EP2123567A1

EP2123567A1 - Envelope for laser printers and process for the manufacturing of the envelope

Info

Publication number: EP2123567A1
Application number: EP08425365A
Authority: EP
Inventors: Mauro Franco Marco Rovelli; Milko Rovelli; Marco Rovelli
Original assignee: Sacchettificio Monzese Srl
Current assignee: Sacchettificio Monzese Srl
Priority date: 2008-05-22
Filing date: 2008-05-22
Publication date: 2009-11-25
Anticipated expiration: 2028-05-22
Also published as: ATE470631T1; DE602008001504D1; EP2123567B1

Abstract

An envelope for laser printing is described. The envelope comprises a first paper sheet having a closure flap of the envelope along a folding line and it comprises a window to view the envelope contents, wherein the window perimeter comprises a first segment and a second segment which extend lengthwise in the direction of the folding line. The envelope further comprises a substantially transparent second paper sheet to close the window, the first and second sheets defining an overlapping region which surrounds the window and in which they overlap. The envelope further comprises a glue layer to glue the first and second sheets, the glue layer being applied to part of the overlapping region around the window perimeter so as to surround the window. The overlapping region comprises a first glue-free region defined between the first segment of the window perimeter and the glue layer surrounding the first segment and comprises a second glue-free region defined between the second segment of the window perimeter and the glue layer surrounding the second region; the width of the first glue-free region is greater than the width of the second glue-free region. Advantageously the width of the first glue-free region is 3 mm or more.

Description

Technical field of the invention

The present invention relates to an envelope and particularly the present invention relates to a window envelope suitable for laser printing and to a manufacturing process of such envelope.

Prior art

Figure 1 shows an open envelope 1 according to the prior art where the closure flap 20, the side flaps 11 and 12 and the lower flap 21 are open. The envelope 1 comprises a paper sheet 2, a substantially rectangular shaped window 4 closed by a transparent sheet 3 in order to view its contents when the envelope is closed and a glue layer 5 to glue the transparent sheet 3 onto the paper sheet 2. The central portion of the paper sheet 2 is the inside face of the envelope 1 when side flaps 11, 12 and lower flap 21 are folded so as to form a closed envelope 1. For example, the window 4 is used to allow the reading of the addressee's address written on a sheet inserted into the closed envelope 1. The transparent sheet 3 is made of plastic material and the generally transparent sheet 3 consists of a material formed of polystyrene beads.
Figure 2 shows the window region of the inside face of the envelope 1 of figure 1 in greater details. The paper sheet 2 and the transparent plastic sheet 3 overlap along an overlapping region around the window 4; a glue layer 5 is applied on a portion of this overlapping region (shown diagonally hatched) along the window perimeter surrounding it in order to glue the transparent plastic sheet 3 to the paper sheet 2. The width L1, L3, L5, L7 of the portion of the overlapping glue-free region defined between the long side 6, the long side 8, the short side 7, the short side 9 of the window 4 and the glue layer 5 is 1 millimetre ("millimetre" hereinafter shortened in "mm") respectively; the width L2, L6, L8 of the glue layer 5 along the long side 6, the short side 7, the short side 9 of the window 4 is 7 mm. respectively, the width L4 of the glue layer 5 along the long side 8 of the window 4 is 8 mm.
Nowadays black-and-white laser printers are very popular and colour laser printers for printing documents in small amounts are quickly widespreading as their costs have greatly reduced.
It is useful to print messages (particularly colour messages) on a transparent window envelope making use of laser printers. For example, an envelope may be used to print (either in colour or black-and-white) ads, the sender's logo or events the sender will take part in. Moreover printing by means of laser printers available on the market allows the message printed on the window envelope to be easily changed.
Therefore in order to be able to print on a transparent window envelope a laser printer is required.
Colour laser printers operate at an internal high temperature (between 120°C and 180°C, for example) and plastics melts at these temperatures. Therefore a plastic window envelope cannot be printed using a laser printer.
Envelopes provided with windows made up of transparent paper, where a transparent paper window is used (instead of a transparent plastic window), so as to prevent the transparent paper from melting due to the high operating temperature of the laser printer are known. This known solution has the drawback of damaging the envelope during laser printing. Actually during the printing, the envelope is drawn by the laser printer rollers and undergoes a compression process and a stretching process; consequently the first creates scratches on the envelope paper sheet (especially near the window sides) while the second creates crimps on the window transparent paper (therefore the crimps are visible from the outside of the envelope which looks damaged).
The Applicant has understood that the reason for the scratches on the envelope paper sheet might be due to the fact that, during the manufacturing process of the envelope, air builds up inside the envelope (therefore the envelope appears slightly swollen). The printer rollers press the envelope and the air inside it, which cannot completely escape the envelope thus creating scratches in the envelope paper. It is also possible that the air attempts to escape the window creating small detached spots between the envelope paper and the transparent paper, thus reducing the reliability of the envelope.
Moreover, the Applicant has noted that crimps on the window transparent paper are probably due to the stretching process of the envelope paper and the window transparent paper. As the transparent paper thickness is much lower than the envelope paper, the stretching of the transparent paper is greater than that of the envelope paper; and (as the transparent paper is glued to the envelope paper all along the window perimeter) crimps on the window transparent paper are thus created. Also the air built up inside the envelope during its manufacturing process may be responsible for the crimps on the window transparent paper. In fact the swelling of the envelope may cause the window transparent paper to stretch and then the laser printer rollers, which carry out the compression of the window transparent paper, create the crimps.
To overcome the scratch problem on the envelope paper sheet, a possible solution (which is not part of the state of the art) is to print using the envelope with its closure flap open to allow the air inside the envelope to flow outside. Experimental tests carried out by the Applicant show how this is only a partial solution to the problem as the air does not completely escape the envelope therefore some scratches still create.
Another possible solution (which is not part of the state of the art) to the problem of crimp formation on the window transparent paper is not to spread the glue onto one or more spots along the window perimeter leaving few spots all around the window capable of absorbing the difference in stretching between the envelope paper and the window transparent paper; moreover these glue-free spots allow the internal air to flow outside of the envelope thus further reducing the scratches formation on the paper sheet of the envelope (caused by the compression of the rollers) and the crimps formation on the window transparent paper. Through experimental tests, the Applicant has found that this solution unfavourably reduces the reliability of the envelope as it is possible to access inside the envelope when it is closed. Another drawback is that during the manufacturing of the envelope, the transparent paper sheet on the window may detach from the envelope paper. In fact envelopes are manufactured from paper reels rotating at high speed and unwound by means of rolls also rotating at high speed; if the glue is not applied along the entire window perimeter, it might not be enough to overcome friction created between the envelope paper (moving at high speed) and the air so that friction may be responsible for the detachment of the transparent paper sheet from the envelope paper.

Summary of the invention

It is an object of the present invention to form an envelope provided with a transparent window which can be used in a laser printer. The object of the present invention is achieved by means of an envelope as defined in the appended claim 1 and by its preferred embodiments described in the dependent claims from 2 to 10.
Further it is an object of the present invention a process for the envelope manufacturing as defined in the appended claim 11 and its preferred embodiments described in the dependent claims from 12 to 14.
Further it is an object of the present invention the use of an envelope as defined in the appended claim 15.
The main idea is to increase the portion of the overlapping glue-free region defined at least between a segment of the window perimeter and the glue surrounding such segment of the window perimeter so as to absorb the different stretching between the envelope paper sheet and the window transparent paper sheet and so as to form possible crimps of the transparent paper sheet in such overlapping glue-free region.
What stated above offers the following advantages:

crimp formation on the window transparent paper and scratch formation of the envelope paper are reduced;
in case of residual crimp formation on the transparent paper, these are limited and only in the glue-free overlapping region, therefore they are not visible outside the envelope;
it is possible to maintain the glue layer applied along the window perimeter so as to surround the whole window thus preserving reliability;
the transparent paper sheet is prevented from detaching from the paper sheet of the envelope during the manufacturing process.

Short description of the drawings

Further characteristics and advantages of the invention will be clearly understood by the following description of the preferred embodiment and of its alternatives provided as way of an example with reference to the accompanying drawings, wherein:

Figure 1 schematically shows an open envelope according to the known art;
Figure 2 schematically shows more in detail the window region of the envelope according to the known art;
Figure 3 schematically shows more in detail the window region of the envelope according to a first embodiment of the invention;
Figure 4 schematically shows more in detail the window region of the envelope according to a second embodiment of the invention;
Figure 5 schematically shows more in detail the window region of the envelope according to a third embodiment of the invention;
Figure 6 schematically shows an open envelope according to the first embodiment of the invention;
Figure 7 schematically shows an envelope according to the first embodiment of the invention with the closure flap open;
Figures 8a - 8d schematically show the manufacturing process of the envelope according to the first embodiment of the invention.

Detailed description of the invention

With reference to Figure 3, the region surrounding the window 4 of the inside face of an envelope 101 according to a first embodiment is shown in detail. Envelope 101 is for letters or for mailing all sorts of commercial documents and comprises a paper sheet 102, a substantially rectangular shaped window 4 (for example having sides 6, 8 100 mm long and sides 7, 9 40 mm long), a transparent paper sheet (also called "film") 103 which overlaps the paper sheet 102 in a region around the window 4, a glue layer 105 along the window perimeter so as to partially surround the window 4 in the overlapping region, to glue the transparent paper sheet 103 to the paper sheet 102. The envelope 101 further comprises a folding line 110 along which a closure flap (not shown in figure 3) to close the envelope 101 is folded.
Figures 3, 4 , 5 show a rectangular shaped window 4, but the invention may also be applied to a window differently shaped where a first segment of the window perimeter and a second segment of the window perimeter can be identified, where the first and second segments extend lengthwise in the direction of the folding line. The expression "extend lengthwise in the direction of the folding line" means that these segments are parallel (or substantially parallel) to the folding line itself but it also refers to segments which are not parallel (or substantially parallel) to the folding line (they may be curved, for example) but in any case they develop between at least two points located on a parallel (or substantially parallel) direction to the folding line.
In the particular case of a substantially rectangular shaped window 4, one of the longer sides (side 6 for example) of the rectangular window 4 is the first segment and the other longer side 4 (side 8 in the example) of the rectangular window 4 is the second segment; therefore in order to explain the invention, hereinafter reference will be made to the first side 6 and to the second side 8 of the rectangular window 4. Moreover in the case of a rectangular window 4, a third segment of the perimeter of the rectangular window 4 is one of the shorter sides (side 7 for example) of the rectangular window 4 and a fourth segment of the perimeter of the rectangular window 4 is the other one of the shorter sides (side 9 in the example).
The first side 6 and second side 8 of the window 4 are substantially parallel to the folding line 110 and the third side 7 and the fourth side 9 of the window 4 are substantially orthogonal to the folding line 110.
The overlapping region between the paper sheet 102 and the transparent paper sheet 103 comprises a first glue-free region 115 (shown in dotted lines in Figure 3) defined between the first side 6 of the window 4 and the glue layer 105 around the first side 6. The overlapping region between the paper sheet 102 and the transparent paper sheet 103 further comprises a second glue-free region 116 (shown squared off in Figure 3) defined between the second side 8 of the window 4 and the glue layer 105 around the second side 8.
Width L101 of the first glue-free region 115 is greater than width L103 of the second glue-free region 116; in fact width L101 is 4 mm while width L103 is 1 mm.
Comparing figure 3 of the first embodiment of the invention to figure 2 of the known art, it is to be noted that width L101 (4 mm) of the first glue-free region 115 is greater than width L1 (1 mm) of the overlapping glue-free region defined between the side 6 of the window 4 of the envelope 1 and the glue layer 5 around the side 6.
During laser printing on the envelope 101, the greater width L101 allows the different stretching between the envelope paper sheet 102 and the window transparent paper sheet 103 to be absorbed thus reducing crimp formation on the window transparent paper 4. Further the greater width L101 of the first glue-free region 115 allows to absorb possible crimps of the transparent paper sheet 103, that is the crimps (if any) of the transparent paper sheet 103 may appear in the first glue-free region 115, therefore not visible outside envelope 101 (which do not look damaged).
The overlapping region between the paper sheet 102 and the transparent paper sheet 103 comprises a third glue-free region (not shown in Figure 3) defined between the third side 7 of the window 4 and the glue layer 105 around the third side 7. Preferably, width L101 (4 mm) of the first region 115 is greater than the width L105 (1 mm) of the third glue-free region.
Preferably the width L101 (4 mm) of the first glue-free region 115 is greater than the width L102 (3 mm) of the glue layer 105 adjoining the first glue-free region 115.
With reference to Figure 4, the region around the window 4 of the envelope 201 according to a second embodiment of the invention is shown in detail.
Comparing figure 4 of the second embodiment of the invention to figure 2 of the known art, it is to be noted that width L203 (2 mm) of the second glue-free region 216 is greater than width L3 (1 mm) of the glue-free part of the overlapping region defined between the enveloper window side 8 and the glue layer 5 around side 8.
During laser printing of the envelope 201, the greater width L203 allows to absorb the different stretching between the envelope paper sheet 102 and the transparent paper sheet 103 of the window 4 even better (as to the first embodiment of the invention), thus further reducing crimp formation on the transparent paper 103 of the window 4. Further the greater width L203 of the second glue-free region 216 allows to absorb possible crimps of the transparent paper sheet 103, that is the crimps (if any) of the transparent paper sheet 103 may appear in the second glue-free region 216, therefore not visible outside the envelope 101 (which do not look damaged).
Preferably, also width L205 of the overlapping region defined between the side 7 of the window 4 and the glue layer 105 around side 7 and width L207 of the overlapping region defined between the side 9 of the window 4 and the glue layer 105 around side 9 are greater than widths L5 andL7 of the known art respectively, as shown in Figure 4.
With reference to Figure 5, the region around the window 4 of the envelope 301 according to a third embodiment of the invention is shown in detail.
Comparing figure 5 of the third embodiment of the invention to figure 2 of the known art, it is to be noted that:

width L301 (3 mm) of the first glue-free region 315 is greater than the width L1 (1 mm) of the glue-free part of the overlapping region defined between the side 6 of the window 4 of the envelope 1 and the glue layer 5 around side 6;
width L303 (3 mm) of the second glue-free region 326 is greater than the width L3 (1 mm) of the glue-free part of the overlapping region defined between side 8 of the window 4 of the envelope 1 and the glue layer 5 around side 8.

Furthermore it is to be noted that width L301 (3 mm) and width L303 (3 mm) are equal.
During laser printing of the envelope 301, the greater widths L301 and L303 allow to absorb the different stretching between the envelope paper sheet 102 and the transparent paper sheet 103 of the window 4 even better (as to the first embodiment of the invention), thus further reducing crimp formation on the transparent paper 103 of the window 4. Furthermore the greater widths L301 and L303 allow to absorb possible crimps of the transparent paper sheet 103, that is the crimps (if any) of the transparent paper sheet 103 may appear in the first glue-free region 315 and in the second glue-free region 326, therefore not visible outside the envelope 301 (which do not look damaged).
Preferably, also the width L305 of the overlapping region defined between the side 7 of the window 4 and the glue layer 105 around side 7 and the width L307 of the overlapping region defined between the side 9 of the window 4 and the glue layer 105 around side 9 are greater than the widths L5 andL7 of the known art respectively, as shown in Figure 5. In this case the width L301, L305, L307 of the glue-free overlapping region adjoining the four sides of the window has increased (as to the known art). This offers a further advantage in that the envelope 301 can be fed into the laser printer from any of its sides that is either from its lower side (numeral reference 122 in figure 7) or one of its short sides ( numeral references 113 and 114 in figure 7) (in the case of a rectangular envelope when the closure flap is closed).
Figures 3, 4, 5 show a substantially rectangular shaped window 4 but the window 4 can be of any shape, for example square, rhomboidal or round shape. Consequently, the glue-free overlapping region defined between a segment of the window perimeter and the glue layer along such segment of the perimeter can be differently shaped from the substantially rectangular shape shown in Figures 3, 4, 5 and can be of a non-constant width (for example a width ranging from a minimum to a maximum value or a first width along a first segment and a second width, greater than the first, along a second segment). In these cases the width of a first part of the glue-free overlapping region defined between a first segment of the window perimeter and the glue layer along the first segment of the perimeter is greater than the width of a second part of the glue-free overlapping region defined between a second segment of the window perimeter and the glue layer along the second segment of the perimeter; alternatively, the minimum width of a first part of the glue-free overlapping region defined between the first segment of the window perimeter and the glue layer along the first segment of the perimeter is 3 mm or more.
Preferably, the glue layer 105 is a water-dispersed continuous phase low water contents synthetic resin; for example the glue layer 105 has a solid contents (that is the percentage or weight of the solid components of the glue layer 105) greater than 70, preferably about 72. Preferably, the viscosity of the glue layer 105 is about 4000 mPa and its density is about 1,01 g/ml.
The low water contents has the advantage to prevent water of the glue layer 105 from transferring to the transparent paper sheet 103 which would be responsible for visible crimps on the transparent paper sheet 103 visible in the window 4 and also would increase crimp formation on the transparent paper sheet 103 inside the window 4.
Preferably, a strong adhesive-bonding glue layer 105, that is having a short setting time enough to overcome the friction originating between the envelope paper (moving at high speed) and the air (during the envelope manufacturing process).
This offers the opportunity of reducing the width of the glue layer overlapping the first glue-free region 115. In fact comparing figure 3 of the first embodiment of the invention to figure 2 of the known art, the width L102 of the glue layer 105 around side 6 of the window 4 of the envelope 101 is lower than the width L2 of t he glue layer 5 around side 6 of the window 4 of the envelope 1; particularly the width L102 is 3 mm while the width L2 is 7 mm.
Preferably, the paper sheet 102 is specifically adapted to internal low moisture laser printers. For example the moisture ratio of the paper sheet 102 is between 3,6 and 4,5 (preferably about 4) and the grammage is between 70 and 120 g/m² (preferably 90). This greatly improves the quality of the printing on the envelope 101.
An advantage of using the transparent sheet of paper 103 is that the envelope can be more easily recycled as it is not required to separate the transparent sheet of paper 103 from the paper sheet 102 (which is necessary with a transparent plastic sheet) because also the transparent paper sheet can be recycled as waste paper.
Preferably, the transparent paper sheet 103 has a melting temperature higher than the internal operating temperature of a laser printer; advantageously the melting temperature of the transparent paper sheet 103 is greater than 180 °C. Preferably the transparent paper sheet 103 has a grammage between 40 and 45 g/m² (preferably about 42,5 g/m²), a thickness between 37 and 47 µm (preferably about 42), an opacity between 25 and 29 (preferably about 25), a moisture degree between 6 and 8,5 (preferably about 7).
Referring to figure 6, an open envelope 101 according to the first embodiment, that is an envelope 101 whose closure flap 120 is open (that is not folded along the folding line 110), where side seams 111 and 112 are not folded along the side folding lines 113 and 114 respectively and where the lower flap 121 is not folded along the lower folding line 122, is shown. The central part of the paper sheet 102 is the inside face of the envelope 101 when the side seams 111, 112 and the lower flap 121 are folded so as to form a closed envelope 101. Preferably the length of the folding lines 110 and 122 is 230 mm and the length of the side folding lines 113 and 114 is 110 mm that is the size of the closed envelope 101 (with the closure flap closed) is 230 mm x 110 mm.
Preferably, as shown in figure 6, the side seam 111 is substantially trapezoidal in shape having a slanting side 125 defining an angle α of at least 50 degrees (and lower than 90 degree) with the extension of the lower folding line 122; advantageously the angle α is about 53 degrees.
Preferably, as shown in figure 6, also the other side seam 112 is substantially trapezoidal in shape having a slanting side 126 defining a angle β of at least 50 degrees (and lower than 90 degrees) with the extension of the lower folding line 122; advantageously the angle β is about 53 degrees.
Figure 6 shows how the length of the side folding lines 113 and 114 of the side seams 111 and 112 is lower than the length of the side folding lines 110 and 122 of the closure flap 120 and of the lower flap 121 but an envelope 101, in which the length of the side folding lines 113 and 114 is greater than the length of the side folding lines 110 and 122, is possible as well.
With reference to figure 7, an envelope 101 according to the first embodiment of the invention with its closure flap 120 open is shown. The side seam 111 is folded along the side folding line 113 and overlaps the central part of the envelope paper sheet 102 in a region 130 (shown with vertical lines in figure 7); therefore the region 130 comprises two layers of the paper sheet 102. The folded side seam 111 overlaps the folded closure flap 121 and the central part of the envelope paper sheet 102 in another region 131 (shown with horizontal lines in figure 7); therefore the region 131 comprises 3 layers of the paper sheet 102. The amplitude of at least 50 degrees of the angle α allows the region 131 consisting of three layers of the paper sheet 102 to be reduced and to increase the region 130 consisting of two layers of the paper sheet 102, thus reducing the thickness of the envelope 101 in the corner of the envelope; this advantageously further reduces scratch formation on the paper sheet of the envelope during laser printing. Moreover the amplitude of at least 50 degrees of the angle α allows to create an opening at the envelope corner thus further improving the flowing of the air from the inside of the envelope to the outside. This further reduces scratch formation on the paper sheet of the envelope during laser printing.
The evaluations pertaining angle α can be applied also to the angle β, therefore the same evaluations as for the regions 130 and 131 of the side seam 111 can be applied to the regions 132 and 133 of the side seam 112 respectively.
With reference to figures 8a, 8b, 8c, 8d a rotary press 500 comprising a plurality of stations for manufacturing a window envelope 101 is shown in the upper part and the operation of each station is shown in the lower part.
The rotary press 500 comprises the following stations linked in series:

an unwinding station 501 adapted to receive a reel 600 of paper 102;
a straightening station 502;
a printing station 503;
a short side creasing station 504;
a window cutting station 505;
a transparent paper gluing and applying station 506;
a side seam cutting station 507;
a side seam folding station 508;
a side seam gluing station 509;
a long side creasing station 510;
a long side cutting station 511;
an envelope back face folding station 512;
a stepping station 513;
a closure flap gluing station 514;
a queuing station 515.

It will be described hereinafter the operation of the rotary press 500 for the manufacturing the window envelope 101.
The unwinding station 501 receives a reel 600 of paper 102, unwinds the reel 600 by rotating it around an axis substantially orthogonal to the plane defined by the reel 600, supplies a paper strip 601 and feeds it in a feeding direction by means of rollers.
The straightening station 502 receives the paper strip 601 and stretches the paper strip 601 in order to straight the paper 102 along the machine path, then supplies a straightened strip 602.
The printing station 503 prints on one and/or both sides of the straightened strip 602 thus supplying a printed strip 603; for example the envelope manufacturer's name or a decoration is printed on one face (which will be the inside face of the envelope 101) or a caption of postage paid is printed on the other face (which will be the outside face of the envelope 101). The printing station 503 is optional; should it not be provided, the straightening station 502 is linked in series to the short side creasing station 504. For sake of simplicity, the printing on strip 603 is not shown in figure 8a.
The short side creasing station 504 receives the printed strip of the paper 102 (or the straightened strip 602), performs the creasing (that is a folding line is created) of the printed strip 603 (or of the straightened strip 602) along the edges (which will be the short sides of the central part 102 of the envelope 101 shown with 113 and 114 in figure 6) of the printed strip 603 (or along the edges of the straightened strip 602) and provides a creased strip 604.
The window cutting station 505 receives the creased strip 604, performs a cut in a part of the creased strip 604 so as to form the window (shown with number 4 in Figg. 3, 4, 5, 6) and supplies a window strip 605.
The transparent paper gluing and applying station 506:

applies the glue layer on a transparent paper strip of the same type of the transparent paper sheet 103 used to seal the window, where the strip is obtained from a reel (of the same type of the transparent paper sheet 103) which unwinds by rotating the reel around its axis;
cuts a part of the transparent paper strip shaped so as to seal the window;
applies the transparent paper cut part onto the window strip 605 around the window 4 to seal it thus forming a strip 606 having a sealed window.

Specifically, the glue layer (the same type of the glue layer 105) is applied onto the transparent paper sheet by means of a cylinder rotating around its axis, where the external surface of the cylinder comprises a relief surface, in which the relief surface is defined between an inner substantially rectangular shape (similar to the substantially rectangular shape of the window) and an outer substantially rectangular shape. More specifically, the cylinder is modified (compared to the cylinder of the known art) so that, when the relief surface is moistened with a glue layer (of the same type as the glue layer 105), when the cylinder rotates around its axis and simultaneously the relief surface is in contact with a planar surface comprising the window, the window perimeter is such to surround the window perimeter and such as to define a first glue-free region (shown with number 115 in Figure 3) defined between a first segment of the perimeter and a segment of the perimeter of the inner rectangular surrounding the first segment of the window perimeter, and the perimeter of the inner rectangular of the relief surface defines a second glue-free region (shown with number 116 in Figure 3) comprised between a second segment of the window perimeter and a segment of the perimeter of the inner rectangular surrounding the second segment of the window perimeter, and in which the width (shown with L101 in Figure 3) of the first glue-free region is greater than the width (shown with L103 in Figure 3) of the second glue-free region; furthermore the width (between the inner and outer rectangular of the relief surface) of the relief surface moistened with the glue layer is such to apply the glue layer around the first and second segments of the window 4 of the envelope 101, as shown with the glue layer 105 in Figure 3. As an alternative, when the relief surface is moistened with the glue layer (the same type as the glue layer 105), when the cylinder is rotated around its axis and simultaneously the relief surface is in contact with a planar surface comprising the window, the perimeter of the inner rectangular of the relief surface is such to surround the window perimeter and to define a first glue-free region (shown with number 115 in Figure 3) defined between a first segment of the window perimeter and a perimeter segment of the inner rectangular surrounding the first segment of the window perimeter and in which the width L101 of the first glue-free region is 3 mm or more; further the width of the relief surface (between the inner and outer rectangular of the relief surface) moistened with the glue layer (the same type as the glue layer 105) is such to apply the glue layer around the first segment of the envelope window 4, as shown with the glue layer 105 in Figure 3.
Therefore the window gluing and applying station 506 performs the following operations:

the relief surface of the cylinder is moistened with the glue layer 105;
the cylinder is rotated around its axis and simultaneously the cylinder relief surface (moistened with the glue layer) comes in contact with the transparent paper strip, thus applying the glue layer onto the transparent paper;
a part of the transparent paper substantially rectangular in shape having a surface greater than the window surface and comprising the glue layer is cut;
the cut part is applied onto the window strip 605 around the window 4, so as to seal it.

Preferably, the cylinder outer surface comprises (over 360°) more than a relief surface shaped as previously described, so as to increase the number of strips 606 with the sealed window manufactured in the time unit. Advantageously the cylinder outer surface comprises three relief surfaces adapted to apply the glue layer 115 onto the transparent paper strip.
The side seam cutting station 507 receives the strip 606 with the sealed window, performs the cutting of a part of the seams of the strip 606 with the sealed window by means of cylinder blades so as to form the side seams (shown with number 111 and 112 in Figure 6) and supplies a strip 607 provided with a sealed window and side seams.
The side seam folding station 508 receives the strip 607 provided with sealed window and side seams, performs the folding of the side seams along the creasing edge (previously provided by the short side creasing station 504) and supplies a strip 608 provided with a sealed window and folded side seams.
The side seam gluing station 509 receives the strip 608 provided with a sealed window and folded side seams, performs the applying of a glue layer onto the folded side seams and supplies a strip 609 provided with a sealed window and glued side seams; preferably the glue layer onto the folded side seams is a water-dispersed synthetic resin having a solid contents of about 47, viscosity of about 1700 mPa and a density of about 1,07 g/ml.
The long side creasing station 510 receives the strip 609 provided with a sealed window and folded side seams, performs the creasing (that is a folding line is created) of the strip 609 between the ends of different side seams (which will be the long sides of the envelope, as shown in Figure 6 for sides 110 and 122 of the envelope 101) with the glue layer being applied so as to create a first and second folding line, supplying a strip 610 provided with a sealed window, folded side seams, the applied glue layer and with the first and second folding line.
The long side cutting station 511 receives the strip 610 provided with the sealed window, the folded side seams, the applied glue layer and with the first and second folding line, performs the cutting along two strip lines 610 so as to form a closure flap (120 in Fig, 6) and a lower flap (121 in Figure 6) supplying an open envelope 611.
The envelope back face folding station 512 receives the open envelope 611, performs the folding of the lower flap along the folding line (previously formed by the long side creasing station 510) and supplies a closed envelope 612.
The stepping station 513 receives a sequence of closed envelopes 612 and assembles them in order to form first stepwise envelopes 613.
The upper flap gluing station 514 receives the first stepwise envelopes 613, applies a glue layer in the upper portion of the closure flap 120 of the envelopes of the first stepwise envelopes 613 and supplies a second stepwise envelopes 614. Preferably, the glue layer on the closure flap 120 is a water-dispersed continuous phase low water contents synthetic resin; for example the glue layer on the closure flap 120 has a solid contents greater than 70, preferably about 72. Preferably the viscosity of the glue layer on the closure flap is about 4000 mPa and its density is about 1,01 g/ml.
The queuing station 515 receives the envelopes of the second set 615 and supplies a line of envelopes 615 ready to be stored in boxes. Specifically the line of envelopes 615 comprises envelopes with the closure flap 120 open.
Advantageously, the envelopes in the line of envelopes 615 are stored in boxes which undergo a conditioning step (not shown in figure 8d). Specifically the conditioning step comprises a first step (also shown as hot air drying) where the envelopes in the boxes are kept at high temperatures (for example higher than 25°C) and are exposed to a strong hot air ventilation; subsequently the conditioning step comprises a second step in which the envelopes in the boxes undergo a cold air ventilation (alternatively during the second step the envelopes in the boxes are kept at temperatures much lower than the temperature of the first step). The envelopes in the boxes are kept in the conditioning step for at least 24 hours (advantageously for 36 hours) afterwards the envelopes 101 according to the first embodiment of the invention are obtained. The hot air ventilation allows the moisture inside the envelopes in the line of envelopes 615 to be reduced (for example the moisture of the glue layer 105 is transferred from the glue layer 105 to the paper sheet 102 or 103 of the envelope, or the moisture in the glue layer on the folded side seams and on the closure flap) and the cold air ventilation allows the temperature of the envelopes in the line of envelopes 615 to settle, interrupting the drying step; in this way the adhesiveness between the envelope paper sheet 102 and the transparent sheet 103 can be improved and the scratch formation of the transparent paper adjoining the portion of transparent paper 103 stuck to the paper 102 is thus reduced.
The evaluations pertaining the rotary press 500 for manufacturing the window envelope 101 also apply to the manufacturing of the window envelope 201 and 301 where the transparent paper gluing and applying station 506 is modified so as to apply the glue layer 105 around the window 4 as shown in the second and third embodiment of the invention. This is achieved modifying (as to the known art) the rotary cylinder used in applying the glue layer onto the transparent strip, that is modifying the inner rectangular and/or the outer rectangular of the relief surface.

Claims

An envelope (101; 201) comprising:
- a first paper sheet (102) comprising a closure flap (120) of the envelope along a folding line (110) and comprising a window (4) to view the envelope contents, wherein the window perimeter comprises a first segment (6) and a second segment (8) which extend lengthwise the folding line;

- a second paper sheet (103) substantially transparent to close the window, the first and second sheets defining an overlapping region (103) which surrounds the window and in which they are overlapped;

- a glue layer (105) to glue the first and the second sheet, the glue layer being applied to part of the overlapping region (103) around the window perimeter in order to surround the window;

the overlapping region comprising a first glue-free region (115) defined between the first segment (6) of the window perimeter and the glue layer surrounding the first segment and comprising a second glue-free region (116) defined between the second segment (8) of the window perimeter and the glue layer surrounding the second segment;

characterized in that the width (L101) of the first glue-free region (115) is greater than the width (L103) of the second glue-free region (116).
An envelope (101, 201) comprising:
- a first paper sheet (102) having a window (4) to view the envelope contents,

- a second paper sheet (103) substantially transparent to close the window, the first and second sheets defining an overlapping region (103) which surrounds the window and in which they are overlapped;

- a glue layer (105) to glue the first and the second sheet, the glue layer being applied to part of the overlapping region around the window perimeter so as to surround the window;
the overlapping region comprising a first glue-free region (115) defined between a first segment of the window perimeter and the glue surrounding the first segment;
characterized in that the width (L101) of the first glue-free region is greater or equal to 3 millimetres.
The envelope according to claim 2, wherein the first paper sheet (102) has a closure flap (120) of the envelope along a folding line (110), and the window perimeter comprises the first segment (6) and a second segment (8) which extend lengthwise the folding line.
The envelope according to claim 1 or 3, wherein the first paper sheet further comprises a lower flap (121) folded along a lower folding line (122) and comprises a side seam (111) folded along a side folding line (113) wherein the angle between the lower folding line and a side (125) of the perimeter of the seam is greater or equal 50 degrees.
The envelope according to at least one of the claims 1, 3 and 4, wherein the first segment (6) of the window perimeter is closer to the folding line (110) than the second segment (8) of the window perimeter.
The envelope according to at least one of the previous claims, wherein the width (L101) of the first glue-free region is greater than the width (L102) of the glue layer adjoining the first glue-free region.
The envelope according to at least one of claims 1 and from 3 to 6, wherein the window perimeter further comprises a third segment (7, 9) substantially orthogonal to the direction of the folding line, the overlapping region comprising a third glue-free region defined between the third segment (7, 9) and the glue layer (105) adjoining the third segment (7, 9),
wherein the width (L101) of the first glue-free region is greater than the width (L105, L107) of the third glue-free region.
The envelope according to at least one of claims 1 and from 3 to 7, wherein the width of the first glue-free region (115) is greater or equal to 4 mm and the width of the second glue-free region is smaller or equal to 2 mm.
The envelope according to at least one of claims 7 and 8, wherein the window perimeter is substantially rectangular, wherein one of the longer sides of the window is the first segment (6), the other one of the longer sides of the window is the second segment (8) and one of the shorter sides of the window is the third segment (7, 9), and wherein the length of the folding line (110) is the greatest length of the envelope sides.
The envelope according to at least one of the previous claims, wherein the glue layer has a solid contents greater than 70 and wherein the substantially transparent second paper sheet has a grammage comprised between 40 and 45 g/m², a thickness comprised between 37 and 47 µm and an opacity comprised between 25 and 29.
A process (500) for manufacturing an envelope (101) including the steps of:
- providing a first paper sheet (102) comprising a closure flap (120) of the envelope along a folding line (110);

- forming in said first paper sheet (102) a window (4) to view the envelope contents, wherein the window perimeter comprises a first segment (6) and a second segment (8) which extend lengthwise the folding line;

- providing a substantially transparent second paper sheet (103) to close the window;

- defining an overlapping region (103) of the first and of the second sheet surrounding the window;

- gluing the first and the second sheet by means of a glue layer (105) applied to part of the overlapping region (103) and around the window perimeter in order to surround the window;
the overlapping region comprising a first glue-free region (115) defined between the first segment (6) of the window perimeter and the glue layer surrounding the first segment and comprising a second glue-free region (116) defined between the second segment (8) of the window perimeter and the glue layer surrounding the second segment;
characterized in that the gluing step is carried out in such a way that the width (L101) of the first glue-free region (115) is greater than the width (L103) of the second glue-free region (116).
The process (500) according to claim 11, wherein:
- the step of providing the first paper sheet comprises: unwinding (501) a segment of a first reel (600) of the type of paper of the first sheet along an advancing direction so as to form a strip of a first type of paper (601);

- the step of forming the window comprises: creating (505) a window (4) in the strip of the first type of paper so as to form a window strip (605);

- the steps of providing a second paper sheet and of gluing comprise:
moistening with the glue layer (105) a relief surface of the outer surface of a cylinder rotating around its axis, the relief surface being defined between an inner geometrical shape and an outer geometrical shape, wherein the perimeter of the inner geometrical shape is such to surround the window;

unwinding a portion of a second reel of a paper type of the second sheet so as to form a strip of a second type of transparent paper;

rotating the cylinder around its axis with the relief surface in contact with the strip of the second type of paper so as to apply the glue layer of the relief surface onto the strip of the second type of paper;

cutting, from the strip of the second paper type bearing the glue layer, the substantially transparent second paper sheet in order to close the window;

gluing (506) the second substantially transparent paper sheet to the strip of the first type of paper in order to close the window.
The process according to at least one of claims 11 and 12, further comprising the step of queuing a plurality of envelopes with the closure flap (120) open.
The process according to at least one of claims from 11 to 13, further comprising the steps of storing the plurality of envelopes into boxes and of exposing the envelopes inside the boxes to an hot air ventilation followed by a cold air ventilation, for at least twenty-four hours, in order to dry the glue layer.
Use of the envelope according to at least one of claims from 1 to 10 for printing messages or information on said first paper sheet by means of a laser printer.