Abstract:
A double belt press having a first, lower circulating plastic belt and a second, upper circulating plastic belt. The plastic belts have a first, low flexural strength, and a product gap is formed between facing strands of the plastic belts for forming a plate-like product. A first circulating lower steel belt and a second circulating upper steel belt are provided and have a second flexural strength which is high in comparison to the flexural strength of the plastic belts. A forming section is formed between the facing strands of the steel belts, which forms part of the product gap. The first, lower steel belt runs parallel and bears against a rear face of the first plastic belt in the forming section during operation, and the second upper steel belt runs parallel and bears against a rear face of the second plastic belt in the forming section during operation.

Description:
FIELD OF THE INVENTION 
     The invention relates to a double belt press with a first, lower revolving plastic belt and with a second, upper revolving plastic belt, the plastic belts having a first, low flexural rigidity, and a product gap between the mutually confronting strands of the plastic belts being formed for the purpose of shaping a plate-like product. The invention also relates to a method for producing a plate-like product using a double belt press. 
     BACKGROUND OF THE INVENTION 
     European patent specification EP 1 045 751 B1 discloses a double belt press with a product gap between two plastic belts running partially in parallel. A thermoplastic preproduct is scattered onto a lower plastic belt in a feed region and is then shaped between the two plastic belts in the product gap into a plate-like product. Bonding of the thermoplastic preproduct into a stable plate-like product takes place by the virtually pressureless welding of the thermoplastic preproduct under a pressure of at most 0.2 bar. This known double belt press has disadvantages when it involves the production of thicker plate-like products and/or the processing of fibrous preproducts, for example during the processing of natural fibers. 
     SUMMARY OF THE INVENTION 
     By virtue of the invention, an improved double belt press and an improved method for producing a plate-like product using a double belt press are to be specified. 
     For this purpose, according to the invention, a double belt press with a first, lower revolving plastic belt and with a second, upper revolving plastic belt is provided, the plastic belts having a first, low flexural rigidity, and a product gap between the mutually confronting strands of the plastic belts being formed for the purpose of shaping a plate-like product, in which double belt press a first revolving lower steel belt and a second revolving upper steel belt are provided, the steel belts having a second flexural rigidity which is high in relation to the flexural rigidity of the plastic belts, a shaping portion being formed between the mutually confronting strands of the steel belts, the shaping portion forming part of the product gap, during operation the first, lower steel belt running in the shaping portion parallel to and so as to bear against a rear side, facing away from the product gap, of the first plastic belt, and during operation the second, upper steel belt running in the shaping portion parallel to and so as to bear against a rear side, facing away from the product gap, of the second plastic belt. 
     By provision of the steel belts which form a shaping portion and which in each case bear partially against a rear side of the plastic belts, the advantages of a double belt press having plastic belts and those of a double belt press having steel belts can be combined in a surprisingly simple way, without the respective disadvantages having to be taken into account. By means of the steel belts, a markedly higher pressure can be exerted in the shaping portion upon the product gap and upon the preproduct located in it than would be possible with plastic belts. In particular, the higher flexural rigidity of the steel belts makes it possible to maintain, even over a relatively long length of the shaping portion, a pressure profile which is highly equalized, as compared with plastic belts. The preproduct itself also stands in the shaping portion, but is always in contact solely with the plastic belts, so that there is no fear of the preproduct adhering to the steel belts. The double belt press according to the invention also affords advantages in energy terms. Thus, a constant temperature can be maintained in the shaping portion for the entire process duration. Since the heat storage capacity of the steel belts is higher than that of plastic belts, it is possible with low losses and also by means of heating devices which do not lie directly in the shaping region. However, the disadvantages occurring in the case of straightforward steel belt presses due to the necessary cooling of the steel belt in different portions of a product gap do not arise in the double belt press according to the invention, since the steel belts, of course, form part of the product gap in the shaping portion only. Cooling of the product in the product gap can then take place, for example, downstream of the shaping portion between the plastic belts. On account of the poor thermal conductivity of plastic, the heat losses of the double belt press according to the invention are therefore considerably lower than in the case of straightforward steel belt presses. By means of the double belt press according to the invention, even very thick plate-like products can consequently be produced easily, and even preproducts requiring a high pressure force in the product gap can be shaped into plate-like products. Nevertheless, the double belt press according to the invention can be implemented cost-effectively, as compared with straightforward steel belt presses, since, of course, the steel belts are required only in the shaping portion and, in terms of the process for producing the plate-like product, can be operated just as easily as conventional double belt presses having plastic belts. According to the invention, further, heating and/or cooling devices for heating or cooling the first and the second plastic belt are provided outside the shaping portion. For example, the plastic belt can be heated in a feed region and also in part of the product gap upstream of the shaping portion. Cooling of the plastic belts can then be carried out easily downstream of the shaping portion. The energy losses are in this case low, since, because of their low heat storage capacity, the plastic belts store little heat energy and transfer it further along the product gap. The plate-like product can consequently be preshaped even outside the shaping portion by means of the steel belts. 
     In a further development of the invention, heating and/or cooling devices for heating or cooling the first and the second steel belt are provided. 
     By heating or cooling the steel belt, the shaping portion as part of the product gap can easily be maintained at an approximately constant temperature. This is because the high heat storage capacity of the steel belts makes it possible to have a uniform temperature over the entire length of the shaping portion. 
     In a further development of the invention, pressure devices are provided in order to apply a pressure force in the shaping portion by means of the steel belts to a product arranged between the plastic belts. 
     Such pressure devices may be, for example, roller chains, pressure rollers, pressure plates or the like running on a rear side of the respective steel belt. Successfully proven pressure devices already known in the case of steel belt presses may be adopted here. If, for example, pressure rollers spaced apart from one another in the longitudinal direction are used, an equalized pressure force profile can be implemented, in spite of the essentially linear contact of the pressure rollers, because of the high flexural rigidity of the steel belts. The profile of the pressure force is in any event substantially more uniform than when a plastic belt is pressed down with low flexural rigidity by a plurality of pressure rollers lying one behind the other. 
     The pressure force advantageously lies in the range of 0.1 bar to 50 bar. 
     As a result of such a pressure range, products which are difficult to process or relatively thick plate-like products can be processed easily. Nevertheless, the costs of the double belt press are kept low, since there is no need for pressure forces lying above 50 bar, such as can readily be achieved by means of conventional steel belt double belt presses. 
     In a further development of the invention, the pressure devices are designed in order to apply in the shaping portion by means of the steel belts a pressure force which is uniform essentially over the entire shaping portion. 
     The provision of what is known as isobaric pressure force in the shaping portion can be implemented without difficulty because of the higher flexural rigidity of the steel belts and their mechanical load-bearing capacity which is generally higher than that of plastic belts. For example, the pressure devices are designed in order to act with pressurized fluid upon a rear side, facing away from the product gap, of the steel belts. 
     In a development of the invention, the plastic belts are provided, at least on their side facing the product gap, with a nonstick coating. 
     Such a nonstick coating may be composed, for example, of Teflon and is expediently coordinated with the type of plate-like product to be produced. By such a nonstick coating being provided, the plate-like product produced can easily be detached from the plastic belts at the end of the product gap, and, for example, the plate-like product can be extracted from the product gap even still in a very hot state. A nonstick coating may also be applied, for example, by a liquid being constantly sprayed on upstream of the product gap. A nonstick coating may also be in the form of what is known as a nanocoating which brings about a considerably reduced contact angle of a product to be processed on the plastic belts. Such a nanocoating may give rise, for example, to what is known as the lotus effect in which a contact angle between a product drop and the plastic belt assumes a very high value of about 160°. This corresponds to extremely low wettability. The aim is preferably to have contact angles of 90° or more than 90°. The contact angle between the margin of a product drop and a surface on which the product drop lies is also designated as the margin angle or wetting angle. 
     In a further development of the invention, at least in the shaping portion, side boundaries for laterally delimiting the product gap are provided. 
     The situation can thus be prevented where the preproduct to be processed flows laterally out between the plastic belts on account of a higher pressure possibly exerted in the shaping portion. 
     In a further development of the invention, a machine stand of the double belt press has a modular set-up and, as seen in the longitudinal direction of the product gap, is formed from a plurality of modules connected releasably to one another. Expediently, the two revolving steel belts and also deflections for the steel belts are arranged inside a modular portion of the machine stand. 
     By means of a modular set-up, double belt presses of different length and with a different set-up can be implemented at little extra outlay in structural terms and are then coordinated especially with a plate-like product to be produced. It becomes considerably easier also to convert the double belt press to another product to be produced. Arranging the two revolving steel belts inside a module and additionally, if appropriate, also heating devices and pressure rollers inside this modular portion makes it possible to retrofit a double belt press with revolving steel belts in order to form the double belt press according to the invention. The heating devices arranged in the module itself can be adapted especially to the requirements of the revolving steel belts. 
     In a further development of the invention, a length of the shaping portion amounts to less than half the length of the product gap, in particular to less than one tenth of the length of the product gap. 
     The double belt press according to the invention can thus be implemented cost-effectively. Steel belts are comparatively costly and also the pressure devices required contribute to high costs. Since a length of the shaping portion amounts to only a fraction of the length of the product gap, the extra costs incurred by the steel belts can be kept low. Nevertheless, the advantages of a higher pressure force in the shaping portion which can be applied by means of the steel belts can be utilized. 
     The problem on which the invention is based is also solved by a method for producing a plate-like product using a double belt press, comprising the following steps: application of at least one preproduct in a feed region to a first plastic belt, introduction of the at least one preproduct into a product gap between the first and the second plastic belt, thermal treatment and/or compression of the at least one preproduct in the product gap, introduction of the at least one preproduct into a shaping portion, the shaping portion forming part of the product gap, and thermal treatment and/or compression of the at least one preproduct in the shaping portion by means of two steel belts which bear in the shaping portion against a rear side, facing away from the product gap, of the plastic belts. 
     The thermal treatment and/or compression of the at least one preproduct in the shaping portion by means of two steel belts make/makes it possible to produce from preproducts even very thick plate-like products and also plate-like products which require a comparatively high pressure during production. These may be, for example, plate-like products which contain natural fibers or otherwise require a somewhat higher pressure for bonding individual particles of the preproducts into a homogeneous mass. Even when increased pressure is not to be applied in the shaping portion, the steel belts make it possible, as seen over the length of the shaping portion, to introduce a uniform heat flow into the preproduct in a simple way. On account of the high heat storage capacity of steel belts, a temperature distribution which is uniform over the shaping portion can thereby be achieved by simple means. In contrast to the use of double belt presses having steel belts which are continuous over the entire length, in this case the heat losses are reduced considerably, since the steel belts can remain at a constant temperature over the entire length of the shaping portion and do not have to be heated and then cooled again in different portions. 
     In a further development of the invention, application, in particular scattering, of at least one thermoplastic preproduct in the feed region is provided. Advantageously, the preproduct is heated in the feed region and/or in the product gap, in order to achieve welding of particles of the at least one thermoplastic preproduct to one another. 
     In a further development of the invention, the thermal treatment and/or compression of the at least one preproduct in the product gap are/is provided outside the shaping region. 
     In a further development of the invention, there is provision for laminating on of at least one layer in the product gap. 
     For example, decorative layers or else thin films or the like which protect the plate-like product against environmental influences can be laminated on. Laminating on can in this case be carried out easily by means of the method according to the invention and can make even tacky products difficult to process processible, since there no longer has to be direct contact between the product and the plastic belts. 
     Further features and advantages of the invention may be gathered from the claims and the following description of preferred embodiments of the invention in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Individual features of the various embodiments illustrated in the figures can in this case be combined with one another in any way, without exceeding the scope of the invention. In the drawings: 
         FIG. 1  shows a diagrammatic illustration of a double belt press according to the invention, and 
         FIG. 2  shows a sectional view of a double belt press according to the invention in a further embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The diagrammatic illustration of a double belt press  10  according to the invention in  FIG. 1  shows a first, lower revolving plastic belt  12  and a second, upper revolving plastic belt  14 . An upper strand of the first plastic belt  12  defines, in a region  16 , a feed region in which a preproduct, for example a thermoplastic preproduct, can be applied to the lower plastic belt  12 . Above this feed region  16  is arranged a scattering device which serves, essentially from a funnel, for delivering the preproduct or a plurality of components in the form of a granulate or in the form of fibers. At the exit of the funnel, a scattering roller  20  is provided which extends over the entire width of the lower plastic belt  12 . The scattering roller  20  is provided on its circumference with bristles, in order to ensure a uniform extraction of the preproduct present as granulate from the funnel  18  and in order then to scatter this preproduct in a uniform layer thickness onto the lower plastic belt  12 . 
     Downstream of the scattering device  18 , a first heating device  22  is provided, which, for example by means of infrared radiation, can cause the scattered preproduct to be heated. 
     There follows, downstream of the feed region  16 , a product gap  24  which extends over a product gap region from a front deflecting roller  26  for the upper plastic belt  14  as far as a rear deflecting roller  28  for the upper plastic belt  14 . A rear deflecting roller  30  for the lower plastic belt  12  lies exactly opposite the rear deflecting roller  28 . The plastic belts  12 ,  14  are guided exactly parallel to one another by means of suitable guide rollers over the entire region of the product gap  24 . For this purpose, merely by way of example, four pressure or guide rollers  32  are illustrated. The pressure rollers  32  and, if appropriate, further rollers prevent the situation where the plastic belts  12 ,  14  move too far apart from one another when a preproduct is present in the product gap  24 . If there is no preproduct in the product gap  24 , the upper plastic belt  14 , because of its comparatively low tension and its low flexural rigidity, lies on the lower plastic belt  12  over wide portions of the product gap  24 . 
     When the double belt press  10  is in operation, the two plastic belts  12 ,  14  revolve at exactly the same speed. There is therefore no relative speed between a preproduct at the start of the product gap  24  or a plate-like product at the end of the product gap  24  and the plastic belts  12 ,  14 . 
     In the initial region of the product gap  24 , pressure and heating devices  34  are provided in each case on a rear side of the lower plastic belt  12  and on a rear side of the upper plastic belt  14 . By means of these pressure and heating devices  34 , a height of the product gap is maintained at the exactly stipulated value, and at the same time the plastic belts  12 ,  14  and therefore also the preproduct located in the product gap are heated. The double belt press  10  illustrated is intended for the processing of thermoplastic materials, and heating by means of the pressure and heating devices  34  consequently causes the individual particles of the preproduct to be welded together in the product gap  24 . A pressure which prevails in the region of the pressure and heating devices  34  in the product gap is in this case comparatively low and is of the order of between 0.2 and 2 bar. At higher pressure forces, either the plastic belts  12 ,  14  become worn very quickly and/or undesirable surface structures are formed in the finished plate-like product, since the plastic belts  12 ,  14 , because of their low flexural rigidity, are not capable of distributing a locally applied pressure over a large area. 
     Downstream of the pressure and heating devices  34 , a first, lower revolving steel belt  36  and a second, upper revolving steel belt  38  are provided. The two revolving steel belts  36 ,  38  are arranged such that the upper strand of the lower steel belt  36  bears against a rear side, facing away from the product gap  24 , of the lower plastic belt  12 . The upper steel belt  38  is arranged such that its lower strand bears against a rear side, opposite the product gap  24 , of the upper plastic belt  14 . By means of the two revolving steel belts  36 ,  38 , a pressure force can thereby be applied to the plastic belts  12 ,  14  in a shaping portion or region  40  which forms part of the product gap  24 , and consequently can be exerted upon the product located in the product gap  24 . The shaping portion or region  40  has an upstream product entry end  40   a  and a downstream product exit end  40   b . The pressure force applied by the steel belts  36  and  38  is in this case equalized over the entire length of the shaping portion  40 , since the steel belts  36 ,  38  have considerably higher flexural rigidity than the plastic belts  12 ,  14 . Furthermore, the upper strand of the lower steel belt  36  and the lower strand of the upper steel belt  38  are pressed against the product gap  24  by means of respective revolving roller chains  42  and  44 . A pressure force in the product gap  24  inside the shaping portion  40  typically lies in this case in a range of 10 bar to 20 bar. Such a pressure is sufficient to bring even preproducts which are difficult to process into the shape of a plate and to achieve a sufficiently high density of the plate-like products produced. For example, fibers, in particular natural fibers, can be processed with thermoplastic filling material to form plate-like products. The pressure force in the shaping portion  40  is then sufficient in order, even when the fibers to be processed have comparatively high rigidity, to achieve a planar surface of the plate-like product produced. 
     In the region of the steel belts  36 ,  38 , heating devices  46  are provided, the heating devices  46  acting upon the lower strand of the lower steel belt  36  and upon the upper strand of the upper steel belt  38 . On account of the comparatively high heat storage capacity of the steel belts  36 ,  38 , the heating devices  46  can be arranged in the position illustrated and therefore comparatively far away from the product gap  24 , since the steel belts  36 ,  38  can store sufficient heat. In addition to the heating devices  46 , there may also be provision whereby the in each case front deflecting roller  48  or  50  for the steel belts  36 ,  38  can be heated. For this purpose, fluid ducts  52  may be provided in the inner space of the deflecting rollers  50 ,  48 . 
     The double belt press  10  according to the invention may in this case also readily be used such that no or almost no pressure force is exerted upon the product gap  24  by means of the steel belts  36 ,  38 , but instead the steel belts  36 ,  38  are merely held in position in order to limit the product gap and consequently the height of the plate-like product produced. In such a case, the steel belts  36 ,  38  are used merely in order to introduce heat into the product gap  24 . 
     Further pressure or guide rollers  32  or, for example, also further heating devices may be provided downstream of the shaping portion  40 . 
     In the double belt press  10  illustrated, cooling devices in the form of spray nozzles  54 , which spray cooling water against an underside of the upper strand of the lower plastic belt  12 , are provided in the end region of the product gap  24 . A rear side, facing away from the product gap  24 , of the upper plastic belt  14  can be cooled in a simple way, for example, by means of a blower  33 . The finished plate-like product can then be extracted at the end of the product gap  24 . 
     The double belt press  10  also has, further, devices for laminating on a coating. For this purpose, a roller  60 , on which a layer  62  to be laminated on is rolled, is also provided upstream of the entry into the product gap  24 . This layer  62 , for example a thin film, is then drawn, together with the preproduct scattered in the feed region  16 , into the product gap  24  and, while it runs through the product gap  24 , is bonded with the preproduct. It is, of course, also possible in this case to laminate one or more layers both onto an underside and onto a topside of the plate-like product. 
     The illustration in  FIG. 2  shows a double belt press  70  according to the invention in a further embodiment of the invention. The basic functioning of the double belt press  70  is the same as described with reference to  FIG. 1  and the double belt press  10 , and therefore this basic functioning is not explained again. The double belt press  70  has a feed region  72  in which a preproduct is applied to a lower plastic belt  74  by means of devices, not illustrated, for example scattering devices. Downstream of the feed region  72 , a product gap region  76  commences, in which a product gap  78  has a constant height with the exception of an entry and an exit region. After running through the product gap  78 , the plate-like product produced can be extracted at its end. 
     A machine frame of the double belt press  70  according to the invention has a modular set-up and is composed overall of eight modules  80  to  94 . The modules  80  to  94  have in each case a closed-off self-supporting frame and are connected releasably to the other modules. When a double belt press  70  according to the invention is being set up or else being converted, individual modules can therefore be inserted or removed, and only the length of the plastic belts  74 ,  75  has to be adapted in this case. 
     The module  80  has a front deflecting unit for the lower plastic belt  74 . Heating devices are provided in the region of the module  82  below the lower plastic belt  74 . The module  84  has a front deflection for the upper plastic belt  75  and also heating devices below the lower plastic belt  74 . Heating devices, especially heating blowers, are provided in the module  86 , in order to heat the lower plastic belt  74  and the upper plastic belt  75  and consequently the preproduct located in the product gap  78  and weld the particles of the latter to one another. Plate-like heating and pressure devices are provided in the module  88  and two pressure rollers  89  are provided centrally in the module  88  in order to set the height of the product in the product gap  78  at a predefined value. In the module  90 , a lower revolving steel belt  96  and an upper revolving steel belt  98  are provided, which run parallel to the product gap in the region of a shaping portion  100  and bear against a respective rear side, facing away from the product gap  78 , of the plastic belts  74 ,  75 . In the module  90 , a pressure force can be exerted by means of the steel belts  96 ,  98  upon the product located in the product gap  78  and/or heat can be introduced into the product gap  78 , the pressure force typically lying in the range of between 10 and 20 bar. In the module  92 , once again, plate-like pressure and heating devices are provided, while alternatively, in module  92 , cooling may also take place. The in each case rear deflection for the lower plastic belt  74  and that for the upper plastic belt  75  are arranged in the module  94 . 
     It can be seen from the illustration in  FIG. 2  that the shaping portion  100  in which the two revolving steel belts are provided is substantially shorter than the length of the product gap  78 . The double belt press according to the invention can therefore be implemented cost-effectively, since the steel belts  96 ,  98  are costly, as compared with the plastic belts  74 ,  75 . Nevertheless, inside the module  90 , a comparatively high pressure force can be exerted upon the product located in the product gap  78  or a large quantity of heat can be introduced. On account of the high flexural rigidity of the steel belts  96 ,  98 , a pressure force can be exerted with an equalized profile over the entire length of the shaping portion  100 . The length of the shaping portion  100  amounts to at most half the length of the product gap and can even amount to less than one tenth of the length of the product gap  78 .