The invention relates to a process and an apparatus for continuous production of nonwovens, particularly mineral wool nonwovens.
In the production of mineral wool nonwovens, e.g. from rock wool or glass wool, not only is the fiberisation process of importance, but also the formation of the nonwoven fabric as such constitutes an important process step. It is customary in this respect for a fibre/gas/air mixture produced by a fiberisation unit to be introduced into a box-like so-called chute to separate the fibres, which chute usually features at the bottom an accumulating conveyor acting as a type of filter screen which is constructed in the form of a gas-permeable, rotating, plane conveyor belt. Under the conveyor belt is located an extraction device which generates a certain partial vacuum. In addition, drum-shaped accumulating conveyors with curved suction surfaces are also known from, for example, German patent specification DE-PS 39 21 399.
If the fibre/gas/air mixture--which can also contain a binder--impinges on the accumulating conveyor, the gas/air mixture is sucked through to below the accumulating conveyor acting as a filter, and the fibres are retained on the conveyor in the form of a nonwoven fabric.
In the known process for nonwoven fabric production, there are generally a plurality of adjacently arranged fiberisation units which produce fibre flows in a manner familiar to a person knowledgeable in the art. For the sake of simplicity, the term "fibre flow" or "fibre stream" used in the following shall refer to the flow bundle comprising fibres, process air, and binder where appropriate, with the term "process air" also covering the propellant gas required in order to draw out the fibres, the secondary air entrained during fiberisation, and any false air which may be sucked into the process for the purpose of cooling following fibre drawing.
Into the space bounded by the accumulating conveyor and the side walls of the chute, are thus introduced from the top fibre flows arranged in the form of adjacent core streams which carry fibres which are in the process of production or which have just been produced. In order to facilitate a directed flow and orderly deposition of the fibres as a nonwoven fabric on the accumulating conveyor, it is therefore necessary to extract the introduced process air from below the accumulating conveyor. By this means, one obtains in the chute a vertical stream of the fibre flows, from which the fibre content is trapped at the accumulating conveyor, as if at a filter, to form a nonwoven fabric which is then conveyed away while the process air continues to flow to extraction devices.
The extraction process under and in the accumulating conveyor presents certain difficulties as extraction has to be performed through the forming wool nonwoven, so that at the beginning of nonwoven formation there is, of necessity, less flow resistance while after partially completed nonwoven formation, a greater level of flow resistance has to be overcome. Directly above the nonwoven formation zone, therefore, a non-uniform flow pattern prevails owing to the spatially differing thicknesses of the nonwoven fabric lying below.
At the entry end of the chute, i.e. above the nonwoven formation zone, the fibre flow pattern is made up of a plurality of core streams, with each core stream initially being readily assignable to an individual fiberisation unit. The core streams which occur immediately below the fiberisation units, which core streams exhibit the energy of the propellant gas flows injected for fibre production and as a result of their elevated velocity represent regions of reduced static pressure, are located in relatively close mutual vicinity and exert a mutual suction effect which can lead to unstable oscillating flows in the individual core streams or in the fibre flow as a whole. The overall result is that, above the accumulating conveyor, there is a heterogeneous, spatially and temporally unstable flow pattern which, although in snapshot terms can be regarded as a downward flow, nevertheless exhibits locally a plurality of different flow components acting in the most varied of directions. The minutest changes in a boundary condition lead in this chaotic flow system to changes in the flow pattern which are difficult to control from the outside, which changes, in turn, adversely affect the degree of uniformity with which the nonwoven is formed and which are therefore undesirable.
In the boundary zone in particular around the fibre flows, fibres exhibiting rapid upward movements can also be observed. These upward streams in the boundary zone of the fibre flows are attributable to the fact that, as a rule, only a certain portion of the process air flowing in from above is completely extracted, while another portion at the side of the actual fibre flows is pushed upward again, or is sucked upward by partial vacuum zones in the region of the injected drawing gas flows. These air streams exhibit high flow velocities in an upward direction and entrain fibres in an upward direction to the area of fiberisation. In the case of fibre production by the blast drawing process, for example, suction of already solidified fibres into the nozzle slot together with the secondary air can lead to massive disruptions to production. In addition, the transport of already solidified fibres into the region of binder injection which, in the blast drawing process, is usually located at the entry zone of the chute, can lead to these fibre elements once again coming into contact with binder and then adhering to the chute wall or falling onto the nonwoven fabric as fibres with an excessive accumulation of binder, for example in the form of highly undesirable lumps.
In order to achieve orderly fibre deposition under these conditions, it is necessary to perform a plurality of fine adjustments for a given production process, so as to optimise, by trial and error, the fibre deposition conditions. Any change in the production conditions leads to the requirement that new fine adjustment be performed.