OBTAINED WITH SAID UNIT"

DESCRIPTION

Technical Field

The present invention relates to paper converting machines, in particular tissue paper converting machines, commonly used to produce rolls of toilet paper, rolls of kitchen towel, paper handkerchiefs and napkins, etc.. More specifically, the invention relates to improvements to ply-bonding units, i.e. units used to join two or more cellulosic plies through mechanical pressure to produce toilet paper, kitchen towels or the like.

State of the art

In the paper converting industry to produce tissue paper articles, such as toilet paper, kitchen towels, paper handkerchiefs and napkins or the like, various systems are used to join two or more cellulosic plies to one another and form a multi-ply product. In some applications, plies are joined by gluing. In other systems ply- bonding units are used, which join the cellulosic plies to one another by applying very high localized pressure that causes a sort of entanglement between the fibers of the mutually adjacent plies. A ply-bonding unit of this type is described, for example, in US patent no. 5,433,817.

These ply-bonding units usually comprise a plurality of ply-bonding wheels aligned with one another, each of which has at least one continuous projection and preferably two continuous projections, cooperating with a common counter-roller for all the ply-bonding wheels. The counter-roller and the ply-bonding wheels define a ply-bonding nip wherethrough two or more plies of cellulosic fiber to be joined to one another are made to pass. Each wheel is pressed by pressing members against the surface of the counter-roller. The projections provided on the ply-bonding wheels define surfaces of small dimension on which the pressure exerted by the wheel and by the counter-roller on the plies passing through the ply-bonding nip is concentrated, so that the pressure required to mechanically join the plies is reached in this zone. To obtain a higher concentration of pressure, the surface of the projections is machined in such a manner as to have individual protuberances, obtained by plastic deformation, chemical etching, laser engraving, chip removal or in another suitable manner. The protuberances are usually arranged according to lines inclined with respect to the axis of the wheel.

This ply-bonding technique has the advantage of not requiring the use of glue, which constitutes a consumable material that affects the cost of the finished product and has some disadvantages, including the fact that it requires the use of specific measures to prevent the machines in which the glue is applied from becoming soiled and the glue from drying when the machine is idle, and other disadvantages known to those skilled in the art.

However, ply-bonding has a disadvantage due to the fact that in the zones in which localized pressure is exerted to obtain entanglement of the fibers forming the plies, the product is subject to deformation involving variation of the thickness. A roll of two-ply toilet paper obtained by ply-bonding will therefore have one or normally two zones, or annular bands, corresponding to the position in which the ply- bonding band is located, wherein the roll has a radial deformation with respect to the remaining portion of the roll. More precisely, the roll has a bulge at each ply -bonding line. This bulge is due to the fact that in the ply-bonded zones the material is compressed with high pressures which cause a decrease of thickness and a consequent increase in the surface of the material. This increase in the surface forms a localized bulge. As the ply-bonding lines extend longitudinally along the web material, when this is wound to form a roll, the ply-bonded zone forms a roll portion with a larger diameter, with consequent overall deformation of the roll relative to the theoretically cylindrical configuration it should have.

The roll will therefore have the shape of a deformed cylinder, which causes difficulties in winding and packaging and an unsightly outer appearance, which has a negative effect on product sales.

WO-A- 1996/09162 describes a method and a device for embossing and joining plies of tissue paper to produce toilet paper or the like. The ply-bonding wheels used to join the plies are designed in such a manner as to form annular ply-bonding bands or zones along the edges of the multi-ply web material, wherein ply-bonding points are masked, i.e. concealed, between embossing points or protuberances of the plies. The ply-bonded protuberances or points are arranged according to a zigzag, or meandering or sinuous, pattern that develops inside the ply-bonding joining bands produced along the edges of the finished product.

Arrangements of ply-bonding tips according to a wavy or curvilinear pattern are also described in EP-A-1533112. In this case, ply-bonding is also used for deco- rative purposes, in substitution of embossing or in combination therewith.

US-A-2009/0297781 describes a method for joining an embossed cellulosic ply to a smooth cellulosic ply through ply-bonding. The object of the method described in this prior art document is that of masking or concealing the ply-bonding, mingling or concealing the ply-bonding points in the embossed pattern or in another decoration produced on the cellulosic ply, according to a technique similar to the one described in WO-A- 1996/09162. Also in this case, in order to conceal the ply-bonded points, these are arranged according to a sinuous line. The line is constituted by the alignment of single ply-bonded points, which can develop not only along the edges but if necessary in the intermediate zone of the embossed web material, according to the type of embossing pattern in which the ply-bonding points must be masked or concealed.

Summary of the invention

The invention proposes a ply-bonding unit that fully or in part overcomes one or more of the problems of prior art ply-bonding units.

In substance, according to the invention, in a ply-bonding unit comprising a plurality of ply-bonding wheels aligned with each other, each of which has at least one continuous projection cooperating with a counter-roller to define a ply-bonding nip wherethrough said at least two plies of cellulosic fibers pass, and wherein each ply-bonding wheel is pressed by pressing members against the counter-roller, said at least one projection has a wavy shape, i.e. along its annular development according to a closed curve about the axis of the ply-bonding wheel it has a variable distance from an intermediate plane of the ply-bonding wheel orthogonal to the axis of rotation of the ply-bonding wheel. In other terms, unlike conventional ply-bonding units, in which each projection has a shape such as to be lying on a plane orthogonal to the axis of the ply-bonding wheel, according to the invention the annular projection or each annular projection provided on the ply-bonding wheel has a wavy shape. Preferably, it always has a pattern according to a closed line and therefore has an annular shape. However, this line does not have a circumferential development lying on a plane orthogonal to the axis of the ply-bonding wheel. On the contrary, this line has an oscillating pattern relative to this plane. In other terms, the projection moves in an axial direction along the circumference of the ply-bonding wheel following a wavy curvilinear pattern exiting first on one side and then on the other from a plane orthogonal to the axis of rotation of the wheel. On the product processed with the ply-bonding wheels configured in this manner, a continuous wavy ply-bonding band is obtained. With a suitable choice of the width of the ply-bonding wheel and of the position and undulation of the projection or projections on the ply-bonding wheel, it is possible to obtain a cellulosic product wherein one or more ply-bonding bands develop according to a wavy pattern contained entirely between the parallel longitudinal edges of the cellulosic material.

According to some preferred embodiments of the invention, each ply-bonding wheel has two projections cooperating with the counter-roller. Preferably, the two projections have a generally symmetrical development relative to an intermediate plane and preferably they are substantially symmetrical with respect to a median plane of ply-bonding wheel, which also constitutes the plane of symmetry of the bearings of the ply-bonding wheel. A configuration of this type has the following advantages. The inclined pattern of each projection tends to generate on the web material a stress having an axial component, i.e. a component parallel to the axis of rota- tion of the wheel, which tends to make the web material drift laterally during processing. By arranging pairs of projections symmetrically relative to a plane orthogonal to the axis of rotation of the respective ply-bonding wheel, the stresses in the axial direction are also symmetrical and therefore are eliminated. Therefore, the resultant of the axial stresses on the web material will be equal to zero, regardless of the num- ber of ply-bonding wheels and of their angular offset.

If the plane of symmetry of the projections of a ply-bonding wheel coincides with the plane of symmetry of the support bearings of the wheel itself, the further advantage of balancing the stresses on the wheel is achieved, eliminating the torque about an axis orthogonal to the axis of rotation of the wheel, which would be gener- ated if the projections were not symmetrical relative to the support bearings of the wheel.

According to a different aspect, the invention relates to a roll of multi-ply cellulosic web material, comprising at least two plies joined to one another by ply- bonding at at least a first ply-bonding band extending longitudinally along the plies and wherealong the two plies are joined by the effect of a localized compression of the plies and an entanglement of the ply fibers, wherein said at least one ply-bonding band has a wavy pattern.

Preferably, the plies forming the web material of the roll are joined by ply- bonding along said first wavy patterned ply-bonding band and along a second ply- wavy patterned bonding band. Preferably, both the wavy ply-bonding lines are contained between the longitudinal edges of the web material and are preferably symmetrical relative to a intermediate straight line between the two wavy lines. In other embodiments, the web material of a roll can be provided with a first continuous wavy ply-bonding line, i.e. fully contained between the two longitudinal edges of the web material, while another wavy ply-bonding band is positioned closer to one of the longitudinal edges and is cut by this edge, portions of said line being cut by the edge corresponding, along the opposite edge, to identical portions of an interrupted ply- bonding band.

According to preferred embodiments of the invention, the projections of the ply-bonding wheel have protuberances or protrusions of limited dimensions, smaller than the transverse dimensions of the projection, so as to obtain a higher concentration of the pressure stresses on the cellulosic plies. Correspondingly, the finished product has points or zones of entanglement of the fibers according to a pattern cor- responding to the pattern according to which the protuberances or protrusions of the ply-bonding wheel or wheels acting on the product are arranged. Preferably, the protrusions of the ply-bonding wheels have a simple geometrical shape, for example truncated-cone or preferably truncated-pyramid shape and are advantageously arranged, for example, according to helical lines, the extension of which is limited to the width of the corresponding projection on the ply-bonding wheel. Preferably, the transverse dimension of each projection of the ply-bonding wheel is a multiple of the dimension (for example of the side or of the diagonal, or of the diameter, in the case of protuberances with a circular section) of the ply-bonding protuberances produced on the projection. In this manner, wavy bands joined by ply bonding are generated on the finished product. Areas, zones or points of entanglement between the cellulosic fibers forming the two or more plies of which the web material is constituted are formed along each band. These zones, areas or points are arranged according to a pattern corresponding to the pattern of the protuberances on the ply-bonding projections of the wheels.

Further advantageous features and embodiments of the invention are indicated in the appended dependent claims and will be described hereunder with reference to the accompanying drawings.

Brief description of the drawings

The invention will be better understood based on the description and accom- panying drawing, which shows a non-limiting practical embodiment of the invention. More in particular, in the drawing:

Fig. 1 shows a front view of a ply-bonding unit according to the invention;

Fig. 2 shows a side view according to II-II of Fig. 1, rotated through 90°; Fig. 3 shows a section of a ply-bonding wheel of the ply-bonding unit;

Fig.3A shows an enlargement of a front portion of the surface of one of the ply-bonding projections present on a ply-bonding wheel;

Fig. 3B shows a local cross section according to B-B of Fig.3A;

Fig. 4 shows a development in the plane of the cylindrical surface of a ply- bonding wheel;

Fig.4A shows a schematic representation of the development in the plane of the cylindrical surface of two adjacent ply-bonding wheels;

Fig. 5 schematically; shows a roll of two-ply web material obtained with a ply- bonding unit according to the invention; and

Fig.6 schematically shows a portion of a roll of paper or other web material, in a modified embodiment.

Detailed description of an embodiment of the invention

Referring initially to Figs. 1 and 2, the ply-bonding unit, indicated as a whole with 1, comprises a support cross beam 3 on which units 5 are mounted, each of which comprises a ply-bonding wheel 7. A unit 1 of this type can be inserted in a paper converting line, for example upstream of a rewinding machine and downstream of an unwinding unit, from which the two plies that form the web material N, and which must be joined to one another by means of the unit 1, are fed.

The ply-bonding wheels 7 are generally co-axial and approximately aligned according to an axis A-A, substantially parallel to an axis B-B of a counter-roller 9 with which the ply-bonding wheels 7 cooperate. In some embodiments, the counter- roller 9 has a smooth surface and cooperates with projections of small cross dimensions provided on each of the ply-bonding wheels 7 of the single ply-bonding units 5. The projections will be described in greater detail hereunder.

Each ply-bonding wheel 7 is carried by an arm 11 pivoted about an axis C-C substantially parallel to the axes A-A and B-B. A pressing member 13, for example a plenum chamber, is arranged between the beam 3 and each arm 11. In some embodiments a single pressing member 13 can be provided for all the ply-bonding wheels 7 or for a group thereof. In other embodiments, a pressing member 13 is pro- vided for each ply-bonding wheel 7 and therefore for each ply-bonding unit 5.

Everything described above is known to those skilled in the art and greater details of ply-bonding units of this kind are described in the aforesaid US patent no. 5,433,817, the content of which is incorporated in the present description.

Fig. 3 shows a section according to a plane containing the axis A-A of a ply- bonding wheel 7 and the pivoting axis C-C of the arm 11. Reference 21 indicates the support pin of the ply-bonding wheel 7 on the respective arm 11. The pin 21 is stationary relative to the arm, i.e. it does not rotate about the axis A-A and the ply- bonding wheel 7 is mounted thereon by means of radial bearings 23, which can there- fore rotate idle about the axis A-A. In a known manner, the ply-bonding wheel 7 has a core 7 A and a sleeve 7B. Elastic elements, three in number in the example illustrated and indicated with the reference 25, are arranged between the core 7A and the sleeve 7B. In some embodiments the elastic elements 25 can be constituted by rubber O-rings. These elastic elements 25 allow a limited degree of freedom of the ply- bonding wheel 7 between the shell 7B and the core 7 A, for purposes known to those skilled in the art.

Characteristically, according to the invention, two projections 31 are provided on the sleeve 7B of each ply-bonding wheel 7, the shape of which on the entire circumference of the ply-bonding wheel 7 is shown in Fig. 4 where a planar develop- ment of the surface of the sleeve 7B is shown. As can be seen in Fig. 4, each projection 31 has a wavy pattern, for example approximately sinusoidal. In the example illustrated, the sinusoidal pattern is limited to a single period of the sinusoid, with a maximum and a minimum. This is sufficient to achieve the object of substantially reducing the deformation of the roll of wound web material.

Reference 31A indicates, for each projection 31, a maximum point of the wavy curve along which the projection 31 develops. Reference 3 IB indicates a minimum point of this curve, relative to a line P-P which constitutes the trace of a median plane orthogonal to the axis A-A of the ply-bonding wheel 7. Preferably, the plane P-P constitutes the plane of symmetry of the ply-bonding wheel. The bearings of the ply-bonding wheel are preferably arranged symmetrically relative to the plane P-P. Preferably, the two projections 31 have a symmetrical, i.e. mirror image, pattern relative to the plane with the trace P-P. This makes it possible to obtain a balanced stress on the bearings of the ply-bonding wheel 7 without generating an overturning torque about an axis orthogonal to the axis of rotation A-A of the ply-bonding wheel. In the illustrated example the minimum points 3 IB of the curves representing the pattern of the projections 31 are located at a minimum distance m (Fig. 4). This distance can be reduced as much as possible in order to make the finished product more uniform.

In fact, the two annular projections preferably do not intersect and do not cross each other. In other terms, the two projections 31 are located one on one side and the other on the opposite side of the plane with the trace P-P, without intersecting said plane. This makes it possible to obtain the further advantage of avoiding the concentration of stresses which would occur in the point in which the projections in- tersect, where the contact surface between the ply-bonding wheel and the counter- roller would be halved with consequent doubling of the pressure.

In some embodiments, the distance "m" corresponds to the minimum distance existing between two adjacent projections 31 of two consecutive ply-bonding wheels 7, as shown schematically in Fig. 4A, which shows the development in the plane of two cylindrical surfaces of two consecutive ply-bonding wheels 7, in the hypothesis in which these ply-bonding wheels are angularly in phase. However, this condition of angular phasing is not necessary and in fact will not be maintained due to the fact that the individual wheels are not constrained torsionally to one another, but each wheel being drawing into rotation separately from the other and can be subject to different slippages relative to one another.

In a known manner, the projections 31 can be provided with knurling or surface engraving to increase the effect of localized pressure, and therefore the entanglement of the fibers by the effect of the pressure applied. Figs. 3A and 3B show a possible configuration of the radial surfaces of the projections 31. In this embodi- ment, the projections have truncated-pyramid shaped protuberances PR, advantageously but not necessarily with a square cross section, arranged according to an appropriate inclination. In the example illustrated, the inclination is 45° relative to the axis of the respective ply-bonding wheel 7. The protuberances PR can be obtained with any processing technique, by plastic deformation, mechanical chip removal, chemical etching, laser engraving or the like. Advantageously, the dimensions of the protuberances PR are smaller than the width of the projection 31 on which they are produced. In the example illustrated, the diagonal dimension of the base of each truncated-pyramid shaped protuberance PR is approximately equal to half or less than half of the width of the wavy annular projection 31. Preferably the arrangement of the protuberances PR and of the projections 31 on each ply-bonding wheel is such that the contact between the ply-bonding wheel and the counter-roller is constant, during rotation of the ply-bonding wheel 7. Moreover, in some preferred embodiments, this arrangement is such that the area of con- tact, i.e. the area on which pressure is exerted by the ply-bonding wheel 7 and the counter-roller 9 is substantially constant. "Substantially constant", means that during rotation the surface of contact varies preferably less than 20% and more preferably less than 15%. A helical arrangement of the protuberances PR is suitable to achieve this result.

Two plies VI and V2, which must be joined by ply-bonding to form the web material N that is subsequently wound to form a roll R, are fed through the ply- bonding nip 2 formed between the counter-roller 9 and the set of ply-bonding wheels 7 (Fig. 5). Typically, the plies that are joined by ply-bonding have a width equal to a multiple of the axial length of the finished rolls R to be produced.

The web material N delivered from the ply-bonding unit 1 is wound downstream of the ply-bonding unit to form logs or reels with a high axial length, for example in a rewinder of known type. The logs are subsequently cut according to cutting planes orthogonal to the axis of winding to form the single rolls of smaller dimensions intended for final consumption. Preferably, the axial length of the ply- bonding wheels 7 of the individual ply-bonding units 5 corresponds approximately to the axial length L of the finished rolls (Fig.5) obtained by cutting the logs produced through winding of the web material N fed from the ply-bonding unit 1. In this manner, two ply-bonding lines or bands, indicated with LM in Fig. 5, will be present on each finished roll. The ply-bonding bands LM have, as a result of the shape of the projections 31 that generated them, a wavy pattern inside the web material N forming the single roll. In this manner, the effect of deformation on the web material N caused by the high localized pressure applied by the protuberances PR of the projections 31 is distributed over the entire axial length L of the roll R, from the lateral flat faces up to the vicinity of the center line lm (see Fig. 5). This gives rise to more even winding and improved shape of the finished product. In practice, in the hypothesis that each roll R is processed by a single ply-bonding wheel 7, the whole axial development of the finished roll R is affected by the deformation caused by the ply- bonding, with the exclusion of a narrow central area having the width lm and of two lateral bands each having the width lm/2. Each band or line of LM remains perfectly visible on the finished product. However, the problem of the formation of a roll R that is radially deformed according to bulges corresponding to the ply-bonding projections is eliminated, as these projections have a wavy pattern and are therefore distributed along the axial development of the roll, giving rise to a substantially constant radial dimension of the finished product.

The same advantage can also be obtained if the wavy ply-bonding bands LM are not centered with respect to the width of the material forming the roll R. Also in the case in which the ply-bonding bands are staggered in such a manner that one of these is located over the edge of a roll, the same effect of increased uniformity of the thickness of the roll is still achieved, provided that the pitch of the pairs of ply- bonding bands substantially correspond to the longitudinal dimension of the roll R. In the case of staggering between ply-bonding bands and edges of the roll, one of the two ply-bonding bands will be cut on an edge and the zones thereof that are outside the edge will be found identical at the opposite edge. Fig. 6 schematically shows a configuration of this type, limited to a portion of web material N. In this figure it can be observed that a continuous wavy ply-bonding band extends on the material in an approximately central position, while a second ply-bonding band intersects the left edge and "exits" from the width of the web material. A portion of a ply-bonding band generated by the adjacent ply-bonding wheel is present along the edge opposite the missing zone of this ply-bonding band.

It is understood that the drawing shows just one example, provided merely as a practical demonstration of the invention, which can vary in its forms and arrangements, without however departing from the scope of the concept underlying the invention. For example, the ply-bonding wheels could arranged according to two alignments parallel to the counter-roller, with an alternated arrangement, with the wheels in even position placed on one side of the counter-roller and the wheels in odd position placed on the opposite side. An arrangement of this type is known, for example, from the prior art documents cited in the introduction of the present description. This makes it possible to decrease the total stress on the counter-roller and obtain increased vicinity or even overlapping between the zones processed by adjacent ply-bonding wheels.

Any reference numbers in the appended claims are provided to facilitate reading of the claims with reference to the description and to the drawing, and do not limit the scope of protection represented by the claims.