EP2591917B1 - Method and device for ink-jet printing on curved container surfaces - Google Patents

Description

The invention relates to a method and apparatus for inkjet printing on curved surfaces of containers, in particular PET bottles or glass bottles.

Inkjet printing on containers, such as beverage bottles and the like, is advantageous because of the freedom of design in the implementation of complex print motifs and the rapid adaptability to different products. Due to the mostly curved container surfaces and the use of line-like printheads, which are common because of the required rapid throughput in beverage filling lines, however, occur unwanted deviations of the generated print image of the respective template, in particular an imprecise printing, which leads to unclean contours in the printed image.

Like from the EP 1 435 296 A1 , of the EP 0 385 624 A1 and the WO 03 002349 It has already been attempted to improve inkjet printing on curved container surfaces by precisely guiding the printheads and surfaces. However, problems remain unresolved when printing on uneven container contours and when using printheads with several rows of nozzles arranged one behind the other. In particular, problems due to different pressure intervals and staggered and with respect to the container surface tilted nozzle lines remain in the described approaches. A method according to the preamble of claim 1 is known from US 2007/0019017 A1 known.

There is therefore a need for improved printing methods and apparatus in this regard.

The stated object is achieved by a method according to claim 1. Thus, the curved surface to be printed is moved relative to at least one row of nozzles aligned transversely or obliquely to the direction of relative movement. Further, ink drops are ejected at ejection timings set for the individual ink droplets depending on the respective pressure distance of the ejecting nozzle.

The ejection times can be adjusted in particular by moving individual pixels on an image template to be printed in or against the print feed. Pixels which are shifted against the direction of printing, which is to be understood in the sense of a printing advance, are printed at an earlier time than non-shifted pixels and vice versa. It is therefore only the print template to adapt without having to adjust the ejection timing of individual nozzles by means of separate control signals individually. The latter, however, is also possible in principle.

As a result, errors in converting two-dimensional print templates into a three-dimensional print can be reduced or compensated for in a simple manner and in the case of flat homogeneous print quality. Such errors can be caused by the relative movement between the print head and the surface to be printed, by a different flight time of the ink drops, by irregular surfaces and / or a design-related offset and oblique impact of ink jets with respect to a predetermined printing position. In particular, PET bottles and glass bottles can have dimensional tolerances due to the production, which can be compensated according to the invention by adjusting the ejection times in order nevertheless to ensure a desired printed image. For cans, especially from sheet metal, this may also be the case.

The relative movement here corresponds essentially to a pressure movement between the print head and the surface to be printed in the sense of a feed in the column direction or row direction. The pressure interval can be identical for all nozzles of the nozzle row. The ejection times of a nozzle row can then also be identical.

Preferably, at least two rows of nozzles are provided one behind the other in the direction of movement, and the ejection timings are set in dependence on a distance between the rows of nozzles. As a result, runtime differences of the nozzle rows can be compensated for until a predetermined printing position is reached over the object surface, as can deviations of the trajectories from an idealized, vertical direction of incidence of the ink droplets on the object surface.

In a particularly favorable development, the ejection times are furthermore set as a function of an incidence angle of the ink droplets formed in each case with the surface. As a result, a geometric offset of individual nozzle rows with respect to a predetermined printing position can be calculated particularly accurately and compensated in the printed image.

Preferably, at least two rows of nozzles in the direction of movement are arranged one behind the other and on both sides of a center position radially aligned with respect to the main axis of the container, and the discharge timings of the ink drops are further dependent on a distance between a transport path of the containers and the center position, a distance between the respective nozzle row and the center position, and set the respective radius of the container. These distances essentially define the catheters of a right-angled triangle whose hypotenuse is formed by an imaginary connecting line from the main axis of the container to the respective row of nozzles.

The transport path is, for example, an orbit of a bottle table. The distance between the center position and the transport path is defined, for example, with respect to the pitch circle of an orbit and / or the trajectory of the container main axes. The center position corresponds to the imaginary position of a radially aligned nozzle row. Based on the above distances, the pressure control can be particularly easily adapted to different container dimensions and container tables.

The transport path of the container surface to be printed may differ from the transport path of an associated holder for the container about a machine axis of rotation, depending on the position of the axis of rotation of the container with respect to its cross section. For containers of substantially circular cross section, the axis of rotation is equal to the longitudinal axis / major axis of the container. When the machine is operated in cycles, the container may be stopped in front of a stationary printhead and rotate in front of it, causing the above relative movement. It can also be overlapping of the two movements, when both the machine and the container located on your turn at the same time.

When the printheads are mounted on the rotating machine, relative movement preferably takes place only by rotation of the container about its longitudinal axis. It would also be conceivable that the print heads are transported at a different peripheral speed than the containers. It is therefore also possible in this regard, a combined relative movement.

Preferably, at least two rows of nozzles in the direction of movement are provided one behind the other, and the rows of nozzles are offset transversely to the direction of movement, in particular by half, for example, in a printhead with two rows of nozzles, or a third, for example, in a printhead with three rows of nozzles, the respective printing resolution along the nozzle rows. The nozzles thus print in the intermediate pressure spaces of the respective leading nozzles. This allows the print quality to be further optimized.

Preferably, the surface in front of the nozzle row is moved about a rotation axis, and the pressure separation is defined with respect to a developable outer surface formed around the rotation axis. This simplifies the calculation of a geometric offset of the individual nozzle rows.

In a particularly advantageous embodiment, a printing original for driving the at least one row of nozzles is created on a development of the lateral surface. This facilitates the adaptation of a two-dimensional print template to the curved printing surface.

In a further preferred embodiment of the invention, the row of nozzles is moved along the surface, and the printing distance is defined with respect to at least one developable surface, which is aligned parallel to an axis of symmetry of the object to be printed. In particular, non-rotationally symmetrical surfaces can also be printed with improved accuracy.

According to the invention, the ejection timings are set by shifting pixels of a print motif on a print original in the direction of movement or against the direction of movement, and the at least one nozzle row is driven on the basis of the shifted pixels. On the print original in the direction of movement offset pixels are printed at different times. The associated ink drops are thus ejected at different times. Thus, different pressure intervals of the nozzles can be compensated by a targeted distortion of the print motif on the print original in the direction of movement.

In addition, the ejection times could be adjusted by assigning individual nozzles or individual pixels of a print original in each case a time offset and the ink droplets are ejected taking into account the respective associated time offset. As a result, a time offset can be assigned to individual nozzles or nozzle rows independently of the print template.

Preferably, a central row of nozzles is provided in a center position radially aligned with respect to the main axis of the container, and the pixels assigned to the central row of nozzles on the master are not displaced or reduced with respect to the direction of movement as pixels associated with rows of nozzles preceding or behind the direction of movement , The central nozzle row can then be used as a reference position with respect to the direction of movement and the adaptation of the ejection times can be simplified.

According to the invention, the pixels on the artwork which are associated with a nozzle row located farther away from the center position at the time of printing or ink ejection are further shifted than the pixels associated with a nozzle row closer to the center position ,

Specifically, the pixels on the artwork which are associated with a nozzle row located farther away from the container surface at the time of printing or ink ejection are further shifted than the pixels associated with a nozzle row located closer to the container surface. The latter also applies in particular Nozzles of a row of nozzles, for example, in each case adjacent nozzles of a row of nozzles, ie nozzles with a distance in the longitudinal direction of the container. This principle applies to printheads with even numbers of rows of nozzles as well as printheads having an odd number of rows of nozzles. Furthermore, this principle can be used in particular in containers which have elevations and / or depressions whose surface deviates from a nominal diameter of the outer surface of the container.

If the printhead has two, four or some other integer multiple of two rows of nozzles, then there is preferably no central row of nozzles.

It can also happen that the pixels on the print original, which are assigned to one nozzle row, are shifted in a different direction, in particular in the opposite direction, than the pixels on the print master, which are assigned to another nozzle row. In particular, these two rows of nozzles are then arranged on opposite sides of the central position.

Preferably, the ejection timings of the ink droplets are adapted to convex and concave radii of curvature along the circumference of the container. This allows even structured surfaces with elevations and depressions to be printed variably and precisely.

The ejection times could also be adjusted depending on the duration of flight of the ink drops and depending on the speed of the surface to be printed in the direction of movement. Likewise, external disturbances, such as due to air turbulence, gravity and friction can be considered. In particular, a different relative movement of the surface with respect to the nozzle row for different pressure intervals can be compensated with a separate correction function. As a result of this correction function, ink droplets are preferably ejected the sooner the greater the printing distance.

Furthermore, the droplet size could be adapted to an angle of incidence of the ink droplets formed with the object surface. As a result, the desired optical density of the ink on the surface can also be achieved for different angles of incidence. In particular, the drops are the larger the more oblique the ink drops hit the surface. The smallest droplet size results here at right angles of incidence.

In a preferred embodiment, the pressure separation is 0.5 to 20 mm, in particular 1 to 7 mm. This allows most commercially available containers to be printed with improved quality.

A further preferred development of the method according to the invention further comprises a step for producing a printing original, in which: a predetermined image grid is laid on the surface to be printed; a print motif is rasterized on the basis of the image raster; and projecting the rasterized print motif onto at least one developable surface to assign print coordinates to projected pixels of the print subject on a surface finish. As a result, the pressure can be optimized on the basis of the print motif screened onto the object. This simplifies the calculation and adaptation of individual correction functions, for example for compensating a geometric offset of individual nozzle rows, differences in flight time, variations in the pressure separation caused by container contours, and the like.

In addition, the step of producing the print template can solve a separate task, namely to be able to create print templates predominantly in an object-oriented manner. This is achieved by the fact that the print motif can first be adapted to the object to be printed from a design point of view, and subsequently a specifically distorted print original is produced. In other words, the printed image can still be examined and optimized in the undistorted state together with the object to be printed.

Preferably, the screened print motif is projected onto at least one lateral surface, wherein the projection origin lies on the rotational axis of the lateral surface. This makes it easier to create print templates and adjust the artwork to different print margins.

In a particularly favorable development of the invention, the step of producing the printing original is carried out by means of a three-dimensional computer model of the surface to be printed. This makes it possible, from a design point of view, to optimize the print image in a particularly comfortable and versatile manner and to save resources.

The stated object is also achieved with an apparatus for the ink-jet printing on curved object surfaces, in particular container surfaces, having the features of claim 16. Preferred developments of the device are further designed for carrying out the developments according to the invention of the described method.

• Fig. 1 eine schematische Ansicht eines Behälters und einer Düsenzeile eines Druckkopfs, gesehen in Druckrichtung;
• Fig. 2 eine gegenüber der Fig. 1 um 90° seitlich geschwenkte Ansicht des Behälters und des Druckkopfs;
• Fig. 3 ein Schema zur Verdeutlichung unterschiedlicher Druckabstände und Flugzeiten der Tintentropfen;
• Fig. 4 ein Schema zur Verdeutlichung unterschiedlicher Druckdichten in Abhängigkeit des Auftreffwinkels und der Tropfengröße;
• Fig. 5 eine schematische Darstellung einer Rasterprojektion auf eine abwickelbare Fläche; und
• Fig. 6 eine Seitenansicht eines Behälters mit einem schematisch gerasterten Druckmotiv.

Preferred embodiments of the invention are shown in the drawing. Show it:

• Fig. 1 a schematic view of a container and a nozzle line of a printhead, seen in the printing direction;
• Fig. 2 one opposite the Fig. 1 90 ° laterally swiveled view of the container and the print head;
• Fig. 3 a scheme to illustrate different pressure intervals and flight times of the ink drops;
• Fig. 4 a scheme to illustrate different pressure densities as a function of the angle of incidence and the droplet size;
• Fig. 5 a schematic representation of a raster projection on a developable surface; and
• Fig. 6 a side view of a container with a schematically screened print motif.

With reference to the Fig. 1 and 2 Hereinafter, a preferred embodiment of the method according to the invention for ink jet printing on container 1, such as beverage bottles, will be described. However, it is also generally suitable for printing on other objects with curved surfaces. Such a surface 2 is in the Fig. 1 schematically illustrated as a portion of the side wall of the container 1. The surface 2 is rotatably positioned in front of a print head 3 with nozzle rows 4 about the main axis 1 'of the container 1. Printers with corresponding rows of nozzles 4 and positioning units for carrying out a printing movement between the print head 3 and the surface 2 to be printed can in this case be arranged in a known manner. According to the invention, these are combined with a control unit (not shown) and / or evaluation unit in order to set the printing times of the rows of nozzles 4 and / or individual nozzles 4a provided on them as a function of the respective printing distance.

As the Fig. 2 can be seen, for example, at least two rows of nozzles 4 in the printing direction 5 are arranged one behind the other, so that in the Fig. 1 only one of the rows of nozzles 4 can be seen. Also indicated is an optional central row of nozzles 4 ', which is aligned in a central position M radially with respect to the main axis 1' of the container 1. The individual nozzles 4a of the nozzle rows 4, 4 'are each arranged with an individual pressure separation 6 to the surface 2. The rows of nozzles 4, 4 'are transversely or obliquely, in particular orthogonal, aligned to the printing direction 5 and define a maximum printing width B of the print head 3. The number of illustrated nozzle rows 4, 4' and the nozzle 4a is merely an example. Likewise, several printheads 3 could be provided. These are then preferably arranged in alignment with each other parallel to the main axis 1 ', in particular in such a way that the nozzle rows 4 are aligned symmetrically to the center position M or an optionally provided middle row of nozzles 4' is aligned exactly radially in the middle position M, as shown in FIG Fig. 2 is indicated.

As the Fig. 2 Furthermore, the printing direction 5 is defined in terms of a printing feed by the relative movement between the print head 3 and the surface 2 to be printed in the region of the print head 3, in the example shown by the tangent to a circumferential line 2 'of the surface 2 at the printing position of a pixel P below the print head. 3

The ejection timings of the ink droplets 9 can be adjusted, for example, depending on a distance between a conveying path of the containers 1 and the center position M, a distance between the respective nozzle row 4 and the center position M, and the respective radius r _{x of} the container 1. These distances essentially define the catheters of a right-angled triangle whose hypotenuse is formed by an imaginary connecting line from the main axis 1 'of the container 1 to the respective nozzle row 4. In the Fig. 2 these catheters correspond to the distance r _N + d _N or the distance X / 2.

In a peripheral portion, there is further exemplified an alternative perimeter line 2 "representing a container surface having concave and convex curvatures In this case, the pressure distance 6 varies along the circumference of the rotating container 1. With a known rotational position of the container 1, the ejection can be achieved the ink drops 9 according to the invention to specifically adapt to depressions and elevations along the container circumference.

As the Fig. 1 indicates, the printing distance 6 can be defined directly as a distance to the surface 2 to be printed. As more fully described below with reference to FIGS FIGS. 5 and 6 However, the printing distance 6 'can also be defined as a virtual size with respect to a developable surface 7, which is, for example, a virtual lateral surface around the main axis 1' of the container 1 to be printed. Suitable lateral surfaces are cylinders, cones, truncated cones and any combinations thereof.

Alternatively to the in the Fig. 1 and 2 indicated rotation of the container 1, the printhead 3 could be moved along the surface 2 along (not shown), in particular if the container to be printed does not have a rotationally symmetrical cross-section but, for example an elliptical cross-section. In this case, the generatrix of a developable surface used to define the printing distance and / or as a printing original could run along an ellipse or the like. The unwindable surface would then be aligned parallel to an axis of symmetry of the container and parallel to the nozzle row 4, 4 '.

In the example of Fig. 2 is the developable surface 7 defined by its radius r _N about the main axis 1 'of the container 1, the position of a pixel P on the surface to be printed 2 by the radius r _X. Further, the print head 3 is positioned at a distance d _N from the unwindable surface 7. In this case, by definition, the printing distance 6 of the pixel P on the radius r _X is equal to d _N + r _N -r _x .

The following are correction functions for correcting printing times as a function of the printing distance on the basis of the Fig. 2 described coordinates. However, corresponding calculations depending on the respective actual or virtual pressure separation 6, 6 'could also be carried out using other coordinate systems. However, the example shown has the advantage that the developable surface 7 is suitable both for defining the virtual printing distance 6 'and for defining a printing original 8.

$$\alpha =\arctan \frac{\left(\frac{X}{2}\right)}{r_N+d_N}$$

$$\mathrm{\Delta }{x}_g=\left(c-r_x\right)\cdot \sin \alpha$$

(1'), (2) in (3) $$\mathrm{\Delta }{x}_g=\left(\sqrt{\left(r_N+d_N\right)+{\left(\frac{X}{2}\right)}^2}-r_x\right)\cdot \sin \left[\arctan \left(\frac{\left(\frac{X}{2}\right)}{r_N+d_N}\right)\right]$$

Between the rows of nozzles 4, a distance X is provided in the pressure direction 5 by design. This can be used to achieve standard print resolutions of, for example, 300 to 600 dpi. Due to the curvature of the surface 2, ink droplets 9 can not impinge on the surface 2 at the same time orthogonally from a plurality of nozzle rows 4 lying one behind the other. This results in a geometric offset of the drops 9 from the nozzle rows 4 caused by the distance X of the rows of nozzles 4. This effect is the stronger the smaller the ratio of the radius of curvature r _X at the printing position to the distance X of the rows of nozzles 4. The problem and its solution will be on hand in the Fig. 2 indicated geometric pressure offset Δx _g in the printing direction 5 of a pixel P on the radius r _{X described} with the following formulas (1) to (3): $${\mathrm{<figure-callout\; id="2"\; label="c"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">c</figure-callout>}}^2={\left(r_N+{\mathrm{<figure-callout\; id="2"\; label="d\; N"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">d\; </figure-callout>}}_N\right)}^2+{\left(\frac{\mathrm{<figure-callout\; id="2"\; label="X"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">X</figure-callout>}}{2}\right)}^2\to \left(1\text{'}\right)c=\sqrt{\left(r_N+d_N\right)+{\left(\frac{\mathrm{<figure-callout\; id="2"\; label="X"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">X</figure-callout>}}{2}\right)}^2}$$

$$\alpha =\arctan \frac{\left(\frac{X}{2}\right)}{r_N+d_N}$$

$$\mathrm{\Delta }{x}_G=\left(c-r_x\right)\cdot \sin \alpha$$

(1 '), (2) in (3) $$\mathrm{\Delta }{x}_G=\left(\sqrt{\left(r_N+d_N\right)+{\left(\frac{\mathrm{<figure-callout\; id="2"\; label="X"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">X</figure-callout>}}{2}\right)}^2}-r_x\right)\cdot \sin \left[\arctan \left(\frac{\left(\frac{X}{2}\right)}{r_N+d_N}\right)\right]$$

As the Fig. 2 As can be seen, the geometric print offset .DELTA.x _g results from the fact that the ink drops 9 then do not hit the surface 2 perpendicularly. The pressure offset Δx _g is thus dependent on the pressure separation 6, 6 'and the distance X of the nozzle rows 4. If, for this purpose, a virtual printing distance 6 'is defined with the aid of the developable surface 7, this preferably also serves as a projection surface for producing the associated printing original 8. The virtual printing distance 6' is equal to r _N - r _X in the example.

Starting from the pressure offset Δx _g , a correction function for the respective nozzle row 4 can be calculated. This correction function can optionally be supplemented by a zero point correction by taking into account the printing offset Δx _N on the developable surface 7, for example by subtraction of the printing offset Δx _g on the surface 2 to be printed. This additionally improves the print quality.

The correction then takes place by means of an adaptation of the ejection time of the ink droplets 9, ie by deliberately delayed or preferred ejection from the rows of nozzles 4 offset from one another in the printing direction 5. According to the invention, the ejection time is determined by moving individual pixels on the printing original 8 in or against the direction of movement 5 , ie the printing direction, reached. This correction is to be calculated separately for each nozzle row 4, for example for corresponding print lines in the printing original 8. The printing offset .DELTA.x _g would be in the example of Fig. 2 for printing an arbitrary pixel with the left nozzle row 4 to add to its printing direction specific image coordinate to subtract for printing the same pixel with the right nozzle row 4 of the image coordinate.

If a middle row of nozzles 4 'is provided in radial alignment at the middle position M, then the center position M can be used as a reference for compensation of the geometrical pressure offset Δx _g . The image coordinates of the central nozzle row 4 'would then not have to be shifted relative to the geometric print offset, but only the image coordinates of the outer nozzle rows 4.

If the pressure separation 6, 6 'differs for individual nozzles 4a due to the contour of the container sidewall, the pressure offset Δx _g can nonetheless be calculated separately for individual nozzles 4a. The ejection times can then be adjusted for individual nozzles 4a according to the above scheme by shifting image coordinates.

$$\mathrm{\Delta }t=\frac{\mathrm{\Delta }y}{v_T};\mathrm{\Delta }y=r_N-r_x$$

$$\mathrm{\Delta }t=\frac{r_N-r_x}{v_T}$$

$$in\left(4\right)\mathrm{\Delta }{x}_t=\frac{r_N-r_x}{v_T}\cdot v_O$$

Another correction function can compensate flight times of the ink drops 9 of different lengths. Here, too, a correction of different pressure intervals 6, 6 ', which during the relative movement between the surface 2 and the rows of nozzles 4 effect that, despite approximately the same drop speed v _T and the same ejection time of the droplets 9, different printing positions P are printed in the printing direction 5. This causes a distorted and / or blurred print image. Without regard to friction losses and associated delays of the drops 9 flight time differences .DELTA.t may be as follows with the formulas (4) and (5) taken into account and a pressure offset caused thereby Ax _t is calculated and corrected if necessary: $$\mathrm{\Delta }{x}_t=\mathrm{\Delta }t\cdot v_O$$

$$\mathrm{\Delta }t=\frac{\mathrm{\Delta }y}{v_T};\mathrm{\Delta }y=r_N-r_x$$

$$\mathrm{\Delta }t=\frac{r_N-r_x}{v_T}$$

$$in\left(4\right)\mathrm{\Delta }{x}_t=\frac{r_N-r_x}{v_T}\cdot v_O$$

As the Fig. 3 indicates, flies an ink droplet 9 to the surface 2, for example by the distance .DELTA.y further than the unwindable surface 7 and thus requires the time .DELTA.t longer to the surface 2. In this period, the surface to be printed 2 moves at the speed v _O on , The resulting displacement Ax _t is analogous to the geometrically caused print offset Ax _g in a correction function for delayed or early ejection of the ink droplets 9 are integrated, for example by appropriate displacement of the corresponding picture elements on the master 8. The speed of the surface 2 can preferably also as a relative value regarding the speed of unwindable Define area 7. This correction is dependent on the printing speed v _O and should be adjusted accordingly.

Under real conditions, friction losses of the droplets in the air also occur. Depending on the droplet diameter, calibration functions, for example for the dependence of the drop velocity on the time of flight, can be determined and incorporated into the correction.

Also, air currents and other environmental influences, such as temperature fluctuations, electrostatic potentials, magnetic fields and the like, may cause depletion of the ink droplets 9 depending on drop size, ink type and printhead type. Further correction functions can also be determined empirically for this purpose and the correction according to the invention of the printing offset by means of coordinate shift can be incorporated in the print original.

The described correction functions can be carried out before the loading of the print motif, but also after the separation of the color channels, for example. Alternatively, individual or all correction functions could also be realized by specifying an explicitly defined time offset for the delayed or early activation of individual nozzle rows 4, 4 'and / or nozzles 4a for varying the ejection times or else only be supplemented by such a correction period.

• fΔx_g(d) geometrische Korrektur;
• fΔx_t(d) Flugzeitkorrektur; und
• fΔx_u(d) Korrektur von Umwelteinflüssen, Luftströmungen und dergleichen

An overall correction function as a function of the actual or virtual pressure separation 6, 6 'could comprise, for example, the following terms, wherein the pressure separation 6, 6' is here denoted by "d" for better readability:

• fΔx _g (d) geometric correction;
• fΔx _t (d) time-of-flight correction; and
• fΔx _u (d) correction of environmental influences, air currents and the like

For each nozzle row 4 and / or individual nozzles 4a, the correction of the x-coordinate of a pixel P in the printing direction 5 can be generally described as follows, for example: $$P_{\left(x\text{'}.y\right)}\left(d\right)=P_{\left(x,y\right)}\left(d\right)+f\mathrm{\Delta }{x}_G\left(d\right)+f\mathrm{\Delta }{x}_t\left(d\right)+f\mathrm{\Delta }{x}_u\left(d\right)$$

The correction then takes place by shifting the pixel P from x to x '.

Correction functions or individual correction values can also be determined empirically by suitably shaped test bodies and / or test patterns. Sign and formula of the correction functions are dependent on the location of the nozzle row relative to the virtual printing position. The virtual printing position is defined, for example, by the position of the axis of rotation of the unwinding under the print head.

An additional possible adaptation of the color density or ink density of the imprint to a three-dimensional contour of the surface 2 to be printed is described below with reference to FIGS Fig. 4 described.

Accordingly, the ink application to curved surfaces is preferably dependent on their local inclination relative to one in the position I of Fig. 4 indicated flat container surface 2 corrected to ensure a uniform optical density of the ink. As the Fig. 4 illustrated at the position II for an inclined surface portion, for example, an oblique impact of the ink drops 9 may not or insufficiently ink-filled gaps in the imprint on the surface 2 cause.

This can be counteracted by adjusting the drop size, for example in the raster image processor, depending on the angle of incidence λ of the ink drops 9. Alternatively, a similar effect can be achieved by deliberately increasing the color saturation in the artwork. It is thus possible to correct the ink density to a target value, as in the FIG. 4 is indicated at position III. Alternatively, by reducing the raster width R to R ', a comparable effect can be obtained as in the US Pat Fig. 3 is indicated at position IV.

Factors for the wetting of the surface 2, such as the surface tension, can preferably be determined empirically under real conditions. For example, surface sections are printed at defined angles of incidence λ and associated correction values for the drop size and / or the screen r are determined. In this case, the outflow of the ink along the inclination can be taken into account.

With reference to the FIGS. 5 and 6 Furthermore, a method according to the invention for producing a printed surface for curved surfaces on a developable surface, which preferably corresponds to the unwindable surface 7 described above, is described below.

The Fig. 5 shows a curved surface 2 to be printed, on which ink droplets 9 are to be placed at a print resolution A, and a developable surface 7, on the development of which, for example, a printing original 8 for printing the surface 2 can be created. For this purpose, individual in the Fig. 5 represented by ink drops 9 pixels of the curved surface 2, starting from a projection center 10, for example the main axis 1 'of the container to be printed 1 corresponds projecting onto the unwindable surface 7. Due to deviating radial distances from the projection center 10, the local printing resolution A 'of the ink droplets 9 on the developable surface 7 differs in places from the local printing resolution A on the curved surface 2.

According to the invention, the position of individual pixels on the printing original 8 independently of one another, and thus also the local resolution A ', can be selectively varied in order to produce the most uniform possible printing resolution A on the surface 2, as in US Pat Fig. 5 is indicated. Here correspond to the radial auxiliary lines 9 'in the Fig. 5 the theoretical trajectories of the ink drops 9 at the respective printing positions on the surface of the second

Accordingly, the virtual printing distance 6 'can be defined as the difference between the radial distances of the pixels (ink droplets in the Fig. 5 ) on the unwindable surface 7 and the associated areas on the surface 2 to be printed.

On the other hand, using conventional printing templates with uniform resolution, different printing distances 6 'would cause a non-uniform printing resolution A on the surface 2. Some ink drops 9 would then overlap, for example, between other ink drops 9 would arise unprinted gaps.

As the Fig. 6 indicates schematically, is counteracted according to the invention by one of the desired printing resolution A corresponding grid 11 is placed on the surface to be printed 2 of the container 1. Preferably, a print motif 12 is designed and / or edited based on the grid 11 in a three-dimensional model of the surface 2. The grid points of the grid 11 are then projected onto the developable surface 7, which results in a spatially different screen ruling or printing resolution A 'on the developable surface 7, depending on the local curvature of the surface 2. By projection, starting from the main axis 1 'of the container 1 and / or another suitable symmetry axis of the developable surface 7, the print motif 12 is purposefully distorted on the printing original 8 and the position of individual pixels is thereby corrected.

For example, the pressure separation 6, 6 'is determined from the three-dimensional model of the surface 2 and the developable projection surface 7. For this purpose, the path lengths between the intersections of the pot trajectory with the surface 2 and the projection surface 7 are determined. As data of the individual pixels, for example, the printing distance 6, 6 ', the pixel coordinates and the associated values of the color channels are stored. The determined pressure distance 6, 6 'can then also be used for the correction functions described above.

The present invention specifically against the print motif 12 distorted artwork templates 8 provide additional creative freedom, and it is a particularly precise printing of the subject 12 allows. In particular, with the help of the three-dimensional model of the surface 2 or the associated container 1, the final printed image can already be assessed visually especially well in the design and modified in a simple manner.

The printing originals 8 according to the invention can be created in computing units separately from the described printing methods on curved object surfaces 2. Thus, a separate technical problem can be solved.

However, a combination with the printing method according to the invention based on a correction of the ejection time as a function of the printing distance 6, 6 'is particularly advantageous. With the help of developable surfaces, the described methods can be combined particularly efficiently.

The described methods and correction functions can be combined as desired in a technically meaningful way.

Claims (16)

1. Method for ink-jet printing on curved surfaces (2) of containers (1), in particular PET bottles or glass bottles, whereby:

   the surface to be printed is moved relative to at least one nozzle row (4, 4') which is oriented transversely or obliquely to the direction of movement (5); and

   ink drops (9) are ejected at ejection times which are set for the individual ink drops as a function of the respective printing distance (6, 6') of the ejecting nozzle (4a),

   characterised in that

   the ejection times are set by shifting image pixels of an associated print motif (12) on a print master (8) in or opposite the direction of movement (5) and the at least one nozzle row (4, 4') is activated on the basis of the shifted image pixels, and

   the image pixels on the print master (8) assigned to a nozzle row which is at a greater distance away from a middle position (M) oriented radially with respect to a main axis (1') of the container at an instant of printing are shifted farther than the image pixels assigned to a nozzle row which is disposed closer to the middle position (M).
2. Method as claimed in claim 1, whereby at least two nozzle rows (4) are provided lying one after the other in the direction of movement (5) and the ejection times are also set as a function of a distance (X) between the nozzle rows.
3. Method as claimed in claim 1 or 2, whereby the ejection times are also set as a function of an impact angle (λ) of the ink drops (9) subtended respectively with the surface (2).
4. Method as claimed in claim 1, whereby at least two nozzle rows (4) are provided lying one after the other in the direction of movement (5) and on either side of a middle position (M) oriented radially with respect to the main axis (1') of the container (1), and the ejection times of the ink drops (9) are also set as a function of a distance between a transport path of the container (1) and the middle position (M), a distance between the respective nozzle row (4) and the middle position (M), and the respective radius (r_x) of the container (1).
5. Method as claimed in at least one of the preceding claims, whereby at least two nozzle rows (4, 4') are provided lying one after the other in the direction of movement (5), and the nozzle rows are offset transversely to the direction of movement, in particular by half or a third of the respective print resolution along the nozzle rows.
6. Method as claimed in at least one of the preceding claims, whereby the surface (2) is moved in front of the nozzle row (4, 4') about an axis of rotation (1') and the printing distance (6') is defined with reference to a lateral surface area (7) formed around the axis of rotation that can be developed.
7. Method as claimed in claim 6, whereby a print master (8) is set up for activating the at least one nozzle row (4, 4') on a development of the lateral surface area (7).
8. Method as claimed in at least one of the preceding claims, whereby the nozzle row (4, 4') is moved along the surface (2) and the printing distance is defined by reference to at least one surface that can be developed which is oriented parallel with an axis of symmetry of the object to be printed.
9. Method as claimed in claim 1, whereby a middle nozzle row (4') is provided in a middle position (M) oriented radially with respect to the main axis (1') of the container (1), and the image pixels on the print master (8) assigned to the middle nozzle row are not shifted or are shifted less relative to the direction of movement (5) than image pixels assigned to nozzle rows (4) lying in front of or behind it relative to the direction of movement.
10. Method as claimed in claim 1 or 9, whereby the image pixels on the print master assigned to a nozzle row, which is disposed farther away from the container surface at an instant of printing, are shifted farther than the image pixels assigned to a nozzle row disposed closer to the container surface.
11. Method as claimed in at least one of the preceding claims, whereby the ejection times of the ink drops (9) are adapted to convex and concave radii of curvature disposed along the container circumference (2").
12. Method as claimed in at least one of the preceding claims, whereby the printing distance (6) is 0.5 to 20 mm, in particular 1 to 7 mm.
13. Method as claimed in at least one of the preceding claims, further comprising a step for producing a print master (8), comprising the sub-steps: laying a predefined raster (11) on the surface (2) to be printed; scanning a print motif (12) on the basis of the raster; and projecting the scanned print motif onto at least one surface (7) that can be developed in order to assign projected image pixels of the print motif to print co-ordinates on a development.
14. Method as claimed in claim 13, whereby the scanned print motif (12) is projected onto at least a lateral surface area (7) and the projection origin (10) lies on the axis of rotation (1') of the lateral surface area.
15. Method as claimed in claim 13 or 14, whereby the step of creating the print master (8) is implemented by means of a three-dimensional computer model of the surface (2) to be printed.
16. Device for ink-jet printing on curved surfaces (2) of containers, in particular PET bottles or glass bottles, comprising:

    - at least one row (4, 4') of ink-jet nozzles (4a) disposed transversely or obliquely with respect to the printing direction (5);

    - a positioning unit for mutually moving a surface to be printed and the ink-jet nozzles; and

    - a control unit for activating the ink-jet nozzles which is configured so that ink drops (9) can be ejected at times which are set for the individual ink drops as a function of the respective printing distance (6, 6') of the ejecting nozzle in accordance with the method as claimed in claim 1.

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