Aqueous inkjet printing method

Abstract

To provide a printing method which prevents misregistration caused by meandering of a plastic film during movement when aqueous ink-jet printing is performed on the plastic film, distortion of printing, and dimensional change and eliminates needs of repetitive adjustment of level differences and gaps between heads.SOLUTION: An aqueous inkjet printing method includes: a step in which prints are made on a plastic film by four-color, CMYK, inks; a step in which the printed plastic film is heated by an infrared ray IR heater which limites its output to such a degree that carbon, a main material, of an india ink of the four color (CMYK) inks does not react to a wavelength of an infrared ray IR fully and only moisture in the inks is evaporated; a step in which the heated plastic film is heated by a hot-air dryer in a manner that the india ink is dried to such a degree that the india ink is not completely dried and does not adhere to a guide roll by low-temperature hot air of 70°C or lower; a step in which prints are made on the heated plastic film with a white ink; and a step in which the plastic film, on which the prints are made with the white ink, is heated at a high temperature and dried completely.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an aqueous inkjet printing method, and particularly to a method optimal for plastic films used for packaging materials such as packaging bags and container lids.

Background Art

[0002] As printing corresponding to printing on packaging materials made of plastic films with small lots and multiple varieties, inkjet printing, which does not require plate making during printing and can generate printing images from digital data, is excellent in on-demand characteristics and is useful for packaging materials that must print various patterns at any time.

[0003] In the above inkjet printing, since aqueous inkjet printing contains an aqueous solvent in addition to a colorant in the ink, it is necessary to remove the aqueous solvent by heat drying after printing.

[0004] Regarding the above, Patent Document 1 describes an aqueous inkjet printing method in which a printing object such as paper, a plastic film, or a fabric passes through a print head portion that discharges ink and travels through a drying path, and a first heating portion and a second heating portion are provided at the tip of the print head portion, and the set temperature of the second heating portion is set higher than the set temperature of the first heating portion.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Disclosure of the Invention

Problems to be Solved by the Invention

[0006] By the way, in the aqueous inkjet printing method using a plastic film as the object to be printed, problems have occurred that do not manifest in the case of paper or fabric. These problems include misregistration due to the meandering of the plastic film during running, misregistration due to printing distortion, misregistration due to dimensional changes, and chronic problems such as repeated adjustment of the head step and gap. Until now, in such situations, there have been many losses before and during printing, which has hindered normal printing.

Means for Solving the Problems

[0007] The present invention was created for the purpose of providing an aqueous inkjet printing method that solves the above problems.

[0008] That is, the aqueous inkjet printing method of the present application is In an aqueous inkjet printing method in which a plastic film to be printed passes through a printing head section that discharges aqueous ink while being given a certain tension and travels along a drying path, A step of printing CMYK four-color inks on the plastic film, A step of heating the plastic film printed with CMYK four-color inks by an infrared IR heating device that limits the output to such an extent that carbon, which is the main raw material of the black ink among the CMYK four-color inks, does not completely react with the wavelength of infrared IR and only the moisture in the ink is evaporated, A step of heating the heated plastic film with a hot air drying device so as to dry it to such an extent that no ink adheres to the guide roll without completely drying the black ink with hot air at a low temperature of 70°C or lower, A step of printing white ink on the heated plastic film, A step of heating the plastic film printed with white ink at a high temperature to completely dry it, characterized by having

[0009] Further, the invention according to claim 2 is characterized in that, in the aqueous inkjet printing method described above, when heating a plastic film printed with white ink at a high temperature, it is heated by an infrared IR heating device with an output limited to 65% or less, and then heated at a high temperature by a hot air drying device.

[0010] Further, the invention according to claim 3 is characterized in that, in the aqueous inkjet printing method described above, the plastic film to be printed travels under a tension of less than 50 N.

[0011] Further, the invention according to claim 4 is characterized in that, in the aqueous inkjet printing method described above, a database is prepared that stores the correlation between the appropriate output of an infrared IR heating device, the attributes of the plastic film, the coverage indicating the usage area, and the film running speed, such that the carbon, which is the main raw material of the black ink among the CMYK four colors, in a plastic film printed with CMYK four-color inks does not completely react with the wavelength of infrared IR and only the moisture in the ink is evaporated. The appropriate output of the infrared IR heating device is automatically calculated by inputting the attributes of the plastic film and the coverage indicating the usage area.

Advantages of the Invention

[0012] In the conventional aqueous inkjet printing method, hot air drying was performed in an environment of 75 to 85°C using a hot air drying device that blows hot air toward the printing surface of the object to be printed.

[0013] The inventor of the present application discovered through experiments that the cause of the above problems in the aqueous inkjet printing method using a plastic film as the object to be printed is that the plastic film undergoes heat shrinkage due to the overly high-temperature hot air drying that is normal for objects to be printed such as paper and fabric. By performing hot air drying at a high temperature of 75 to 85°C, the plastic film undergoes heat shrinkage, resulting in chronic defects such as the following phenomenon 1 (snaking during running), phenomenon 2 (printing distortion), phenomenon 3 (dimensional change) leading to misregistration, and phenomenon 4 (repeated adjustment of head steps and gaps), as well as the need for adjustment.

[0014] In order to fundamentally solve the above problems, it is necessary to lower the drying temperature to 70°C or lower. However, if the drying temperature is lowered to 70°C or lower, the water-based ink will be insufficiently dried and adhere to the roll, making printing impossible (hereinafter referred to as "roll seizure"). It is impossible to achieve drying conditions under which the water-based ink is not roll-seized without heat-shrinking the plastic film, and there has been no way to solve this problem until now.

[0015] The inventor of the present application conceived, through experiments, the temperature conditions and heating means essential for an invention that fundamentally solves this chronic problem.

[0016] First, regarding the heating means, in the present invention, the configuration in which a drying device was arranged and dried after printing the inks of CMYK four colors and white ink has been changed to a configuration in which drying is performed individually each time each ink is printed. That is, in the water-based inkjet printing apparatus for implementing the present invention, a primary unit for printing and drying the CMYK four-color inks and a secondary unit for printing and drying the white ink thereafter are provided, and individual drying devices are arranged in each unit.

[0017] Regarding the drying after printing the CMYK four-color inks, by drying with low-temperature hot air at 70°C or lower without completely drying the ink so that the ink does not adhere to the guide roll, heat shrinkage of the plastic film in the path until the next white ink is printed is prevented, and problems such as misregistration due to meandering during the running of the plastic film, misregistration due to printing distortion, misregistration due to dimensional changes, and repeated adjustment of the head step and gap are solved.

[0018] Regarding the drying in the above-mentioned primary unit, the inventor of the present application conceived of adopting a known infrared IR method (such as Patent Document 1). However, when drying color ink with infrared rays, especially black ink reacts too much to the wavelength of infrared rays and the black ink gets heated and melts. Therefore, the inventor of the present application focused on the condition of stopping at the energy that only dries the moisture in the black ink without the carbon, which is the main raw material of the black ink, reacting completely to the wavelength of infrared IR, and discovered it. Evaporate only the moisture in the ink of CMYK four colors with infrared IR, and dry it with hot air at a low temperature of 70°C or less without completely drying the black ink in particular so that the ink is not taken up by the guide roll, and then use high-temperature hot air in the subsequent white ink unit to completely dry white and CMKY four colors. That is, the drying condition of evaporating only the moisture of CMYK four colors (especially black ink) at a low temperature of 70°C or less using infrared IR was found in the above-mentioned primary unit of CMYK four colors. That is, the fact that it was confirmed that drying was performed in a state where the ink was not taken up by the guide roll of the primary unit of CMYK four colors became the trigger for the invention. That is, since there was a process of drying only CMYK four colors first, it was possible to create a state where it was not completely dried with infrared IR even at a low temperature of 70°C or less. As a result, the phenomenon in which the conventional problem of black ink reacting too much to the wavelength of infrared IR and getting heated and melting could be avoided.

[0019] According to the present invention, it becomes possible to use infrared IR for a known black ink mainly made of carbon.

Brief Explanation of Drawings

[0020]

Figure 1

Best Mode for Carrying Out the Invention

[0021] Hereinafter, the chronic quality defect phenomenon when printing a plastic film with aqueous inkjet by the inventor of the present application will be described.

[0022] (Chronic Defect Phenomenon No. 1: Misregistration due to Snakiness during Running) During the printing process, the deviation of the slanting state for CMYK four colors with respect to white is ±0.3 to 0.5 mm in the front-back and left-right directions. According to the experiments of the inventor of the present application, at the conventional hot air drying temperature of 75 to 85 °C, both heat-resistant OPP and general-purpose OPP continue to slant back and forth and left and right slowly while running even when a constant tension is applied in the drying furnace. As a result of the experiments, it was clarified that this is due to the heat shrinkage of the plastic film at high temperatures due to the orientation during the production of the plastic film.

[0023] The water-based inkjet printing device does not control the tension in a constant manner. Instead, by adjusting the driving nip roll to a constant rotational speed, the running of the plastic film flows smoothly without problems. That is, even if there is a tension fluctuation due to the heat shrinkage of the plastic film, the driving nip roll remains at a constant rotation, so the fluctuation of the plastic film directly results in the phenomenon of slanting left and right and back and forth. Originally, it is not tension control, but the pulling rate (rotation rate of all driving nip rolls) is set to match the target tension. In this case, the target tension is set between 20 and 50 N. Ordinarily, a high tension of 50 N or more is required to eliminate slanting, but if the tension is too strong, wrinkles are likely to appear on the plastic film, and the straight line in the width direction of the printing also curves, causing problems in terms of design. Therefore, it is necessary to use a low tension of less than 50 N (20 to 50 N). However, in this tension range, due to the influence of the heat shrinkage of the plastic film, the flow extends and the width contracts, which directly results in the phenomenon of slanting left and right and back and forth. In the gravure machine and flexo machine, the tension control method is different, and since printing is performed while being nipped by the plate, even at a high tension (50 N or more), the straight line in the width direction of the printing does not curve and normal printing is performed, and the phenomenon of slanting left and right and back and forth does not occur. That is, it is a phenomenon peculiar to the water-based inkjet printing device.

[0024] The plastic films in question are both general-purpose OPP, which is prone to heat shrinkage and is less stable than heat-resistant OPP, which is more stable than general-purpose OPP and is other than PET with stable heat shrinkage. General-purpose OPP is likely to cause significant heat shrinkage during running and meandering left and right when the drying temperature is too high at 75 to 85 °C. Similarly, heat-resistant OPP is less likely to meander, but when the alignment of CMYK four colors and white is strict (±0.3 mm) in terms of design, misalignment occurs. Therefore, it is necessary to find the optimal conditions under which both general-purpose OPP and heat-resistant OPP can be stably printed.

[0025] (Chronic Defect Phenomenon Part 2: Misalignment Due to Printing Distortion) In water-based inkjet printing, since the printed material is directly put into the drying oven, when the plastic film undergoes heat shrinkage due to high-temperature drying in the drying oven, even if the wind speed difference of the hot air in the width direction of the nozzle and the assembly of the guide roll and the machine body in the drying oven are within the allowable range and within the accuracy, the CMYK four-color printing will have a left-right difference in the width direction and be distorted in terms of design. When printing white in this distorted state, the positions of the CMYK four colors will have a left-right difference from the white at the normal position and will be misaligned. It was a chronic defect that was likely to cause non-adjustable distortion by the twisting roll in the machine.

[0026] The cause of this printing distortion was clarified from the experimental results to be hot air drying at too high a temperature. When both general-purpose OPP and heat-resistant OPP are dried at a high temperature (75 to 85 °C), it affects the orientation during the film formation of the plastic film. For example, a shape perpendicular to the printing design will be distorted left and right and subtly become a rhombus. Of course, it also has some influence on the assembly accuracy of the printing device and the left-right wind speed difference of the hot air from the drying nozzle. However, in the case of this water-based inkjet printing device, it was clarified that even within the accuracy of the machine manufacturer, the plastic film undergoes heat shrinkage due to hot air drying at too high a temperature, resulting in distortion.

[0027] The target plastic films are both general-purpose OPP, which is prone to heat shrinkage except for PET with stable heat shrinkage, and heat-resistant OPP, which is slightly more stable than the former. General-purpose OPP is likely to cause a distortion phenomenon with a left-right difference due to significant heat shrinkage during running at a drying temperature as high as 75 - 85°C. Similarly, although heat-resistant OPP is less likely to generate distortion, misregistration occurs when the tolerance for CMYK four colors and white in terms of design is strict (±0.3 mm). Therefore, it is necessary to find the optimal conditions under which both general-purpose OPP and heat-resistant OPP can be stably printed.

[0028] (Chronic Defect Phenomenon No. 3: Misregistration Due to Dimension Change) In water-based inkjet printing, there is no printing plate in the inkjet method, and the ink is ejected non-contact for printing. The material runs through the drying oven under a constant tension until the ink dries. However, since it is necessary to dry the water-based ink at a high temperature (75 - 85°C) to completely dry it, the dimensions of the CMYK four colors change significantly due to the influence of this high temperature. After drying the CMYK four colors and printing white in the next unit, there is a dimensional difference between the CMYK four colors with changed dimensions and the white in the original data, so misregistration will occur if printed as it is. Therefore, usually, while making trial prints, the dimensions of each other are confirmed, and especially the dimension of white is corrected in the data to start printing.

[0029] The cause of this dimensional difference was clarified from the results of this experiment to be hot air drying at too high a temperature. When both general-purpose OPP and heat-resistant OPP are dried at a high temperature (75 - 85°C), the plastic film undergoes heat shrinkage, resulting in a change in the dimensions of the CMYK four colors. It was clarified that due to the conventional hot air drying at too high a temperature, the plastic film undergoes heat shrinkage, causing a dimensional difference between the CMYK four colors and white.

[0030] The target plastic film is both general-purpose OPP, which is prone to heat shrinkage except for PET with stable heat shrinkage, and heat-resistant OPP, which is slightly more stable. General-purpose OPP significantly undergoes heat shrinkage during running at a drying temperature as high as 75 - 85°C, making it easy for the dimensional changes of CMYK four colors to become large after drying. Similarly, although heat-resistant OPP is less prone to heat shrinkage than general-purpose OPP, when the alignment of CMYK four colors and white is strict (±0.3 mm) in terms of design, alignment errors occur. Therefore, it is necessary to find the optimal conditions under which both general-purpose OPP and heat-resistant OPP can be stably printed.

[0031] (Chronic Defect Phenomenon No. 4: Repeated Adjustment of Head Step and Gap) Since water-based inkjet printing uses multiple heads for the same color, it is necessary to individually adjust the step (alignment in the flow direction) and gap (alignment in the width direction) between adjacent heads for each head. However, even if adjusted carefully, once the machine is stopped and printing is done again, the adjustment results cannot be reproduced, and it is necessary to repeat the adjustment, which is not suitable for mass production.

[0032] The reason why the adjustment of this head step (alignment in the flow direction) and gap (alignment in the width direction) cannot be reproduced was clarified from the results of this experiment as being due to hot air drying at too high a temperature. Even if the step and gap were adjusted between individual heads first and confirmed by proof printing, adjustment would be required at another location and could not be reproduced repeatedly. Whether it is general-purpose OPP or heat-resistant OPP, when dried at a high temperature (75 - 85°C), the plastic film undergoes heat shrinkage, and the plastic film changes subtly with each repetition, so the previous state could not be reproduced.

[0033] Figure 1 is a conceptual diagram of a water-based inkjet printing apparatus for implementing the water-based inkjet printing method of the present invention. In the figure, reference symbol F is the plastic film to be printed. This plastic film passes through a printing head section that ejects ink while being given a tension of 50 N here and travels through a drying path to be printed.

[0034] In the present invention, the printing head for printing CMYK four colors and the printing head 12 for printing white ink are respectively housed in the primary unit 1 and the secondary unit 10, and individual drying paths are provided in each unit.

[0035] That is, a drying path consisting of an infrared IR heating device 3 and a subsequent hot air drying device 4 is provided after the printing head 2 for printing CMYK four colors in the primary unit 1, and a drying path consisting of an infrared IR heating device 13 and a subsequent hot air drying device 14 is provided after the printing head 12 for printing white ink in the secondary unit 10.

[0036] In the above drying path, in the infrared IR heating device 3 in the primary unit 1, the output is adjusted so that carbon, which is the main raw material of the black ink among the CMYK four colors, does not completely react to the wavelength of infrared IR and only the moisture in the ink is evaporated. In the hot air drying device 4, the drying is limited to a level where the ink does not adhere to the guide roll with hot air at a low temperature of 70°C or less without completely drying the black ink.

[0037] On the other hand, in the drying path of the secondary unit 10, after printing with CMYK four colors, the plastic film F printed with white ink by the printing head 12 is completely dried. Here, an infrared IR heating device 13 and a subsequent hot air drying device 14 are provided. In this case, if there is a sufficient distance in the drying path by the hot air drying device 14, the infrared IR heating device 13 becomes unnecessary.

[0038] Regarding the appropriate output of the infrared IR heating device where carbon, which is the main raw material of the black ink among the CMYK four colors, does not completely react to the wavelength of infrared IR and only the moisture in the ink is evaporated, the inventor of the present application has clarified through experiments that there is a correlation with the attributes of the plastic film (for example, general-purpose OPP that is prone to heat shrinkage and heat-resistant OPP that is slightly more stable), the coverage indicating the usage area, and the film running speed.

[0039] Therefore, if a database storing the above-mentioned correlation is prepared, a preset function can be added to automatically calculate the appropriate output of the infrared IR heating device by inputting the attributes of the plastic film and the coverage indicating the usage area.

[0040] In addition, regarding the appropriate output of the infrared IR heating device for a plastic film printed with CMYK four-color inks, where carbon, which is the main raw material of the black ink among the CMYK four colors, does not fully react to the wavelength of the infrared IR and only the moisture in the ink is evaporated, as a result of experiments, the inventor of the present application has clarified that the pulling rate (the rotation rate of all driving nip rolls) varies depending on the attributes of the plastic film (for example, general-purpose OPP that is prone to heat shrinkage and heat-resistant OPP that is slightly more stable, etc.) in order to maintain the optimum tension.

[0041] Therefore, if a database storing the correlation between the above-mentioned attributes of the plastic film and the pulling rate is prepared, a preset function can be added to automatically calculate the pulling rate that results in the optimum tension when using the infrared IR heating device by inputting the attributes of the plastic film and the coverage indicating the usage area.

[0042] The inventor of the present application conducted the following experiment using the above-mentioned aqueous inkjet printing device. <Experiment content> ◎Plastic film (printing substrate) · Heat-resistant OPP #20 (Toyobo P2171) · General-purpose OPP #20 (Toppan U-1) (Experiment machine (1) Experimental conditions) Aqueous inkjet printing conditions: speed 50 m / min, tension around 40 N, paper width 760 mm Drying conditions for CMYK four colors · Heat-resistant OPP = primary unit IR output 90% or more + drying temperature 70°C or less · General-purpose OPP = primary unit IR output 90% or more + drying temperature 70°C or less (Experiment machine (2) Experimental conditions) Aqueous inkjet printing conditions: speed 50 m / min, tension around 40 N, paper width 760 mm Drying Conditions for CMYK Four Colors · Heat-resistant OPP = IR output of the primary unit is 70% or more + drying temperature is 70°C or less · General-purpose OPP = IR output of the primary unit is 70% or more + drying temperature is 70°C or less ◎ Specifications of Infrared IR Heating Device · Test machine (1): 3.8 kw × 6 units (wavelength approximately 2.0 μm) · Test machine (2): 3.8 kw × 9 units (wavelength approximately 2.0 μm) ◎ Two Kinds of Printing Designs (Coverage: Four Colors 120 - 140%, White 140%) · Design A with a Strict Tolerance: Sinuous movement left - right, front - back ±0.3 mm · Design B with a Strict Head Step and Gap: Reproduce by repeating after adjustment ◎ White Drying Conditions (Low IR Output + High - Temperature Hot - Air Drying) Same conditions for both test machines (1) and (2): IR output of the secondary unit is 65% or less + 80°C or more

[0043] <Experimental Results> (Results of Test Machine (1)) · Heat-resistant OPP = IR output of the primary unit is 90% or more + 70°C or less → All of the above chronic defective phenomena 1 - 4 are eliminated · General-purpose OPP = IR output of the primary unit is 90% or more + 70°C or less → All of the above chronic defective phenomena 1 - 4 are eliminated (Results of Test Machine (2)) · Heat-resistant OPP = IR output of the primary unit is 70% or more + 70°C or less → All of the above chronic defective phenomena 1 - 4 are eliminated · General-purpose OPP = IR output of the primary unit is 70% or more + 70°C or less → All of the above chronic defective phenomena 1 - 4 are eliminated

[0044] (Experimental Consideration) Experiments were conducted under the same conditions for both test machines (1) and (2), except for the difference in the specifications of the infrared IR heating device. The difference in the specifications of the infrared IR heating device lies in that the IR lamp of test machine (1) has 6 units, while the IR lamp of test machine (2) has 9 units.

[0045] In the case of six IR lamps of the experimental machine (1), even when the IR output was increased to 90% or more, there was no damage to the plastic film. By combining drying at a low temperature of 70°C or lower, the aqueous ink could be dried at a film running speed of 50 m / min. In the case of nine IR lamps of the experimental machine (2), when the IR output was increased to 90% or more, damage occurred to the heat-resistant OPP plastic film and strong flow wrinkles were generated. Therefore, the output was set to 70% or more to eliminate the plastic film wrinkles. For general-purpose OPP as well, when the IR output was 90% or more, damage occurred to the plastic film and strong flow wrinkles were generated. Therefore, the output was set to 70% or more to eliminate the plastic film wrinkles. By combining drying at a low temperature of 70°C or lower with the appropriate IR output for both heat-resistant OPP and general-purpose OPP, the aqueous ink could be sufficiently dried at a speed of 50 m / min.

[0046] By optimizing the output according to the infrared IR heating device specifications and combining drying at a low temperature of 70°C or lower, all chronic defects such as phenomenon 1 (snaking during running), phenomenon 2 (printing distortion), phenomenon 3 (dimension change), and phenomenon 4 (repeated adjustment of head steps and gaps) caused by heat shrinkage of the plastic film at high temperatures were resolved.

Explanation of symbols

[0047] F Film 1 Primary unit 2 CMYK ink printing head 3 Infrared IR heating device 4 Hot air drying device 10 Secondary unit 12 CMYK ink printing head 13 Infrared IR heating device 14 Hot air drying device

Claims

1. In an aqueous inkjet printing method in which a plastic film to be printed travels through a drying path while passing through a printing head unit that discharges aqueous ink while a constant tension is applied to the plastic film, a step of printing CMYK four-color ink on the plastic film, a step of heating the plastic film printed with CMYK four-color ink with an infrared IR heating device that limits the output so that carbon, which is the main raw material of black ink among the CMYK four colors, does not completely react with the wavelength of infrared IR and only the moisture in the ink is evaporated, a step of heating the heated plastic film with a hot air drying device so as to dry it to such an extent that ink does not adhere to the guide roll without completely drying the black ink with hot air at a low temperature of 70°C or less, a step of printing white ink on the heated plastic film, a step of heating the plastic film printed with white ink at a high temperature to completely dry it, An aqueous inkjet printing method characterized by comprising the above steps.

2. The aqueous inkjet printing method according to Claim 1, wherein when heating the plastic film printed with white ink at a high temperature, after heating with an infrared IR heating device with an output limited to 65% or less, it is heated at a high temperature with a hot air drying device.

3. The aqueous inkjet printing method according to Claim 1 or 2, wherein the plastic film to be printed travels with a tension of less than 50 N.

4. Prepare a database that stores the correlation between the appropriate output of an infrared IR heating device that limits the output so that carbon, which is the main raw material of black ink among the CMYK four colors, does not completely react with the wavelength of infrared IR and only the moisture in the ink is evaporated, and the attributes of the plastic film, the coverage indicating the use area, and the film running speed, The aqueous inkjet printing method according to Claim 1 or 2, wherein the appropriate output of the infrared IR heating device is automatically calculated by inputting the attributes of the plastic film and the coverage indicating the use area.

Images