Abstract:
A method and a device for the dry forming of a fiber web, in which a multiplicity of fibers or fiber mixtures are fed to a forming head by means of an air flow is described. The forming head produces a fiber stream which is introduced into a clearance of a forming zone between the forming head and a laydown belt. To obtain as uniform a construction of the fiber layer as possible during the laying down of the fibers, the fibers of the fiber stream run through the clearance within the forming zone with free sections of different lengths. To this end, the forming head and the laydown belt are held in a non-parallel arrangement, with the result that the clearance is formed by different spacings between the laydown belt and the forming outlet of the forming head.

Description:
This application is a continuation-in-part of and claims the benefit of priority from PCT application PCT/EP2011/064659 filed Aug. 25, 2011 and German Patent Application DE 10 2010 035 944.0 filed Aug. 31, 2010, the disclosure of each is hereby incorporated by reference in its entirety. 
    
    
     The present invention relates to a method for the dry forming of a fiber web as well as to an apparatus for carrying out the method. 
     BACKGROUND 
     It is known for the production of non-woven fabrics that the fibers are laid down to a fiber layer on a laydown belt by means of an air flow. This method, ordinarily described as air laid among experts, is based on the fact that the fibers or fiber mixtures are placed uniformly distributed on the surface of a laydown belt by means of a forming head. The zone covered by the forming head on the laydown belt is ordinarily referred to as a forming zone, in which the fibers meet on the laydown belt. Such a method and device are described in, for example, WO 2004/106604 A1. In the case of the known methods and the known device, a multiplicity of fibers or fiber mixtures is fed to a forming head by means of an air flow. Within the forming head means are provided for mixing and distributing the fibers. On the underside of the forming head a forming outlet is constructed which ordinarily is arranged at a short distance above the laydown belt. In this connection, a clearance is formed between the forming head and the laydown belt, the clearance serving to guide a fiber stream escaping from the forming outlet. The laying down of the fibers on the laydown belt is supported by a suction device which absorbs the air of the fiber stream and conducts it away. The fiber layer forming on the surface of the laydown belt is continuously conveyed via the laydown belt out of the forming zone, so that a fiber layer is formed which is subsequently fed to a further treatment, for example solidification. 
     Depending on the fiber type and fiber size used in such methods, irregularities can arise in the laying down of the fibers, with the irregularities being referred to as beaching. Such irregularities in the fiber distribution are generally attributed to the fact that the distribution and laying down of the fibers is influenced by secondary air flows from the surroundings which are absorbed from the surroundings into the forming zone via the suction device. 
     In order to eliminate such irregularities in the laying down of the fibers, it is known for example from WO 2006/131122 A1 to influence the suction flow of the suction device in sub-regions of the forming zone. In the case of the known methods and the known device, a guide plate is assigned on an inflow side of the forming zone of the suction device, with the guide plate influencing the suction flow underneath the laying down belt. It is noted that air turbulence arising through suctioned secondary air from the surroundings on the inflow side of the forming zone is supposed to be prevented. However, as a result of the use of such a guide plate, there are differing suction flows in the forming zone which leads to differing laydown behavior of the fibers within the forming zone. 
     The phenomenon of beaching also could not be ruled out from other systems, such as those known for example from WO 2003/016622 A1. In this connection, the forming head on the inflow side and the outflow side each have sealing rollers, which are in contact with the surface of the laydown belt or the surface of the fiber layer. As a result, it possible to prevent to a great extent an influx in the secondary air from the surroundings. However, in this connection it is disadvantageous that the fiber layer on the surface of the laydown belt is condensed directly on the outflow side by the sealing roller arranged there. 
     SUMMARY 
     Hence, the invention addresses the problem of creating a generic method as well as a generic device for the dry forming of a fiber web with which a high uniformity of fiber distribution can be achieved within the fiber layer. 
     The invention is based on the understanding that the laying down of the fibers is influenced by a fiber stream transverse to a laydown belt essentially through the reorientation of the fibers from an essentially vertical movement to a horizontal movement defined by the laydown belt. Thus it became apparent to the inventors that the residence time of the fibers until impinging on the laydown belt has an influence on the development of the fiber layer. In this respect the laying down of the fibers and the structure of the fiber layer could be advantageously improved by having the fibers of the fiber stream within the forming zone run through the clearance with free sections of different lengths. In this way zones could be realized in which the fibers had greater latitude for the reorientation through long free sections. 
     The method variant in which the fibers of a fiber stream produced on an inflow side of the forming zone run through a longer free section than the fibers of the fiber stream produced on an outflow side of the forming zone is particularly advantageous. In addition, the great distance between the forming head and the laydown belt on the inflow side can prevent the turbulence effects caused by the inflowing secondary air. On the other hand, the secondary air can be used in a supporting manner for reorientation and laying down of the fibers. 
     In order to obtain a uniform modification of the free sections within the forming zone, the method variant is preferably used in which case the fiber stream is produced by a forming head inclined vis-à-vis the laydown belt, wherein the free sections of the fibers within the clearance between the inflow side and the outflow side are continuously changing. With this arrangement it is possible to make advantageous use of a horizontally aligned laydown belt for receiving and development of the fiber layer, so that a redistribution of the fibers in the fiber layer cannot occur during transport on an inclined laydown belt. 
     In order to suppress counter-effects through other secondary air flows within the forming zone, the method variant is particular advantageous in which case the clearance of the forming zone on the outflow side for guiding the fibers is screened by at least one screening means vis-à-vis the surroundings. 
     In contrast, the clearance of the forming zone on the inflow side for guiding the fibers to the surroundings is kept open. With this, a secondary air flow can be deliberately produced which acts in the direction of the material flow of the laydown belt. Thus, advantageous pre-orientations can be produced on the fiber stream in the direction of the material flow. 
     In order to be able to generate a uniform fiber layer over the entire width of the laydown belt, in accordance with an advantageous improvement of the inventive method, provision is made that the clearance of the forming zone between the inflow side and the outflow side for guiding the fibers to the surroundings is kept closed. In this way secondary air flows occurring on the long side of the laydown belt can be prevented. 
     For carrying out of the inventive method in the case of the inventive device the forming head and the laydown belt are kept in a non-parallel arrangement, so that the clearance is formed by differing distances between the laydown belt and the forming outlet of the forming head. 
     In this connection, the arrangement of the forming head and of the laydown belt is preferably constructed in such a way that the distance between the laydown belt and the forming outlet of the forming head on an inflow side of the forming zone is greater than distance between the laydown belt and the forming outlet of the forming head on an outflow side of the forming zone. In addition, the larger free section for reorientation is realized in the inflow region of the forming zone. 
     The forming head is preferably held on a inclined plane vis-à-vis the laydown belt so that the distance between the laydown belt and the forming outlet of the forming head from the inflow side to the outflow side of the forming zone is continually changing. With this, a continuous reduction of the free section in the direction of the material flow of the laydown belt can be achieved. Thus, the reduced suction effect due to the already formed fiber layers toward the outflow side of the forming zone can be compensated. The fibers can be received with essentially the same kinetic energy on the surface of the laydown belt or of the fiber layer. 
     In order to obtain a setting of the free sections in the forming zone optimized for formation of the fiber layer dependent on the fibers and fiber mixtures, the forming head is advantageously held by an adjustable retainer, as a result of which the degree and/or the height of the inclined plane of the forming head can be set. 
     In order to suppress as much as possible the entry of secondary air on the outflow side, two alternative device variants of the inventive device can be employed. In the case of a first variant, at least one screening means is arranged on the outflow side of the forming head, through which the clearance can be screened vis-à-vis the surroundings. Such screening means are preferably formed by driven sealing rollers which are held in contact with a fiber layer on the laydown belt. This variant is however only suitable when a pre-compression of the fiber layer on the surface of the laydown belt is harmless for any further processing. 
     For sensitive fiber layers, the device variant is preferably implemented in which case an outflow opening is formed on the outflow side of the forming zone between the forming head and the laydown belt. Such outflow openings are preferably implemented with a small gap height which, depending on the thickness of the fiber layer, can range from 4 mm to 20 mm. 
     In order to be able to use the secondary flow of ambient air necessary for reorientation, the inventive device is preferably constructed in such a way that an inflow opening is formed on the inflow side of the forming zone between the forming head and the laydown belt. 
     In this connection, the inflow opening is preferably constructed with a gap height ranging between 40 mm to 400 mm. Thus, preferably laminar secondary flows of the ambient air can be introduced into the forming zone. 
     The peripheral regions of the forming zone are preferably sealed in accordance with the advantageous improvement of the invention, wherein the clearance to both long sides of the laydown belt vis-à-vis the surroundings is sealed by sealing means between the forming head and the laydown belt. Thus, a uniform fiber layer can be produced over the entire width of the laydown belt. 
     The fiber stream is preferably produced at the forming outlet of the forming head through a perforated plate or stressed screen cloth, which makes possible a homogenized distribution of the fibers over the entire forming zone. 
     The inventive method and the inventive device are suitable for the laying down of all fibers and fiber mixtures. For example, synthetic and natural fibers or mixtures of synthetic and natural fibers can be laid down to fiber layers. Due to the high uniformity of the produced fiber layer in the process, preferably even the finest parts such as for example a powder can be advantageously integrated into the mixture. 
     The inventive method will be explained in more detail with the help of some exemplary embodiments of the inventive device making reference to the attached figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows schematically a cross-section view of a first exemplary embodiment of the inventive device for carrying out the inventive method. 
         FIG. 2  shows schematically a cross-section view of a further exemplary embodiment of the inventive device for carrying out the inventive method. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows schematically a first exemplary embodiment of the inventive device for carrying out the inventive method. The exemplary embodiment shows a mixing chamber  1  that is connected via a fiber inlet  2  to a fiber feed not shown in the figure. The fiber inlet  2  can contain one or more connections in order to feed one or more fibers or fiber mixtures by means of an air flow of the mixing chamber  1 . The mixing chamber  1  is connected on an underside to a forming head  3 . The forming head  3  includes several means (not shown in detail here) to uniformly distribute the fibers or fiber mixtures and conduct them away as a fiber stream via a forming outlet  4  constructed on the underside. The forming outlet  4  preferably includes a perforated plate  5 . In the process, the distribution takes place within the forming head preferably via several driven wings, such as is known for example from WO 2004/106604. In this respect, WO 2004/106604 is incorporated herein by reference. 
     The forming head  3  is arranged above a laydown belt  8  at an inclined plane  21 . The laydown belt  8  is essentially horizontally aligned, so that on an inflow side  10  a greater distance between the forming head  3  and the laydown belt  8  is set than on the opposing outflow side  11 . The distance to the inflow side  10  is marked with the identification letters letter S E . In contrast the distance between the forming outlet  4  and the laydown belt  8  is marked with the identification letters S A . 
     The location of the forming head  3  or the location of the inclined plane  21  can be set via a retainer  19  of the forming head  3 . The retainer  19  is in this exemplary embodiment formed by two actuators  20 . 1  and  20 . 2 , each of which engages on support arm  22 . 1  and  22 . 2 , with the support arms being connected to the forming head  3 . Thus, through a parallel actuation of the actuators  20 . 1  and  20 . 2  the height of the forming head can be set relative to the laydown belt  8  and thus the height of the inclined plane  21 . By means of unilateral actuation of the actuators  20 . 1  or  20 . 2  it is possible to set the angular position of the forming head  3  and with it the degree of the inclined plane  21  relative to the laydown belt  8 . In each case, a modification of the distances between the forming outlet  4  and the laydown belt  8  occurs. 
     The laydown belt  8  is gas permeable and is continuously fed in a material conveying direction via several guide rollers  9 , with the material conveying direction being identified by a double arrow. In this respect, the laydown belt  8  continuously runs through the forming zone  6  from the inflow side  10  to the outflow side  11 . In the process, the fibers are laid down on the surface of the laydown belt  8  to a fiber layer  23 . 
     Below the laydown belt, a suction device  16  is arranged with the suction device being connected via a suction channel  17  to a vacuum source not shown in the figure. 
     The forming outlet  4  of the forming head  3  is in this case rectangularly constructed, so that an essentially rectangular forming zone  6  is constructed above the laydown belt  8 . The clearance  7  of the forming zone  6  is in this exemplary embodiment only connected to the surroundings via an inflow opening  14  on the inflow side  10 . On the opposing outflow side  11 , a screening means  12  in the form of a sealing roller  13  is arranged between the forming head  3  and the laydown belt  8 . The absorption of secondary air from the surroundings can be prevented in this way. In addition, in the region of the long sides of the forming head  3 , there are two opposing separating plates  15  provided, which seal the clearance  7  of the forming zone  6  to both long sides of the laydown belt  8  vis-à-vis the surroundings. 
     In the case of the exemplary embodiment of the inventive device shown in  FIG. 1 , a synthetic fiber, for example, is fed with a powder jointly via an air flow of the mixing chamber  1 . Within the mixing chamber  1 , static or dynamic means can be constructed, which implement a premixing of the fibers. Subsequently, the mixture of fiber and powder is guided via the air flow to the forming head  3 . Within the forming head  3 , a distribution of the fiber and powder mixture takes place via the distribution means, with the mixture then being guided as a fiber stream into the clearance  7  via the forming outlet  4 . Within the forming zone  6 , a continuously active suction flow is generated via the suction device  16 , with the suction flow on the one hand collecting the fibers entering into the clearance  7  and on the other hand generating a secondary air flow from the surroundings on the inflow side  10 . In the guiding of the fibers within the clearance  7 , the fibers of the fiber stream in the region of the inflow side  10  run through a longer free section until being laid down on the laydown belt  8 . By way of contrast, the fibers on the opposing outflow side  11  are guided on a shorter free section. Thus the fibers guided in the region of the inflow side  10  receive a higher residence time in order to execute the transition from a vertically oriented movement to a horizontally oriented movement. Thus, the fibers can be laid down by the influence of a secondary air flow on the inflow side with a slight pre-orientation in material flow direction. This turns out to be particularly advantageous in particular in the formation of a uniform fiber layer  23 . 
     Depending on the fiber type and fiber mixtures, it turns out that the distance S E  on the inflow side  10  for formation of the inflow opening  14  should be in a range from 40 mm to 400 mm. Too small a distance between the forming head  3  and the laydown belt  8  has the disadvantage that the absorbed secondary air leads to severe turbulence. Too great a distance between the forming head  3  and the laydown belt  8  on the inflow side  10  increasingly reduces the influence of the secondary air, so that this should likewise be avoided. 
     On the opposing outflow side  11  of the forming head  3  the absorption of a secondary air is prevented by the sealing roller  13 . In this respect, only the influence of the secondary air permitted via the inflow opening  14  remains, with the secondary air being able to be used purposefully for the improvement of the fiber layers. 
     The inventive method and the inventive device are thus particularly well suited for achieving a high uniformity in the production of fiber layers that are formed of a multiplicity of single finite fiber pieces. In this connection, synthetic or natural fibers or mixtures of synthetic and natural fibers can be laid. 
       FIG. 2  shows a further exemplary embodiment of the inventive device for carrying out the inventive method. The exemplary embodiment of the inventive device shown in  FIG. 2  is essentially identical to the exemplary embodiment in accordance with  FIG. 1 , so that only the differences will be explained here and otherwise reference is made to the aforementioned description. 
     In the exemplary embodiment shown in  FIG. 2 , the forming head  3  is likewise held on an inclined plane vis-à-vis the laydown belt  8 , so that on the inflow side  10  a greater distance arises between the forming outlet  4  and the laydown belt  8  than vis-à-vis the outflow side  11 . The distance on the inflow side is marked with the identification letter S E  and on the outflow side with the identification letter S A . In this connection, on the outflow side  11  between the forming head  3  and the laydown belt  8  an outflow opening  18  is formed, which connects the clearance  7  of the forming zone  6  to the surroundings. Likewise, on the opposing inflow side  10 , an open inflow opening  14  is shown that is likewise connected to the surroundings. However, through the inclined arrangement of the forming head  3  the outflow opening  18  a significantly lower gap height than the opposing inflow opening  14  is provided. Thus, depending on the fiber and fiber type, the outflow opening  18  is constructed in such a way that a distance in the magnitude of 4 mm to 20 mm ensues between the forming outlet  4  and the laydown belt  8 . The gap height of the outflow opening  18  is arranged or set in the process essentially to the thickness of the fiber layer which is produced on the surface of the laydown belt. 
     For the setting of the inflow opening  14  and the outflow opening  18 , the forming head  3  is likewise adjustable via an adjustable retainer  19 . The retainer  19  is in this connection identical to the aforementioned exemplary embodiment, so that no further explanation will be given here. 
     In the exemplary embodiment of the inventive device shown in  FIG. 2 , the forming zone and thus the clearance  7  are only screened from the surroundings by the separating plates  15  arranged on the long sides. No additional screening means are provided on either the inflow side  10  or the outflow side  11 . 
     In the exemplary embodiment shown in  FIG. 2 , the fibers within the fiber stream are likewise guided in free sections of differing length within the clearance, so that the residence times for running through the free sections in the inflow region of the forming zone are greater than in the outflow region. In this connection, in this respect the secondary air effects can be used jointly in order to obtain a favorable reorientation of the movement sequences in single fibers. Through the narrow gap on the outflow side it is possible to minimize the absorbed secondary air so that undesired disturbing effects can be avoided. 
     The above disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments, which fall within the true spirit and scope of the present invention. Thus, to the maximum extent allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description. While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents 
     REFERENCE LIST 
     
         
           1  Mixing chamber 
           2  Fiber inlet 
           3  Forming head 
           4  Forming outlet 
           5  Perforated plate 
           6  Forming zone 
           7  Clearance 
           8  Laydown belt 
           9  Guide rollers 
           10  Inflow side 
           11  Outflow side 
           12  Screening means 
           13  Sealing roller 
           14  Inflow opening 
           15  Separating plate 
           16  Suction device 
           17  Suction channel 
           18  Outflow opening 
           19  Retainer 
           20 . 1 ,  20 . 2  Actuator 
           21  Inclined plane 
           22 . 1 ,  22 . 2  Support arm 
           23  Fiber layer 
         Distance S E , S A