STITCHING METHODS AND SYSTEMS

Abstract

A method includes providing a printhead for printing an image on a surface from two different printhead positions relative to the surface, the two different printhead positions at first and second orientations, wherein the at least one printhead and the surface move relative to each other along a print path having two potentially overlapping swathes, determining an overlap area on the surface for the two overlapping swathes wherein the overlap area is to be printed on from either of the two different printhead positions, determining for each position in the overlap area at which an inkjet drop may be printed the angle of incidence at the surface of that drop, and selecting a stitch point or region in the overlap area wherein the difference between the angles of incidence at the surface of drops from the two printhead positions is kept within an acceptable limit.

Claims

1. A method comprising: providing at least one 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, the two different adjacent printhead positions at first and second orientations relative to the print path direction, wherein the at least one drop-on-demand printhead and the curved surface move relative to each other along the print path; determining an overlap area on the curved surface for two overlapping swathes of the print path, wherein the two overlapping swathes are parallel to a center axis of the curved surface, wherein the overlap area is to be printed on from either of the two different printhead positions; determining for a plurality of locations in the overlap area at which an inkjet drop may be printed an angle of incidence at the curved surface of the drop; and selecting a stitch point or region in the overlap area wherein a difference between the angles of incidence at the curved surface of drops from the two different adjacent printhead positions, respectively, is kept within a predetermined parameter range, such that a difference between the respective dot pitches in the stitch region of the two different adjacent printhead positions is kept within an acceptable limit; wherein the two different adjacent printhead positions correspond to a same printhead of the at least one drop-on demand printhead, the overlapping swathes corresponding to different passes of the same printhead in the print path direction, or wherein the at least one drop-on-demand printhead comprises two printheads and the two different adjacent printhead positions correspond, respectively, to the two printheads.

2. The method according to claim 1, wherein the two printheads have equal nozzle pitches.

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

4. The method according to claim 3, wherein the two print heads have different nozzle pitches.

5. The method according to claim 1, comprising stitching the two swathes at the selected stitch point.

6. A control system for at least one 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, the two different adjacent printhead positions at first and second orientations relative to the print path direction, wherein the at least one drop-on-demand printhead and the curved surface are arranged to move relative to each other along the print path; the control system comprising a processor configured to: determine an overlap area on the curved surface for the two overlapping swathes of the print path, wherein the two overlapping swathes are parallel to a center axis of the curved surface, wherein the overlap area is to be printed on from either of the two different printhead positions; determine for a plurality of locations in the overlap area at which an inkjet drop may be printed an angle of incidence at the curved surface of the drop; and select a stich point in the overlap area wherein a difference between the angles of incidence at the curved surface of drops from the two different adjacent printhead positions, respectively, is kept within a predetermined parameter range, such that a difference between the respective dot pitches in the stitch region of the two different adjacent printhead positions is kept within an acceptable limit; wherein the two different adjacent printhead positions correspond to a same printhead of the at least one drop-on-demand printhead, the overlapping swathes corresponding to different passes of the same printhead in the print path direction, or wherein the at least one drop-on-demand printhead comprises two printheads and the two different adjacent printhead positions correspond, respectively, to the two printheads.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Aspects of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which:

[0023] FIG. 1 is a schematic illustration of how the angle of incidence of ink droplets at a surface affects the pitch of the printed dots.

[0024] FIG. 2 is a schematic illustration of a printing system for printing onto a surface such that the stitch region is achieved by inkjet drops that have differing angles of incidence at the surface from position A and position B;

[0025] FIG. 3 is a schematic illustration of a printing system for printing onto a surface such that a stitch region is achieved by inkjet drops that have a similar angle of incidence at the surface from position A and position B.

[0026] FIG. 4 shows two Print Head positions, A and B, which are arranged so that a proportion of the nozzles of a Print Head at each position might print ink droplets on the same region of the Curved Surface.

DETAILED DESCRIPTION

[0027] An example printing system comprising at least one printhead is provided to coat or decorate a “shape”, representing an object to be printed on and including a curved surface. The object may have a nominal shape (nominal object surface) subject to tolerances in the order of a few hundred microns.

[0028] A “print path” describes the movement of a printhead (including an arrangement of nozzles) relative to a surface for printing. Accordingly, a print path is the relative movement of a nozzle arrangement and the object (shape) during the printing process. The “print path direction” is in the direction of print (direction of the printing process). A “path” describes the plurality of locations on the surface which are to pass under a nozzle. Determining the path of an individual nozzle provides the locus the nozzle traces across the target surface. This relative motion is equivalent even if the arrangement of nozzles is static and the object moves, or both move providing a relative motion.

[0029] Printing onto the object requires generating printhead nozzle data or print image generation. Typically, the printing of the image is wider than the width of one printhead, therefore multiple swathes, one from each printhead, are required to cover the target surface area. The print path has potentially overlapping neighboring swathes.

[0030] Good stitching of each neighboring swathe is critical because the human eye is very good at detecting discontinuities in a printed image, especially in areas of homogeneous color. The stitched area of two print paths (the overlap area of the potential adjacent swathes) is typically 20-40 pixels (e.g. 2-4 mm) but it will be appreciated that this can be smaller or much larger.

[0031] The nozzles of inkjet print heads are typically arranged so that they are evenly spaced across the head, and will therefore print ink droplets on to a flat surface at a constant pitch. For some head designs, this is achieved by placing the nozzles in a line at regular intervals, and these are known as linear heads. On other head designs, known as 2-D heads, the nozzles are placed in a two-dimensional array so that they are still evenly spaced along the print head but also displaced in the print direction. These nozzle arrangements are well known, and it will be obvious to those skilled in the art that the present invention applies equally to heads of both designs.

[0032] It is also well known that, especially for linear head designs, heads can be used to print in an orientation other than orthogonal to the print direction. This technique is known as ‘raking’ and can be used to modify the effective nozzle pitch of the head. This technique is well known, and it will be obvious to those skilled in the art that the present invention applies equally to print systems using heads which are oriented orthogonally or which are raked or both.

[0033] In FIG. 1, Nozzle 1 and Nozzle 2 are adjacent nozzles of the Print Head, separated by a distance known as the Native Pitch of the head. The nozzles eject ink droplets along substantially parallel trajectories which are incident at the surface at angle q. The Surface Pitch is the distance between the points where the ink droplets land. The relationship between the Native Pitch (NP) and the Surface Pitch (SP) is: SP=NP/cosine(q).

[0034] Unlike stitching onto flat surfaces where swathes are printed by inkjet drops that have the same angle of incidence at the surface, stitching on to arbitrary curved surfaces typically results in swathes printed by inkjet drops that have differing angles of incidence at the surface. With reference to FIG. 2, consider the case of two positions for a printhead, and a curved surface (i.e. shape or object) on to which a printhead prints from each position. Note that we distinguish only the positions, and the print might either be by the same printhead moving between the two positions, or by two printheads printing in parallel. In other words, when we refer to “two printheads” A, B this covers the case whereby the same print head has moved to a different area.

[0035] In the example of FIG. 2, it can be seen that the two print positions are such that in the stitch region, Print Head A is printing almost perpendicular to the surface, but Print Head B is printing at an incident angle θ of approximately 45 degrees. Assuming that the print heads have the same nozzle pitch, the drops from Print Head B will have a dot pitch at the surface which is greater than the nozzle pitch by the factor 1/cos(θ), which in this case would be 1/)cos(45° ˜=1.41.

[0036] Such a difference between the angles of incidence at the surface of drops from the two printhead positions will give rise to a significant difference between the corresponding dot pitches at the surface. Such a difference between the dot pitches would cause a density shift at the stitch point of a flat stitch, and would make it impossible to employ an interleaved stitch. It is therefore advantageous to arrange the geometry of the print so that the difference between the dot pitches is kept within an acceptable limit, which is dependent on the quality requirements for the print. For flat stitches, the maximum dot pitch difference is determined by the maximum acceptable density shift. For an interleaved stitch, the maximum acceptable dot pitch difference also depends on the width of the stitch zone. Typically for interleaving to be effective, dot positions should not vary from nominal by more than about 10% of dot pitch, so in this case for a stitch zone 10 nozzles wide the acceptable limit for the difference in surface dot pitches would be 1%.

[0037] With reference to FIG. 3, the relative positions of the print heads and the surface are as shown. In this case, the stitch region has been chosen so that the angles θ.sub.A, θ.sub.B of incidence at the surface of drops from each print head position are equal, or approximately so. Advantageously, the absolute difference between θ.sub.A and θ.sub.B is kept small so the surface dot pitch in the stitch region for the two positions will match, making it easier to match densities in the two swathes, and to use an interleaved stitch if required.

[0038] FIG. 4 shows two Print Head positions, A and B, which are arranged so that a proportion of the nozzles of each might print ink droplets on the same region of the Curved Surface. It is apparent that at the extremes of this region of overlap, Potential Stitch Positions 1 and 3, the angle of incidence at the surface of drops from Position A will differ markedly from that of drops from Position B. Furthermore, for a continuously curving surface, that there will be a point or region between those extremes, Potential Stitch Position 3 at which such differences between the angles of incidence will be small.

[0039] The method of selection for the stitch region will now be described with reference to FIG. 4. Two printhead positions A and B are such that there is an overlap of the projected inkjet drops from printhead position A and printhead position B, as projected onto the surface of the object. As such there are is a plurality of nozzle positions N.sub.A that can be selected from printhead positioned at A and a plurality of nozzle positions N.sub.B that can be positioned at B, such that the inkjet drop contribution from printhead position A and that from printhead position B result in printhead position A creating a projected inkjet drop that is the closest neighbor to that resulting from printhead position B, in other words creating projected inkjet drops that neighbor each other in the stitch region. The selection of the most optimal nozzles N.sub.A N.sub.B from printhead positions A and that from position B respectively is such that their projected drops are both neighboring and the absolute difference of the angles of incidence |θ.sub.A−θ.sub.B| of the inkjet drop from printhead position A and that from printhead position B is kept small.

[0040] Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.