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Method and device at a paper making machine

This invention relates to a method of forming a paper web in a fourdrinier paper making machine and to a method for carrying out the method.

It is of extreme importance for the properties of the paper made, that the paper web is formed in the wet section of the machine under controlled conditions.

Normally the stock is ejected in the form of a free jet from the head box onto the wire where the stock is dewatered and a paper web is formed. The forming of the web is influenced by many different' disturbing factors, such as e.g. incomplete dispersion of the fibres in the stock, non-uniform flow of the pulp out of the head box, difference in rate between the pulp jet and the wire, non-uniform dewatering because of unsuitable or deficient dewatering is especially difficult to cope with the two firstmentioned ones of said disturbing factors. For geometric-mechanic reasons, the fibres have a tendency to flocculate. This flocculation tendency is accentuated at increasing fibre concentration and fibre length. For being able to make a paper with good formation, the fibre flocks must be well dispersed in the stock. This can be achieved by a very low fibre concentration, but most often this is not attractive, because it necessarily implies the handling of large flow amounts. A degradation of the fibre flocks can be effected by means of a fine turbulence in the stock flow. Machine manufacturers, therefore, try to establish in the head box such flow geometries, which yield a low-scale turbulence of sufficient intensity. Experience from practical operation, however, shows that this has created a dilemma. The turbulence generated often has a relatively wide spectrum, i.e. a turbulence of relatively high scale mixed with a low-scale one. The, low-scale turbulence decays rapidly and thereby gives rise to rapid re-flocking. The large whirls, which are rich in energy, have a longer duration and often have the opportunity of participating in the flow out of the head box. Too high a turbulence level in the jet ejected from the head box results in a change of the jet geometry (which originally is determined by the geometry of the slice). The thickness of the stock jet shows local variations in time and simultaneourly across the machine. As the substance of the sheet formed depends on the thickness of the stock layer across the wire, the substance, consequently, varies from one position to the other in the paper web.

The problems referred to above, which often involve insufficient deflocculation of the stock, because the necessary turbulence level would unacceptably disturb the web formation on the wire, are, of course, more serious on a fourdrinier machine than on a double-wire machine. On the latter machine, the free jet length generally is short, and dewatering takes place rapidly, so that thickness variations in the stock layer have not sufficient time for growing to the same extent as on a fourdrinier machine.

The dewatering to a fixed state of the individual fibres in a fibre bed on a fourdrinier machine is carried out by different types of dewatering members, forming tables, table rolls, foils, wet suction boxes. In addition to their primary object of dewatering, these members have in common that they, to a greater or smaller extent, introduce distrubances into the stock layer. As an example, dewatering by means of foils may be described. A foil strip is positioned at a certain angle relative to the wire so as to form a diverging space with the wire in the direction of the machine. When the wire with the stock layer rushes ahead over the strip, a vacuum is formed in the diverging space which brings about the dewatering. A greater or smaller amount of the drained water follows along with the wire on the lower side thereof to the next foil strip, at the front edge of which the water is scraped off. This scraping off of water gives rise to a pressure pulse, which is directed upward against the wire and the web already formed and lying on the wire. The size of the pressure pulse is a function of the water amount scraped off, the scrape angle and the wire speed. For reasons already mentioned, in the stock flowing out from the head box there exists often a state of flocculation, which is unacceptable for the paper formation. The pressure pulses produced at the front edge of the foil strips introduce shear stresses into the stock on the wire, which stresses yield a positive deflocculation effect at an early stage of web formation. This effect, however, is difficult to control, and too strong pressure pulses at a somewhat later stage of web formation can destroy a fibre network already formed on the wire and thereby negatively affect the web formation.

Various methods and constructions have been proposed for solving the aforesaid problems. It is known, for example, to apply a slice on a head box in such a manner, that an upper slice lip extends forward over the wire in its direction of movement and over a dewatering member located beneath the wire. The intention is to establish a converging space between the upper lip and the wire, which space is adapted to the dewatering rate, so that the stock flow can be maintained constant in this space. This results during the greater part of the dewatering process in a stock layer, which is well-defined by the extended upper lip and the wire, and in which hydrodynamic disturbances generated in the head box have no possibility of developing. The converging space between the extended upper lip and the wire can be so defined with respect to its shape, that the upper lip is stiff and the wire is supported by a dewatering member providing the wire with a given tension. The dewatering member may be a suction breast roll or a plane suction box, the appearance of which latter may vary. The open area in the suction box cover may be a hole pattern or slits extending transversely to the machine. All suction box covers have in common, that the open area and, respectively, land area are arranged so that the wire is supported in a manner, which implies a minimum of deflection in the suction zones. The suction box can be divided into sections, so that varying vacuum levels can be applied in the different sections. By this arrangement the dewatering rate can be controlled so as to be adapted to the converging forming space. However, as mentioned before with reference to foils, a support beneath the wire during a dewatering phase implies that pressure pulses are directed upward against the wire and can have a degradation effect on the web formed. The situation is aggravated additionally in that the fibre network formed is not exposed, either, to stabilizing suction forces above the land areas.

The present invention has the object to eliminate to the greatest possible extent the aforesaid drawbacks arising in connection with paper web formation. This object has been achieved, in that the formation takes place in a space between a flexible upper lip pro jecting from the slice and a portion of the wire which portion is unsupported at least at the end of the formation as set forth in the attached claims.

Due to at least the end of the formation of the paper web taking place in a space between a flexible upper lip and an unsupported portion of the wire, the hydrodynamic disturbances are damped and, therefore, a more uniform substance of the paper web can be maintained. The converging forming space adapts itself automatically to the mode of the dewatering process. The stock concentration in the head box can be increased without interfering with the forming process. This is especially advantageous when paper with high bulk is made. The dewatering is effected by vacuum on the lower surface of the wire during the formation. Due to the fact that the unsupported wire portion extends past the flexible upper lip at maintained vacuum, no disturbances arise in the paper web when it leaves the forming space. The liquid film developing between the flexible upper lip and the web is sucked down into the web as soon as the web leaves the forming space,

In addition to an undisturbed web formation, the invention also renders a high dewatering capacity possible. This implies the possibility of shortening the wire section of the paper making machine.

The invention can also be applied to the making of multi-layer paper, where the stock is transferred to the wire through two or more slices and where preferably a flexible upper lip is provided at the upper portion of every. slice in such a manner, that the upper lip for a layer higher up extends past the upper lip for a layer lying below.

The present invention is described in the following by way of some embodiments, with reference to the accompanying drawings, in which Figs. 1 - 3 are sections through three embodiments of the
Fig. 4 shows an embodiment for the making of multi-layer
paper, and
Fig. 5 shows a further embodiment of the device.

Fig. 1 shows a slice 1 on a head box (not shown). The stock 2 is ejected through the slice onto a wire 3 passing over a breast roll 4. A flexible upper lip 5 extends from the upper portion of the slice and is attached to the slice by a fastening means 6. The slice opening can be adjusted by a setting means 7. The slice 1 is to be directed so as to form a small angle between the stock jet and the wire, which angle preferably should be smaller than 15 .

Between the flexible upper lip 5 and the wire, a forming space 8 for the paper v/eb is formed. Beneath said space 8 a suction box 9 is located which extends across the entire width of the web and is upwardly open, so that the wire passes unsupported over the suction box. The suction box and the unsupported wire portion extend through a distance past the upper lip 5. The length of this excess distance should be at least 10% of the length of the forming space.

Owing to the vacuum in the suction box, the stock is dewatered rapidly in the forming space. The vacuum in the suction box should be 0.2 - 1.0 mm water-column, preferably about 0,5 mm water-column,

Due to the controlled forming of the paper web in the space between the flexible upper lip and the unsupported wire portion, the stock concentration in the head box can be maintained within 0,1 - 1 .0%, preferably within 0.3 - 0.8%, when paper with a low substance is being made, and within 1.0 - 2.0%, preferably 1.3 - 1.7%, at the making of pulp sheet.

The flexible upper lip 5 shown in Fig. 1 has a smooth lower surface. In order to increase the micro-turbulence in the stock in the initial part of the forming space, the lower surface of the upper lip can be provided with unevennesses 10, as shown in Fig. 2, which introduce shear forces into the flow. These forces have a defloccu-lation effect on the stock and thereby improve the web formation.
Corresponding details in Fig. 2 are designated with the same reference numerals as in Fig. 1, According to Fig. 2 the upper lip 5 is attached so that lip exchange can be carried out more simply and rapidly.

Fig. 3 shows a further embodiment of the slice 1, where the flexible upper lip 5 is provided along the attached edge with a thickened portion 11, which fits into a groove 12 in the fastening means 6. Said groove 12 extends along the entire width of the slice and simplifies the exchange of the upper lip 5.

Fig. 4 shows an embodiment for the making of multi-layer paper. The head box 13 is formed with three chambers 13a, b and c having three slices la, b and c, each of which is provided with a flexible upper lip 5a, b and c. The lowermost upper lip 5σ extends through a distance beyond the unsupported portion of the wire 3. The central upper lip 5b extends through a distance past the upper lip 5a, and the uppermost upper lip 5c extends through a distance past the upper lip 5b. When the paper web includes another number of layers, the device is designed .accordingly.

Fig. 5 shows a further embodiment where, like the embodiment of Fig. 2, an increased microturbulence in the stock at the beginning of the forming space is obtained. According to this embodiment a number of dewatering strips 14 arranged under the wire at the beginning of the suction zone. The function of the strips 14 are to introduce hydrodynamic shear fields in the flow next- to the wire. These have a deflocculating effect on the stock. At the same time the already formed fibre network will be loosened which will facilitate the continued dewatering. Otherwise this embodiment corresponds to the embodiment of Fig. 1.