EP3445590B1 - Method and device for digitally printing on three-dimensional objects - Google Patents

Description

The invention relates to a method for digitally printing 3-dimensional objects, in particular bottles, cans or other hollow bodies, by means of at least one print head, wherein the object to be printed moves relative to the print head for printing, in particular rotates, and a print template, preferably in a digitization step, is broken down into a plurality of printing dots (pixels) and the printing dots are stored in a printing raster consisting of image columns and image lines, the printing raster being used to control the print head during printing in order to apply a print image to the object to be printed.

In digital printing processes, the print image is transferred directly from a computer to a printing machine without using a static printing form. These methods include inkjet printing in particular, in which small droplets of ink are fired from the nozzles of the print head in a targeted manner onto the surface to be printed in order to produce a print image there.

To determine the positions at which the individual drops of paint are sprayed onto the print object, the image to be printed (image or print template) is first rasterized. Rasterizing is a software-supported process in which the print template is "converted" into print data. The core is the "raster image processing". The term rasterization is based on the fact that an image is divided into discrete image points (pixels) with fixed distances. A grid-like printing grid with grid cells or meshes is used for this purpose. The corresponding color information of the respective discrete image point is stored for the respective cell.

The result of the rasterization is a raster graphic consisting of a raster-shaped arrangement of pixels.

To generate a print raster from a print template, the print template can be optically scanned, for example by means of a scanner, and subdivided or broken down into pixels. It is also part of the state of the art to convert graphics generated on the computer (for example vector graphics) directly into raster graphics. The coordinates assigned to the image points and the color information stored for the respective coordinates are sent to the program control for the spray nozzles of the print head for generating the print image.

The raster process, also called image scanning, can be described in such a way that a virtual raster consisting of rows and columns is placed over the print template and the color values (intensity values) are stored _{in the individual raster cells with the associated raster coordinates (row X i} , column Y _j) . During this process, the print template is read into the grid or the print grid. The result is a matrix with color information stored in the cells.

The print template is usually recorded using a two-dimensional, Cartesian coordinate system (Cartesian image grid). Such a coordinate system is formed from mutually orthogonal axes X and Y and has a grid-like structure with rectangular cells. The resolution of the image is determined by the size of the grid cells. For example, the distance between two horizontal grid lines determines the print resolution in the vertical direction. The distance between two vertical grid lines determines the print resolution in the horizontal direction. The so-called quantization takes place in a subsequent step. Quantization is understood to mean the evaluation of the image point, i.e. the brightness (intensity) and, if applicable, the color tone of a pixel by means of a specified amount of gray values or colors in the individual raster cells. The color information is saved with the associated coordinate of the grid (row X _i , column Y _j ).

The digital image data are used to control the print head. The print head moves along the grid coordinates of the print image and creates print points at the points specified by the print grid in accordance with the color information stored for the individual print point (e.g. quantity, color). The result is a print image that consists of pixels arranged in a grid.

The print head has at least one print nozzle, but generally several print nozzles which are arranged next to one another in a row of nozzles, the row of nozzles extending in the direction of the width of the print head. If there is only one row of nozzles, it is a single-row printhead. The distance between the two outermost nozzles in the row determines the effective print head width. With an even arrangement in the row, the individual pressure nozzles are each offset by one nozzle spacing in the direction of the row. The native printhead resolution of the single-row printhead along the printhead width is given by the nozzle spacing.

Several rows of nozzles running parallel next to one another, each with the same number of nozzles, are also common (multi-row printhead). The print nozzles of a second row are offset from those of the first row in the direction of the print head width, with two-row nozzles being offset by half the nozzle spacing. The native printhead resolution in the direction of the printhead width can thus be doubled for a two-row printhead compared to a single-row printhead with the same nozzle spacing.

All printing dots of a row (line) of the printing screen are normally printed by the same nozzle, which moves relative to the printing surface parallel to one of the printing screen axes. However, if the print motif is wider than the effective print head width, the image must be divided into sections and printed in parts. First a first image part and then a second image part offset from the first image part is printed.

However, this procedure has the disadvantage that the approach between the two partial images, also called print segments, is easily recognizable. In order to counteract this problem, a process called "stitching" is often used. The printing segments / partial images of two successive printing steps are no longer flush with one another, but rather overlap in an overlapping area. Therefore, in two successive printing steps, two partial images are not simply printed in a completely flush manner next to one another. Instead, in the first printing step, the printing required to achieve the desired image is only partially carried out in the overlap area. The missing part of the print is added in the following, second print step. Outside the transition areas, the image parts are printed in one printing step. The image quality is increased because the boundaries of the printing segments of successive printing steps are less clearly recognizable due to the overlap area.

The stitching reduces the effective usable length of the printhead, but is gladly accepted because the image quality can be increased. However, this method makes it necessary to interrupt the printing process in order to relocate the printing object to a second printing position. This procedure is therefore referred to as the successive application of image parts in cycles. However, this can lead to tolerance-related faults, which is due, among other things, to the displacement of the object to be printed. This can be illustrated by the fact that with a print resolution of 720 dpi (1 dpi = 1 point per inch) the print points are only about 3/100 mm apart. Servomotors known from the prior art have a tolerance of 1/100 mm, which can also be referred to as resolution. This means that the approach can deviate by 1/100 mm from the printed image in front of it, which means 33% of the printing point distance. This can also be perceived by an inexperienced eye, as it is able to recognize positional deviations of a few micrometers. In particular, such an offset is also easy to recognize because the stitching approach is the same for all printing colors in order to utilize the maximum printing width and therefore all colors at the same point have the same offset problems. Furthermore, when printing with vertically arranged print heads, the effect of gravity becomes visible. Typically there is a negative pressure of approximately 10 mbar in the individual nozzle chambers in order to avoid an unintentional escape of ink from the pressure nozzles. The current typical print head width of around 70 mm therefore results in a pressure difference of 7 mbar between an uppermost and a lowermost print nozzle of a print head with several print nozzles when the print nozzles are arranged vertically one above the other. As a result, the drop volume is no longer uniform when printing. The lower pressure nozzles with the higher internal pressure (corresponding to the lower negative pressure) print a slightly larger drop than the upper nozzles with a lower internal pressure. This is noticeable in the color intensity, since more color is applied in an area printed by the lower print nozzles than in an area printed by the upper print nozzles. This is particularly noticeable when, because of the stitching, drops of the lower and upper pressure nozzles adjoin one another. This reinforces the visual impression of error in the viewer.

The DE 35 26 769 A1 describes a method for printing containers in which the container rotates in front of the print head and in the direction of its axis of rotation is moved. The individual colored dots are applied along parallel helical lines. This means that there is no longer any need to interrupt the printing process. As a result of the relative movements, however, the printing result can be impaired if the print head applies the individual colors according to the values in the print raster. The nozzles or nozzle heads are offset from one another in the direction of the longitudinal axis of the container and are therefore not in one plane.

There is stitching even in such helical printing. However, this is not horizontal as in section printing, but follows the helical movement. Moving the printheads, as shown in the DE 35 26 769 A1 is described, therefore has the consequence that the stitching area is again identical for all colors and therefore easily recognizable.

It is the object of the present invention to improve the quality of digital prints on 3-dimensional objects.

This object is achieved by a method with the features of claim 1.

Advantageous refinements of the invention are the subject matter of the subclaims.

The invention is based on the idea of enabling seamless printing of infinite image lengths on 3-dimensional objects without the printing having to be divided into individual work cycles. An essential idea of the invention is that the print raster, which is used to control the print head during printing and into which the print template is read, is not rectangular, but curved or distorted and that the image lines and the image columns or the X-axis and the Y-axis not perpendicular, but run obliquely to each other. It is an inclined line grid.

In the inclined line grid, the rows and columns or the X-axis and the Y-axis do not run orthogonally, but rather, for example, at an angle of less than 90 ° to one another. The curvature of the printing screen can be compared with a distortion of a Cartesian coordinate system. A regular rectangular print raster can be distorted by offsetting one of the opposite sides of a rectangle by a certain distance (offset / shift), so that a parallelogram with correspondingly parallelogram-shaped raster cells is created. The sides adjoining the offset side undertake a pivoting movement like a parallelogram guide. The printing grid or the individual printing grid cells can also have the shape of a rhombus, a special shape of the parallelogram.

The curved print grid is used like a regular print grid to control the print head. The print head, for example, scans the image lines (X-axis) and applies the print medium to the object for each image point, in accordance with the _{information stored in the raster cell (X i} , Y _j ) for the image point. Reading the print template into the curved print raster, however, enables the print result to be improved when the print head moves relative to the object to be printed.

According to a further embodiment of the invention, the print raster or the print raster cells each have the shape of a parallelogram or a rhombus.

The curved print raster is particularly advantageous in the case of multi-axis movements of the print head relative to the object to be printed. Curvatures of the print image can be avoided and compensated for in advance by using a distorted grid when digitizing the print template. It is ensured that each printhead nozzle sets the correct pressure point on the printing surface. A further embodiment of the invention therefore provides that the object to be printed or the surface to be printed not only moves along or around an axis relative to one or more print heads, so for example rotates, but that there is a composite multi-axis movement relative to the Printhead completes. Of course, the print head can also perform a multi-axis movement around the print object.

Compound multi-axis movements take place along and around single or multiple axes. They can also be referred to as layered movements. This means that this is not a purely translational or a purely rotational movement, but in particular a combination of relative displacement and relative rotation during printing. The term "printing" denotes the process during which the print head applies the print medium to the surface to be printed. During printing, the object to be printed can be displaced along a first axis, while it simultaneously rotates about one or more axes of rotation. The axis of rotation can of course coincide with the first axis of displacement. In principle, however, the proposed method can also be used in the case of a 1-dimensional relative movement between the print head and the object.

According to a further embodiment of the invention, the movement of the object to be printed is a helical movement along and around an axis. According to the invention, it is therefore provided that the object, while it is being printed by the print head, rotates in front of the print head and is simultaneously moved along its axis of rotation. The shifting movement is relative relative to the printhead, for example, can be done relatively up or down. The helical movement divides the image into oblique strips, which are joined together seamlessly or seamlessly and allow a complete print without interruption or repositioning of the object. This avoids the error of repositioning the printhead when printing in sections and the quality of the stitching is significantly improved.

If the pressure point grid corresponds to a parallelogram grid, the X-axis extends virtually helically around the object, with the printhead being guided along a helical path relative to the object. The printing grid can be placed virtually on the outside of the container / object, where it extends helically around the container. The relative movement of the print head follows the print raster or is moved along the print raster and applies the print medium according to the information of the corresponding print raster cell.

Due to the curved grid and the screw movement, it is no longer necessary to subdivide the print image into individual print segments or to break it down into rectangular strips that are printed one after the other on the surface by moving the object between two image sections in an additional movement in order to to be printed further after a standstill. Instead, the printing can be continuous. This leads to a significantly better print image.

According to the invention, the printing medium can be applied using the multi-pass or single-pass method. In the multi-pass process, each line / print raster cell or mesh to be printed is applied several times, with a pattern or image being built up in several steps. That is, the print medium for the print raster cell is made in several passes or steps applied. However, printing with multiple passes also allows printing with a resolution greater than the native resolution of the printhead, in that further points are set between points that have already been set. In the single-pass process, the print image is printed in just one printing process, without the printhead having to scan the printing surface a second time.

According to a further embodiment of the invention, different colors are applied to the surface of the object to be printed at the same time, without intermediate hardening between individual colors. Multi-color printing is a technique for creating colored printed matter. The most common form of multi-color printing is four-color printing with the standardized basic colors cyan, magenta, yellow and black (CMYK), the process colors that are sprayed onto the object through the nozzles of the print head. The 4 printing nozzles for the basic colors CMYK are particularly preferably activated at the same time and therefore allow immediate curing after the print has been applied.

Another embodiment of the invention provides that at least individual print points of the print image are applied in several steps, the distribution of the total amount of print medium of the individual print points to the individual steps taking place in a random manner. The determination of the amount of pressure medium to be applied for the pressure point can be done by a random generator. Alternatively, an algorithm can also be used that distributes the amount of print medium to the individual steps. In this case it is possible under certain circumstances that one or more of the mentioned pressure nozzles which can be used for printing the pressure point cell do not apply any pressure medium to the pressure point cell. The random or algorithm-controlled distribution of the amount of printing medium to individual steps can be used in principle for all printing processes in which printing dots are applied in several sub-steps. The irregular, inconsistent distribution of the pressure medium application over several Steps reduces the perceptibility of errors caused by failure or malfunction of nozzles or positioning inaccuracies. In particular, the risk of transitions or printing band boundaries being recognizable is reduced. According to the invention, the algorithm can be set up to take into account the failure of a pressure nozzle, in that this pressure nozzle is no longer used and the other pressure nozzles to be used to print the pressure point compensate for the non-use. For this purpose, the intended amount of pressure medium from the failed nozzle can be distributed to the other nozzles. This can increase the running times of machines until maintenance.

In order to achieve an optimal and economical use of the print head capacity in the single-pass method, a further embodiment of the invention provides that in the case of a helical relative movement between a single-row print head and the object, the length of the movement or the displacement along the axis of rotation per revolution of the Object (in the following: slope) corresponds to the product that results from the number of nozzles of the printhead multiplied by the nozzle spacing. The expansion of the pressure nozzle arrangement is therefore used as a measure of the gradient for the continuously applied pressure surface. The print image grid is thus optimally adapted to the movement and resolution of the print head. In this case, no area of the print surface is passed more than once by print nozzles (single-pass method).

Regardless of the diameter of a particularly rotationally symmetrical object that is continuously printed by a relative helical movement, the print image always finds its starting point after one revolution and the curved or parallelogram-shaped print raster ensures an optimal print image even with this multi-dimensional relative movement.

For a single-row print head with n print nozzles that are offset by one nozzle spacing in the row direction, the curved print grid is preferably formed from a rectangular print grid by distorting the rectangular print grid into a parallelogram or a rhombus, the distortion with the n -fold of the nozzle spacing correlated.

For color printing, several print head modules or print heads can be mounted one behind the other or next to each other in the running direction of the print surface in a single-pass printing system, which e.g. is guided past the print heads in a helical movement relative to the print heads. The print head modules are each assigned a basic color, in particular cyan, magenta and yellow and optionally black. Print head modules with a special color can be added for special printing applications.

According to the invention, however, the slope can also be reduced by a factor or a fixed value, which results in an overlap of the printed images after one revolution, the width of which results from the reduction in the slope. The greater the degree of reduction in the slope, the greater the overlap. By means of the overlap, transitions between the orders of different nozzles can be blurred. For example, an overlap can be used to blur a transition at a boundary between an application from an upper and a lower nozzle after a 360 ° rotation. In a subsequent rotation, overlaying print points in the overlap area can be distributed to the individual partial prints of the overlap area according to an algorithm or by means of a random generator. The distribution of the amount of ink in this stitching-like process does not follow a fixed pattern.

For multi-pass processes in particular, it is advantageous if, according to a further embodiment, the movement or displacement of the object along the axis of rotation is directly related to the resolution of the print image and the print nozzle density in the displacement direction. According to the invention, at least individual print points of the print image are applied in several steps, with the pitch (axial offset per revolution) used for the helical movement corresponding to the number of nozzles multiplied by the nozzle spacing and divided by the number of steps for a multipass.

It is also possible that only some of the pressure nozzles are used. According to the invention it is provided, for example, that only every second print nozzle of the print head is used. Due to the half-pitch of the printing helix compared to the "single pass" method, twice as many revolutions are required for the same height of the printing area, but half of the printing nozzles can remain deactivated for the same printing density. It is particularly advantageous to use the pressure nozzles not used in the previous rotation for two successive rotations in the later rotation and to deactivate the previously inserted half of the pressure nozzles (alternating nozzle use). Positioning inaccuracies and nozzle errors are less noticeable as a result.

The use of only some of the print head's print nozzles (for example only 100 out of 1000) is also advantageous when objects with a very small diameter are to be printed. As a result, given the desired vertical print density, the ratio of print helix pitch and circumference of the object can be matched to one another.

Another embodiment of the invention provides that the relative speed at which the print head and the object move relative to one another move, changed or varied during printing. In particular, it is provided that the object executes a helical movement relative to the print head during printing and that the pitch of the helical movement is varied during printing. In other words, the length by which the object is displaced during printing during one revolution of the object along its axis of rotation can vary. This makes it possible to print objects with complex shapes and / or to print special images. With the variation of the slope, it is possible, for example, to react to variations in the outer diameter of a container, so that a uniform print image results.

According to an advantageous embodiment of the invention, the resolution can also be varied with the change in the slope. In particular, the print image or a region of the print image can be applied in a higher resolution than the native resolution of the print head. The pitch (axial offset per revolution) used when printing the area for the helical movement is the number of nozzles multiplied by the nozzle spacing and divided by the number a multiple of the native resolution.

A further embodiment of the invention provides, however, that the print head or heads are aligned such that the nozzle arrangement extends in a direction parallel to the relative axis of rotation. In a printing device, the printing nozzles can be arranged one below the other, that is to say vertically. The vertical arrangement allows the pressure medium filled into the printhead to be ejected by gravity at the lower nozzle with a higher pressure than at the upper nozzle. This usually results in other droplet sizes, which are reflected in the color intensity. With clocked, offset printing, approaches would become visible. If you print with several colors, the effect is clearer because this applies to all colors. By the method according to the invention the offset effect is avoided because, on the one hand, no horizontal line is formed and, on the other hand, each color can be applied at a different point in multi-color printing. In particular, in the case of multicolor printing, the application of the color can be started for different colors, particularly preferably for each of the colors, in each case at different points on the object to be printed. The printing nozzles of different colors, particularly preferably of all colors, thus begin the application of paint in each case at points spatially offset from one another. This is the case in particular with a helical relative movement between the print head and the object. Due to the helical relative movement, the colors are effectively distributed and do not appear in the same place as when printing with approaches. This is particularly the case when, for example, the lowermost nozzle of a plurality of print heads are each at the same height, in particular when the print heads are arranged in a ring around a cylindrical body. Then the helical attachments of the different colors do not run on the same line when the print heads start printing at the same time, but are offset from one another. As a result, in 4-color printing, the approach of each color is masked by 3 other colors, because their approaches are each in a different place. It is preferably proposed to modify the print template read into the print raster for a first print head and / or a second print head in such a way that the print starts of the various print heads on the object to be printed are offset from one another. The print start is therefore not for both print heads at the beginning of the print image on the object to be printed. In particular, the start of printing of the second printhead can be offset by an angle with respect to an axis of rotation about which the object to be printed is rotated relative to the start of printing of the first printhead. The second print head only prints the omitted starting area of the print image afterwards, which enables 360 ° printing again. This is possible, for example, when the object to be printed is rotated further relative to the second print head and that of the second print head initially omitted starting area is in front of the second printhead. In order to achieve that the print starts of the first print head and the second print head are offset from one another on the object to be printed, it is generally sufficient that the print template read into the print raster is only modified for the first print head or the second print head.

Particularly preferably, an area of the original for the first printhead and / or for the second printhead or the second printhead that is not initially to be printed is cut off and attached to a previous end of the original that was read into the raster. This creates a continuous 360 ° image for the corresponding print head without jumping from the end at 360 ° to the beginning at 0 °.

This is explained in more detail using an example. If the second print head, for example for the color magenta, is shifted by 90 ° with respect to the axis of rotation compared to the first print head, for example for the color cyan, then the original print template read into the print raster for the second print head, in this example for the Color magenta, can be modified as follows: In the print template read into the print raster, an initial area is cut out for the second print head, which corresponds to 0 ° to 90 ° of the print image to be applied with respect to the axis of rotation. So that the modified print template reaches from 0 ° to 360 ° again, on the one hand the area not cut off is shifted by 90 ° to the beginning and on the other hand the cut area is attached to one end of the area which is not cut off. The previously cut off starting area now forms an area from 270 ° to 360 ° of the print template for the second print head. After the modification, the information for the second print head is shifted cyclically by an angle of 90 ° compared to that for the first print head. The print template for the second print head, here for the color magenta, is, so to speak, compared to the print template for the first print head, here for the color blue "out of phase". Thanks to this modification, it is possible, in particular, for the first print head and the second print head to be arranged at the same height in relation to the axis of rotation and still be able to start printing on the object at the same time. The print start of the second print head (in the example cyan) is offset by 90 ° compared to the print start of the first print head (in the example magenta) for better quality stitching.

In addition, a print start of a third print head can particularly preferably be offset by an angle with respect to the print start of the second print head. In particular, this angle can be equal to the angle at which the start of printing of the second print head is offset with respect to the start of printing of the first print head.

Most preferably, the print template read into the print raster is modified for the various print heads in such a way that the print starts of all print heads on the object to be printed are each offset from one another.

For a clean print image, it has proven to be advantageous to place the print start below the print image to be printed. As soon as the point of the object to be printed reaches the first nozzle of the print head during its movement in the axial direction, the first point is set.

According to a further embodiment of the invention, the object is rotated about an axis of rotation during printing and the direction of rotation is reversed during the printing process. As a result, the printing surface or individual areas of the printing surface can be printed with a higher resolution. Alternatively or additionally, the object can be moved axially during printing along an axis, preferably the axis of rotation, and the direction of movement along the axis can be reversed during the printing process. The reversal of the shift or the rotation can take place in the area of one or more print heads. This saves time, especially with multi-pass procedures.

Another embodiment of the invention provides that pinning and / or curing of the ink take place during the printing process and / or after printing. In particular, the printed image can be pinned and / or hardened during or between or after the individual ink applications. After applying the printing medium (e.g. ink or other color media), the object can be hardened by appropriate means. These include, for example, radiation sources such as a UV lamp, chemical agents such as crosslinking or hardening components, or thermal sources for evaporating the liquid components. The object can also rotate about an axis of rotation during curing, and the direction of rotation can be reversed during the curing process.

The invention also relates to a device for digitally printing 3-dimensional objects, in particular bottles, cans or other hollow bodies, which is set up to carry out one of the methods described here. According to the invention, such a device has a receptacle for the object to be printed, at least one drive device with which the receptacle is axially displaceable in a displacement direction and rotatable in a direction of rotation about an axis of rotation, at least two print heads and a controller for controlling the drive device and the print heads . The control is set up in such a way that it shifts the receptacle with the object arranged on it both in the displacement direction and rotates it in the direction of rotation during printing. The receptacle can be a holder, in particular a turntable or the like, which guides the object in particular in a composite multi-axis movement.

In the case of multi-color printing, at least two print heads are at the same height position in the direction of displacement. They are therefore in one plane and have no axial offset. However, they are arranged offset in the circumferential direction about the axis of rotation.

Preferably, the at least two print heads lying in one plane start printing at the same time. The oblique stripes that you print on the object to be printed when printing in a helical manner and that extend helically around the object to be printed then do not fit exactly on top of one another. Instead, they are offset from one another, even after one complete and several revolutions of the object to be printed. The strip edges of a first inclined strip of a first print head are therefore in the interior of at least a second inclined strip of a second print head and are masked in this way. This makes the print image sharper and its quality is particularly good. In particular, it is less affected by irregularities in the shape of the object to be printed.

The device can be set up to control the multi-axis movement of the object in such a way that a surface to be printed can be printed by all print heads during the printing process. For this purpose, for example, the positions of the print heads can be selected as a function of the axial offset per revolution so that the print heads can start printing at the same axial height of the object.

The invention is explained in more detail below using an exemplary embodiment and the drawing.

Fig. 1

schematisch das Bedrucken eines rotationssymmetrischen Behälters nach einer ersten Ausführungsform der Erfindung; und

Fig. 2a-e

schematisch das Rastern eines Bildmotivs nach einer weiteren Ausführungsform der Erfindung.

Show it:

Fig. 1

schematically the printing of a rotationally symmetrical container according to a first embodiment of the invention; and

Figures 2a-e

schematically the rasterization of a picture motif according to a further embodiment of the invention.

Figure 1 shows a rotationally symmetrical 3-dimensional object 1 to be printed in the form of a bottle. The bottle 1 is received in a receptacle 2 in the form of a turntable, the turntable 2 being drivable in rotation about an axis of rotation 3, so that the bottle 2 rotates about its axis of symmetry or center axis that coincides with the axis of rotation 3 in the longitudinal direction (here in the vertical Direction) coincides. The turntable 2 is part of a drive device 4, which is only shown schematically and which can be moved up and down in the vertical direction, that is to say along the axis of rotation 3, which is indicated by arrows 5.

On the outside of the bottle 1, an area 6 is marked which extends over the outer circumference of the bottle 1 and which is to be printed with a print image. A print head 7 is arranged next to the bottle 1. Different positions of the print head 7 in the vertical direction relative to the bottle 1 are identified by a), b) and c). However, it is the same printhead at different stages of the printing process.

The print head 7 has a number of print nozzles 8, the arrangement of which extends in the height direction (displacement direction) along the extension B (effective print head width). Adjacent (directly adjacent) nozzles in a row of nozzles extending parallel to the axis 3 have one Distance T. The row of pressure nozzles is aligned vertically and parallel to axis 3. The extent of the pressure area 6 or the pressure surface is greater in the axial direction (in the height direction) than that of the nozzle arrangement B.

The image to be printed is in digital form and is divided into a virtual grid of image points, which consists of image columns and image lines, by means of software known from the prior art. The print grid is used to control the print nozzles. By applying droplets, ink from the nozzles 8 is applied to the bottle 1 in the form of pressure points in accordance with the specifications of the pressure dot grid, so that the print motif is printed as a grid motif on the outside of the bottle 1.

The bottle 1 is held with the surface 6 to be printed at a small distance from the print head 7 and rotates around its central axis or around the axis of rotation 3 along the direction of rotation R. This rotational movement is paired with a displacement movement 5 along the axis of rotation 3 (here downwards), so that the print head 7 is offset upwards relative to the object 1. It is therefore a composite or superimposed multi-axis movement that combines an axial movement with a rotary movement. As a result, the surface 6 to be printed moves helically past the print head 7. It is a relative movement in the form of a helix line. One can also speak of a pressure helix.

The print head 7 is thereby moved relative to the bottle 1 from position a) via position b) to position c). Printing begins when the first (uppermost) nozzle 8 of the print head 7 sets the first pixel of the bottom row of the print dot grid at position a). The following print points for the lower edge of the print dot grid are printed by the uppermost nozzle 8 until the second nozzle from the top reaches the lower edge of the print area. From this point on, the second nozzle will also print a helix line below that of the first nozzle. After a certain displacement of the printhead, the lowest nozzle also enters the printing area. From this point in time, an oblique strip 10 is printed on the bottle, which extends helically around the bottle. The oblique solid lines 9 in Figure 1 characterize the pressure course of the strip 10 applied by the print head 7 on the front side 11 of the bottle 1. The inclined strip 10 merges seamlessly into the strip printed during the previous rotation, so that a seamless and clean print image results.

The slope, i.e. the relative displacement h between the print head 7 and bottle 1 in the axial direction per revolution of the bottle 1 around the axis of rotation 3 results from the sum of the extent B of the nozzle arrangement 8 and the distance T between the nozzles 8 (h = B + T ). This creates a continuous pressure of a compression strip 10 surrounding the outside of the bottle 1 in a helical manner, with the width L in the axial direction, which corresponds to the dimension B.

In position c), the print head 7 has almost left the printing area, with the lowermost nozzle of the print head 7 applying the final droplets in accordance with the programming.

In the context of the invention, the method described above can also be carried out the other way around, in that the bottle is guided upward in the axial direction during printing. In this case, printing starts at the top of the area to be printed 6.

In addition, the slope can also be halved by halving the distance of the relative displacement per revolution. In this way, intermediate points can also be set and the resolution used for printing can be increased.

Fig. 2a shows an analogue print motif (print template 12. This picture motif is rasterized for raster printing.

How Figure 2b shows, a virtual Cartesian grid 13 is placed over the image motif. The grid 13 is in the form of an XY grid and is divided into individual grid cells 14. The grid cells 14 are rectangular. The color information of the print motif in the individual grid cells is read in and stored in the grid cells. The print template is now available in locked form. During the printing process, the print head virtually traverses the print raster and prints the individual image points according to the specifications that are stored in the raster for the individual image points.

Figure 2c shows analogously to Figure 2b the rasterization of the motif 12 Fig. 2a into a printing raster 15. The printing raster 15 is not a Cartesian raster with rectangular cells, but a non-orthogonal two-dimensional raster in which the coordinate axes X, Y are not perpendicular to one another but are at an angle 16 at an angle to one another .

To generate the printing raster 15, the in Figure 2b Cartesian raster 13 shown are stretched before the print motif is read in by one of the axes X or Y is shifted. In the case shown, the Y axis has been shifted by a distance 17. As a result, the entire grid was distorted like a parallel guide. The formerly rectangular cells 14 of the grid now have the shape of a parallelogram.

The parallelogram 15 is large enough that it completely encloses the analog image motif when it is virtually superimposed on it. Analogously to the Cartesian grid 13, the image area is divided into columns of the image area by vertical grid lines (parallel to the Y-axis) or inclined grid lines (parallel to the X-axis) and image area lines divided. You have to consider the size of a pixel, which at 720 dpi is 0.03 x 0.03 mm. A horizontal line would therefore be just as step-shaped as a sloping line in the Cartesian grid. For this, the inclined line is printed more precisely in the parallelogram grid. The deviation when a horizontal line is printed in the parallelogram grid corresponds at most to the distance between 2 nozzles (in our example 1/100 mm) and is not noticeable to the eye.

In the present case, the parallelogram 15, which is placed over the print motif, is arranged opposite the Cartesian grid 13 in such a way that the Y grid lines of the parallelogram 15 run parallel to the Y grid lines of the Cartesian grid 13, whereas the X grid lines of the the two grids 12, 15 are inclined to one another. In other words, it is basically possible to distort a raster used to rasterize a print motif and to read the image data into the raster.

During the rasterization, the image motif is divided into individual parallelograms 14 (image area cells) due to the inclined (X) and vertical (Y) raster lines, and the corresponding color information is stored in the raster 15.

During printing, the image information is read out in the print cells 14 at the raster positions (image column X _i and image line Y _j ) and used to control the print head. The parallelogram raster is processed like a Cartesian raster for the print data.

Fig. 2d shows the print result 18 (raster-like print image) when the parallelogram raster 15 is processed as a Cartesian raster, the print head being moved parallel to the X axis and no relative movement between the print head and the surface to be printed in the Y direction. The printed motif is sheared or printed at an angle.

Fig. 2e shows the print result 19 (raster-like print image) of a combined rotating and shifting movement of the object 1 to be printed relative to the print head 7 when the parallelogram raster 15 is off Figure 2c is used to control the print head 7. When the image is applied, the displacement of the surface to be printed is superimposed on the rotational movement. This results in a pressure helix gradient α of the pressure lines of the individual pressure nozzles per revolution (see Fig. 1 ). The printing helix pitch α correlates with or corresponds to the pitch angle 16 of the printing grid 15. Due to the printing helix pitch α, the bottle is displaced by the distance h after one revolution - in accordance with the definition of the pitch given above. The print motif Fig. 2e is approximately transformed back and the individual pressure points 20 are applied according to the desired result.

The necessary consideration of the relative movement of the body to be printed along its axis of rotation to the print head took place when generating the print data from the image motif 12 by means of the Y-axis of the raster sheared around the print helix pitch α.

List of reference symbols

1

Print object (bottle)

2

Recording (turntable)

3

Axis of rotation

4th

Drive device

5

Direction of movement

6th

area to be printed (print area)

7th

Printhead

8th

Pressure nozzles

9

Pressure gradient on the front

10

Print strips

11

front

12th

Print template (image)

13th

Cartesian grid

14th

Print screen

15th

Grid cell

16

angle

17th

shift

18th

Print image (print result)

19th

Print image (print result)

20th

Pressure points

B.

Expansion pressure nozzles

L.

Strip width

R.

Direction of rotation

T

Distance between adjacent pressure nozzles in a row of nozzles

H

shift

α

Pressure helix slope

Claims (15)

1. Method for digitally printing on 3-dimensional objects (1), particularly bottles, cans or other hollow bodies, by means of at least one printing head (7), wherein the object (1) to be printed moves relative to the printing head (7) for printing, wherein a printing template (12) is broken down into a multiplicity of printing dots (14) and the printing dots (14) are stored in a printing raster (15) consisting of image columns and image rows, wherein the printing raster (15) is used for controlling the printing head (7) during printing in order to apply a print image onto the object (1) to be printed, characterized in that the printing raster (15) is curved and the image rows and the image columns do not extend perpendicularly but obliquely to one another; and in that the printing template (12) is read into the curved printing raster (15).
2. Method according to claim 1, characterized in that the printing raster (15) has the form of a parallel programme or a rhombus.
3. Method according to any one of the preceding claims, wherein the printing head (7) comprises a row of printing nozzles with a number n of printing nozzles (8) with a spacing (T) between adjacent printing nozzles (8), characterized in that the curved printing raster (15) is formed from a rectangular printing raster (13), wherein the rectangular printing raster (13) is distorted into a parallelogram or a rhombus, wherein the distortion (17) correlates with n times the spacing (T).
4. Method according to any one of the preceding claims, characterized in that an area (6) of the object (1) to be printed is moved relative to the printing head (7) in a composite multi-axial motion during printing.
5. Method according to any one of the preceding claims, characterized in that the object (1) is rotated relative to the printing head (7) and simultaneously moved along its rotational axis (3) during printing such that the object (1) carries out a helical motion relative to the printing head (8).
6. Method according to claim 5, characterized in that a pitch of the helical motion for a single-pass-printing corresponds to the number of printing nozzles (8) multiplied by the spacing (T) between two adjacent printing nozzles.
7. Method according to claim 5, wherein individual printing dots of a print image (19) are applied in multiple steps, characterized in that a pitch corresponds to the number of printing nozzles (8) multiplied by the spacing (T) between two adjacent printing nozzles (8) and divided by the number of steps.
8. Method according to claim 5, characterized in in that a pitch of the helical motion corresponds to the number of printing nozzles (8) multiplied by the spacing (T) between two adjacent printing nozzles (8) and divided by the number of a multiple of the native resolution of the printing head so that an area of the print image (19) is applied with a higher resolution than a native resolution of the printing head.
9. Method according to any one of claims 5 to 8, characterized in that pinning and/or curing of the ink takes place during the process of printing and/or after printing,
   and/or that a rotating direction of the object (1) can be reversed during the process of printing and/or curing the ink,
   and/or that a pitch of the helical motion is varied during printing.
10. Method according to any one of the preceding claims, characterized in that at least individual printing dots (20) of the print image (19) are applied in multiple steps, wherein the distribution of the overall quantity of the print medium of the printing dots (20) over the individual steps takes place randomly controlled.
11. Method according to any one of the preceding claims, characterized in that the printing template read into the printing raster is respectively modified for a first printing head and/or a second printing head in such a way that the printing starts of the different printing heads on the object to be printed are offset to one another.
12. Method according to claim 11, characterized in in that an area, which is not to be printed initially, of the printing template read into the printing raster for the first printing head and/or for the second printing head is cut off and placed at a previous end of the printing template read into the printing raster.
13. Method according to claim 11 or claim 12, characterized in that a printing start of a third and/or fourth printing head is offset relative to the print starts of the second printing head by an angle.
14. Device for digitally printing on 3-dimensional objects (1), particularly bottles, cans or other hollow bodies, by means of a method according to any one of the preceding claims, having a receptacle (2) for the object to be printed, at least one drive unit (4), by means of which the receptacle (2) can be displaced axially in a displacement direction (5) and rotated about a rotational axis (3) in a rotating direction (R), at least two printing heads (7), and a control for controlling the drive unit (4) and the printing heads (7), wherein the control is configured such that it displaces, in the displacement direction (5), as well as s well as rotates, in the rotating (R), during printing the receptacle (2) with the object (1) to be printed arranged thereon, wherein the at least two printing heads (7) are positioned at the same height position in the displacement direction (5).
15. Device according to claim 16, characterized in that the control is configured such that the at least two printing heads (7) simultaneously start printing on the object (1) to be printed.

Images