The invention is comprised in the field of high-security forms, for example, for sending security codes by mail. More specifically, the invention relates to a tamper-proof form comprising a sheet with a security pattern on its surface, foldable and unfoldable between an open state and a closed state along at least one fold line, and said security pattern being arranged such that it covers a code area in the closed state. State of the Art

Forms for sending security codes are known on the market. Said forms consist of a sheet of paper folded one or several times containing a code therein. The sheet also has a security pattern on its surface, arranged such that once the form is folded the pattern covers the code and prevents seeing it when it is regarded backlit. These forms are generally provided with gummed lines on their edges to keep them closed.

One of the main drawbacks of the type of solutions set forth above is that the forms can be easily tampered with. By applying steam or other techniques, it is possible to open them to see the code and to then close them again, such that the final recipient has been the subject of an intrusion without even knowing about it.

For the purpose of overcoming such problems, there are solutions containing internal adhesive elements covering and concealing the code. However, such solutions have not only a high manufacturing cost, but furthermore they can be easily tampered with and enable accessing the code without leaving a trace. In another type of forms, the code is printed from the outside once the form is closed by means of a matrix type printer or the like. The printer forms the characters of the code on the surface without ink and a tracing element provided on the inner face of the sheet of the form transmits those characters to the inner face of the sheet opposite the face having the tracing element. This process has the advantage that during the writing process no one sees the code, but in contrast, printing can leave prints caused by pressure on the outer surface of the sheet which allow inferring the code by simply shading it in with a pencil. Accordingly, it is necessary to provide a form providing high security against illicit access to the information it contains, and which does not involve a high manufacturing cost.

Summary of the Invention

The purpose of the invention is to provide a tamper-proof form of the type indicated above, which makes any attempt to intrude in order to access the code unequivocally evident to the final recipient such that said recipient can take the suitable measures.

In the context of the invention any numerical, alphanumerical or graphic sequence that is to be kept secret, for example, a pin number or a password, is understood to be a code. Another object of the invention is to be a low-cost system.

This can be achieved by means of a tamper-proof form of the type indicated above, characterized in that it further comprises a first gummed band arranged around the code area, a second gummed band arranged such that the first time the sheet is in the closed state, the first and second bands keep the sheet in the closed state by mutual adhesion, and a first perforated line extending along the outer perimeter of the second gummed band, such that when the sheet goes from the closed state to the open state for the first time, said sheet tears along the first perforated line, whereas the first and second gummed bands remain attached to one another to form a security window, the code area remaining concealed by said security window.

This tearing of the security window caused by the action of opening the form makes it irreversibly evident that someone has attempted to access the code area. Therefore, a recipient receiving a form of this type that has been tampered with would have reliable proof of the intrusion, because the part of the sheet on which the first perforated line is provided will clearly be torn. In fact, the glue must be strong enough so that in any situation, the first perforated line always tears, even if only slightly, when opening the form from the closed state, and so that the tearing always occurs before the first and second gummed bands start to become detached from one another.

In addition, the elements that achieve the preceding effect are formed on the sheet itself by means of simple processes, for example, die cutting and applying of glue, and such elements act in the actual form closing process, so the manufacturing cost is very low.

Preferred embodiments have been provided based on the invention defined in the main claim, the features of said preferred embodiments being described in the dependent claims.

From a form design and manufacturing point of view, it would be appropriate to have a reference between the gummed bands that must complement one another, thereby simplifying the design and reducing the risk of errors in misalignment. As a result, in a preferred embodiment, the second gummed band is mirror-symmetrical to the first gummed band with respect to the fold line.

In an alternative embodiment, the edge adjacent to the fold line of the first and second gummed bands coincides with said fold line. This enables the first perforated line not having to surround the entire gummed band, resulting in a simpler die cutting process. Once the form has been opened for the first time, the user must pull off the window conceal the code area. This may be difficult if the first and second gummed bands are held strongly attached to one another. To prevent this problem, in an advantageous embodiment, the form of the invention comprises a second perforated line extending at least partially along the inner perimeter of the second gummed band, and also extending around said code area in the closed state of the sheet to form a detachable strip.

Given that the final recipient must tear off the part of the sheet covering the code area to enable accessing same, this detachable strip makes access easier because the tearing will not depend on the mutual adhesion strength between the first and second gummed bands.

It may not be very evident to the user that said strip has to be pulled off, so in a preferred embodiment, the detachable strip has a detachment indicator intended for marking the point at which the recipient can pull on the strip.

A priori, the detachable strip can be detached at any point of the second perforated line. Nevertheless, to enable more comfortably removing the strip, the second perforated line preferably has a tab formed along its perimeter that is suitable for causing said second perforated line to tear by pulling on it.

This tab allows the recipient to hold one end of the part of the sheet covering the code area, such that it is more comfortable to tear it by simply pulling on said tab. If there is a detachment indicator, it is preferably placed such that it points to the tab.

In an advantageous embodiment, the first perforated line forms a tear tip at the end farthest away from the fold line. The security window defined by the perforated line therefore goes from being narrower to wider in the direction for opening the form. This initial narrow tear tip is a point where stress is accumulated, so it is the point from which the first perforated line starts to be torn when opening the form. Accordingly, if the form is to be opened, the opening thereof is made easier with the tear tip. In contrast, in the event of an illicit opening, tampering using a much lower force, which furthermore leaves a very clear mark, becomes evident.

Preferably, the tear tip of the first perforated line is formed by at least one straight line not parallel to the fold line. The tear tip thereby forms a steep angle, this shape favoring the accumulation of stress. Accordingly, said tip being the controlled tear point of the first perforated line is favored.

In another alternative embodiment, the first band extends along the entire horizontal extension of the code area, being interrupted at its horizontal ends. As a result, once the form has been opened and the security window has been released, the part of the sheet covering the code area is opened at least on one side, which allows the recipient of the form to introduce a finger or an implement that allows easily tearing said part of the sheet.

If possible intruders know of the existence of the proof of tampering that the form has, they can attempt to take any type of measure. For that purpose and for the purpose of increasing security, it is appropriate for such proof of tampering to not be evident from the outside with the form in the initial closed state. Accordingly, in an advantageous embodiment the form has at least two fold lines, and the first perforated line is arranged in the sheet such that the first perforated line is not visible in the closed state of the form. As said perforated line is the most evident detail of the existence of the proof of tampering, hiding it between the folds of the form makes it more difficult for the possible intruder to know of its existence. Therefore, if said intruder attempts to open the form, the possibilities of tearing said perforated line, and therefore the proof of tampering, increase.

The invention also covers other features relating to detail illustrated in the detailed description of an embodiment of the invention and in the attached drawings. Brief Description of the Drawings

The advantages and features of the invention are apparent from the following description in which, in a non-limiting manner with respect to the scope of the main claim, preferred embodiments of the invention are described in reference to the drawings.

Figures 1 A to 1 C show schematic views of the different states of a tamper-proof form.

Figure 1 A shows the inner face of the form in the initial open state.

Figure 1 B shows a perspective view of the form in the process of going from a closed state to an open state.

Figure 1 C shows the inner face of the form once it has gone from the closed state to the open state.

Figure 2 shows a schematic view of another embodiment of the form having two fold lines and an alternative configuration of the security window.

Figures 3A and 3B are detailed views of an example of the security window.

Figure 3A corresponds to the initial open state of the form.

Figure 3B shows the open state, once the form has already been closed and opened, and once the first perforated line has been torn.

Figures 4A, 4B and 4C show detailed views of another embodiment in which the detachable area forms a tear tip to make it easier to tear the first perforated line when opening the form for the first time. Detailed Description of Embodiments of the Invention

The sequence of Figures 1 A to 1 C shows a first preferred embodiment herein described of the tamper-proof form 1 . In this embodiment, the sheet 2 has a single fold line 4 along which it is folded in two equal segments. So there are thereby defined an inner face 7 containing the code area 5 to be hidden, and an outer face 8 covered by a security pattern 3 intended to hinder seeing the code, for example, when regarding it backlit, when the form 1 is closed. Though not shown in the drawings, said pattern 3 can be located in both segments of the sheet 2 for further security.

Likewise, on the inner face 7 of the sheet 2 there is provided a first gummed band 10 arranged around the code area 5, completely surrounding it. In this preferred embodiment, this first band 10 is mirror-symmetrical to another second gummed band 1 1 with respect to the fold line 4.

Surrounding the second gummed band 1 1 there are two perforated lines formed by die cutting on the sheet 2 itself. As can be seen in the drawings, a first external perforated line 12 and a second internal perforated line 14 are provided.

Finally, gummed closing lines 6 are also provided on the edges of the inner face 7 of the form 1 , intended for keeping the sheet 2 closed. Pressure- or heat-activated glues are normally used in gummed areas of the form 1 , i.e., either closing lines 6 or the first and second gummed bands 10 and 1 1 .

Figure 1 A shows the components arranged on the inner face 7 of the sheet 2 for this embodiment. The drawing shows the initial open state, with the code already printed in the code area 5 and before closing the form 1 for the first time to hide said code. The code is usually printed with a conventional printer of the state of the art, i.e., a laser printer or an inkjet printer. This prevents the problem of the marking or engraving of the outer face 8 that appeared with the matrix printers and that allowed deducing the security code by shading in the area with a pencil or by looking at the reflection of the engraving caused on the surface of the paper by the matrix printer. On the other hand, other forms of representing a code that are not written are not ruled out. Therefore, the code could also be, for example, an adhesive provided with a hologram. When closed for the first time, the closing lines 6 and the first and second gummed bands 10 and 1 1 adhere to one another, respectively. Accordingly, the code area 5 is concealed and as a result of the security pattern 3 of the outer face 8, it is not possible to determine the code when regarding it backlit. This is the state used for sending the form 1 .

When an attempt is made to open the form 1 to access the code, the first step is to open the sheet 2, as shown in Figure 1 B. At this point, the user pulls on a segment of the sheet 2 and the sheet 2 tears along the first perforated line 12, given that the first and second gummed bands 10 and 1 1 remain adhered to one another. If this opening is an attempted intrusion, there will be visibly evident proof of the attempted illicit access to the code of the form 1 .

Finally, Figure 1 C shows the form 1 once it has been opened entirely and the area surrounded by the first perforated line 12 has been completely detached. At this point, both gummed bands 10 and 1 1 remain attached, forming a security window 13 having a detachable strip 15 therein and which the user must detach to be able to access the code area 5. In the embodiment herein described, this security window 13 has a second die cut perforated line 14 to make it easier to remove the detachable strip 15. The drawing shows the detachable strip 15 being practically detached, providing access to the code. It should be mentioned that the second perforated line 14 is not essential for solving the problem with inviolability of the code area 5. Nevertheless, this solution makes opening this area much easier.

Figure 2 shows another embodiment of the form 1 . In this case, the sheet 2 has several fold lines 4, despite the fact that only one of them acts as a line of symmetry for the first and second gummed bands 10 and 1 1 . In the example, the sheet 2 is first folded along the fold line 4 of symmetry, adhering the first and second gummed bands 10 and 1 1 , and then the sheet 2 is folded again with respect to the other fold line 4. The person skilled in the art would not have any problems in seeing that the folding in C herein described can be replaced with other types of folding. For example, folding could be done in the form of a Z by outwardly folding the upper segment 18 of the sheet. In this case, the gummed closing lines 6 of said upper segment 18 would have to be located on the face opposite the one depicted in the drawing. In any case, by providing sufficient fold lines 4 and providing them in a suitable manner, the person skilled in the art could create configurations to do multiple folding in the form of a C, a Z or in the form of any figure that may be advantageous for the specific application.

This multiple folding also allows the overlay of several security patterns 3 not shown in the drawings which cover the code area 5, which results in it being better hidden. In particular, in the embodiment shown in Figure 2 security patterns 3 can be provided on the entire outer face 8 of the sheet 2, and also on the inner face 7, i.e., in the segment that is free of the inner face 7 of the sheet 2.

In the embodiment herein described, the first and second gummed bands 10 and 1 1 are open at their horizontal ends 9, being mirror-symmetrical to one another. Each of the bands 10 and 1 1 is therefore really made up of two equally sized lines parallel to the fold line 4. In this case, the code area 5 is located between both lines of the first gummed band 10.

When the gummed bands 10 and 1 1 of this embodiment are attached to one another, they create a security window 13 in which the detachable strip 15 is open at both horizontal ends, such that the user can introduce a finger or an implement which allows easily detaching it. The opening of the horizontal ends 9 is small enough so that the security code cannot be read.

Figures 3A and 3B show another embodiment in which the detachable strip 15 has a tab 16 at one of its ends formed on the second perforated line 14. In the example, the tab 16 has a semicircular shape and is die cut on the sheet 2. Once the form 1 is closed and the security window 13 is created, this tab 16 allows the user to pull on the detachable strip 15 and easily tear the second perforated line 14, thereby revealing the code area 5. Also to help the user identify the most optimal point for pulling on the detachable strip 15, said strip 15 has a detachment indicator 19 in the form of a triangle. The shape of the tab 6 and of the detachment indicator 19 is evidently irrelevant for the invention and despite the fact that in this embodiment it has been depicted in the form of a semicircle and triangle, respectively, they could have other shapes, such as elliptical, triangular, polygonal or others.

Another embodiment is shown in Figures 4A, 4B and 4C. In it, the first perforated line 12 surrounding the second gummed band 1 1 forms a tear tip 17 at its end farthest away from the fold line 4. In the embodiment, the tip 17 is formed by the attachment of two straight lines not parallel to the fold line 4 forming an angle at said tip 17.

Figures 4B and 4C show probable ways of tearing the first perforated line 12 for this embodiment, depending on the area in which the user pulls on the sheet 2 when opening the form 1 .

The person skilled in the art will understand that multiple variations with respect to the embodiments herein described are possible, without departing from the scope of the main claim as a result. In particular, and in a non-limiting manner, variations with respect to the shape of the sheet, the number and distribution of fold lines, as well as the shape of said folding, the shape and position of the code area, as well as of the gummed bands and perforated lines, the position and size of the security patterns, the shape of the detachable strip and the tab, of the tear tip, as well as the presence or absence of gummed closing lines and the distribution thereof, can be envisaged.

Finally, despite being a rather uncommon embodiment, the person skilled in the art would also understand placing the code area on the inner face of the detachable strip 15 as being equivalent. In this embodiment, the recipient would access the code with the strip remaining and discarding the rest of the form.