Printing machine-readable indicia on cardboard substrates

Abstract

Printing apparatus 10 has a laser marker 12, a printer 14 and a controller 16 that causes the laser marker 12 to irradiate a first region of a substrate to change a colour and / or a surface characteris

Description

This invention relates to printing apparatus, to a method of printing, and to a computer program executable by printing apparatus to carry out such a method, in particular for printing machine-readable indicia on cardboard substrates. Background to the Invention Printing an indicium intended to be machine-readable, such as a bar code or a data matrix, on a cardboard substrate presents two potential problems. First, when printing on a natural-coloured cardboard substrate, e.g., an unbleached cardboard box, regardless of whether a light- or darkcoloured ink is chosen, a contrast of a printed indicium with a natural colour of the cardboard substrate will be low. Second, when printing on a coated cardboard substrate with an ink that relies at least in part on absorption into the substrate for drying, such as a water- or oil-based ink, the ink tends to spread across the coated surface of the substrate rather than being absorbed into the substrate, which results in a blurred outline of a printed indicium. Machine vision systems typically struggle to read indicia with low contrast and / or blurred outlines. WO 2023 / 165960 'Cajo' is concerned with methods of marking indicia intended to be machine-readable on a natural-coloured cardboard substrate using a carbon dioxide laser. Cajo suggests three such methods, namely exposing regions of the substrate to laser radiation with an energy per unit area that lightens those regions relative to a natural hue of the substrate, as shown in Figure 3 of Cajo; exposing regions of the substrate to laser radiation with an energy per unit area that darkens those regions relative to the natural hue of the substrate, as shown in Figure 4 of Cajo; and exposing some regions of the substrate to laser radiation that lightens those regions and exposing other regions of the substrate to laser radiation that darkens those regions, as shown in Figure 5 of Cajo. The first two methods form indicia with relatively low contrast, because the lightened or darkened regions of the substrate are surrounded by regions with the natural hue of the substrate. The third method forms indicia with relatively high contrast, because the lightened regions of the substrate are surrounded by darkened regions, or vice versa. Such indicia would be expected to be reliably machine-readable, but would require a laser with relatively complex, and therefore expensive, control to obtain the lightened and darkened regions of the substrate, for example by modulating an output power of the laser and / or a speed at which a laser beam moves relative to the substrate. Summary of the Invention According to a first aspect of the invention there is provided printing apparatus comprising a laser marker, a printer and a controller operable to cause the laser marker to irradiate a first region of a substrate to change a colour and / or a surface characteristic of the first region, and to cause the printer to deposit ink on a second region of the substrate, wherein the second region is adjacent to or forms part of the first region. The invention can provide printing apparatus that can print a machine-readable indicium on a naturalcoloured cardboard substrate using an inexpensive laser marker and a printer, by changing a colour of a first region of the substrate to a first colour using the laser marker and depositing ink of a second, contrasting colour on a second region of the substrate, which second region is adjacent to or forms part of the first region. The invention can further provide printing apparatus that can print a machine-readable indicium on a coated cardboard substrate using ink that relies at least in part on absorption into the substrate for drying, such as water- or oil-based ink, by removing a coating of a first region of the substrate using the laser marker and depositing the water- or oil-based ink on a second region of the substrate, which second region forms part of the first region. In preferred embodiments of the invention the laser marker is a carbon dioxide laser marker, although it may be possible to use a different laser marker such as a diode laser marker. The lasers of carbon dioxide laser markers usually have a wavelength of 10.6 pm. Preferably, however, a laser of the laser marker has a wavelength of 9.3 pm. Although it is envisaged that the printer may be of any type, such as a thermal transfer printer or a pad printer, preferably the printer is an inkjet printer such as a valvejet printer or a continuous inkjet printer, and more preferably still a piezo inkjet printer. The controller may be operable to cause the printer to deposit ink on the second region of the substrate before causing the laser marker to irradiate the first region of the substrate. Preferably, however, the controller is operable to cause the laser marker to irradiate the first region of the substrate before causing the printer to deposit ink on the second region of the substrate. Where the controller is so operable, the second region can form part of the first region, such that exact register of the first and second regions is not necessary, and the design of the controller can be simplified. The controller may advantageously be operable to cause the laser marker to irradiate the first region of the substrate to darken a colour of the first region. Darkening a colour of the first region of the substrate typically requires more energy per unit area than lightening a colour of the first region of the substrate, but can be achieved with a relatively wide range of laser technologies. Preferably, therefore, the controller is operable to cause the laser marker to irradiate the first region of the substrate to lighten a colour of the first region. In addition to requiring less energy per unit area than darkening a colour of the first region of the substrate, where the controller is so operable, the printer may advantageously be configured to deposit inks of two or more colours on the second region of the substrate. The controller may advantageously be operable to cause the laser marker to irradiate the first region of the substrate to remove a coating of the first region. Where the controller is operable to cause the laser marker to irradiate the first region to remove the coating of the first region, it is essential that the second region on which the ink is deposited form part of the first region. Where the controller is operable to cause the laser marker to irradiate the first region to change the colour of the first region, energy consumption of the printing apparatus may be reduced, at the expense of increased complexity of the controller, by depositing ink on a second region that is adjacent to the first region. That is to say, any region on which ink is or is to be deposited is not irradiated by the laser marker. Preferably, however, for simplicity of the controller, the second region on which the ink is deposited forms part of the first region. That is to say, the region on which the ink is deposited is irradiated by the laser marker, typically before, but potentially after, the ink is deposited. The controller may advantageously be operable to cause the laser marker to irradiate the first region of the substrate by scanning a laser beam in generally perpendicular scan directions relative to the substrate. Mechanisms for scanning a laser beam in generally perpendicular scan directions relative to a substrate are well known and readily available, albeit expensive. Preferably, therefore, the controller is operable to cause the laser marker to irradiate the first region of the substrate by repeatedly scanning a laser beam in a fast scan direction while the substrate is moved relative to the printing apparatus in a slow scan direction that is generally perpendicular to the fast scan direction. It may also be possible, using a laser marker of sufficient power and means for spreading a bame of the laser marker, to cause the laser marker to irradiate the first portion of the substrate by spreading a laser beam in a first direction while the substrate is moved relative to the printing apparatus in a second direction that is non-parallel to, and preferably generally perpendicular to, the first direction. It will be appreciated that the controller may be operable to cause the laser marker to irradiate two or more first regions of the substrate and / or to cause the printer to deposit ink on two or more second regions. As an example, when printing a data matrix, the controller may be operable to cause the laser marker to irradiate one first region of the substrate that surrounds a second region on which ink is deposited to form the data matrix, and one or more further first regions that are surrounded by the second region. As another example, when printing a bar code, the controller may be operable to cause the printer to deposit ink on multiple second regions that constitute bars of the bar code and are surrounded by a first region. According to a second aspect of the invention there is provided a method of printing, the method comprising a first step of causing a laser marker to irradiate a first region of a substrate to change a colour and / or a surface characteristic of the first region and a second step of causing a printer to deposit ink on a second region of the substrate, wherein the second region is adjacent to or forms part of the first region. Preferably the first step precedes the second step. Where the first step precedes the second step, preferably the second region forms part of the first region. In one embodiment the first step comprises causing the laser marker to irradiate the first region of the substrate to lighten a colour of the first region. In another embodiment the first step comprises causing the laser marker to irradiate the first region of the substrate to remove a coating of the first region. Where the first step precedes the second step, it is envisaged that the first step of causing the laser marker to irradiate the first region of the substrate could be carried out an hour, a day or an even longer period of time before the second step of causing the printer to deposit ink on the second region of the substrate is carried out. Preferably, however, the method further comprises transporting the substrate along a production line and the steps of causing the laser marker to irradiate the first region and causing the printer to deposit ink on the second region of the substrate are carried out as the substrate is transported along the production line past the laser marker and printer, the printer being located downstream from the laser marker in a transport direction of the production line. That is to say, the second step is carried out within minutes, and preferably within seconds, of the first step. The first step may advantageously comprise causing the laser marker repeatedly to scan a laser beam in a fast scan direction while the substrate is transported relative to the laser marker in a slow scan direction that is generally perpendicular to the fast scan direction. Preferably the second step comprises causing the printer to deposit ink of two or more colours on the second region of the substrate. The step of causing the printer to deposit ink on the second region of the substrate may advantageously comprise causing the printer to deposit an ink that relies at least in part on absorption into the substrate for drying, such as a water- or oil-based ink, on the second region of the substrate. Inks that rely at least in part on absorption into a substrate for drying, such as water- or oil-based inks, are particularly well suited to printing on cardboard substrates because the carriers of such inks simply soak into such substrates without requiring the use of drying or curing apparatus. The inventors believe that the method of the invention may also be applicable to clear or translucent substrates, such as the cellulose compostable material sold by Futamura Group of Japan under the trade mark NATUREFLEX. Such clear or translucent substrates give rise to the same problem of low contrast for machine vision systems as natural-coloured cardboard substrates. Causing a laser marker to irradiate a region of such a clear or translucent substrate can cause that region of the substrate to whiten and / or become opaque, possibly due to the formation of micro-foams that increase light scattering, giving the region a white and / or opaque appearance. According to a third aspect of the invention there is provided a computer program executable by printing apparatus to carry out the method of the second aspect of the invention. Brief Description of the Drawings The invention will now be described, by way of example, with reference to the attached drawing figures, in which: Figure 1 is a schematic perspective view of a production line that includes the printing apparatus of the invention; Figure 2 is a schematic representation of a portion of a cardboard box printed with a machine-readable indicium using the printing apparatus of Figure 1; and Figure 3 is a flow chart of a computer program to carry out the method of the invention. Detailed Description of Embodiments The printing apparatus 10 shown in Figure 1 comprises a ** what type? ** carbon dioxide laser marker 12, a Cx350i piezo inkjet printer 14 (both from Domino Printing Sciences PLC of Cambridge, England) and a controller in the form of a programmed personal computer (PC) 16. The printing apparatus 10 is positioned to one side of a conveyor belt 18 that transports cardboard boxes 20, 22 past the printing apparatus 10 in a direction indicated by arrow 24. A rotary encoder (not shown) and a product detector (not shown) are connected to inputs 26, 28 of the PC 16 and transmit signals to the PC 16 indicative, respectively, of movement of the conveyor belt 18 and the presence of a cardboard box in a marking zone 30 of the printing apparatus 10. In use of the printing apparatus 10, cardboard boxes 20, 22 are transported by conveyor belt 18 past the product detector, which, in a manner that will be familiar to the skilled person, detects when a leading edge of a cardboard box enters the marking zone 30 and transmits a signal to the PC 16. As the cardboard box advances in the direction of the arrow 24, the rotary encoder transmits signals to the PC 16 indicative of the movement of the cardboard box along the path of the conveyor belt 18. When the signals transmitted by the rotary encoder to the PC 16 indicate that the cardboard box has advanced a predetermined distance after the leading edge of the cardboard box entered the marking zone 30, the PC 16 controls the laser marker 12 to scan a laser beam in a direction perpendicular to the surface of the conveyor belt 18 to scribe a vertical line on a side of the cardboard box that faces the laser marker 12. As the cardboard box continues to advance along the path of the conveyor belt 18, the PC 16 receives signals from the rotary encoder and, in dependence upon the signals received from the rotary encoder, controls the laser marker to scan the laser beam to scribe a series of adjacent, vertical lines on the side of the cardboard box that faces the laser marker 12. The direction perpendicular to the surface of the conveyor belt 18 in which the laser beam is scanned to scribe vertical lines on the side of the cardboard box is a fast scan direction and the direction, indicated by arrow 24, in which the cardboard box is transported, is a slow scan direction. The scribing of the vertical lines by the laser marker 12 continues until the signals received by the PC 16 from the rotary encoder indicate that the cardboard box has advanced a further predetermined distance along the path of the conveyor belt 18, resulting in a rectangular, lightened region of the side of the cardboard box with vertical edges of the region extending perpendicular to the surface of the conveyor belt 18 and horizontal edges of the region extending parallel to the surface of the conveyor belt 18. Although in this description the vertical lines are all the same length, the invention is not limited to this and, by appropriate control of the start and end points of the vertical lines, lightened regions of any shape can be formed, such as a parallelogram or indeed any arbitrary shape. In dependence upon the signals received from the rotary encoder, the PC 16 controls the printer 14 to print a series of vertical strokes on the lightened region of the side of the box to mark an indicium in the form of a bar code within the lightened region. It will be appreciated that a bar code is only one example of what may be printed on the lightened region, other examples being text, images, graphics or other machine-readable indicia such as data matrices. It will also be appreciated that it is not necessary for the entire lightened region of the side of the cardboard box to be formed before the printer 14 starts to print the series of vertical strokes on the lightened region; the printer 14 may start to print the series of the strokes on a leading portion of the lightened region at the same time as the laser marker 12 forms a trailing portion of the lightened region. Turning to Figure 2, this represents a portion 50 of the side of the cardboard box on which the lightened region 52 has been formed and a bar code 54 has been printed using black oil-based ink in the lightened region 52. Compared with the natural brown colour (indicated by hatching in Figure 2) of the cardboard substrate, the lightened region 52 provides improved contrast with the black colour of the bar code 54, and correspondingly improved machine-readability of the bar code 54. The bar code 54 is printed using black ink on a lightened region 52 of a cardboard box. The bar code 54 could, of course, have been printed using light-coloured ink on a darkened region of the cardboard box. Nevertheless, it is preferable to print using dark-coloured ink on a lightened region of the cardboard box because darkening a given region of the box requires more laser energy than lightening the same region, and light-coloured inks with good opacity typically contain titanium dioxide pigment, which places greater demands on a printer than the pigments used in dark-coloured inks. Moreover, where the laser marker lightens the region of the box, it is possible using multiple printheads or a printhead with multiple rows of ink jetting nozzles, to print multicolour images with good contrast on a natural-coloured cardboard box. To quantify the improvement in contrast, the colour of a natural-coloured cardboard substrate was measured using an XRite spectrophotometer to be L* = 60.8, a* = 10.5, b* = 21.8, OD = 0.82. A region of the cardboard substrate was lightened by scribing parallel lines with a line pitch of 0.43 mm on the substrate using a D620i 9.3 pm wavelength carbon dioxide laser marker with an output power of 60 W and fitted with an i-Tech 15 scan head and 450 mm focal length objective lens. The distance from the objective lens to the substrate was 420 mm. The laser spot diameter and power density were calculated to be 485 pm and 325 Wmm'2 and the beam scanning speed was 14.8 ms1. The colour of the lightened region was measured using the XRite spectrophotometer to be L* = 70, a* = 5.7, b* = 15.1, OD = 0.55. Bar codes were printed on the natural-coloured cardboard substrate and on the lightened portion using oil-based I-962BK ink jetted from a Cx350i piezo inkjet printer (both from Domino Printing Sciences PLC of Cambridge, England). The resulting bar codes were verified according to the ISO / IEC15416 standard using an LVS-9510 bar code verifier. The LVS-9510 bar code verifier gives several measurements of the readability of a bar code, including a contrast measurement. The bar code printed on the natural-coloured cardboard substrate had a contrast of 1.7 whereas the bar code printed on the lightened portion had a contrast of 2.3. Figure 2 could equally well represent a portion 50 of the side of a coated cardboard box (some cardboard boxes, particularly those intended for use in humid climates, are manufactured with a coat of varnish to protect the cardboard proper of the box) from a region 52 of which a coating has been removed by irradiation by the laser marker and a bar code 54 has been printed using black oil-based ink in the irradiated region 52. Compared with the coated region (indicated by hatching in Figure 2) of the cardboard substrate, the irradiated region 52 provides reduced spreading of the oil-based ink of the bar code 54, and correspondingly improved machine readability of the bar code 54. Where the coating is coloured, the irradiation by the laser marker may both remove the coating and change a colour of the substrate, by exposing a region of the substrate beneath the coating. A coated cardboard substrate, the coating having an orange colour, was irradiated to scribe parallel lines with a line pitch of 0.43 mm on the substrate using a D620i 9.3 pm wavelength carbon dioxide laser marker with an output power of 60 W and fitted with an i-Tech 15 scan head and 325 mm focal length objective lens. The distance from the objective lens to the substrate was 320 mm. The laser spot diameter and power density were calculated to be 350 pm and 624 Wmm 2 and the beam scanning speed was 5.8 ms1. The orange-coloured coating was more or less removed, exposing the cardboard proper, which was white-coloured. Data matrices were printed on the exposed cardboard using oil-based I-962BK ink jetted from a Ricoh MH5320 Gen6 piezo inkjet printhead operated by a controller from Meteor Inkjet Ltd of Cambridge, England. The resulting data matrices achieved grade B (possible grades are A, B, C, D and FAIL), which is considerably better than would be expected using an oil-based ink on a coated cardboard substrate. Turning finally to Figure 3, this shows the method steps carried out by a computer program executed by the controller 16. At step 100 the program starts and at step 102 determines whether a leading edge of a cardboard box has triggered a product detector, indicating that a cardboard box has entered the marking zone of the printing apparatus. If so, execution proceeds to steps 104 and 106, which are the first steps of two parallel threads; if not, execution returns to step 100. At step 104 the program awaits a pulse from the rotary encoder. Upon detecting an encoder pulse, at step 108 the program determines whether a first target count of encoder pulses has been reached, indicating that a leading edge of a region to be marked of the cardboard box is aligned with the laser marker. If so, execution proceeds to step 110; if not, execution returns to step 104. To avoid confusion, execution of the thread starting at step 104 will be described in full before describing execution of the thread starting at step 106, even though the threads are executed in parallel. At step 110 the program obtains start and end points of a line (or lines if a broken line is to be scribed on the cardboard box) and scans the laser beam, turning the laser beam on and off and / or controlling the scanning speed of the laser to scribe a line or lines in accordance with the start and end points. At step 112 the program determines whether all the required lines have been scribed. If so, execution of the thread ends at step 114; if not, execution returns to step 110 to obtain start and end points of the next line to be scribed. Turning to the thread starting at step 106, the program awaits a pulse from the rotary encoder. Upon detecting an encoder pulse, at step 116 the program determines whether a second target count of encoder pulses, greater than the first target count, has been reached, indicating that a leading portion of the marked region of the cardboard box is aligned with the ink jetting nozzles of the printer. If so, execution proceeds to step 118; if not, execution returns to step 106. At step 118 the program obtains pixel data and causes the printer to jet a stroke of ink droplets on the marked region in accordance with the pixel data. At step 120 the program determines whether all the required strokes have been printed. If so, execution returns to step 100; if not, execution returns to step 118 to obtain pixel data for the next stroke to be printed. The threaded approach described above enables the printer to start printing on a leading portion of the marked region of the cardboard box as a trailing portion of the marked region is marked by the laser marker. Where a non-threaded approach is preferred, execution can pass to step 106 from step 114, instead of step 102, provided that the second target value is increased appropriately. It will be appreciated that the above description relates only to two embodiments of the invention, and that the invention encompasses other embodiments as defined by the claims.

Claims

1. Printing apparatus comprising a laser marker, a printer and a controller operable to cause the laser marker to irradiate a first region of a substrate to change a colour and / or a surface characteristic of the first region, and to cause the printer to deposit ink on a second region of the substrate, wherein the second region is adjacent to or forms part of the first region.

2. Printing apparatus according to claim 2, wherein a laser of the laser marker has a wavelength of 9.3 pm.

3. Printing apparatus according to any preceding claim, wherein the printer is an inkjet printer.

4. Printing apparatus according to any preceding claim, wherein the controller is operable to cause the laser marker to irradiate the first region of the substrate before causing the printer to deposit ink on the second region of the substrate.

5. Printing apparatus according to claim 4, wherein the second region forms part of the first region.

6. Printing apparatus according to any preceding claim, wherein the controller is operable to cause the laser marker to irradiate the first region of the substrate to lighten a colour of the first region.

7. Printing apparatus according to any preceding claim, wherein the controller is operable to cause the laser marker to irradiate the first region of the substrate to remove a coating of the first region.

8. Printing apparatus according to any preceding claim, wherein the controller is operable to cause the laser marker to irradiate the first region of the substrate by repeatedly scanning a laser beam in a fast scan direction while the substrate is moved relative to the printing apparatus in a slow scan direction that is generally perpendicular to the fast scan direction.

9. A method of printing, the method comprising a first step of causing a laser marker to irradiate a first region of a substrate to change a colour and / or a surface characteristic of the firstportion and a second step of causing a printer to deposit ink on a second region of the substrate, wherein the second region is adjacent to or forms part of the first region.

10. A method according to claim 9, wherein the first step precedes the second step.

11. A method according to claim 10, wherein the second region forms part of the first region.

12. A method according to any of claims 9 to 11, wherein the first step comprises causing the laser marker to irradiate the first region of the substrate to lighten a colour of the first region.

13. A method according to any of claims 9 to 12, wherein the first step comprises causing the laser marker to irradiate the first region of the substrate to remove a coating of the first region.

14. A method according to any of claims 9 to 13, wherein the first step comprises causing the laser marker repeatedly to scan a laser beam in a fast scan direction while the substrate is transported relative to the laser marker in a slow scan direction that is generally perpendicular to the fast scan direction.

15. A method according to any of claims 9 to 14, wherein the second step comprises causing the printer to deposit ink of two or more colours on the second region of the substrate.

16. A method according to any of claims 9 to 15, wherein the second step comprises causing the printer to deposit an ink that relies at least in part on absorption into the substrate for drying on the second region of the substrate.

17. A computer program executable by printing apparatus to carry out the method of any of claims 9 to 16.

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