬ imately from 40°C to 50°C. As a result, the time taken for the casting material to be heated to die gelation temperature in the mould and for the reaction of d e casting material to commence is considerably reduced. On die otiier hand, die measure of connecting the microwave oven direcdy upstream of the casting mould and tims substantially heating the casting material before it enters the mould prevents the reaction of the casting material from commencing before the casting material has been fed into the mould. It is thus possible for the duration of a cycle, tiiat is to say the period for which the mould is occupied in order to produce a single casting, to be shortened considerably. At die same time it is possible to produce in that manner castings that are free from bubbles and cracks,

that is to say that suffer no loss in quality.

The plant according to the invention is a very important field of application for the micro¬ wave oven 4 according to the invention which is connected direcdy upstream of the casting mould 5. An example embodiment of tiiat microwave oven 4 is explained in detail in the following with reference to Fig. 2. Whereas in Fig. 1 the channel through which the casting material flows in the oven is essentially U-shaped so as to minimise the space requirement, another example embodiment of the oven in which the channel is of a straight design will be explained witii reference to Fig. 2.

For the purpose of clarity, the oven housing has not been shown in Fig.2. Instead, tiiere have been shown diagrammatically only those parts of that example embodiment of the oven according to the invention that are necessary for the purposes of understanding. The Figure shows a channel 40 with an inlet 41 and an outiet 42 for the casting material. Arranged in the channel 40 is a separate pipe 43 through which the casting material flows. In order to heat die casting material by means of microwaves several heating units 44 are arranged along the channel. A heating unit 44 in this case comprises two microwave radiators 44a and 44b, which in turn each comprise a magnetron 440 as HF generator which feeds die microwave radiation by means of its antenna 441 into a waveguide 442 connected to die magnetron 440, which waveguide guides die radiation to the channel 40 and opens into that channel so that in that manner the radiation generated by die magnetron 440 is coupled into the channel 40. The waveguide 442 is so arranged tiiat it couples the microwave radiation into the channel transversely to the direction of flow of the casting material to be heated. The antenna 441 feeds die radiation with symmetrical output into the two "branches" of the annular waveguide 442. The homogeneity of the energy coupled into the casting material is thereby increased and with it die homogeneity of the temperature of the casting material. The substantially annular waveguide 442 has a rectangular cross-section in die example embodiment described here. A waveguide 442 having a differendy shaped cross-section is, however, equally possible. The important factor is to ensure the excitation of electromagnetic fields tiiat have a stable wave shape. The dimensions of die waveguide 442 and die frequency (or die wavelength) of the micro¬ wave radiation generated by the magnetron 440 are tuned to one another. The pipe 43 preferably consists of a very low loss material for the microwave radiation, that is, a material that has only minimal dielectric losses with respect to the microwave radiation used. A suitable material for such a pipe is, for example, Teflon. The channel 40, on the other hand, consists of a material that reflects the microwave radiation, that is, a material

of good conductivity, for example aluminium, so that the microwave radiation coupled into the channel can be propagated in die channel 40. As a result of the interaction between the microwave radiation and die casting material, the energy of the microwave radiation is for the most part converted into diermal energy, which ultimately results in the casting material which flows through being heated.

As can be seen from Fig. 2, several such heating units are arranged along the channel 40, d e units being arranged in such a manner that adjacent microwave radiators are essen¬ tially decoupled from one anotiier. In the example embodiment described here, die wave¬ guides of adjacent radiators are so arranged along die channel 40 that they open into the channel each offset at the circumference of the channel by an angle α of approximately 90°. That arrangement can be seen especially well in Fig. 4, which shows a view along the line IV-IV of Fig. 2. Generally, the angle α is so selected tiiat the wave types excited by each radiator are propagated in die channel 40 with as different as possible a polarisation for each radiator. An especially effective decoupling of adjacent heating units 44 is produced witii an angle α of approximately 90°. A good decoupling is of importance especially with regard to favourable operating regions of the magnetron. A further improvement of the decoupling may comprise electrically phase-displaced operation of the individual magnetrons.

A further measure with a view to as effective and reliable as possible a decoupling of adjacent heating units is also shown in Fig. 2. It comprises the provision in the channel of decoupling diaphragms 45 that are arranged essentially at right angles to the direction of flow of the casting material, and between individual heating units 44 viewed in the direction of flow. Obviously, in each case a separate decoupling diaphragm 45 can be arranged between all the microwave radiators but, for simplification, a diaphragm 45 has been shown only between adjacent heating units 44 in Fig. 2. Apart from its decoupling action die decoupling diaphragm 45 also has a further advantage: it can facilitate the insertion of the pipe 43 into the channel 40, especially if it is funnel-shaped. In addition, it can support die pipe 43 inserted into the channel 40. Especially when the passage through the diaphragm 45 is funnel-shaped, die insertion of the pipe 43, which normally consists of a material that is transparent to die microwave radiation, into the channel 40 is substan¬ tially facilitated by means of the diaphragm 45. A Teflon pipe is especially suitable for the described heating process.

Decoupling by the funnel-shaped diaphragm 45 is effected by coupling the forward wave

with the return wave in such a manner that a large degree of attenuation occurs. The diaphragm 45 leading in the form of a funnel towards its passsage for the pipe 43 is so designed that die generatrix of the funnel in the longitudinal direction complies with an exponential function having a negative exponent, as shown in Fig. 3. In particular, the generatrix of the funnel of the diaphragm 43 in the longitudinal direction complies with the function

a(z) = a _! x exp -(3.13 x 10 ^' Φ x k x z x (1- f) ² )

in which z denotes the coordinate on die longitudinal axis L, the point z=0 on die longi¬ tudinal axis L coinciding with the beginning of the funnel of the diaphragm. Also, a(z) denotes the distance of die respective point of the generatrix of the funnel from the longi¬ tudinal axis L, a _! denotes die distance from the longitudinal axis at the beginning of the funnel, i.e. when z=0, k denotes the wave number, φ is the attenuation in dB of d e return component of the wave compared with die forward component, f _c denotes the mininimum possible frequency, i.e. the lower limiting frequency, and f denotes the actual frequency of the wave. With such diaphragms it is possible, even in the case of small diaphragm lengths in the direction of the longitudinal axis L, nevertheless to achieve a good attenuation effect of the incident wave in small lengths (diaphragm length in the direction of the longi¬ tudinal axis L ≤ 20 mm).

In Fig. 2 there is provided in die vicinity of the outlet 42 a thermoelement 46 which measures the temperature of the heated casting material. That thermoelement 46 is connected to a rapid regulating means 47 in the control means 7, which acts on die magnetrons 440, there being shown in Fig. 2 only two connections to the magnetrons 440 to represent the connections to all magnetrons. When the temperature of the heated casting material at the outiet 42 is too high, the energy generated by die magnetrons is reduced, since the pipe 43 may become blocked if the casting material reacts in the pipe 43. The possibility exists of taking measurements of the temperature distribution along the channel 40 so as to be able to establish especially desired output profiles or temperature profiles by means of optimised regulation.

In order to improve interaction between the casting material and die electromagnetic field and tiius to achieve a higher efficiency of the oven, the patii along which the casting material is transported may extend in the shape of a helix about die longitudinal axis L. For that purpose the pipe 43 may be designed in the form of a helix about the longitudinal

axis L as indicated in Fig. 5.

In a practical example embodiment, there may be used as casting material a mixture of hexahydrophthalic acid diglycidyl ester, hexahydrophthalic acid anhydride, benzyl¬ dimethylamine and a silanised quartz powder, in a ratio of approximately 100 parts by weight of hexahydrophdialic acid diglycidyl ester : approximately 90 parts by weight of hexahydrophdialic acid anhydride : approximately 0.5 parts by weight of benzyldimethyl¬ amine : approximately 285 parts by weight of silanised quartz powder. The temperature in the supply tank may be approximately from 30°C to 60°C, preferably approximately from 40°C to 50°C. During the preheating operation in the microwave oven the material can then be heated to approximately from 90°C to 110°C, preferably to approximately from 95°C to 100°C. The temperature of the casting mould 5 may tiien be approximately from 130°C to 150°C, preferably approximately from 140°C to 145°C. The flow rate in that case may be approximately from 4.5 to 5 kg min with a difference in die temperature of the casting material at the inlet and oudet of the oven of 60°C. Obviously, with the same oven (same microwave output), greater temperature differences can be achieved with a lower rate of flow, and smaller temperature differences with a higher rate of flow. In general, higher flow rates with a constant or greater temperature difference ΔT can be achieved by using more powerful HF generators and/or by a cascade-like connection of several ovens in series with one another. The series connection of the ovens is rendered possible by their modular construction. As shown in Fig. 2, six microwave radiators may be provided which each emit an output of 1.26 kilowatts, the frequency preferably lying in the range of approximately from 900 MHz to 30 GHz, especially approximately 2.45 GHz ± 10 MHz. It is, however, also possible for other frequencies to be used, die geometric dimensions of the oven and die frequencies employed being tuned to one another in each frequency range. Using such a mixture at the temperatures mentioned, it is possible to produce bubble- and crack-free castings reliably and quickly.

A further example embodiment of die channel and a few further details of an oven accord¬ ing to the invention are explained in the following with reference to Figs. 6 to 11. The oven comprises a channel 140 that has an inlet for several pipes, in this case a forward pipe 141 and a return pipe 142 (Fig.7), through which the casting material to be heated flows, as well as an outlet for those pipes. The pipes 141 and 142 are arranged at a particular distance from the longitudinal axis of the channel which is determined in the manner explained hereinafter. The casting material flows through the channel 140 first of all through the forward pipe 141. A U-shaped deflection (not shown) of the casting

material heated on the forward patii through the channel 1 0 may be effected at the outiet, and the casting material then flows back through the channel 140 again, through the return pipe 142. In that manner the radiation coupled into the channel is used twice. This simp¬ lifies the control or regulation of the output of the magnetrons. If the temperature of the resin is measured at die oudet of the return pipe 142 and it is established tiiat it is too high or too low, the output of the magnetrons has to be regulated by a smaller amount than would be necessary if they were to be used only once, thus rendering possible more rapid regulation of die output. In addition, the homogeneity of the temperature distribution in the casting material is thereby increased.

The channel 140 itself comprises several, in this case three, individual adjacent heating units 143, 144 and 145, which are joined (e.g. welded) to one another, there being provided at each of die joining points, between the individual heating units 143, 144, 145, (and also at die inlet and at the oudet), a metallic closing wall 146. A resonance chamber for the wave is thereby defined in each case. The pipes 141 and 142 are guided dirough corresponding funnel-shaped openings 1461 and 1462 (Fig. 9 and Fig. 10) in the closing wall 146 (in a similar manner to that in the example embodiment described further above). As a result of the funnel-shaped construction of those openings in the closing wall 146, insertion of the pipes is facilitated during installation and the pipes are also supported tiiereby.

Each individual heating unit, for example the heating unit 143, comprises a microwave radiator 147 witii a magnetron as generator and a, for example, rectangular waveguide 1471 (Fig. 8) connected tiiereto into which the microwave radiation of the magnetron is fed by means of an antenna 1472. The same applies to the heating units 144 and 145. The waveguide 1471 guides to the channel 140 the radiation which has been coupled into it from the magnetron. Since the waveguide opens into the channel 140 (Fig. 8), it couples the radiation into the channel, that opening and coupling occurring transversely to the direction of flow of the casting material to be heated.

There is provided in the waveguide 1471 a so-called tuning screw 1473. That tuning screw represents an open circuit for the forward wave in the waveguide 1471 and a short circuit for the return wave in the waveguide 1471 returning in the direction of the generator (magnetron). In that manner the generator is protected against reflections that derive, for example, from temperature-dependent variations in the material properties of the casting material. The generator can thus be operated in a stable manner in a favourable and

reliable operating region (output and oscillation stability).

The tuning screw 1473 is displaceable in a slot 1474 (Fig. 7) so that when there are different high-frequency ratios in the transition plane between the waveguide 1471 and die channel 140 it is possible to effect optimal adaptation of the output. It can also be displaced in the direction into and out of the waveguide 1471 (Fig. 8) and can conse¬ quently always be optimally adjusted for different frequencies.

Worthy of special note is the length 1 and the diameter d-j of the, for example, hollow cylindrical portion of the heating unit (Fig. 8) that forms a portion of the channel 140. The length 1 is smaller than half the wavelength of the microwave radiation used for the heating. This is worthy of note insofar as, with a particular spacing between two closing walls 146 and witii a particular diameter dj of the portion of the heating unit which forms a portion of the channel 140, only the wave types most favourable for the proposed heating process can be propagated between the closing walls 146. Those may be, for example, waves of the type TM _lln . The two pipes 141 and 142 (Fig. 9) are so arranged that their longitudinal axes extend at a certain distance b from one another, so that the electrical field components of die waves tiien have a maximum on the longitudinal axis of the two pipes, which results in a very good transfer of energy to the casting material flowing in the pipes 141 and 142. The efficiency, i.e. the ratio of the thermal energy produced in the casting material to the electrical energy fed into the magnetron, may in that case be up to 70 %.

The internal diameter dj of die channel portion is so selected tiiat it is approximately

di = n x λ _g /1.236,

n denoting a natural number (1,2,3,...) and λ _g die wavelengtii of die radiation in the wave¬ guide 1471. The channel portion 140 furthermore has a length 1 selected roughly in the region of

l = dV2

it being possible for that length 1 to vary about that value d 2 by a constant A which depends on die frequency of the radiation and on die casting material flowing through the pipe. The constant A is inversely proportional to the frequency used and to die dielectric

constant €Q X € _r of the casting material, thus

A ~ 1 / (e ₀ x e _r x f).

The selection of the lengtii 1 of the channel portion 140 is such that the electrical field component of die wave has a minimum at the passage through the closing wall. In principle it is possible to do without separate decoupling measures, but even so the closing walls 146 naturally have to be provided between die individual units in order to define the resonance chamber for the wave propagating therein. The closing wall 146 does not, however, need to have a funnel with an exponential curve.

The distance b between the longitudinal axes of the two pipes 141 and 142 is expediently selected as a function of the size of the diameter d. (Fig.7) of die pipes 141 and 142. For pipe diameters d,. in die range d./4 < d. < d 2, die distance b between die longitudinal axes of the pipes may be

b = d _j /2

and, for pipe diameters d. in the range d. < dj/4, the distance b between the longitudinal axes of the pipes may be

die factor c, depending on the size of die pipe diameter d,., lying in the range 0.5 < c < 1.2.

In that case, too, it is in principle possible to use pipes that extend in the shape of a helix around die respective longitudinal axis, so as to lengthen the path over which the casting material is heated in the oven. Care must then, of course, be taken to ensure a correspond¬ ing distribution of the maxima of the electrical field component of the wave in the resonance chamber.

It should also be noted tiiat the axes of adjacent waveguides in the example embodiment discussed here include an angle of β = 45° (Fig. 7), that angle, however, being completely arbitrary and being determined, simply for constructional reasons, such that adjacent magnetrons and die waveguides connected thereto do not obstruct one another spatially and can be arranged in a space-saving manner. The choice of the angle β between the axes

of adjacent waveguides has nothing, however, to do witii the decoupling of adjacent magnetrons.

Finally, Fig. 11 shows a further example embodiment of the channel and a few further details of die oven according to the invention. It is possible to see here that two modules, as shown in Fig. 6, which each comprise three individual heating units, are assembled in modular fashion to form a channel having six heating units. That is of advantage especially if larger amounts of a casting material have to be heated wid in a short period, and consequendy more energy in the form of microwave radiation has to be supplied, since also a larger amount of casting material has to be heated. The modular construction is very advantageous since the individual modules are simple to assemble and consequent¬ ly it is possible for ovens of even greater output also to be constructed using the same modules simply by connecting several individual modules one after another.