How the quality of printed matter has a lot to do with the quality of originals, technological measures, printer performance, paper quality, and technical quality of the operator. Therefore, strengthening the quality inspection and control of the printing process is very important to improve the printing quality of products. Meaning. So, how should the quality check of the halftone printed matter with halftone dots be performed? This is the inspection knowledge necessary for print quality management and control. The author specifically talks about the quality inspection of the screen prints for readers' reference.
First, we must recognize the items and requirements of print quality inspection 1. The concept and requirements of tone value. The tone of the screen print is the density range of the image or the density step of the image. We know that the tone of the screen print is determined by the whiteness of the paper and the density contrast of the effective maximum density of the four color plates superimposed. Therefore, the better the printability, the larger the density range, and the better the tone reproduction; conversely, the worse the printability, the smaller the density range, and the worse the tone reproduction. The printing process practice shows that there are many factors that affect tone reproduction. The tone value of the original, the characteristics of paper, ink, and machine, as well as the process design and printing conditions during film and printing will all affect the reproduction of the product. quality. According to national industry standards, the density of fine products on the ground: 0.85 to 1.10 for yellow, 1.25 to 1.50 for magenta, 1.30 to 1.55 for cyan, 1.40 to 1.70 for black; general product density: 0.80 to 1.05 for yellow, magenta It is 1.15 to 1.40, cyan is 1.25 to 1.50, and black is between 1.20 to 1.50. Brightness reproduction of fine products is 2% to 4% of the dot area; brightness reproduction of general products is 3% to 5% of the dot area.
2. Concepts and requirements of print layout level and clarity. The level of the printed layout refers to the level of brightness that can be detected by the naked eye within the range of the density that can be copied. The sharpness of printed products refers to the difference in hue of adjacent details. Under the premise of overall picture coordination, the more obvious the difference in hue of adjacent details, the better the clarity of printed matter. In order to make the layout of printed products clear, the levels of light, middle and dark tones should be clearly separated. This requires high quality of originals, good quality of separations, and specifications of imposition, copy, and printing. Accurate, at the same time, it is required to print overprint accurately, the dot increase value is small (controlled within the allowable value range), the relative contrast value is controlled within the standard range, taking printed fine products as an example, the K value of the yellow version should be controlled at 0.25 Between 0.35 and 0.35, magenta, cyan and black plates are controlled between 0.35 and 0.45; the yellow value of general product yellow plate is controlled between 0.20 and 0.30, and magenta, cyan and black plates are controlled between 0.30 and 0.40. In this way, the gradation and sharpness of the printed layout can be better expressed, and the copy quality of the product is also good.
3. Dot angle, enlargement value and overprint quality requirements. High-quality prints require clear dots, accurate angles, and no ghosting and moire. The expanded value of 50% of the dots of fine products is controlled between 10% and 20%; the expanded value of 50% of the dots of general printed products is controlled between 10% and 25%. Anilox overprinting must be accurate, and the contour and position of the layout image of multi-color overprinting should achieve more accurate registration. The overprinting tolerance of fine printing products is ≤0.10mm; the overprinting tolerance of general printing products is ≤0.20mm, This is also the basic requirement for quality control of screen printing.
4. Inspection requirements for color and appearance quality of printed products. To achieve the color reproduction of halftone dot printing, the effect of gray balance should be achieved, and the color of the printed product should be consistent with the original, with a sense of authenticity, naturalness and harmony. The allowable error of the solid density of different printed sheets of the same batch of products is: cyan and magenta ≤0.15, black ≤0.20, yellow ≤0.10. In order to ensure the quality of color reproduction, we should carefully control the quality of the original, the quality of separations, the quality of the ink, and the operating technology. The appearance quality of the printed product mainly refers to the appearance characteristics of the product, and the surface quality conditions that can be directly felt visually, such as ink stains, spots, pastes, flower patterns, scratches on the layout, ink color is too thick or light, and dirty Quality conditions such as printing registration, dimensional error, and consistency of plate tones. Printing products that meet the printing quality standards require the plate tones of adjacent plates to be basically the same without visual chromatic aberration. In addition, the layout of the plates must be accurate. The allowable error of fine products is ≤0.5mm, and the allowable error of general products is ≤1.0mm. In addition, the layout of the entire printed product must be clean and free from obvious dirt; the text on the layout must be complete and clear, and the position of the text and graphics accurate, which is the basic requirement for the quality inspection of the appearance of the screen printing plate.
Second, several elements of the quality inspection of the printed matter 1. Detection of the increased value of the dot. The so-called dot increase value refers to the difference between the clear dot area on the printed product and the effective dot area of the corresponding part on the original. Under the conditions of printing pressure, whether the pressure is normal or not, a certain degree of expansion of the printing dots will be inevitable. The performance of the tone, gradation, sharpness, and color of the printed image mainly depends on the effect of the process of overprinting and combining the dots in different areas of each color plate. Obviously, the dot arrangement and combination, overprinting are accurate and standardized, and the dot area expansion value is within the allowable range. The level, sharpness, and color rendering of the printed image are also good. On the contrary, the printing process effect is necessarily poor.
The calculation formula for detecting the increase in the network dots is:
(1) Dot area of printed products A (%) = 100 × [1-10- (Dt-D0)] / [1-10-(Ds-D0)]
Note: In the formula, D0 is the reflection density value of the non-printed part on the printed matter; Ds is the reflection density value of the solid on the printed matter; Dt is the reflection density value of the halftone dot part on the printed matter.
(2) Dot area of color separation sheet A (%) = 100 × [1-10- (Dt-D0)] / [1-10- (Ds-D0)]
Note: In the formula, D0 is the transmission density value of the blank mesh-adjusted film; Ds is the transmission density value of the solid on the film; Dt is the transmission density value of the film dots.
2. Control the relative contrast value. The relative contrast value is abbreviated as K value, which is an index for controlling the tone of the image. It refers to the ratio of the difference between the integral density of a certain dot area and the solid density between the solid density value and the middle to dark tone on the screen. The larger the ratio, the larger the K value, which ranges from 0 to 1. When calculating the relative contrast value, the solid density DV can be measured first, and then the dot integral density DR can be measured. Its calculation formula is:
The density of color prints is basically a logarithmic curve. The change in the coverage of the halftone dot area is linear from small dots to 100% halftone dots. The halftone dot area converted to density is a logarithmic curve, but it may not be a pure logarithmic curve during proofing or printing. Process conditions show that under the correct conditions of the original, if the curve is too dark, the color tone of the screen will have a deep, dull, gray, and dark visual sense, and its layering will be poor. In this way, the dot expansion value becomes larger, and the K value becomes smaller. On the contrary, if the curve is not convex enough, the color tone of the screen will have a light, thin, gray, and flat visual sense, so that the layout of the printed product will also lack a sense of hierarchy. , The dot expansion value is too small, and the K value exceeds the allowable range. Therefore, it can be said that controlling the K value is not only an indicator for controlling the intermediate tone of the image to a dark tone, but also can control the tone of the entire image to ensure the stability of the quality of the screen printing product.
3. Control the density value in the field. The process practice clearly shows that the solid density value on the printed sheet increases correspondingly with the increase of the thickness of the printing ink layer. When the printing ink layer increases to a certain thickness value, the solid density of the ink will also reach a maximum limit. Then, in this case, if you continue to increase the thickness of the ink layer, the solid density value will no longer increase, and the dot expansion value will increase significantly. This shows that as long as the solid density value is controlled, it is almost equivalent to controlling the thickness of the ink layer and the dot increase value, as well as controlling the tone value of the image. It can be seen that the measurement and control of the actual density of printed products is a crucial part of printing quality control. Due to differences in printing pressure, pad characteristics, paper, and ink, the actual density values will also vary. The maximum solid density values of the four colors Y, M, C, and B (K) are different on the same sheet of paper. On different types of paper, even with the same ink, the same machine, and even the same ink layer thickness for printing, the actual density values are not exactly the same. On the other hand, the density of the printed ink layer varies depending on the wet and dry degree. For example, just after the ink layer on the printed sheet is printed, the solid density value measured when the ink color layer is still in a wet state is higher, and when the ink layer is dried, the solid density value is lowered. This situation where the solid density value decreases as the ink layer dries is commonly referred to as the ink density dry back phenomenon. For papers with poor gloss, the dryness of ink density is more obvious. Therefore, as the density value of printing quality control is of great significance, the density of the solid sample should not be measured with the freshly printed sample, and the density of the solid sample should be measured with the dried sample to ensure the accuracy of the test data. As long as the solid density value of the ink color block on the sample is measured during the printing process, the thickness of the printing ink layer can be judged as appropriate. For this reason, it is necessary to accurately measure and control the density of the printed products in order to make the product quality control in the printing process better.
In summary, only by correctly understanding and mastering the items and elements of the print quality inspection, and through the control of process technology, can the print product quality be more reliably guaranteed. Only when the printed matter is scientifically and reasonably inspected correctly can the process quality level of the screen print be fundamentally improved to meet the customer's needs.