The present invention relates to a sheet packaging material, according to the preamble of claim 1, in particular web packaging material for packaging pourable food products, having a register mark for detecting the position of the material in a packaging machine.

As it is known, packages containing pourable food products such as milk, fruit juice, tomato sauce or wine are made in packaging machines from a web of packaging material or from sheet material blanks.

In the case of continuous web material, the web, which is fed off a reel and sterilized through an aseptic chamber, is longitudinally bent and sealed to form a continuous, vertical tube which is filled with the food product and transversally sealed to form so-called pillow packs by pairs of sealing jaws that are cyclically brought into contact with the tube.

Pillow packs are then separated from one another by cutting along the transversal sealing lines, and folded into their fmal, substantially parallelepiped shape.

During forming, the web material is subjected to folding steps along predetermined creasing lines that are embossed on the web material; in order to ensure that the forming steps are performed in register with the crease line pattern, the web longitudinal position is detected by sensing the position of optically detectable indicia, e.g. printed marks, that are repeated along the web packaging material at a pitch (repeatlength) that is equal to the repeatlength of the crease pattern. In response to the position of the register mark being detected later or earlier than a reference time, a so-called "design correction" device acts on the packages being formed to draw more or less material in the feed direction.

Printed marks to detect the longitudinal position of the web material have been used for many years, are simple to produce and easy to read; in facts, the printed mark is simply a portion of the package design and thus does not bring about any additional cost. However, being a portion of the printed design means that the register mark itself is not perfectly in register with the crease pattern, because printing and creasing steps are two different, successive steps in web material production and, although relative shifts between the two steps are kept to a minimum, there are inherent tolerances in the production process.

EP-A-0 772 760 discloses a method and device for detecting the position of a crease line of a packaging web material by means of a laser reflection technology, in particular by using two photodetectors detecting the light reflected by the material along two directions inclined with respect to the light incidence direction and by processing the signals thus obtained.

If a crease line is used as a "register mark", the position of the web material in a given, e.g. longitudinal, direction can be detected by this method, provided that the line is correctly identified with respect to the other crease lines forming the crease line pattern. This would require to use an additional reference code, e.g. a printed code, to trigger a "reading window".

Also the position of the web material in a transversal direction perpendicular to the feed direction must generally be controlled; this is the case both when the web material is still flat but undergoes preliminary operations in the machine, such as cutting, applying removable tabs or opening devices, and when the tube is already formed, to control "tube twist", i.e. torsional movement of the tube about its longitudinal axis.

Different optically readable indicia are used in these cases, such as the position of a web edge, or prelaminated holes for opening devices, or the longitudinal seal of the tube; however, these indicia are difficult to detect, and in any case require dedicated sensors.

Attempts have been made in the past to solve the afore-mentioned problems.

WO 03/037729 and WO 03/037722 disclose the use of crease lines for detecting both the longitudinal and transverse position of the web material. However, the solution provided is not entirely satisfactory because it has proven to be difficult to "select" the right crease lines to be detected among the various lines constituting the crease pattern without triggering a "reading window" by means of a printed mark of conventional type. Although effective to eliminate the "print-to-crease" tolerances, this solution still requires two detection systems, one for printed indicia, the other for crease lines.

DE-A-10246449 and EP-A-1 406 061 disclose a method of detecting the longitudinal and transverse position of a web packaging material by reading two mutually intersecting embossed lines by means of laser differential reflection technology similar to that disclosed in above-mentioned EP-A-0 772 760. This method, however, cannot provide all the data that are necessary to control the web feed because no information can be obtained on the web speed, that is an essential parameter to be detected and must be determined in some other way.

WO 2004/018190 discloses a sheet packaging material as defined in the preamble of claim 1.

An object of the present invention is to devise a sheet packaging material provided with a register mark which overcomes the drawbacks of the prior art, i.e. that is in register with the crease pattern, can be selectively read with respect to the crease pattern, and does not require "external" auxiliary indicia to fully establish the longitudinal and transverse position of the sheet material, as well as its speed.

A further object of the present invention is to devise a sheet packaging material provided with a register mark that can be manufactured without substantial increase of the production costs compared with known materials presently in use.

This object is attained by a sheet packaging material as claimed in claim 1.

Preferred embodiments of the present invention will be described hereinafter, by way of non-limiting examples and with reference to the appended drawings, in which:

• Figure 1 discloses a portion of a web packaging material in accordance with an embodiment of the present invention;
• Figure 2 shows, schematically, a forming/filling machine for producing aseptic packages from the web material of figure 1;
• Figure 3 shows a detail of the material of figure 1, at an enlarged scale, including a register mark;
• Figure 4 is an enlarged, partial cross section of the web material taken along the line IV-IV in figure 3;
• Figure 5 is a schematic, perspective view of a detecting device for detecting the register mark of figure 3;
• Figures 6, 7 and 8 show schematically three different scanning situations in which the register mark is respectively centred, shifted on one side and shifted on the other side with respect to the detecting device;
• Figure 9 is a diagram showing signals generated by the detecting device in cases corresponding to figures 6, 7 and 8;
• Figures 10, 11 and 12 disclose alternative embodiments of the register mark;
• Figure 13 is a cross section of the web material taken along the line XIII- XIII in figure 12;
• Figures 14 and 15 disclose alternative embodiments of a part of the register mark.

Numeral 1 in figure 1 indicates a portion of a sheet packaging material 2 fed in the form of a continuous web 3.

Web 3 of material 2 comprises a crease line pattern 4 and a printed design 5, which are repeated at intervals R equal to the length of material required to produce one package. Crease line pattern 4 includes, in a known manner, a plurality of longitudinal crease lines 4a, corresponding to the vertical corners of the finished packages, and a plurality of transverse crease lines 4b corresponding to the horizontal corners of the package and the base of the transversal seal portions or "fins".

Inclined crease lines 4c, mainly inclined by 45° but also present at different angles with respect to the longitudinal direction, appear in the top and bottom part of the pattern and define the folding lines for the package flaps.

Web 3 can be used on a machine 6, shown schematically in figure 2, for producing aseptic packages, and on which web 3 is unwound off a reel 7 and fed through an aseptic chamber (not shown), where it is sterilized, and through an assembly 8 by which it is folded and sealead longitudinally to form, in known manner, a continuous vertical tube 9.

Tube 9 of packaging material is filled continuously with the pourable food product by means of a known filling device 10, and is then fed to a forming and transverse sealing station 14 where it is gripped between pairs of jaws (not shown) that seal the tube transversely to form pillow packs 15.

Pillow packs 15 are then separated by cutting the sealing portion between the packs, and are fed to a final folding station 16 where they are folded mechanically to form the fmished packages 17.

The packages are formed by folding the material along crease lines 4a, 4b, 4c, and by controlling material feed by means of an optical detecting device 18 for "reading" register marks 19 located on the material at intervals R.

According to the present invention, each register mark 19 includes at least four inclined lines 21, 22, 23, 24 defined by topographic surface modifications of the web material obtained during the creasing process and detectable by means of detecting device 18, that is conveniently based on differential laser reflection technology according to the principle of EP-A-0 772 760, as better described hereinafter.

Two of the lines (21, 23) are parallel to one another and inclined with respect to the transverse direction by an angle α in a first direction, the other two lines (22, 24) are parallel to one another and inclined with respect to the longitudinal direction by the same angle α in the opposite direction. Angle α conveniently ranges between 15° and 35°. These angle values are sufficiently far from the inclination of standard crease lines 4a, 4b, 4c to provide for good recognition of lines 21, 22, 23, 24 by detecting device 18.

In the embodiment of figure 3, the four lines 21, 22, 23, 24 are defined by compression crease lines, i.e. lines obtained by using a creasing cylinder with radially protruding ribs and a purely cylindrical counter-cylinder, i.e. without any corresponding indentation. The result of this is a local compression on one side only of the web material 3 (see enlarged, cross-section detail of figure 4) having a substantially trapezoidal shape.

This technology has proven to be advantageous over standard creasing technologies using male-female creasing tools because the crease line lateral edges result to be sharp, as opposed to bulging edges that are generally obtained by standard creasing processes.

Further, according to the embodiment of figure 3, lines 21, 23 and 22, 24 intersect at their midpoints, so as to define two "crosses" 27, 28 that are offset from one another in the longitudinal direction. At the intersection points, in order to minimize light scattering, the lines may be locally interrupted, leaving a substantially non-compressed zone 29 as shown in figures 14 or 15.

Figure 5 is a schematic view showing a possible embodiment of detecting device 18.

A central laser beam emitter 30 includes a single laser source and a diffracting lens unit, both not shown in detail as known per se, to generate two planar laser beams 31, 32 that are directed perpendicularly to the web material 3 and intersect along an axis B. The two laser beams 31, 32 can be thought of a single laser beam 35 that is X-shaped in cross section; of course, the two branches of the "X" have the same inclination as 21, 23 and 22, 24 respectively.

Detecting device 18 further includes two pairs of laser detectors 33, 34, 35, 36 arranged around emitter 30. Laser detectors 33, 35 are located on opposite sides on the emitter 30, on a plane containing axis B and arranged at an angle to the plane of laser beam 31, i.e. to lines 21, 23, so as to detect differential reflection produced when beam 31 passes on lines 21, 23. Laser detectors 34, 36 are located on opposite sides on the emitter 30, on a plane containing axis B and arranged at an angle to the plane of laser beam 32, i.e. to lines 22, 24, so as to detect differential reflection produced when the beam 32 passes on lines 22, 24.

Detecting device 18 is connected to a control unit 40 which controls web feed in response to signals received from laser detectors 33, 34, 35, 36.

Figures 6 to 8 show the working principle of web material longitudinal and transverse position detection in accordance with the present invention.

In the case of Figure 6, the transversal position of the web material 3 is centred, i.e. the centres of crosses 27, 28 of the register mark 19 are aligned with axis B. In this case, being T1, T2, T3 and T4 the times when lines 21, 22, 23 and 24 are detected, T1 equals T2 and T3 equals T4. Lateral offset is then zero.

In the case of Figures 7, T1 will precede T2 and T3 will precede T4; in the case of Figure 8, T2 will precede T1 and T4 will precede T3.

The three situations are reflected in figures 9 a), b) and c) where differential reflection signals are plotted against time. In each situation, the difference ΔT_1,2 between T1 and T2 (if any) equals the difference between ΔT_3,4 between T3 and T4.

With reference to the scheme of figure 8, and being:

• D = distance between the two cross centres,
• W = one half of the cross width in the transverse direction,
• E = the lateral offset of the web material with respect to its transverse reference position of Figure 6; and
• A = the distance covered by the web material during time ΔT_1,2,

$$\left[\mathrm{1}\right]\mathrm{E}\mathrm{/}\mathrm{A}\mathrm{=}\mathrm{W}\mathrm{/}\mathrm{D}\mathrm{,}\mathrm{and}$$

$$\left[\mathrm{2}\right]\mathrm{W}\mathrm{=}\mathrm{D}\mathrm{/}\mathrm{2}\mathrm{tg\alpha }\mathrm{,}$$

$$\left[3\right]\mathrm{E}\mathrm{=}\mathrm{A}\mathrm{/}\mathrm{2}\mathrm{tg\alpha }\mathrm{.}$$

Furthermore, being D the distance covered during time ΔT_1,3 = T3-T1, $$\left[4\right]\mathrm{A}\mathrm{=}\mathrm{D}*{\mathrm{\Delta T}}_{1,2}/{\mathrm{\Delta T}}_{1,3}\mathrm{.}$$

If the value of α is, for example, 26.5°, then tgα=0.5, in which case expressions [3] and [4] are further simplified to read: $$\left[5\right]\mathrm{E}\mathrm{=}\mathrm{A}\mathrm{=}\mathrm{D}*{\mathrm{\Delta T}}_{1,2}/{\mathrm{\Delta T}}_{1,3}$$

The foregoing explains how transversal deviations can be calculated by control unit 40 and corresponding control signals can be generated.

Web speed can also be easily calculated, for control purposes, on the grounds of the followign expression: $$\left[6\right]\mathrm{v}\mathrm{=}\mathrm{D}/{\mathrm{\Delta T}}_{1,3}$$

Finally, as it is well known in the art per se, control unit 40 generates a register pulse signal T (figure 2) to control longitudinal web feed (design correction). The register mark 19 of the present invention allows to make such a signal independent from lateral displacement and web speed, as well as to avoid any jitter (deviations in signal timing).

Referring again to the diagrams of figures 9 b) and c), the following approach may be used.

A register pulse T is generated at a time corresponding to the "reading" of the second cross 28 when no lateral displacement exists. This is done by reading the first three pulses coming form laser detectors at times T1, T2, T3 (or T2, T1, T4 as the case may be), calculating ΔT_1,2 and applying a delay of ½ ΔT_1,2 to the reading of T3 or T4, whichever comes first. This delayed pulse is pulse T. Therefore, pulse T will be jitter-free and can be easily compared to a clock pulse, in a known manner, to determine whether the web material is "fast" or "slow" and thereby control the design correction mechanism.

Figures 10, 11 and 12 disclose different embodiments of the register mark 19, that are however all based on the use of topography modifications on web material surface forming two pairs of parallel lines.

The example of figure 10 is similar to that already described, with the only difference that lines 21, 24 have one common end to form a V and that, similarly, lines 22, 23 have an opposite common end so as to form an inverted V. Lines 21,22 and 23,24 intersect at respective intersection points P1 and P2. According to this configuration, within the range of lateral shift comprised between P1 and P2 lines 21, 22, 24 and 23 are always detected in this order, which simplifies the individual line recognition in case when substantial light scattering by the web material occurs, whereby lines 21, 23 are detected also by detectors 34, 36 and, viceversa, lines 22, 24 are detected also by detectors 33, 35. If this is the case, only two detectors can be used, as opposed to four.

The arrangement of figure 11, in which the two inverted V's are not partially superimposed but rather distinct from one another, ensures that that lines 21, 24, 23, 22 are always read in this order; although the overall dimensions of the register mark are larger, this arrangement allows a broader range of correctable transverse shifts.

Since the line order is fixed, two detectors only can be used in this case as well if the light scattering is high enough.

Finally, the arrangement of figure 12 is very similar to that of figure 3 in terms of line geometry; again, lines 21, 22, 23 and 24 form two crosses shifted from one another in the web feed direction. However, in this case lines 21 to 24 are not defined by compression crease lines, but rather by transition lines between different-level surfaces 43, 44. In particular, such lines are defined by the sides of a plurality of embossed portions 43 that alternate with unmodified surface portions 44 of the web material. Such lines are, in cross-section, "steps" between embossed portions 43 and unmodified portions 44 (figure 13).

In order to produce lines 21, 22, 23, 24, embossed portions 43 are defined by three isosceles triangles 50, 51, 52 and two rectangular triangles 54, 55 that are equal to one half of an isosceles triangle. Two of the isosceles triangles 50, 51 have their bases aligned in the web feed direction and a common base corner, the third isosceles triangle 52 has its base corners coincident with the vertex corners of the first two triangles. The rectangular triangles 54, 55 are located upstream and downstream of the third isosceles triangle in the web longitudinal direction, each with its short cathetus aligned in the web longitudinal direction and its hypotenuse aligned with one side of a respective isosceles triangle 51, 50.

The asymmetrical reflection pattern that originates when the laser beam scans a "step" allows to detect which specific line 21, 22, 23, 24 is being detected, because lines 21, 23 that have a first, e.g. "high to low", step profile will originate a first reflection pattern and lines 22, 24 that have a second, e.g. "low to high" step profile, will originate a second reflection pattern. As a consequence, if light scattering is sufficiently high only two detectors are enough, which allows the use of a simpler and cheaper detecting device 18.

On the grounds of the foregoing description, the advantages of the present invention over the prior art are clear. Register marks 19 are in perfect register with the crease pattern, can be selectively read with respect to the crease pattern, and do not require "external" reference or auxiliary indicia, such as printed optical codes, to fully establish the longitudinal and transverse position of the web material and its speed.