DE102014006991A1 - Apparatus for printing with an ink jet printhead on a curved surface of an obiect - Google Patents

Abstract

An inventive device for printing with an inkjet printhead on a curved surface of an object, e.g. B. on a motor vehicle body, the print head (1) comprising at least one row of nozzles (2a, 2b) with nozzles (2) for ejecting ink jet drops (F) and wherein the print head (1) and / or the object ( B) is moved by a movement device (E), preferably a 6-axis robot, to generate a relative speed component (vHead) between the print head (1) and surface (C), characterized in that the print head (1) and / or the row of nozzles (2a, 2b) about an axis (3) parallel to the row of nozzles (2a, 2b) by an angle (β) against the direction of the relative speed component (vHead) between the print head (1) and the surface (C) , d. H. against the forward movement, is inclined in such a way that ejected ink jet drops (F) receive a relative speed component (vJetZ) between drops (F) and surface (C), which the relative speed component (vHead) between printhead (1) and surface (C) essentially compensated. Speed-dependent print image distortions can thus be reduced or avoided in an advantageous manner without changing the print image data.

Description

The present invention relates to an apparatus for printing with an ink-jet printhead on a curved surface of an object having the features of the preamble of claim 1. Furthermore, the present invention relates to a method of printing with an ink-jet printhead on a curved surface of an object with the Features of the preamble of claim 9.

The invention is in the technical field of inkjet printing or inkjet printing.

It is already known, objects with free-form surfaces such. B. to paint a body of a vehicle monochrome or multi-colored. A multi-colored coating can already be done during production or later. It is also already known to provide a body with colorful pictures. For example, advertising is provided on the side or the rear of the vehicle.

The known prior art in this technical field comprises the following: The DE 37 37 455 A1 discloses a device for generating color patterns. By means of a computer-controlled and guided by a robot inkjet system z. B. raster images in full color (3 basic colors) produced as body decorations. The pictures can also be customized.

The DE 10 2008 053 178 A1 discloses a coating apparatus for bodies with a multi-axis robot and an ink jet printhead mounted thereon. In one embodiment, a plurality of print heads are provided, which are guided together on a robot and are pivotable relative to each other, which allows adaptation to curved component surfaces.

When printing curved surfaces with inkjet printheads, disadvantages can occur due to undesired distortions of the printed image. Such print defects should be avoided in order to achieve better print quality. The underlying technical problem is in the 1A to 1C shown.

From the DE 40 33 816 A1 an ink jet printer with a marking head is known, which can be statically adapted to the surface to be printed by a corresponding curvature of a nozzle carrier.

The WO 2004/007203 A1 teaches, however, to adjust the print data so that an expected distortion is compensated. First, test patterns are printed and their distortions are analyzed. The analysis results are included in the compensation procedure.

Finally, the reveals US 5,043,740 a compensation method in which the nozzles of the inkjet printhead are driven at different times. This document very well illustrates the problem of distorted printed lines in its introduction.

The said compensation methods based on the pressure data or nozzle controls may also have disadvantages, for. For example, they are computationally intensive and inadequate in those situations where there are deviations in the movement of the printhead from its nominal values (eg position, orientation, speed, acceleration) during printing.

Against this background, it is an object of the present invention to provide an improved over the prior art apparatus and a corresponding method which / which makes it possible, speed-dependent print image interference when printing with an inkjet printhead on a curved surface of an object in a simple manner and to reduce or avoid with high reliability.

An inventive solution to this problem is a device having the features of main claim 1. A further inventive solution to this problem is a method having the features of main claim 9.

An apparatus according to the invention for printing with an inkjet printhead on a curved surface of an object, the printhead comprising at least one row of nozzles with nozzles for ejecting inkjet droplets, and wherein the printhead and / or the object are moved by a moving device, preferably a robot arm Generation of a relative speed component between printhead and surface is characterized in that the printhead and / or the nozzle row is inclined about an axis parallel to the nozzle row by an angle β against the direction of the relative velocity component between the printhead and the surface such that ejected ink jet drops receive a relative velocity component between the drop and the surface that substantially compensates for the relative velocity component between the print head and the surface

The device according to the invention advantageously allows speed-dependent print image disturbances when printing with an inkjet print head on a curved surface an object in a simple manner and with high reliability to reduce or avoid.

According to the invention, an adaptation of the print data or the control of the nozzles is not made, as taught in the prior art, but an inclination of the print head and / or the nozzle row against the forward movement or the main movement direction of the print head is set such that the compensation " at the level of the drop-speed vectors. The individual drops receive an additional velocity component due to the inclination of the print head and / or the nozzle row when ejecting the drops, which (by vector addition) causes the drops to move in the direction of the object surface as if they were from a stationary print head been ejected. Straight lines contained in the printed image, which are printed for example by simultaneous ejection of the nozzles of a row of nozzles, are therefore printed perpendicular to the surface as a straight line and not distorted from above.

The term "relative speed component between printhead and surface" means (in the case of the movement of the printhead) the component of the relative velocity vector between the printhead and the surface parallel to a forward movement or the main direction of movement of the printhead. The forward movement of the printhead is to be distinguished from a lateral movement (laterally, i.e. in the longitudinal direction of the nozzle row) and an up-and-down movement of the printhead (approach and removal from the surface). The same applies to the case that not the print head, but the object is moved.

The term "relative velocity component between drops and surface" is in the description of the figures 2A explained.

With a device according to the invention can be z. B. surfaces of vehicles, ie z. B. body parts, trouble-free printing. In this case, a robot arm, in particular with six axes, is preferably used.

The invention is also in 2.5-D printers z. B. for printing on mobile phone shells, toys or robotic machines for printing surfaces of smaller objects can be used advantageously.

Such types of printing machines can, for. B. x-y or x-y-z tables with linear guides, similar to machine tools have.

Advantageous and therefore preferred developments of this invention will become apparent from the appended subclaims and from the description and the accompanying drawings.

A preferred development of the device according to the invention can be characterized in that the sine of the angle β is substantially proportional to the amount of the relative velocity component between the print head and the surface, in particular that the sine of the angle β and the amount of the relative velocity component between the print head and the surface are essentially constant. This means that the printhead or nozzle row incline is increased at higher printhead speeds, and vice versa. The required inclination can preferably be set by appropriate activation of the movement device or the robot arm or its tool holder. When using a conventional 6-axis robot with 3 position and 3 angular degrees of freedom thus advantageously no additional actuators for the inclination adjustment needs to be provided.

A preferred embodiment of the device according to the invention can be characterized in that at least two ink-jet printheads are provided for printing different colors. The tilt adjustment can be done together for the printheads, z. B. by the robot arm.

A preferred development of the device according to the invention can be characterized in that the inclination of the ink jet printheads is separately adjustable. For this purpose, separate actuators may be provided on the tool holder of the print head. Such a solution can be particularly advantageous in the case of 4-color printing since the installation space increases due to the four print heads arranged one behind the other and the spaced-apart print heads can be adapted separately to a changing surface curvature.

A preferred embodiment of the device according to the invention can be characterized in that a print head is provided with at least two rows of nozzles. The tilt adjustment can be done together for the nozzle rows, z. B. by the robot arm or by an actuator for the print head.

A preferred embodiment of the device according to the invention may be characterized in that the inclination of each nozzle row is adjustable relative to the print head, in particular that the inclination of each nozzle row is adjustable separately relative to the print head. As a result, a tilting movement of the print head as a whole is advantageously prevented. The inclination adjustment of the individual nozzle rows can be due to their low Mass also advantageously be faster and more accurate. It can be provided separate actuators for the individual tilt adjustment of the nozzle rows.

A preferred embodiment of the device according to the invention can be characterized in that the nozzle rows are mounted with an inclination on the respective print head. This technical solution is particularly advantageous when the printhead is always moved forward during printing with the same substantially constant preferred speed or design speed. Then the head does not need to be inclined or inclined at a lower angle, because the rows of nozzles preassembled with the inclination matching the constant preferred speed already eject the drops with the compensating speed component. And even at other speeds low printhead inclinations are sufficient. Slight speed deviations can then still be achieved by tilting the printhead, e.g. B. be corrected by appropriate control of the robot arm. The total inclination β = β _D + β _U is then composed of the inclination of the nozzle row β _D and the head inclination β _U. With the inclination of the nozzle row for design speed 300 mm / s can z. For example, over the entire speed range of 200 mm / s to 600 mm / s, the required print head inclination can be reduced by 50% to 100%.

A preferred embodiment of the device according to the invention can be characterized in that the inkjet printheads are mutually displaceable. This provides the advantage that the distances of the individual printheads to the surface for all printheads in the desired spacing window (between minimum and maximum allowable distance) can be maintained.

The inventive device or its preferred developments can also be used in production lines for vehicles or other goods, such. B. toys or trays of consumer electronic goods (cell phones, cameras, etc.), be integrated and make the printing "inline". The invention therefore also relates to production lines with integrated device according to the invention.

The above apparatus according to the invention can also be described by the following synonymous main claim or its combination of features: Inkjet printhead for printing on a curved surface of an object, wherein the printhead or its rows of nozzles are inclined to compensate for the forward movement of the printhead during printing ,

A method according to the invention for printing with an inkjet printhead on a curved surface of an object, wherein the printhead comprises at least one row of nozzles with nozzles for ejecting inkjet droplets, and wherein the printhead and / or the object are moved by a moving device, preferably a robot arm a relative speed component between the print head and the surface is characterized in that the print head and / or the nozzle row and / or the object about an axis parallel to the nozzle row by an angle β against the direction of the relative velocity component between the print head and the surface is tilted such that ejected inkjet drops receive a relative velocity component between the droplet and the surface that substantially compensates for the relative velocity component between the printhead and the surface.

The method according to the invention, like the device, advantageously makes it possible to reduce or avoid speed-dependent print image defects when printing with an inkjet print head on a curved surface of an object in a simple manner and with high reliability.

A preferred further development of the method according to the invention can be characterized in that the inkjet printhead is arranged on the robot arm and guided along the surface while maintaining a minimum distance. The abovementioned method according to the invention can also be described by the following synonymous main claim or its combination of features : A method of printing with an ink jet printhead on a curved surface of an object, wherein the printhead, its rows of nozzles, or the object for compensating the forward movement of the printhead or the object are tilted during printing.

The invention as such as well as structurally and / or functionally advantageous developments of the invention will be described below with reference to the accompanying drawings with reference to at least one preferred embodiment. In the drawings, corresponding elements are provided with the same reference numerals.

The drawings show:

1A . 1B . 1C A schematic view of an embodiment of a device without speed compensation;

2A . 2 B A schematic view of a preferred embodiment of a device according to the invention;

3 A schematic view of another preferred embodiment of a device according to the invention; and

4A . 4B A schematic view of another preferred embodiment of a device according to the invention.

1A shows a side view of an apparatus A with an inkjet printhead 1 when printing on the curved surface C of an object B, e.g. B. a pipe. The printhead has two rows of nozzles 2a and 2 B with respective ink jet nozzles 2 on (better recognizable in 1B ). The printhead 1 However, it may also have only one row of nozzles or more than two rows of nozzles.

In the figures, xTool, yTool and zTool denote the mutually orthogonal axes of a coordinate system fixed to the print head (printhead coordinate system). In the preferred embodiment, the inkjet printhead A is moved by a robot arm E to produce a relative velocity component between printhead A and surface C. The axes xTool, yTool and zTool can be understood as the axes of the tool coordinate system of the robot arm E.

In the figures, the vector vHead denotes the speed and direction of movement of the print head A during printing (forward movement or main movement). When printing on curved surfaces C, the direction of movement and speed may change during printing. vHead is then a function of time t. Relevant for the compensation according to the invention is then at any time the corresponding instantaneous value of vHead.

The from the nozzles 2 leaking ink droplets F have a speed component vJet in the direction of the surface C, which corresponds to the airspeed of the ink drops F with the head is stopped and the manufacturer's instructions or by measurements z. B. can be determined with a high-speed camera. It is typically several meters per second. The vectorial addition of the velocity component vHead of the printhead movement to vJet results in the resulting velocity vector vRes of the ink droplets F. Tan (α) = vHead / vJet.

1B shows a front view of the device A and the object B from 1A (The zTool axis and the vHead motion vector come out perpendicular to the paper plane). Here is the different pressure difference of the nozzles 2 to recognize the surface C. The contour of the tube B represents a simple circle when viewed from the front. However, the invention is also applicable to other objects and freeform surfaces with correspondingly more complex contours when printing. In 1A is z. B. recognizable that the tube B also in the direction of movement of the print head 1 may have positive and negative curvatures.

In 1C is a plan view of the printhead 1 When printing at speed vHead is shown on a tube B in the longitudinal direction, with the printhead 1 is aligned parallel to the direction of movement, ie the axis 3 a nozzle line 2a is aligned substantially perpendicular to the direction of movement. On the surface then, due to the finite (ie non-infinite) velocity vJet, the ejected ink droplet F enters in conjunction with the different pressure distance of the individual nozzles 2 to the surface C distortions, which affect the print quality.

By way of illustration, in the example of the 1C all nozzles 2 of the printhead 1 At equidistant intervals ink is emitted at the same time (in other words, straight lines should be printed perpendicular to the surface from above). The pressure distance, ie the distance between the nozzle 2 and impact point of the nozzle 2 ejected ink drops F on the surface C, is for the individual nozzles 2 different and in the example for the outer nozzles 2 larger than for those in the middle of the printhead 1 , Therefore, ink pens F of the outer nozzles fly longer until their impact point. Since the ink plugs F also with a velocity component in the direction of movement of the printhead 1 flying, they are offset from the ink drops F from the middle nozzles offset from the surface. The projection of the printed lines perpendicular to the printing web and surface C is then not straight as desired and looking at the pipe from above according to the in 1C As a result, undesired curved or distorted lines result 4 , The zTool axis points in the printing direction or direction of movement of the print head 1 , the axis yTool along the axis 3 the nozzle rows 2a and 2 B of the printhead 1 and the axis xTool points into the plane of the paper and stands in the middle of the tube perpendicular to the surface C of the tube B.

2A shows a side view of the printhead 1 when printing with the Speed compensation according to the invention. This is the printhead 1 around the axis yTool, or one to the nozzle row 2a or 2 B parallel axis 3 rotated by the angle β and thereby inclined backwards (against the direction of the relative speed component between the print head and the surface) that the resulting velocity vector vRes of the ink drops F is perpendicular to the speed vector vHead of the print head. We have sin (β) = vHead / vJet and the angle β can be calculated from the arcsine according to β = arcsin (vHead / vJet). If vHead is a function of time then β (t) = arcsin (vHead (t) / vJet). From the known or measurable values for vHead and VJet can thus be the (possibly time-dependent) inclination of the printhead 1 calculate and the printhead 1 tilt accordingly (if necessary, time-dependent).

The velocity vector vJet (between droplet F and surface C) is composed of the two components vJetX and vJetZ. vJetX points to the vector xTool with the printhead not tilted 1 (see. 1A ) and vJetZ points in the direction of the vector zTool with the printhead not tilted 1 (also see 1A ). vJetZ forms the relative velocity component between droplet F and surface C.

In 2 B is the printhead 1 when printing with the speed compensation 2A shown. The xTool axis no longer points vertically into the plane of the paper, but is slightly tilted backwards by the angle β. Accordingly, the axis zTool no longer points exactly in the direction of the motion vector vHead, but is slightly tilted downwards by the angle β. Due to the motion compensation, the ink drops F no longer have a velocity component in the direction of movement of the print head 1 so that they hit the surface without any offset in the direction of movement. The projection of the printed lines 5 perpendicular to the printing web and surface C is then straight and therefore the view of the tube B from above, the desired straight lines 5 ,

In 3 is a device A with a plurality of printheads arranged one behind the other 1 represented, with the various printing inks, z. B. YMCK, can be printed in one step. The printheads 1 are variable or fixed against each other. Thus, the speed compensation according to the invention can be realized without disturbing increase of the maximum pressure distance to the surface C.

In order to minimize the maximum printing distance D at different speeds vHead when using the speed compensation, the printheads can 1 depending on the speed vHead optimally be shifted against each other. Alternatively, the printheads leuch can be rigidly displaced relative to each other by a fixed amount to z. B. at the most commonly used speed vHead to achieve the lowest possible maximum pressure distance D.

4A shows a side view of a printhead 1 when printing with the speed compensation according to the invention, in which the ink is not inclined printhead 1 at an angle β at the speed vJet from the printhead 1 exit. The nozzles can spray at a fixed angle β, the z. For example, the compensation angle of the speed compensation according to the invention may be vHead at the most commonly used speed. For this purpose, the nozzle rows 2a and 2 B with the corresponding inclination β on the printhead 1 be (pre-) mounted. Alternatively, the spray angle can be variably adjusted in order to optimally set the spray angle at the lowest possible pressure difference D when using the speed compensation according to the invention at different speeds v Head. For this purpose, the inclinations of the nozzle rows 2a and 2 B be set. This can be z. B. by an actuator, preferably an electric actuator, are effected. The spray angle can z. B. be adjustable by rotatably mounted rows of nozzles in the printheads, which are moved by so-called piezo stacks. The spray angle may alternatively be adjustable by the deflection of electrically charged ink in an electric field. If xTool and yTool of the tool coordinate system are aligned in the direction of the spray velocity vector vJet for printhead printheads, those described above with reference to 2A given equations also for the printhead 1 in this 4A , If instead the in 4A is selected, the head is for velocity compensation according to the invention by the spray angle less to rotate or tilt.

In 4B is a device A with a plurality of printheads arranged one behind the other 1 according to 4A represented with respective spray angle β, with the various inks can be printed in one step. The spray angle β can be fixed or variably adjustable. In addition, it may be provided that the printheads 1 can be displaced relative to one another in order to allow an even better adaptation to the surface C to be printed, and thus to keep the maximum pressure separation D as low as possible with or without the use of the speed compensation.

Without the speed compensation according to the invention, at a speed vJet, the ink droplets F are in the direction xTool with vJet = 4 m / s, a vHead speed of the printhead 1 with vHead = 0.5 m / s and a pressure difference DeltaD (difference between the pressure distances of the individual nozzles 2 from the respective point of impact of the ejected ink drops F on the surface C) of delta D = 8 mm, the pressure points of the nozzles 2 z. By up to vHead / vJet · delta D = 1 mm in the direction of movement of the printhead 1 offset against each other. At 360 dpi print resolution, this corresponds to more than 14 print dots. The velocity compensation avoids not only this unwanted offset, but also a mispositioning, which would result without the consideration of the velocity component vHead in the control of the print head.

In the preferred embodiment, the robot path or trajectory of the printhead 1 planned beforehand as a course of the positions and orientations of the tool coordinate system defined by the axes xTool, yTool and zTool, and from this planning a program is generated which then determines the planned movements of the print head 1 executes and control the nozzles 2 of the printhead 1 synchronizes with the web, so that the printed image is printed at the desired locations on the surface C. The movement speed vHead of the print head and the motion compensation calculated therefrom are also predefined in the program. The motion compensation is implicit in the orientation or inclination of the tool coordinate system along the path.

In an alternative embodiment, the paths are calculated without motion compensation and the motion compensation is calculated and executed only during program execution from the velocity vector vHead and the orientation of the tool coordinate system defined by axes xTool, yTool and zTool. This offers the advantage that the speed compensation for the actual speed can be carried out even if the actual programmed speed deviates. Such deviations can z. B. arise when the robot program is executed only at reduced speed in test mode or when jerk or acceleration restrictions lead to the reduction of the web speed.

LIST OF REFERENCE NUMBERS

1

Inkjet printhead

2

An ink jet nozzle

2a

nozzle row

2 B

nozzle row

3

axis

4

printed curved lines

5

printed straight lines

A

contraption

B

object

C

Surface of the object

D

printing pitch

e

robot arm

F

Inkjet drops

xTool

x-axis of the printhead coordinate system

Ytool

y axis of the printhead coordinate system

zTool

z axis of the printhead coordinate system

VHEAD

Speed vector of the printhead

Vjet

Speed vector of drops

vJetX

Component of the velocity vector of the drops

vJetZ

Component of the velocity vector of the drops

vRes

resulting velocity vector

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 3737455 A1 [0004]
• DE 102008053178 A1 [0005]
• DE 4033816 A1 [0007]
• WO 2004/007203 A1 [0008]
• US 5043740 [0009]

Claims (10)

Apparatus for printing with an ink-jet printhead on a curved surface of an object, wherein the printhead ( 1 ) at least one nozzle row ( 2a . 2 B ) with nozzles ( 2 ) for ejecting ink-jet drops (F), and wherein the print head ( 1 ) and / or the object (B) from a movement device (E) for generating a relative velocity component (vHead) between the print head ( 1 ) and surface (C) is moved, characterized in that the print head ( 1 ) and / or the nozzle row ( 2a . 2 B ) around a nozzle row ( 2a . 2 B ) parallel axis ( 3 ) by an angle (β) against the direction of the relative velocity component (vHead) between the print head ( 1 ) and surface (C) is tilted so that ejected ink jet drops (F) receive a relative velocity component (vJetZ) between droplet (F) and surface (C) which indicates the relative velocity component (vHead) between printhead (10). 1 ) and surface (C) are substantially compensated. Device according to Claim 1, characterized in that the sine of the angle (β) is substantially proportional to the magnitude of the relative velocity component (vHead) between the print head ( 1 ) and surface (C), in particular that the angle (β) and the amount of the relative velocity component (vHead) between print head ( 1 ) and surface (C) are substantially constant. Apparatus according to claim 1 or 2, characterized in that at least two ink-jet printheads ( 1 ) are provided for printing different colors. Apparatus according to claim 3, characterized in that the inclination of the inkjet printheads ( 1 ) is separately adjustable. Device according to one of the preceding claims, characterized in that a print head ( 1 ) with at least two rows of nozzles ( 2a . 2 B ) is provided. Apparatus according to claim 5, characterized in that the inclination of each nozzle row ( 2a . 2 B ) relative to the print head ( 1 ) is adjustable, in particular that the inclination of each nozzle row ( 2a . 2 B ) relative to the print head ( 1 ) is separately adjustable. Apparatus according to claim 5 or 6, characterized in that the nozzle rows ( 2a . 2 B ) with a tilt on the respective print head ( 1 ) are mounted. Device according to one of the preceding claims, characterized in that the inkjet printheads ( 1 ) are mutually displaceable. Method for printing with an inkjet printhead on a curved surface of an object, wherein the printhead ( 1 ) at least one nozzle row ( 2a . 2 B ) with nozzles ( 2 ) for ejecting ink-jet drops (F), and wherein the print head ( 1 ) and / or the object (B) from a movement device (E) for generating a relative velocity component (vHead) between the print head ( 1 ) and surface (C) is moved, characterized in that the print head ( 1 ) and / or the nozzle row ( 2a . 2 B ) and / or the object (B) around a nozzle row ( 2a . 2 B ) parallel axis ( 3 ) by an angle (β) against the direction of the relative velocity component (vHead) between the print head ( 1 ) and surface (C) is tilted such that ejected ink jet drops (F) receive a relative velocity component (vJetZ) between droplet (F) and surface (C) that indicates the relative velocity component (vHead) between printhead (14). 1 ) and surface (C) are substantially compensated. A method according to claim 9, characterized in that the inkjet printhead ( 1 ) is arranged on the movement device (E) and is guided by this while maintaining a minimum distance along the surface (C).

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