A method for formulating standards for flexographic water-based inks

Abstract

The present invention discloses a method for establishing a standard for flexographic water-based ink, comprising the following steps: Step 1: determining a target color density; Step 2: testing the ink dot gain rate; Step 3: testing the ink dot and solid density difference; Step 4: printing solid color samples of each of the colors C, M, Y, and K using the ink determined in Step 3, and measuring the Lab value of the printed color samples, with the measured Lab value serving as the standard value for the flexographic water-based ink. The present invention can form internal enterprise standards, facilitate accurate ink performance evaluation and identification of improvement directions when purchasing flexographic water-based ink, shield against input influences from the supply side, maintain process stability, simplify process adjustment, and improve production efficiency.

Description

Technical Field

[0001] The present invention relates to a printing quality control process, and more particularly to a screening method for flexographic water-based ink replacing offset printing ink. Background Art

[0002] Preprinting refers to the printing of corrugated cardboard's reel-type paper prior to production. The printed paper is then rolled and sent to the corrugated box production equipment for laminating and forming. This process involves preprinting the desired pattern before the corrugated cardboard is made. Existing preprinting equipment mostly uses flexographic printing (flexo) or gravure printing (gravure). Flexo is widely used for its balanced performance, including ink thickness, image detail, and process cost. Due to increasingly stringent environmental requirements for printed products in both domestic and international markets, the use of water-based inks is increasing. Water-based inks are currently the most widely used in flexographic printing, leading many customers to request the conversion of existing offset printing orders to flexo. However, due to the inherent differences between flexographic and offset printing processes, achieving the same results with flexographic printing presents significant technical challenges. The difference in printing performance between flexographic and offset inks significantly impacts the transition from offset to flexo. To address this, it is necessary to first determine the standards for flexographic water-based inks that can replace offset inks. Unlike offset inks, which are governed by the ISO 2846-1 standard, there is currently no unified industry standard for flexographic water-based inks. Each brand of ink manufacturer independently customizes its own standards, causing significant inconvenience for downstream users. Changing ink brands often requires a series of process adjustments. Therefore, it is crucial for printers producing pre-printed products to customize their own standards for flexographic water-based inks based on their needs. This helps them accurately determine whether the inks provided by suppliers meet their requirements, provide recommendations for improvements, and maintain process stability. Summary of the Invention

[0003] There are differences in the printing performance of flexographic water-based ink and offset ink, and the industry currently lacks product standards for flexographic water-based ink, which restricts the promotion of the market demand for "offset to flexographic". To overcome these defects, the present invention provides a method for screening flexographic water-based ink that can form a basis for internal enterprise quality judgment of flexographic water-based ink, thereby being more conducive to ensuring production quality stability and production efficiency, and replacing offset ink.

[0004] The technical solution of the present invention is: a method for screening flexographic water-based ink that replaces offset ink, the flexible plate used for flexographic printing is a resin plate, and the screening method comprises the following steps:

[0005] Step 1. Determine the target color density. Use various anilox roller specifications to print solid flexo color samples for each of the C, M, Y, and K colors. Test the color density of the color samples generated by each anilox roller specification. Use the color density of the solid offset color patches for each of the C, M, Y, and K colors as a reference. Select the anilox roller corresponding to the flexo color sample with the color density closest to the reference as the reference anilox roller for further testing.

[0006] Step 2. Perform ink dot gain rate detection; perform flexographic color printing on the reference anilox roller selected in step 1, and print test dots with a dot area rate of 50% for each color ink of C, M, Y, and K, respectively. Measure the dot gain rate of the test dots and select an ink with a dot gain rate of 22 ± 3% to continue testing;

[0007] Step 3. Detect the difference between ink dots and solid density; measure the color density D of the color sample with 75% dot area rate printed by each color ink. 75 And the solid color density D of the same color S , using the formula ΔD=1-D 75 / D S Calculate the relative density difference ΔD and select ink with a ΔD of 35% to 45% to continue testing;

[0008] Step 4. Use the ink determined in step 3 to print solid color samples of C, M, Y, and K colors, and measure the Lab values ​​of the printed color samples. The measured Lab values ​​are used as the standard values ​​for flexographic water-based inks.

[0009] In the flexographic printing process, the printing plate is made of a flexible resin material. During printing, the plate is directly pressed against the substrate, transferring the ink from the plate to the substrate. Because the resin material is much thicker and more elastic than offset printing plates, flexographic plates are more susceptible to deformation during printing, causing the printed dots to deform and expand, resulting in distortion compared to the same pattern produced by offset printing. Furthermore, in flexographic printing, the anilox roller draws ink from an ink fountain, and the printing plate receives the ink directly from the anilox roller. In offset printing, however, the ink passes through numerous rubber rollers for distribution, distribution, and transfer from the ink fountain to the final substrate. This ink transfer path is much longer than in flexographic printing, and the ink layer thickness in flexographic printing is significantly thicker than that in offset printing. Therefore, the process differences between flexographic and offset printing inevitably lead to different printing results. However, color density and Lab values ​​are parameters used to quantitatively characterize color characteristics. Therefore, the idea behind this method is to use the color density of the four primary offset ink colors (C, M, Y, and K) as a reference. By screening process conditions, the color density of the four primary flexographic water-based inks can be brought close to the reference value of offset inks. In this method, step one allows the selection of anilox rollers that can provide an appropriate ink layer thickness, so that the solid density of each color produced by the flexographic process can be compared with the solid density of each color produced by offset printing. The selected anilox rollers are then used to print color samples for further testing, and anilox rollers of other specifications are eliminated. Steps two and three further evaluate the dot printing performance of the flexographic water-based inks to select flexographic water-based inks that meet the "offset-to-flex" conversion requirements. The 50% dot is the dot that best reflects the degree of dot gain. Offset printing also experiences dot gain when printing at 50% dot density. As mentioned above, flexographic plates are more susceptible to deformation, making dot gain more pronounced in flexo than in offset. Considering the inherent differences between flexo and offset printing, flexographic water-based inks that maintain a dot gain within the range of 22±3% after printing are considered eligible for "offset-to-flexo" conversion. Similarly, 75% dot density best reflects the density difference between dotted and solid text. Considering the inherent differences between flexo and offset printing, flexographic water-based inks that maintain a density difference within the range of 36%-45% are considered eligible for "offset-to-flexo" conversion. These constraints allow for the selection of suitable flexographic water-based inks. These inks are then subjected to Lab value measurement to obtain intuitive, quantitative parameters, facilitating performance evaluation and identifying improvement areas, enabling more efficient and convenient communication and consultation with ink suppliers.

[0010] Preferably, the resin plate used in steps 1, 2, and 4 has a Shore hardness of 67 to 69. This resin plate has a higher hardness, a relatively small ink loading capacity, and a relatively small deformation, which facilitates the reproduction of fine images and texts, thereby closer to the effect of offset printing. It also has a relatively higher press run and a longer service life.

[0011] Preferably, a custom segmented anilox roller is used in step 1, with each segment corresponding to a specific anilox roller line count specification. The standard anilox roller is custom-made as a segmented roller, integrating commonly used anilox specifications onto a single roller. During color sample printing, color samples corresponding to various anilox roller specifications are generated simultaneously, improving color sample printing efficiency.

[0012] Alternatively, multiple anilox rollers of the same specification can be used to print the color samples separately in step 1. This allows the use of existing anilox rollers for color sample printing, eliminating the need for custom-made ones and significantly saving costs.

[0013] Preferably, the present method for screening flexographic water-based inks that replace offset inks further includes designing a comprehensive test sample sheet, which includes a solid color block area, a color scale, and an overprint effect area. A flexible resin plate is produced based on the comprehensive test sample sheet, and then the comprehensive test sample sheet is printed. The solid color block area of ​​the sample sheet provides solid color blocks of various colors, the color scale provides dot samples of each color from 5% to 95%, and the overprint effect area presents the color range after four-color overprinting, demonstrating the color gamut size and the color rendering power. Therefore, the comprehensive test sample sheet includes the test objects required for steps two to four. Use of this comprehensive test sample sheet can greatly simplify the testing process, eliminating the need for separate color sample printing for each step.

[0014] Preferably, the dot gain ratio in step 2 is measured using a spectrodensitometer. A spectrodensitometer can accurately measure the dot coverage ratio of a specific printing area, thereby accurately determining whether the dot gain of larger dots is within the allowable range. Compared with manual judgment based on experience, the dot coverage ratio data on the printed material measured using a spectrodensitometer is more accurate.

[0015] Preferably, the color sample printing is carried out on a pre-printing production line. Directly using production equipment rather than proofing equipment to perform the test in this method is closer to actual production and the test results are more accurate.

[0016] As an advantage, 200 coated paper is used for color sample printing in each step. 200 coated paper is currently the main printing material for many pre-printed product printing manufacturers and is relatively representative.

[0017] The beneficial effects of the present invention are:

[0018] Forming internal enterprise standards facilitates accurate evaluation of ink performance and identification of improvement directions when purchasing flexographic water-based inks, shields the impact of input from the supply side, helps maintain process stability, simplifies process adjustment, and improves production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the structure of the segmented anilox roller in the present invention;

[0020] Figure 2Schematic diagram of the arrangement of the flexible resin plate on the plate cylinder in the present invention;

[0021] Figure 3 A curve diagram showing the relationship between the anilox roller specifications, the flexible resin plate type and the solid density in the present invention;

[0022] Figure 4 This is a set of test data in Example 1.

[0023] In the figure, 1-plate cylinder, 2-flexible resin plate. DETAILED DESCRIPTION

[0024] The present invention will be further described below with reference to specific embodiments of the drawings.

[0025] Example 1:

[0026] A method for screening flexographic water-based inks that replace offset inks, wherein the flexible plate used in flexographic printing is a resin plate, and the screening method comprises the following steps:

[0027] Step 1. Determine the target color density. Use various anilox roller specifications to print solid flexo color samples for each of the C, M, Y, and K colors. Test the color density of the color samples generated by each anilox roller specification. Use the color density of the solid offset color patches for each of the C, M, Y, and K colors as a reference. Select the anilox roller corresponding to the flexo color sample with the color density closest to the reference as the reference anilox roller for further testing.

[0028] Step 2. Perform ink dot gain rate detection; use the reference anilox roller selected in step 1 to perform dot flexographic color printing, and print test dots with a dot area rate of 50% for each color ink of C, M, Y, and K. Measure the dot gain rate of the test dots and select an ink with a dot gain rate of 22±3% to continue testing. The dot gain rate value here is the average value after measuring 10 sets of data. The dot gain rate is measured by a spectrodensitometer;

[0029] Step 3. Detect the difference between ink dots and solid density; measure the density D of the dot sample with 75% dot area rate printed by each color ink. 75 And the solid color sample density D of the same color S , using the formula ΔD=1-D 75 / D S Calculate the relative density difference ΔD, select ink with ΔD of 36% to 45% to continue testing, where D 75 、D S The values ​​are the average values ​​after measuring 10 sets of data;

[0030] Step 4. Use the ink determined in step 3 to print solid color samples of C, M, Y, and K colors, and measure the Lab value of the printed color samples. The measured Lab value is used as the standard value of flexo water-based ink. The Lab value here is the average value after measuring 10 sets of data.

[0031] The color sample printing in the above steps is performed on a pre-printing line using 200 coated paper as the substrate. This method for screening flexographic water-based inks as an alternative to offset inks also includes designing a comprehensive test sheet with a solid color block area, a color scale, and an overprint effect area. The relevant measurements in steps 2 through 4 are performed on this comprehensive test sheet.

[0032] Step 1 includes four sub-steps:

[0033] Steps a. Customize the segmented anilox roller, that is, divide one anilox roller into seven segments, corresponding to seven anilox roller line number specifications of 300 lines / inch, 400 lines / inch, 500 lines / inch, 600 lines / inch, 700 lines / inch, 800 lines / inch, and 900 lines / inch, respectively. Figure 1 As shown;

[0034] Step b. Prepare three types of Shore hardness flexible resin plates 2 with 67 degrees, 42 degrees, and 35 degrees, cut them into long strips, and stick three flexible resin plates 2 on the plate cylinder 1 of the pre-printing production line. The three flexible resin plates 2 are evenly spaced on the plate cylinder 1 and are distributed in an equilateral triangle relative to the axis of the plate cylinder 1, such as Figure 2 As shown; the length direction of the flexible resin plate 2 is in the same direction as the axial direction of the plate cylinder 1; the flexible resin plate 2 has been completed and carries a strip-shaped solid pattern;

[0035] Step c. Start printing to obtain C, M, Y, and K color samples, one sample for each color, and each sample has three solid monochrome stripes. Measure all solid patterns separately to obtain their respective solid color density values;

[0036] In step d, the measured solid color density values ​​are compared one by one with the solid color density values ​​of the four colors C, M, Y, and K specified in the relevant standards for offset printing. Those that fall within the numerical range are considered to meet the requirements, and the anilox roller specifications corresponding to the solid color density values ​​that meet the requirements are selected.

[0037] In this embodiment, according to Figure 3The results shown indicate that combining printing plates of varying hardness with anilox rollers of varying specifications can produce overlapping results. For example, a higher-hardness printing plate paired with a lower-line-count anilox roller, or a lower-hardness printing plate paired with a higher-line-count anilox roller, can both achieve the desired solid color density. In this case, it's important to consider factors such as the company's usage habits, cost, and future product positioning, prioritizing the solution that minimizes the impact on existing processes. In this example, a 600-line-per-inch anilox roller was selected as the baseline for the three colors C, M, and Y, and a 700-line-per-inch anilox roller was selected for black K. The selected flexographic resin plate had a Shore A hardness of 67.

[0038] According to the above standards, two brands of flexographic water-based inks were evaluated. The dot gain rates of the 50% dot samples of the C color ink of Brand 1 and Brand 2 were 19% and 22% respectively, while the color density of the 75% dot sample was 75 Solid color sample color density D S The density difference ΔD is as follows: Figure 4 The dot gain rates of the 50% dot samples of the K ink of Brand 1 and Brand 2 are 22% and 25% respectively, while the color density D of the 75% dot sample is 22%. 75 Solid color sample color density D S The density difference ΔD is as follows: Figure 4 The values ​​are shown as 47.87% and 46.28%, respectively. Therefore, the K inks of Brand 1 and Brand 2 can be deemed as non-compliant due to the excessive density difference ΔD. Based on this, feedback was given to the supplier, suggesting that the proportion of black colorant be reduced. This can reduce costs, improve printing quality, and avoid the problem of severe discoloration caused by poor ink adhesion. The color density D of the 75% dot sample of the M ink of Brand 1 and Brand 2 is shown as 75 Solid color sample color density D S The density difference ΔD is as follows: Figure 4 The values ​​shown are 36.0% and 45.0%, so the M inks of Brand 1 and Brand 2 can both be considered as meeting the standard. The 75% dot density D of the Y inks of Brand 1 and Brand 2 75 Solid color sample color density D S The density difference ΔD is as follows: Figure 4 As shown, the percentages are 32.93% and 30.82%, respectively. Therefore, the Y inks of Brand 1 and Brand 2 can be deemed as not meeting the standards. Based on this, feedback was given to the supplier, suggesting that the proportion of yellow colorant be increased.

[0039] Example 2:

[0040] In step 1, seven single-size anilox rollers (300, 400, 500, 600, 700, 800, and 900 lines per inch) were used to print color samples of four colors: C, M, Y, and K. In step b, three Shore hardnesses of 68, 45, and 37 were prepared, and a Shore hardness of 68 was ultimately selected. The remaining steps were the same as in Example 1.

[0041] Example 3:

[0042] In step b, three flexible resin plates with Shore hardnesses of 69, 50, and 40 were prepared, and the flexible resin plate with Shore hardness of 69 was finally selected.

Claims

1. A method for screening flexographic water-based inks that replace offset inks, wherein the flexible plate used for flexographic printing is a resin plate, characterized in that The following steps are involved: Step 1. Determine the target color density; Use anilox rollers of various specifications to print solid color patches of C, M, Y, and K colors in flexo printing. Test the color density of the color samples generated by each anilox roller specification. Use the color density of the solid color patches of C, M, Y, and K colors in offset printing as a reference. Select the anilox roller corresponding to the flexo color sample with the color density closest to the reference as the reference anilox roller for further testing. Step 2. Perform ink dot gain rate detection; perform flexographic color printing on the reference anilox roller selected in step 1, and print test dots with a dot area rate of 50% for each color ink of C, M, Y, and K, respectively. Measure the dot gain rate of the test dots and select an ink with a dot gain rate of 22 ± 3% to continue testing; Step 3. Detect the difference between ink dots and solid density; measure the color density D of the color sample with 75% dot area rate printed by each color ink. 75 And the solid color density D of the same color S , using the formula ΔD=1-D 75 / D S Calculate the relative density difference ΔD and select ink with a ΔD of 36% to 45% to continue testing; Step 4. Use the ink determined in step 3 to print solid color samples of C, M, Y, and K colors, and measure the Lab value of the printed color samples. The measured Lab value is used as the standard value of flexographic water-based ink.

2. The method for screening flexographic water-based ink for replacing offset ink according to claim 1, characterized in that The Shore hardness of the resin plate used in steps 1, 2, and 4 is 67 to 69 degrees.

3. The method for screening flexographic water-based ink for replacing offset ink according to claim 1, characterized in that In step one, a custom segmented anilox roller is used, with each segment corresponding to a line count specification of the anilox roller.

4. The method for screening flexographic water-based ink for replacing offset ink according to claim 1, characterized in that In step 1, multiple anilox rollers of the same specification are used to print color samples.

5. The method for screening flexographic water-based ink for replacing offset ink according to claim 1, characterized in that It also includes a designed comprehensive test sample, which has a solid color block area, a color gradient and an overprint effect area.

6. The method for screening flexographic water-based ink for replacing offset ink according to claim 1, characterized in that The dot gain ratio in step 2 is measured by a spectrodensitometer.

7. The method for screening flexographic water-based ink for replacing offset ink according to any one of claims 1 to 6, characterized in that Color sample printing is carried out on the pre-printing production line.

8. The method for screening flexographic water-based ink for replacing offset ink according to any one of claims 1 to 6, characterized in that 200 coated paper was used for color sample printing in each step.

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