DIGITAL PRINTING MACHINE AND METHOD FOR PRODUCING AND PRINTING A WORKPIECE

Abstract

Digital printing machine for printing workpieces, including a print head carrier to which a print head for dispensing ink droplets in a printing direction and a drying unit for curing the ink droplets are attached, wherein the print head and the drying unit define a working space in which an application of a print image to an outer surface of a workpiece with the print head and a drying of the print image on the workpiece with the drying unit is provided, wherein the drying unit provides electromagnetic waves for a photochemical polymerization of the ink droplets, wherein the drying unit includes a radiation source to provide electromagnetic waves with an intensity maximum at a wavelength from the group of: 395 nanometers, 385 nanometers, 365 nanometers.

Claims

1. A digital printing machine for printing workpieces, having a print head carrier to which a print head for dispensing ink droplets in a printing direction and a drying unit for curing the ink droplets are attached, wherein the printing head and the drying unit define a working space in which an application of a printing image to an outer surface of a workpiece with the printing head and a drying of the printing image on the workpiece with the drying unit is provided, wherein the drying unit provides electromagnetic waves for photochemical polymerization of the ink droplets and wherein the drying unit comprises a radiation source to provide electromagnetic waves having an intensity maximum at a wavelength from the group: 395 nanometers, 385 nanometers, 365 nanometers.

2. The digital printing machine according to claim 1, wherein the radiation source is a light-emitting diode comprising a semiconductor from the group: aluminum nitride, aluminum gallium nitride, aluminum gallium indium nitride, diamond, to provide monochromatic electromagnetic waves.

3. The digital printing machine according to claim 1, wherein the radiation source is configured at 50 percent of the maximum radiation intensity for providing electromagnetic waves in a wavelength interval of less than 13 nanometers and/or at 25 percent of the maximum radiation intensity for providing electromagnetic waves in a wavelength interval of less than 20 nanometers.

4. The digital printing machine according to claim 1, wherein a short-pass filter from the group: absorption filter, dichroic filter, with a cut-off wavelength from the group: greater than 400 nanometers, greater than 390 nanometers, greater than 370 nanometers, is arranged between the radiation source and the working space.

5. The digital printing machine according to claim 1, wherein the print head carrier is fixed to a machine frame on which a conveying device for workpieces is arranged to supply workpieces into the working space and to rotate the workpiece in the working space about an axis of rotation oriented transversely to the printing direction.

6. A method for producing and printing a workpiece from a transparent or translucent material, the method having the steps: providing a workpiece in a working space of a digital printing machine; dispensing ink droplets from a print head onto a printing area of an outer surface of the workpiece; producing a printed image on the outer surface by rotating the workpiece about an axis of rotation; and curing the ink droplets by irradiating at least a partial area of the print image with electromagnetic waves provided by a radiation source whose intensity maximum is at a to wavelength from the group of: 395 nanometers, 385 nanometers, 365 nanometers.

7. The method according to claim 6, wherein the workpiece is made of a glass material which, in a wavelength range smaller than 400 nanometers, has an optical transmission of less than 25 percent.

8. The method according to claim 6, wherein the workpiece is made of plastic, the plastic having an absorber for ultraviolet radiation selected from the group: 2-(2-hydroxyphenyl)-2H-benzotriazoles, (2-hydroxyphenyl)-s-triazines, hydroxybenzophenones, oxalanilides, titanium dioxide, iron oxide, zinc oxide, cadmium stearate.

9. The method according to claim 6, wherein, during the rotation of the workpiece about the axis of rotation, a distance between the outer surface of the workpiece, which is provided with the printed image, and the print head is constant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] An advantageous embodiment of the invention is shown in the drawing. Here shows:

[0027] FIG. 1 a strictly schematic side view of a digital printing machine with a print head carrier, a print head, a drying unit as well as a workpiece which is received on a rotatably mounted spindle, and

[0028] FIG. 2 a strictly schematic front view of the digital printing machine according to FIG. 1, wherein the print head carrier is not shown.

DETAILED DESCRIPTION

[0029] A digital printing machine 1 shown strictly schematically in FIGS. 1 and 2 comprises a print head carrier 2 shown only schematically, to which a print head 3 also shown only schematically and a drying unit 4 shown schematically are fixedly attached. The print head carrier 2 is connected to a machine frame 5, which is also shown only schematically and which is stationary in a manner not shown in more detail on a floor plate of a production hall which is not shown.

[0030] A workpiece rotary table 6, shown only symbolically, is mounted on the machine frame 5 so as to be rotatably movable about an axis of rotation 9, wherein the workpiece rotary table 6 may in practice be of disc-shaped design, for example, and is provided on a radially outer circumferential surface with a plurality of radially aligned spindles, of which only one spindle 7 is shown in FIG. 1 as an example. The spindle 7 is accommodated on the workpiece rotary table 6 so as to be rotatable about an axis of rotation 10 and, in purely exemplary fashion, is of circular-cylindrical profile. The spindle 7 serves to receive a purely exemplary circular sleeve-shaped workpiece 8, which may be, for example, a plastic vessel made of a transparent or translucent plastic material.

[0031] The print head 3 is provided on an underside 20 opposite to an outer surface 12 of the workpiece 8 behind a plurality of ink nozzles, not shown, which are arranged along a straight line at equal pitch, said straight line being aligned parallel to the axis of rotation 10. Each of the ink nozzles can be individually controlled by a controller for the respective print head 3, which controller is not shown, and thereby enables a droplet of ink, which is not shown, to be dispensed in a printing direction 11. Purely exemplarily, the spindle 7 with the workpiece 8 received thereon and the print head 3 are aligned with respect to each other during an execution of a printing process in such a way that the printing direction 11 is identical with a surface normal to the outer surface 12 of the workpiece 8. Due to the arrangement of the ink nozzles, which are not shown, the print head 3 can discharge a freely selectable number of ink droplets onto the outer surface 12 of the workpiece 8 along the straight line which is aligned parallel to the axis of rotation 10. Thus, to create a printed image on the outer surface 12, it is intended to rotate the workpiece 8 about the axis of rotation 10 so that the printed image can be created by a plurality of juxtaposed ink droplets. The area of the outer surface 12 of the workpiece 8 which can be printed by the print head 3 is also referred to as the printing area 15, and is in the form of a circular cylindrical section.

[0032] Opposite the print head 3, the drying unit 4 is arranged, as can be seen in particular from the illustration in FIG. 2. Together with the print head 3, the drying unit 4 delimits a working space 22 into which the spindle 7 provided with the respective workpiece 8 can be swiveled by a rotation of the workpiece rotary table 6 about the axis of rotation 9. For this purpose, the workpiece rotary table 6 performs a rotary step movement in which a sequence of a pivoting movement and a standstill phase is provided, the printing of the workpiece 8 being carried out during the standstill phase and the workpiece 8 being set into a relative movement with respect to the print head 3 during this standstill phase by the rotation of the spindle 7 about the axis of rotation 10.

[0033] The drying unit 4 comprises a housing 16 which is provided with a recess 17 in which, purely by way of example, a plurality of radiation sources 18 in the form of light-emitting diodes are arranged. Each of the radiation sources 18 is thereby provided for the provision of electromagnetic waves with a spectral wavelength distribution in which an intensity maximum lies at the wavelength of 395 nanometers, preferably of 385 nanometers, in particular of 365 nanometers. Preferably, all radiation sources 18 are of identical design and accordingly each have the same spectral wavelength distribution.

[0034] The radiation sources 18 are designed and arranged in the recess 17 in such a way that a central beam 21 of the respective radiation source 18, which indicates the spatial direction in which radiation source 18 has its maximum intensity, is aligned parallel and, in particular, coaxially with the printing direction 11 of the respective opposite ink nozzle.

[0035] The recess 17 in the housing 16 is covered by a filter 19 whose optical properties are selected such that wavelengths of the electromagnetic waves provided by radiation source 18 which lie above (are longer than) a predetermined cut-off wavelength of the filter 19 are at least almost completely blocked. Depending on the design of the filter 19, this is achieved by absorption of the electromagnetic waves or by reflection of the electromagnetic waves. Purely by way of example, it is provided that the cut-off wavelength of the filter 19 is located a few nanometers above the wavelength at which the radiation source 18 has its intensity maximum.

[0036] The workpiece 8 is preferably made of an optically transparent or an optically translucent material, in particular glass or plastic or a composite of glass and plastic, and therefore has the property that visible light can pass through the workpiece 8 with low loss. The workpiece 8 thus forms a waveguide for electromagnetic waves which wavelengths are located in a wavelength range from 380 nanometers to 780 nanometers. To avoid onward transmission of electromagnetic waves, which are provided by the drying unit 4 to the outer surface 12 of the workpiece 8 for drying the ink droplets, as far as the print head 3, the workpiece 8 is designed by suitable material selection in a manner by which onward transmission of electromagnetic waves with a wavelength of less than 400 nanometers, preferably with a wavelength of less than 390 nanometers, in particular with a wavelength of less than 370 nanometers, is at least largely prevented, even when the workpiece 8 is transparent or translucent.

[0037] Such properties can be realized when glass is used as the material for the workpiece 8 by means of corresponding absorbers, which are preferably of such a nature that the absorbers do not change, or only slightly change, the other properties of the glass material used. When plastic is used for the workpiece 8, absorbers can likewise be used which are adapted to the respective plastic material.

[0038] Accordingly, when the printing machine 1 and the workpiece 8 are considered together, the result is a printing system 30 which, based on the characteristics summarized below, enables printing of transparent or translucent workpieces by the ink jet printing method with a guarantee of long cleaning intervals for cleaning the print head. The ink for the ink droplets emitted by the print head 3 through the ink jet nozzles (not shown) in the printing direction 11 onto the printing area 15 of the workpiece 8 is configured for polymerization with electromagnetic waves whose wavelengths are less than 400 nanometers, preferably less than 390 nanometers, in particular less than 370 nanometers.

[0039] The workpiece 8 is made of a transparent material, in particular glass and/or plastic, the materials used for this purpose ensuring at least partial absorption for electromagnetic waves whose wavelengths are smaller than 400 nanometers, preferably smaller than 390 nanometers, in particular smaller than 370 nanometers, by means of corresponding absorbers.

[0040] The at least one radiation source 18 is designed to provide electromagnetic waves having an intensity maximum at a wavelength of 395 nanometers, preferably at a wavelength of 385 nanometers, in particular at a wavelength of 365 nanometers.

[0041] It is further provided that between the at least one radiation source 18 and the working space 22 defined by the print head 3 and the drying unit 4, a filter 19 is arranged which is designed as a short-pass filter with a cut-off wavelength greater than 400 nanometers, preferably with a cut-off wavelength greater than 390 nanometers, in particular with a cut-off wavelength greater than 370 nanometers.