AND RELATED METHOD

DESCRIPTION TECHNICAL FIELD

[0001] The present invention relates to paper converting machines, especially to tissue paper converting machines, commonly used for producing rolls of toilet paper, kitchen towels, napkins, handkerchiefs etc. More specifically, the invention relates to improvements to the so-called mechanical ply-bonding units, i.e. the units used for bonding, through mechanical pressure, two or more cellulose plies for producing toilet paper, kitchen towels and the like.

BACKGROUND ART

[0002] In the paper converting industry, for producing tissue paper products such as toilet paper, kitchen towels, napkins, handkerchiefs and the like, several systems are used for bonding two or more cellulose plies and forming a multi-ply product. In some applications, the plies are bonded by gluing. In other systems, so-called mechanical ply-bonding units are used, that mechanically bond the cellulose plies by applying localized significant pressure, which causes a sort of fusion of the fibers of adjacent plies. Mechanical ply-bonding units of this type are disclosed, for instance, in US 5.433.817 and WO 2012/032555.

[0003] These mechanical ply-bonding units usually comprise a plurality of mechanical ply-bonding wheels, which are aligned to one another and each of which has a knurled side surface, i.e. a surface engraved so as to form a plurality of small protuberances, usually arranged according to a geometrical pattern, and separated by cavities or grooves.

[0004] The anvil roller and the mechanical ply-bonding wheels define a mechanical ply-bonding nip, through which two or more cellulose plies to be bonded pass. Each wheel is pushed against the surface of the anvil roller by means of pressure members. The protuberances on the knurled surfaces of the mechanical ply-bonding wheels define head surfaces of small dimensions, where the pressure, exerted by the wheel and the anvil roller on the plies passing through the mechanical ply-bonding nip, is concentrated, so as to have, in this area, the pressure necessary for mechanically bonding the plies.

[0005] The advantage of this mechanical ply-bonding technique is that it does not require glue, that are consumables affecting the cost of the finished products and having some drawbacks, among which the fact of requiring specific measures to avoid that the machines where glue is applied get dirty and that the glue dries when the machines are stopped, and other inconveniences known to those skilled in the art.

[0006] However, mechanical ply-bonding also has some drawbacks. In particular, when using the mechanical ply-bonding unit, cellulose fibers accumulate in the small engravings or cavities that separate from one another the protuberances of the knurled surfaces of each wheel. Over the time, these fibers form significant masses, which are very compact and difficult to be removed. In some cases, this accumulation, together with the strong pushes onto the mechanical ply-bonding wheels, can result in the breakage of the wheels, as the accumulated cellulose fibers prevent a uniform surface contact of the wheels, causing strong vibrations.

[0007] A need therefore exists of improving the mechanical ply-bonding units in order to solve or alleviate one or more problems of the prior art, and in particular to avoid or to reduce accumulation of fibers on the knurled surfaces of the mechanical ply-bonding wheels.

SUMMARY

[0008] According to a first aspect, a mechanical ply-bonding member for a ply- bonding unit is provided, having a side surface comprising a plurality of protuberances spaced by cavities, wherein the cavities are at least partially filled with a yielding material.

[0009] The mechanical ply-bonding member can be a member co-acting with an anvil roller, or a mechanical ply-bonding wheel or a mechanical ply-bonding roller co-acting with one or more anvil wheels, that can be either smooth or engraved.

[0010] The yielding material can fill the cavities of the mechanical ply-bonding member completely or only for part of the depth thereof. This prevents the cellulose fibers from accumulating in the cavities between the protuberances of the engraved or knurled surfaces of the mechanical ply-bonding member, resulting in a non- uniform contact between the mechanical ply-bonding member and the anvil member, with which the mechanical ply-bonding member co-acts.

[0011] Advantageously, the yielding material is an elastic material, especially a polymer. In this way, in case of compression, due to the presence of cellulose fibers or other debris accumulated in or on the cavities, the elastic return, i.e. the return to the original shape, causes the ejection of the fibers and the debris.

[0012] According to a further aspect, a mechanical ply-bonding unit is disclosed herein, comprising: a support; at least one mechanical ply-bonding member, as defined above, mounted on the support; at least one anvil member co acting with the mechanical ply-bonding member; pressure members adapted to push the mechanical ply-bonding member against the anvil member.

[0013] Preferably, the mechanical ply-bonding member is a mechanical ply- bonding wheel and the anvil member is an anvil roller. In other embodiments, the mechanical ply-bonding member is a roller and the anvil member is a wheel or a series of wheels.

[0014] The anvil member has preferably a smooth surface co-acting with the mechanical ply-bonding member, so as to reduce the wear thereof. However, also the anvil member can have an engraved surface with protuberances co-acting with the mechanical ply-bonding member. In this case, also the anvil member can have cavities interposed between protuberances, in these cavities a yielding material being arranged, for example an elastically yielding material.

[0015] In general,“yielding material” means a material that, under the work pressure of the members of the mechanical ply-bonding unit, is subjected to a greater compressive deformation than that of the mechanical ply-bonding member and of the anvil member. The yielding material is for example an elastically yielding material, for instance a polymer, a natural or synthetic rubber, or any other material that is adapted to be attached to the surface (for example a metallic surface) of the mechanical ply-bonding member and is enough yielding when subjected to a compressive force.

[0016] According to a further aspect, a method is disclosed herein for bonding, through mechanical bonding, a plurality of plies of web material, the method comprising the following steps: feeding the plies to a mechanical ply-bonding nip between a mechanical ply-bonding member and an anvil member, wherein the mechanical ply-bonding member comprises a side surface provided with a plurality of protuberances spaced by cavities, and wherein the cavities are at least partially filled with a yielding material; pushing the mechanical ply-bonding member against the anvil member, thus generating a pressure onto the plies that causes the mechanical j oining thereof.

[0017] A method for producing a mechanical ply-bonding member is also disclosed herein, comprising the following steps: engraving a side surface of a mechanical ply- bonding member and forming thereon an area provided with protuberances and cavities; applying, at least to the bottom of the cavities, a yielding material. [0018] Further aspects and features of the invention are described below and defined in the attached claims, forming an integral part of the present description.

BRIEF DESCRIPTION OF THE DRAWING

[0019] The invention will be better understood by following the description and the accompanying drawing, which shows a non-limiting example of embodiment of the invention. More in particular, in the drawing:

Fig. 1 is a plan view, according to I-I of Fig.2, of a mechanical ply-bonding unit;

Fig. 2 is a side view according to II- II of Fig. 1;

Fig.3 is an enlargement of a front portion of the knurled surface of a mechanical ply-bonding wheel;

Fig. 4 shows a local cross-section according to IV-IV of Fig. 3; and

Fig. 5 is a sectional schematic enlargement of a multi-ply web material.

DETAIFED DESCRIPTION OF AN EMBODIMENT

[0020] Below, a mechanical ply-bonding unit is described, comprising a plurality of mechanical ply-bonding wheels co- acting with an anvil roller. However, it shall be understood that at least some of the advantages of the present invention can be achieved also with mechanical ply-bonding units wherein the mechanical ply- bonding member is an engraved roller, i.e. a roller provided with mechanical ply- bonding protuberances on the cylindrical side surface thereof, co-acting with an anvil member comprising one or more wheels with smooth side surface.

[0021] With initial reference to Figs. 1 and 2, the mechanical ply-bonding unit 1 comprises a transverse support beam 3, on which units 5 are mounted, each of which comprises a mechanical ply-bonding wheel 7. A mechanical ply-bonding unit 1 of this type can be arranged in a paper converting line, for example upstream of a rewinder and downstream of an unwinder, from which the two plies VI, V2 (Fig. 5) to be bonded by the unit 1 for forming the web material N are fed. In some cases, the web material N processed by a mechanical ply-bonding unit 1 can be also formed by bonding more than two plies.

[0022] Usually, the mechanical ply-bonding wheels 7 are coaxial and approximately aligned according to an axis A-A substantially parallel to an axis B-B of an anvil roller 9, with which the mechanical ply-bonding wheels 7 co-act. Each mechanical ply-bonding wheel 7 forms, with the anvil roller 9, a mechanical ply- bonding nip 10 (Fig. 2), through which the plies VI and V2 pass to form, by mechanical bonding, the web material N.

[0023] In some embodiments, the anvil roller 9 has a smooth surface and co-acts with protuberances provided on the side surfaces of the mechanical ply-bonding wheels 7. In some embodiments, each mechanical ply-bonding wheel 7 comprises one or more projections 7.1, of small transverse dimensions. The projections 7.1 can be round in shape and lie on a plane orthogonal to the axis A-A, as described for instance in US 5.433.817. In other embodiments, the projections 7.1 may have a wavy pattern around the axis A-A, as disclosed for instance in WO2012/032555, for the purposes described therein. The side surfaces of the projections 7.1 are provided with engravings defining protuberances, arranged for example according to alignments inclined with respect to the axis A-A of the respective wheel, as it will be described in greater detail below.

[0024] In further embodiments, the mechanical ply-bonding wheels may have a side surface that has no projections and is provided, on the whole extension thereof, with engravings and corresponding protuberances, or it may be formed by annular portions, each of which having an outer cylindrical surface provided with protuberances, as disclosed in US 5.433.817. In further embodiments, each mechanical ply-bonding wheel comprises protuberances forming a decoration, as disclosed in EP1533112. In further embodiments, the mechanical ply-bonding unit 1 comprises an anvil roller 9 provided with embossing tips and co-acting with smooth or engraved mechanical ply-bonding wheels, as disclosed in EP2069136. The embossing or mechanical ply-bonding protuberances of the anvil roller 9 may form a uniform background or a decorative pattern.

[0025] Each mechanical ply-bonding wheel 7 can be brought by an arm 11 hinged around an axis C-C substantially parallel to the axes A-A and B-B.

[0026] One or more actuators can be also provided, for pushing the mechanical ply- bonding wheels 7 against the anvil roller 9. For example, pneumatic actuators can be provided, preferably more actuators acting separately on single mechanical ply- bonding wheels or on groups of mechanical ply-bonding wheels 7. In the illustrated embodiment, see in particular Fig. 2, between the beam 3 and each arm 11 an actuator is arranged, in the form of a pressure member 13, for example an expandable bladder. Even if, in principle, a single pressure member 13 can be provided for all the mechanical ply-bonding wheels 7, in preferred embodiments one pressure member 13 is provided for each mechanical ply-bonding wheel 7, and therefore for each mechanical ply -bonding unit 5. In alternative embodiments, each single mechanical ply-bonding wheel 7 can be moved towards the anvil roller 9 by means of a respective pneumatic cylinder-piston actuator or other individual actuator.

[0027] What described above is known to those skilled in the art, and more details on this type of mechanical ply-bonding units are disclosed, for example, in US 5.433.817 and in WO2012/032555, where possible embodiments of the inside of the mechanical ply-bonding wheels are also disclosed.

[0028] Practically, the plies VI, V2 of the web material N, passing through the mechanical ply-bonding nips 10 formed by the mechanical ply-bonding wheels 7 and by the anvil roller 9, are pressed with high pressure between the mechanical ply- bonding wheels 7 and the anvil roller, resulting in bonding of the two plies VI, V2.

[0029] In order to have higher pressure and therefore more efficient mechanical ply-bonding, it is advisable to reduce the contact surface between the mechanical ply-bonding wheel 7 and the anvil roller. To this end, the mechanical ply-bonding wheels 7 may be provided with projections 7.1.

[0030] Moreover, the contact surface of the mechanical ply-bonding wheel 7 with the anvil roller 9 is reduced by providing protuberances, forming a knurling, on the side surface of the mechanical ply-bonding wheel 7.

[0031] Fig. 3 shows a detail of the area indicated by the arrow III of Fig. 2. More in particular, Fig. 3 shows a detail of the side surface of one of the projections 7.1 of a mechanical ply-bonding wheel 7. Fig. 4 is a cross-section according to the line IV-IV of Fig. 3. As shown in Figs. 3 and 4, the surface of each mechanical ply-bonding wheel 7, and in particular the surface of each projection 7.1, has a knurled side surface, i.e. a surface with a plurality of protuberances 21.“Side surface” means the surface of the mechanical ply -bonding wheel 7 coming into contact with the anvil roller 9 to generate the pressure necessary to bond the paper plies VI and V2. The protuberances 21 may be formed by engraving a substantially cylindrical start surface. Engraving can be through chip-removal processing, laser, chemical etching, or in any other suitable manner, known to those skilled in the art. The reference number 23 indicates the cavities defined by engraving and surrounding each protuberance 21.

[0032] In the embodiment of Figs. 3 and 4, the protuberances have approximately frusto-pyramidal shape, for example with a square base and head surfaces 21.1 (see Fig. 4). This shape is particularly simple and can be obtained also with mechanical tools, for example with a gear hob. In this way, the pressure between the mechanical ply-bonding wheels 7 and the anvil roller 9 is concentrated on the head surfaces 21.1 of the protuberances 21 and, with a limited push by means of the pushing members 13, high pressure values can be obtained in the area of contact between the head surfaces 21.1 and the anvil roller 9. [0033] Characteristically, the cavities 23 are at least partially filled with an elastically or plastically yielding material, anchored to the metallic material forming the mechanical ply-bonding wheel 7. In the cross-section of Fig. 4, the reference number 25 indicates the yielding material, for example an elastomer resin, filling the cavities 23.

[0034] In some embodiments, the yielding material 25 may be silicone (polysiloxane) of suitable consistency and elastic characteristics, or a silicone rubber or any other material adapted to adhere to the surface of the cavities 23 and having elastic deformation properties for performing what described below.

[0035] The cavities 23 may be filled with elastically yielding material completely or only partially. Fig. 4 shows, just by way of non-limiting example, cavities completely filled and cavities partially filled with yielding material 25.

[0036] The cavities 23, which can be all interconnected, as shown Fig. 4, being for example formed by the intersection of engraving helical channels, have preferably constant depth, indicated with P in Fig. 4. However, this is not necessary, and engravings can be also provided, wherein the cavities have different depths in different areas of the mechanical ply -bonding wheel 7. In general, the yielding material 25 fills the cavities 23 for at least part of the depth P thereof. Advantageously, for greater efficiency, the elastically yielding material 25 is applied on the whole bottom of the cavities 23, even if it does not necessarily fill the cavities up to the top of the protuberances 21.

[0037] From a constructional point of view, it could be preferable to completely fill the cavities 23 with yielding material 25, and, in case, to cover also the head surfaces 21.1 of the protuberances 21. When the mechanical ply -bonding wheel 7 is used for the first time, for example by making it working on the anvil roller 9 with no interposed web material N, the excess of yielding material 25 covering the head surfaces 21.1 of the protuberances 21 can be removed thanks to the wear effect in the area of contact with the anvil roller 9.

[0038] During normal operation of the mechanical ply-bonding unit 1, cellulose fibers released from the plies VI, V2 forming the web material N tend to accumulate in the hollow spaces constituted by the cavities 23 between the protuberances 21. If these hollow spaces are completely filled with yielding material 25, this prevents the fibers from accumulating. Vice versa, if the cavities 23 are not completely filled with yielding material 25, the material tends to deform due to the pressure of the anvil roller 9 and any accumulated fibers. When the pressure generated by the co-action of the mechanical ply-bonding wheel 7 and the anvil roller 9 ends, the yielding material 25 tends to expand, ejecting the accumulated fibers from the cavities 23 or making the removal thereof easier during the normal maintenance and cleaning of the mechanical ply-bonding unit 1.

[0039] Consequently, the fibers do not accumulate in the cavities 23 and are not compressed in the cavities of the side surfaces of the wheels. Practically, the yielding material, preferably elastically yielding, accumulates strain energy in the nip between the mechanical ply-bonding wheel 7 and the anvil roller 9 in case of presence of fibers, and releases elastic energy, expanding when the pressure in the mechanical ply-bonding nip ends, thus ejecting the fibers and any other debris from the cavities 23. If the yielding material completely fills the cavities 23, any accumulation of cellulose fibers is avoided. [0040] The invention has been described with reference to various embodiments; however, it will be clearly apparent to those skilled in the art that modifications, changes and omissions can be done to the invention, without however departing from the scope of protection thereof as defined in the attached claims.