Paper machine arranged to form a short transfer nip

Abstract

The present invention relates to a paper making machine, comprising a forming section (101) for forming a fibrous web (W) from stock injected from a head box into a forming nip (FN) and a drying section (102) The drying section comprises a structured fabric (10) transporting the fibrous web (W), a Yankee drying cylinder (20) and a press arrangement (30) arranged to form an extended press nip that is a transfer nip (TN) with the Yankee drying cylinder (20) where the fibrous web (W) is transferred from the structured fabric (10) to the Yankee drying cylinder (20). The press arrangement (30) comprises a press body (31) arranged to extend against the Yankee drying cylinder (20) into the transfer nip (TN) formed between the press body (31) and the Yankee drying cylinder (20), wherein the press arrangement (30) also comprises an actuator (32) for extending the press body (31) into the transfer nip (TN) in order to load the transfer nip (TN). The extension of the transfer nip (TN) in a machine direction (MD) is ≤ 20 mm.

Description

[0001] PAPER MACHINE ARRANGED TO FORM A SHORT TRANSFER NIP

[0002] TECHNICAL FIELD

[0003] The present invention relates to a paper making machine having a press arrangement arranged to form a short transfer nip with a Yankee drying cylinder in the drying section of the paper making machine.

[0004] The paper making machine may be a machine for making tissue, structured paper, or structured tissue.

[0005] BACKGROUND

[0006] When manufacturing tissue paper, a paper web is formed in a forming section and is then then dried in a drying section where the paper web is transferred from a structured fabric to a final drying step on a Yankee dryer. The transfer to the Yankee dryer takes place by a transfer nip traditionally formed between the Yankee dryer and a transfer roll.

[0007] In known solutions, a transfer nip in the drying section of the paper making machine is typically formed using a transfer roll and a counter roll, typically a Yankee drying cylinder. The transfer roll may have a polymer cover that is ether plain or grooved. Some transfer rolls have internal cooling to keep the cover from overheating. The transfer roll cover needs to have a crown and a dub on both sides to provide a good crown fit with the counter roll. The diameter of the transfer roll and counter roll in combination with the transfer roll cover thickness, cover hardness and potential grooves sets the nip profile at a certain linear load.

[0008] As is well known in the art, a transfer in the drying section is a sensitive process since the properties of the web, including its dryness, needs to be well controlled in order for the web to attach well to the Yankee drying cylinder in the transfer. For these reasons, it is crucial to the success of the transfer that the nip is as short as possible. During a transfer of a fibrous web in the drying section, the transfer point should preferably act like a line across the machine where the entire fibrous web, i.e. the entire sheet width of the fibrous web, transfers from one fabric or roll to the next fabric or roll instantly. Of course, an instant nip is only possible in theory but not in practice. However, it is desirable that the length of the transfer nip in the machine direction is as short as possible.

[0009] In existing solutions for providing a short transfer nip in the drying section of a paper machine, a small radius transfer roll is typically used in the transfer nip. Of course, this is a working solution for achieving a short nip because a decreased roll radius will generate a decrease in length of the transfer nip in the machine direction. However, when designing a nip using a small radius transfer roll, the roll needs to be used in combination with a hard thin cover and the combination of the transfer roll and the hard cover gives an unyielding profile without any room for adaptation to imperfections in the process such as machine clothing thickness variation, wear or coating build-up on the Yankee drying cylinder surface. Another problem is that if there is a local high spot, all pressure is focused on this spot leaving areas around the high spot with low to no pressure. Any of these problems may lead to a deficient web transfer and an inferior end product and / or decrease in yield.

[0010] There is therefore a need for improvements within this area.

[0011] SUMMARY

[0012] The object of the present invention is to eliminate or at least to minimize the problems discussed above. This is achieved by a paper making machine according to the appended independent claim, having a press arrangement arranged to form a short extended transfer nip with a Yankee drying cylinder in the drying section of the paper making machine.

[0013] The invention includes a paper making machine for making paper. The paper making machine, or paper machine, comprises a forming section for forming a fibrous web from stock injected from a head box into a forming nip and a drying section. The drying section comprises a structured fabric transporting the fibrous web, a Yankee drying cylinder and a press arrangement arranged to form an extended press nip with the Yankee drying cylinder where the fibrous web is transferred from the structured fabric to the Yankee drying cylinder. The extended press nip is hence a transfer nip, i.e. an extended transfer nip. It is noted that the extended press nip is a non-dewatering transfer nip.

[0014] The press arrangement comprises a press body that is arranged to extend against the Yankee drying cylinder into the transfer nip formed between the press body and the Yankee drying cylinder. The press arrangement also comprises an actuator for extending the press body into the transfer nip in order to load the transfer nip. The extension of the transfer nip in a machine direction is < 20 mm, or preferably even < 15 mm.

[0015] Suitably, by utilizing a press body arranged to extend against the Yankee drying cylinder into the transfer nip and an actuator for extending the press body into the transfer nip in order to load the transfer nip, an extended transfer nip is achieved. The use of an extended nip enables the nip to be very short in the machine direction while at the same time improving the uniformity of the nip compared to the prior solution of using a thin, small radius, transfer roll with a hard cover. Additionally, the extended transfer nip advantageously can adapt to and compensate for both crowning and dub, as well as any imperfections or variations in the process such as machine clothing thickness variation, wear or coating build-up on the Yankee drying cylinder surface, compared to using a prior art transfer roll with a hard cover. In other words, the need for a crown and dub is removed. At the same time, the top loading surface of the extended nip press can be designed for a very short nip, while not sacrificing the runnability. A further advantage is that the transfer nip is not locked in to one linear load and pressure based on a fixed roll and roll cover design but can be adjusted, even on the run if needed. That the linear load and press profile can be more freely adjusted further advantageously enables using less coating add-on. It further contributes to a decrease of creeping performance and gives room for adaptation to imperfections in the process. Furthermore, any problems with local high spots are reduced since the pressure is more evenly distributed compared to using a prior art transfer roll with a hard cover. Thereby, an improved web transfer is suitably achieved, leading to an improved end product quality and improved yield.

[0016] Furthermore, the shorter the nip length is with maintained pressure, the lower the resulting linear load of the transfer nip will be. A lowering in the linear load of the transfer nip advantageously contributes to reducing wear of the flexible jacket and other in-between layers included in the transfer nip. Suitably, a lower linear load further reduces the risk that components included in the transfer nip move in relation to each other, for example that the structured fabric stretches or contracts during or after the transfer nip (as seen in the machine direction), or that any of the pressing bodies move. Although such movements are typically very small, even micro movements, they may still negatively affect the process and the resulting paper product as the movements may lead to the adhesive coating on the Yankee drying cylinder adhering to the fibrous web during the transfer nip. This in turn results in the finished paper product containing traces of coating, which is undesirable and negatively affects the quality of the paper product, and in that more coating needs to be added to the Yankee drying cylinder, which is both expensive and wasteful.

[0017] The press body may comprise a hard, substantially non-flexible material, for example a metal material. Suitably, this enables a shorter transfer nip compared to if a softer (flexible, elastically deformable) press body is used. In this case, the actuator may be a linear push unit for pushing the press body into the extended transfer nip. Alternatively, or additionally, the press arrangement may in this case further comprise at least one tube arranged on the side of the press body facing away from the extended transfer nip, the at least one tube being configured to be pressurized for controlled expanding of the at least one tube in a direction of the press body to push the press body into the extended transfer nip, wherein the actuator is a pressurization device for pressurizing the at least one tube. The working area of the press body may further be adaptable to an opposed surface of the Yankee drying cylinder. The working area may for example comprise an elastically deformable material, such as a polymer. Thereby, a more uniform pressure distribution over the width transfer nip is suitably achieved.

[0018] Alternatively, the press body may be elastically deformable and comprise at least one pressure chamber arranged to being set under pressure in order to load the extended transfer nip. In this case, the actuator is suitably a pressurization device for pressurizing the at least one pressure chamber. The elastically deformable press body may comprise a polymer material. Also in these embodiments, the working area of the press body may be adaptable to an opposed surface of the Yankee drying cylinder, and may for example comprise an elastically deformable material, such as a polymer. Thereby, a more uniform pressure distribution over the width transfer nip is achieved.

[0019] The drying section may also comprise at least one through air dryer, TAD. Thereby, additional drying of the fibrous web is achieved in an efficient manner.

[0020] In an embodiment, an upper surface of the press body comprises a working area, or working surface that forms the extended transfer nip together with the Yankee drying cylinder when the press body extends against the Yankee drying cylinder into the transfer nip. Suitably, the extension of the working area in the machine direction is < 20 mm, or even < 15 mm, providing a very short transfer nip.

[0021] For any of the embodiments, the working area may be adaptable to an opposed surface of the Yankee drying cylinder. Suitably, as the press body comprises a flexible material, part or all of the surface on the upper side of the press body, at least including the working area may thus be adaptable to the opposed surface, or meeting surface, of the Yankee drying cylinder. Thereby, the extended transfer nip TN can advantageously adapt to and compensate for any imperfections or variations in the transfer process as well as remove the need for both crowning and dub. The adaptable part of the surface on the upper side may suitably comprise an elastically deformable material, such as a polymer. The extension of the working surface in the machine direction MD, and the corresponding extension of the extended transfer nip TP in the machine direction MD, is preferably > 3 mm. Suitably, a very short transfer nip it thereby enabled, without risking burning the flexible jacket that is typically arranged between the press arrangement and the structured fabric, and against which the press body of the press arrangement presses when the press body extends towards the Yankee drying cylinder into the transfer nip.

[0022] Many additional benefits and advantages of the present invention will be readily understood by the skilled person in view of the detailed description below.

[0023] DRAWINGS

[0024] The invention will now be described in more detail with reference to the appended drawings, wherein:

[0025] Fig. 1 discloses schematically a paper machine according to the invention;

[0026] Fig. 2 discloses a schematic cross-sectional view from the side of an extended nip press forming a short transfer nip with a Yankee dryer according to a first embodiment of the invention;

[0027] Fig. 3a discloses a schematic cross-sectional view from the side of the extended nip press with a short working surface and the Yankee dryer of Fig. 2 in a rest position where the transfer nip is not active;

[0028] Fig. 3b discloses a schematic cross-sectional view from the side of the extended nip press with an even shorter working surface and the Yankee dryer of Fig. 2 in a rest position where the transfer nip is not active;

[0029] Fig. 4a discloses a schematic cross-sectional view from the side of an extended nip press forming a short transfer nip with a Yankee dryer according to a second embodiment of the invention; Fig. 4b discloses a schematic cross-sectional view from the side of the extended nip press and the Yankee dryer of Fig. 4a in a rest position where the transfer nip is not active;

[0030] Fig. 5a discloses a schematic cross-sectional view from the side of an extended nip press forming a short transfer nip with a Yankee dryer according to a third embodiment of the invention;

[0031] Fig. 5b discloses a schematic cross-sectional view from the side of the extended nip press and the Yankee dryer of Fig. 5a in a rest position where the transfer nip is not active;

[0032] Fig. 6a discloses a schematic cross-sectional view from the side of an extended nip press forming a short transfer nip with a Yankee dryer according to a fourth embodiment of the invention;

[0033] Fig. 6b discloses a schematic cross-sectional view from the side of the extended nip press and the Yankee dryer of Fig. 6a in a rest position where the transfer nip is not active;

[0034] Fig 7 is a graph showing examples of pressure distributions of transfer nips using prior art paper machines and examples of pressure distributions of extended transfer nips using a paper machine according to embodiments of the invention; and

[0035] Fig. 8 schematically shows the pressure distribution of an extended transfer nip formed using a paper making machine according to the invention.

[0036] All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the respective embodiments, whereas other parts may be omitted or merely suggested. Any reference number appearing in multiple drawings refers to the same object or feature throughout the drawings, unless otherwise indicated. DETAILED DESCRIPTION

[0037] When referring to a “paper machine”, “paper making machine” or a “tissue paper machine” herein, this is to be understood as a machine suitable for producing paper from a pulp.

[0038] It is to be noted that all dimensions given herein are to be understood as being within manufacturing tolerances or at least not differing more than 10 %. Also, when it is stated that a feature is substantially constant or substantially uniform, this is also to be understood as being constant or uniform within manufacturing tolerances or at least not differing more than 10 %.

[0039] A paper making machine 100, also referred to as a paper machine 100, according to the invention will now be described briefly with reference to Fig. 1, followed by a more detailed description of a transfer nip TN of the paper machine 100 with reference to Fig. 2 onwards. Since the present invention concerns the transfer nip TN in a drying section 102, other sections and components of the paper machine 100 will be described only in passing.

[0040] Fig. 1 discloses schematically a paper machine 100 comprising a forming section 101, the drying section 102 and a reel section 103. In the forming section 101, stock is injected into a forming nip to form a fibrous web, or paper web, W, as is already well known in the art, and the fibrous web W is transported on a fabric in a machine direction MD to the drying section 102 where moisture is removed to create a finished paper P. The paper P is then transferred to the reel section 103 where the paper P is reeled onto a roll before being removed from the paper machine 100 as a finished tissue paper product. The machine direction MD is defined as a direction from the forming section 101 to the reel section 103, i.e. a main direction of travel of the fibrous web W. Hereinafter, the terms fibrous web and paper web may be used interchangeably.

[0041] As long as the paper web W is newly formed and wet, it must be transported on a fabric that serves to support and protect the paper web W. After forming, the paper web W may in some embodiments travel also through a press section 104 before reaching the drying section 102. The drying section 102 comprises at least one drying roll in the form of a Yankee dryer, or Yankee drying cylinder, 20 (see Fig. 2 onwards), but there may also be at least one other drying roll such as a through-air dryer, TAD. The design and operation of the Yankee dryer 20, as well as a TAD or any other kind of dryer is already well known in the art and will not be described further herein.

[0042] Furthermore, moisture may be removed through at least one dewatering nip in the drying section 102. While transported in the drying section 102, the paper web W is supported by a structured fabric 10 (see Fig. 2 onwards) but there may also be at least one other fabric on which the paper web W is transported between the forming section 101 and the drying section 102, such as e.g. a felt. When the paper web W is transferred from one fabric to another, this takes place in a transfer nip where pressure is applied to cause the paper web W to release one fabric and instead adhere to the next.

[0043] In the drying section 102, the paper web W is transferred to the Yankee dryer 20 from the structured fabric 10 in the transfer nip TN as shown in Fig. 2, which discloses a schematic cross-sectional view from the side of an extended nip press forming a short transfer nip with a Yankee dryer according to a first embodiment of the invention. The web W is typically doctored from the Yankee diyer 20 by a doctor blade and is passed in the form of the finished paper P to a reel-up.

[0044] Turning again to Fig. 2, the drying section 102 of the paper machine 100 according to the invention comprises the structured fabric 10 transporting the fibrous web W, the Yankee drying cylinder, or Yankee dryer, 20, and a press arrangement 30 arranged to form an extended press nip that is a transfer nip TN with the Yankee drying cylinder 20 where the fibrous web W is transferred from the structured fabric 10 to the Yankee drying cylinder 20. The press arrangement 30 comprises a press body 31 arranged to extend against the Yankee drying cylinder 20 into the transfer nip TN formed between the press body 31 and the Yankee drying cylinder 20. The press arrangement 30 also comprises an actuator 32 for extending the press body 31 into the transfer nip TN in order to load the transfer nip TN. The extension of the transfer nip TN in a machine direction MD is < 20 mm, preferably even < 15 mm. In contrast to transfer nips known from the prior art, the transfer nip TN of the present invention is in the form of an extended nip press in which a press arrangement 30 creates an extended press nip by extending a press body 31 towards the Yankee dryer 20 to form the transfer nip TN. In the prior art, extended nips are known only as dewatering nips, but the present inventors have realized that there are numerous advantages to using an extended nip press to transfer the paper web W without dewatering it. In particular, using the extended nip press with the press arrangement 30 enables changing the pressure and thereby the linear load in the nip, thereby giving operators of the paper machine 100 the possibility of designing the nip depending on paper grade, coating on the Yankee dryer, moisture content at the transfer nip, and other factors. It also enables a shorter nip so that transfer to the Yankee diyer 20 is rendered swift and accurate. In other words, using an extended nip as a transfer nip enables the transfer nip to have such a short extension in the machine direction MD, and thereby improves the speed and accuracy with which the transfer onto the Yankee drying cylinder 20 is performed.

[0045] When the term “extended nip press” or “extended transfer nip” is used herein, this is to be understood as an arrangement where the press body 31 is extended into the nip. In contrast, a prior art transfer nip instead comprises a transfer roll.

[0046] The extended transfer nip TN formed between the Yankee drying cylinder 20 and the press body 31 is a non-dewatering transfer nip.

[0047] The press arrangement 30 of Fig. 2 comprises a support 34 in which the press body 31 is arranged, and further comprises at least one actuator 32 for extending the press body 31 into the nip. The press body 31 may be a press shoe.

[0048] Fig. 2 also shows a flexible jacket 70 with an interior side 71 against which the press body 31 of the press arrangement 30 presses and an outside 72 that contacts the structured fabric 10 on which the paper web W is held. The press arrangement 30 also suitably comprises a supply of lubricating fluid (e.g. oil) that is supplied to an upper side of the press body 31 to decrease friction between the press body 31 and the interior side 71 of the flexible jacket 70.

[0049] The flexible jacket 70 typically comprises polyurethane and is shaped as a tube that extends in a cross-machine direction, said cross-machine direction being a direction that is perpendicular to the machine direction MD. Thus, the flexible jacket 70 has an axial direction that coincides with the cross-machine direction and at its axial ends the flexible jacket 70 is normally connected to end walls that are rotatably arranged so that the flexible jacket 70 is able to rotate. Suitably, a source of pressurized air may be connected so that an enclosed space formed by the flexible jacket 70 and the end walls may be filled with pressurized air. The flexible jacket 70 and the press arrangement 30 may together be seen as an extended nip roll 80. The press body 31 is elongated and extends along the interior surface 71 of the flexible jacket 70 in a direction transversal to a direction of rotation of the flexible jacket 70 in order to be able to press against an entire width of the paper web W in the transfer nip TN. For this purpose, the press body 31 may have a length in the direction transversal to a direction of rotation of the flexible jacket 70 that is equal to or larger than an intended width of the paper web W, but in some embodiments the press body 31 may instead have a length that is shorter than the intended width of the paper web W.

[0050] The press arrangement 30 of the extended nip press is in Fig. 2 shown in an active state, i.e. when the transfer nip TN is formed by the press body 31 extending towards the Yankee dryer 20 so that the structured fabric 10 carrying the paper web W is pressed against the Yankee dryer 20 and the paper web W is transferred from one to the other.

[0051] Figs. 3a and 3b show the same components as Fig. 2 except for the flexible jacket 70, the structured fabric 10 and the paper web W that have been removed to show the press arrangement 30 of the extended nip press more clearly. Thus, Figs. 3a and 3b shows a rest position of the press arrangement 30 where the transfer nip TN is not active. In Fig. 3a, a surface 21 of the Yankee dryer 20 is shown, and on an upper side 37 of the press body 31 a working surface, or working area, 36 is disclosed. This working surface 36 is a part of the upper side 37 that is active in the transfer nip TN, i.e. that is extended into the transfer nip TN and transfers a force to the paper web W when the transfer nip TN is active. The press body 31 in some embodiments also comprises an entry surface 35 and / or exit surface 38 that are not active in the transfer nip TN. Such entry and / or exit surfaces 25, 28 are hence areas that are not active in, i.e. do not form part of, the extended transfer nip, i.e. that are non-pressurized during transfer. If the upper side 37 of the press body 31 comprises an entry surface 35 and / or an exit surface 38, the length of these surfaces are not included in the extension of the transfer nip (TN) in the machine direction (MD). In other words, the extension of the working surface, or working area, 36 in the machine direction MD is < 20 mm.

[0052] Fig. 3b, in likeness with Fig. 3a, discloses a schematic cross-sectional view from the side of the extended nip press and the Yankee dryer 20 of Fig. 2 in a rest position where the transfer nip TN is not active. Fig. 3b differs from Fig. 3a only in that the extension of the working surface 36 in the machine direction is illustrated as even shorter compared to in Fig. 3a. Fig. 3b thereby represents embodiments wherein the transfer nip TN and the extension of the working surface, or working area, 36 in the machine direction MD is < 15 mm.

[0053] When terms such as “upper”, “lower”, “top” and “bottom” are used in the following this is to be understood as being in relation to a first direction D that may be seen as a direction upwards. The direction D is indicated in Figs. 3a and 3b. Thus, an upper side is a side that faces in the first direction D whereas a lower side faces in a direction opposite to the first direction D.

[0054] For other embodiments of the invention, they will be described mainly in those features that differ from each other to avoid repetition. Therefore, it is to be understood that all features not specifically stated as differing from one embodiment to another are to be seen as similar or identical. Also, features from one embodiment may freely be incorporated into another embodiment where technically feasible. With reference to Figs. 4a to 6b, different options for the press body 31 and actuator 32 and embodiments of the invention adapted to these options will now be described.

[0055] The press body 31 may comprise a hard, substantially non-flexible material, for example a metal material comprising e.g. bronze, yellow metal materials, aluminum and / or stainless steel. Suitably, this enables a slightly shorter transfer nip TN compared to if a softer (flexible, elastically deformable) press body 31 is used. If the press body 31 comprises a non-flexible material such as metal, the actuator 32 suitably comprises an external pressure source (shown in dashed lines underneath the press body 31 in Figs. 2, 3a and 3b) arranged to extend the press body 31 into the extended transfer nip TN. Fig. 4a discloses a schematic cross-sectional view from the side of an extended nip press forming a short transfer nip with a Yankee dryer according to a second embodiment of the invention, wherein the press body 31 comprises a hard material such as metal. In this embodiment, the external pressure source is in the form of a displacement device 33 such as a mechanical or hydraulic actuator that is configured to operate the press body 31 by moving it towards the Yankee dryer 20 to extend it into the transfer nip TN. The displacement device 33 may be a linear push unit for pushing and thereby extending the press body 31 into the extended transfer nip TN. Fig. 4b shows the rest position and Fig. 4a shows the active position where the transfer nip TN is formed between the press body 31 and the Yankee dryer 20. In Fig. 4a, the displacement device 33 is shown as comprising or being connected to and using an arm 33’ for moving the press body 31, but it is to be noted that the displacement of the press body 31 can be achieved also in other ways, such as by the displacement device 33 expanding or a plurality of arms or supports contacting the press body 31 to move it into the nip. As shown in Fig. 4a, the external pressure source, here the displacement device 33, may be communicatively connected and controllable by to a controller 90. The controller is then in turn configured to control the displacement device 33 to extend the press body into the transfer nip TN. Fig. 4b discloses a schematic cross-sectional view from the side of the extended nip press and the Yankee dryer of Fig. 4a in a rest position where the transfer nip is not active. Alternatively, if the press body 31 comprises a non-flexible material such as metal, the external pressure source may comprise at least one tube 40 being configured to be pressurized by a pressurization device 41 that supplies a pressurized fluid to cause the at least one tube 40 to expand and push the press body 31 into the transfer nip TN. This third embodiment of the invention is illustrated in Figs. 5a and 5b, wherein Fig. 5a discloses a schematic cross- sectional view from the side of an extended nip press forming a short transfer nip with a Yankee dryer according to the third embodiment of the invention and Fig. 5b discloses a schematic cross-sectional view from the side of the extended nip press and the Yankee dryer of Fig. 5a in a rest position where the transfer nip is not active. As illustrated in Figs. 5a and 5b, the external pressure source, here the pressurization device 41, may be communicatively connected and controllable by to a controller 90. The controller is then in turn configured to control the pressurization device 41 to supply the pressurized fluid to the at least one tube 40, thereby causing the at least one tube 40 to expand and push the press body 31 into the transfer nip TN. Alternatively, or additionally, to using a linear push unit and / or at least one expandable tube, any other suitably external pressure source may be used. In the second and third embodiments, part or all of the surface on the upper side 37 of the hard press body 31, at least including the working area 36, may be adaptable to the opposed surface, or meeting surface, 21 of the Yankee drying cylinder 20. Thereby, the extended transfer nip TN can advantageously adapt to and compensate for any imperfections or variations in the transfer process as well as remove the need for both crowning and dub. The adaptable part of the surface on the upper side 37 may suitably comprise an elastically deformable material, such as a polymer.

[0056] Alternatively, the press body 31 may comprise a flexible material, or be elastically deformable. In this case, the actuator 32 suitably comprises an internal pressure chamber comprised in the press body 31 (shown in dashed lines inside the press body 31 in Figs. 2, 3a and 3b) and a pressurization device that supplies a pressurized fluid to cause the press body 31 to stretch and extend into the nip. The press body 31 may in this case for example comprise a polymer material. Fig. 6a discloses a schematic cross-sectional view from the side of an extended nip press forming a short transfer nip with a Yankee dryer according to a fourth embodiment of the invention, wherein the press body 31 comprises a flexible material, and Fig. 6b discloses a schematic cross-sectional view from the side of the extended nip press and the Yankee dryer of Fig. 6a in a rest position where the transfer nip is not active. In this embodiment, the actuator 32 comprises an internal pressure chamber 50 and a pressurization device 51. As illustrated in Figs. 6a and 6b, the pressurization device 51 may be communicatively connected and controllable by to a controller 90. The controller is then in turn configured to control the pressurization device 51 to supply the pressurized fluid to the internal pressure chamber 50, thereby causing the internal pressure chamber 50 to expand or stretch and extend into the transfer nip TN. As the press body 31 comprises a flexible material, part or all of the surface on the upper side 37 of the press body 31, at least including the working area 36, may also in this fourth embodiment be adaptable to the opposed surface, or meeting surface, 21 of the Yankee drying cylinder 20. Thereby, the extended transfer nip TN can advantageously adapt to and compensate for any imperfections or variations in the transfer process as well as remove the need for both crowning and dub. The adaptable part of the surface on the upper side 37 may suitably comprise an elastically deformable material, such as a polymer.

[0057] For all embodiments herein, the extension of the transfer nip TN in the machine direction MD is < 20 mm, and in some cases also < 15 mm. Similarly, for all embodiments herein it is thus true that the extension of the working area 36 in the machine direction MD is < 20 mm, and in some cases also < 15 mm. The lower boundary for the extension of the transfer nip TN in the machine direction MD is, besides the thickness and hardness of in-between layers, also dependent on the minimum extension of the working surface 36 in the machine direction MD that can be used without burning the flexible jacket 70. For the purpose of this invention, the extension of the working surface 36 in the machine direction MD, and the extension of the extended transfer nip TP in the machine direction MD, is therefor set to > 3 mm.

[0058] The press body 31 according to any embodiment herein, and more specifically the working area or working surface 36, may be planar, curved with short radius, a combination of planar and curved, have a thin peak, have more than one peak, or have any other top geometiy that is suitable for achieving a short transfer nip. The extension of the working area or working surface 36 in the machine direction MD can be substantially equal to the extension of the resulting extended transfer nip TN in the machine direction MD, or the working area 36 extension may be shorter than the extension of the resulting extended transfer nip TN, due to thickness and hardness of the in-between layers comprising the fibrous web W and the structured fabric 10.

[0059] Fig. 7 is a graph showing examples of examples of pressure distributions of transfer nips using prior art paper machines with transfer rolls compared to examples of pressure distributions of extended transfer nips using a paper machine according to embodiments of the invention. As can be seen from Fig. 7, the exemplary transfer nips using transfer rolls result in a nip extension in the machine direction MD of about 65 mm at a linear load of 40 kN / m (illustrated by the single unbroken line), and a nip extension in the machine direction MD of about 55 mm at a linear load of 90 kN / m (illustrated by the double unbroken line). The graph further shows that the peak pressure of these transfer nips is 1 MPa at the applied linear load of 90 kN / m and about 1.8 MPa at the applied linear load of 40 kN / m. In contrast, the nip pressure distribution of embodiments of the invention, here exemplified by the dotted line and the dashed line, form much shorter extended transfer nips TN with relatively higher peak pressures under lower linear loads. Thereby, clearly improved transfers are achieved. In Fig. 7, the dotted line illustrates the pressure distribution of a short press body 31 having a working area 36 extension in the machine direction MD of about 20 mm and achieving an extended transfer nip TN of the same length. This extended transfer nip TN advantageously has a peak nip pressure of about 1.4 MPa and the linear load applied is only 25 kN / m. The dashed line illustrates the pressure distribution when a press body 31 with an even shorter working area 36 is used, in this example extending only about 10 mm in the machine direction MD and resulting in an extended transfer nip TN having an extension of only about 16 mm in the machine direction MD. This extended transfer nip TN advantageously has a peak nip pressure of about 2.2 MPa while the linear load applied is only 20 kN / m.

[0060] Fig. 8 schematically shows the pressure distribution of a short extended transfer nip TN formed using a paper making machine 100 according to embodiments of the invention. In Fig. 8, the patterned areas PD of the press body 31 and the Yankee drying cylinder 20 are pressurized while the white areas of the press body 31 and the Yankee drying cylinder 20 are not pressurized. The figure thereby shows that the working surface 36 is pressurized during the transfer nip, while the surrounding surfaces 35, 38 are not pressurized and thus are not active in the extended transfer nip TN. It is to be noted that features from the various embodiments described herein may freely be combined, unless it is explicitly stated that such a combination would be unsuitable.

Claims

CLAIMS1. A paper making machine (100) for making paper, the machine comprising:- a forming section (101) for forming a fibrous web (W) from stock injected from a head box into a forming nip (FN);- a drying section (102), comprising:- a structured fabric (10) transporting the fibrous web (W);- a Yankee drying cylinder (20);- and a press arrangement (30) arranged to form an extended press nip that is a transfer nip (TN) with the Yankee drying cylinder (20) where the fibrous web (W) is transferred from the structured fabric (10) to the Yankee drying cylinder (20), wherein the press arrangement (30) comprises a press body (31) arranged to extend against the Yankee drying cylinder (20) into the transfer nip (TN) formed between the press body (31) and the Yankee drying cylinder (20), wherein the press arrangement (30) also comprises an actuator (32) for extending the press body (31) into the transfer nip (TN) in order to load the transfer nip (TN), characterized in that the extension of the transfer nip (TN) in a machine direction (MD) is < 20 mm.

2. The paper making machine (100) of claim 1, wherein an upper side (37) of the press body (31) comprises a working area (36) that is active in the extended transfer nip (TN) when the press body (31) extends against the Yankee drying cylinder (20) into the transfer nip (TN).

3. The paper making machine (100) of claim 2, wherein the extension of the working area (36) in the machine direction (MD) is < 20 mm.

4. The paper making machine (100) of any one of the preceding claims, wherein the extension of the transfer nip (TN) in the machine direction (MD) is < 15 mm.

5. The paper making machine (100) of claim 4, wherein the extension of the working area (36) in the machine direction (MD) is < 15 mm.

6. The paper making machine (100) of any one of the preceding claims, wherein the transfer nip (TN) formed between the Yankee drying cylinder (20) and the press body (31) is a non-dewatering transfer nip.

7. The paper making machine (100) of any one of the preceding claims, wherein the press body (31) is made of metal, wherein the actuator (32) is a displacement device (33) configured to operate the press body (31) by moving it towards the Yankee dryer (20) to extend it into the extended transfer nip (TN), or wherein the press arrangement (30) further comprises at least one tube (40) arranged on the side of the press body (30) facing away from the extended transfer nip (TN), the at least one tube (40) being configured to be pressurized for controlled expanding of the at least one tube (40) in a direction of the press body (31) to push the press body (31) into the extended transfer nip (TN), wherein the actuator (32) is a pressurization device (41) for pressurizing the at least one tube (40).

8. The paper making machine (100) of claim 7, wherein the working area (36) is adaptable to an opposed surface (21) of the Yankee drying cylinder (20).

9. The paper making machine (100) of claim 7 or 8, wherein the working area (36) comprises an elastically deformable material, such as a polymer.

10. The paper making machine (100) of any one of the claims 1 to 3, wherein the press body (31) is elastically deformable and comprises atleast one pressure chamber (50) arranged to being set under pressure in order to load the extended transfer nip (TN), wherein the actuator (32) is a pressurization device (51) for pressurizing the at least one pressure chamber (50).

11. The paper making machine (100) of claim 10, wherein the press body (31) comprises a polymer material.

12. The paper making machine (100) of claim 11, wherein the working area (36) is adaptable to an opposed surface (21) of the Yankee drying cylinder (20).

13. The paper making machine (100) of claim 11 or 12, wherein the working area (36) comprises an elastically deformable material, such as a polymer.

14. The paper making machine (100) of any one of the preceding claims, wherein the drying section (102) also comprises at least one through air dryer, TAD.

15. The paper making machine (100) of any one of the preceding claims, wherein the extension of the transfer nip (TN) in the machine direction (MD) is > 3 mm.

16. The paper making machine (100) of claim 15, wherein the extension of working area (36) in the machine direction (MD) is > 3 mm.

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