STITCHING METHOD AND SYSTEM

Abstract

The method includes providing at least one printhead for printing an image on a surface from two different printhead positions relative to the surface, the two different printhead positions being in first and second orientations, and the at least one printhead and the surface moving relatively along a print path, determining an overlap region on the surface of two overlapping swaths, the overlap region being printed from either of the two different printheads, and for each position within the overlap region at which an inkjet drop is printed, determining a projection angle at the surface of that drop, and selecting a stitch point or region within the overlap region where the difference between the projection angles at the curved surface of the drops from each of the two printhead positions is maintained within a tolerance.

Description

[Technical field]

[0001] Aspects of the present invention relate generally to inkjet printing systems, and more particularly to methods and systems that enable efficient image stitching between different swaths of a printed image when printing on curved surfaces. [Background technology]

[0002] Modern inkjet printing systems typically include a printhead with a number of drop ejection devices, also called "nozzles," that form a nozzle array. Each nozzle typically includes an actuator configured to eject ink from the nozzle upon actuation. Such actuators include, for example, piezoelectric actuators. The actuators are driven by drive electronics (electronic drive circuitry) that provide a voltage waveform or common drive signal configured to eject ink from the nozzle. For example, an actuation event generates a pressure pulse in the ink chamber of the nozzle, which in turn dispenses an ink drop.

[0003] In many applications, drive electronics supply a common drive signal to many nozzles, and a separate or integrated controller provides the printhead with data switching that determines which of the individual nozzles eject ink for a particular instance of an actuation event. The data for a group of nozzles associated with a shared actuation event is called "stripe data."

[0004] By arranging a coordinated sequence of drive signals and switching inputs, the printhead creates an image on the substrate in the form of a pixel array as the printhead and substrate (object surface) move relative to one another. This is applicable to, but not limited to, single pass and scanning (multiple pass) printing systems. The data of such coordinated sequences of actuation events, which are one or more instances of "stripe data", are called "swath data". The area addressed by each printhead during printing is typically known as a "swath".

[0005] Existing "stitching" techniques use precise drop placement to achieve the appearance of continuity between different swaths of a printed image. On flat surfaces, it is often sufficient to print adjacent swaths with a single border (known as flat stitching or butt stitching). In this situation, the only requirement is that the swaths are precisely registered. Also, on flat surfaces where some variation in drop size from different heads is observed, it is common to interleave a few rows of drops from two adjacent swaths to create a gradual transition between potentially different color densities. Summary of the Invention [Problem to be solved by the invention]

[0006] A curved surface typically presents the problem that inkjet drops are no longer ejected perpendicular to the surface. This means that the pitch at the surface between drops from adjacent nozzles is no longer the same as the nozzle pitch, but varies with the angle at which these drops are incident on the surface. As a result, differences in color density at the flat stitch boundaries between adjacent swaths will typically occur due to differences in dot pitch resulting from adjacent swaths being printed by inkjet drops that are incident on the surface at different angles. As a result, in both flat stitch and interleaved stitch, the boundaries between adjacent swaths may have gradations in density due to the different dot pitches.

[0007] DE 102018003096 recognises the problem that image pixels on a 3D surface "have different distances and orientations relative to the 3D surface" and attempts to solve the problem by adjusting the drop size. EP1990206A2 discusses the change in density of a single print path of a printhead translating linearly over a surface that moves away by having a radius or curvature. CN 103722886 discusses the change in density of a single print path of a printhead translating linearly over a surface that moves away by having a radius or curvature. on demand It discloses a method of spraying onto a surface rather than a printing method.

[0008] It is to these and other problems that the present invention seeks to solve. In particular, aspects of the present invention provide methods and systems for managing the above differences and achieving stitching with characteristics similar to those obtained when printing on a flat surface where the inkjet drops are incident perpendicular to the surface. [Means for solving the problem]

[0009] The method of the present invention includes the steps of providing first and second drop-on-demand printheads for printing an image on a curved surface from two different adjacent printhead locations with respect to two print path directions at first and second orientations relative to a print path direction relative to the curved surface; moving the curved surface relative to each of the first and second drop-on-demand printheads along a print path; determining an overlap region on the curved surface for two overlapping swaths of the printing path parallel to a central axis of the curved surface, determining an overlap region on the curved surface for two overlapping swaths of the printing path, a first swath corresponding to a first area addressed during printing by a first drop-on-demand printhead from a first printhead position, and a second swath corresponding to a second area addressed during printing by a second drop-on-demand printhead from a second printhead position, where the first swath and the second swath overlap; The two overlapping swarfs are parallel to a central axis of the curved surface, and the overlap area is printing from one of two different print head positions; determining a projection angle of an inkjet drop on the curved surface for a plurality of locations within the overlap region where the inkjet drop may be printed; The process comprises the steps of: selecting stitch points or areas within an overlap region on a curved surface for two overlapping swaths of a printing path where a first swath and a second swath overlap, such that a difference in the projection angle on the curved surface of drops from two different adjacent print head positions is maintained within a predetermined parameter range, thereby maintaining a difference in respective dot pitches within the stitch regions of the two different adjacent print head positions within an acceptable limit.

[0010] The method of the present invention is advantageous for printing on curved surfaces of various shapes. The advantage of the method of the present invention is that it allows the geometry of the print to be arranged such that the difference between the dot pitches is kept within the tolerance limits (e.g., within a predetermined parameter range) that meets the required print quality. Then, by keeping the difference between the first and second projection angles small, the stitch points or areas can be selected such that the projected dot pitches corresponding to the two print head positions closely match. In other words, since the difference between the projection pitches at the two print head positions is kept small, the dot pitches in the stitch areas of the two adjacent print head positions match within a tolerance range. There is no known prior art that allows the stitch points from each area to be at points where the difference in the projection angle, i.e., the dot pitch, is kept within a tolerance range, and this is an important feature of the present invention that is different from the prior art.

[0011] In other embodiments, two different adjacent printhead locations correspond to at least one same printhead, i.e., the same printhead passes multiple times along the printing path.

[0012] Alternatively, two different adjacent printhead positions correspond to the two printheads, respectively. The two printheads may have equal nozzle pitches. Preferably, the two printheads have the same orientation with respect to the print path direction. In one example, both of the two printheads are perpendicular to the printing process direction (direction of relative movement). Alternatively, the two separate printheads have different "native" nozzle pitches, resulting in different printed dot pitches. In another embodiment, the method of the present invention further comprises stitching the two swaths together at the selected stitch point.

[0013] A second independent embodiment is a control system for at least one printhead for printing an image on a curved surface from two different adjacent printhead positions in a first and second orientation with respect to a printpath direction relative to the curved surface, the at least one printhead and the curved surface being arranged to move relative to each other along the printpath, and for two overlapping swaths of the printpath, determining an overlap region on the curved surface to be printed from one of the two different printhead positions, the control system comprising a processor adapted to determine a plurality of positions in the overlap region where inkjet drops are printed at angles at which the drops strike the curved surface, and to select stitch points in the overlap region where the difference between the respective angles at which the drops strike the curved surface from the two different adjacent printhead positions is kept within predetermined parameters, whereby the difference between the respective dot pitches in the stitch region of the two different adjacent printhead positions is kept within a tolerance range. Preferred features of each of the independent claims are set out in the dependent claims. Aspects of the invention will now be described by way of example with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0014] [Figure 1] 1 is a schematic diagram showing how the projection angle of an ink drop on a surface affects the pitch of a printed dot. [Diagram 2] FIG. 1 is a schematic diagram of a printing system for printing on a surface such that a stitched region is achieved by inkjet drops having different projection angles onto the surface from position A and position B. [Diagram 3] FIG. 1 is a schematic diagram of a printing system for printing on a surface such that a stitched area is achieved by inkjet drops having the same projection angle onto the surface from position A and position B. [Figure 4] FIG. 1 is a schematic diagram showing two printhead positions A and B, positioned so that some of the printhead nozzles at each position print ink drops onto the same area of ​​the curved surface. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] An exemplary printing system with at least one printhead is provided for coating or decorating the "feature" of a printed object, including curved surfaces, which may be subject to tolerances on the order of hundreds of microns around a nominal shape (nominal object surface). A "printing path" refers to the movement of the print head (including the nozzle arrangement) relative to the surface for printing. Thus, a printing pass is the relative movement of the nozzle arrangement and the object (shape) during the printing process. A "printing path direction" refers to the printing direction (print process direction). A "path" refers to the multiple locations on the surface that pass under a nozzle. Determining the path of an individual nozzle gives the trajectory that the nozzle traces across the target surface. This relative motion is equivalent if the nozzle arrangement is static and the object moves, or if both move to provide the relative motion.

[0016] Printing on an object requires the generation of printhead nozzle data or the generation of a print image. Typically, printing an image is wider than the width of a single printhead, so multiple swaths are required to cover the target surface area, one from each printhead. A printing pass may have adjacent swaths overlapping. The human eye is very good at detecting discontinuities in a printed image, especially in areas of uniform color, so it is important to properly stitch each adjacent swath together. The stitched area of ​​two printing passes (the overlap area of ​​potential adjacent swaths) is typically 20-40 pixels (e.g., 2-4 mm), although it is understood that this can be smaller or larger.

[0017] The nozzles of an inkjet printhead are typically evenly spaced across the head, so that they print ink drops onto a flat surface at a uniform pitch. In some head designs, this is achieved by arranging the nozzles in a line at regular intervals, and these are known as linear heads. In other head designs, known as 2-D heads, the nozzles are arranged in a two-dimensional array, so that they are evenly spaced along the printhead, but also shifted in the printing direction. These nozzle configurations are well known, and it will be apparent to one skilled in the art that the present invention applies equally to heads of both designs.

[0018] It is also well known, especially with linear head designs, that the head can be used to print in an orientation other than perpendicular to the print direction. This technique is known as "raking" and can be used to change the effective nozzle pitch of the head. This technique is well known and it will be apparent to one skilled in the art that the invention applies equally to printing systems that use orthogonally oriented heads, raked heads, or both.

[0019] In Figure 1, nozzle 1 and nozzle 2 are adjacent nozzles in a printhead, separated by a distance called the head's native pitch. The nozzles eject ink drops along substantially parallel trajectories that are incident on a surface at an angle θ. The surface pitch is the distance between the points where the ink drops land. The relationship between native pitch (NP) and surface pitch (SP) is SP=NP / cosine(θ).

[0020] Unlike stitching on a flat surface, where swaths are printed by inkjet drops with the same angle of incidence on the surface, stitching on any curved surface typically involves swaths being printed by inkjet drops with different angles of incidence on the surface. Consider the case of two positions of a printhead and a curved surface (i.e., a shape or object) that the printhead prints from each position, with reference to Figure 2. Note that we only distinguish between positions. Printing can be due to the same printhead moving between the two positions or due to two printheads printing in parallel. In other words, when we refer to "two printheads" as A, B, this covers the case where the same printhead moves to different areas.

[0021] In the example in Figure 2, we can see that in the stitch area, print head A prints almost perpendicular to the surface, while print head B prints at a projection angle θ of about 45 degrees. Assuming that the print heads have the same nozzle pitch, the drops from print head B have a dot pitch on the surface that is larger than the nozzle pitch by a factor 1 / cos(θ), which in this case is 1 / cos(45°)~=1.41.

[0022] Such a difference in the projection angles at the surface of the drops from the two printhead positions will cause a significant difference between the corresponding dot pitches at the surface. Such a difference in dot pitch will cause density shifts at the stitching points of the flat stitch, making it impossible to employ interleaved stitching. It is therefore advantageous to adjust the shape of the print so that the difference between the dot pitches, which depends on the quality requirements of the print, is kept within a tolerance range. For flat stitching, the maximum difference in dot pitches is determined by the maximum allowable density shift. For interleaved stitching, the maximum allowable dot pitch difference also depends on the width of the stitch zone. Typically, for interleaving to be effective, it is necessary that the dot positions do not vary from the nominal value by more than about 10% of the dot pitch. Thus, in this case, for a stitch zone 10 nozzles wide, the tolerance limit for the difference in surface dot pitch is 1%.

[0023] The relative positions of the print head and the surface are shown in FIG. 3. In this case, the stitch area is determined by the projection angle θ of the drop from each print head position onto the surface. A θ B are chosen to be equal or nearly equal. Advantageously, θ A and θ B The absolute difference in is kept small so that the surface dot pitches of the stitched areas in the two locations match, making it easy to match the densities of the two swaths and use interleaved stitching if necessary.

[0024] Figure 4 shows two printhead positions, A and B, arranged so that a percentage of the nozzles in each will print ink drops on the same area of ​​the curved surface. It is clear that at the extremes of this overlap area, potential stitch positions 1 and 3, the projection angle on the surface of the drops from position A will be significantly different than the projection angle of the drops from position B. Furthermore, for a continuously curved surface, there may be points or regions between these extremes, with potential stitch position 3 showing a smaller difference between the projection angles.

[0025] Now, referring to Fig. 4, a method for selecting a stitch area will be described. Two printhead positions A and B are such that, as projected onto a surface, there is an overlap of inkjet drops projected from printhead position A and printhead position B. In this way, there are multiple nozzle positions N that can be selected from the print head arranged in A. A and multiple nozzle positions N arranged at B B , where the inkjet drop contribution from printhead position A matches the inkjet drop contribution from printhead position B. As a result, printhead position A creates projected inkjet drops that are closest to the ones obtained from printhead position B, i.e., projected inkjet drops that are adjacent to each other in the stitch area. The optimal nozzle N from printhead positions A and B A N BThe choice of is based on the absolute difference in the projection angle of the inkjet drops from the printhead position A [θ A -θ B ] and the output from print head position B is kept small.

[0026] Those skilled in the art will be able to make modifications and substitutions in light of the disclosure that are believed to be within the scope of the appended claims. Each feature disclosed or illustrated in this specification may be incorporated into the present invention alone or in any suitable combination with other features disclosed or illustrated in this specification.

Claims

1. providing first and second drop-on-demand printheads for printing an image on a curved surface from two different adjacent printhead locations with respect to two print path directions at first and second orientations relative to a print path direction for the curved surface; moving the curved surface relative to each of the first and second drop-on-demand printheads along a print path; determining an overlap region on the curved surface for two overlapping swaths of a printing path, a first swath corresponding to a first area addressed by a first drop-on-demand printhead from a first printhead position during printing, and a second swath corresponding to a second area addressed by a second drop-on-demand printhead from a second printhead position during printing, where the first swath and the second swath overlap; The two overlapping swarfs are parallel to a central axis of the curved surface, and the overlap area is printing from either of two different print head positions; determining a projection angle of an inkjet drop on the curved surface for a plurality of locations within the overlap region where the inkjet drop may be printed; selecting stitch points or areas within an overlap region on a curved surface for two overlapping swaths of a printing path where a first swath and a second swath overlap, such that a difference in projection angle on the curved surface of drops from two different adjacent print head positions is maintained within a predetermined parameter range and a difference in respective dot pitches within the stitch region of the two different adjacent print head positions is maintained within an acceptable limit; A method comprising:

2. two different adjacent printhead locations correspond to the same printhead of the first drop-on-demand printhead and the second drop-on-demand printhead, and overlapping swaths correspond to different passes of the same printhead in a print path direction; Alternatively, the method of claim 1, wherein the first and second drop-on-demand printheads include two printheads, and the two different adjacent printhead locations correspond to the two printheads, respectively.

3. The method of claim 1 , wherein the two printheads have the same orientation relative to the print path direction.

4. The method of any of claims 1 to 3, wherein the two printheads have equal nozzle pitch.

5. The method of any one of claims 1 to 3, wherein the two printheads have different nozzle pitches.

6. The method of claim 1 further comprising the step of stitching the two swaths at a selected stitch point.

7. a control system for a first drop-on-demand printhead and a second drop-on-demand printhead for printing an image on a curved surface from two different adjacent printhead positions with respect to a print path direction relative to the curved surface, two different adjacent printheads are positioned in first and second directions relative to a print path direction, and the curved surface is arranged to move along the print path relative to each of the first and second drop-on-demand printheads; a first swath corresponding to a first area addressed during printing by a first drop-on-demand printhead from a first printhead position; a second swath corresponding to a second area addressed during printing by a second drop-on-demand printhead from a second printhead position; and determining an overlap region on the curved surface for two overlapping swaths of a printing path where the first swath and the second swath overlap; where the two overlapping swaths are parallel to the central axis of the surface, The overlap area is Printed from one of two different printhead positions, determining a projection angle of the inkjet drop at the curved surface for a plurality of locations within the overlap region where the inkjet drop may be printed; selecting stitch points in an overlap region on the curved surface for two overlapping swaths of the printing path where the first swath and the second swath overlap; the difference in the projection angles at the curvature of droplets from two different adjacent printhead positions is held within predetermined parameters, The difference in dot pitch in the stitched area of ​​two different adjacent print head positions is kept within a tolerance. a processor configured to: Control system.

8. two different adjacent printhead locations correspond to the same printhead of the first drop-on-demand printhead and the second drop-on-demand printhead, and the overlapping swaths correspond to different passes of the same printhead in the print path direction; Alternatively, the first and second drop-on-demand printheads comprise two printheads, and the two different adjacent printhead locations correspond to the two printheads, respectively.

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