EP2247418B1 - Method and device for preheating a pressed material mat during manufacture of wood material boards - Google Patents

Abstract

The invention relates to a method for preheating a pressed material mat (14) spread on an endlessly and continuously circulating molding band (6) during manufacture of wood material boards, wherein microwaves from one or both press surface sides are beamed into the pressed material mat (14) to preheat the pressed material mat (14) and the pressed material mat (14) is compacted and hardened by application of pressure and heat after transfer to a continuously operating press (1). The invention consists of microwaves in a frequency range of 2400-2500 MHz being used to heat the pressed material mat (14), wherein the microwaves for each press surface side are generated from 20 to 300 microwave generators (26) with magnetrons (20) of a respective output of 3 to 50 kW. A device for preheating pressed material mats (14) is also provided, in which 20 to 300 microwave generators (26) with magnetrons (20) with an output of 3 to 50 kW and with a frequency range of 2400-2500 MHz are arranged in a conveyor furnace (4) per area side.

Description

The invention relates to a method according to claim 1 for preheating a pressed material mat scattered on an endless continuously circulating forming belt and a device according to claim 15 for preheating a pressed material mat scattered on an endless continuously circulating forming belt during the production of wood-based panels.

From the patent literature and industry, the use of high-frequency technology as a means for preheating Span- or Fasergut for the purpose of reducing the pressing factor during the subsequent pressing process to increase the production capacity is well known. It is known US 4,018,642 A the use of the microwave for plywood, particle boards, chipboard and waffle boards as heat energy, with traveling waves are selectively introduced via so-called wave rectifier with a frequency in the range of 100 to 10,000 MHz to the pressed material. These U.S. Patent 4,018,642 essentially deals with the preheating and curing of alkaline resins and similar glue compositions. The efficiency is usually less than 50%. Thus, it does not make economic sense to use this type of heating for through hardening of a pressed material mat, but only for preheating of poured and possibly pre-compacted pressed material mats. The main problems and dangers of high-frequency heating are not necessarily uniform heating of the pressed material mat, control difficulties of the supplied high-frequency energy and breakdowns occurring. To overcome these difficulties, targeted consolidation measures between the Devices for producing wood-based panels or laminated veneer sheets with microwave preheating are also known DE 197 18 772 A1 . US 5892208 or DE 196 27 024 A1 , With these devices, the preheating of the material to be pressed (pressed material mat, pressed material strand) have been successfully carried out by means of microwaves for some time in the industry. This technology has proven itself in particular in processes for the production of very thick wood-based panels or laminated veneer lumber panels with thicknesses of up to 150 mm, which would not be economical to produce without a preheating device. The microwave preheating units used are predominantly continuous furnaces. Since in the production of wood-based panels, the plate width is many times greater than the plate thicknesses, the microwaves are radiated perpendicular to the wood material plate plane. The plate widths are usually between 1200 and 3900 mm and the plate thicknesses at 30 to 150 mm. The generation of the microwaves takes place in microwave generators, in which the high-frequency modulation and the magnetron tubes are housed. Due to the high microwave power requirement, several generators are required for a preheating device, which usually have an output power of 75-100 kW per generator and are housed in closed electrical control rooms next to the production plant. From there, the microwaves generated are guided by means of hollow waveguides to the actual heating cell in the production plant, wherein a hollow waveguide is necessary for each generator. In order to achieve the most uniform possible heat distribution in the continuous pressed material, guided in the hollow waveguides microwaves are branched coming from the individual generators and thus multiplied by the number of energy-carrying hollow waveguides, thus a narrow grid of feed points below and can be realized above the heating cell. Today is a branch 1 in 2, that is, the energy coming from four generators, which is initially performed in four waveguides, is subdivided on up to 8 hollow waveguides, which open into 8 feed points. The feed into the heating cell takes place by means of round hollow waveguides, which are mounted vertically below and above the heating cell. For each feed point, a measuring and control device is required, with which the phase angle of the microwave is tuned. The investment cost for such a Mikrowavevorwärmeinrichtung is very high and has therefore so far only successful in installations for the production of veneer lumber panels.

With the DE 101 57 601 A1 a device for heating, of pressed material by means of microwave energy was created, with which the investment costs can be reduced, the system availability increased and the control effort should be reduced. This object has been achieved in that the microwave preheating device consists of a heating furnace formed as a continuous furnace, in which the feeding of the microwaves into the pressed material via successively arranged rod antennas with reflection screens, lying horizontally and transversely to the production direction above and / or below the material to be pressed are mounted within the heating cell, wherein the rod antennas are each assigned to the opposite surfaces of the material to be pressed reflection surfaces. The supply of microwaves can continue to be done by means of hollow waveguide from the generators to the heating cell, due to the radiation characteristics of the rod antenna, usually no additional branching of the hollow waveguide coming from the generators is necessary, that is, the number of feed points corresponds to the number of generators. For the transition from hollow waveguide to rod antenna specially developed waveguide transitions are used. This kind of preheating has indeed Proven in principle, but still suffers disadvantages in terms of the extensive space and the high power consumption of individual components.

From the experience and the patent literature, the following frequency ranges for high frequency and microwave can be found in the described industrial application. It is usually understood that a frequency of less than 300 MHz as high frequency, a frequency of 300 MHz to 300,000 MHz as the microwave frequency.

In the DE 694 19 631 T2 finds a high frequency wave with 13.56 MHz and a power of 8 kW use. Out DE 44 12 515 A1 there is an indication of an operating frequency of 21.12 MHz or 13.56 MHz.

Out CA 2 443 799 C Microwave heaters with a frequency band of 915 MHz are known, in which case the microwaves are introduced directly in the inlet gap (region of the tapered press nip in the inlet of a continuously operating press) in the pressed material mat. In addition to a very complicated structure, problems have also shown by uncontrollable reflections on the steel strips in operation.

Basically, the state of the art lacks concrete statements regarding an optimum frequency range in connection with a necessary power consumption or radiation capacity and in conjunction with the necessary number of generators for heating a press material mat with differentiated properties at a given speed. In general, one reads in the patent literature: The exact equipment of the microwave device for this or that method is left to the skilled person (on site), data on the frequency are limited to the range microwave or contain sizes across several powers. These statements do not give the person skilled in the art any indication of the implementation of a teaching with respect to these parameters from the patent literature concerning a optimally usable and useful frequency. It has been found that the skilled person has been virtually left alone and in a range of frequencies when using microwaves over several powers (3x10 ² MHz to 3x10 ⁶ MHz) can decide which frequency could be chosen.

As already indicated, it is further disadvantageous that large plant technical effort to ensure the radiation safety for the staff and the machines must be made when the high or microwave frequencies in separate systems (usually next to the main power connections) are generated and waveguides only for Application must be performed in the production plant. In addition to a massive waste of useful space, costly radiation detectors must be installed in a security area against possible damage to these so-called waveguides. All of this complicates the minimum maintenance (on sight) and promotes a high cost of repairs and downtime. The failure of a preheating system alone results in a business loss despite continuing production of up to 30%, since the press factor increases significantly without preheating and the production speed must be reduced by one third.

The object of the present invention is to provide a method and a device which makes it possible to provide a high efficiency for the heating of Pressgutmatten with a suitable frequency, wherein the heating is uniform and energetic as environmentally and economically as possible to make before This press material mat is pressed in a continuously operating press. At the same time, the method and the device make it possible to use components of lower power consumption. The apparatus provided in this context is usable with the method as well independent functional and should have easily replaceable components and a high resistance to interference.

The solution for creating a method is that microwaves are used in a frequency range of 2400 - 2500 MHz for heating the press mat, the microwaves for each press surface side from 20 to 300 microwave generators with a respective power of 3 to 50 kW are generated.

The solution for a device for carrying out the method or as an independent device is that 20 to 300 microwave generators with magnetrons of a power of 3 to 50 kW and with a frequency range of 2400 - 2500 MHz are arranged in a continuous furnace per press surface side.

Preference is given to using this method and a suitable system press material mats with a basis weight of 2 to 40 kg / m ² heated, which are moved at a feed rate of 50 to 2000 m / s. The mat height is after pre-pressing in the MDF board production at 40 to 350 mm and in chipboard production at 30 to 200 mm. Orientated scattered chipboard (OSB) can be used without pre-pressing in a height of 50 to 500 mm. In a preferred embodiment, magnetrons with a power of 6 to 20 kW are particularly suitable for this frame data of the pressed material mat to be heated. The frequency used is in the ISM (Industrial Science Medicine Band) band and is an internationally recognized and approval-free frequency band for microwaves.

Experiments have now shown that advantageously at a microwave length of 12 cm, a large amount of microwaves is absorbed in a pressed material mat up to a penetration depth of 200 mm. These physical conditions could also be checked by calculation; This is called a depth of penetration "d", which is by definition called the distance from the surface at which the energy of the waves has dropped to 1 / e = 0.37, this being approximately 37% of the field strength prevailing in the outer layers of material E "corresponds. $$d=\frac{c}{\pi \sqrt{{\mathrm{\epsilon \text{'}}}_r}\sqrt{2\left(\sqrt{1+{\tan }^2\delta }-1\right)}}\cdot \frac{1}{f}$$

ergibt sich die Formel $$d=\frac{3\cdot {10}^8\frac{m}{s}}{\pi \sqrt{3,5}\sqrt{2\left(\sqrt{1+{0,11428}^2}-1\right)}}\cdot \frac{1}{2,45\cdot {10}^9\frac{1}{s}}$$

For the following existing boundary conditions
f = frequency = 2.45 GHz,
c = speed of light ≈ 3 * 10 ^ 8 m / s
ε ' _r ≈ 3.5
ε " _r ≈ 0.4, where $${\tan }^{}\delta =\frac{{\mathrm{\epsilon \text{'}\text{'}}}_r}{{\mathrm{\epsilon \text{'}}}_r}=0.11428$$

the formula results $$d=\frac{3\cdot {10}^{\mathrm{8th}}\frac{m}{s}}{\pi \sqrt{3.5}\sqrt{2\left(\sqrt{1+{0.11428}^2}-1\right)}}\cdot \frac{1}{2.45\cdot {10}^9\frac{1}{s}}$$

The thus calculable penetration depth is d = 0.183m.

The hitherto customary high-frequency devices have the disadvantage that a large amount of radiation comes out of the pressed material mat again or simply passes through without heating the pressed material mat. Therefore, reflectors must be arranged on the other side after the press material mat. Include extensive calculations for the best possible radiation and appropriate control and regulatory effort. In the microwave radiation has a through Calculation and corresponding experiments surprisingly shown that in a pre-compacted MDF press material mat or similar material, a penetration depth of about 200 mm at a frequency of 2450 MHz is present. In the OSB production, a pre-compaction is not provided. With a 400 mm high pressed material mat with a two-sided irradiation on the first 200 mm already in the first round about 60% of the energy is converted into heat output and leads to an optimized efficiency during the heating. At the same time, half as high and smaller press material mats can be run at a significantly higher production speed, as a radiation entering from both sides is optimally absorbed and twice as much power is available.

The large number of generators that are necessary for the device and the method advantageously result in a small size of the radiation openings at the microwave frequency used. This is approximately at a 2 x 5 cm opening. For this reason, it is also possible to arrange a plurality of generators in width and in a small space. The waveguide neck at the exit are preferably covered to be protected from a possible dust. When using the usual high-frequency radiation for heating Pressgutmatten, 930 MHz required much larger waveguide, so that a larger number of generators or waveguides over the width of a pressed material mat would not be buildable. A microwave generator is preferably modular and can be easily disassembled on site into parts for repair or replacement. It is also possible to provide a whole microwave generator (including magnetron, circulator and tuner, etc.) as a module and to provide it with quick-release fasteners for mounting and dismounting. Thus, failed microwave generators can be quickly and easily removed from the device and replaced with new ones. An exchange of parts in the previously used high-frequency systems includes a very extensive repair, for which in addition to high personnel costs and large lifting and mounting equipment must be used. Just the effort to bring the necessary materials or staff in a three-shift operation in case of failure on site is costly and costs a lot of time. In contrast, the replacement of a modular microwave generator is easy to accomplish by one or two people easily and takes little time. Such modules can easily be kept available due to their size and during operation of the system is usually always a fitter on site.

In the plant or in the device, a metal detector may be arranged to examine the pressed material mat before microwave heating to metallic parts. Particularly critical are metallic parts whose dimensions are greater in length than ¼ of the wavelength (about 40 mm). Sparks during heating may cause fires in the pressed material mat. Since non-magnetic metal parts can also lead to such reactions and these can not be removed from the press material mat via a conventional magnetic separator, it must be possible to dispose of the press material mat for disposal before heating the press material mat, or the microwave generators are switched off when a detected piece of metal passes through and the discharge of the thus not heated Pressgutmatte can then be done shortly before the press. Nevertheless, it is necessary to check the passing pressed material mat for sparks or fires. This is done with conventional sensors and measuring technology. At the same time, means for extinguishing fires are advantageously present in the device or already integrated in the production hall on site.

In a preferred embodiment of the device, the following technical conditions result:

The overall efficiency of a continuous furnace with microwave generation results from three different efficiencies. η _ges = η ₁ * η ₂ * η ₃ η ₁ corresponds to the efficiency of the transformer, which converts the mains voltage locally into a DC voltage. η ₂ corresponds to the efficiency of the magnetrons used in the microwave generators, which convert the high voltage into microwave radiation and η ₃ is the efficiency of converting the microwave radiation into thermal power in the pressed material mat and corresponds to the temperature increase. In this case occur as a loss, for example, the leakage radiation, reflected power, the absorber power and the like.

Usually η ₁ and η ₂ are specified by the respective manufacturers and in the preferred embodiment have the values η ₁ = 0.95 and η ₂ = 0.70. η ₃ could be determined in laboratory tests and depends to a great extent on the boundary conditions (eg plastic tapes) and the material to be heated. The present material is a mixture of scattered fibers and / or chips which have been precompressed for venting and have a relatively low moisture content.

In the experiments, under laboratory conditions at a throughput of 1 kg / s and a heating by 20 K, a heat output in the product of 36 kW has been shown, which corresponds to an efficiency η ₃ = 0.60. In a further experiment with 0.5 kg / s, a warming of 40 K with constant heat output was achieved, which confirmed the efficiency. Surrounded by a large plant with a throughput of 18 t / h atro and a mat width after side trimming from 1850 to 2150 mm, the requirement is that 18 t of the raw material in the spreading machines per hour from an average temperature of 30 ° Celsius to 60 ° C have to be warmed up by the device.

Thus, at a throughput of 5 kg / s and a desired heating T = 30K, a heat output in the product of 270 kW results. Assuming an efficiency η ₃ = 0.60 results in a total efficiency of η _ges = 0.40 and a total connected load of 675 kW. The required number of magnetrons and their power then results in a further conversion to 450 kW. Divided into a selected number of magnetrons, for example, there are 50 magnetrons with a respective power of 9 kW. Accordingly, 25 magnetrons are installed in respective microwave generators in the device per press surface side. Experience has shown that the installation space is by far sufficient, so that even expansion possibilities are given to double the capacity, for example, and / or to install microwave generators or magnetrons in reserve in order to alternately use a set. Thus, unforeseen overheating conditions in the device and common equipment problems associated with 24/7 continuous operation can be avoided.

The person skilled in the art can communicate that appropriate control and regulating mechanisms and remote maintenance should be provided for such a device. It is also sensible to provide a control circuit which, in accordance with the throughput n kg / s, adapts the power of the microwave generators and ensures optimum and energy-saving use. In addition, values about the moisture content of the pressed material mat, density, speed and the like must be included in this control loop in order to enable meaningful control. Corresponding measurement technology can then be provided in the device.

In a further preferred embodiment, the following structure of the device is given.

The forming belt has a greater width than the microwave belt used in the continuous furnace. The latter is preferably made of Kevlar®. This circumstance results from the need to allow a very wide spread, which is then trimmed by 10-20%, since the edges of a scattered press mat usually have inhomogeneities such as scattering errors or unwanted increases in density. For example, a 2500 mm wide Pressgutmatte before the Enema in the pre-press at 2250 mm width trimmed. Accordingly, it is sufficient if the microwave band in the continuous furnace has a width of 2300 mm. This is advantageous in the necessary design of the sealing of edge radiations from the microwave generation in the continuous furnace. Advantageously, on the longitudinal sides of stationary and movable in the inlet and outlet of the continuous furnace absorption means or - elements are provided which absorb the edge and scattered radiation. Particular attention must be paid to the moisture retention in the pressed material mat and in order to avoid moisture loss during heating by evaporation of the moisture, it may be necessary to provide an endless circulating plastic strip resting on the pressed material mat. The heating by means of the microwaves advantageously causes a uniform temperature distribution of +/- 7 ° C in the pressed material mat 14 over the length and width.

Further advantageous measures and embodiments of the subject matter of the invention will become apparent from the dependent claims and the following description with the drawing.

Figur 1

eine schematische Seitenansicht einer Anlage zur Herstellung von Werkstoffplatten von der Streuung einer Pressgutmatte auf ein Formband bis hin zum Beginn einer kontinuierlich arbeitenden Doppelbandpresse,

Figur 2

eine vergrößerte Darstellung einer Vorrichtung zur Vorwärmung einer Pressgutmatte mittels Mikrowellen nach Figur 1 und

Figur 3

eine Draufsicht auf eine Vorrichtung zur Vorwärmung einer Pressgutmatte mit schematischer Anordnung der Mikrowellenerzeuger.

Show it:

FIG. 1

a schematic side view of a plant for the production of material plates from the dispersion of a pressed product mat on a forming belt to the beginning of a continuous double belt press,

FIG. 2

an enlarged view of a device for preheating a Pressgutmatte by means of microwaves FIG. 1 and

FIG. 3

a plan view of a device for preheating a pressed material mat with a schematic arrangement of the microwave generator.

In FIG. 1 is a production plant for the production of material plates from a Pressgutmatte 14 shown schematically in a side view. It consists in its main parts of one or more scattering stations 16, from which a Pressgutmatte 14 is continuously scattered in one or more layers on a forming belt 6. In production direction 3 there is a pre-press 17, consisting of an over the forming belt 6 endlessly circulating hold-down belt 19. To support the forming belt 6 at higher Niederhaltedrücken may be arranged underneath an endless with circumferential guide belt 18. In the exemplary embodiment, a continuously operating press 1 is shown, which is designed as a double belt press with rotating steel bands 7 and heated press / heating plates 2. The revolving steel belts 7 are compared to the press / heating plates 2 by means of rolling elements 5, for example, parallel to each other and endlessly guided rolling rods, supported.

The continuous furnace 4 is arranged immediately in front of the incoming steel strips 5 of the continuously operating press 1. In this case, the pressed material mat 14 is passed for a passage through the continuous furnace 4 of the forming belt 6 on the lower plastic belt 11 and optionally clamped depending on the type and design of the continuous furnace 4 with a top circumferential plastic belt 8. The absorber stones 25 arranged on both sides of the microwave generator 26 can be raised and lowered by means of the height adjustment 12 and are adjusted depending on the height of the pressed material mat passing through. The height adjustment for the top circumferential plastic band 8 is not shown. The task of the upper plastic belt 8 is to protect the continuous furnace 4 from increased dust formation by the pressed material mat 14 and to prevent the pressed material mat 14 from springing back to its original state during pre-compression through the pre-press 17 during transport. Also, the upper plastic band 8 can prevent escape of moisture during preheating.

Depending on the overall structure of the production plant, it is possible to execute the forming belt 6 as a microwaveable mold belt 6 and the press material mat 14 without a transfer through the continuous furnace 4 drive. Microwaveable mold or plastic bands 6, 8, 11 are characterized in that they only heat by about 10 ° in a passage through the range of the microwave generator 26. Suitable for this purpose, for example, a microwave band KEVLAR® with a one- or two-sided Teflon coating.

As FIG. 2 shows, a simple device of the continuous furnace 4 is constructed as follows. At a lower frame 23 there is the circulation of the lower plastic belt 11 with associated drive 11. In this case, the mold belt 6 passes the Pressgutmatte 14 on the lower plastic belt 11. The gap between the two rotating endless belts can be bridged easily in a Pressgutmatte 14, otherwise means are provided which ensure that a pressed material mat 14 undamaged survives the transition to the lower plastic strip 11 of the continuous furnace 4. In the upper frame 24, a height adjustment 12 is arranged for the inlet 27 and outlet 28 of the continuous furnace 4 provided Absorbtionselemente 25 to properly shield the microwave radiation generated by the microwave generator 26 in order to preheat different heights of Pressgutmatten 14 can. In the same way, the inlet 27 and the outlet 28 can be adjusted in width. This width adjustment and the height adjustment for the upper circumferentially arranged plastic belt 8 are not shown. The absorption elements 25 can be designed, for example, as absorber stones or water containers. In addition to the Absorbtionselementen 25 but also reflectors (such as perforated plates or other suitable means) may be provided or a combination of both possibilities. Preferably, the reflectors are arranged such that they reintroduce the scattered radiation directly into the pressed material mat 14. Furthermore, sensors 29 can be arranged which detect the height and the width of the pressed material mat 14 and adjust the inlet 27 and the outlet 28 of the pressed material mat 4 accordingly.

On the support frame 15, the microwave generator 26 are arranged in the middle of the continuous furnace 4. A microwave generator 26 consists at least of a magnetron 20, an associated circulator 21 and a tuner 22. The tuner 22 takes care of the fine adjustment of the microwave radiation or its orientation, whereas the circulator 21 receives retroreflective microwaves and feeds them to further use. In most cases, water from the water cooling 9 is heated to absorb the excess microwave radiation. At 13, the metal detector of the device is shown. This can also be arranged depending on the design of the system directly above the forming belt 6 in front of the continuous furnace 4. Preferably, in this case, a discharge or a broaching possibility of an offset with metal pieces Pressgutmatte before the continuous furnace 4 is given. Alternatively, or even if the metal detector 13 is disposed within the circulation of the plastic bands 8, 11 in front of the absorber bricks, the microwave generators 26 are briefly switched off during the passage of a piece of metal and the part of the pressed material mat 14, which has not been heated, over a short in the direction of production disposed of disposed in front of the press 1 drop.

In the top view FIG. 3 If one takes away the multiplicity of the necessary microwave generators 26 across the width of a press material mat 14, which is conveyed in the direction of production 3 in the direction of continuously operating press 1. It is clear to the person skilled in the art that the irradiation of the microwaves must be carried out from the pressing surface sides, which subsequently come into contact with the steel strips 7 of the press 1. A microwave radiation over the narrow or longitudinal surfaces of the edge of the pressed material mat is not useful due to the theoretically and practically determined penetration depth.

With regard to the ease of maintenance of the system, it is preferably provided in the continuous furnace 4, the individual parts, such as Magnetron 20, Circulator 21 and tuner 22, a microwave generator 26 modular build and provide for quick replacement in case of failure or maintenance.

Alternatively or in combination, it would be advantageous if each microwave generator 26 is constructed as a separate module in the continuous furnace 4 and possibly has quick-release closures for disassembly and assembly. To increase operational safety, it is preferably possible to arrange sensors for spark and / or fire detection in and / or on the pressed material mat 14 in or on the continuous furnace 4 and / or to provide means for extinguishing a fire.

LIST OF REFERENCE NUMBERS

1.

continuously working press

Second

Press / heating plate in 1

Third

production direction

4th

Continuous furnace

5th

rolling elements

6th

forming belt

7th

steel strips

8th.

Upper plastic band

9th

water cooling

10th

Drive for 11

11th

lower plastic band

12th

height adjustment

13th

metal detector

14th

press material mat

15th

Support frame for 26

16th

spreading station

17th

pre-press

18th

Guide band below

19th

Hold down belt

20th

magnetron

21st

circulator

22nd

tuner

23rd

Frame below

24th

Frame above

25th

absorbing elements

26th

microwave generator

27th

enema

28th

outlet

29th

sensors

Claims (25)

1. A method for preheating a pressed material mat (14) scattered onto an endless, continuously revolving continuous forming conveyor (6) in the course of the production of wood-based panels, with microwaves being radiated from one or both sides of the press surface into the pressed material mat (14) for preheating the pressed material mat (14), and with the pressed material mat (14) being pressed and hardened under application of pressure and heat after transfer to a continuously working press (1), that for heating the pressed material mat (14) microwaves are used in a frequency range of 2400 to 2500 MHz, characterized in that microwaves for each side of the press surface are generated by 20 to 300 microwave generators (26) with magnetrons (20) of a respective power of 3 to 50 kW.
2. A method according to claim 1, characterized in that the heating by means of microwaves produces an even temperature distribution of +/- 7°C in the pressed material mat (14) over the length and width.
3. A method according to claim 1, characterized in that the pressed material mat (14) is examined for metallic parts prior to heating, with especially metallic parts being sought which are larger in their dimensions than ¼ of the wavelength.
4. A method according to one or several of the preceding claims, characterized in that inlet (27) and/or the outlet (28) of the continuous furnace (4) are adjusted automatically in respect of height and width to the pressed material mat (14).
5. A method according to one or several of the preceding claims, characterized in that the continuous forming conveyor (6) is compatible with microwaves and guides the pressed material mat (14) directly through the continuous furnace (4).
6. A method according to one or several of the preceding claims, characterized in that the plastic belt (6, 8, 11) used in the continuous furnace (4) heats up less than 10°C in one passage through the machine.
7. A method according to one or several of the preceding claims, characterized in that an escape of the humidity from the pressed material mat (14) is prevented by using an upper, endlessly revolving plastic belt (8) in the continuous furnace (4).
8. A method according to one or several of the preceding claims, characterized in that the absorption elements (25) in the continuous furnace (4) are moved as closely as possible to the pressed material mat (14) during passage.
9. A method according to one or several of the preceding claims, characterized in that absorber bricks or water containers are used as absorber elements.
10. A method according to one or several of the preceding claims, characterized in that reflectors introduce excess scattered radiation back into the pressed material mat (14).
11. A method according to one or several of the preceding claims, characterized in that the microwave generators (26) are automatically deactivated in regions of the continuous furnace (14) in which no pressed material mat (14) is conveyed and/or in which metallic foreign bodies are determined.
12. A method according to one or several of the preceding claims, characterized in that the necessary cooling power is converted via heat recovery for long-distance heating.
13. A method according to one or several of the preceding claims, characterized in that during the passage of the pressed material mat (14) through the continuous furnace (4) it is checked for sparks or fires.
14. A method according to one or several of the preceding claims, characterized in that occurring sparks and/or fires are quenched automatically.
15. An apparatus for preheating a pressed material mat (14) scattered onto an endless, continuously revolving continuous forming conveyor (6) in the course of the production of wood-based panels, with the apparatus being arranged as a continuous furnace (4) in which microwave generators (26) are arranged for the generation of directed microwaves with a frequency range of 2400 to 2500 MHz onto one or both surface sides of the pressed material mat (14) for preheating the pressed material mat (14), characterized in that 20 to 300 microwave generators (26) with magnetrons (20) with a power of 3 to 50 kW are arranged per press surface side in the continuous furnace (4).
16. An apparatus according to claim 15, characterized in that a metal separator (13) is arranged in the direction of production (3) before the continuous furnace (4).
17. An apparatus according to claim 15 or 16, characterized in that sensors (29) for determining the width and/or the height of the pressed material mat (14) are arranged in or before the continuous furnace (4).
18. An apparatus according to one or several of the preceding claims 15 to 17, characterized in that the inlet (27) and/or the outlet (28) of the continuous furnace (4) is variably arranged in respect of height and/or width.
19. An apparatus according to one or several of the preceding claims 15 to 18, characterized in that movable absorption elements (25) are arranged for changing the inlet (27) or the outlet (28).
20. An apparatus according to one or several of the preceding claims 15 to 19, characterized in that absorber bricks and/or water containers are arranged as absorption elements (25).
21. An apparatus according to one or several of the preceding claims 15 to 20, characterized in that reflectors are arranged in the continuous furnace (4) in addition to or instead of the absorption elements (25).
22. An apparatus according to one or several of the preceding claims 15 to 21, characterized in that the individual parts such as magnetron (20), circulator (21) and tuner (22) of a microwave generator (26) are modularly arranged in the continuous furnace (4) and are suitable for rapid exchange in case of defect or maintenance.
23. An apparatus according to one or several of the preceding claims 15 to 22, characterized in that each microwave generator (26) is arranged as a separate module in the continuous furnace (4).
24. An apparatus according to one or several of the preceding claims 15 to 23, characterized in that sensors for recognizing sparks and/or fires in and/or on the pressed material mat (14) are arranged in or on the continuous furnace (4).
25. An apparatus according to one or several of the preceding claims 15 to 24, characterized in that means for quenching a fire are provided in or on the continuous furnace (4).

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