MATERIAL- AND APPARATUS FOR

CARRYING OUT THE METHOD Technical Field

The present invention relates to a method for forming a web of material, by depositing particles, such as fibres, distributed in and carried by a gaseous medium onto a particle-laying surface, whereupon a strea ^' m of material, comprising the gaseous medium and said particles, is conducted along an inner cylind¬ rical wall of a vessel, from an inlet to an outlet, from which outlet the particles pass to the particle- laying surface. Background Art

The main problem of conducting such a stream of material, in which stream the particles are separate from one another to at least a substantial extent, between the inlet and outlet of said vessel, is one of maintaining the density of the material flow, i.e. of preventing the stream of material from expanding or contracting while, at the same time, maintaining the velocity of the particles substantially constant. It is extremely important that the velocity of the particles is held constant, because if each individual particle is subjected to retardation force? while travelling from the inlet to the region of the outlet of said vessel, the. particles will stick together and form aggregates or floes . When the web formed on the particle-laying surface is a fibre web, these floes will impair the fluffiness of the web and will result in an uneven product.

A method of laying fibres in web form onto a fibre-laying surface is known, for example, from SE Patent Specification No. 169 259. By means of the apparatus described in this Patent Specification

the stream of material is led tangentially into a cylindrical drum, in a circular path. Part of the ingoing stream of material is taken out through an outlet in the drum, while the remainder of the stream travels a full revolution and is rejoined with the ingoing stream. The stream of material is greatly contracted during this circulatory movement of the stream, and there is no way of preventing the fibres from adhering to one another and forming floes. Consequently, a prime object of the present .. invention is to provide a method of the kind described in the introduction with which the turbulent motion of the stream is maintained without slowing down the particles carried by the gaseous medium to any appreciable extent, while, at the same time, changing the motional mode of the stream of material to a helical movement mode.

Brief disclosure of the invention

This object is primarily realised by first imparting a given velocity to the stream of material in said inlet, and then, by maintaining a gas flow in the axial direction of the vessel , guiding the stream of material in a helical ^' path of substantially constant pitch and with substantially constant particle velocity, to the region of said outlet.

The particles in the helically moving stream of material will be acted upon by centrifugal forces and by forces created by the central and axial air flow. The gas-flow in the said helical path will be so affected by the axial flow of air that its speed will remain constant, or at least substantially constant, and consequently the particles will move at a constant speed throughout the whole of the transport distance, thereby effectively preventing flocculation or the like.

Brief description of the drawings

The invention also relates to apparatus for carrying out the method.

The invention will now be described in more detail with reference to the accompanying drawing, in which

Figure 1 is a simple illustration of the invention,

Figure 2 is a top-plan view of apparatus according to the invention, and

Figure '3 illustrates an outlet means for the vessel shown in Figure 2.

Best mode of carrying out the invention Figure 1 illustrates a vessel 1 having an inner cylindrical wall 2 and an internal cylinder diameter 2R. The vessel 1 has an inlet 3 for a stream of material 4, comprising fibres or other particles uniformly distributed in a gas stream. The stream of material 4 is injected in a tangential direction against the wall 2 at a velocity V, and preferably also at an oblique angle relative to the axis of the cylinder. It is fully possible, however, to feed the stream of material tangentially in a plane extending at right angles to the cylinder axis, where¬ upon the stream of material, unless special measures are taken, will describe a circular path, as set forth in the aforementioned SE Patent Specification No. 169 259. In accordance with the invention a flow of gas having velocity W is drawn by suction through the vessel 1 in the direction of the cylinder axis, through a suction opening 5. This axial flow of air acts upon the gaseous vehicle of said stream of material, so that said gaseous vehicle describes a rotary movement with laminar flow along the wall 2 and imparts to the gas vortex formed a

force which acts axially towards the region of the particle outlet 6. The stream of material

4 is given a desired pitch L within each section of length of the cylinder, in dependance upon the velocity W of the axial air flow. In turn, the velocity

W-iSf-contingent upon the density of the material stream, the diameter 2R of the vessel and upon the input velocity V. In order to achieve the result intended, i.e. a substantially constant flow velocity in' the helical path -followed by the stream of material 4 and to maintain a desired, normally variable pitch L along the distance travelled by said material stream, said material stream should

3 have a particle concentration of 50-600-gram. fibre/m gas and preferably 120-360 grams fibre/m gas. With particle concentrations in excess of 600 grams fibre/ 3 gas it is necessary to employ extremely high input velocities V, and the effect of the resultant centri¬ fugal forces may then be so great as to cause a certain degree of flocculation in the gas layer located near¬ est the wall 2. With particle concentration of

3 50-600 gram fibre/m gas it is possible to operate at inlet velocities of the order of 5-200 m/s, preferably 10-50m/s. The outlet 6 of the vessel 1 is located above a moveable particle-laying surface 7, which is suitabl gas-permeable and which is connected to a suction box 8, said suction box being connected to a source of suction (not shown) via a pipe 9. When the stream of material 4 reaches the region 10 adjacent the outlet 6, that part of the axial air flow which has reached this region will be divided into a part air-flow through the suction opening 5 and an air flow out or in through the outlet 6, depending upon the setting of a throttle valve 16, arranged in the outlet 5 and the suction force

OMFI sA wπ ^> o

from the suction box 8 and the pitch L, and the velocity of the material stream will consequently decrease to an extent such that the fibres are moved' out through the outlet 6 and laid on the' fibre-laying surface

7. Throughout the whole of the transport path between the inlet 3 and said region 10, all fibres in the stream of material 4 have been kept in motion at a substantially unchanged velocity, and have consequentl not been able to form floes. The distance between the point at which retardation is commenced above the outlet 6 and the particle laying surface 7 can be made sufficiently short to totally obviate the risk of flocculation.

The thickness of the steam of material, which may, for example, be up to 80% of the radius of said vessel, can be adjusted by changing the velocity in the helical gas stream, which follows the wall of the vessel, and the axial air ^: flow. The width of the web is determined, _' for example, by the width of an injection nozzle. *

Figure 2 illustrates in a simple manner an apparatus for producing a uniform and homogenous layer of cellulose fibres used as absorbent material in, for example, diapers or sanitary towels. Cellulose fibres are fed pneumatically from a store or a shredder 11 to, for example, a jet ejector 12, from which a stream of gas carrying mutually discrete fibres is fed through a line 13 to an acceleration nozzle 14, by means of which the stream of material 4 is injected into the vessel. In the illustrated embodiment, the vessel 1 has an air inlet 15, and said axial air flow is drawn, by means of a fan 24, through said inlet towards the suction outlet 5, said outlet 5 suitably being provided with said adjustable throttle valve 16 for regulating the axial air flow. The vessel 1 is provided with an outlet pipe^ 17 which is arranged tangentially relative to

the wall of the vessel 1 and which exhibits an outlet gap 18. The outlet gap 18 extends in the axial direction ofthe vessel 1 and has a length which corresponds to the width of the homogenous fibre web 19, said web being laid on an endless belt 20, which is driven in the direction of the arrow shown. The belt 20 suitably comprises a wire cloth having a suction box arranged therebeneath.

In order to separate out those fibre floes which are not broken down in the acceleration nozzle, or in some other suitable device, prior to feeding the material -to the vessel 1, the aforementioned outlet part may be provided with any known, suitable separating means. A suitable means intended for this purpose is illustrated in Figure 3. Thus, there may be arranged a lower slightly curved surface 22, which the individual fibres' follow (the Coanda effect) and a separating tongue 23, over which the fibre floes, which have a larger mass than an individual fibre and thus greater kinetic energy, are thrown and returned, for example to the shredder 11 via a line 21, while the individual, lightweight fibres are fed to the driven belt 20 and form thereupon a uniform and fully homogenous fibre web, with' the fibures lying substantially parallel to one another

In the aforegoing it has been assumed that the stream of material is fed tangentially into the vessel. Although this is to be preferred, it is also possible to introduce the stream of material axially, into the vessel with the aid of a vortex forming defice arranged to impart a rotary movement to the gas stream carrying said fibres . In order to direct stream of material purely axially so that said stream moves tangentially, or at least substantially tangentially, on the inner wall surface of the cylindrical vessel, it is possible.

for example, to use well known techniques with air jets arranged to guide said stream into the desired mode of motion. It is also possible to feed in a plurality of material streams lying helically adjacent one another and having mutually the same pitch. Theor- etically, it is possible to place such material streams without mutual spacing therebetween, thereby to provide a continuous layer which moves helically along the inner surface of the vessel.

A further, and particularly important advantage afforded by the invention is that the 'fibres in the injected gaseous medium will be aligned in the direction of the helical path of said gaseous medium and.will therefore have a higher degree of parallel¬ ism than fibres laid by means of known apparatus. In order to eliminate as far as possible friction between the fibres present in the gas stream and the inner surfaces of the vessel 1, said inner surfaces are made as smooth as possible and, for example, are polished. The thickness of the ingoing stream of material and its velocity, and also the pitchof the helix are regulated partly by the velocity of the injected stream and the amount of gas therein, partly by the radius of the vessel, and partly by the velocity of the axial air flow.