Forming of a Paper or Board Web in a Twin-Wire Former or in a Twin-Wire Section of a Former

Abstract

Fibrous stock is supplied by a paper or board machine headbox ( 1 ) between looped forming wires ( 11, 12 ). Water is removed in a two part first dewatering zone (Z 1 ), the first part formed by a suction forming roll engaging a forming wire ( 12 ) and the second part formed by a fixed forming shoe engaging opposite a forming wire ( 11 ). The wires ( 11, 12 ) are guided so the first dewatering zone causes essentially non-pulsating dewatering in the stock. Water is removed in a second dewatering zone (Z 2 ) formed by fixed dewatering blades ( 21 ) on the other side of the wires ( 11, 12 ) and supported against the stock therebetween ( 21 ) and on the opposite side of the wires ( 11, 12 ) by dewatering blades ( 24 ), loaded in a controlled manner against the fixed dewatering blades ( 21 ) at gaps ( 22 ) between these so that pulsating dewatering is caused in the second zone (Z 2 ).

1 . A twin-wire forming section of a paper or board machine, comprising: 
 a headbox;    a first forming wire forming a first wire loop and extending from a first breast roll to a second forming roll;    a second forming wire forming a second wire loop extending from a first forming roll to the second forming roll then to a guide roll, wherein the first forming roll is spaced further from the headbox than the first breast roll is spaced from the headbox;    the second forming wire and the first forming wire forming a twin-wire dewatering area therebetween, said twin-wire dewatering area being located after the first forming wire passes over the first breast roll, and defining a beginning where the second forming wire and the first forming wire together pass over the first forming roll, the dewatering area arranged in to at least a first dewatering zone, and a second successive dewatering zone;    wherein the headbox is arranged to form a lip jet directed to engage only the first forming wire at a location spaced from the first breast roll and before the twin-wire dewatering area, such that a lip jet will travel only with the first forming wire until the beginning of the twin-wire dewatering area;    wherein the first forming roll is a suction roll having an open surface and a suction zone limited by crosswise seals, the suction zone engaging the second forming wire where the second forming wire and the first forming wire together pass over the first forming roll in the first dewatering zone;    a fixed forming shoe in the first dewatering zone following the suction zone having a curved surface deck and a leading edge, the curved surface deck in supporting engagement with the first forming wire wherein the second forming wire is unsupported over the fixed forming shoe;    wherein the second successive dewatering zone is formed by a plurality of fixed dewatering blades engaging one of the forming wires in the cross-machine direction, wherein the fixed dewatering blades define gaps therebetween;    a plurality of movable dewatering blades mounted for a loading motion in a controlled manner against a fibrous stock located between the first forming wire and the second wire, the movable dewatering blades being opposite the gaps and engaging the other forming wire at the gaps in between the fixed dewatering blades, so causing pulsating dewatering in the fibrous stock in the second dewatering zone; and    wherein the fixed forming shoe is provided with an essentially open surface and a source of underpressure arranged under the forming shoe so as to cause essentially non-pulsating dewatering in the fibrous stock traveling in between the forming wires.    
   
   
       2 . The apparatus of  claim 1 , wherein the open surface area of the deck of the fixed forming shoe is at least 50 percent of the total surface area of the shoe.  
   
   
       3 . The apparatus of  claim 1 , wherein the open surface area of the deck of the fixed forming shoe is formed of holes extending through the deck.  
   
   
       4 . The apparatus of  claim 1 , wherein the holes extending through the deck and forming the open surface area of the deck of the fixed forming shoe are arranged at an angle in relation to the top surface of the deck and against a first direction and wherein the first wire and the second wire are mounted to travel in the first direction.  
   
   
       5 . The apparatus of  claim 1 , wherein the radius of curvature of the deck of the fixed forming shoe is between 600 mm and 4000 mm.  
   
   
       6 . The apparatus of  claim 1 , wherein the plurality of fixed dewatering blades engage the second forming wire in the cross-machine direction and wherein the plurality of movable dewatering blades opposite the gaps engage the first forming wire at the gaps in between the fixed dewatering blades.  
   
   
       7 . The apparatus of  claim 1 , wherein the plurality of fixed dewatering blades engage the first forming wire in the cross-machine direction and wherein the plurality of movable dewatering blades opposite the gaps engage the second forming wire at the gaps in between the fixed dewatering blades.  
   
   
       8 . A method of forming a web in a twin-wire formation section of a paper or board machine, comprising the steps of: 
 supplying a lip jet of stock from a headbox to a first forming wire forming a first wire loop so that the stock travels only on the first forming wire;    moving a second forming wire forming a second wire loop against a suction forming roll having an open surface and a suction zone limited by crosswise seals, the second forming wire engaging the suction zone, wherein the second forming wire, the first forming wire and the stock therebetween are passed together over the suction forming roll and dewater the stock in a first part of a first dewatering zone beginning over the suction zone;    moving the first forming wire against a fixed forming shoe forming a second part of the first dewatering zone, the forming shoe having a deck having a curved surface, the first forming wire being supported by and moving against an area defined by the curved surface of the forming shoe deck;    further dewatering the stock in the second part of the first dewatering zone, between the first forming wire and the second forming wire on the curved surface of the forming shoe deck, the second forming wire being unsupported in the area defined by the curved surface of the shoe;    wherein essentially non-pulsating dewatering is preformed in the first dewatering zone;    removing water from the stock in a successive second dewatering zone through which the first forming wire, the second forming wire and the stock therebetween travel, the second dewatering zone formed by fixed dewatering blades which extend in a cross-machine direction, and engage one of the forming wires, the fixed dewatering blades defining gaps therebetween; and    supporting against the other of the forming wires movable dewatering blades, which are loaded in a controlled fashion opposite the gaps and between the fixed dewatering blades against the fibrous stock between the forming wires so that the second dewatering zone causes pulsating dewatering in the fibrous stock.    
   
   
       9 . The method of  claim 8 , further comprising the step of applying underpressure to the fibrous stock through the deck of the fixed forming shoe.  
   
   
       10 . The method of  claim 8 , wherein the fixed dewatering blades engage the second forming wire in the cross-machine direction and wherein the movable dewatering blades opposite the gaps engage the first forming wire at the gaps in between the fixed dewatering blades.  
   
   
       11 . The method of  claim 8 , wherein the fixed dewatering blades engage the first forming wire in the cross-machine direction and wherein the movable dewatering blades opposite the gaps engage the second forming wire at the gaps in between the fixed dewatering blades.