The present invention relates to a fiber distributor for forming an air-laid fiber web on a running endless forming wire comprising a suction unit positioned under the forming wire, a housing positioned above the forming wire, and having at least one fiber inlet, and an improved base having a number of flow openings defined by partitions which diverge in a downward direction, and a number of rotational wings positioned above this base for distributing the fibers along the upper side of the base.
Fiber distributors of the types disclosed herein are used extensively in systems where the fiber layer on the forming wire is subsequently subjected to a number of processes which convert the fiber layer to a continuous web in the form of, for example, paper and synthetic paper materials of the kind typically used for the production of various paper products and hygienic articles.
The fibers are fed into the fiber housing through the fiber inlet and are driven in a flow over the upper side of the base by the wings which, during operation, rotate in such a way that the fibers are evenly distributed over the total area of the base.
At the same time, the suction unit generates an air flow through the openings in the base and the forming wire. This air flow successively pulls fibers with it down through the openings in the base. As the openings in the forming wire are smaller in size than the openings in the base, the majority of these fibers lie in a desiredly even layer on the upper side of the forming wire, or on a fiber layer formed in advance on the forming wire. The forming wire continuously carries the fiber layer on to the following processes mentioned above.
The base conventionally comprises a net with a quadratic mesh. When the fibers comprise of, or contain, short cellulose fibers, the mesh must be dimensioned with a correspondingly small mesh aperture. A fiber distributor such as this therefore has a comparatively small capacity.
One proposal to solve this problem is disclosed in U.S. Pat. No. 4,355,066. This patent describes a fiber distributor for forming short-fibered cellulose pulp on a forming wire via a rectangularly meshed base net. Thus, each flow opening in this known base net has both a small and a large dimension, which means that the flow area of the individual flow openings and thus the capacity of the net is increased correspondingly.
Certain characteristics of the fiber distributor and of the fibers themselves make designing an ideal base net difficult. The base net must be able to withstand the differential pressure generated by the suction unit. This means that the thread of the base net must have a suitable thickness in order to resist the resulting comparatively large load. Additionally, for reasons of economy and strength, a mixture of cheap cellulose fibers and more expensive, but longer synthetic fibers are often used to produce fiber web.
It has become apparent that when using the rectangularly meshed net mentioned in U.S. Pat. No. 4,355,066 with a sufficiently thick base net thread, the short and long fibers become stuck and block the net openings when passing through the narrow gap between the thick thread of this net. The long synthetic fibers also tend to get wound up in the thread of the net. This means that the fiber distributor is periodically out-of-service, and that the structure of the fiber layer on the forming wire is very uneven.
European Patent EP-A-0 226 939 concerns an apparatus for forming fibrous webs with advances consisting of controlled formation of light and heavy fluff zones and improved gas flow regulation. The patent describes a fiber distributor which has a flow channel housing for depositing the fibers onto an upstream surface of a forming wire which is located on an outer peripheral rim section of a cylindrical drum assembly. One of the embodiments contains a inner drum ring or attenuating layer with a number of small openings which could be small tapering or conical bores similar to those described in the instant patent. The only purpose of this attenuating layer is to diminishing the pressure differential exerted on the forming surface of the forming wire from a vacuum from a central vacuum duct. Therefore, the small bores will, of necessity, only occupy a limited part of the total area of the attenuating layer and is inapplicable to a forming wire.
The present invention now provides an improved fiber distributor which overcomes the shortcomings of the prior art.
The present invention relates to a fiber distributor for forming an air-laid fiber web on a running endless forming wire which, during operation, preferably is horizontal, comprising a suction unit positioned under the forming wire, a housing positioned above the forming wire and having at least one fiber inlet, and a base having a number of flow openings, and a number of rotational wings positioned above this base for distributing the fibers along the upper side of the base, wherein the base is designed with flow opening defined by partitions which diverge in a downwards direction.
Advantageously, the two opposite partitions in a flow opening preferentially form an angle of between 5 and 35xc2x0 and more preferentially between 10 and 25xc2x0. Also, each opening typically has a quadratic or rectangular area. Preferably, two of the opposite sides of each opening are extending mainly parallel with the transport direction of the forming wire. Alternatively, two of the opposite sides of each opening forms an angle with the transport direction of the forming wire.
The base is preferably a grid having grid bars, each having two sides converging in a downwards direction and each forming a partition in a flow opening. Advantageously, the height of each grid bar is between 1 and 5 times greater than its width at the upper side of the grid. Typically, the crossed grid bars are joined at cross points by welding or soldering, and the base is coated with a fluorocarbon such as Teflon.