Multi-ply paper or board is usually made by feeding the stock flows one above the other from the multi-layer headbox onto the wire, on which the web begins to form as water drains off from the suspensions positioned aboveone another. There are at least two stock flows, for example two or three, and at least two of these stock flows have different compositions, which may have different fibre compositions or the same fibre composition, but they differ from one another as to other additives, the quality and/or quantities of which may be different.

In paper and board manufacturing technology it is known that by influencing the location of the different raw material components in the thickness direction of the paper or board web, significant quality changes can be achieved. An example of a common application is the improvement of the product's stiffness by imitating the so-called I- girder structure. In this case, a high-bulk (low density) raw material is directed into the product's middle layer. In this way the surface layers of the product can be kept far apart from each other, and by feeding a raw material with a high modulus of elasticity into the surface layer, the stiffness of the product can be improved, compared to a situation where different raw material components are dispersed evenly in the thickness, or Z-direction of the paper or board.

The layered structure is mostly used in board machines that form the web by means of separate web forming units and where the layers are combined into one board web. Due to the developments in multi-layer headboxes and gap formers, multi-layer web formation has also become possible. Thus the utilisation of different layered structures has opened up new possibilities for influencing the paper's quality properties.

When utilising the layering technique, completely different stock components can be used for the middle and surface layers of the paper (stock layering). Alternatively, the different fibre fractions of a stock can be used if fractionation technique has been used in stock preparation. It is characteristic of the layering of different stock compositions that the processes of the preparing the stock intended for the different layers are kept separate. In order to prevent the mixing of the middle and surface layer stocks, there must be separate short cycles for the different layers. The weakness of this technology is its high investment costs, which are due to the complicated nature of the process.

It is thus known in the prior art to use different stocks to form different layers in such a manner that at least two different stocks are fed into the multi-layer headbox along different fresh stock lines, whereby the stocks are brought separately, each from its own stock chest or the like source through the different production phases.

Another possibility for using the layering technique is additive layering in the short cycle of the paper machine, which is described in the article Haggblom-Ahnger, U., Layering of office paper, Paperi ja Puu 80 (1998) 7, pages 508-513. This method aims, through different process management chemicals, for example, to influence the fines and filler distribution in the Z-direction of the paper. The process required by additive layering is significantly simpler and cheaper compared with the process required by stock layering, but the possibilities to influence the quality of the product and the economy of the production process are also more restricted.

Different kinds of stocks have different fibre dimensions and properties, but even the fibres in a single type of stock differ from one another. A fractionation technique can be used to separate the different fibre fractions from each other and, as a result, the quality of the paper can be influenced by varying the placing of different fibre fractions in the paper web. If fractionation is conducted in the stock preparation area the result is a complex and expensive process solution. If fractionation is conducted during the short cycle of the paper or board machine this can also easily result in overly complex process solutions.

A Finnish patent 92729, the European equivalent of which is patent application 651092, and the European patent application 745721 discloses a stock feed system of a multi- layer headbox, in which stock derived from the same fresh stock container is used for the different layers, but in which the necessary additives, such as chemicals and fillers, are fed separately into each of the stock flows branching off from the same fresh stock container. In this way a simple stock cycle with a single source of initial fresh stock can be used, and with the appropriate additives at least two different stock concepts can be created.

The international patent application publication WO 98/17860 it has been suggested concerning this single stock system that fractionation be added to the process in such a fashion that after the machine screen there is a stock fractionation device for each of the lines branching off from the same basic stock line, with which devices the stock can be divided into different fractions according to the lengths of the fibres. In the fractionation devices, the long fibres separated from the stock can be transferred into the flow forming the middle layer and the short fibres can be transferred into the flows forming the surface layers. The rejects of the surface layer stock flows, which are separated in the above mentioned stock flows by means of screens and are comprised of long fibres, are fed into the flow forming the middle layer before the fractioning screen on this stock line. The accept from this screen is comprised of long fibres, which are allowed to flow along the stock line into the appropriate manifold pipe, and the reject fractions from the screen, which are comprised of short fibres, are distributed into the stock flows intended for the surface layers.

The above mentioned solution requires fractionation devices between the machine screens and the headbox manifold pipes. When three different stock feed lines are used to form three-layered paper, fractionation devices must be used at least on the stock feed lines for both surface layers, if long fibres are to be transferred to the middle layer.

Dividing and unifying the flows near the headbox causes control-related problems in the process.

The Finnish patent publication 75200, the equivalent of which is the US patent 4 781 793, presents a method for producing multi-ply paper in which two different pulps,

chemical and mechanical, with different fibre contents, are mixed together in the stock preparation phase. From the mixing chest the stock is fed to fractionation through a pressure screen, after which the long fibred fractions are further refined and fractioned, and the shorter fibre obtained from this process is mixed with short-fibred fraction from the first fractionation, and from this the middle layer of the paper is formed in the multi- layer headbox, into which the long-fibred fraction is fed to form the surface layers. The default condition in this process is the existence of two different pulps, from which a basic stock can be formed by mixing, and the said stock that can be fractioned in the stock preparation system. This method requires many storage chests and fractionation devices.

The fractionation technique has also been used in the US patents 5 916 417 and 5 503 710, which describe the production of multi-ply board. A stock is fractioned at least once so that the fibres of the separated fractions end up in different layers.

The purpose of the invention is to eliminate the above mentioned problems and present a method for producing multi-ply paper, in which from the same stock different parallel flows with different fibre compositions can be obtained, from which parallel flows two- ply or multi-ply paper can be formed. To realize this goal the method as claimed in the invention is characterised by what is presented in the attached patent claims in claim 1.

Different fibre and fines fractions are formed and separated using suitable machines in the short cycle of the paper or board machine and they are fed into the storage chest in such a way that they remain essentially different. Fractions are formed and separated into different fractions, preferably by a centrifugal cleaning technique, in a combined process formed by a deculator and a centrifugal cleaner in the short cycle of a paper or board machine, whereby existing equipment can to a large extent be used.

It is necessary for the application of the invention that the stock or the stock mixture fed into the fractionation contains different fibres, which differ from each other in at least one of the following fibre properties: fibre content, fibre width, thickness of fibre wall, linear density, fines content.

Different fibre fractions are produced by means of the centrifugal cleaning technique, which can separate different fractions on the basis of fibre density differences, specific area differences, and in some cases fibre length differences. To this a known process solution can be applied, in which process the first phases (normally 2-4) of the centrifugal cleaning system are carried out in connection with the deculator and the accepts of these phases are fed into the deculator.

The centrifugal cleaners are dimensioned so that the accepts from the centrifugal cleaner phases fed into the deculator form the desired fibre fractions. In the deculator,"spaces" are separated for the middle layer and the surface layer stocks, by means of partition walls. The fibre fractions are formed in the centrifugal cleaners, the accepts from which are fed into the spaces intended for the middle and surface layers in the deculator. From the deculator different fractions flow in separate pipes into the headbox feed pumps. In this way different fibre fractions can be directed into the middle and surface layers.

With many paper and board grades, the aim is to feed the filler and/or fines as well as the shorter fibre fraction into the surface layers. In this way the qualities of the paper's surface layer can be improved with a view to certain converting processes. This kind of layering has a positive effect, for example on the quality of printed products. This invention gives improved possibilities for the layering of fines compared with state of the art techniques, and the layering of different fibre fractions can be accomplished by a simpler process.

The surface and middle layer stock differences created by means of the process solution according to the invention can be characterised by the following fibre dimension differences: Surface layer Middle layer Fines content higher lower Fibre length shorter longer Thickness of fibre wall thinner thicker Linear density lower higher

The invention is explained in more detail referring to the attached illustrations, in which Figure 1 depicts diagrammatically the basic principle of the method, Figure 2 depicts, in a more precise diagram, one embodiment of the invention concerning the separation phase, and Figure 3 depicts, in a more precise diagram, another embodiment of the invention concerning the separation phase.

Figure 1 presents the basic principle of the method. The diagram presents the short cycle of a paper machine, in which the stock derives from the same source and it is fed diluted with circulation water from the short cycle as stock flow M, the consistency of which is typically approximately 0.2-1.5%, into the first separation phase 1, from which the first fraction D is fed into the storage chest 2, which is deculator 2 operating in a known way, and the second fraction is fed further into the second separation phase 1, from which the first fraction B is fed into the deculator 2 and the second fraction is removed.

The deculator is an air extractor used in the short cycle of the paper machine, which removes air from the stock on the basis of the partial vacuum in the chamber of the deculator and the temperature of the stock. The air pump creating the partial vacuum is marked with the reference number 7. The fractions D, B obtained in the separation phases and fed into deculator 2 are kept separate, at least to the extent that their composition is kept different from one another, which shows in the properties of the fibres and in the fines content. From deculator 2 fractions D and B are fed as separate flows towards the paper machine's multi-layer headbox 4 in a consistency that corresponds essentially to the forming consistency for the paper. To these flows small amounts of dilution water can still be added. Fraction D, separated in the first phase, before the headbox, is divided into two more separate similar fractions A and C, which are fed simultaneously with fraction B towards the headbox as separate flows, the fractions passing through machine screens 3. From the fractions D, A, C obtained from the first separation phase, the surface layers of the layered paper are formed, and from the fraction B obtained from the second separation phase 1, the middle layer of the paper is formed. The fractions are fed in a known way into the multi-layer headbox 4, in which they form flows one above another according to the principle described above.

The structure of the multi-layer headbox 4 is in itself already known in the prior art and

thus it is not described in detail here, and for this part the above-mentioned publications are referred to. The pumps for pumping the stock are marked with the letter P.

The first separation phase 1 is dimensioned so that fines together with a fibre fraction, the central wall thickness and fibre content of which are less than that of the original source stock, are fed into the surface layer fraction. Similarly, less fines are fed into the middle layer along with a fibre fraction in which the average fibre wall thickness and fibre content is higher than that of the original source stock. By feeding of fines and a fibre fraction of smaller fibre dimensions into the surface layers, the roughness and average pore size of the surface layer is reduced. When using chemical pulps, an advantage is also the higher modulus of elasticity of the surface layer, which adds to the stiffness of the paper.

Figure 2 presents in a more precise diagram the above mentioned principle according to an embodiment of the invention. Centrifugal cleaners are used as separation phases 1, and the lighter fraction from these cleaners is always fed into deculator 2, from which the stock is taken out and fed as a suitable fraction into the headbox. The finer material separated in the first phase is fed into deculator 2 into its own compartment 2a, which has been separated by means of a partition wall 5 from compartment 2b, into which the finer material separated in the next separation phases 1 is fed, which material has been acquired from the coarser fraction, the"reject"of the first separation phase 1. From the two consecutive separation phases 1, fractions B 1 and B2, the"accepts"deriving from the same stock flow, are fed into compartment 2b. The second compartment 2b in the deculator is separated by means of a partition wall 5 from a third compartment 2c, which has been connected to overflow 6, which is fed into the wire pit. From the first compartment 2a the finer fraction is fed to the headbox as fraction D, forming the surface layers, which is then divided before the headbox into fractions A and C, which are similar to each other. From the second compartment 2b a stock flow is fed towards the headbox as fraction B, which forms the middle layer and which has coarser fibres.

The flows formed by the different fractions are kept separate and they are fed into their respective manifold pipes 4a, 4b and 4c into the headbox 4. From the manifold pipes the stock flows are fed as separate flows along the known internal structures of the headbox into the slice cone 4d, and after whose separating walls they are again combined. From

the headbox 4 the stock is fed onto the wire, on which it begins to form a layered product. The water filtered off on the wire drains in a known manner, as a part of the short cycle, into the wire pit.

Figure 3 presents another embodiment which also has three consecutive separation phases 1, so that in the first phase the finer fraction from the same stock flow M is separated into the compartment 2a of deculator 2, and the remaining fraction is fed through two consecutive separation phases in which the separated finer fractions B 1, B2 form a coarser fraction compared with the previous one, which is then fed into its own compartment 2b in deculator 2 as the stock fraction B, forming the middle layer.

Between compartments 2a, 2b is a third compartment 2c separated with partition walls 5 which is connected to overflow 6, which is fed into the wire pit. The solution presented in Figure 3 has the advantage that the stock fractions D and B separated in the consecutive separation phases do not mix, since there is between them compartment 2c, to which they are both connected by way of the overflow over partition wall 5.

Further, Figures 2 and 3 show with an unbroken line LI, how the dilution water containing filler and fines taken from above is fed from the wire pit to mix with the fraction D obtained first, into compartment 2a in deculator 2, and with a broken line, how this can also be fed into the fraction B obtained next, into the deculator compartment 2b. In addition, Figure 2 shows with a broken line L2 how, before separation, a small part of stock flow M can be fed when needed either directly into the first fraction D or into the next fraction B obtained, which also increases the possibilities for regulating the compositions of the stock fractions.

The invention facilitates the possibilities to use a fractionation technique in the layering web formation of paper or board. In addition, the double stock preparation systems and short cycles usually required are unnecessary. The invention can be applied with minor changes to existing short stock cycles. Especially if centrifugal cleaners and a deculator are already being used in the short cycle, the fractionation can be realized by connecting the centrifugal cleaners in an appropriate manner to the deculator, in which partition walls can be installed to create compartments in the deculator for the different fractions.

Both of the fractions can also be arranged to have their own deculators, both of which are provided with an overflow and from which the respective fractions are taken into the multi-layer headbox of the paper or board machine.

The invention is not limited to the use of any particular pulp: it can be used with all pulps and stock mixtures, which are separable by their compositions into different fractions in the short cycle.

For example, softwood sulphate pulp typically contains different fibre fractions. The spring and summer wood fibres of softwood pulp differ from each other, for example, as to the thickness of the fibre wall, and also fibres deriving from different parts of the tree stem differ from each as to the thickness and fibre content of the fibre wall. Thus, the same raw wood material may already contain different fibres that can be separated from each other into fractions with different fibre compositions.

The stock mixture often comprises, in addition to softwood sulphate pulp, also short fibre pulp, which differs as to its fibre dimensions from the softwood pulp used as reinforcement pulp. Because the stock mixture used for producing paper often contains fibres of different fibre dimensions, the invention can be widely applied to different stock mixtures, the fibres of which derive from at least two different raw wood materials and/or two different stock preparation methods. The following stock mixtures can be mentioned as examples: -softwood sulphate/hardwood sulphate, -softwood sulphate/groundwood pulp (groundwood pulp, pressure groundwood pulp), -softwood sulphate/refiner mechanical pulp (refiner mechanical pulp, thermomechanical pulp TMP), -CTMP With these stock mixtures, for example the following multi-ply paper types can be produced: -wood-containing and wood-free papers which are coated in further processes, -wood-free uncoated and coated printing paper,

-wood-containing uncoated and coated printing paper (SC, LWC, MWC) -folding boxboard, graphic boards, liquid packaging board.

The term"same source of stock"refers generally in this application to fibre stock deriving from the same raw wood material or plant-based raw material, a mixture of the above-mentioned raw materials which has been formed according, for example, to the desired properties of the finished product, the paper or board, or from a mixture of stocks derived from different stock preparation methods, the raw materials of which may be the same or different.