FR2727442A1 - Blanket for wet section of a paper-making machine - Google Patents

Abstract

The blanket, for the wet section of a papermaking machine, has a woven base (1) and a superimposed knitted structure (4), covered by a non-woven layer (7), with needle bonding to hold the layers together. The knitted layer (4) is a three-dimensional structure with two parallel knitted surfaces (41, 42) parallel to the base layer (1). The surfaces (41, 42) are joined by yarns (43) at right angles to them. Pref. the woven base layer (1) has warps (2) and wefts (3) of polyamide. The knitted layer (4) is e.g. of polyamide, polyester, polyether ether ketone, etc

Description

STATIONARY FELT FOR WET PART OF MANUFACTURING MACHINE
OF PAPER
The present invention relates to a stationery felt for a wet part of a paper-making machine.

 It will be recalled that paper making machines schematically comprise three parts: - a first part for the formation, from paper pulp, of a sheet of paper in aqueous phase, with water filtration, - a second part, in the wet phase, for pressing the sheet of paper thus obtained and extracting water from the latter, by passing the sheet of paper, transported by a conveyor belt, between pressure rollers, - a third part, in the dry phase, for drying the sheet of paper stripped of its water, with vapor-air filtration, the sheet of paper being brought for this purpose by another conveyor belt on heated cylinders.

 The paper is made continuously, in these three parts successively.

 In each of these parts, conveyor belts are used which each time perform a different function, in addition to that of conveying the sheet of paper being formed; in particular, the conveyor belts of the second and third parts have the function respectively of eliminating the excess water, expressed by the passage between the pressure rollers, and of applying the sheet of which the water has been partly expressed on the heated cylinders.

 In the paper industry, these conveyor belts having particular functions in the second and third parts of the papermaking machine are called "felt for wet part" and "felt for dry part" respectively.

 The present invention relates more particularly to felts for wet areas.

 A felt for a known wet part, and described in patent US Pat. No. 4,427,734, comprises: - a base fabric or canvas comprising warp threads crossed with weft threads, these threads thus forming a woven fabric, this basic fabric serving to ensure the dimensional stability and the tensile strength of the felt, the latter being subjected to significant forces during its passage between the pressing rollers, - a knitted structure superimposed on the base fabric, this structure serving to give the felt a certain incompressibility, which theoretically allows the felt to keep a relatively constant thickness allowing it to ensure its water absorption function for a longer time, - a sheet of nonwoven fibers, superimposed on the knitted structure (and possibly also another layer of fibers interposed between the base fabric and the knitted structure) and linked by needling to the assembly constituted by the base fabric and the knitted structure, this sheet serving to ensure the cohesion of the whole.

 The choice of the structure of a stationery felt for a wet part results in most cases from a compromise between two essential properties that such a felt must have: high compressibility and a water-carrying capacity, for a mass per given unit of felt area, high. These two properties are in fact incompatible and lead to contradictory choices in terms of the structure of the felt.

 The known felt described above, the result of such a compromise, poses a number of problems.

 Firstly, because of the preference given in the choice of its structure to the water accommodation capacity, it offers limited compressibility, so that it dehydrates badly by pressure (it is recalled that, to eliminate the water retained in the felt, this is done by dewatering the felt by pressure then by vacuum suction using suction boxes) and can therefore not be used when the water content to be extracted from the sheet is very high. This low compressibility also causes problems known as "rewetting at the pinch outlet": when the felt leaves the pinch zone between the pressing rollers, it relaxes, and during this relaxation, part of the water it The stored material rises to its surface because the knitted structure does not have sufficient water accommodation capacity to retain all the absorbed water.

 Furthermore, its air permeability is also low, so that its suction cleaning by means of the suction boxes is made difficult.

 In addition, it has a drawback, for example in the event of the paper sheet breaking, when paper "plugs" become blocked between one of the rollers and the surface of the felt: these plugs then rub on the felt, this friction giving off heat and causing a fusion of the fibers of the sheet (generally made of synthetic material) leading to buffing of the latter, and therefore causing the gradual disappearance of the permeability of the felt which can no longer fulfill its function and must To be replaced.

 Finally, the known felt does not make it possible to satisfactorily absorb the vibrations of the paper-making machine: in order to avoid the latter, the machine must be slowed down, which reduces its efficiency.

 An object of the present invention is to develop a stationery felt for a wet part in which both the compressibility and the water-holding capacity are increased compared to known felts.

 Another object of the present invention is to develop such a felt which also has increased air permeability compared to prior art felts, in order to facilitate cleaning by suction by means of suction boxes.

 Another object of the present invention is to produce such a felt making it possible to avoid the melting bands due to the dough plugs.

 Finally, another object of the present invention is to produce such a felt making it possible to dampen the vibrations of the paper-making machine.

 To this end, the present invention provides a stationery felt for a wet part of a paper-making machine comprising: - a base fabric comprising warp threads interwoven with weft threads, these threads thus forming a woven fabric, - a structure knitted superimposed on said base fabric, - a sheet of non-woven fibers, superimposed on said knitted structure and linked by needling to the assembly comprising said base fabric and said knitted structure, characterized in that said knitted structure consists of a three-dimensional knitting comprising two knitted faces, parallel to each other and to said base fabric, and linked to each other by threads which are substantially perpendicular to them.

 Thanks to the felt according to the invention, which uses a three-dimensional knit, the water-carrying capacity, relative to the mass per unit area of the felt, is greatly increased compared to the felts of the prior art. In fact, when the felt is compressed between the pressing rollers, there is a large residual void in three-dimensional knitting, not only in the stitches of the two knitted faces, but also in the space between the latter and the threads which connect them; the felt according to the invention therefore has a very large void volume, which gives it a water accommodation capacity significantly greater than that of known felt (in which the knitted layer does not have such a void volume), which increases its water absorption and reduces its resistance to water flow.

 Thus, the felt according to the invention can be used in applications where the water content to be extracted from the paper sheet is high.

 Furthermore1 three-dimensional knitting has a significantly higher compressibility than that of ordinary knits, due to the orientation of the threads connecting its two faces. The resulting compressibility of the felt according to the invention is therefore much higher than that of known felts, which gives the felt a certain elasticity, so that the initial characteristics of the felt are maintained over time, especially in terms of its thickness. , its air permeability and its water permeability.

 On the other hand, the compressibility and elasticity of the felt provide the latter with a damping power which enables it to absorb the vibrations of the papermaking machine, so that the latter can operate without being slowed down, d 'where increased yield from paper production.

 In addition, the very high compressibility of the felt according to the invention, due to the three-dimensional structure of the knit, ensures relaxation at the pinch outlet and thus prevents the water contained in the sheet from going up in the sheet of paper, which reduces rewetting problems at the pinch outlet.

 In addition, when a paper plug is blocked between the pressure rollers, the high compressibility of the knitted fabric allows it to be absorbed very quickly in the gripper line, which reduces the friction time of this plug on the felt, and allows therefore to avoid the problems of premature wear of known felts.

 Finally, it can be noted that, with comparable performance on the paper making machine compared to the felts of the prior art, the felt according to the invention is more flexible and lighter, which allows on the one hand a more flexible introduction. easy in the papermaking machine, this reducing the machine downtime when changing the felt, and on the other hand greater ease of handling.

 Advantageously, a fabric of woven threads can be inserted between the three-dimensional knitting and the web of non-woven fibers, in order to reduce the marking caused by the three-dimensional knitting. It is recalled here that the constituent of the felt which risks marking the sheet of paper is that located immediately below the sheet of fibers. When using a three-dimensional knit, which necessarily has junctions because it is a flat knit, that is to say made in flat bands, these junctions tend to mark the paper, while a woven fabric, which can being made circularly, that is to say without a junction (like a sock), marks it less.

 This additional woven fabric further increases the tensile strength and dimensional stability of the felt.

 Alternatively, a knitted structure in the form of an ordinary knitted fabric can be inserted between the three-dimensional knitted fabric and the web of non-woven fibers. This layer also makes it possible to reduce the marking, because ordinary knits can be produced in a circular fashion, therefore without joining, and because of the scrambling effect due to the multiplicity of their stitches, makes it possible to avoid too much marking.

 According to an advantageous characteristic of the present invention, an oriented knitted structure or a woven fabric of yarns can be arranged below the base fabric, on the side of the face of the latter opposite to that above which the knitted fabric is superimposed. three-dimensional.

 This arrangement improves the wear resistance of the base fabric.

 In addition, when using an oriented knitted structure, if the felt is oriented so that its direction of travel in the papermaking machine is confused with the orientation of the oriented knitted structure, the parallel threads between them of this first, which give it its oriented character, allow the recovery of part of the water from the felt between the latter and the pressure roller, this water then being sucked. This avoids having to use a roller with grooves or blind holes to ensure the same function, that is to say the increase in the capacity of water accommodation in the clamp line (contact line of the pressure rollers).

 We get the same result when we replace this knitted structure oriented by a woven fabric.

 Finally, three-dimensional knitting can be of symmetrical structure with open or closed meshes, or asymmetrical.

 When it is closed mesh, on one side or both (depending on whether it is asymmetrical or symmetrical), its surface finish is smoother, which allows less marking of the paper. Thus, when it is asymmetrical, it can be arranged so that its side with closed meshes is turned towards the sheet of paper.

 When it is open meshed, on one side or both, its water absorption capacity is greater.

 Other characteristics and advantages of the present invention will appear in the following description of various embodiments of the present invention, given by way of illustration and in no way limitative.

 In the following figures: - Figure i shows in section a first embodiment of a felt according to the invention, - Figure 2 shows in section a second embodiment of a felt according to the invention, - Figure 3 shows in section a third embodiment of a felt according to the invention, - Figure 4 shows in perspective with partial anchoring a symmetrical three-dimensional knitted fabric with closed mesh usable in the present invention, - Figure 5 shows in perspective a knitting symmetrical three-dimensional open mesh usable in the present invention, - Figure 6 shows a top view of an ordinary knitted fabric, - Figure 7 shows a top view of a knitted fabric with directional orientation.

 In all these figures, the common elements have the same reference numbers.

 FIG. 1 represents a section of a felt 10 according to the invention comprising: - a base fabric or canvas 1 comprising a layer of warp threads 2 crossed with weft threads 3, these threads thus forming a woven fabric, - a knitted fabric three-dimensional 4 superimposed on the base fabric i, and the structure of which will be detailed below, - a sheet of non-woven fibers 7, superimposed on the three-dimensional knit 4 and bonded by needling to the assembly comprising the base fabric 1 and the three-dimensional knit 4.

 The weft 3 and warp 2 threads can be formed from monofilament or multifilament threads, for example made of polyamide, and more generally of a plastic material, from 0.2 to 0.5 mm in diameter. Obviously, one can choose for the base fabric 1 any suitable thread in this area and well known to those skilled in the art.

 An example of a three-dimensional knit usable in the felt of the invention is shown in FIG. 4. This three-dimensional knit, referenced 40, is said to be symmetrical and with closed stitches. It consists of two parallel faces 41 and 42 knitted, that is to say each consisting of a network of interlaced loops 44 forming stitches 45, in the manner of an ordinary knit, and connected together by threads 43 which are perpendicular to them. The yarns constituting the knitted fabric 40 can be monofilament or multifilament yarns, or else yarns knitted on themselves, such as those described in patent EP-0 059 973.

 It will be understood that such a structure is very compressible, and that it has a very large capacity for housing water between its faces 41 and 42.

 The three-dimensional knit 40 is symmetrical, because its two faces 41 and 42 are identical: in the example of FIG. 4, they are with closed stitches, that is to say that the ratio between the cumulative surface of the openings and the total surface of the faces 41 or 42 is less than 25%.

 One could also use in a felt according to the invention a symmetrical three-dimensional knit with open stitches, that is to say in which the ratio between the cumulative surface of the openings and the total surface is greater than 25%. An example of such a knitting is given in FIG. 5: this three-dimensional knitting 40 'has two faces 41' and 42 'linked together by threads 43' of the same type as the threads 43.

 Alternatively, the three-dimensional knitting used in a felt according to the invention can be asymmetrical, that is to say have one face with open stitches, and one face with closed stitches.

 It will be better understood why the three-dimensional knitted fabric 4 used in the felt 10 according to the invention has a greater water-carrying capacity than that of an ordinary knitted fabric by comparing the structures of the three-dimensional knitted fabrics 40 and 40 ′ in FIGS. 4 and 5 with the structure of an ordinary knit 60, shown in FIG. 6.

 The so-called ordinary knitted fabric 60 of FIG. 6 consists of a network of interlaced loops 61, forming stitches 62. It is clear that such a structure can only provide limited compressibility, and that it also has a very small water accommodation capacity compared to a structure such as those of FIGS. 4 and 5, in which not only the faces 41 and 42 (or 41 ′ and 42 ′) consist of ordinary knits, but also the the fact that they are connected by vertical wires 43 (or 43 ') increases the space available for the water housing.

 In addition, the existence of these vertical threads gives on the one hand an increased compressibility, which ensures the maintenance over time of the initial characteristics of the felt, in particular in terms of thickness, air permeability and air permeability. water, and on the other hand an elasticity which ensures resistance to vibrations and limits rewetting at the pinch outlet.

 The felt according to the invention therefore has better properties than those of known felts.

 In FIG. 1, the ply 7 has a thickness for example of 2 to 3 mm above the three-dimensional knit 4. It serves on the one hand to bind the felt, that is to say to hold together the base fabric 1 and the three-dimensional knit 4, and on the other hand to give the felt 10 a non-marking surface for the paper. It will be recalled that the paper comes into contact with the felt 10, in a paper-making machine, on the face of this latter carrying the ply 7.

 The structure of the felt 10 according to the invention which has just been described can obviously give rise to many variants, as long as these variants all include a woven base fabric, a three-dimensional knit and a web of fibers. These variants depend on the application which will be made of the felt, and in particular on the dimensional stability and the tensile strength to be given to the latter, as well as on the marking which it is desired or not to obtain on the paper.

 Figures 2 and 3 give two non-limiting examples of possible variants of the felt 10 according to the present invention.

 More specifically, the felt 10 'of FIG. 2 comprises, as base fabric 1', a weaving of weft threads 3 'and two layers of warp threads 2'. This double chain structure gives felt 10 'increased tensile strength and dimensional stability.

 Furthermore, above the three-dimensional knit 4 and below the fiber ply 7, there is another woven fabric 8, identical to fabric 1, that is to say with a single warp. This fabric 8 also serves to improve the tensile strength and the dimensional stability, but it can also serve to avoid the marking due to the junction of the three-dimensional knitting 4. In fact, the three-dimensional knits are fabricated in plan, so that , for large widths such as those necessary in paper making machines up to 10 meters), it is often essential to connect two three-dimensional knits; such a junction risks marking the paper undesirably. To avoid this, we can therefore use the fabric 8, which even if it has "knees" 80 at the passage of the warp threads above the weft threads, establishes a "bridging" of the inherent defect in knitting and therefore marks less and more regularly than three-dimensional knitting 4.

 To avoid marking, and according to a variant not illustrated of the present invention, the fabric 8 can be replaced by an ordinary knitted fabric; it is indeed well known that knitwear marks much less than woven fabrics, due to the scrambling effect produced by their surface made of intertwined loops forming meshes which can be of small dimensions.

 Figure 3 shows another variant of the present invention. In this variant, the 10 "felt comprises: - for the base fabric, a double warp fabric such as the fabric 1 'in FIG. 2, - the three-dimensional knitting 4, - an ordinary knitting 8" above the knitting three-dimensional 4, on the paper side, for the reasons explained above, - a sheet of fibers 7, - under the fabric 1 ', a knitted fabric 9 known as oriented or directionally oriented, the structure of which is shown in detail in FIG. 7 .

 It can be seen in this figure that the knit 9 is made up of parallel threads 91, in the manner of weft threads, with which a thread 92 is intertwined to form stitches 93 in the manner of an ordinary knit. Such a directionally oriented knitted fabric has the property of being very mechanically stable in the direction of orientation of the threads 91, so that it strengthens the dimensional stability and the tensile strength of the 10 "felt. Furthermore, thanks to the wires 91, which come, in the paper-making machine, directly into contact with one of the pressure rollers, a possibility of housing water is offered at this level, which avoids having to use rollers with grooves or blind holes, the machining of which is much more delicate than that of rollers with a smooth surface.

 To play a function similar to that of directional knitting 9, another woven fabric (not shown) can be used.

 In all the variants of the invention, the three-dimensional knitted fabric can consist of polyamide, polyester, polyether-ether ketone yarns, and more generally, of any suitable plastic material, this not being limiting.

 Obviously, the invention is not limited to the embodiments which have just been described.

 In particular, three-dimensional knitting can be "sandwiched" between two layers of fibers.

 Furthermore, it is possible to use, for each woven fabric (basic or additional) of the felt according to the invention, a fabric with a single or multiple chain, or else a complex obtained by superposition of several of these woven fabrics.

 Finally, any means can be replaced by equivalent means without departing from the scope of the invention.

Claims (7)

ROIENDICATIONS 1 / Stationery felt for the wet part of a paper-making machine comprising: - a base fabric (1) comprising warp threads (2) crossed with weft threads (3), these threads thus forming a woven fabric, - a knitted structure superimposed on said base fabric (1), - a sheet of non-woven fibers (7), superimposed on said knitted structure and bonded by needling to the assembly comprising said base fabric (1) and said knitted structure, characterized in that said knitted structure consists of a three-dimensional knit (4) comprising two knitted faces (41, 42), mutually parallel and to said base fabric (1), and linked together by threads (43) which their are substantially perpendicular.  2 / felt according to claim 1 characterized in that said base fabric (1) is chosen from single warp fabrics and from those with multiple superimposed chains.  3 / felt according to one of claims 1 or 2 characterized in that another ply of fibers is disposed between said base fabric and said three-dimensional knit, and linked to the latter by needling.  4 / felt according to one of claims 1 to 3 characterized in that a fabric of woven son (8) is inserted between said three-dimensional knit (4) and said sheet of non-woven fibers (7). 5 / felt according to one of claims 1 to 3 characterized in that a knitted structure forming an ordinary knit (8 ") is inserted between said three-dimensional knit (4) and said sheet of non-woven fibers (7).  6 / felt according to one of claims 1 to 5 characterized in that a knitted oriented structure (9) is arranged below said base fabric (1), on the side of the face of this latter opposite to that above which is superimposed on said three-dimensional knit (4).  7 / felt according to one of claims 1 to 5 characterized in that a woven son fabric is disposed below said base fabric (1), on the side of the face of the latter opposite to that above which is superimposed on said three-dimensional knit (4).  81 Felt according to one of claims 1 to 7 characterized in that said three-dimensional knit (4) is of symmetrical structure with open or closed meshes, or asymmetrical.