The imprinting of a three-dimensional structure into the surface of a paper web, in particular of a tissue web, in particular of hand tissue, is known (see, for, example, WO 99/47749, WO 01/18307). It is further known that a very good paper quality can be achieved by a so-called through-air drying (TAD) process. However, it is disadvantageous that the use of TAD dryers is very complex and correspondingly expensive to pur- chase and operate.

To make the highest quality tissue and toweling products, it is necessary to develop products that are high in bulk, high in absorbancy, yet still have adequate strength. The normal papermaking processes, which in- cludes shoe and roll presses for dewatering a wet sheet, do not provide a bulky, absorbant sheet. Instead, they provide a strong, "flat"sheet that is typical of old technology low cost tissue.

Several techniques are used to develop sheet bulk. Generally, the fiber web or sheet is first formed on or vacuumed into a special embossing or imprinting fabric. This fabric is rough, due to its coarse weave. The wet sheet conforms to this fabric, and in doing so this increases the overall bulk of the sheet. Next, air is pulled through the sheet using vacuum, or low pressure. This airflow mechanically dewaters the sheet. Finally, hot air is blown through the sheet to dry it. The hot air dryer is called a Through Air Dryer (TAD for short). A TAD is usually made up of two large drums that are under vacuum that pull heated air through the sheet drying it. These are very expensive units costing 10's of millions, to install.

As shown in earlier times, one way to get high bulk is to emboss (mold) the sheet while it is wet. This can be done either by forming the sheet on a rough forming (or moldirig) fabric, or it can be formed"flat"in a conven- tional manor and then it can be vacuumed into a embossing fabric. Either way, the sheet surface takes on the approximate shape of the embossing fabric surface. After the sheet is molded, it must be dried to its final state.

Drying is usually a two step process, where water is first removed me- chanically, and then the remaining water is removed using heat.

The problem is that it is difficult to mechanically remove water from the sheet without destroying its molded structure. If the sheet and fabric are pressed for example, little water is removed since the embossing fabric adsorbs and then rewets the sheet after pressing. If the sheet is removed from the embossing fabric and then pressed, more water is removed, but the sheet bulk and adsorbancy is lost since the sheet becomes flatter.

The situation is slightly better if the sheet and embossing fabric are passed over a vacuum box. In this case, most prior art shows that the embossing fabric is on the vacuum side, supporting the sheet as air is pulled through it. The action of the vacuum removes water from the sheet, but after the water leaves the sheet, the embossing fabric retains much of it. Later, when the vacuum is removed, water passes back into the sheet, rewetting it. With this technology, the highest solids obtained for the sheet with, low basis weights is less than 25 % and more likely close to 20 %.

Never the less, vacuum dewatering has been used since it retains the sheet structure. However because the sheet is so wet, this technology uses a lot of energy, in the form of a hot air, to dry the sheet.

It is an aim of the invention to provide an improved method and an im- proved apparatus of the kind initially mentioned with which in particular a high quality of the end product can be achieved in an economic and correspondingly favorably priced manner even without the use of a larger TAD drying apparatus. In this connection, a corresponding quality should be reached in particular with respect to the water retention capacity, the water absorption rate, the bulk, softness, etc.

This object is satisfied in accordance with the invention by a method for manufacturing a fiber web, in particular a web of tissue or hygiene mate- rial, provided with a three-dimensional surface structure, in which the fiber web is pressed, e. g. sucked, at a dry content of <35%, in particular <30%, and preferably <256/o onto an imprinting fabric by means of a first pressure field, and is thereby pre-imprinted, and in which the fiber web is guided through at least one pressure field (third pressure field) provided for dewatering and/or drying said fiber web.

As a result of this embodiment, a lasting three-dimensional surface struc- ture is produced in the relevant fiber web, i. e. in particular in the relevant paper web, tissue web, or hygiene paper web, which is also present in the desired manner in the web, i. e. for example in the paper, even after the drying process. The use of a complex and correspondingly expensive TAD process is no longer required. In particular a lasting surface structure of, for example, of a tissue web or of a hygiene paper web can now also be produced downstream of the forming region or forming zone even without such a TAD drying apparatus.

Preferably, the fiber web is once more pressed onto an imprinting fabric by means of a second pressure field in order to fix strength without destroy- ing the three-dimensional surface structure. The fiber web is preferably guided between the first and the second pressure field through said at least one third pressure Filed. Preferably, the same imprinting fabric is used in said first pressure field and said second pressure field.

The imprinting or structured fabric could be a woven or a casted fabric in, a continous loop and can, for example, be a TAD (through-air-drying) fabric or an imprinting membrane.

The fiber web is generally pre-imprinted downstream of the forming re- gion.

It is of advantage in certain cases for the fiber web to be formed on the imprinting fabric used for the pre-imprinting. The fiber web can, however, also be transferred onto the imprinting fabric used for the pre-imprinting.

In accordance with a preferred embodiment, at least the first pressure field is produced by means of at least one suction or pressure element arranged on the side of the imprinting fabric remote from the fiber web in order to suck or press the fiber web into the surface structure of the im- printing fabric. In this connection, in particular a so-called wet suction box or pressure box can be used as the suction or pressure element.

It is also of advantage for the fiber web to be pressed gently in the second pressure field, i. e. preferably over an extended nip in the web running direction.

The second pressure field is preferably produced by means of a press nip.

To effect the most gentle possible pressing of the web, this press nip can, for example, be produced between a dryer cylinder and an opposing ele- ment, with the fiber web guided through the press nip being in contact with the surface of the dryer cylinder and contacting the imprinting fabric with its other side. In particular a, so-called Yankee cylinder can be used as the dryer cylinder. In particular a shoe press unit, which includes a flexible sleeve guided via a press shoe in the region of the press nip, can be used as an opposing element interacting with the dryer cylinder, with a shoe pressing roll provided with a flexible roll sleeve preferably being used as the shoe press unit. However, a press roll or a suction pressing roll can, for example, also be used as an opposing element interacting with the dryer cylinder.

A preferred practical embodiment of the method in accordance with the invention is characterized in that the pre-imprinted fiber web is dried on the dryer cylinder, or the Yankee cylinder, the fiber web is creped and/or the fiber web is subsequently wound up In accordance with a preferred embodiment of the method in accordance with the invention, the dry content at which the fiber web is pre-imprinted and/or the dry content at which the three-dimensional surface structure is created is selected in each case at <30%, in particular <25%, in particu- lar < 15%, and preferably < 10%. The water retention capacity and the bulk, among other things, are thus lastingly increased, which means that the desired imprinting is also still present on the use of the end product, for example of a relevant web of tissue or hygiene material. In particular the advantage of a higher water retention capacity for towel tissue (towel paper) is also still effective on the use of the relevant end product.

The third pressure field is preferably provided between said first pressure field and said second pressure field.

In accordance with a preferred practical embodiment of the method of the invention, a drying apparatus is used in order to provide said third pres- sure field.

In accordance with a preferred practical embodiment of the method of the invention, a suction or pressure device is used as a drying apparatus. The fiber web can, for example, be guided together with ah imprinting fabric both, through the third pressure field and the second pressure field. It is of advantage in this connection if the suction or pressure device has a curved surface and if the fiber web and the imprinting fabric are guided over this curved surface.

A suction roll can, for example, be used as the suction device. Such L suction device can have a pressurized hood to support the vacuum effect of the suction device.

According to another preferred practical embodiment of the method of the invention said third pressure field is provided by a gas press, preferably an air press. Such an gas or air press can, for example, comprise an arrangement of at least four rolls or a U-shaped box.

In addition it is advantageous to operate the gas or air press for displace- ment dewatering with a pressure in the chamber of > 30 psi, preferably > 40 psi.

In general, one or more third pressure fields can be provided. The third pressure fields can, e. g. be provided by drying apparatus of a different kind. For example, one of the drying apparatus can comprise a gas or air press whereas another drying apparatus may comprise a suction roll or the like.

Further advantages result in the use of a press shoe due to the relatively long press nip, since a better transfer of the fiber web to the Yankee cylin- der is achieved over a longer nip providing a longer dwell time.

The imprinting fabric can in particular be guided via the suction element or the wet suction box upstream of the suction device, i. e. for example the suction roll, in order to suck the fiber web into the three-dimensional surface structure of the imprinting fabric and thus to imprint this struc- ture onto the imprinting fabric. At the same time, the relevant suction element results in a corresponding increase in the dry content.

It is also of advantage for the length of the press nip of the shoe press including the dryer cylinder and the shoe press unit observed in the web running direction to be selected larger than a value of approximately 80 mm and for the shoe press to be designed such that a pressure profile results over the press nip length with a maximum pressing pressure which is smaller or equal to a value of approximately 2.5 MPa. A gentle pressing is thus ensured with which it is avoided that the structure pro- duced in the fiber web, e. g. in the tissue web or in the hygiene paper web, is again smoothed out.

As already mentioned, a suction roll, with which a pressure hood is pref- erably associated,, can, for example, be used between the suction element producing the'first pressure field and the press nip.

In accordance with a preferred practical embodiment of the method in accordance with the invention, at least one dewatering fabric with zonally different fabric permeability is used in the forming region. The relevant dewatering fabric can in particular be provided as an outer fabric. A corre- sponding embodiment of the method is in particular of advantage in the manufacture'of towel tissue. The fabric produces a fine structure which increases the water absorbing rate and which provides an increased water retention capacity in conjunction with the imprinting in accordance with the invention.

In certain cases, it is of advantage if a former with two circulating dewa- tering fabrics is used, e. g. twin wire former, which run together to form a pulp run in gap and are guided over a forming element such as in particu- lar a forming roll, and if a dewatering fabric with zonally different fabric permeability is used as an outer fabric not coming into contact with the forming element and/or as an inner fabric. In this connection, an imprint- ing fabric can be used as an inner fabric, for example, and preferably a dewatering fabric with zonally different fabric permeability as an outer fabric. It is, for example, also possible for the fiber web preferably to be transferred from the inner fabric to an imprinting fabric.

In wet imprinting in a tissue machine provided with an imprinting fabric, it is in particular a question of achieving the desired dry content. The web can, for example, be wet imprinted by means of the imprinting fabric using a suction box upstream of the press.

To now avoid the three-dimensional surface structure, which was pre- imprinted by the wet imprinting in the region of the wet suction box, being destroyed again by a short-term high pressure in the press nip in coopera- tion with a press felt, as is the case e. g. with a conventional suction press roll or press roll, in accordance with an advantageous practical embodi- ment of the method in accordance with the invention, there is guided through the press nip the imprinting fabric, e. g. a TAD fabric or an im- printing membrane, which is structured such that a smaller contact area portion formed by raised or closed zones (solid portions between the holes) results for this imprinting fabric in comparison with the non-contact area portion formed by recessed zones or holes and accordingly a smaller contact area portion of the fiber web is pressed in the press nip. The smaller contact area portion of raised or closed zones produces the web regions of high density for the strength, whereas the larger surface portion of recessed zones or holes, which remains at least substantially un- pressed, provides the desired water absorption capacity and the desired bulk such as has previously only been achieved by a complex and expen- sive TAD drying.

In this connection, an imprinting fabric can advantageously be used in which the contact area portion of raised or closed zones is <40% and preferably lies in a range from approximately 20 to approximately 30%, and in particular at approximately 25%. The contact area need not to be the same as the open area or the void volume. The open area or the void volume of a fabric can be independent of the contact area.

An imprinting fabric is expediently used in which the raised zones and the recessed zones result through offsets, i. e. through intersections of picks and ends, of a fabric cloth. As already mentioned, an imprinting mem- brane can, for example, also be used in which the raised and recessed zones result through the holes. It is of advantage in this case that 100% of the surface is pressed around the holes and a higher strength results.

The relevant, imprinting fabric can again be guided together with the fiber web, for example, over a dryer cylinder, in particular a Yankee cylinder. In particular a shoe-pressing unit can again be used as the opposing element interacting with the dryer cylinder. The length of the press nip observed in the web running direction and the pressure profile resulting over the press nip length can also in particular be again selected such as was set down above.

It has been found that with the method in accordance with the invention, a water absorbing capacity (g H20/g fibers) higher by at least 50% and a bulk (cm3/g) higher by 100% can be achieved with the same tensile strength when an imprinting fabric is used instead of a conventional felt in the press nip.

The quality of the paper results as a consequence of the lower pressing of the web as a consequence of the smaller area proportion of raised zones, and not due to a TAD dryer. The permeability of the web results from the stretching of the web into the structure of the imprinting fabric by means of the suction element, whereby so-called pillows are produced which correspondingly increase the water absorbing capacity and the bulk. A relatively complex and correspondingly expensive TAD dryer is therefore no longer necessary for this.

The function of the TAD drum and of the through-air system consists of drying the web and, for this reason, the above mentioned alternate drying apparatus (third pressure field) is preferable, since the third pressure field can be retrofitted to a conventional machine at lower cost than TAD.

To achieve the desired dryness, in accordance with an advantageous embodiment of the method in accordance with the invention, at least one felt with a foamed layer wrapping a suction roll is used for dewatering the web. In this connection, the foam coating can in particular be selected such that the mean pore size in a range from approximately 3 to approx- mately 6 llm results. The corresponding capillary action is therefore util- ized for dewatering. The felt is provided with a special foam layer which gives the surface very small pores whose diameters can lie in the range set forth from approximately 3 to approximately 6 um. The air permeability of this felt is very low. The natural capillary action is used for dewatering the web while this is in contact with the felt.

In accordance with an advantageous embodiment of the method in accor- dance with the invention, a so-called SPECTRA membrane is used for dewatering the web, said SPECTRA membrane preferably being laminated or otherwise attached to an air distribution layer, and with this SPECTRA membrane preferably being used together with a conventional, in particu- lar woven, fabric.

Such a SPECTRA membrane can in particular be designed and manufac- tured as is described in GB 2 305 156 A in connection with its Figure 3 and in GB 2 235 705 B. The two publications just cited are hereby incor- porated in the present application by reference.

The SPECTRA membrane can therefore in particular be a membrane with a regular, non-woven mesh structure through which suction is possible. It can be provide with spun reinforcement threads which extend through the mesh structure in the web running direction (cf. in particular Figure 3 of GB 2 305 156 A). This SPECTRA membrane can in particular be a porous, reinforced membrane made from a composite, with spun threads or yarns extending in the machine direction forming the reinforcing elements and the surrounding matrix material including fluid passages, completely encapsulating the spun threads and connecting them to one another, spun thread for spun thread, to produce the non-woven SPECTRA mem- brane (cf. in particular GB 2 235 705 B). In other respects, the SPECTRA membrane can also in particular be designed and manufactured as is described in GB 2 305 156 A and GB 2 235 705 B.

As mentioned above, the SPECTRA membrane can, e. g. , be laminated or otherwise attached to an air distribution layer.

Since the SPECTRA membrane has a relatively coarse cast structure, it is of advantage for it to'preferably be used together with a conventional, in particular woven, fabric arranged between the SPECTRA membrane and for example a through flow cylinder. The distribution of the air flow is thus substantially improved, i. e. a more uniform distribution of the air/gas is achieved, and the drying is thus more uniform. This effect is advantageous when the surface of the through-flow cylinder has an open area of <25% and large land areas are provided between the holes.

Such a SPECTRA membrane can therefore in particular be used instead of the felt with a foamed layer. An anti-rewetting effect is utilized for dewater- ing instead of the capillary effect, in addition to.

In accordance with a further advantageous alternative embodiment of the method in accordance with the invention a so-called anti-rewetting mem- brane or anti-rewetting fabric (or anti-rewet fabric) can also be used for dewatering the web.

The anti-rewetting membrane can in particular include the following: at least one air distribution fabric layer ; with such an air distri- bution fabric layer being configured for coming into contact with the fiber web; and - a perforated film layer, which can consist of a polyester film or of a plastic film, wherein the perforated film layer has a first film side and a second film side and wherein the first film side can be laminated or applied to the relevant air distribution fabric layer.

The perforated film layer can also be brought into direct contact with the paper web, with in this case, however, the positive effect being substantially lower. A respective air distribution fabric layer can include a plain weave (linen bond) or a fabric of a plu- rality of floating threads (multi-float weave, multi-strand bond; weave type). The perforated film layer can include a series of per- foration holes, which are spaced apart as closely as possibly, be- ing separated from the others by a perforation space, with each air distribution fabric layer having an associated kind of material bond or weave and with the kind of material bond or weave hav- ing the ability to disperse the air over a distance greater than the perforation space. That means the weave repeat length should be equal to or larger than the perforation space. The bond kind or weave kind interval distance can in particular also be larger than the perforation space. The perforation film layer can have a se- ries of perforation holes, with the perforation film layer being able to have, for example, approximately 40,000 holes per m2.

The perforation film layer can in particular have a series of per- foration holes, with the perforation film layer being able to have, for example, less than approximately 200,000 holes per m2. The perforation film layer can have an open area, for example, in the range from approximately 1% to approximately 30% and pref- erably in a range from, for example, approximately 5% to ap- proximately 15%. The perforated film layer can, for example, have a thickness of less than approximately 0. 04 inches, with the thickness, for example, being less than approximately 0.005 inches. Moreover, the anti-rewetting membrane can, for example, include a first air distribution fabric layer and a second air dis- tribution fabric layer, with the first air distribution fabric layer being able to be laminated or applied to the first film side and with the second air distribution fabric layer being able to be laminated or applied to the second film side. A respective air dis- tribution material layer could be, for example, a fabric with satin weave.

The anti-rewetting membrane can be used together with a conventional, in particular woven, fabric or also without an additional fabric or the like.

The method in accordance with the invention thus also provides the ad- vantage that substantially higher dry contents of the tissue web are achieved even upstream of the dryer cylinder, in particular the Yankee cylinder, by avoiding the rewetting as a consequence of the embodiment of the method in accordance with the invention, while retaining the high specific bulk which is important for tissue. It is of particular advantage if the web is wet imprinted at a low dry content upstream of a dewatering unit or dewatering apparatus.

A pressure difference of the gas between the two sides of the paper web is absolutely necessary for the wet imprinting. The use of a suction box is particularly advantageous. The use of a pressure box with compressed air is also possible (pressure field one).

As already mentioned, the anti-rewetting, membrane does not necessarily have to be used together with a conventional, in particular woven, fabric, since such an anti-rewetting membrane also has a good flow distribution effect.

A clothing, e. g. a fabric, felt with a foamed layer, a SPECTRA membrane- preferably together with a conventional, in particular woven, fabric-or an anti-rewetting membrane with or without a conventional, in particular woven, fabric, can be guided together with an imprinting fabric, e. g. a TAD fabric or an imprinting membrane, and a fiber web interposed therebe- tween around a suction roll, with the clothing preferably being in contact with the suction roll.

The clothing with a foamed layer, SPECTRA membrane, preferably to- gether with a conventional, in particular woven, fabric or an anti-rewetting membrane with or without a conventional, in particular woven, fabric, can, for example, wrap a suction roll with a diameter from, for example, approximately 2 to 3 m, or a plurality of suction rolls with smaller diame- ters, preferably two suction rolls each with a diameter of, for example, approximately 2 m. The dwelling time of the web in the region of the suc- tion roll or suction rolls should expediently be larger than approximately 0. 15 s and less than approximately 0.40 s.

The relevant suction roll can have, for example, a vacuum applied to its lower side or a suction roll with an associated siphon extractor can be used. In particular with a lower diameter, the water can, for example, also be spun into a channel by centrifugal force. The water can in particular also be blown off by means of an air knife.

Dewatering while utilizing the capillary effect is admittedly already de- scribed in US 5 701 682, but the relevant capillary element is here a part of the suction roll, which is disadvantageous for the conditioning of the capillary element.

The advantage of using a foamed fabric is to have better conditions for cleaning. The run of the fabric can be adapted for conditioning. The clean- ing device can be arranged apart from the suction roll, i. e. apart from the process (no disturbance).

Despite the utilization of the capillary effect or of the anti-rewetting effect for the dewatering, the suction device can in particular have a pressurized hood to support the underpressure effect of the suction device and to be able to work at higher temperatures (e. g.-140°C).

In accordance with a further preferred embodiment of the method in accordance with the invention, to drive out water by means of gas pres- sure, e. g. by an air press, the fiber web is guided together with an imprint- ing fabric at least once, possibly twice, through a pressure space which e. g. is bounded by at least four rolls arranged in parallel and lateral seal plates into which compressed gas is fed. In this connection, the fiber web is preferably guided together and in contact with the imprinting fabric between membranes through the pressure space, with preferably an air distribution membrane and an anti-rewetting membrane being used. The basic principle of such a displacement press in which the water in the fiber web is displaced by air, is described, for example, in DE 19946972.

As already mentioned above, the displacement press can alternatively comprise a U-shaped box.

A method in accordance with the invention for manufacturing a fiber web, in particular a web for tissue or hygiene material, which can be used alone or in combination with one or more of the above described methods, is , characterized in that water is driven out of said fiber web by means of a displacement dewatering process, and a clothing arrangement is used which comprises, as regarded in the direction of the displacement fluid flow, the following elements : a membrane, an imprinting fabric, said fiber web, and an anti-rewet fabric; and in which said clothing arrangement is, in the direction of the displacement fluid flow,. followed by an open surface of a counter means.

Suction means can be associated with said counter means. The counter means can, for example, comprise a vented roll, an open box, i. e. box with a sloted or drilled cover, or the like.

A fabric can be associated with the open surface of said counter means in order to provide a fluid distribution effect. The anti-rewet fabric can, for example, include at least one fluid or air distribution fabric layer, with said distribution fabric layer being configured for contacting the open surface of said counter means.

A method in accordance with the invention for dewatering a fiber web, in particular a web of tissue or hygiene material, is characterized in that to drive out water by means of gas pressure, e. g. by using an air press, the fiber web is guided together with an imprinting fabric at least once, and possibly twice, through a pressure space which is bounded e. g. by at least four rolls arranged in parallel and into which a compressed gas is fed, and in that the fiber web is guided together with the imprinting fabric between membranes through the pressure space, with preferably an air distribu- tion membrane and an anti-rewetting membrane being used. As men- tioned above, also a U-shaped box can be used.

An apparatus in accordance with the invention for manufacturing a fiber web, in particular a web of tissue, or hygiene material, provided with a three-dimensional surface structure is characterized in that the fiber web is pressed at a dry content of <35%, in particular <-30%, and preferably <25% onto an imprinting fabric, e. g. by suction, by means of a first print- ing field and is thereby pre-imprintecl, and in which the fiber web is guided through at least one pressure field (third pressure field) provided for dewatering and/or drying said fiber web. Preferably, the fiber web is once more pressed onto an imprinting fabric by means of a further pres- sure field (second pressure field) in order to fix strength without destroy- ing the three-dimensional surface structure. The fiber web is preferably guided between the first pressure field (I) and the second pressure field (II) through said at least one third pressure field (III). Preferably, the same imprinting fabric is used in said first pressure field (I) and in said second pressure field (II).

Preferred embodiments of this apparatus in accordance with the invention are set forth in the dependent claims.

An apparatus in accordance with the invention for manufacturing a fiber web, in particular a web or tissue of hygiene material, is characterized in that it comprises a displacement dewatering device for driving water out of said fiber web and a clothing arrangement including, as regarded in the direction of the displacement fluid flow, the following elements: a mem- brane, an imprinting fabric, said fiber web, and an anti-rewet fabric.

Preferably, said clothing arrangement is followed, in the direction of the displacement fluid flow, by an e. g. vented roll with an open surface.

Preferred embodiments of this apparatus in accordance with the invention are set forth in the dependent claims.

An apparatus in accordance with the invention for dewatering a fiber web, in particular a web of tissue or hygiene material, is characterized in that, to drive out water by means of gas pressure, the fiber web is guided to- gether with an imprinting fabric at least once, and possibly twice, through a pressure space which is bounded by e. g. at least four rolls arranged in parallel and into which a compressed gas can be led, and in that the fiber web is guided together with the imprinting fabric and between membranes through the pressure space, with preferably an air distribution membrane and an anti-rewetting membrane being used.

One aspect of the invention as defined in the claims is the fabric order.

According to a preferred embodiment, a corresponding clothing arrange- ment can include, as regarded in the direction of the displacement fluid flow, the following elements: a membrane, an imprinting or embossing fabric, the fiber web or sheet, and an anti-rewet fabric. Consequently, the following fabric order could, for example, be used: mem- brane/molding/sheet/anti-rewet layer. Such a fabric order can in general be applied to vacuum assisted displacement dewatering (i. e. use of mem- brane/molding/sheet/anti-rewet fabric/vacuum box or the like). The mentioned fabric order can, for example, be applied to displacement presses of different types. For example, a corresponding displacement press can comprise a U-shaped box, a cluster of four or more rolls. Spe- cifically a tandem (two or more displacement presses) or the like can be provided. An embossing or imprinting fabric is not in any case necessary.

A non-molding transfer fabric could be used or the membrane or the anti rewet layer could be a transfer fabric. Such an embodiment without an imprinting fabric is not specific to tissue alone. Another aspect of the invention is the use of a membrane used to mold or not mold (for graphic paper) sheet, with an anti-rewet fabric under the sheet.

The membrane according to the present invention reduces air flow, makes it possible to build pressure, reduce process air cost, presses in emboss- ing or, imprinting fabric, prevents blowing off the paper web from the imprinting fabric (reduced air flow) and makes it possible to generate further mechanical pressure which causes high strength areas in sheet.

The embossing or imprinting fabric carries the sheet or fiber web through the process. The imprinting fabric needs a pattern, surface energy, open area and/or surface texture that holds sheet without letting the sheet transfer to anti-rewet layer. It further concentrates membrane pressure into specific areas. The structure of imprinting fabric causes pressure pattern that rates high strength areas in sheet. The unpressed areas give bulk to the sheet despite pressing. Most of sheet is not pressed. The im- printing fabric can balance sheet strength with sheet adsorbancy depend- ing on imprinting fabric structure. The imprinting fabric releases its water into sheet thus it has no water to rewet the sheet. The imprinting fabric can carry sheet through drying process. If this is done, drying will take less energy than current TAD technology since imprinting fabric and sheet are at a much dryer level. For lowest air consumption, imprinting fabric mainly allows vertical flow of air.

The anti-rewet fabric prevents rewet of sheet. The air flow from displace- ment process isolates water. The. anti-rewet fabric does not pick up sheet from imprinting fabric. It protects sheet from process water after the displacement or gas press : The present invention provides a new process that has many of the advan- tages of the known processes, without some of the disadvantages. This invention creates a sheet with high bulk, but does it using less energy and small, simpler equipment. More importantly, it can be added as a rebuild to an extending."Yankee"tissue machine making flat sheet. Furthermore, it can reduce energy consumption.

The invention can be used in particular with crescent formers, duo form- ers, C wrap formers, S wrap formers and in the manufacture of single layer and/or multi-layer and multi-ply tissue.

The invention will be described in more detail in the following with refer- ence to embodiments and to the drawing, in which are shown: Figure 1 a schematic part representation of an embodiment of an apparatus for manufacturing a fiber web provided with a three-dimensional surface structure in which a dewatering apparatus (third pressure field) is additionally provided in which the capillary action of a felt with a foamed layer, the action of a SPECTRA membrane, preferably with an associ- ated conventional, in particular woven, fabric, or the action of an anti-rewetting membrane with or without a conventional, in particular woven, fabric is utilized for dewatering; Figure la a schematic representation of the dewatering apparatus with a SPECTRA membrane or an anti-rewetting membrane, op- tionally with an additional conventional fabric; Figure 2 a schematic part representation of a further embodiment of an apparatus for manufacturing a fiber web provided with a three-dimensional surface structure in which a dewatering apparatus is additionally provided in which the capillary ac- tion of a felt with a foamed layer, the action of a SPECTRA membrane, preferably with an associated conventional, in particular woven, fabric, or the action of a anti-rewetting membrane with or without a conventional, in particular woven, fabric is utilized for dewatering; Figure 2a a variant with a pick-up or separation element for a better web transfer; Figure 3 a schematic part representation of an embodiment of an apparatus for manufacturing a fiber web provided with a three-dimensional surface structure in which a displacement press is additionally provided ; Figure 4 a schematic part representation of a further embodiment with a displacement press; Figure 5 a schematic part representation of a further embodiment with a displacement press; Figure 6 a schematic part representation of an imprinting fabric with a smaller area proportion of raised zones in comparison with the area proportion of recessed zones; Figure 7 a schematic section through a press nip through which the imprinting fabric shown in Fig. 5 is led together with the fiber web, and Figures 8 to 15 graphs illustrating advantages of some of the aspects of the present invention.

In the embodiment as described in the following at least one imprinting fabric is-used which could be a woven TAD (through-air-drying) or casted SPECTRA fabric in a continuous loop.

Figure 1 shows in a schematic part representation an embodiment of an apparatus 10 for manufacturing a fiber web 12 provided with a three- dimensional surface structure in. which a dewatering apparatus 34 (third pressure field) is provided in which, for example, the capillary action of a felt 36 with a foamed layer is utilized for dewatering. In this connection, the foam coating can in particular be selected such that the mean pore size results in a range from approximately 3 to approximately 6 ! lm.

Figure 1 shows, as an example for conditioning means or a cleaning device 80, water shower nozzles or air nozzles. One of the advantages of a foamed fabric is that it is easily accessible for cleaning purposes. That is, the fabric can be cleaned from the outside, from the inside or form both sides.

As cleaning devices also suction means, e. g. pipe suction means, alone or in combination with water shower nozzles and/or air nozzles, could be used.

Instead of a felt with a foamed layer, a so-called SPECTRA membrane can, for example, also be used, with this SPECTRA membrane preferably being used together with a conventional, in particular woven, fabric. Alterna- tively, a so-called anti-rewetting membrane can also be used. Such an anti-rewetting membrane can be used together with a conventional, in particular woven, fabric or also without such an additional fabric or the like.

In the present case, * the felt 36 with a foamed layer is guided together with an imprinting fabric 14 and a fiber web 12 interposed therebetween about a larger suction roll 38, with the felt 36 preferably being in contact with the suction roll 38. The suction roll 38 wrapped, for example, by the felt 36 with a foamed layer can, for example, have a diameter from approxi- mately 2 to approximately 3 m. The suction roll 38 can have a vacuum applied to its lower side. Generally, a siphon extractor can also be associ- ated with the suction roll 38. Or a tray 40 can be used to take off the water and/or air, which can be blow out of the mantle of the roll.

In the forming region, at least one dewatering fabric with zonally different fabric permeability can be provided.

In the present case, a former with two peripheral dewatering fabrics 14,42 is provided, with the inner fabric 14 simultaneously serving as the im- printing fabric. The two dewatering fabrics 14,42 run together while forming a pulp run in gap and are guided over a forming element 46 such as in particular a forming roll.

In the present case, the imprinting fabric 14 is used as the inner fabric of the former which comes into contact with the forming element 46. The outer fabric 42, which does not come into contact with the forming ele- ment 46, can in particular be provided as a dewatering fabric with zonally different fabric permeability.

The fiber suspension is introduced into the pulp run in gap 44 by means of a head box 48. A pick-up element or separation element 50 which can be configured such that it acts as part of the pressure field I, is provided downstream of the forming element 46 and the web is held on the imprint- ing fabric 14 by this during the separation from the dewatering fabric 42.

A suction element 16 (solid representation) as the other part of pressure field I is preferably provided upstream of the apparatus 34 with capillary action or, for example, of the action of a SPECTRA membrane or of an anti-rewetting membrane with or without an additional conventional fabric and the fiber web 12 is sucked into the 3-dimensional structure of the imprinting fabric 14 by this. This suction element can, however, also be arranged between the apparatus 34 with, for example, capillary action, etc. and the suction device or suction roll 30 (broken line representation 16') to prevent the paper web of separating from the imprinting fabric.

The fiber web 12 and the imprinting fabric 14 are guided through the press nip 18 (pressure field II) formed between a dryer cylinder 20 and a shoe press unit 22. The shoe press unit 22 includes a flexible sleeve 26 guided over a press shoe 24 in the region of the press nip 18. The imprint- ing fabric 14 and the fiber web 12 are guided upstream of the press nip about a suction device 30 which can in particular be a suction roll. The dryer cylinder 20 can in particular be a Yankee cylinder. In this connec- tion, a dryer hood 52 can be associated with this dryer cylinder 20.

In the present example, the dry content of the fiber web upstream of the dewatering apparatus 34 amounts to approximately 10 to approximately 25% ; in the region downstream of this apparatus 34, for example ap- proximately 30 to approximately 40°M,.

The fiber web 12 is therefore in particular pressed, e. g. sucked, at a dry content of <30%, in particular <25%, in particular <15% and preferably < 10%, onto the imprinting fabric or structured fabric 14 by means of a first pressure field I in the region of the suction element 16 and/or in the region of the separation element 50 and is thereby pre-imprinted, in par- ticular, and is subsequently once more pressed onto the imprinting fabric 14 by means of a further pressure field II in the region of the press nip 18 in order to fix and/or increase strength without destroying the 3- dimensional structure of the sheet and for the transfer to the drying cylin- der.

Figure la shows in a schematic representation the dewatering apparatus 34 with a SPECTRA membrane 36 which is used, in the present example together with a conventional, in particular woven, fabric 76. In this Figure la, a vacuum producing apparatus such as in particular the suction cylinder or the large suction roll 38 and the imprinting fabric or imprint- ing fabric 14 can also again be recognized.

The embodiment shown in Figure 2 initially differs from that in accor- dance with Figure 1 in that the fiber web 12 is taken over by the imprint- ing fabric 14 from an inner fabric 54 of the former. In the present case, for example, this inner fabric 54 or preferably the outer fabric 42 of the for- mer can again be provided as a dewatering fabric with zonally different fabric permeability. The two peripheral dewatering fabrics 42,54 again run together while forming a pulp run in gap 44, with them again being guided via a forming element 46 such as in particular a solid or suction forming roll. The pulp run in gap 44 is again charged with fiber suspen- sion by means of a head box 48. In contrast to the embodiment in accor- dance with Figure 1, the fiber suspension is, however, supplied from below in the present case.

A pick-up element or separation element 50 is again provided within the loop of the imprinting fabric 14 and the fiber web 12 is held on the im- printing fabric by this on the separation from the inner fabric 54 of the former.

The suction element 16 provided within the loop of the imprinting fabric 14 is arranged upstream of the dewatering apparatus 34 with a capillary action or, for example, of the action of a SPECTRA membrane or of an anti-rewetting membrane with or without an additional, conventional fabric, with generally, however, an arrangement downstream of this appa- ratus 34 also being possible.

The dry content of the fiber web in the present example amounts to ap- proximately 10 to approximately 25% in the region of the pick-up element 50, approximately 15 to approximately 30% in the region upstream of the dewatering apparatus 34 and approximately 35 to approximately 45% in. the region downstream of this apparatus 34. In this case, e. g. a pressing roll 30 (suction roll) can be provided instead of a shoe press unit. The pressing roll can also be a solid roll.

The turning roll 29 provided adjacent to the dewatering apparatus 34 can also be a suction roll for a better web transfer.

Another variant with a pick-up element or a separation element for a better web transfer is shown in Figure 2a.

In another respect, this embodiment can have at least substantially the same design as that in accordance with Figure 1. Elements corresponding to one another are assigned the same reference numerals.

Figure 3 shows in a schematic part representation an embodiment of the apparatus 10 in which a displacement press 56 (pressure field III) is pro- vided. In this connection, to drive out water by means of gas pressure, the fiber web 12 is guided together with the imprinting fabric 14 at least once through a pressure space 58 which is bounded by at least four rolls 60- 66 arranged in parallel and into which compressed gas can be led. Conse- quently,'the embodiment of Figure 3 differs from that of Figures 1 and 2 in that such a roll arrangement 60-66 defining the pressure space 58 is used. In this connection, the fiber web 12 is preferably guided through the pressure space 58 together with the imprinting fabric 14 and a membrane 72 for air distribution as well as an anti rewetting membrane 36. The fiber web is sandwiched between the imprinting fabric 14 and the anti- rewetting membrane.

The imprinting'fabric 14 could be a SPECTRA membrane in which case all the air is forced vertically through the sheet because it is a cast structure without cross over points. Cross flows in between the membrane and therefore air leakage in machine direction is eliminated.

In the present case, the imprinting fabric 14 forms the inner fabric of the former which in turn includes a forming element 46 such as in particular a forming roll in whose region the inner fabric provided as the imprinting fabric 14 and the outer fabric 42 run together while forming a pulp run in gap 44 which is charged with fiber suspension by means of a head box 48.

Subsequently to the air press 56, the fiber web 12 is again guided with the imprinting fabric 14 over a suction device 30, in particular a suction roll, and through the press nip 18 (pressure field II) formed between a dryer cylinder 20, in particular a Yankee cylinder, and a shoe press unit 22. In the example shown, a dryer hood 52 is again associated with the dryer cylinder or Yankee cylinder 20.

In the present case, the first pressure field I, through which the fiber web 12 is pressed onto the imprinting belt 14 and correspondingly pre- imprinted at a dry content of in particular <30%, in particular <25%, in particular <15%, and preferably <10%, can be produced by the suction element 16.

Figure 4 shows in a schematic part representation a further embodiment with a displacement or air press 56.

The embodiment initially differs from that in accordance with Figure 3 in that the inner fabric 78 of the former is provided separately from the imprinting fabric 14 and the fiber web 12 is transferred to the imprinting fabric 14 from the inner fabric 78. Moreover, the fiber suspension is poured into the pulp run in gap 44 diagonally from the bottom to the top by means of the head box 48.

Furthermore, in the present example, the suction device 30 provided in the embodiment in accordance with Figure 3 is omitted. Instead of the shoe press unit 22, a conventional press roll 28, solid or suction roll, is provided, for example, which forms the press nip 18 with the dryer cylin- der 20, in particular the Yankee cylinder.

The membrane 72 can, for example, be a fine membrane fir air distribu- tion and the membrane 36 can, for example, be a laminated coarse cast structure SPECTRA membrane and/or an anti-rewetting membrane.

In another respect, this embodiment shown in Figure 4 can again have at least substantially the same design as that in Figure 3.

Figure 5 shows in a schematic part representation a further embodiment of the apparatus with a displacement press 56.

In this case, the displacement press 56 comprises a U-shaped box 82. The air pressure within the U-shaped box 82 provides an air flow 84 through the membrane 72, preferably an air distribution membrane, the imprint- ing fabric 14, the fiber web 12 and the membrane 36, as regarded in the direction of the air flow 84. The membrane 36 can, for example, be a SPECTRA membrane or an anti-rewetting membrane.

As can be recognized, for example, with reference to Figures 6 and 7, the respective imprinting fabric 14, e. g. a woven fabric with raised Knuckles (cf. in particular the left hand part of Fig. 6) or imprinting membrane (cf. in particular the right hand part of Fig. 6), guided through the press nip 18 can be structured such that for this imprinting fabric 14 a smaller area proportion of raised or closed zones 68 results in comparison with the area proportion of recessed zones or holes 74 and accordingly a smaller area proportion of the fiber web 12 is pressed in the press nip 18.

In this connection, the contact area proportion of raised or closed zones 68 can in particular be <40% and can preferably lie in a range from ap- proximately 20 to approximately 30% and in particular at approximately 25%. The contact area need not to be the same as the open area or the void volume. The open area or the void volume of a fabric can be inde- pendent of the contact area.

The raised zones 68 and the recessed zones can result, for example, due to offsets, i. e. due to intersection points of picks and ends, of a woven fabric. In the case of the pressing membrane reproduced in the right hand part of Figure 6, a corresponding structure arises due to the holes 74.

Figure 6 shows a schematic part representation of a corresponding im- printing fabric 14, e. g. imprinting fabric or imprinting membrane, with a smaller area proportion of raised or closed zones 68 in comparison with an area proportion of recessed zones or holes 74.

The thickness d of the imprinting membrane shown in the right hand part of Figure 6 can amount, for example, approximately to 1 to approximately 3 mm. The membrane expediently consists of a material resistant to the fiber chemistry. It can consist, for example, of polyurethane.

Figure 7 shows a schematic section through a press nip 18 through which the imprinting fabric 14 shown in Figure 5 is guided together with the fiber web 12. In this connection, this imprinting fabric 14 is in contact with the flexible sleeve 26 of the shoe press unit which is guided in the region of the press nip 18 over a press shoe 24 via which the desired pressing force can be applied.

The fiber web 12 contacts the dryer cylinder 20, preferably a Yankee cylinder.

Moreover, in Figure 7, the pressing zones 70 resulting as a consequence of the raised zones 68 can be recognized.

The fiber web 12 is already imprinted upstream of the nip. As can be recognized with reference to Figure 7, it already contacts the imprinting fabric upstream of the nip.

Some of the above mentioned aspects of the present invention are exem- plified in more detail in the following: Recently, the applicant has been developing a new mechanical process for dewatering paper using high pressure air. Prior to these developments, no continuous method was available for pressing a sheet of paper using the pressures which can now be. developed.

One of the presses which can, e. g, be used as an air press is called a BCP (Beck Cluster Press). Its preferred state is shown in Figure 8.

The center of the 4 roll cluster along with roll ends seals form"a chamber" that can be pressurized. The web passes through the nips into the pres- sure chamber. While in the chamber, the web feels a pressure gradient between chamber and the vented main roll.

Because of this gradient, air flows from the chamber, through the web and into the vented main roll. The motion of the air through the web, and the pressure of the chamber, dewaters the sheet. The extent of sheet dewater- ing depends on the web make up and pressing conditions such as pres- sure, speed, and temperature.

Published research has shown that good sheet dewatering occurs when the sheet is mechanically pressed and at the same time, air is passed through the sheet. This process is called"displacement dewatering". A "membrane"fabric can be used as the upper most layer in the web. The membrane reduces airflow to the level needed for dewatering, and at the same time, acts like a piston, to convert air pressure into mechanical pressure. Thus, the membrane acts to press and to control airflow through the rest of the web and sheet.

After the membrane layer, the following layers can be varied to influence pressing conditions. For example, consider the web passing through the BCP displacement press as constructed in the layers shown below : High Pressure air Membrane Sheet Imprinting fabric Support fabric Vented roll For projected commercial displacement pressing conditions, using this configuration the sheet will have solids exiting the BCP in the range of 20 % for a 20-30 GSM tissue sheet. With this solids content, there is no advantage of the displacement pressing method over conventional press- ing methods. The sheet is simply too wet.

If, however, the basis weight of the sheet is increased with this web con- figuration, as shown in Figure 9, the sheet solids increase. This means the displacement pressing process is capable of high solids, but at low sheet basis weight the sheet solids will be very low, due to the sheet re- adsorbing water after pressing (cf. Figure 9).

According to one embodiment of the present invention the imprinting fabric is put on the high pressure side, next to the membrane. The web configuration was changed to: High Pressure Air Membrane Imprinting fabric Sheet Support fabric Vented roll (low pressure) This configuration indeed, did increase sheet solids for the tissue sheets.

Sheet solids increased from under 20% to about 32 % for the 22 GSM tissue sheet. Again interestingly, increasing sheet basis weight causes increases in solids content (cf. Figure 10).

Figure 11 shows a"Sweet"plot for the two web configurations. The Sweet plot is a way of estimating the amount of rewet in such cases. To make a Sweet plot, one plots 1/ (Sheet Basis Weight) on the x axis against sheet dryness on the y axis. The Y intercept from such a plot indicates the theoretical maximum solids attainable if no rewet existed.

This Sweet plot shows that both web conditions would yield solids of about 51 % if there were no rewet present. Thus, by moving the embossing fabric on top of the sheet, we had greatly increased sheet solids for low basis weight tissue, but the Sweet plot shows that our 32 % tissue solids is a long way from the maximum solids of 51 % attainable for our condi- tions.

Additional research led the applicant develop an anti-rewet fabric that virtually eliminates sheet rewet. This fabric, placed underneath the sheet (cf. Figure 12), vastly inhibits water from passing back into the sheet after displacement pressing.

By putting the anti-rewet fabric under the tissue sheet, a gain in sheet dryness was seen. We get the improvement in solids as shown in figure 13.

By using the imprinting fabric on the top of the sheet, and the anti-rewet fabric on the bottom of the sheet, rewet has been greatly decreased, so that a 25 GSM tissue sheet now has solids close to the values predicted by Sweet.

From the above discussion, it can be seen that one aspect of the present invention is the order and type of fabrics used in the displacement press- ing process. One object of the present invention is to reach the highest sheet solids possible, at the lowest cost and without greatly affecting the bulk of the sheet. The fabric positions and types are one part of attaining this goal. By putting the imprinting fabric on the top of the sheet, and the anti-rewet layer underneath, high solids can be reached through me- chanical removal of water. To create the mechanical pressure and limit the air flow a membrane with a low permeability is used. The permeability is e. g. less than 15 cfm, preferably less than 10 cfm, preferably less than 8 cfm, measured by TAPPI test method TIP 0404-20. In addition it is advan- tageous to operate the air press for displacement dewatering with a pres- sure in the chamber of > 30 psi, preferably > 40 psi.

Mechanical removal of water is much cheaper than evaporative drying, so an object of the present invention is to reach the highest solids possible without evaporative drying. For our process, we have found that the amount of air that is passed through the sheet is best measured as a film thickness of atmospheric air. As the film thickness of air pushed through the sheet increases, the water removal process progresses. The more air pushed through the sheet, the dryer the sheet becomes. This behavior for the displacement pressing process is shown in Figure 14.

From this plot, we can see that initially, a thin air film will remove a lot of water. But as the dryness of the sheet increases, it takes more and more air to remove water from the sheet.

There are two fundamentally different dewatering mechanisms taking place. The first mechanism is the displacement pressing phase. During this phase, water primarily leaves the sheet as a liquid. The water moves out of the sheet and into the anti-rewet layer and/or the vented roll. In general, it takes less than 5"and generally 5"or less of air film (thickness) to remove water in the displacement pressing phase.

To increase sheet dryness in the displacement pressing phase, air pres- sure should be increased. Increasing air pressure increases mechanical pressure, which increases the ultimate dryness attainable by the process.

There are limits to this as is seen in Figure 15.

From this graph it is obvious that vacuum dewatering (which is a low- pressure process) as is being done currently by most TAD processes, will only give low dryness sheets. If the objective is to remove the most water in the displacement pressing phase, it's important to use pressure that is high enough for the paper being dewatered.

Reference numeral list 10 apparatus 12 fiber web 14 imprinting fabric 16 suction element 18 press nip 20 dryer cylinder, Yankee cylinder 20'surface 22 opposing element, shoe press unit 24 press shoe 26 flexible sleeve, flexible roll sleeve 29 turning roll 30 suction device, suction roll 34 dewatering apparatus with capillary action or anti-rewetting action 36 felt with foamed layer, SPECTRA membrane or anti-rewetting membrane 38 large suction roll 40 vacuum, box 42 dewatering fabric 44 pulp run in gap 46 forming element, forming roll 48 head box 50 pick-up element or separation element 52 dryer hood 54 inner dewatering fabric 56 air press 58 pressure space 60 roll 62 roll 64 roll 66 roll 68 raised zones 70 pressing zones 72 air distribution membrane 74 holes 76 conventional fabric 78 inner fabric 80 cleaning device 82 U-shaped box 84 air flow d thickness L web running direction I first pressure field II further pressure field