Patent Description:
Corrugated paper sheets are known to be used on their own or even coupled with one or more smooth, i.e., non-corrugated, paper sheets, which give the corrugated paper sheet shape stability.

A known technique for making corrugated paper sheets consists in passing a smooth paper sheet between a pair of cylinders, each affected on the whole side surface thereof by radially protruding veinings running parallel to the cylinder axis. The veinings are separated by grooves, in which the veinings of the other cylinder engage, forming a sort of toothed coupling between one another.

If a paper sheet is passed between the two cylinders, it is subjected to a deformation process, which turns it into a sheet corrugated transversely to the passage direction therebetween.

If it is then necessary to obtain a multilayer sheet of corrugated cardboard, two non-corrugated paper sheets are applied to one or both sides of the corrugated paper sheet, which stabilize the shape of the corrugated sheet and globally form the multilayer sheet of corrugated cardboard.

Obviously, the same technique is also applicable to obtain a continuous web of corrugated cardboard, provided that the two cylinders are fed with a continuous web instead of a paper sheet and provided that one or two continuous non-corrugated paper webs are applied thereto, following the passage between the cylinders.

This technique has proven to be valid and is currently very widespread, but has a limitation in that the cardboard obtained is corrugated only in the direction transverse to the forming direction. Furthermore, the web or the multilayer sheet obtained, although having a sufficient folding resistance in the direction orthogonal to the corrugations, it has a limited folding resistance in the direction parallel to the corrugations. As a result, to obtain the same folding resistance in both directions, individual cardboard sheets need to be produced by gluing them together so that they have the corrugations oriented at <NUM>° with respect to one another, and this with a non-continuous process.

Another drawback consists in that, in any case, this known technique does not allow to obtain a continuous paper web corrugated in the longitudinal direction.

To eliminate this limitation, passing a continuous paper web between a pair of cylinders affected by a plurality of circumferential veinings and coupled to one another so that the veinings of a cylinder enter in the grooves of the other cylinder has already been provided, for example in <CIT>. If now a paper web is passed between the two cylinders, this has to follow the profile of the two cylinders and, when exiting therefrom, it should theoretically have assumed a longitudinally corrugated shape.

A drawback of this known solution consists in that the web passing between the two cylinders is affected by the corrugation process over the whole width thereof at the same time, and this determines a frequent longitudinal tearing of the web itself, with a consequent unacceptable production waste, which significantly affects the final cost of the product obtained.

To avoid this drawback, creating the longitudinal corrugations on the paper web starting from the central band towards the edges has already been provided. This solution also has the drawback of causing a significant variation in the overall width of the exiting corrugated web, with respect to the entering flat web. As a result, to obtain an exiting longitudinally corrugated web with a width suitable for satisfying the standard requirements of this industry, it is necessary to feed the forming machine with a paper web with a width far greater than the standard one, with the need to provide, upstream of the forming rollers, a structure supporting the paper roll to be processed with a bulkiness significantly greater than the bulkiness of the forming machine itself. Furthermore, given an equal width of the entering flat web, the width of the longitudinally corrugated web thus obtained is influenced by the shape and size of the corrugations.

Another drawback of this known solution consists in the very poor shape stability of the corrugated web, which tends to elastically regain the flat shape thereof unless it is stabilized, immediately after the forming, by applying a flat paper web thereto.

<CIT> and <CIT> relate to a process for obtaining a paper having a smooth and easily printable surface which may be extended both in the transverse direction and in the longitudinal direction. In particular, this is obtained by passing the starting web between a rigid cylinder provided with veinings and a cylinder externally coated with rubber so as to simultaneously carry out the embossing on the surface in contact with the rigid cylinder and the creping on the surface in contact with the rubber-coated cylinder. The exiting paper has a surface - the one that has come into contact with the rigid cylinder - which is substantially flat and smooth with recesses at the veinings of the rigid cylinder, while the other surface - the opposite one, which has come into contact with the rubber-coated cylinder - is flat with superficial ripples.

<CIT> relates to a method of creping paper. in which a web a is corrugated or waved longitudinally e.g. by passing it between rollers band , includes the provision of means to advance the paper at the crests of the corrugations and to retard it at the troughs or vice versa so that transverse and oblique crepings are produced.

<CIT> provides for passing a fabric between a web of a soft material and a rigid cylinder (i.e., of a non-elastic material) which is grooved and is heated. Furthermore, such solution provides for introducing a covering cloth or an endless continuous web of a flexible material between the rigid cylinder and the fabric to be treated, so as to create a ripple effect on the fabric, and this for ornamental purposes only.

It is the object of the invention to provide a process for making continuous webs of longitudinally corrugated paper devoid of all the drawbacks of the conventional technique and more specifically for making a continuous web of longitudinally corrugated paper with shape stability and with a width equal to the width of the starting flat web.

It is another object of the invention to provide a process for making continuous webs of longitudinally corrugated paper in a simple manner and with a low cost.

It is another object of the invention to provide a process which allows to achieve high production speeds.

It is another object of the invention to provide a process for making continuous webs of longitudinally corrugated paper, with different and improved features.

It is another object of the invention to provide a process for making continuous webs of longitudinally corrugated paper, with which to prepare multilayer corrugated cardboards of different types.

It is another object of the invention to provide a continuous process with which it is possible to produce multilayers having increased flexural resistance, by virtue of the coupling of transversely corrugated webs with longitudinally corrugated webs.

It is another object of the invention to provide a process which may be implemented in a simple manner and at a low cost.

It is another object of the invention to provide a process having an alternative and/or improved characterization with respect to conventional solutions, both in terms of construction and function.

All these objects, considered both individually and in any combination thereof, as well as others which will result from the following description, are achieved, in accordance with the invention, by a process for producing a continuous web of a fibrous material, in particular of paper, longitudinally corrugated, as defined in claim <NUM>.

The present invention is further clarified below in some of the preferred embodiments thereof, given purely by way of explanation and not by way of limitation, with reference to the accompanying drawings, in which:.

<FIG> shows a conventional apparatus for producing a continuous web of multilayer cardboard comprising a first external web <NUM> of a fibrous material in general and of paper in particular, unwound from a first reel <NUM>, a second external web <NUM> of paper, unwound from a second reel <NUM> and a third web <NUM>, interposed between the two external webs <NUM>, <NUM> and formed by a transversely corrugated paper web. It is obtained starting from a flat web <NUM>, unwound from a reel <NUM> and passed between two forming cylinders <NUM>, <NUM>' affected on the side surface by a plurality of transverse veinings alternating with transverse grooves complementary to the veinings of the other forming cylinder <NUM>', <NUM>.

Unlike the web <NUM>, corrugated in the transverse direction and not very resistant to foldings parallel to the corrugations, the present invention provides a process and a plant with which a paper web may be obtained, corrugated in the longitudinal direction and resistant to foldings both parallel and orthogonal to the corrugations. Suitably, according to the present invention, the corrugations <NUM> affect the whole thickness of the paper web thus obtained and, in particular, both surfaces <NUM>, <NUM> thereof. In particular, the process in accordance with the invention uses:.

Suitably, the presser element may comprise a cylinder <NUM> coated in an elastically compressible material (see <FIG>) or a web <NUM> of an elastically compressible material which is pressed against the rigid cylinder <NUM> by a delimiting element which may be defined by a rotating element <NUM> (see <FIG>) or by a fixed element (see <FIG>).

The process in accordance with the invention provides that, when a continuous web <NUM> of a non-corrugated fibrous material is passed between said rigid cylinder <NUM> and the presser, said web is also subjected, on at least one segment of the passage thereof between said rigid cylinder <NUM> and said presser, to a reduction of the speed of the surface <NUM> thereof in contact with said presser with respect to the speed of the surface <NUM> thereof in contact with said rigid cylinder.

Suitably, the method in accordance with the invention provides that the force for maintaining the presser adherent to said rigid cylinder and the speed reduction between the two surfaces <NUM>, <NUM> of the web <NUM> are mutually defined, so that:.

Suitably, the rigid cylinder <NUM> and the presser advance in directions opposite to one another.

The rigid cylinder <NUM> is affected on the whole side surface thereof by a plurality of circumferential grooves <NUM> which are flanked and shaped so as to define a substantially corrugated profile reproducing the pattern of the corrugations <NUM> to be substantially obtained on the whole thickness of the web itself, and in particular on both surfaces <NUM>, <NUM> of the exiting paper web.

These grooves <NUM> preferably have a circumferential pattern and may suitably be continuous or discontinuous, and may have, in section, a curved corrugated pattern, preferably sinusoidal, or square wave, or of any other shape.

Also the size of the grooves <NUM> may vary a lot, depending on the features of the corrugations <NUM> to be formed on the whole thickness of the paper web - and therefore on both surfaces <NUM>, <NUM> of the web itself - according to market requirements.

The grooves <NUM> have a depth and/or a pitch not smaller than <NUM> and, preferably, have a pitch and/or a depth of between about <NUM> and <NUM> and, more particularly, of between about <NUM> and <NUM>.

Advantageously, the width (i.e., the transverse development) of the grooves <NUM> is substantially corresponding to the distance (always in the transverse direction) provided between the adjacent grooves <NUM>. Suitably, this allows to obtain, at the exit a paper web <NUM> with corrugations <NUM> in which portions of concavities, opposite with respect to that of the corresponding adjacent portions, alternate with constant pitch. In particular, preferably, in the exiting paper web <NUM>, the width of the segments raised (and deriving from the entry of the paper into the grooves <NUM>) substantially corresponds to the width of the remaining segments (i.e., those not raised).

Preferably, the presser of an elastically compressible material <NUM> has a substantially smooth external surface.

Advantageously, the presser of an elastically compressible material <NUM> has a hardness of between <NUM> and <NUM> PJ and preferably of between <NUM> and <NUM> PJ.

Suitably, the continuous paper web <NUM> - which is sent entering into the cylinder <NUM> and into the presser - has a degree of dryness of <NUM>-<NUM>%, preferably of <NUM>-<NUM>%. Advantageously, said continuous paper web <NUM>, may be of any type, even of recycled waste fibers.

Furthermore, said entering entering paper web <NUM> may be the one which is obtained directly in the paper mill, or may even be the one which, after being dried, is sent to the converter for subsequent processing; in the latter case, then, the paper web is previously wetted, before being sent entering into the cylinder <NUM> and into the presser. Suitably, this wetting may be obtained by vaporizing or by spraying on the web <NUM> water or another suitable liquid or by passing the web itself through a tank containing this liquid.

Advantageously, in a first embodiment, shown in <FIG>, a pair of forming cylinders <NUM>, <NUM>, is provided, of which the cylinder <NUM> is made of a rigid material, preferably steel, while the cylinder <NUM> is made, at least externally, of an elastically compressible material, for example rubber or even steel, but with an external rubber coating. Preferably, but not necessarily, the rigid cylinder <NUM> is located in a lower position with respect to the cylinder of an elastically compressible material <NUM>.

Both cylinders <NUM>, <NUM> are supported by a suitable structure (not shown) and are associated with respective motors, which drive them in rotation in a direction opposite to one another at different peripheral speeds. Suitably, the cylinders <NUM>, <NUM> are actuated so as to rotate at different speeds, and respectively at a speed V<NUM> for the rigid cylinder <NUM> and at a speed V<NUM> < V<NUM> for the cylinder <NUM> of an elastically compressible material. Preferably, but not necessarily, the cylinder of an elastically compressible material <NUM> and the rigid cylinder <NUM> have approximately the same diameter.

The apparatus described herein operates as follows: when a continuous paper web <NUM> is passed between the two cylinders <NUM> and <NUM>, the combined effect of the pressure existing in the contact area (nip) thereof and the different speeds thereof ensures that the cylinder <NUM> of an elastically compressible material presses the surface <NUM> of the paper web <NUM> against the rigid cylinder <NUM> to an extent suitable to lead the paper itself to be inserted into the grooves <NUM> (even without necessarily adhering to the bottom of the latter) and this effect, combined with the difference in peripheral speed of the two cylinders <NUM>, <NUM> - and therefore the different speed which the two surfaces of the paper web <NUM> are subjected to - causes a transfer of energy from the cylinder of an elastically compressible material <NUM> to the surface <NUM> of the paper itself, to an extent sufficient both to deform the paper web <NUM>, to imprint longitudinal corrugations <NUM> thereto along the whole width thereof and along the whole thickness thereof, and to create, in particular on the bottom of these corrugations thus obtained, a series of transverse ripples <NUM> which stabilize the shape of the corrugations themselves, even without the need to apply a flat paper sheet to the corrugated paper sheet as necessarily required by the conventional technique.

Suitably, a portion of web obtained by the process described herein is shown in <FIG>. In particular, as shown in <FIG>, the longitudinal corrugations <NUM> affect the whole thickness of the exiting paper web <NUM>, and therefore both the surfaces <NUM> and <NUM> of the web itself.

Suitably, as shown in <FIG>, the transverse ripples <NUM> mainly and especially affect the surface <NUM> of the exiting paper web <NUM>, i.e., the surface of the latter which has come into contact with the rigid cylinder <NUM>. Suitably, the transverse ripples <NUM> may also affect the surface <NUM> of the exiting paper web <NUM>, i.e., the surface of the latter which has come into contact with the cylinder <NUM>.

Suitably, the exiting paper web <NUM> has a thickness which has a substantially corrugated pattern and which remains substantially constant both in the longitudinal direction and in the transverse direction. Suitably, the thickness of the paper web <NUM>, exiting from the passage defined between the rigid cylinder <NUM> and the presser, is greater with respect to that of the paper web entering into said passage.

The longitudinal braking given by the different speeds of the two cylinders <NUM>, <NUM> has a double effect. The first effect consists in that the longitudinal braking determines a longitudinal compaction which is immediately used to obtain a greater transverse development, which in turn allows to obtain the corrugation without reducing the width of the exiting corrugated web. In other words, according to the present invention, if the entering web has a width La and a length Lu, at the exit, the web itself has a width which is still La and a length Lu' with Lu' < Lu. The second effect consists in creating the transverse ripples <NUM> which give stability to the obtained corrugations.

In the embodiment shown in <FIG>, the rigid cylinder <NUM>, which is circumferentially grooved, cooperates with a continuous web <NUM> of an elastically compressible material instead of with a cylinder of an elastically compressible material.

Preferably, but not necessarily, the rigid cylinder <NUM> is positioned in a lower position with respect to the web <NUM>. Suitably, the rigid cylinder <NUM> has one or more of the features described for the previous embodiment.

Advantageously, the web <NUM> is stretched between a series of return rollers, at least one of which is motorized, and is advanced, as in the case of the previous embodiment, at a peripheral speed V<NUM> <V<NUM>, where V<NUM> is the speed of rotation of the rigid cylinder. More particularly, the web <NUM> is stretched between a first roller <NUM>, which presses it against the cylinder <NUM>, a second roller <NUM> and a third roller <NUM>, which are respectively located upstream and downstream of the first roller <NUM>.

Also in this case, the apparatus operates as in the previous case, in the sense that the lower peripheral speed V<NUM> of the web <NUM>, with respect to the peripheral speed V<NUM> of the cylinder <NUM>, causes on the paper web <NUM>, interposed between the two, a braking effect, which, combined with the pressure exerted by the web itself on the paper web <NUM>, in addition to pressing it on the bottom of the circumferential grooves <NUM> of the cylinder <NUM> to create the longitudinal corrugations <NUM> affecting the whole thickness of the web itself, creates on the bottom of the corrugations thus obtained a series of transverse ripples <NUM> which stabilize the shape of the corrugations themselves.

Suitably, also in this case, the portion of web obtained is shown in <FIG>. In particular, also in this case, the longitudinal corrugations <NUM> affect the whole thickness of the exiting paper web <NUM>, and therefore both the surfaces <NUM> and <NUM> of the web itself. Suitably, as shown in <FIG>, the transverse ripples <NUM> mainly and especially affect the surface <NUM> of the exiting paper web <NUM>, i.e., the surface of the latter which has come into contact with the rigid cylinder <NUM>. Suitably, the transverse ripples <NUM> may also affect the surface <NUM> of the exiting paper web <NUM>, i.e., the surface of the latter which has come into contact with the web <NUM>.

In the embodiment shown in <FIG>, as in the case of the previous embodiment, a web <NUM>, made of an elastically compressible material and stretched between two or more return rollers, is associated to the circumferentially grooved rigid cylinder <NUM>.

Suitably, in such embodiment, the web <NUM> is supported and stretched by at least one return element <NUM> which is fixed, i.e., non-rotating. Advantageously, this fixed return element <NUM> is defined by a fixed bar. Preferably, the return bar <NUM> has a significantly smaller diameter with respect to the diameter of the grooved cylinder <NUM>.

Suitably, the fixed return element <NUM> is positioned and configured so as to locally press the web <NUM> against said cylinder <NUM> so as to significantly reduce the thickness thereof and to locally increase, consequently, the peripheral speed thereof in the short area of contact with the grooved cylinder <NUM>.

The various quantities involved are sized so that the speed of the web <NUM> at the point of passage between the roller <NUM> or the fixed element <NUM> and the cylinder <NUM> is equal to the peripheral speed V<NUM> of the lower cylinder <NUM>.

Also, as soon as the web <NUM> leaves the passage delimited by the fixed element <NUM> and by the cylinder <NUM>, it elastically regains the original thickness thereof, with an abrupt increase in thickness and a consequent abrupt decrease in speed, determining a sort of slowing of the paper web <NUM> with which it is in contact, and with a formation of corrugations <NUM> and transverse ripples <NUM>, in a similar manner to what already described in the two previous examples.

Advantageously, in an embodiment not shown in the drawings, said corrugated web, which exits the passage gap between the presser of an elastically compressible material and the rigid cylinder <NUM>, is passed between two stabilizing rollers, preferably of steel, with a corrugated profile for pressing the corrugations of said web so as to stabilize and stiffen the corrugations themselves or, possibly, so as to vary and/or define the final shape (which may have a sinusoidal, trapezoidal, V or square/rectangular profile) of said corrugations.

Advantageously, the pitch of the corrugated profile of the two subsequent stabilizing rollers corresponds to that of the rigid cylinder <NUM>. Advantageously, the corrugated profile of the two subsequent stabilizing rollers may have a shape and/or a depth equal to or different from those of the rigid cylinder <NUM>. Suitably, the subsequent stabilizing rollers are activated at a speed corresponding to that of the corrugated web exiting from the passage gap between the presser of an elastically compressible material and the rigid cylinder <NUM>.

Suitably, in all embodiments, the longitudinal braking - which derives from the different speeds between the surface <NUM> of the web in contact with the rigid cylinder <NUM> and the surface <NUM> of the web which is in contact with the presser element (which may be defined by a cylinder <NUM> provided, at least externally, with a layer of an elastically compressible material or may be defined by a web <NUM> of an elastically compressible material which is pressed against the rigid cylinder <NUM> by a rotating element <NUM> and/or by a fixed element <NUM>) - simultaneously has two effects connected to one another:.

Suitably, in order to obtain such double effect, the longitudinal braking given by the different peripheral speeds of the cylinder <NUM> with respect to the presser element <NUM> or <NUM> is defined on the basis of the force provided to maintain the presser itself adherent to said rigid cylinder <NUM>, and vice versa. In other words, to obtain such double effect, the braking and the force to maintain the presser adherent to said rigid cylinder <NUM> must be suitably and mutually defined and correlated. Suitably, the greater the braking is, the greater the force is required to maintain the presser itself adherent to said rigid cylinder <NUM> in order to obtain the aforesaid double effect.

Advantageously, the difference between the peripheral speeds of the cylinder <NUM> with respect to the presser element <NUM> or <NUM> is about <NUM>-<NUM>% while the force for maintaining the presser adherent to said rigid cylinder <NUM> is such that the linear pressure at the contact area (nip) is about <NUM>-<NUM> per linear centimeter of length of the contact area (nip).

Suitably, said braking and said force for maintaining the presser adherent to said rigid cylinder <NUM> depend on the moisture content of the fibers of the web <NUM> of a fibrous material when entering, on the weight of the web <NUM> of a fibrous material when entering, on the hardness of the layer of an elastically compressible material of the presser and on the size of the rigid cylinder <NUM> and of the presser.

In the embodiment shown in <FIG> the forming cylinder <NUM> is hollow, has the side surface permeable and/or perforated <NUM> and houses therein a fixed suction chamber <NUM>. It is connected to a suction pump (not shown) and is shaped as a cylindrical section with the two radial walls closed, and is open on the side facing towards the side surface <NUM> of the cylinder <NUM> and is affected along the perimeter edge by suitable sealing means sliding with respect to said side surface <NUM>.

The circumferential development of the opening of the suction chamber <NUM> facing towards the side surface <NUM> of the cylinder <NUM> is approximately equal to the circumferential length of the contact band between the cylinder <NUM> and the corrugated paper web <NUM> being formed.

The function of this suction chamber <NUM> is to ensure the adherence of said paper web <NUM> to the corrugated surface of the cylinder <NUM>, even in the case of great hardness of the rubber forming or covering the cylinder <NUM> and/or in the case of deep corrugations, which are often desired, given the high dimensional stability which they give to the corrugated web.

Naturally, the suction chamber <NUM> may advantageously be provided also in the case of a presser consisting of a rubber web <NUM> instead of a rubberized cylinder <NUM>.

In the embodiment shown in <FIG>, the present disclosure provides that the cylinder <NUM>, in addition to having the side surface affected by the circumferential grooves <NUM> aimed at creating the longitudinal corrugations <NUM> in the continuous web, is affected by circumferential incisions <NUM> in the grooves <NUM>. They are designed to form in the longitudinal corrugations <NUM> of the corrugated web a series of small veinings <NUM>, which run parallel to the corrugations themselves and give thereto a greater dimensional stability, by virtue of the increase of the deformation resistance thereof, in particular the compression resistance.

Since the paper web <NUM> to be transversely corrugated must have a certain plasticity to be able to enter into the small incisions <NUM> obtained in the transverse grooves <NUM> of the rigid cylinder <NUM>, and since the water content present in the paper web <NUM> may not be sufficient to give this plasticity required, it is also provided that, before the introduction thereof between the cylinder <NUM> and the presser <NUM>, <NUM> it is subjected to wetting, to an extent suitable for bringing the dry content thereof to the optimum value, which is generally of between <NUM> and <NUM>%, and is preferably of between <NUM> and <NUM>%. Suitably, this wetting may be obtained by vaporizing or by spraying on the web <NUM> water or another suitable liquid or by passing the web itself through a tank containing this liquid.

Regardless of the type of apparatus used and the operating parameters selected, in any case, the process in accordance with the invention and the machine allowing it to be implemented are somewhat more advantageous than conventional techniques for making the continuous web of longitudinally corrugated paper, and in particular:.

In particular, unlike <CIT> and <CIT>, the present invention allows to obtain paper with an overall pattern which is longitudinally corrugated for the whole thickness thereof and, in particular, in which the two opposite surfaces of the paper have both a longitudinally corrugated pattern. On the contrary, the solutions of <CIT> and <CIT> envisage carrying out an embossing, such to create recesses which exclusively affect the external surface of the paper web which comes into contact with the rigid cylinder, and therefore do not create longitudinal corrugations affecting the whole thickness of the web itself; in particular, this is due to the fact that in <CIT> and <CIT> the pressure between the rigid cylinder and the presser at the contact area (nip) thereof is very light, for example, it is of about <NUM> per linear centimeter of nip (i.e., <NUM> pounds per linear inch). Furthermore, the present invention allows to obtain paper with transverse ripples which are present also and especially at the surface which was in contact with the rigid cylinder, while in <CIT> and <CIT> the transverse ripples are present only at the surface which was in contact with the element of an elastically compressible material while the other surface (the one which was in contact with the rigid cylinder) must necessarily be smooth in order to be printed. Preferably, the longitudinally corrugated paper web, obtained by the process described herein, may then be wound in a roll, just as it has been formed, and be transferred to a user, or may be coupled with another material to form a multilayer web with particular features.

In particular, a paper web affected by transverse corrugations may be preferably applied to the web affected by the longitudinal corrugations, preferably in the same line and continuously, and this with an apparatus shown in <FIG>, which differs from a conventional apparatus, of the type shown in <FIG>, in that it carries out the coupling of a paper web <NUM>, corrugated transversely with conventional techniques, with a paper web <NUM> corrugated longitudinally with techniques in accordance with the invention.

However, it is understood that other couplings may be carried out, preferably in the same line and continuously, using differently obtained paper webs, such as, for example, a longitudinally corrugated paper web <NUM> with a flat paper web <NUM> (see <FIG>) or a longitudinally corrugated paper web <NUM> with a flat paper web <NUM> and with a subsequent transversely corrugated paper web <NUM> (see <FIG>) or a longitudinally corrugated paper web <NUM> with two flat paper webs <NUM>, <NUM>', to one of which is then applied a transversely corrugated paper web <NUM> and a subsequent flat paper web <NUM>" (see <FIG>).

Furthermore, one or both of the external flat paper webs <NUM>, <NUM>', if made of extensible paper, may be affected by drawings <NUM> or by other ornamental and/or structural motifs, obtained by stretching between embossing rollers, precisely exploiting the extensibility features of the webs themselves (see <FIG>).

Advantageously, a layer of a polymeric material may be applied, preferably continuously, to the web obtained in accordance with the invention and affected by the longitudinal corrugations <NUM> and by the transverse ripples <NUM>.

Claim 1:
A process for making a continuous web of a longitudinally corrugated fibrous material comprising the steps that a continuous web (<NUM>) of a non-corrugated fibrous material is passed between a rigid cylinder (<NUM>), which is affected on the side surface thereof by circumferential grooves <NUM> which are flanked and shaped so as to define a substantially corrugated profile reproducing the pattern of the corrugations (<NUM>) to be obtained substantially on the whole thickness of the web itself, and a presser (<NUM>, <NUM>) comprising an external layer which is of an elastically compressible material and which is maintained adherent to said rigid cylinder (<NUM>) with a force suitable for making said continuous web (<NUM>) penetrate into said grooves (<NUM>) and thus obtaining said corrugations (<NUM>) affecting the whole thickness of the web itself, and at the same time said continuous web (<NUM>) is subjected, on at least one segment of the passage thereof between said rigid cylinder (<NUM>) and said presser (<NUM>, <NUM>), to a reduction of the speed of the surface (<NUM>) thereof in contact with said presser (<NUM>, <NUM>) with respect to the speed of the surface (<NUM>) thereof in contact with said rigid cylinder (<NUM>), said force for maintaining said presser (<NUM>, <NUM>) adherent to said rigid cylinder (<NUM>) and said speed reduction between said two surfaces (<NUM>, <NUM>) of the web (<NUM>) being mutually defined so that the web (<NUM>) exiting from said passage has:
- a longitudinally corrugated pattern on both the surfaces (<NUM>, <NUM>) thereof,
- transverse ripples (<NUM>) at least at the corrugated surface (<NUM>) which was in contact with said rigid cylinder (<NUM>),
said process being further characterized in that:
- the difference between the speed of the surface (<NUM>) of said web (<NUM>) in contact with said presser (<NUM>, <NUM>) with respect to the speed of the surface (<NUM>) thereof in contact with said rigid cylinder is about <NUM>-<NUM>%,
- said force for maintaining the presser adherent to said rigid cylinder (<NUM>) is such to generate, at the contact area (nip), a linear pressure of about <NUM>-<NUM> per linear centimeter of length of the nip.