The present invention relates to a method for binding a bundle of sheets of paper, for example to form a book or similar, in particular to obtain a book or folder whose sheets can be opened flat and the printing runs seamlessly across the two open sheets. This last-mentioned is particularly desired for composing books of photographs, magazines and similar with photographs and illustrations that run across the entire width of the open book.

A method as described in BE 2012/0759 and BE 20113/0014 is already known, whereby an edge of the bundle of sheets of paper is introduced into the binding spine and fastened therein, whereby for the binding use is made of a bundle of sheets where a strip of each sheet is double folded separately along the same line to form a fold line that extends parallel to and at a distance from the aforementioned edge.

Double folded here means that the strip is first folded in the one direction and then folded in the other direction.

An advantage of such a method is that the fold line enables the sheets to be folded along this fold line when the book is opened and that when the bundle of sheets is bound in the spine with their sharp fold lines against one another, the sheets can be opened flat and the printing can continue across two adjacent fold lines as good as seamlessly across opened sheets.

Another advantage is that when the bundle is opened there is no gap between the opened sheets, so that the adhesive or stitching or staples that are used to bind the bundle in the spine are not visible and thus do not interfere with the printing that continues from the one sheet to the other sheet .

However, practice has shown that although such a method works well and can bring about the desired advantages for paper with a weight of 120 grams or 140 grams per square metre, the results for paper with a weight of less than 120 grams per square metre are not so good.

This means that the method will yield very good results for thicker sheets, but the results are not so good for thinner sheets. More specifically the opening of such sheets along the fold line in a bundle will proceed less smoothly. As a result it will often be possible that the pages do not lie perfectly flat.

The purpose of the present invention is to provide a solution to this.

To this end the invention concerns a method for binding a bundle of sheets of paper, whereby an edge of the bundle of sheets is bound, whereby for binding use is made of a bundle of sheets in which a strip of each of the sheets is double folded separately beforehand along the same line to form a fold line that extends parallel to and at a distance from the aforementioned edge, whereby according to the invention, for a sheet of paper with a density of less than 120 grams per square metre, after having been double folded the strip is folded at least once more along the aforementioned fold line.

The advantage of such a method is that by additionally folding the sheet, the results are better and comparable to a thicker sheet with a weight of 120 grams or 140 grams per square metre.

Note that the problem and the solution to it is not obvious to a person skilled in the art and in principle even goes against intuition.

Indeed, as practice has shown that the known method gives the desired result for thicker sheets, a person skilled in the art would be least of all inclined to fold a thinner sheet once again.

Nevertheless such a method according to the invention ensures that the desired result is achieved for sheets with a weight of less than 120 grams per square metre.

The invention also concerns a bundle of sheets of paper, whereby a strip of each of the sheets is double folded separately along the same line to form a fold line that extends parallel to and at a distance from an edge, whereby for a sheet of paper with a weight of less than 120 grams per square metre, after having been double folded the strip is folded at least once again along the aforementioned fold line .

Furthermore the invention also concerns a book or folder with a binding spine and a bundle of sheets bound therein, whereby the bound bundle is a bundle according to the invention .

With the intention of better showing the characteristics of the invention, a preferred method for binding a bundle of sheets according to the invention is described hereinafter by way of an example, without any limiting nature, with reference to the accompanying drawings, wherein: Figures 1 to 7 schematically show successive steps of a method according to the invention;

figures 8 to 10 schematically show the bundle of figure 7 when leafing through it;

figure 11 schematically shows a view according to the arrow Fll in figure 7;

figure 12 schematically shows the folded sheets of figure 11, but in a separated state;

figure 13 schematically shows an alternative embodiment of figure 12;

figure 14 schematically shows a variant of figure 7.

Figures 1 to 4 schematically show a method for producing a double-folded sheet of paper 1. To this end an edge 2 of a sheet 1 is folded in one direction along a fold line 3 that is parallel to the edge 2 and at a distance A therefrom, as can be seen in figure 2. In this way a strip 4 is formed that is demarcated by a fold line 3. Then the strip 4 is folded in the other direction along the sharp fold line 3 as can be seen in figure 3.

In this way a double-folded sheet 1 is made. Preferably the strip 4 for forming a sharp fold line 3 is completely folded back twice. In this way the fold line 3 forms a flexible hinge as it were, along which the sheet 1 can be flexibly turned back and forth with respect to the strip 4.

The distance A between the edge 2 and the fold line 3 is equal to the width of the strip 4. The distance A is preferably between 10 and 24 millimetres. According to the invention, when the sheet has a weight of less than 120 grams per square metre, the strip 4 is folded again as shown in figure 2.

Additionally, but not necessarily, the strip 4 can be folded again in the other direction, as shown in figure 3.

In this way the strip 4 is double folded twice.

Finally, as shown in figure 4, the strip 4 is folded back to be in line with the sheet 1. However, it is not excluded that the strip 4 is folded again a number of times. Indeed, depending on the type of paper and the weight of the paper the strip 4 is folded back as many times until the desired result is obtained, this means until the sheet 1 can be easily and readily folded at the location of the fold line 3.

Figures 5 to 7 schematically show a method for binding a bundle 5 of double-folded sheets 1.

In the following double-folded sheets 1 also means sheets 1 that, after having been double folded, are folded again at least once as described above. As can be seen in figure 5, the bundle 5 consists of a number of double-folded sheets 1 that are placed with their sides against one another, whereby the edges 2 of the sheets 1 to be bound are aligned with respect to one another .

Preferably the bundle 5 comprises at least one sheet 1 with a density of less than 120 grams per square metre.

According to the invention it is not excluded that the sheets 1 have a different density. For example, some of the sheets 1 can have a density of less than 120 grams per square metre and some of the sheets 1 have a density greater than or equal to 120 grams per square metre. Hereby in the example shown the distance A is the same for all sheets. However, it is also possible for the sheets 1 of the bundle 5 to be double folded at different distances from the edge 2 to be bound, whereby the first sheet is folded at a distance A, the second sheet at a distance A plus approximately the thickness of one sheet 1, the third sheet at a distance A plus approximately twice the thickness of one sheet and so on until the middle of the bundle, after which the distances decrease symmetrically such that the last sheet 1 of the bundle 5 is folded at a distance A from the edge.

In order to bind the bundle 5, in this example use is made of a binding spine 7.

In this case the binding spine 7 consists of a U-shaped part 8, for example of metal with a layer of hot-melt adhesive 10 on the base 9.

In this case, the binding spine 7 is further provided with two endpapers 11 of a book or folder, for example of cardboard that are provided on either side with a covering 12.

It is clear that the binding spine 7 can also take on other forms such as a V-shaped binding spine for example or can only be provided with one endpaper 11.

The bundle 5 is placed in the U-shaped part 8 of the binding spine 7, whereby the edges 2 of the double-folded sheets 1 are placed against the layer of hot-melt adhesive 10, as shown in figure 6. Hereby the fold line 3 of the double-folded sheets 1 is somewhat above the U-shaped part 8. This occurs because the distance A primarily corresponds to the depth of the binding spine 7 or is somewhat greater than the depth of the binding spine 7.

Then the layer of hot-melt adhesive 10 is heated by means of a heat source 13. The hot-melt adhesive 10 will hereby melt and become liquid, such that the double-folded sheets 1 of the bundle 5 can be introduced therein.

After the hot-melt adhesive 10 has solidified, the bundle 5 is attached to the bundle 7 by the hot-melt adhesive 10. Then, as can be seen in figure 7, the arms 14 of the U- shaped part 8 of the binding spine 7 are squeezed together. This will ensure an extra sturdy fixation of the bundle 5 in the binding spine 7 and the close fit of the bundle 5 against the free edges of the arms 14 without a gap being visible between the two.

It is also clear that the squeezing shut of the arms 14 of the U-shaped part 8 of the binding spine 7 can be omitted. The bound bundle 5 is now finished into a folder with bound sheets 1.

As can be seen in figure 7, upon opening the bundle 5 the double-folded sheets 1 will extend in a single plane. By applying a method according to the invention, not only will the thicker sheets with a weight of 120 or 140 grams per square metre for example, but also thinner sheets with a weight of less than 120 grams per square metre, give the desired result.

In reality the result is better than shown in the drawings, which give a somewhat distorted picture because a certain thickness has been given to the sheets for clarity.

Figures 8 to 10 schematically show how the U-shaped part 8 of the binding spine 7 will turn while being leafed through . When an endpaper 11 is opened, it is will be able to be folded to against the arm 14 of the U-shaped part 8, as can be seen in figure 8.

When leafing through further, the arm 14 of the U-shaped part 8 will move away from this endpaper 11 because the U- shaped part 8 will turn according to arrow B towards the other endpaper 11. This is shown in figure 9.

When leafing through further, as shown in figure 10, the U- shaped part 8 of the binding spine will turn further until the other arm 14 of the binding spine 7 is finally against the other endpaper 11.

This turning of the binding spine 7 will be done automatically as it were while leafing through the bundle 5 and will ensure that the sheets 1 extend flat when the bundle 5 is opened at a certain page.

As presented in figure 11, the sides 6 of the double-folded sheets 1 are printed, for example with a part of an illustration 15 or photograph, whereby the illustration 15 as good as seamlessly runs across from the one double- folded sheet 1 to the other double-folded sheet 1 to thus form one continuous illustration. In this way large photographs can extend over two sheets 1 in a photo album and this without an interfering gap between the two.

Figure 12 shows two printed double-folded sheets, whereby after binding the printing 15 on both sheets 1 together will form a whole.

Hereby the printing 15 is printed up to the fold line 3 whereby the strip 4 remains unprinted. Figure 13 shows a variant of this, whereby a part 16 of the strips 4 are also printed with an overlapping part of the printing 15.

Figure 14 shows a variant, whereby the bundle 5 is bound by making use of a file folder. Such file folders are produced by Leitz® for example. To this end the sheets 1 are provided with the necessary perforations and the distance A is preferably equal to 20 mm. It is clear that in the example of figure 5 the hot-melt adhesive 10 in the binding spine 7 can be omitted and that the bundle 5 can be stapled before being placed in the binding spine 7.

It is also clear that it is possible that, in order to bind the bundle 5 of sheets 1, the bundle 5 is stapled at a distance from the edge 2 that is smaller than the distance A without making further use of a binding spine 7. It is clear that it is not excluded that for the method according to the invention use is made of a bundle 5 of sheets 1 of a different thickness, whereby for the sheets 1 with a greater thickness the strip 4 is double folded and for the sheets with a smaller thickness the strip 4 is then folded or double folded again.

It is clear that it is not excluded that the method according to the invention is also applied for sheets (1) with a density of 120 grams per square metre or greater.

The present invention is by no means limited to the embodiments described as an example and shown in the drawings, but a method for binding bundles of sheets according to the invention can be realised in different variants without departing from the scope of the invention.