Image forming method and single-pass inkjet printer

Abstract

The present invention aims to provide an image forming method and a single-pass inkjet printer that enable high-speed printing while preventing the occurrence of streaks in printed materials, even when forming solid-color images. [Solution] A method for forming a solid pattern image using a single-pass inkjet printer, wherein the ink ejected from a single head includes at least two or more different droplet sizes.

Description

Technical Field

[0001] The present invention relates to an image forming method for single - pass type image formation using inkjet ink and a single - pass type inkjet printer.

Background Art

[0002] When printing while continuously conveying a printing substrate, there are a scanning method and a single - pass method. Since there is no need for scanning, the single - pass method is particularly suitable for high - speed printing when continuously conveying a web - like printing substrate. Examples of inkjet printers for single - pass type image formation include those described in Patent Document 1. In a single - pass type inkjet printer, ink is ejected from an inkjet head having a plurality of inkjet nozzles arranged in a line, and when performing multicolor printing, inkjet heads for ejecting ink of each color (such as white (W), black (B), cyan (C), magenta (M), yellow (Y), etc.) are respectively used.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In printing by a single - pass type inkjet printer, compared with printing by a scanning type inkjet printer (also called a multi - pass inkjet printer), there is a problem that lines called streaks in the conveyance direction are likely to occur on the printed matter. In particular, there is a problem that streaks are likely to occur in solid - color areas.

[0005] The causes of streaks in the transport direction on printed materials include non-discharge due to blockage of ink ejection nozzles, misalignment of ink droplet landing positions (so-called flight curves), misalignment of the inkjet head mounting position, meandering of the printing substrate, and inhibition of dot coalescence due to poor wettability of the printing substrate. These non-uniformities occurring in the width direction of the printing substrate are continuously manifested in the transport direction during transport. In particular, in areas with solid patterns, which are often expressed with monotonous colors without continuous changes in dot density, the non-uniformities occurring in the width direction are more pronounced, making streaks more noticeable.

[0006] Furthermore, while streaks in the transport direction are a problem for all colors (W, B, C, M, Y) when performing multi-color printing, white (W) is often used as a base to achieve opacity and is therefore mostly used for solid patterns. In addition, when printing on the reverse side, it is necessary to level uniformly not only on the substrate but also on the B, C, M, and Y colors, which have different surface conditions, which makes the aforementioned problem of streaks in the transport direction more likely to occur. One possible measure to prevent streaks in printed materials is to create an ink composition that spreads and wets the substrate after it lands. However, this would require lowering the surface tension of the ink, which would create problems in stably dispensing the ink. When the surface tension of the ink is low, it becomes very fluid, making it difficult to control the ink during dispensing and thus difficult to achieve stable dispensing. Another measure to prevent streaks in printed materials is to increase the amount of ink ejected. In this case, the drying load on the ink increases because the ejected ink needs to dry. Water-based inks, in particular, are slow to dry, so it becomes necessary to reduce the printing speed. Nowadays, printing speeds of 50 m / min or more are required, and sometimes even 80 m / min or more, but if the printing speed is reduced as described above, high printing speeds of 50 m / min or more cannot be achieved. On the other hand, if the ink drying zone is enlarged to enable high-speed printing, the inkjet printing machine itself becomes larger, which is a problem. [Means for solving the problem]

[0007] The inventors diligently investigated measures to prevent streaks in printed materials and discovered that when forming a solid pattern image in a single-pass inkjet printer, using an image forming method in which the ink ejected from a single head includes at least two or more different droplet sizes results in streaky image formation on printed materials, leading to the present invention. In other words, the present invention has been made in view of the problems of the prior art described above, and aims to provide an image forming method and a single-pass inkjet printer that enable high-speed printing while preventing the occurrence of streaks in solid pattern areas of printed materials. [Effects of the Invention]

[0008] The present invention offers the significant advantage of providing an image forming method and a single-pass inkjet printer that enable high-speed printing while preventing the occurrence of streaks in printed materials, even when forming solid-color images. According to the present invention, by using two or more different droplet sizes to prevent streaks, the drying and curing properties of the ink are superior compared to the case where only large droplets are used to increase the ink volume and prevent streaks. For example, this enables printing speeds of 50 m / min or more, or high-speed printing of 80 m / min or more. [Brief explanation of the drawing]

[0009] [Figure 1] This is a cross-sectional diagram illustrating one embodiment of the inkjet printer of the present invention. [Figure 2] This is a view from below of the line-shaped inkjet head for white ink in a single-pass inkjet printer. [Figure 3] This is an enlarged schematic diagram showing an example of nozzle arrangement for an inkjet head used for white ink. [Figure 4] This is a view from below of a linear inkjet head for color ink in a single-pass inkjet printer. [Figure 5] This is an enlarged schematic diagram showing an example of nozzle arrangement for an inkjet head used for color inks. [Figure 6] According to the present invention, the image forming method is as follows. [Figure 7] This is an example of creating 2-bit TIFF data. [Figure 8] This is a schematic diagram illustrating an example of creating an image with the same tonal range using inks of different droplet sizes. [Figure 9] This photograph shows an example of 4-value ink ejection based on 2-bit TIFF data. [Figure 10] This is an example of creating 1-bit TIFF data. [Figure 11] This is a schematic diagram showing an example of forming a solid image using one type of ink droplet size. [Figure 12] This photograph shows an example of binary ink ejection based on 1-bit TIFF data. [Modes for carrying out the invention]

[0010] The embodiments of the present invention are described below, but these embodiments are illustrative examples, and it goes without saying that various modifications are possible as long as they do not deviate from the technical concept of the present invention. The same components are denoted by the same reference numerals. Unless otherwise specified in this specification, "A~B" representing a numerical range means "greater than or equal to A, and less than or equal to B". The solid pattern image formation method of this disclosure is characterized in that the ink ejected from a single head includes at least two or more different droplet sizes. Furthermore, a preferred embodiment of the present invention is to assign the inks of different droplet sizes in a multi-bit format to create multi-bit data for printing, and to form a solid pattern image based on the multi-bit data for printing. In this specification, a solid pattern is intended to be covered with ink without gaps while leaving the base material exposed, and is also intended to be an image area having 40 to 300% in total or individually of each of the BCMYW colors. In this specification, the dot density is a value obtained by assuming a certain area, considering that when the area is completely filled with dots at the highest density at which any one of the BCMYW colors can be dot-printed, it is 100%, and expressing the number of dots in a certain area as a percentage. The total dot density represents the sum of the dot densities of each of the BCMYW colors. That is, when a certain area is completely filled with only cyan (C) ink, the dot density is 100%, and when the area is further covered with magenta (M) ink at a dot density of 100%, the total dot density of the area is 200%.

[0011] [Operation mechanism of the present invention] As described above, the reason for the streaks occurring in the conveyance direction is that the non-uniformity occurring in the width direction of the printing substrate, which occurs for various reasons, continuously appears in the conveyance direction due to conveyance. Therefore, by ejecting ink containing at least two or more different droplet sizes onto the solid pattern portion from one head, non-uniformity is intentionally generated in the conveyance direction, and the non-uniformity occurring in the width direction of the printing substrate is canceled out by the non-uniformity in the conveyance direction, so that it is considered that the streaks occurring in the printing direction disappear.

[0012] In order to eject at least two or more different droplet sizes, the ejection onto the solid pattern portion can be assigned in a multi-bit format to create printing multi-bit data, and based on the printing multi-bit data, it can be controlled by ejecting ink from the head. Assigning in a multi-bit format in this specification is intended to be a mode in which printing data is created as multi-bit data and the multi-bit data is used for controlling the droplet size of the ink ejected from the head. The multi-bit format in this specification is intended to be two bits or more. As the multi-bit format, a 2-bit or 3-bit format is preferably suitable. As the at least two or more different droplet sizes, when the first droplet size is A pL and the second droplet size is B pL, it is preferable that B is different from A by ±20% or more.

[0013] In printing with an inkjet printer, a digital image is created by a dot pattern of ejected ink droplets. In this specification, image formation is intended to mean that a digital image is created by a dot pattern of ejected ink droplets. As the web-like printing substrate, in addition to a transparent film, an opaque web-like printing substrate such as paper or non-woven fabric can also be applied. As the web-like printing substrate of a transparent film, a transparent film using a web-like synthetic resin film such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), or PP (polypropylene) can be preferably used.

[0014] [Inkjet Printer] The inkjet printer of the present disclosure may be a single-pass inkjet printer. FIG. 1 is a cross-sectional structure explanatory view showing one embodiment of the inkjet printer of the present invention. In FIG. 1, reference numeral 10 indicates the inkjet printer (single-pass inkjet printer) of the present invention. The inkjet printer 10 may include a unwinding portion 14 of a web-like printing substrate 12, a single-pass inkjet head portion 18 including single-pass inkjet heads 18a to 18e that eject inkjet ink in a single-pass manner onto the surface 16 of the unwound web-like printing substrate 12, and a winding portion 20 provided adjacent to the unwinding portion for winding up the web-like printing substrate 12. The web-like printing substrate 12 is conveyed from the upstream side U to the downstream side D of the single-pass inkjet head portion 18. The term "web-like printing substrate" in this specification is used in contrast to a "sheet-like substrate (sheet-like substrate, sheet substrate)", and is intended to mean a substrate suitable for feeding out a roll-like substrate for printing and winding it up in a roll-like manner.

[0015] The unwinding section 14 itself has a known configuration in which an unwinding roll is rotated by a drive belt, as described in Patent Document 1. The adjacent winding section 20 itself has a known configuration in which a winding roll is rotated by a drive belt, as described in Patent Document 1. The web-like printing substrate 12 can be any web-like printing substrate, but it is preferably a transparent, opaque, or colored film. There are no particular limitations on the material of the film, but for example, films made of synthetic resins such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), PP (polypropylene), PE (polyethylene), PS (polystyrene), and NY (nylon) can be suitably used as the web-like printing substrate of the film. Alternatively, metal films made of metals such as aluminum can also be suitably used. As shown in Figure 1, the single-pass inkjet heads 18a to 18e are inkjet heads having multiple inkjet nozzles arranged in a line. Inkjet head 18a ejects B (black), inkjet head 18b ejects C (cyan), inkjet head 18c ejects M (magenta), inkjet head 18d ejects Y (yellow), and inkjet head 18e ejects W (white). The single-pass inkjet head 18e ejects white ink. The single-pass inkjet head 18e is equipped with nozzles 102 as shown in Figures 2 and 3.

[0016] Figure 2 shows an example of a linear inkjet head in a single-pass inkjet printer. In Figure 2, reference numeral 100 denotes the underside of the inkjet head. Reference numeral 102 denotes a nozzle for ejecting ink from the inkjet head. Figure 3 shows an enlarged schematic diagram of the arrangement of the nozzles 102 of the inkjet head. As shown in Figure 3, the nozzle arrangement of each row of nozzles 102 is slightly offset from the previous row with respect to the direction of travel of the transported web-shaped printing substrate. In other words, taking Figure 3 as an example, if the entire head has 360 DPI (nozzle pitch 0.2822 mm), the four rows A to D, which correspond to 90 DPI (nozzle pitch 0.0706 mm), are slightly offset to achieve a total DPI equivalent to 360 for the entire head. Although a high DPI is achieved by slightly offsetting each row in this way, there was a problem that if the mounting position of the head was tilted even slightly from the normal position, the amount of offset in the landing position of each row would differ for each row, making it easy for streaks in the transport direction to occur. This technology solves that problem. In this specification, "slightly offset" means that the centers of the nozzles provided in the same head are not in a straight line with respect to the transport direction of the substrate. Furthermore, the inkjet head of a single-pass inkjet printer is fixed and does not oscillate like that of a scanning type printer.

[0017] Reference numeral 100 denotes the underside of the single-pass inkjet head 18e. For example, the nozzles for white ink (nozzles from which white ink is ejected) shown in Figures 10 and 11 are divided into four rows, A to D, with 384 nozzles per row. Therefore, the single-pass inkjet head 18e has a total of 1536 nozzles. In the illustrated example, the resolution of the white inkjet head is shown to be 360 ​​dpi. The illustrated example is merely illustrative, and it is possible to use an inkjet head with a resolution other than 360 dpi as the white inkjet head. On the other hand, the single-pass inkjet heads 18a, 18b, 18c, and 18d each eject inks of the following colors: B (black), C (cyan), M (magenta), and Y (yellow).

[0018] The single-pass inkjet heads 18a, 18b, 18c, and 18d each have nozzles 106 as shown in Figure 4. Reference numeral 104 denotes the underside of each of the single-pass inkjet heads 18a, 18b, 18c, and 18d. For example, the nozzles for color ink (nozzles from which color ink is ejected) shown in Figure 5 are divided into 32 rows labeled A-Z and a-f. The use of uppercase and lowercase letters indicates that there are 26 uppercase letters (A-Z) and 6 lowercase letters (a-f), totaling 32 rows. Each row of the single-pass inkjet heads 18a, 18b, 18c, and 18d has 64 nozzles. Therefore, in the case of the single-pass inkjet heads 18a, 18b, 18c, and 18d, there are a total of 2048 nozzles per inkjet head. In the illustrated example, the resolution of the inkjet head for color ink is shown to be 1200 dpi. The illustrations are for illustrative purposes only, and it is possible to use inkjet heads with resolutions other than 1200 dpi for color inks.

[0019] As shown in Figure 5, in the single-pass inkjet heads 18a, 18b, 18c, and 18d for color inks, the nozzle pitch within each of the 32 rows A-Z and a-f is 0.6773 mm. In the illustrated example, the resolution of the color inkjet head is 1200 dpi, and since 1 inch = 25.4 mm, the pitch between adjacent nozzles (for example, the nozzle in row B next to the nozzle in row A) is 25.4 ÷ 1200 = 0.02117 mm. Furthermore, the nozzle arrangement in each of the 32 rows A-Z and a-f is slightly offset from the previous row relative to the direction of travel of the transported web-like print substrate, so that the nozzle arrangement does not overlap with the direction of travel of the transported web-like print substrate. The inkjet head of a single-pass inkjet printer is fixed and does not oscillate like a scanning type. Furthermore, each of the multiple single-pass inkjet heads 18a to 18e is an inkjet head capable of ejecting droplets of different sizes (2 to a maximum of 8 different droplet sizes). That is, for example, single-pass inkjet head 18a is capable of ejecting 2 to a maximum of 8 different droplet sizes, and single-pass inkjet head 18b is also capable of ejecting 2 to a maximum of 8 different droplet sizes. Similarly, the other inkjet heads 18c to 18e are also capable of ejecting 2 to a maximum of 8 different droplet sizes.

[0020] To produce droplets of different sizes, the volume of ink to be ejected (in pL) can be changed. Each color has its own ink storage tank (not shown in the diagram), and the inkjet ink of each color is ejected from each inkjet head 18a to 18e.

[0021] There are no particular restrictions on the method of ejecting ink from the nozzles of the inkjet heads 18a to 18e. For example, a piezoelectric method that controls the amount of ink ejected by controlling the voltage applied by a piezoelectric element, a thermal (bubble) method that ejects ink by heating with a heater, and a valve method that ejects ink by pressurizing it with a valve are all preferred. Water-based ink or UV ink is preferred as the inkjet ink. The drying unit is a heating mechanism in the case of water-based ink, and an ultraviolet curing mechanism in the case of UV ink. Water-based and UV inks have the problem of being more difficult to dry than oil-based inks (solvent inks). Therefore, it is preferable to install the drying unit downstream of the inkjet head, and a configuration in which the drying unit is installed downstream of each inkjet head can be used to accelerate ink drying. The illustrated example shows the case of water-based ink.

[0022] Furthermore, it is preferable that drying acceleration units 60a to 60g are provided next to each inkjet head 18a to 18e to promote the drying of the ink on the surface of the web-shaped printing substrate 12 from which the inkjet ink has been ejected. In the illustrated example, since an example of water-based ink is shown as the inkjet ink, the drying acceleration units 60a to 60g are considered to be surface heating units. For example, the surface heating unit described in Patent Document 1 can be applied as the drying acceleration units 60a to 60g, and the hot air blowing means described in Patent Document 1 can be applied as the surface heating units 60a to 60g. When a hot air blowing means is used as the surface heating unit that is the drying acceleration unit, hot air at a temperature of about 40°C to 80°C, for example, 70°C, is applied to the surface of the web-shaped printing substrate 12. The time for applying the hot air is about 2 to 3 seconds when the printing speed is 15 m / min, but this can be appropriately changed depending on the temperature of the hot air.

[0023] When using UV ink as the inkjet ink, the drying acceleration sections 60a to 60g become UV curing sections that cure and dry the ink by irradiating it with ultraviolet light. Furthermore, the web-shaped printing substrate 12 from which the water-based ink has been ejected is transported through the drying zone 22. A dryer can be used as the drying zone 22, and in the illustrated example, a heating box 24 is shown. The heating box 24 is covered with an insulating wall 26 with an insulating structure and has an inlet 28 for the web-shaped printing substrate 12 and an outlet 30 for the web-shaped printing substrate 12. When UV ink is used as the inkjet ink, the drying zone 22 becomes an ultraviolet curing zone for curing and drying the ink by irradiating it with ultraviolet light. For the means of irradiating with ultraviolet light, known ultraviolet irradiation means can be used. The heating box 24 is preheated to 60°C to 70°C. Heating inside the heating box 24 can be done by using hot air or by using various known heaters. In the illustrated example, a configuration in which the heating box 24 is heated with hot air is shown.

[0024] Furthermore, various rollers 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, and 58 are provided for transporting the web-shaped printing substrate 12, and the web-shaped printing substrate 12 is transported via these rollers and sent to the adjacent winding section 20. Note that the symbol O represents the inkjet printer operator. The web-like printing substrate 12 can be any web-like printing substrate, but for example, in addition to transparent films, opaque web-like printing substrates such as paper and nonwoven fabrics can also be used as the web-like printing substrate. There are no special limitations on the material of the film, but for example, films made of synthetic resins such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), PP (polypropylene), PE (polyethylene), PS (polystyrene), and NY (nylon) can be suitably used as web-like printing substrates for films. Alternatively, metal films made of metals such as aluminum can also be suitably used. The aforementioned web-like printing substrate may be an anchor-coated substrate or an un-anchor-coated substrate. However, even with an un-anchor-coated substrate, the image forming method of the present invention is expected to enable high-speed printing while preventing the occurrence of streaks in the printed material, even when forming solid patterns.

[0025] [Image forming method] One embodiment of the present invention is an image forming method that prevents streaking by applying an image forming method, in which ink containing at least two different droplet sizes is ejected from a single head, to a solid pattern. Another embodiment of the present invention is an image forming method that prevents streaking by applying a ejection technique using 2 or more bits of data, which is a conventional technique for reducing roughness in the representation of gradients where the dot density changes continuously, to a solid pattern. Furthermore, the single-pass inkjet printer of the present invention is a single-pass inkjet printer that performs solid image formation, wherein the ejected ink includes at least two different droplet sizes, the inks of the different droplet sizes are assigned in a multi-bit format to create multi-bit data for printing, and the image of the solid pattern area is formed based on the multi-bit data for printing.

[0026] In this invention, an image is formed by ejecting ink of at least two different droplet sizes. In a more preferred embodiment of this invention, the inks of different droplet sizes are assigned in a multi-bit format to create multi-bit data for printing, and the image of the solid pattern area is formed based on the multi-bit data for printing. Conventionally, solid pattern areas, which do not have a continuous change in dot density and are intended to form a seamless ink coating without exposing the substrate, were represented with only one type of droplet size, resulting in streaks in the printed material. However, it has been found that by ejecting inks of at least two different droplet sizes to form the image, it is possible to prevent the occurrence of streaks in the printed material.

[0027] Figure 6 shows a flowchart of the image forming method of the present invention. In the present invention, at least two or more inks with different droplet sizes are used as the ink to be ejected. In one embodiment of the present invention, at least two or more inks with different droplet sizes are assigned in a multi-bit format (S101 in Figure 6). When assigning in a multi-bit format, if performed from vector data, a tone curve (also called a gradient curve) is set. A tone curve is a graph of density change drawn when the density (%) of the input level (original image value) when an image is input to an inkjet printer is plotted on the horizontal axis and the density (%) of the output level (newly adjusted value) when output from the inkjet printer is plotted on the vertical axis. If the data when an image is input to an inkjet printer is a PDF file, the density (%) will be that of the PDF file. When setting the tone curve, pay attention to the percentage of the 100% gradation on the gradient scale (i.e., the solid color area) to be output, and to the degree to which dot gain should be considered. The tone curve setting greatly affects the reproducibility of the print.

[0028] Furthermore, when assigning in the multi-bit format described above, halftone settings are also performed. Halftone is the representation of the original image using dots, and in this invention, since at least two or more inks with different droplet sizes are used, the ratio of the droplet sizes used is also adjusted when forming the image. Furthermore, when assigning inks of at least two different droplet sizes in a multi-bit format, it is also possible to convert existing raster data, such as solid areas represented by 1 bit (e.g., 100% large droplets), to a 2-bit representation (e.g., large / medium / small droplets = 33 / 33 / 33). Furthermore, the term "image formation" refers to the process in which an inkjet printer creates a digital image from a group of ejected ink droplets. Therefore, in this specification, "printing" and "image formation" are synonymous.

[0029] In the present invention, it is preferable that the image formation process includes at least a solid area, and that the multi-bit data for printing is created for the solid area. The aforementioned multi-bit format includes any multi-bit format, but a 2-bit or 3-bit format is preferred, for example.

[0030] In the case of the above-mentioned two or more different droplet sizes, if the first droplet size is A pL and the second droplet size is B pL, it is preferable that B differs from A by ±20% or more. That is, it is preferable that B differs from A by 80% or less, or by 120% or more. Furthermore, if the three or more different droplet sizes of ink are white ink, if the first droplet size is A pL, the second droplet size is B pL, and the third droplet size is C pL, it is preferable that B differs from A by ±20% or more, and C differs from B by ±20% or more. When the aforementioned inks of at least two different droplet sizes are white inks, if the first droplet size is A pL and the second droplet size is B pL, it is preferable that B differs from A by ±20% or more. That is, it is preferable that B differs from A by 80% or less, or by 120% or more. Furthermore, when the inks of three or more different droplet sizes are white inks, if the first droplet size is A pL, the second droplet size is B pL, and the third droplet size is C pL, it is preferable that B differs from A by ±20% or more, and C differs from B by ±20% or more. For example, in the case of three different droplet sizes, the first droplet size can be small: 20 pL, the second droplet size can be medium: 30 pL, and the third droplet size can be large: 40 pL.

[0031] If the above-mentioned inks of at least two different droplet sizes are color inks, and the first droplet size is A pL and the second droplet size is B pL, it is preferable that B differs from A by ±20% or more. That is, it is preferable that B differs from A by 80% or less, or by 120% or more. Furthermore, if the inks of three or more different droplet sizes are white inks, and the first droplet size is A pL, the second droplet size is B pL, and the third droplet size is C pL, it is preferable that B differs from A by ±20% or more, and C differs from B by ±20% or more. For example, if there are three different droplet sizes, the first droplet size can be small (2 pL), the second droplet size medium (3 pL), and the third droplet size large (4 pL). Furthermore, it is preferable that the at least two different droplet sizes of ink consist of at least two different droplet sizes of white ink and / or at least two different droplet sizes of color ink, and that the smallest droplet size of the white ink is larger than the largest droplet size of the color ink. In other words, the droplet size of the white ink is larger than the droplet size of the color ink. Furthermore, while there is no particular limit to the number of different droplet sizes as long as there are two or more, it is preferable to assign three droplet sizes—small, medium, and large—if the aim is to reduce streaks more efficiently. Using only two sizes, small and large, reduces the streaking effect. If four or more droplet sizes are assigned, it is necessary to allow selection from five patterns, including non-discharged droplets, which exceeds the upper limit of 2 bits (4 patterns, or 2 squared), making image formation complicated.

[0032] Furthermore, when assigning inks of different droplet sizes in the multi-bit format to create the multi-bit data for printing, the assignment in the multi-bit format may be done regularly or randomly. A combination of regular and random assignments may also be used. In this way, multi-bit data for printing is created (S102 in Figure 6). When creating multi-bit data for printing, the image to be printed should be created as electronic data in advance using commercially available software. The electronic data of the image to be printed is, for example, in PDF format. For example, based on the electronic data of the image to be printed in PDF format, multi-bit data for printing is created. This multi-bit data for printing is, for example, a 2-bit TIFF file. Then, based on the multi-bit data for printing, ink is ejected from the inkjet head to form an image (S103 in Figure 6).

[0033] The aforementioned web-like printing substrate is preferably a web-like printing substrate made of synthetic resin. As the synthetic resin web-like printing substrate, in addition to opaque films, transparent films can also be used, as can opaque web-like printing substrates such as paper and nonwoven fabrics. As a transparent film web-like printing substrate, transparent films using web-like synthetic resin films such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PP (polypropylene) can be suitably used. The single-pass inkjet printer of the present invention has the above configuration, and in which an image of the same gradation is formed on the web-like printing substrate by ejecting ink from a plurality of fixed linear inkjet heads, in which the nozzle arrangement of each row is slightly shifted from the previous row with respect to the direction of travel of the transported web-like printing substrate, wherein the ejected ink includes at least two or more different droplet sizes, the inks of the different droplet sizes are assigned in a multi-bit format to create multi-bit data for printing, and an image of the same gradation is formed based on the multi-bit data for printing.

[0034] [Printed material] By forming an image using the image forming method of the present invention or the inkjet printer of the present invention, a printed material having a solid pattern can be obtained. [Examples]

[0035] The present invention will be described in more detail below with reference to examples, but it goes without saying that these examples are provided illustratively and should not be interpreted as limiting.

[0036] <Evaluation of print results> Presence or absence of tendons ○: No visible lines or grooves ×: The lines can be seen with the naked eye. dryness ○: No ink overflow or tackiness of the coating can be observed after printing. ×: Ink overflow or tackiness of the coating can be observed after printing.

[0037] [Example 1] 2-bit conversion with white ink <Examples of three different droplet sizes using white ink> The first droplet size is small: 20 pL, the second droplet size is medium: 30 pL, and the third droplet size is large: 40 pL. <Tone curve settings> The output density of the 100% density portion of the PDF was set to 97% using a tone curve. <Halftone setting> The halftone settings for the 97% output concentration portion were set to 33% for small droplets, 33% for medium droplets, and 33% for large droplets.

[0038] <Preparing the electronic data for the design to be printed> When creating multi-bit data for printing, the images to be printed were first created as electronic data in PDF format using commercially available software. <2-bit conversion> Two bits can represent four values. Waveforms for small, medium, and large droplets were assigned to light gray, dark gray, and black, respectively. The four values ​​were set to white (00), light gray (10), dark gray (01), and black (11). A colorless, transparent synthetic resin web-like printing substrate was prepared, and 2-bit TIFF data, as shown in Figure 7, was created from the electronic data of the design to be printed using the three colors: light gray, dark gray, and black. In addition, the TIFF data shown in Figure 7 does not use the color corresponding to white (00). Based on 2-bit TIFF data, ink was ejected from multiple fixed linear inkjet heads, each with a nozzle arrangement slightly shifted relative to the direction of travel of the transported web-like printing substrate, to form an image with 100% uniform gradation on a gradient scale. In this way, solid areas were formed using three different droplet sizes of ink, as shown in Figure 8. A magnified photograph of the solid area is shown in Figure 9. No streaks were visible to the naked eye.

[0039] [Comparative Example 1] <One type of droplet size> The droplet size was limited to large droplets: 30 pL only. <Preparing the electronic data for the design to be printed> One bit can represent two values. The waveform of a large droplet is mapped to black. The binary values ​​were set to white (0) and black (1). A colorless, transparent synthetic resin web-like printing substrate was prepared, and the 1-bit TIFF data shown in Figure 10 was created from the electronic data of the design to be printed in only one color, black. Based on the example of creating 1-bit TIFF data shown in Figure 10, a solid area was imaged using one type of ink droplet size, as shown in Figure 11. A magnified photograph of the solid area is shown in Figure 12. Streaks were visible to the naked eye. As described above, in Example 1, by forming an image using inks of different droplet sizes within the same tonal range, it was possible to prevent the occurrence of streaks in the printed material. On the other hand, in Comparative Example 1, where the amount of ink ejected per unit area was the same, but an image was formed in the same tonal range using only one type of ink droplet size, it was not possible to prevent the occurrence of streaks in the printed material. The evaluation was conducted using the same method as described above, except that the color tone, substrate, bit depth, and ink droplet size assignments were changed as shown in Table 1. The results are also shown in Table 1.

[0040] [Table 1]

[0041] A comparison of Examples 2 and 6 with Comparative Examples 2 and 3 demonstrates that this method effectively eliminates streaks even when the substrate or undercoat changes. A comparison of Examples 7, 8, and 9 with Comparative Examples 5, 6, and 7 demonstrated that this method effectively eliminates streaks even when the color tone or head resolution changes. A comparison of Example 1 and Comparative Example 4 demonstrates that this method effectively eliminates streaks with a small amount of ink ejection while also ensuring good drying properties. [Explanation of Symbols]

[0042] 10 Inkjet Printers 18 Inkjet Heads

Claims

1. A method for forming a solid pattern image using a single-pass inkjet printer, An image forming method in which ink ejected from a single head includes at least two or more different droplet sizes.

2. The image forming method according to claim 1, wherein the different droplet sizes are used to assign ink ejected from a single head in a multi-bit format to create multi-bit data for printing, and a solid pattern image is formed based on the multi-bit data for printing.

3. The image forming method according to claim 1, wherein the multi-bit format is a 2-bit or 3-bit format.

4. The image forming method according to claim 1, wherein, of the at least two different droplet sizes, the first droplet size is A pL and the second droplet size is B pL, and B is within a range of ±20% or more different from A.

5. The image forming method according to claim 1, wherein, in creating the multi-bit data for printing by assigning inks of different droplet sizes in the multi-bit format, the assignment of the multi-bit format is regular and / or random, and the proportion of each droplet size is arbitrarily set and carried out.

6. The image forming method according to claim 1, wherein the printing substrate is a synthetic resin substrate.

7. A single-pass inkjet printer in which ink ejected from a single print head contains at least two or more different droplet sizes to form solid-colored images.

8. The single-pass inkjet printer according to claim 7, wherein the different droplet sizes assign ink ejected from a single head in a multi-bit format to create multi-bit data for printing, and a solid pattern image is formed based on the multi-bit data for printing.

9. The single-pass inkjet printer according to claim 7, wherein the multi-bit format is a 2-bit or 3-bit format.

10. The single-pass inkjet printer according to claim 7, wherein, of the at least two different droplet sizes, the first droplet size is A pL and the second droplet size is B pL, and B is within a range of ±20% or more different from A.

11. The single-pass inkjet printer according to claim 7, wherein, in creating the multi-bit data for printing by assigning inks of different droplet sizes in the multi-bit format, the assignment of the multi-bit format is regular and / or random, and the proportion of each droplet size is arbitrarily set and carried out.

12. The single-pass inkjet printer according to claim 7, wherein the printing substrate is a synthetic resin substrate.

13. A method for manufacturing a printed material, comprising the image forming method described in any one of claims 1 to 6, or the step of printing an image using a single-pass inkjet printer described in any one of claims 7 to 12.

Images