EP3681724B1 - Direct printer for applying a printed layer to containers - Google Patents

Description

The present invention relates to a direct printing machine for applying a printing layer to a container according to claims 1 and 8 and to a direct printing method for applying a printing layer to a container according to claim 12.

State of the art

Direct printing processes for printing on containers are known from the state of the art over a wide range of resolutions.

In particular, the EP 2 089 234 B1 a direct printing process in which a primer is applied to the container before a print image is applied to it. From this it is known that ink drops with a resolution of 200 to 1200 drops/inch (dpi) and also different drop sizes can be applied to the surface of the container in order to form, for example, a primer or at least part of the print image. Although the known process basically allows the application of a primer and the subsequent application of a print image, due to the usable resolutions of a maximum of 1200 dpi and a drop size of 10 to 200 µm, it cannot be guaranteed over the entire surface of the container that the applied print layer or primer is flat or almost flat. Such unevenness can lead to the printing ink running down when a print image is subsequently applied, in accordance with the relief created by the primer, and the print quality can thus be reduced.

The EN 10 2014 206 730 A1 describes a device and a method for inkjet printing on containers. According to this, containers are transported in holders rotating on a container table along at least one inkjet printing station with several print heads that are arranged one behind the other in the direction of rotation. Furthermore, the containers rotate in such a way that partial wall segments of the side wall of the containers rotate in front of at least one associated print head in the opposite direction to the direction of rotation of the container table. The print heads are activated in such a way that partial prints are produced on the partial wall segments that complement each other to form a print image. This means that high-quality partial prints can be combined to form a full print image with high machine performance.

The EP 2 860 036 A1 relates to a printing device for printing on a peripheral surface of an object, with at least two print heads, each of which has at least one row arrangement of ink dosing elements, in particular ink nozzles, which are each designed for an individually predeterminable dispensing of ink onto the object. It is provided that at least one of the print heads is arranged movably along an extension axis of the ink dosing elements on a print head carrier and that the movably mounted print head is assigned an electrically controllable adjustment device for adjusting a relative position with respect to the at least one further print head.

The EN 10 2014 206730 A1 discloses an inclination of print heads in a device for printing containers using inkjet. This arrangement improves the print resolution compared to individual orthogonally positioned nozzle fields.

The EN 10 2015 215224 A1 describes a process in which a container is first printed over the entire printing area to increase the print resolution. The direct print head is then moved half a distance along the rows of nozzles using a displacement unit, and the container is then printed again over the entire printing area.

Task

Based on the known state of the art, the technical problem to be solved is to improve direct printing processes and direct printing machines for printing on containers with regard to the quality of the printed images.

Solution

This object is achieved by the direct printing machines for applying a printing layer to containers according to claim 1 or claim 8 and the direct printing method for applying a printing layer to a container according to claim 12. Advantageous further developments of the invention are covered in the subclaims.

The direct printing machine according to the invention comprises the technical feature that the print head is aligned in such a way that the rows of nozzles form an angle greater than 0° with a plane perpendicular to the transport direction of the containers. This means that the print head is inclined in relation to the container in the transport direction. This reduces the distance between the individual nozzles in the rows of nozzles, at least in the vertical direction, so that the pixel density in this direction can be increased beyond the initial pixel density of the print head. Depending on the angle of attack, for example, twice the ink drop density can be achieved, at least in the vertical direction. This means that a much more uniform layer can be produced when the printing layer is applied, so that a color layer later applied to the printing layer can be produced more evenly and thus of higher quality. The term "printing layer" is to be understood as any layer applied to the surface of a container using a printing module of a direct printing machine. This includes in particular colored layers (for example yellow, red or blue), but also functional layers such as primers that are applied to the container before the actual print motif is applied. This primer can already be part of the image (for example, it can be white or have certain other optical properties or it can also be any color), but this does not have to be the case. The primer can also be designed as a transparent layer or at least as a partially transparent layer and, for example, influence the adhesive properties of the color layers applied to it for the printed image. For example, the primer can consist of a material that reduces or increases the surface tension compared to the actual surface of the container, which can be advantageous for wetting with the actual printing inks.

The fact that the nozzle rows form an angle greater than 0° with a plane perpendicular to the transport direction should not be understood as an arbitrary misalignment, for example due to an incompletely precise alignment of a printing module arranged vertically elsewhere (the nozzle rows run parallel to the plane perpendicular to the transport direction as planned), but rather as intentional inclinations. These will therefore generally be larger, even significantly larger, than the error tolerances acceptable for vertical alignment and are in the range of several degrees, in particular 5° to 30°.

In one embodiment, the nozzle rows are arranged in a plane in the print head that is perpendicular to the transport plane. This means that the print head cannot be moved in the direction of the transport plane or in the direction of a container is not tilted but stands straight. Distortions of the print image can thus be avoided.

In a further embodiment, the print head has a normal resolution of 1200 dpi. The normal resolution means that this is the resolution that can be achieved on a flat substrate when the ink drops hit vertically. By setting the print head at an angle and thus increasing the resolution on the substrate at least in the vertical direction, an even higher resolution than 1200 dpi, in particular 1440 dpi, can be achieved, which means an even more even distribution of the applied print layer on the surface of the container.

It can further be provided that the printing machine comprises two print heads arranged one after the other in the transport direction, which are offset from one another perpendicular to the transport plane and each have a normal resolution of 600 dpi, whereby both print heads comprise at least two rows of nozzles and are aligned such that the rows of nozzles form an angle greater than 0° with the plane perpendicular to the transport direction of the containers. By vertically offsetting the two print heads from one another and simultaneously slanting them according to the invention, a suitable offset of the ink drops emitted by the print heads can be achieved, so that even with the comparatively low resolution of 600 dpi/print head, an ink drop density can be achieved on the surface of the container that is significantly greater than the sum of the ink drop densities of the individual print heads, for example significantly greater than 1200 dpi, so that a layer that is as flat as possible is applied.

In a further development of this embodiment, the print heads are offset from one another by a distance h, which corresponds to half the distance between two adjacent nozzles in a row of nozzles in the direction of the surface normal of the transport plane. This means that the ink drops of the second print head are applied to the surface of the container precisely in the spaces between the ink drops of the first print head. This makes it possible to produce a print layer that is as uniform as possible, which makes it easier to apply the subsequent print layers and leads to an improved print image.

Furthermore, the print head(s) can be designed in such a way that they can produce ink drops with a diameter of 10 to 50 µm. The diameter of the ink drops refers to the dimensions of an ink drop at normal pressure and room temperature in equilibrium, i.e. without any total forces acting on the ink drop.

Furthermore, in embodiments with two print heads, one of the print heads can be designed to eject ink drops with a smaller diameter than the other print head. This makes it possible to create embodiments in which the gaps between the relatively large ink drops are filled by the smaller ink drops, thus creating a surface that is as flat as possible.

Another direct printing machine according to the invention for applying a printing layer to containers has the technical feature that the print heads are arranged perpendicular to the transport plane and offset from one another by a distance h that is smaller than the distance between two adjacent nozzles in the nozzle row. The ink drop density can thus be doubled in this direction (i.e. perpendicular to the transport plane of the containers). In this way, a printing layer that is as uniform as possible can be applied, which can improve the quality of print images applied to this printing layer.

Furthermore, it can be provided that the distance h is equal to half the distance between two adjacent nozzles in a row of nozzles.

Furthermore, one of the print heads can be designed to eject ink drops with a smaller diameter than the other print head. If the ink drops from the print head designed to eject smaller ink drops hit the gaps between the ink drops of the other print head exactly, an even print layer can be achieved because the smaller ink drops fill the gaps.

It can also be provided that the print heads have a normal resolution of 720 dpi and/or at least four rows of nozzles. The maximum resolutions achievable with such print heads show good results in terms of the homogeneity of the applied print layer and thus improve the conditions for print images or further print layers to be applied thereon.

Furthermore, it is provided that in each of the direct printing machines described so far, each print head is assigned an adjustment device and this is designed to change the relative position of the print head to the transport plane. In particular, a change in the position perpendicular to the transport direction and a change in the angle enclosed by a plane perpendicular to the transport direction and the rows of nozzles are intended. In this way, the settings of the print heads can be adapted to the requirements of the printing layer for the containers, for example depending on the type of container or the level of quality to be achieved.

The method according to the invention for printing containers comprises the features that the containers are transported in a direct printing machine according to one of the preceding embodiments along a transport path defining a transport plane and are provided with the printing layer during transport.

"During transport" does not necessarily mean that the containers experience a movement along a transport direction in the transport device during the printing process. The containers can also remain stationary or at least rotate relative to the print heads during the printing process.

In one embodiment of the method, it is provided that the ink drops applied to the container have a volume of 3 to 30 pl and/or a diameter of 10 to 50 µm and/or a distance between the centers after application to the container of 15 to 20 µm, preferably 17.5 µm, at normal temperature and normal pressure.

In a further embodiment, the print heads are aligned relative to the transport plane using an adjustment device to a predetermined value. This makes it possible to adapt the print heads to different requirements.

Short description of the characters

Fig. 1a + b

schematic representation of a known arrangement of a print head ( Fig. 1a ) and a print head according to the invention ( Fig. 1b );

Fig. 2a - c

schematic representation of an embodiment of the invention with two print heads and different ink drop distributions;

Fig.3

Schematic representation of another embodiment with two print heads.

Detailed description

In Fig. 1a an arrangement of a container 30 on a transport path 31 is shown. The container 30 is transported along the arrow 32 shown. The transport path shown here is shown as an example as a conveyor belt on which the containers 30 are transported. Such transport is not mandatory. Alternatively, transport can also be carried out using neck handling or on a stand or turntable in conjunction with a centering device. All of these embodiments for transporting the containers are already known from the prior art and can be used together with the invention described here.

It is also understood that each of these special designs for transport can be assigned a "transport plane" in which the containers are transported. This does not have to coincide with an area of the transport device or be explicitly defined by it. In principle, for example, any plane that runs perpendicular to the longitudinal axis of a container transported in the transport device can be regarded as the transport plane of the container. The longitudinal axis of the container usually extends from the bottom of the container to the opening and, in particular in the case of rotationally symmetrical or essentially rotationally symmetrical containers, corresponds to their axis of symmetry.

In the prior art, a print head 1 in the direct printing machine (which is not shown in full here) is arranged in such a way that the individual nozzles of the nozzle rows 11 and 12 point in the direction of the surface of a container guided past the print head. The individual print nozzles of the nozzle rows 11 and 12 can then, as shown here in dashed lines, dispense ink drops and apply them to the surface of the container. This can produce a print image or a coating on the surface of the container. Various printing inks (particularly with different colors) and functional substances that are present as liquids under normal conditions (pressure, temperature, etc.) and can be applied to the surface of the container are already known from the prior art.

In the prior art, the nozzle rows 11 and 12 are arranged together with the print head 1 in such a way that the nozzle rows 11 and 12 or an imaginary connecting line of the individual nozzles of each nozzle row run parallel to a plane 33, which in turn is perpendicular to the transport direction of the containers. The maximum resolution in the vertical direction starting from the transport plane, i.e. in the direction of the surface normal n ₁ , for an applied print image is therefore equal to the resolution of the print head in this direction, and is therefore given by the distance between the individual nozzles in the nozzle rows in the direction n ₁ .

In contrast, the Fig. 1b an embodiment of a direct printing machine (here at least the transport device 131 and a print head 101) according to the invention. The properties of the transport device are compared to those in Fig. 1a unchanged. However, the print head 101 is compared to the Fig. 1a arranged in such a way that the nozzle rows 111 and 112 form an angle α with the plane 133 and not, as in Fig.1 shown, run parallel to it. This reduces the direction of the surface normal n ₁ of the transport plane the distance between adjacent nozzles in a row of nozzles, so that the image resolution or the density with which ink drops can be emitted in this direction can be increased.

In particular, the angle can be in a range between 5° and 30°, which for common print heads with a maximum resolution of 1200 dpi allows an increase to at least 1400 dpi and thus an ink drop density that is 1/6 higher, allowing a significantly more even print layer to be applied.

Although not shown here, the plane of the nozzle rows, which is designated by 113, does not have to be perpendicular to the transport plane defined by the transport device 131 (i.e. it does not have to run parallel to n ₁ ). In some embodiments, the print head can therefore also be inclined relative to the transport plane, for example in cases where the neck area of the container is to be printed. Methods can also be provided in which the print head is moved along the contour of the container and between a position in which the nozzle rows in the plane 113 are perpendicular to the transport plane and a position in which the plane 113 with the nozzle rows arranged therein encloses an angle of less than 90° with the transport plane. Such movements leave the angle α invariant with respect to the plane 133 perpendicular to the transport plane, so that the increased density of the ink drops perpendicular to the transport plane can be maintained.

Even if not explicitly shown here, one or more adjustment devices can be provided which can adjust the alignment of the print head as a whole or at least adjust the alignment of the nozzle rows so that the angle α to the plane 133 is changed. These adjustment devices can be actuators or similar which are connected to a control unit, such as a computer on which a control program runs, which makes these settings either on the basis of user input and/or based on the container to be printed and controls the adjustment device accordingly.

Fig. 2a - c show an embodiment of the invention in which two print heads according to the Fig. 1b described embodiment are provided in the printing machine. A container 130 in a transport path 131, as already described, passes along the transport direction 132 first the print head 201 and then the print head 202. Both can either have rows of nozzles arranged obliquely at the same angle with respect to the plane perpendicular to the transport direction (for example 25°) or the angles can be different.

In addition, the print heads 201 and 202 are offset from each other perpendicular to the transport plane, i.e. parallel to the surface normal n ₁ . This is also shown in Fig. 2a The height offset is h and can preferably be a fraction of the offset of two nozzles of adjacent nozzle rows along the normal n ₁ . Depending on the choice of the height difference h, the vertical distance between the nozzles of the nozzle heads can be adjusted so that the density of the ink drops that ultimately form the print layer on the container is further increased. This embodiment is particularly advantageous if the individual print heads 201 and 202 only have a comparatively low resolution, for example 600 dpi. With this embodiment, the resulting density of ink drops in the direction of the normal n ₁ can then be increased to over 1200 dpi.

Analogous to Fig. 1b In the embodiment described, each of the print heads is assigned an adjustment device, which is preferably designed not only to change the angle of the nozzle rows to the plane perpendicular to the transport direction, but also to influence the height difference h. Analogous to Fig. 1b For this purpose, a corresponding control unit, for example a computer, can be connected to one or more servo drives per printing module and effect appropriate control.

The Fig. 2b and 2c show embodiments of ink drop distributions on the printed surface of a container, as achieved with the print heads according to Fig. 2a can be generated.

In Fig. 2b a container with the printed image 250 is shown, with a section 251 being shown in schematically enlarged form. Between two rows of ink drops 211 and 212, which were applied to the container by the same print head, another, slightly offset row of ink drops 221 can be seen. This was applied by the second print head.

In the Fig. 2b In the embodiment shown, the print heads 201 and 202 produce ink drops of the same size, which all have a diameter of 10 - 50 µm, for example.

Alternatively, in the embodiment according to Fig. 2c also shows a section 252 of a print image 250 or a print layer 250 on the surface of a container 130. Here, however, the sizes of the ink drops of the different print heads are different. While the ink drops of the ink drop rows 211, 212 have the same diameter (for example 50 µm) and were applied with the same print head, the ink drops of the ink drop row 221 have a smaller diameter and are additionally in the gaps between the individual ink drops of the ink drop rows 211 and 212. The size of these ink drops can be, for example, only 10 µm.

Preferably, the ink drops of the ink drop row 221 are applied to the surface of the container in such a way that the connecting lines of the center of such an ink drop with the centers of the directly adjacent ink drops from the ink drop rows 211 and 212 enclose an angle of exactly 60°, so that the ink drop of the ink drop row 221 is positioned exactly in the gap between the ink drops of the ink drop row 211 and the ink drops of the ink drop row 212.

In principle, the examples described so far and the following examples for the size of the ink drops are not to be understood as limiting. The ink drops can basically have a size of 10 - 50 µm. In addition, they can have a volume under normal conditions (normal temperature and normal pressure) of 3 - 30 pl and on the surface of the container the centers of the ink drops can have a distance from each other of, for example, 15 - 20 µm, preferably 17.5 µm. These statements also apply to the descriptions of the subsequent embodiments.

Fig.3 shows a further embodiment in which two print heads 301 and 302 are also arranged one after the other in the transport direction of the containers 130, so that the container 130 passes them one after the other during transport in the direction 132. In this embodiment, the rows of nozzles are arranged so that they run parallel to the surface normal of the transport plane, here designated n _1. However, as also shown in Fig.3 shown, the print heads are spaced apart in the direction of the surface normal n ₁ , so that two nozzles from the two print heads that correspond to one another (for example the first nozzle in the nozzle row) have a distance h in the direction of the surface normal n ₁ that is smaller than the distance between two immediately adjacent nozzles in a nozzle row of the print head. Preferably, this distance is exactly half the distance between two adjacent nozzles in a nozzle row, so that the ink drops that are emitted by the second print head 302 in the transport direction of the container 130 reach exactly the gaps between the ink drops of the first print head 301.

In principle, the ink drop density can be doubled. In particular, print heads with a dpi number in the direction of the surface normal n ₁ of 720 dpi can be used here, so that the total density can be increased to 1440 dpi.

Here too, an adjustment device (not shown in detail) is provided, which is designed, for example, in the form of one or more servo motors and can adjust the distance h between the print heads by moving either only one or both print heads.

While the above description was based on general inks, implementations in which UV-curable inks are used are preferred. These are common inks that are already used for printing on containers. These cure when irradiated with UV light from suitable sources and have very good adhesive properties with respect to PET. Since the present invention can also be used in particular in conjunction with PET-based containers, good results can be achieved in this way.

The measurement of the parameters specified for the ink drops, in particular drop size, volume, diameter, contact angle on the surface of the container, can also be carried out using a Drop Shape Analyzer DSA30 Micro available from KRÜSS. For this purpose, a test body can be equipped with a transparent PET film that is, for example, 25-50µm thick. The film can have the shape of a label and can be fixed to the container using adhesive strips, for example.

The test body prepared in this way is then subjected to a test print, preferably using only every fifth or every tenth print nozzle of a print head, so that the ink drops on the test body or film are as isolated as possible. The film is then removed from the test body and analyzed with the Drop Shape Analyzer DSA30, which enables height, width, diameter and contact angle measurements to be achieved.

In order to compensate for possible deviations within the operating tolerance of the print heads during measurement, such tests can be carried out several times, at least 30 times, preferably 300 times, and by subsequently applying a normal distribution or other statistical methods, the mean value of the parameters relevant to the ink drops and any deviations (in particular standard deviations) can be determined.

As an alternative to applying a film to a test container, a container made of PET can be used, which is cut up after printing and from which a printed piece is further used for measurement according to the procedure described above.

Since pretreatment of the container and posttreatment of the printed container can also influence the properties of the ink drops, it is preferable to use a test container that is pretreated according to the desired production and posttreated according to the desired production after printing and before measuring/analyzing.

The volume of an ink drop can be measured by weighing. A test substrate of known mass is provided with, for example, 10,000 or 100,000 ink drops and the test substrate printed in this way is then weighed. The mass difference can be used to determine the total mass of the ink drops and thus the average mass of an ink drop. The volume of an individual ink drop can be calculated from this value using the known density of the ink drops, for example at room temperature and normal pressure.

Claims (14)

1. Direct printing machine for applying a print layer on containers (130) in the beverage processing industry, with at least one printing head (101), which comprises at least two nozzle rows (111, 112) for applying ink drops on a surface of a container, wherein the printing head is aligned such that the nozzle rows enclose an angle larger than 0° with a plane (133) perpendicular to a transport direction (132) of the containers, wherein each printing head (301, 302) is associated with an adjustment device, which is configured to change the relative position of the printing head to the transport plane.
2. Direct printing machine according to claim 1, wherein a plane, in which the nozzle rows (111, 112) are arranged in the printing head (101), is perpendicular to the transport plane.
3. Direct printing machine according to claim 1 or 2, wherein the printing head (101) exhibits a normal resolution of 1200dpi.
4. Direct printing machine according to claim 1 or 2, wherein the printing machine comprises two printing heads (201, 202) that are arranged subsequently in transport direction of the containers, which are perpendicularly displaced to each other with respect to the transport plane and each exhibit a normal resolution of 600dpi, wherein both printing heads comprise at least two nozzle rows and are arranged such that the nozzle rows enclose an angle larger than 0° with the plane that is perpendicular to the transport direction of the containers.
5. Direct printing machine according to claim 4, wherein the printing heads (201, 202) are displaced by a distance h with respect to each other, which corresponds to half the distance of two neighboring nozzles of a nozzle row in direction of the surface normal of the transport plane.
6. Direct printing machine according to one of claims 1 to 5, wherein the printing head or the printing heads are configured for applying ink drops with a diameter of 10 to 50µm.
7. Direct printing machine according to claim 4 or 5, wherein one of the printing heads (201, 202) is configured for applying ink drops with a smaller diameter than the other printing head.
8. Direct printing machine for applying a print layer on containers (130), such as bottles in the beverage processing industry, with two printing heads (301, 302) that are arranged subsequently in transport direction of the containers, each having at least two nozzle rows, wherein the printing heads are displaced perpendicular to the transport plane by a distance h with respect to each other, which is smaller than the distance between two neighboring nozzles of a nozzle row, wherein each printing head (301, 302) is associated with an adjustment device that is configured to change the relative position of the printing head with respect to the transport plane.
9. Direct printing machine according to claim 8, wherein the distance h is equal to half the distance between two neighboring nozzles of a nozzle row.
10. Direct printing machine according to claim 8 or 9, wherein one of the printing heads (301, 302) is configured for applying ink drops with a smaller diameter than the other printing head.
11. Direct printing machine according to one of claims 8 to 10, wherein the printing heads (301, 302) exhibit a normal resolution of 720dpi and/or at least four nozzle rows.
12. Direct printing method for applying a print layer on a container (130), such as a bottle in the beverage processing industry, wherein the containers are transported along a transport track (131), which defines a transport plane, and are provided with the print layer during the transport, wherein the containers undergo a movement along a transport direction during the transport in the transport device during the printing process or the containers stand still during the printing process with respect to the printing heads or the containers undergo a rotation.
13. Direct printing method according to claim 12, wherein the ink drops applied to the container exhibit a volume of 3-30pl and/or a diameter of 10 to 50µm and/or a distance of the middle points after application to the container of 15 to 20µm, preferred 17.5 µm, at normal temperature and normal pressure.
14. Direct printing method according to 12 or 13, wherein the orientation of the printing heads (101, 201, 202, 301, 302) with respect to the transport plane is set by means of an adjustment device to a preset value.

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