TECHNICAL FIELD

The present invention relates to a flexurally rigid sheet material, a cardboard comprising the flexurally rigid sheet material and a mould for producing a flexurally rigid sheet material. BACKGROUND

Corrugated board is frequently used in the production of boxes and packages of different types and sizes. The strength of packaging material depends on the chosen material and their structure in three dimensions. The common feature of all types of corrugated board is that they consist of one or more layers of fluted paper stuck to one layer or between several layers of flat paper; so called liners.

The corrugation is done by forming undulating flutes in one sheet. The corrugated sheet is then glued to two plain papers. Through this arrangement the bending resistance is increased in the orientation of the flutes. However, in the perpendicular orientation it is not.

It would be desirable to produce a similar type of board but with more equal strength properties in all orientations. In order to increase the strength of the packaging material in more than one orientation corrugated composites consisting of more than one layer can be used. For instance, two different corrugated sheets can be stacked on each other, the flutes being perpendicularly oriented.

Preferably the corrugated board has a "flute pattern" which maximise the minimal rigidity in any given orientation across the corrugated board obtained by one layer only. For example EP0424526 and US20040076798 are describing "flute patterns" which are improving the rigidity in different orientations, but those "flute patterns" are still showing a significant regularity which has a negative impact on maximising the minimal rigidity in any given orientation across the corrugated board. Another problem is that the corrugation of flutes creates visible stripes/lines on the liner along the flutes, for instance when printing on an assembled corrugated board. For some applications this is obviously not desired, the human eye detects such "defects" easily. Figure 1 shows a corrugated composite with one layer corrugated sheet and two "plain" papers on either side of the corrugated sheet. The bending resistance is strong in one direction, i.e. turning it 90 degrees from where it has its maximum bending resistance will lead to the bending line where it has its minimum bending resistance. One way to alleviate this problem is by combining two corrugated sheets such that the flutes of the corrugated sheets are arranged perpendicular to one another as can be seen in figure 2. Depending on desired strength of the corrugated composite additional layers could be introduced. Thus, further layers with the flutes in other angles could be added to increase the bending resistance even more. Figure 6 shows one measure of strength that is defined as the number of crossings of the ridges in various orientations. As can be seen in the figure the number of crossing varies with the angle along the different bending lines, here represented by black lines.

Figure 7 shows the bending resistance of the corrugation in figure 6. On the x-axis is the angle of the bending line and on the y-axis the bending resistance. It is apparent that along the bending line that is parallel to the y-axis, the bending resistance is at a minimum.

SUMMARY OF THE INVENTION

According to the present disclosure, a flexurally rigid sheet material is proposed comprising a "flute pattern" showing essentially equal strength properties in any given orientation. One way of creating an irregular pattern is by using mathematical transforms and to filter an initial pattern in an iterative process to obtain an irregular pattern with the desired properties.

In a first embodiment of the present disclosure, a flexurally rigid sheet material is provided comprising a two-dimensional basal plane and a multiplicity of protrusions in the basal plane; each protrusion having a height extending in a direction perpendicular to the basal plane and a two-dimensional contour having an essentially uniform width along an extension of the protrusion in the basal plane. Each two-dimensional contour has a continuous curvature. In a set of adjacent protrusions, at least one protrusion has a two-dimensional contour including a plurality of asymmetrical branches along an extension of the protrusion in the basal plane. The pattern formed by the multiplicity of protrusions provides flexural rigidity in essentially any direction of the basal plane. The disclosed pattern provides advantages over existing patterns in that the the lack of symmetry of the pattern provides for an improved bending resistance in the flexurally rigid sheet material. The bending resistance of the sheet material will be the same, or at least nearly the same, in all directions, irrespective of the orientation of the bending of the sheet.

In accordance with an aspect of the embodiment, the protrusion is a non-linear flute and transitions between the protrusions and basal plane forming ridges and grooves of the flutes are smooth and continuous.

The smooth and continuous transitions enhance the property of an improved bending resistance in the sheet material, irrespective of the orientation of the bending of the sheet.

In accordance with a further aspect of the embodiment, a distance between a top plane, defined by the uppermost points of the ridges, and the basal plane, defined by the lowermost points of the grooves is equal to or smaller than the width of the grooves.

In an example, the non-linear flutes has a pattern generated by using Fourier transform and inverse Fourier transform in an iterative process on a digital image consisting of a plurality of pixels. The iterative process comprises: Fourier transform the digital image with a filter, inverse transform the result of the filtering, set the image to the threshold result of the inverse, and repeat previous steps a set number of times. Repeating the steps of the process a set number of times will give an iterative process. The process will generate a pattern that is non-linear and non-symmetrical. By using Fourier transform, the filter can be designed so that certain wave lengths are amplified while others are suppressed.

A pattern generated by the Fourier transform process is periodic and thereby allows two patterns to be aligned and matched back to front and top to bottom. Moreover, a Fourier transform process gives that the generated pattern has no sharp corners. The flutes in the pattern are curved and the curvatures are irregular. In the pattern of the present invention all curvatures are smooth.

In a further aspect of the invention the sheet is a paper sheet or a metal sheet. Thereby the corrugated sheet can be used for different purposes depending on the desired features. Using paper gives the possibility to produce corrugated cardboard. Using paper, there smooth transitions between the ridges and grooves gives the advantage that there is little risk of creating breaks in the material during rolling of the paper sheet material. Paper has the advantages of being light weight, moderately priced, easily produced and readily available on the market.

Using metal sheets there is little risk of creating breaks in the material during embossing. Moreover, metal plates are possible to permanently deform.

In a second embodiment of the present disclosure, a cardboard material is produced comprising the flexurally rigid sheet material arranged with a liner sheet on at least one side.

The pattern of the present invention does not have the problem of the flutes creating lines visible through the liner sheet. When attaching a liner sheet to a corrugated sheet this could be done by applying an adhesive to the corrugated sheet. If the corrugated sheet has a regular pattern, for instance stripes, the pattern will be more easily detected.

Also, if printing is performed after the liner sheet has been attached to the corrugated sheet the printing will depend on the support of the flutes. Again, if the flutes have a regular pattern, the printing will be affected such that the flute pattern will appear in the printing. The non-linear and non-symmetrical pattern of the present invention fools the human eye. Hence, the surface will appear more even to the human eye. This is especially desirable when printing on the cardboard material.

In a third embodiment of the present dislosure, a mould for producing a flexurally rigid sheet material is provided. The mould includes a three-dimensional roller structure having a cylinder basal plane and a multiplicity of protrusions in the cylinder basal plane, each protrusion having a height extending in a direction perpendicular to the cylinder basal plane and a two- dimensional contour having a uniform width along an extension of the protrusion in the cylinder basal plane. The two-dimensional contour of a protrusion has a continuous curvature. The two-dimensional contour of a protrusion in a set of adjacent protrusions includes one or more branches along an extension of the protrusion in the cylinder basal plane. A sheet formed by the roller has a pattern such that the sheet exhibits flexural rigidity in essentially any direction of a basal plane of the sheet. Another aspect of the invention is that the mould comprises two mould parts, one positive and one negative, made in the form of rolls. An effect of this is that the risk of breaking the sheet material is reduced. Only one mould and a soft counter support will increase the load on some parts of the sheet material as compared to when two moulds are used which result in a more controlled load. A benefit from the two mould parts being in the form of rolls is that a continuous sheet of material is possible to produce in a continuous process.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described with the reference to the drawings. Figure 1 shows a one layer cardboard material according to prior art. Figure 2 shows a two layer cardboard material according to prior art.

Figure 3 is a partial perspective view of the structure according to the present invention. Figure 4 shows a part of a pattern of the present invention. Figure 5 shows the flowchart of the iterative process.

Figure 6 shows a graph of the number of crossed flutes in various orientations according to prior art.

Figure 7 is the measure of the bending resistance in various orientations according to prior art.

Figure 8 shows a graph of the number of crossed flutes in various orientations according to the present invention.

Figure 9 is the measure of the bending resistance in various orientations according to the present invention.

Figure 10 discloses an exemplary mould.

DETAILED DESCRIPTION

In the following a number of embodiments of the invention are shown and described.

Figure 3 shows the mould pattern for embossing. Figure 10 discloses an exemplary view of a mould having a mould pattern. The structure of the flexurally rigid sheet material is embossed by pressing one mould with the shown pattern against a flexible element with the sheet in between or by pressing two moulds together, one with a positive pattern and one with a negative pattern, or by using two rollers, one positive and one negative. The structure can also be embossed by pressing one flat mould together with one roller. Figure 3 further shows that the transitions between the grooves and the ridges are smooth.

When the sheet material is subjected to the embossing, the width and length of the resulting sheet is diminished. It is not a problem in the length orientation but the input material has to be fed to the machine faster than the embossed material exits from it. It is more difficult in the cross direction perpendicular to the movement of the material. Since the material is embossed also perpendicularly across the material, the width of the material is reduced when it exits from the machine. If the material is very rigid and not deformable there is a risk for breaks in the material despite the smoothness in the embossed pattern. In order to avoid this issue the fibres could be slightly loosened from each other, for example through application of heat and/or humidity. Another way to circumvent the problem is to use pre-micro-crepe paper. Such paper can be stretched up to as much as approximately 20%. If such paper is used with the micro-creping in the machine feeding direction the width will remain the same before and after the embossing and minimise the risk of breaks.

Figure 4 shows one example of a part of a pattern in two dimensions. The number of patterns is virtually infinite since it depends on the starting parameters for an iterative algorithm which is described in detail below. The pattern has the property to be matching back to front and top to bottom. This makes it possible to create infinitely large uniform patterns through tiling. Since the patterns have this matching property there are no visible borders between the tiles. This makes it possible to produce a roll with a continuous pattern without interruptions. The generated pattern will vary with the starting image (P) and the filter (F) settings. Figure 5 shows an example of an algorithm that generates the pattern. The inputs to the algorithm are N, P and F. N denotes the number of iterations that the algorithm must run. P is an initial starting picture in the format of a digital image comprising a plurality of pixels. The picture could be any type of animation or photographic image representing a pattern, an object, a scenery or any other type of representation implying a plurality of heterogeneous pixels in the image. The filter F is a two dimensional frequency band filter, for example created by the difference of Gaussians, a so called DoG filter. Let n be the loop counter of the algorithm. The algorithm starts by inputting the variables N, P and F. The intermediate variable Q is first set equal to P and the loop counter is set to N. Then, for each iteration n is decremented by 1 until n reaches zero and the algorithm ends. Q is Fourier transformed and filtered by multiplication by the filter F.

The result of filtering is inverse Fourier transformed and thresholded at zero. The result of this operation is a binary image with values 0 and 1 depending on the thresholding. Assuming that n has not reached zero Q is set to the thresholded result (P) subtracted by 0.5. Continuing the loop, n is decremented again and Q is Fourier transformed and filtered and P is set to the new thresholded inverse Fourier transformation. Should n now equal zero the algorithm ends and the result is equal to P, the binary result after thresholding. If n still is larger than zero the loop continues until n reaches zero.

The running variable Q approaches a stable form as the iteration continues. In the algorithm in figure 5 the iteration is not stopped until the loop counter reaches zero. However, in order to limit the number of iterations, the absolute difference between consecutive Q:s can be used instead of N for stopping when the difference has decreased to a preset value. The number of iterations, N, can be set to a wide range of values. In order for Q to stabilize it is preferred to set N to a value of one hundred or more. The higher the value the more distinct the pattern will be in terms of its structure with white and black areas. As previously described the resulting pattern has the property to be matching back to front and top to bottom. This is a property of the Fourier transform applied in the disclosed iterative process.

Figure 8 shows one measure of strength that is defined as the number of crossings of the ridges in various orientations. As can be seen in the figure there are approximately the same number of crossings along all bending lines, here represented by black lines. The main advantage of the pattern of the present invention over conventional corrugated material is the irregularity of the pattern apparent in figure 8.

Figure 9 shows the bending resistance of one pattern of the present invention depending on the orientation of the flutes relative the bending line. It is shown that the resistance is approximately the same in all orientations. Depending on the generated pattern the graph will be slightly different but the general characteristics of the graph will be the same.

In one embodiment of the invention the filter is a frequency band pass filter. Possibly a low pass filter and/or high pass filter can be used. In another embodiment the distance between a top plane, defined by the uppermost points of the ridges, and a bottom plane, defined by the lowermost points of the grooves is equal to or wider than the width of the grooves.

A three dimensional printer can be used as one step in the manufacturing of the moulds.

Figure 10 discloses an exemplary mould 100 that may be used in the forming of the corrugated sheet. The mould could have a variety of sizes, from the size disclosed by the image of Figure 10, to a size suitable for introduction for embossing in paper mill production. The mould includes an imprinted pattern 110 generated according to the above described method and applied to the mould. For an application in paper mill production, the relationship between the dimensions of the mould pattern and the dimension of the roller mould will differ significantly compared to what is disclosed in exemplary Figure 10. The mould has a three-dimensional roller structure having a cylinder basal plane and a multiplicity of protrusions in the cylinder basal plane, each protrusion having a height extending in a direction perpendicular to the cylinder basal plane and a two-dimensional contour having a uniform width along an extension of the protrusion in the cylinder basal plane. The protrusions on the mould form a pattern 110, wherein each two-dimensional contour in the pattern has a continuous curvature. A protrusion in a set of adjacent protrusions has a two- dimensional contour including one or more branches along an extension of the protrusion in the cylinder basal plane. Moulding of a sheet material with use of the mould, possible in cooperation with a mould having an inverted pattern, results in a flexurally rigid sheet material exhibiting flexurally rigidity in essentially any direction of a basal plane of the sheet.

The invention is not limited to the specific embodiments presented, but includes all variations within the scope of the present claims. For instance, the number of iterations can be varied and the iterative filtering can be done using Fourier methods, other mathematical transform methods or direct filtering using convolution. Accordingly, the drawings and the description thereto are to be regarded as illustrative in nature, and not restrictive.

Further, the terms "cardboard" and "paper" are considered to encompass all different kinds of board, paperboard, pasteboard, composition board and cartonnage. The terms "one positive" and "one negative" refers to the two parts of the mould that are compatible and fits together when placed with the mould side against each other.