Method and apparatus for producing corrugated cardboard sheets and webs

Abstract

Method of producing corrugated cardboard sheets and strips characterized in that a sheet or strip of paper (2) extensible at least longitudinally from 6% to 70%, preferably from 10% to 30%, is passed between a cylinder (6), provided on the entire lateral surface with a plurality of teeth developing parallel to the axis of the cylinder itself, and a contrast member (4,10,12), which moves simultaneously with said cylinder (4) so that said sheet or said tape (2) is dynamically held in the stretch between at least two transversal bands held by teeth of said cylinder and by cooperating portions of said contrast member and is at the same time subjected to longitudinal stretching to be conformed to the path imposed by each tooth.

Description

[0001] METHOD AND APPARATUS FOR PRODUCING CORRUGATED CARDBOARD SHEETS AND WEBS.

[0002] The present invention concerns a method and apparatus for producing corrugated cardboard sheets and webs.

[0003] The so-called corrugated cartons are known in the technical packaging sector, which generally comprise a first sheet of paper which is corrugated, i.e. provided over its entire surface with a sequence of parallel waves having constant pitch and amplitude, and a second sheet ( liner) of smooth paper, glued to the crests of the waves of the first sheet and essentially having the function of stabilizing them and possibly constituting a printing support.

[0004] Corrugated cardboards are also known which, instead of having a single liner, have two, glued to both sides of the corrugated paper sheet, and multilayer corrugated cardboards are also known, i.e. made up of several corrugated sheets, interspersed with the same number of liners.

[0005] The materials with which the corrugated sheets and liners are made are traditional and include virgin fiber papers, semi-chemical pulp papers, recycled papers and their mixes, etc.

[0006] The techniques for making corrugated sheets are also traditional and involve passing a ribbon or a sheet of normal paper between two counter-rotating cylinders provided with parallel teeth that mesh with each other and are capable of transforming the flat sheet into a sheet provided with a plurality of parallel corrugations which are then stabilized in their shape following the gluing of the liner.

[0007] A drawback of the known corrugated cardboard consists in the fact that the corrugated sheets that form them must have the longitudinal dimension, i.e. the dimension orthogonal to the axis of the corrugations, significantly greater than the corresponding dimension of the corrugated cardboard obtained, since the corrugations require a quantity of material, the greater the amplitude of the waves and the smaller their pitch. By way of example, a corrugated cardboard with waves having a width from crest to valley equal to 2.48 mm and a pitch equal to 6.51 mm can result in a length of the sheet to be corrugated equal to approximately 130% of the length of the corrugated cardboard obtained, and this greater length constitutes the so-called take up factor , which in the example referred to is equal to 1.30.

[0008] Another drawback of the known corrugated cardboard is linked to the method used to make the corrugated paper sheet. This method, in fact, provides, as it has been said, that a sheet of smooth paper is passed between two parallel cylinders, provided with a plurality of teeth which develop parallel to the axis of the cylinders themselves over their entire axial length and they mesh with each other during their rotation. In this way the sheet of paper is dragged to arrange itself between the teeth which mesh together and to assume the wavy shape imposed by them. However, from the moment in which the sheet of paper begins to be engaged between homologous teeth of the two cylinders, to the moment in which it ceases to be engaged by them, it is also subjected to a longitudinal stretching of an amount linked to the height of the cylinders and to the number of pairs of teeth that simultaneously hold the sheet of paper and that, with the rotation of the two cylinders, distance the gripping lines of the latter. If this stretching exceeds the values of 1 % - 3%, which are the typical values of the elongation of normal paper, it causes tearing of the same and in addition to creating a defective product, and therefore to be discarded, it involves expensive machine downtime for restore its correct functioning.

[0009] In order to prevent this stretching from causing tearing of the sheet of paper, various measures have been suggested up to now. One of these consists in avoiding feeding the two cylinders with paper kept taut, as this tension would inevitably lead to an increase in the stretching of the paper between the teeth of the cylinders. However, feeding the cylinders with unstretched paper results in irregular feeding with equally irregular and unsatisfactory formation of corrugations.

[0010] In order to avoid this inconvenience, it has also been proposed to preliminarily subject the sheet of paper to be corrugated to wetting and preheating to make it mouldable and therefore capable of reducing the risk of tearing as it passes between the toothed cylinders. This is a measure that has proven useful, even if not decisive, but which has led to plant complications with a consequent increase in production costs.

[0011] Another well-known measure to reduce the risk of tearing the sheet of paper during the corrugation phase consists in creating the two toothed cylinders with teeth of limited height. This measure also reduced the risks of tearing the sheet of paper, but led to obtaining waves of limited height and therefore corrugated cardboard of limited thickness and unsuitable, as such, for use in situations in which a high thickness is required of corrugated cardboard.

[0012] Another well-known measure consists in limiting the number of teeth of the two toothed cylinders engaged at the same time. Also in this way it was possible to reduce the risk of lacerations but, to obtain a limited number of teeth engaged at the same time, it was necessary to use cylinders with teeth sufficiently spread apart, which is only possible if at least one of the two toothed cylinders has a small diameter. This solved the problem to some extent, although the limited diameter of the cylinder and its long length resulted in its limited resistance to bending, in particular if one takes into account that the two toothed cylinders must be pressed together with a certain force , which may be incompatible with the stresses involved. For this reason it has already been proposed to counteract these possible flexions with a third cylinder which has a diameter substantially equal to that of the corrugating cylinder with a larger diameter and is coupled to the corrugating cylinder with a smaller diameter on the opposite side to that coupled with the corrugating cylinder with a larger diameter. However, this third cylinder, in addition to complicating the construction of the system, also causes interference between the cylinders with consequent vibrations, noise and wear.

[0013] Another drawback of traditional methods of producing corrugated cardboard sheets is linked to the instability of the waves obtained and the consequent need to stabilize them immediately after their formation with the application of the liner. In practice, immediately downstream of the two toothed cylinders, a roller applies thin bands of glue on the top of the waves obtained, on which the liner is then applied. This, however, resulted in a certain operational slowness, due to the need to wait for the glue to dry, and therefore resulted in a low production and storage speed of the corrugated cardboard.

[0014] Another drawback of traditional corrugated cardboard production methods consists in the fact that when the sheet of paper that generates the corrugated sheet is made to adhere to the teeth of the forming toothed cylinders, it is subjected to a sudden deviation, which is compacted on the internal side by at least 7% and on the external side it is lengthened to a substantially similar extent, as highlighted for example in and this requires that the paper be softened beforehand to avoid the negative effects of tearing on the external side and stretching on the internal side.

[0015] The aim of the invention is to produce corrugated cardboard without having to incur the drawbacks just mentioned.

[0016] In particular, an object of the invention is to produce sheets of corrugated cardboard with a take up factor much lower than that currently imposed by known corrugated cardboards, and more particularly less than 20% and preferably less than 15%.

[0017] Another aim of the invention is to produce sheets of corrugated cardboard without having to subject the sheets of paper to be corrugated to preliminary wetting and wetting treatments of preheating.

[0018] Another aim of the invention is to produce sheets of corrugated cardboard by eliminating the risk of tears in the paper sheet during the formation of the corrugations and in this way significantly reducing both machine downtime, inevitably linked to such tears, and processing waste. Another aim of the invention is to produce sheets of corrugated cardboard with the possibility of creating corrugations of a width greater than that of current corrugated cardboard.

[0019] Another aim of the invention is to produce corrugated cardboard sheets which ensure the stability of the corrugations even in the absence of liners.

[0020] Another aim of the invention is to produce sheets of corrugated cardboard with a production speed higher than the current ones and in any case not lower than 300 - 450 meters per minute (mpm), i.e. the usual production speeds of paper tapes with traditional paper machines, paper.

[0021] Another aim of the invention is to produce sheets of corrugated cardboard with simpler systems than traditional ones.

[0022] These aims and others which will result from the following description can be jointly or separately achieved according to the invention with a method of producing corrugated cardboard sheets as defined in claim 1 and with an apparatus as defined in claim 9.

[0023] The present invention is further clarified below in some of its preferred practical embodiments reported for purely illustrative and non-limiting aims with reference to the attached drawings, in which: figure 1 shows a schematic longitudinal section of a method of forming a corrugated paper ribbon in a first embodiment, figure 2 shows it in the same view as fig. 2 in a second embodiment, e figure 3 shows it in the same view as fig. 1 in a third embodiment.

[0024] In this description we will talk indifferently about sheets of paper or paper tapes, given that the production of corrugated cardboard can involve both sheets of defined length, which can then also be used individually, and sheets of indefinite length, which are then cut to size, based on usage needs.

[0025] In the embodiment illustrated schematically in fig. 1 the method according to the invention involves feeding a paper tape 2 having an extensibility at least in the longitudinal direction, i.e. in the direction of advancement and processing of the tape itself, between 6% and 70% and preferably between 10% and 30%, to a pair of counter-rotating parallel cylinders 4.6, made of substantially rigid material, for example steel, and each provided with a plurality of teeth developing parallel to the axes of the cylinders themselves and interpenetrating each other, so as to be able to engage the paper ribbon 2 to drag it forward and at the same time to shape it in accordance with the wavy path defined between their teeth. In this phase the belt 2 is subjected to a longitudinal stretching, and therefore to a plastic deformation, as it is gripped by pairs of teeth of the two cylinders 4,6 in correspondence with several transversal bands, which following the rotation of the themselves are not fixed but move, spacing the bands themselves apart as the ribbon passes from its initial flat configuration to its final wavy configuration.

[0026] However, thanks to the extensibility of the ribbon itself, this stretching does not lead to any tearing, but rather to its plastic deformation which also performs the important function of stabilizing the shape of the corrugations. Therefore, the corrugated band 2' coming out of the two cylinders 4,6 is provided with a stable transverse corrugation, of substantially constant width and pitch and corresponding to the width and pitch of the teeth of the two forming cylinders 4,6.

[0027] The localized stretching of the belt 2 during its passage between the two cylinders 4,6 is evidently due to the multiple dynamic contact of the belt itself with the pairs of teeth simultaneously meshed with each other, and the number of teeth of each cylinder simultaneously meshed with teeth of the other cylinder is larger the greater the diameter of the cylinders 4.6, with the same dimensions of the teeth. Furthermore, unlike the toothed cylinders used according to the traditional technique, the toothed cylinders used according to the present invention, thanks to the use of the longitudinally extensible paper tape, are practically not subject to any limitation in their diameter.

[0028] In some cases it is preferable for the belt 2, with which the toothed cylinders 4,6 are fed, to be subjected to retention, which in this way favors the entry of the belt itself between the teeth and a more regular formation of the corrugations.

[0029] The fact that according to the present invention this phase of formation of the corrugations occurs by longitudinal stretching of the paper tape 2 involves a substantial modification of the traditional technique, in the sense that, while according to the traditional technique the paper tape is folded and accumulated by the teeth which with the rotation of the two cylinders dynamically impose the path of the tape itself, according to the present invention the paper tape is folded and stretched longitudinally. An important effect of this difference is that, while according to the traditional technique the corrugations of the paper tape are unstable and can only be stabilized with the immediate application of the liner or liner, with the method according to the present invention the corrugations are stable and do not require any immediate application of the liner.

[0030] In turn, the elimination of the need to apply the liner immediately after the formation of the corrugations leads to a significant increase in the production speed of the tape itself, given that it is no longer necessary to wait for the glue that holds the liner to the tape to dry. corrugated, and the production line of this can be exploited more intensively.

[0031] In the embodiment illustrated in fig. 2 the extensible paper tape 2 is fed to a pair of cylinders 6,8, of which the lower cylinder 6 is substantially rigid, for example because it is made of steel, and is provided on its entire lateral surface with teeth, while the cylinder upper cylinder 8 is covered by a layer 10 of elastically yielding material, in particular natural or synthetic rubber, into which the teeth of the lower cylinder 6 can sink.

[0032] The two cylinders 6,8 are made to rotate substantially with the same peripheral speed and in a discordant direction, and with their rotation they drag the longitudinally extensible paper ribbon 2, which is placed between them, and at the same time they subject it to deformation for stretching, which is also accentuated by the greater friction of the paper tape with the layer of elastically yielding material 10. Therefore, at the exit of the cylinders 6,8 the tape 2' is provided with a plurality of transversal undulations, which thanks to the with which they were obtained are intact and stable.

[0033] In the embodiment illustrated in fig. 3, the longitudinally extensible paper ribbon 2 is fed to a transversal corrugation forming unit, comprising a lower cylinder 6 made of rigid material, preferably steel, and provided with a plurality of transverse teeth, and an upper continuous belt 12, made of elastically yielding material, in particular natural or synthetic rubber, into which the teeth of the lower cylinder 6 can sink.

[0034] In this case the elastic carpet, in addition to cooperating with the upper cylinder to form the transverse corrugations by stretching the longitudinally extensible paper tape and in addition to creating greater friction between paper and rubber, also involves less wear of the elastic layer.

[0035] In all the embodiments described, the paper tape 2 can be made up of any type of long or short fiber paper or their mixtures, semi-chemical paper or recycled paper, provided that it is extensible at least in the machine direction from 6% to 70%. and preferably from 10% to 30%. Depending on the processing technique adopted, the paper ribbon 2 can be smooth or provided with surface scratches, which were formed during the production phase of the ribbon itself and which will evidently also be present in the corrugated paper ribbon 2' obtained.

[0036] The following examples, referring to the first embodiment of the invention illustrated in fig. 1 , will further clarify the advantageous differences of the same compared to the state of the art.

[0037] Example 1

[0038] A pair of 4.6 cylinders having equal diameter of 350 mm with 181 teeth (height of the tooth 4.3 mm, width of the tooth 3.61 mm at the base and 1 .31 mm at the top) was fed with sheets of paper 2 having a longitudinal extensibility coefficient (in the machine direction) of 20%. These teeth were chosen with dimensions much larger than those of the teeth used for the production of traditional corrugated cardboard with a take up factor of approximately 1.75. The sheets of paper 2 having a length of 50 cm were only shortened by 5 cm and therefore by approximately 10%. Furthermore, the corrugations obtained were stable even without the application of liners.

[0039] Example 2

[0040] A pair of 4.6 cylinders with an equal diameter of 650 mm and 314 teeth (pitch 6.5 mm and height 2.6 mm, wave type B, according to the traditional terminology defined for example inb^Ps; / ^^^...:^eypgPgg^iS^D:de / wissgr weypappg / bauprjnzip / brweNe.html ) was fed with sheets of paper 2 having a longitudinal extensibility coefficient of 20%.

[0041] The take-up factor of a standard paper would have been equal to 1.32, i.e. the standard paper would have undergone a shortening of 32%. However, the shortening of the stretchable paper sheets 2 was only 5% and led to obtaining corrugated sheets 2' with stable corrugations even without liner.

[0042] Example 3

[0043] A pair of cylinders 4.6 with an equal diameter of 650 mm and with 314 teeth (pitch 6.5 mm and height 2.6 mm, wave type B) was fed with sheets of paper 2 having a longitudinal extensibility coefficient of 6%. The take-up factor with a standard card would have been equal to 1.32, meaning this would have been shortened by 32% compared to its original length. The sheets of stretchable paper 2, on the other hand, only shortened by 10% ( Take up factor = 1.10).

[0044] Example 4

[0045] A pair of cylinders 4.6 with an equal diameter of 408 mm and 204 teeth (pitch 6.29 mm and height 2.6 mm, wave type B) was fed with sheets of paper 2 having a coefficient of longitudinal extensibility of 35%. The take-up factor with a standard card would have been equal to 1.35, meaning this would have been shortened by 35% compared to its original length. The sheets of stretchable paper 2 did not shorten.

[0046] Example 5

[0047] In a pair of traditional cylinders for the production of corrugated cardboard with a corrugation height of 2.2 mm and a pitch of 4.8 mm, and more specifically in a pair of toothed cylinders, one of which has a diameter of 325 mm and the other diameter of 540 mm, the cylinder with a smaller diameter was replaced with a cylinder with a diameter of 540 mm. In this case the interpenetration of the teeth is increased to 7 teeth. The pair of cylinders was fed with sheets of paper 2 having a longitudinal extensibility equal to 8% and no tearing of the paper was found. Furthermore, using the same motor that drove the smaller diameter cylinder (with angular speed equal to 294 rpm and peripheral speed equal to 300 mpm ) and which after the replacement drove the new cylinder at the same angular speed (with peripheral speed of almost 500 mpm ) an hourly production was obtained which, in addition to being waste-free, was approximately 65% higher, and the further advantage of eliminating any bending of the smaller diameter cylinder was also obtained.

[0048] Example 6

[0049] In order to be able to use corrugated cardboard, which in itself can be defined as substantially two-dimensional, to obtain three-dimensional shapes (containers, trays, tubs, etc.), for example with traditional embossing methods, it may be advantageous to achieve this with paper extensible both longitudinally and transversally. In this case it might be preferable that the transversal corrugations of the corrugated cardboard do not continuously affect the entire width of the sheet or paper ribbon and therefore that the teeth of the forming cylinder or cylinders do not affect the entire axial length of the cylinders themselves , and can have the more general form of reliefs obtained on one cylinder, which correspond to complementary cavities obtained on the other cylinder, if the apparatus includes two coupled metal cylinders.

[0050] In a configuration of this type, a pair of equal 4.6 metal cylinders with a diameter of 650 mm, provided with reliefs and complementary cavities, were fed with a sheet of paper having a longitudinal extensibility of 20% and a transversal extensibility of 12%. It was achieved with a take up factor essentially the same as 1.0 a sheet of cardboard provided over its entire surface with a plurality of corrugations of reduced length but stable shape.

Claims

C L A I M S1 . Method of producing corrugated cardboard sheets and strips characterized in that a sheet or strip of paper (2) extensible at least longitudinally from 6% to 70%, preferably from 10% to 30%, is passed between a cylinder (6), provided on the entire lateral surface with a plurality of teeth developing parallel to the axis of the cylinder itself, and a contrast member (4,10,12), which moves simultaneously with said cylinder (4) so that said sheet or said tape (2) is dynamically held in the stretch between at least two transversal bands held by teeth of said cylinder and by cooperating portions of said contrast member and is at the same time subjected to longitudinal stretching to be conformed to the path imposed by each tooth.

2. Method according to claim 1 characterized in that at least one cylinder (6) is used which is provided on the entire lateral surface with a plurality of teeth extending continuously over the entire axial length of the cylinder itself.

3. Method according to claim 1 and / or 2 characterized in that the stretching of said paper sheet or ribbon (2) is promoted by breaking its advancement caused by the engagement of the latter by said teeth and said contrast member (4,10,12).

4. Method according to one or more of the previous claims characterized in that said sheet or ribbon of paper (2) is passed between a pair of counter-rotating toothed cylinders (4,6).

5. Method according to one or more of claims 1 to 3 characterized in that the sheet or ribbon of paper (2) is passed between a cylinder (6) provided with a plurality of teeth and a roller (8) having a layer external (10) of elastically yielding material with a thickness equal to at least the height of said teeth.

6. Method according to one or more of claims 1 to 3 characterized in that the sheet or ribbon of paper (2) is passed between a cylinder (6) provided with a plurality of teeth and a continuous belt (12) made of elastically yielding material and having a thickness equal to at least the height of said teeth.

7. Method according to claim 5 and / or 6 characterized in that said elastically yielding material is constituted by natural or synthetic rubber.

8. Method according to one or more of the previous claims characterized in that a sheet or ribbon of paper (2) which is extensible by at least 6% also in the transversal direction is used.

9. Apparatus for producing corrugated cardboard sheets and strips characterized by comprising:- a cylinder (6) provided on its entire lateral surface with a plurality of teeth developing parallel to the axis of the cylinder itself,- a contrast member (4,10,12) movable in synchronism with the teeth of said cylinder (6), so as to have at least two of these teeth dynamically engaged with said contrast member to define a wavy passage having a substantially compliant trend to the corrugations to be formed in said sheet or strip (2), - means for feeding to said cylinder (6) and to said contrast member (4,10,12) a sheet or ribbon of paper (2) which can be extended at least longitudinally.

10. Apparatus according to claim 9 characterized in that it also includes means for retaining said sheet or ribbon of paper which must feed said toothed cylinder (6) and said contrast member (4,10,12).

11. Apparatus according to claim 9 characterized in that said contrast member is constituted by a roller (8) covered by an external layer (10) of elastically yielding material having a thickness not less than the height of the teeth of said toothed cylinder (6).

12. Apparatus according to claim 9 characterized in that said contrast member is constituted by a continuous belt (12) made of elastically yielding material and having a thickness not less than the height of the teeth of said toothed cylinder (6).

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