The invention relates to a multi-layer web former according to the preamble of claim 1 for forming a three-layer paper web, advantageously a fine paper web.

In prior art there is known a multi-layer headbox whose slice opening is divided by means of flexible lamellae into three parts placed one upon the other. By means of the headbox it is possible to produce three superimposed slice jets which may have different compositions and different properties. However, several drawbacks are associated with the device disclosed. By the action of turbulence, the different layers of a multi-layer web are mixed with one another at the termination of the lamellae and desired layer purity is not achieved. It is difficult to control the thickness of an individual layer.

Another commonly used method of making a multi-layer web is based on the use of several separate web forming units in which the different layers of the web are caused to be drained in a layer by layer fashion either onto one another or onto separate wires, in which case they are combined with one another after partial dewatering. As an example of the latter, reference is made to FI patent application 950336, in which two layers of the web are formed in separate gap formers and the layers are combined with each other after partial dewatering. A problem in this arrangement is the poor bonding of the different layers of the web to one another.

FI patent application 2785/72 discloses a web forming section in which the middle layer of the web is formed in a gap former and the top and bottom layers of the web are caused to be drained onto this middle layer in the next two web forming stages.

The additional layers are sprayed into a gap between the middle layer of the web, which is situated on a smooth forming roll, and an outer wire, in which connection dewatering on the forming roll takes place solely in the direction of the new addi-

tional layer. As the dewatering members following the forming roll also remove water only in the direction of the new additional layer of the web, good bonding of the different layers of the web to one another cannot be ensured by the proposed arrangement.

US patent 3, 856, 618 discloses an arrangement of the same type where the top layer and the bottom layer of the web are sprayed into a space between the middle layer of the web, which is situated against an open forming roll, and an outer wire. The principal removal of water from the newly formed additional layer of the web takes place in the direction of the added layer in the region of a forming shoe following the forming roll.

US patent 3,844, 881 discloses a two-layer web former in which both forming rolls are suction rolls. The middle layer of the web is caused to be drained by one-sided dewatering in a gap between a wire situated on a forming roll and an extended upper lip of a headbox, and a top layer is sprayed immediately after that into a gap between the middle layer and a wire situated on the second suction forming roll, in which connection removal of water from the formed new layer takes place, on one hand, by the action of centrifugal forces through the entire web and, on the other hand, by the action of suction into the forming roll. The dewatering at the first forming stage is not very efficient, wherefore it is necessary to use a suction roll also as the second forming roll.

An object of the present invention is to solve the drawbacks associated with the known multi-layer web formers described above.

Particularly, the objective is to provide a multi-layer web former by means of which it is possible to make fine paper which has good z-direction strength and the different layers of which can have different properties, when needed.

With a view to achieving these objectives and those that will come out later, the multi-layer web former according to the invention is characterized in what is stated in the characterizing clause of claim 1.

A fine paper web is formed by means of the multi-layer web former in accordance with the invention in three stages such that the middle layer of the web is formed at first in a conventional gap former and the top and bottom layers of the web are caused to be drained onto this middle layer in subsequent stages. Thus, the web forming section comprises three successive web forming units, in all of which the removal of water from the web is arranged to take place in both directions through both forming wires confining the web. In each web forming unit, drainage begins at a forming roll, which is either a suction roll or an open roll without suction. Since part of the dewatering takes place through the previously formed layers of the web, the different layers bond very well to one another. The two-sided dewatering when forming the top and bottom layers of the web also has a favourable effect on the z- direction distribution of fines and fillers in the web.

As compared with a multi-layer headbox, the multi-layer web former in accordance with the invention can provide relatively good layer purity and uniform layer thickness. The possibilities of regulating the structure of paper in the z-direction separately in each layer are good. Regulation may be accomplished, among other things, by controlling the turbulence of a stock suspension, the speed difference between a slice jet and a wire and the filler content or filler type of the stock suspension.

In the following, the invention will be described with reference to figures 1 and 2 of the accompanying drawing, to the details of which the invention is not intended to be confined.

Figure 1 shows a multi-layer web former in accordance with the invention for forming a three-layer paper web.

Figure 2 shows a multi-layer web former variant in which twin-wire zones extend in a vertical direction.

The multi-layer web former in accordance with the invention comprises three successive web forming units, each of which comprises a headbox and two wire loops between which is formed a twin-wire forming and dewatering zone in whose area a forming roll and a number of dewatering elements are acting. The middle layer of the web is formed in the first web forming unit and the top and bottom layers of the web are caused to be drained onto the upper and lower surfaces of the middle layer in the following two units.

As shown in Fig. 1, four wire loops 10,18,20 and 30 placed in two rows and guided by guide rolls form with one another three twin-wire zones 14,24 and 34 which are situated one after the other in the running direction of the web and into which a pulp suspension is sprayed from three headboxes 12,22 and 32 in order to form the three layers of the web.

The first web forming unit comprises a first endless wire 10 which travels while guided by guide rolls, one of which is an open forming roll 16 provided with a suction zone or without a suction zone. A second wire 18 is arranged in the immedi- ate vicinity of the first wire 10 such that the wires 10 and 18 travel some distance together forming a twin-wire web forming zone 14 between them. The twin-wire zone 14 begins with a forming gap confined between the forming roll 16 and the lower wire 18 pressed against it. The wires 10 and 18 have a common wrap angle on the periphery of the forming roll 16.

A stock suspension intended to form the middle layer of the web is sprayed from a first headbox 12 into the forming gap between the wires 10 and 18. Dewatering in the gap takes place in both directions through both wires 10 and 18 defining the forming zone. Centrifugal force and the pressure caused by the tension of the wire p = T/R, where T = the tension of the wire and R = the radius of the forming roll, cause water to be drained through the outer wire 18 outwards from the forming

roll 16, and at the same time, the suction acting in the roll 16 and the pressure caused by the tension of the wire cause water to be drained through the inner wire 10 in the direction of the forming roll 16.

The forming roll 16 is followed by a number of dewatering elements in the twin- wire portion 14, by means of which elements the dry solids content of the web is increased before the next layer is caused to be drained onto the middle layer of the web. In Fig. 1, the forming roll 16 is followed by a rib loading unit 17, by means of which water is removed from the web through both of its surfaces at the same time as the pressure pulses caused by the set of ribs improve the formation of the web.

A suction box 19 is placed inside the second wire loop 18 close to the point where the joint run of the wires 10 and 18 ends in order to ensure that the web follows the second wire 18 to the second web forming stage in which the top layer of the web is caused to be drained onto the middle layer of the web supported by the wire 18.

The second web forming unit comprises a second headbox 22 and a third wire 20 which together with the second wire 18 defines a second twin-wire web forming zone 24. A second forming roll 26 is placed inside the third wire loop 20 and it is advantageously a roll with an open structure. Alternatively, a suction roll may also be used as the forming roll.

A stock suspension intended to form the top layer of the web is sprayed from the second headbox 22 onto the middle layer of the web running on support of the second wire 18 and water is removed from this new stock layer in the forming gap curving over the second forming roll 26. The removal of water from the added layer takes place, on one hand, through the third wire 20 in the direction of the open forming roll 26 and, on the other hand, through the middle layer of the web in the direction away from the forming roll 26. Advantageously, the proportion of the water being removed in the direction of the open forming roll 26 is higher than the proportion of the water draining through the middle layer of the web.

The second forming roll 26 is followed by a number of dewatering elements, which in Fig. 1 comprise a rib unit 27 placed inside the second wire loop 18.

Close to the point of separation of the wires 18 and 20, inside the third wire loop 20, a suction box 29 is placed to ensure that the web follows the third wire 20, which carries the web to the third web forming stage.

The third web forming unit comprises a headbox 32 and a fourth wire 30 which together with the third wire 20 defines a third twin-wire web forming and dewatering zone 34. A third forming roll 36 is placed inside the fourth wire loop 30, on the periphery of which roll the wires 20 and 30 travel some distance together, in which connection a curved forming gap is formed between them.

A stock suspension intended to form the bottom layer of the web is sprayed from the third headbox 32 onto the lower surface of the web carried by the third wire 20, and immediately after that water is removed from the web in the forming gap between the wires 20 and 30. The third forming roll 36 is also preferably an open roll, so that draining takes place, on one hand, in the direction of the new layer towards the forming roll 36 and, on the other hand, through the previously formed two web layers in the direction away from the forming roll 36.

The forming roll 36 in the web forming zone 34 is followed by a number of dewatering elements, such as a rib unit 37, which is placed inside the third wire loop 20 to remove water from the newly formed bottom layer of the web through the previously formed two web layers. Separation of the web from the third wire 20 is ensured by means of a suction box 39 placed inside the fourth wire loop 30. The fourth wire carries the web to a pick-up point (not depicted) from which the web is transferred to a press section.

When fine paper having, for example, a grammage of 80 g/m2 is manufactured by means of the multi-layer web former illustrated in Fig. 1, the middle layer of the web, the grammage of which is then about 40 g/m2, is manufactured in the first web

forming unit, and the top and bottom layers of the web, the grammage of which both is at least 20 g/m2, are caused to be drained onto both sides of the middle layer in the subsequent two web forming stages. All the three stock layers can be made of the same kind of stock suspension or, alternatively, stock suspensions of different compositions may be used in different layers. The parameters of each headbox and web forming portion may be regulated separately, whereby the different layers of the web may be provided with different properties. The removal of water from the web takes place in stages and always more or less in the direction of both surfaces of the web. Since part of draining takes place through the previously formed layers, good bonding of the different layers of the web to one another is achieved.

Fig. 2 shows a second embodiment of the multi-layer web former in accordance with the invention, which former differs from the arrangement shown in Fig. 1 mainly in that four successive wire loops 50,58,60 and 70 are placed one after the other such that the web runs substantially in a vertical direction in all three twin-wire web forming and dewatering zones 54,64 and 74. This arrangement makes it possible to shorten the space required by the web forming section in the horizontal direction and, in addition, the one-sided effect of the gravity of the earth on the dewatering in the twin-wire portions can be eliminated.

In the first web forming unit, a stock jet is sprayed from a headbox 52 upwards into a curved forming gap between a first 50 and a second wire 58 meeting each other on a suction forming roll 56, in which gap the removal of water takes place in two directions. The forming roll 56 is followed by a loadable rib unit 57, which dewaters the web and, at the same time, improves the formation of the web. At the end of the twin-wire zone 54, the web runs over a leading roll 59, which advantageously comprises a suction zone to ensure that the web follows the second wire 58 after the twin-wire zone 54.

In the second web forming unit, a headbox 62 is arranged to spray a stock into a gap between the middle layer of the web formed at the preceding stage and a third wire 60, which gap curves over a second forming roll 66 downwards. The open forming

roll 66 placed inside the third wire loop is followed by a rib unit 67 placed inside the second wire loop 58 and by a leading roll 69 placed inside the third wire loop and provided with a suction zone.

At the third web forming stage, a stock jet is again sprayed upwards from a headbox 72 into a space between the middle layer of the web and a fourth forming wire 70.

In the third twin-wire zone 74, an open forming roll 76, a rib unit 77 placed inside the third wire loop 60 and a leading roll 79 placed inside the fourth wire loop 70 and provided with a suction box operate as dewatering members. The fourth forming wire 70 carries the web further to a press section of the paper machine.