Field of the invention

The present invention relates to a converting machine for producing packaging containers such as flat-packed or folding boxes. In particular, the invention relates to an inspection system for detecting positions and alignments of printed colors and coatings.

Background of the invention Converting machines can be configured to produce packaging containers such as flat-packed or folding boxes from sheets substrates which are printed, cut and scored to form blanks. These blanks can subsequently be folded and assembled into three-dimensional boxes. The boxes are designed to be folded either manually or automatically in a folder-gluer machine. When the packaging containers and boxes are provided with a printed motif comprising a plurality of colors and various coatings, it is required that each color and coating is in the correct position on the blank and that the colors and coatings are aligned with each other.

To align the colors is referred to as setting the printing register and is often done by printing reference marks in the margins of the blanks and using a camera system to capture an image of a printed reference mark. The displacements of different elements in the reference mark can then be determined. Based on this information, printing units in the converting machine can be either manually or automatically adjusted. However, known systems have difficulties in detecting transparent coatings such as varnishes.

Summary of the Invention

In view of the above-mentioned problems, it is an object of the present invention to provide an inspection device which is capable of detecting coatings with a high precision. It would also be advantageous to provide an inspection device which additionally detects different colors with high precision.

This object is solved by an inspection device according to claim 1.

According to a first aspect of the present invention, there is provided an inspection device for checking the position of at least one coating on a blank transported through a converting machine, the inspection device comprising a camera configured to capture an image of a portion of the blank provided with a reference mark comprising at least one coating, wherein an optical axis of the camera is arranged at a first angle in relation to a vertical axis defined by a normal vector of a surface of the blank, and wherein the inspection device comprises an illumination system comprising a first illumination module having at least one illumination unit, and wherein the illumination unit is configured to emit incident light rays towards a measuring point on the surface of the blank, said emitted light rays forming a second angle in relation to the vertical axis, and wherein the first and second angles are selected such that incident light rays from the illumination unit are directed to the reference mark and specular reflected light rays from the reference mark are captured by the camera.

The present invention is based on a realization that reflective coatings can be detected by creating a mirror effect from the coatings which is received into the camera. The angle of the optical axis enables the camera to capture specular reflected light rays, such that the reflective coatings can be detected.

The specular reflected light rays are captured by the camera as they are directed into an entrance pupil of a camera lens.

The entrance pupil is an optical aperture from the blank. Hence from the object side. In other words, the entrance pupil can be defined as an optical opening into the camera through which light can enter.

The surface of the blank may be a horizontal surface. Consequently, the vertical axis may be coinciding with the gravitational direction. The measuring point is located on the surface of the blank which is provided with the printed reference mark.

The term “coating” within the context of this application is transparent, i.e. colorless. It is the transparent nature of the coating which makes it invisible for a conventional camera system. The reference mark can be a composed reference mark comprising a plurality of individual reference marks. The reference mark thus comprises at least one individual reference mark printed with a coating. Each individual reference mark may be printed by a separate flexographic printing cylinder.

The first illumination module may be configured to illuminate a reference mark which generates a specular reflection. The reference mark which generates a specular reflection may comprise a varnish.

In an embodiment, the first illumination unit of the first illumination module comprises a diffusive layer. The first illumination unit may further comprise a plurality of light sources arranged side-by-side on a circuit board, and wherein the light sources are covered by the diffusive layer.

In an advantageous embodiment, the reference mark comprises at least a first individual reference mark and a second individual reference mark, and wherein the inspection device comprises a second illumination module configured to illuminate an individual reference mark configured to generate a diffuse reflection of light rays, the second illumination module comprising at least one illumination unit which is arranged at a third angle in relation to the vertical axis, the third angle being selected such that incident light rays from the at least one illumination unit are directed to the reference mark and specular reflected light rays from the reference mark are directed outside of the entrance pupil of the camera.

The second individual reference mark is printed in a color. The colors are printed with an opaque ink comprising at least one colorant, such as a dye or pigment. The colors typically generate a diffuse reflection of light.

The second illumination module may comprise at least two illumination units, wherein a first and second illumination units are arranged on opposite sides of the optical axis of the camera. The first and second illumination units are preferably elongated and comprise a plurality of light sources arranged in a line. The longitudinal extension of the illumination units is arranged perpendicular in relation to the direction of transportation of the blank.

The first and second illumination units can be positioned at large angles towards the field of view on the blank. This enables light rays from both sides to be received outside the entrance pupil of the camera and a homogeneous illumination can be achieved.

In an embodiment, the first and second illumination units only comprise light sources located at the extremities of their elongated extension. In such a way, the illumination units are arranged in a square around the optical axis of the camera. This provides an equal distribution of the illumination units around the camera. As a result, a homogeneous illumination in the field of vision of the camera can be provided.

The light intensity from the first and second illumination modules can be varied. By varying the light intensity, the balance between glare and strong illumination of the reference mark can be optimized to achieve a precise captured image of the reference mark.

In an embodiment, the first illumination module can be disabled. This may be advantageous if the reference mark does not comprise a reflective coating.

In another embodiment, the second illumination module can be disabled. If the blank contains low-contrast color marks against a background color of the blank, but the marks exhibit different reflection characteristics as the background, it may be advantageous to illuminate only with the first illumination module.

In a preferred embodiment, the inspection device is mounted inside a housing shroud. The inspection device may further comprise a slide rail extending transversely in relation to the transportation path of the blank, and wherein the inspection device is configured to be displaced along the slide rail. The slide rail extends in perpendicular over or under the transportation path of the blank.

In an embodiment, the camera is triggered by a time signal from a control unit which is issued when an optical sensor registers a detection of a front leading edge of the blank, and wherein the time signal corresponds to an arrival time of the reference mark at a reflective illuminated area of the field of view of the camera: The reflective illuminated area can be provided by the first illumination module. Brief description of the drawings

The invention will now be described by way of example and with reference to embodiments shown in the enclosed drawings, where the same reference numerals will be used for similar elements and in which:

Figure 1a is a schematic plan view of a blank suitable for producing a box; Figure 1b is a detailed view of a first type of a reference mark at an edge of the blank;

Figure 1c is a detailed view of a second type of a reference mark;

Figure 2 is a schematic perspective view of a converting machine in the configuration of a rotary die-cutter; Figure 3 is a schematic diagram of a flexographic printing module and inspection device of an embodiment of the present invention;

Figure 4 is schematic perspective view of an inspection device according to an embodiment of the present invention;

Figure 5 is an exploded view showing the inspection device of figure 4; Figure 6 is a schematic cross-sectional view illustrating the mounting of a camera inside the inspection device;

Figure 7a is a schematic cross-sectional view showing emitted light rays from a first illumination module against a blank;

Figure 7b is a schematic cross-sectional view showing emitted light rays from a second illumination module against a blank;

Figure 8 is a schematic cross-sectional view illustrating the mounting of the inspection device inside the converting machine; and Figures 9a and 9b are images taken with and without a first illumination module of the preset invention.

Detailed description

Figure 1a illustrates an example of a blank 1 for a flat-packed or folding box. The blank 1 may be produced from cardboard, paperboard, plastics or like.

The blank 1 can be produced in a converting machine 10, such as the one illustrated in figure 2. The converting machine 10 is in the configuration of a rotary die-cutting machine 10. At an entry position of the converting machine 10, sheet substrates 1 are placed in a feeder module 14 and are transported in a direction of transportation T through the converting machine 10 in order to undergo a series of operations which print, cut and crease the sheet substrates 1 to form the blanks 1. Hence, within the context of this application, the term “blank” applies as the sheet substrate 1 has been provided with a printed motif from at least one printing unit. The direction of transportation T is defined from the inlet to the outlet of the converting machine 10. The blank 1 is transported along a transportation path P, which can be defined as the trajectory of the blank 1 through the converting machine 10.

From the inlet of the converting machine 10 and in a downstream direction along the direction of transportation T, the converting machine 10 may comprise a prefeeder 12, a feeder module 14, printing module 15, a die-cutting module 18, a bundle stacker module 20 and a palletizer-breaker 22. Optionally, a dryer module 13 (see figure 3) can be provided after the printing module 15 and is configured to dry the ink before the blank 1 enters into the die-cutting module 18. A main operator interface 11 may also be provided in the proximity of the converting machine 10.

As illustrated in figure 3, the printing module 15 comprises a plurality of flexographic printing units 16a to 16e. Each flexographic printing unit 16 comprises a flexographic printing assembly including a flexographic printing cylinder, and is configured to print an individual motif in a separate color or coating on the sheet substrate 1. The individual motifs together form the final motif 2 on the blank 1. Typically, at least four flexographic printing units 16a to 16d are provided in order to enable printing with different colors according to a large color palette. As best seen in figures 1b and 1c, a reference mark 30 is printed by the flexographic printing units 16 together with the motif 2. Each flexographic printing unit 16 is configured to print an individual reference mark 30’ at the same time as an individual motif is being printed onto the sheet substrate 1. In such a way, a composed reference mark 30 is created by the different flexographic printing units 16. The reference mark 30 is preferably located at a front leading edge 4 of the blank 1.

Optionally, an additional second reference mark 34 can be provided on the rear edge 6 of the blank 1. A reference mark 30 on the front leading edge 4 and a second reference mark 34 on the rear edge 6 facilitates the determination of rotational displacement shifts of the blank 1 in the flexographic printing module 15.

As shown in figure 1 b, the reference mark 30 may comprise a grid 36 and a plurality of dot-shaped individual reference marks 30’ arranged in the grid 36. The grid 36 is typically printed by the first flexographic printing unit 16a together with a separate dot-shaped reference mark 30’ of a first color. The grid 36 is provided with a predetermined height H and length L.

As the sheet substrate 1 is conveyed through the flexographic printing module 15, each flexographic printing unit 16 is printing a dot-shaped reference mark 30’ in the grid 36. If the colors and coatings are aligned and thus perfectly registered, each dot-shaped reference mark 30’ is located in a predetermined position in the grid 36, such as in the center of the grid 36.

Alternatively, as illustrated in figure 1c, the grid 36 can be excluded and only a dot shaped reference mark 30’ is printed by each flexographic printing unit 16. This reference mark 30 displays the positions and alignment of the different colors and coatings in two dimensions by interrelated distances in X- and Y- coordinates.

As illustrated in figure 3, the converting machine 10 comprises a printing quality control system 40 configured to detect the positions of the individual motifs and the alignment between the different individual motifs transferred from each flexographic printing unit 16 onto the sheet substrate 1. The printing quality control system 40 comprises an inspection device 42, a control unit 44 and a memory 46.

The printing quality control system 40 is configured to detect and measure a longitudinal displacement and a lateral displacement between the different colors and coatings in the reference mark 30. The longitudinal displacement is in the direction of transportation T, and the lateral displacement is in a direction perpendicular to the direction of transportation T. In such a way, the printing register, i.e. the positions and the alignment between the different colors and coatings can be determined. If the printing units 16 are not correctly registered in relation to each other, the final motif 2 will show a misalignment of individual motifs printed in different colors.

The printing quality control system 40 is configured to calculate longitudinal and lateral displacements and send corrective information to a central control system 48 of the converting machine 10. The corrective information includes required adjustments in the angular and lateral positions of the printing cylinders of the printing module 15. The converting machine 10 may be configured to automatically adjust the angular and lateral positions of the printing cylinders. Alternatively, the printing quality system 40 can display corrective information needed for a manual adjustment of the printing module 15 on a machine interface 11.

If the printing quality control system 40 detects defective blanks 1 with misaligned colors and coatings, the central control system 48 can send information to an ejector module 17, which discards the blank 1.

As best seen in figures 4, 6, 7a and 7b, the inspection device 42 comprises an imaging system 49 and an illumination system 50. The imaging system 49 can be a camera 49 with an active pixel sensor (e.g. a CMOS sensor) having an interface protocol configured to deliver images to the control unit 44. The camera 49 is configured to receive light rays from the blank 1 within its field of view 51.

As illustrated in figure 8, the inspection device 42 can be mounted to a slide rail system 45, also referred to as a “sliding rail system 45”. The slide rail system 45 comprises a longitudinal slide rail 47 extending in a direction perpendicular to the direction of transportation T.

Referring back to figure 3, an optical sensor 52 may be placed upstream and in proximity with the camera 49 and can be configured to detect the arrival of the front leading edge 4 of the blank 1. The camera 49 is triggered by a time signal from the control unit 44 which is issued when the optical sensor 52 registers a detection of the front leading edge 4 of the blank 1. The inspection device 42 is mounted downstream of the flexographic printing module 15. As illustrated in figure 3, the inspection device 42 is located below the transportation path P of the blank 1. However, it is also possible to position the inspection device 42 above the above transportation path P of the blank 1. The inspection device 42 is thus located such that the illumination system 50 and the field of view 51 of the camera 49 are directed towards a printed side of blank 1. If the converting machine 10 is provided with a dryer module 13, the inspection device 42 can be located after the flexographic printing module 15 and the dryer module 13. Alternatively, the inspection device can be located between the flexographic printing module 15 and the dryer module 13.

As best seen in figures 6, 7a and 7b, the camera 49 has an optical axis A, which is a straight line passing through the geometrical center of a lens 53 of the camera 49. The optical axis A is arranged at a first angle f in relation to the direction defined by a normal vector N of the printed sheet surface of the blank 1.

As illustrated in figure 4, the illumination system 50 comprises a first illumination module 56 comprising at least one illumination unit 57. As best seen in figure 7a, light emitted from the first illumination unit 57 towards a measurement point Pm on the blank 1 forms a second angle -a in relation to a vertical axis V defined by the normal vector N of the sheet surface of the blank 1. The second angle -a is a negative angle. The measurement point Pm is preferably located in the reference mark 30.

The absolute value of the second angle -a and the first angle f may be equal. However, the first angle f of the optical axis A is a positive angle.

Within the context of this application, a positive angle results from counterclockwise rotation from the vertical axis V. Consequently, a negative angle results from clockwise rotation from the vertical axis V.

As illustrated in figure 7a, a blank 1 with a reflective surface is illuminated by the first illumination unit 57. The second angle -a of the first illumination unit 57 is selected such that incident light rays from the first illumination unit 57 are directed to the reference mark 30 and specular reflected light rays from the reference mark 30 are directed into an entrance pupil 55 of the camera lens 53. Coatings such as varnishes are highly reflective, which makes them difficult to detect without creating a “mirror-reflection” effect into the entrance pupil 55 of the camera lens 53. These types of coatings generate a specular reflection when illuminated.

The first illumination unit 57 is configured to emit diffused light rays which are directed towards the reference mark 30 from multiple directions. This ensures that some reflected specular light rays are received through the entrance pupil 55 of the camera lens 53. The first illumination unit 57 comprises at least one light source 58 and a diffusive layer 59. The diffusive layer 59 is positioned over the at least one light source 58. The diffusive layer 59 is configured to scatter the transmitted light rays from the light source 58 and provide a homogenous radiating surface of diffused light. The diffusive layer 59 can be made from an optically diffusive material, such as Polymethylmethacrylat.

In the illustrated embodiment, the first illumination unit 57 is configured such that only light rays which are reflected by a portion of the field of view 51 on the blank 1 are received through the entrance pupil 55 of the camera lens 53. This portion is referred to as reflective illuminated area Ria. Hence, the reflective illuminated area Ria on the blank 1 is of a smaller surface area than area of the field of view 51 on the blank 1. The reference mark 30 thus needs to be placed in the reflective illuminated area Ria of the field of view 51 on the blank when an image of a reflective reference mark 30 is captured by the camera 49.

The camera 49 can be triggered by a time signal from the control unit 44 which is issued when the optical sensor 52 registers a detection of the front leading edge 4 of the blank 1. The signal can be set to a time which corresponds to an arrival time of the reference mark 30 at the reflective illuminated area Ria of the field of view 51.

In an embodiment, the first illumination unit 57 can be elongated with a plurality of light sources 58 arranged side by side. The longitudinal extension of the illumination unit 57 is arranged perpendicular in relation to the direction of transportation T. The longitudinal direction of the light sources is also arranged perpendicular in relation to the direction of transportation T and coincides with a longitudinal extension of the reference mark 30 on the blankl . The light sources 58 can be arranged in a row or in a plurality of rows. The light sources 58 may be arranged on a printed circuit board (PCB). The distances between the light sources are selected such that a homogeneous illumination of the diffusive layer 59 is obtained.

When the inspection device 42 is mounted in the converting machine 10, the camera axis A is arranged at the first angle f in relation to the vertical axis V. The horizontal axis is defined by the printed surface on the blank 1 and the vertical axis is perpendicular thereto. The first angle f enables the camera 49 to capture specular light rays reflected at an angle from the reference mark 30. The first angle f may be between 1° and 15°, and preferably about 5°.

In a preferred embodiment, a second illumination module 60 is also provided. The second illumination module 60 is configured to illuminate printed colors which generate a diffuse reflection when illuminated with specular light rays.

These types of colors include for instance water-based or solvent-based inks. Due to the diffuse reflection of the light rays from the reference mark 30, the camera 49 will receive reflected light rays into the entrance pupil 55 of the camera lens 53. The second illumination module 60 is configured to provide a homogeneous illumination of the reference mark 30 on the blank 1.

As best seen in figures 4 and 7b, which illustrate a reflective surface on a blank 1 which is illuminated with the second illumination module 60. The second illumination module 60 comprises at least one illumination unit 62, 63, 65 which is arranged to emit light at a third angle b in relation to the vertical axis V defined by a normal vector N of the surface of the blank 1. The third angle b is selected such that incident light rays from the at least one illumination unit 62, 63, 65 of the second illumination module 60 are directed to the reference mark 30 and specular reflected light rays from the reference mark 30 are directed outside the entrance pupil 55 of the camera lens 53. This enables the camera 49 to capture a sharp image of the reference mark 30 without glare. Hence, when illuminating a reflective surface, the specular reflected light rays are not received in the entrance pupil 55 of the camera lens 53. The full field of view 51 on the blank 1 may be illuminated by the second illumination module. The at least one illumination unit 62, 63, 65 can be elongated and may comprise a plurality of light sources 64 arranged in a line. The longitudinal extension of the at least one illumination unit 62, 63, 65 is arranged perpendicular in relation to the direction of transportation T of the blank 1.

The at least one illumination unit 62, 63, 65 of the second illumination module 60 may comprise a continuous line of light sources 64 arranged at a constant distance from each other. Alternatively, the at least one illumination unit 62, 63, 65 may only comprise light sources 64 located at the extremities of the line. In such a way, the light sources 64 are arranged in a square around the camera 49.

In an embodiment, an additional second illumination unit 63 is arranged on an opposite side of the optical axis A of the camera 49 in relation to the first illumination unit 62. In such a way, a further improved and homogenous illumination of the field of view 51 on the blank 1 can be achieved. In an embodiment, a third illumination unit 65 is further provided on at least one of the sides of the camera 49.

Each illumination unit 62, 63, 65 may be configured to emit light towards the blank 1 at a different third angle b. Hence, in the illustrated example in figure 7b, there are three illumination units 62, 63, 65 and their respective angles of emitted light can be referred to as b1, b2, b3. These angles are selected such that the specular reflection of light rays is directed outside of the entrance pupil of the camera 49. As long as the reflected light rays are not received in the entrance pupil 55 of the camera 49, the angles b1, b2, b3 may all be different.

The first illumination module 56 and the second illumination module 60 can be operated at the same time, whereby the camera 49 captures one image of the reference mark 30. Alternatively, either the first illumination module 56 or the second illumination module 60 is operated and an image can be captured by the camera 49. In another embodiment, only one of a plurality of illumination units 62, 63, 65 of the second illumination module 60 is operated.

For inks generating a diffuse reflection when illuminated, the first illumination module 56 can be disabled. Depending on the color and reflective characteristics of the coatings, it can thus be sufficient to only illuminate the reference mark 30 with the second illumination module 60. In such a way, reflections from reflective surfaces on the blank can be avoided. This is illustrated in figure 9b where the blank 1, when illuminated with the first illumination module 56, generates reflections in smooth areas which have been unintentionally rubbed by friction in the converting machine 10. In figure 9a, the blank 1 is only illuminated with the second illumination module 60 and displays less reflections.

The light intensity from the first 56 and second illumination modules 60 can be varied. This makes it possible to adapt the illumination settings depending on the reference mark characteristics. Especially for reflective coatings (inks or varnishes), the illumination can be calibrated to obtain a detectable reflection.

As best seen in figures 5 and 6, the camera 49 is mounted inside an external housing shroud 70 of the inspection device 42. On a top side of the housing shroud 70, a cover 72 is provided. The cover 72 is provided with a transparent surface 73, such as a glass surface 73. The external housing shroud 70 is designed to provide a hermetically sealed housing which is arranged around and encloses the camera 49. The level of sealing level may for instance be IP64.

The external housing shroud 70 may comprise a wall 74 with varying thickness. The varying thickness enable a larger traversing wall for fasteners 71 and provides rigidity to the wall 74. The wall 74 of the housing shroud 70 may further comprise a biased section 75, which forms the first angle f with the longitudinal extension of the external housing shroud 70. This allows the optical axis A of the camera 49 to form the first angle f with the vertical axis V. The vertical axis V coincides with the longitudinal direction of the external housing shroud 70, and the camera 49 is thus directed through an opening 76 in the cover 72 arranged in-between the illumination modules 56, 60.

A thermoelectric element 78 is arranged between the camera 49 and the external housing shroud 70. The thermoelectric element 78 can be a Peltier element 78. The camera 49 comprises an optical module 49a and an electronic processing module 49b. The electronic processing module 49b comprises electronic parts that are heat sensitive. The camera 49 is preferably arranged within the inspection device 42 such that the electronic parts are arranged close to the thermoelectric element 78. In such a way, the electronic processing module 49b is thermally connected to the thermoelectric element 78. An isolating inner housing 80 is arranged inside the external housing shroud 70 and is configured to enclose the camera 49. The inner housing 80 may comprise a first housing part 80a arranged around the electronic processing module 49b of the camera 49. A second housing part 80b can be arranged around the optical module 49a of the camera 49. The second housing part 80b can be tubular.

The first housing part 80a may comprise a recess 82, in which the second housing part 80b is partially received. This allows for a modular design and access to the optical module 49a of the camera 49 without dismantling the first housing part 80a.

As best seen in figure 5, the first housing part 80a comprises an isolating portion 83 and a thermally conductive portion 84. The thermally conductive portion 84 comprises a heat-conductive plate 84, for instance a metal plate. For example, the heat-conductive plate 84 can be made from aluminum or silver. The thermoelectric element 78 is positioned between the heat-conductive plate 84 and the external housing shroud 70. The heat-conductive plate 84 distributes and spreads the cold from the thermoelectric element 78 to the electronic processing module 49b of the camera 49. The camera is fixed to the external housing shroud 70 by at least one fastener 71. In the illustrated embodiment, a plurality of fasteners 71, such as four fasteners 71 connect the inner housing 80 of the camera 49 to the external housing shroud 70.

The thermoelectric element 78 produces a warm side and a cold side when a current is applied over the two sides. Consequently, the cold side of the thermoelectric element 78 is in contact with the thermally conductive plate 84 and the warm side of the thermoelectric element 78 is in contact with the external housing shroud 70. In such a way, the processing module 49b of the camera 49 is cooled, while the external housing shroud 70 can be used to transfer the heat away from the thermoelectric element 78.

As best seen in figure 3, the inspection device 42 may be placed under a vacuum transfer 9 of the flexographic printing module 15. Alternatively, the vacuum transfer can be located below the inspection device 42. The vacuum suction force from the vacuum transfer 9 induces an airflow over the external housing shroud 70 which provides a heat transfer to the ambient air. A dryer module 13 may be located after the flexographic printing module 15 to ensure that the ink is dried before the blank 1 travels to subsequent modules such as a die-cutting or a folding module. The dryer module 13 operates by blowing hot air at the printed side of the blank 1. By integrating the thermoelectric element 78 as previously described into the present inspection device 42, a cooling effect can be achieved to reduce heating the camera 49 with stray heat from the dryer. Moreover, a dust-free environment for the camera 49 can be obtained.