PRINTING APPARATUS AND METHOD FOR CONTROLLING A PRINTING APPARATUS

Abstract

 The invention provides a printing apparatus and a printing method. The printing apparatus comprises a first print head comprising a number n of first nozzles for discharging ink to a substrate transported through the printing apparatus in a transport direction, wherein the first nozzles are indexed by an index i=1...n. It further comprises a second print head comprising an equal number n of second nozzles for discharging ink to the substrate, wherein the second nozzles are indexed by an index j=1...n. The second print head is arranged parallel to the first print head. A control unit is configured to control a first nozzle i and a second nozzle j to discharge ink on a substrate position (P) in a single pass. Herein the second print head is shifted in a parallel direction to the first print head such that j = i + k, wherein k is an integer unequal to zero.

Description

FIELD OF THE INVENTION

[0001] The present invention generally pertains to a printing apparatus, comprising at least two print heads and a controller. The invention also pertains to a method for controlling the printing apparatus.

BACKGROUND ART

[0002] Inkjet printers, both for single stage printing and for scanningwise printing, typically comprise at least two print heads in a print head assembly, wherein each print head comprises a row of nozzles for supplying ink to a substrate being transported in a transport direction. The print heads are arranged parallel to one another with their nozzles being arranged in aligned positions. For example, the first print head comprises a number n nozzles and the second print head comprises the same number n nozzles. A specific nozzle i of the first print head is provided for marking one specific position at the substrate which is sometimes referred to as a substrate pixel. A nozzle j of the second print head is provided for marking the same substrate pixel at a later point of time in the same pass. This nozzle is aligned with the nozzle i of the first print head and therefore j=i.

[0003] A problem with this type of printers is that linear errors or faults can occur from a deviating single nozzle. In particular, if nozzles of several print heads show systematic deviations, i.e. on the same position within the print heads, these linear errors are enhanced. This situation occurs if there is a systematic error in the production of the print heads. Even if a nozzle failure compensation by neighboring nozzles is applied, a linear error may be visible, if this compensation is applied several times on the same substrate line of pixels.

[0004] US 6 341 840 B1 describes a multipass printing method carried out by a print head comprising a row of nozzles. The method comprises printing a first sub-image in a first printing stage by moving a substrate relative to a print head, wherein the nozzles address each second pixel in a row of pixels, and wherein adjacent nozzles address complementing pixels of adjacent rows. In a second stage, the print head is moved relative to the substrate with an opposite orientation of the row of nozzles. Thereby, linear defects resulting from a deviation from a single nozzle within the row of nozzles are at least partially masked.

[0005] However, it is not always possible to address a single substrate position, or substrate pixel, more than once, such as in single pass printing methods. Still there is a need to mitigate the effect of systematic errors in the print heads of a print head assembly to diminish the visibility of linear errors.

SUMMARY OF THE INVENTION

[0006] In a first aspect of the present invention, a printing apparatus is provided. The printing apparatus comprises a first print head, a second print head, forming a print head assembly, and a control unit. The first print head comprises an array of n first nozzles for discharging ink to a substrate, wherein the first nozzles are indexed by an index i=1 ...n. The second print head comprises an array of n second nozzles for discharging ink to the substrate, wherein the second nozzles are indexed by an index j=1...n. The second print head is arranged parallel to the first print head and is shifted in the direction of the array of nozzles relative to the first print head such that a second nozzle of index j is aligned with a first nozzle of index i = j + k, wherein k is an integer unequal to zero. The control unit is configured to control the first nozzle i and the second nozzle j to discharge ink on a substrate position in a single pass. This alignment has as a consequence, that there are nozzles at the ends of the first and second print head that have no corresponding nozzle in another print head of the assembly. This means that the printable image width becomes somewhat smaller in order to mitigate the effect of systematic deviations in the print heads.

[0007] In a second aspect of the present invention, a printing method is provided which comprises transporting a substrate along a transport direction through a printing apparatus, marking one or more pixels of the substrate by discharging ink through one or more first nozzles of a first print head which comprises a number n of first nozzles, wherein the first nozzles are indexed by an index i=1...n, and marking said one or more pixels of the substrate by discharging ink through one or more second nozzles of a second print head which comprises a number n of second nozzles, wherein the second print head is arranged parallel to the first print head and shifted relative to the first printing head such that a second nozzle of index j is aligned with a first nozzle of index i = j + k, wherein k is an integer unequal to zero.

[0008] It is one of the ideas of the invention to arrange a first and a second print head next to each other, or perhabs better behind each other, in a transport direction, wherein the nozzles of the print heads are shifted transverse to the transport direction about at least one position. In other words, a nozzle of index j = 1 of the second print head is aligned with a nozzle of index i ≥ 2 of the first print head. Thereby, systematic errors or malfunctions of nozzles comprising the same index i = j are smeared or distributed over two substrate pixels. This improves print quality. Since print heads can be used without constructive adaption, the improved arrangement of print heads provides a simple and efficient solution for improving print quality.

[0009] It should be understood that the method according to the second aspect is usable with the printing apparatus according to the first aspect and vice versa. The method may be modified or adapted according to all variations and modifications described with respect to the printing apparatus according to the first aspect and vice versa.

[0010] Additional advantages, and the solution of additional problems, will be apparent from the subject-matter of the dependent claims as well as from the description and the drawings.

[0011] According to one embodiment, the first print head is configured to discharge ink of a first color through the first nozzles and the second print head is configured to discharge ink of a second color through the second nozzles. Thus, aligned first and second nozzles which address the same pixel of the substrate may discharge different colors so as to achieve a desired resulting color of the pixel. On the other hand, also in the case of the first and second color being the same, the invention is clearly advantageous.

[0012] According to one embodiment, the integer k=1. That is, a second nozzle of index j=1 is aligned in the transport direction with a first nozzle of index i=2. Thus, the second print head is shifted only about one nozzle position. Hence, a maximum number of overlapping nozzles of the first and second print heads is achieved and a maximum number of nozzles can actually be used for discharging ink to the substrate. This further improves efficiency of the printing apparatus and method.

[0013] According to one embodiment, the first and the second printing head are arranged stationary with respect to a direction extending transverse to the transport direction. In particular, the first and second print heads may extend over a width transverse to the transport direction and being fixed in their positions. Thus, a single pass printing apparatus and method may be provided allowing for improved printing quality.

[0014] The term "printing" is used herein as a general term for any marking of a recording medium or substrate with any marking material. As a synonym, the term "image forming" may be used. Recording mediums may comprise paper, flexible plastic, metal foils, textiles and so on. Marking materials may comprise ink, metal, varnish, toner and so on. "Marking" may be any process by which the marking material is applied to the recording medium.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will become more fully understood from the detailed description given herein below and the accompanying schematic drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

Fig. 1

schematically shows a representation of a printing apparatus according to an embodiment of the invention in a first operating state;

Fig. 2

schematically shows a representation flow chart of a printing method according to an embodiment of the invention; and

Fig. 3

schematically shows the printing apparatus of Fig. 1 in a second operating state.

DETAILED DESCRIPTION OF THE DRAWINGS

[0016] The present invention will now be described with reference to the accompanying drawings, wherein the same reference numerals have been used to identify the same or similar elements throughout the several views, and in some instances throughout the several embodiments.

[0017] Fig. 1 schematically shows a representation of a printing apparatus 1. The printing apparatus 1 comprises a first print head 10, a second print head 20, a controller 30, and a transporting device 40 for transporting a recording medium or substrate 2 through the printing apparatus 1.

[0018] As is schematically shown in Fig. 1, the transporting device 40 feeds or transports a substrate 2, e.g. a sheet of paper, along a transport direction T through the printing apparatus 1. For example, the transporting device 40 may be realized as a pair of rotatable rollers which transport the substrate 2.

[0019] The first and second print heads 10, 20 each comprise a plurality of nozzles 11, 21, wherein the nozzles of the first print head 10 will be referred to as first nozzles 11 and the nozzles of the second print head 20 will be referred to as second nozzles 21 in the following. The first print head 10 is configured to discharge ink to the substrate through the first nozzles 11 and the second print head 20 is 10 is configured to discharge ink to the substrate through the second nozzles 21.

[0020] As schematically shown in Fig. 1, the first nozzles 11 are indexed by an index i, wherein i is an integer number between 1 and n. In the example of Fig. 1, n = 5, that is, the exemplary first print head 10 of Fig. 1 comprises five first nozzles 11₁, 11₂, 11₃, 11₄, 11₅. It should be understood that the number n = 5 is merely an example which has been chosen to simplify explanation of the invention. In a practical apparatus, n may be in the range of about 5000 to 15000. Each of the first nozzles 11 is provided for discharging ink to a specific location of the substrate 2 with respect to a lateral direction extending transverse to the transport direction T. This location is referred to as a pixel line P. The first print head is configured to discharge ink of a first color through the first nozzles 11.

[0021] Similary, the second nozzles 21 are indexed by an index j, wherein j is an integer number between 1 and n. In the example of Fig. 1, the exemplary second print head 20 of Fig. 1 comprises five second nozzles 21₁, 21₂, 21₃, 21₄, 21₅. Each of the second nozzles 21 is provided for discharging ink to a respective pixel P of the substrate. The second print head is configured to discharge ink of a second color through the second nozzles 21, wherein the second color is different from the first color.

[0022] As is schematically shown in Fig. 1, the second print head 20 is arranged distanced to the first print head 10 in the transport direction T and the first and second print heads 10, 20 are arranged parallel to each other. As is shown in Fig. 1, the second print head 20 is shifted to the first printing head 10 in the lateral direction. As exemplarily shown in Fig. 1, the second print head 20 is shifted in the lateral direction one nozzle 11. That is, the second nozzle 21₁ having index j=1 is aligned with the first nozzle 11₂ having index i=2 in the transport direction T. Generally, the second printing head 20 is shifted relative to the first print head 10 such that a second nozzle 21 of index j=1 is aligned with a first nozzle 11 of index i=1+k, wherein k is an integer unequal to zero. Thus, in Fig. 1 which shows the second print head 20 to be shifted about one nozzle index, k = 1.

[0023] The first and the second print head 10, 20 are arranged stationary relative to each other. Other embodiments, wherein the print heads are arranged in a print head assembly that scans the substrate in a scanning type printing apparatus, are readily derivable from this embodiment. The first and the second print head 10, 20 are arranged stationary with respect to the lateral direction.

[0024] The control unit 30 is functionally connected to the first and second print heads 10, 20 via a data connection. The data connection may be a wireless connection, such as a WIFI connection, or a wire bound connection. The control unit 30 is configured to generate command signals which cause the printing heads 10, 20 to discharge ink through one or more of the first and second nozzles 11, 21. The control unit 30 may comprise a processor (not shown), such as a CPU, and a data memory (not shown), in particular a non-transitory data memory.

[0025] The control unit 30 configured to control the aligned first and second nozzles 11, 21 to discharge ink to corresponding pixels P of the substrate. That is, the control unit 30 controls the print heads 10, 20 such that ink is only discharged through those first and second nozzles 11, 21 which are arranged aligned to each other. For example, in Fig. 1, only the first nozzles 11₂ to 11₅ having indices i=2...5 and second nozzles 21₁ to 21₄ having indices j=2...4 are controlled to discharge ink while first nozzle 11₁ having index i=1 and second nozzle 21₅ having index 5 are controlled to be closed.

[0026] Fig. 1 shows a first operational state of the printing apparatus 1. In the first operational state all of the first nozzles 11₂ to 11₅ which are aligned with corresponding second nozzles 21₁ to 21₄ are in an operational, non-error condition, that is, they allow discharging ink. As symbolically shown in Fig. 1 by the crosswise hatching of the pixels P, all addressed pixels P are colored as desired.

[0027] Fig. 2 shows a second operational state of the printing apparatus 1. In the second operational state at least one of the first nozzles 11₂ to 11₅ which are aligned with corresponding second nozzles 21₁ to 21₄ and at least one of those second nozzles 21₁ to 21₄ are in a non-operational, error condition, that is, they do not allow discharging ink. In Fig. 2, a situation is shown where first and second nozzles 11, 21 having the same index i=j are in a non-operational condition. In the example of Fig. 3, the nozzles 11₃ and 21₃ are in the non-operational condition and therefore are illustrated in dashed lines. This situation can occur for example due to systematic failures in the production of the print heads 10, 20. Since the first and second print heads 10, 20 are shifted relative to each other such that the first and second nozzles 11, 21 are displaced relative to each other about at least one index number, nozzles 11, 21 having the same index number are not aligned. Therefore, if nozzles 11, 21 having the same index number are in a non-operating condition, the respective pixel P still can be marked at least by the aligned nozzle 11, 21 of the first or second print head 10, 20. As exemplarily shown in Fig. 2, even though first nozzle 11₃ and the second nozzle 21₃ are in a non-operating condition, the corresponding pixels P are marked by second nozzle 21₂ and first nozzle 11₄. Thereby, a linear error E in the printed image I is smeared or distributed over more pixels P which is optically more attractive than a completely void pixel.

[0028] Fig. 3 schematically shows a flow chart of a printing method. This method, by way of example, will be explained by referring to the above described apparatus.

[0029] In step S1, the substrate 2 is transported along a transport direction T through the printing apparatus 1 so as to pass the first print head 10 and to pass subsequently the second print head 20. Transporting of the substrate 2 may be performed by means of the transport device 40.

[0030] In step S2, the controller controls the first print head 10 to discharge ink through one or more of the first nozzles 11 so as to mark one or more pixels P of the substrate 2. In particular, only those of the first nozzles which are aligned with a second nozzle 21 of the second print head 20, are controlled to discharge ink.

[0031] The substrate 2 is transported in the transport direction T after marking some pixels P by the first print head 10 and in step S3, the controller controls the second print head 20 to discharge ink through one or more of the second nozzles 21 so as to mark said one or more pixels P of the substrate 2 that have been marked by the first print head 10.

[0032] Thereby, the pixels P can be marked with ink in a desired pattern, as is schematically shown in Fig. 1 and Fig. 2.

[0033] While detailed embodiments of the present invention are disclosed herein, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. In particular, features presented and described in separate dependent claims may be applied in combination and any advantageous combination of such claims is herewith disclosed.

[0034] Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. The terms "a" or "an", as used herein, are defined as one or as more than one. The term plurality, as used herein, is defined as two or as more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language).

[0035] It will be evident that the described embodiments may be varied in many ways. All such modifications as would be evident to one skilled in the art starting from what is explicitly described are intended to be included.

Claims

1. A printing apparatus (1), comprising:

a first print head (10) comprising an array of n first nozzles (11) for discharging ink to a substrate (2), the first nozzles (11) being indexed by an index i=1...n;

a second print head (20) comprising an array of n second nozzles (21) for discharging ink to the substrate (2), the second nozzles (21) being indexed by an index j=1...n, the second print head (21) being arranged parallel to the first print head (10);

a control unit (30) configured to control a first nozzle i and a second nozzle j to discharge ink on a substrate position (P) in a single pass,

wherein the second print head is shifted in a parallel direction to the first print head such that j = i + k, wherein k is an integer unequal to zero.

2. The printing apparatus (1) of claim 1,
wherein the first print head (10) is configured to discharge ink of a first color through the first nozzles (11), and
wherein the second print head (20) is configured to discharge ink of a second color through the second nozzles (21).

3. The printing apparatus (1) of any one of the preceding claims,
wherein k=1.

4. The printing apparatus (1) of any one of the preceding claims,
wherein the first and the second print head (10; 20) are arranged stationary with respect to a direction extending transverse to the transport direction (T).

5. A printing method, comprising:

transporting (S1) a substrate (2) along a transport direction (T) through a printing apparatus (1);

marking (S2) one or more pixels (P) of the substrate (2) by discharging ink through one or more first nozzles (11) of a first print head (10) which comprises a number n of first nozzles (11), the first nozzles (11) being indexed by an index i=1...n;

marking (S3) said one or more pixels (P) of the substrate by discharging ink through one or more second nozzles (21) of a second print head (20) which comprises a number n of second nozzles (21), wherein the second print head (20) is distanced to the first print head (10) in the transport direction (T) and shifted relative to the first print head (10) such that a second nozzle (21) of index j=1 is aligned with a first nozzle (11) of index i=1+k, wherein k is an integer unequal zero.

6. The printing method of claim 5,
wherein the first and the second print head (10; 20) are arranged stationary with respect to a direction extending transverse to the transport direction (T).

7. The printing method of claim 5 or 6,
wherein the first print head (10) is configured to discharge ink of a first color through the first nozzles (11), and
the second print head (20) is configured to discharge ink of a second color through the second nozzles (21).

8. The printing method of any one of claims 5 to 7,
wherein k=1.

Drawing