Quality of printed matter is closely related to the quality of the original manuscript, process measures, printing machine performance, paper quality, and the technical competence of operators. Therefore, strengthening the quality inspection and control during the printing process is of great significance for improving the printing quality of products. So, how should the quality inspection of halftone screen printed matter be carried out? This is essential knowledge for the management and control of printing quality. The author will share some work experience on the quality inspection of halftone screen printed matter for readers' reference.
I. Understanding the Items and Requirements of Print Quality Inspection
Concept and Requirements of Gradation Values: Gradation in halftone printed matter refers to the density range of the image or the density difference of the image. The gradation of halftone printed matter is determined by the contrast of the effective maximum density of the paper's whiteness and the four-color overprint. Therefore, better printing adaptability results in a larger density range and better gradation reproduction. On the contrary, poor printing adaptability results in a smaller density range and poorer gradation reproduction. According to national industry standards, the standard for fine product field density is as follows: yellow 0.85-1.10, magenta 1.25-1.50, cyan 1.30-1.55, black 1.40-1.70. For general products, the standard is yellow 0.80-1.05, magenta 1.15-1.40, cyan 1.25-1.50, black 1.20-1.50. The highlight reproduction for fine products is 2%-4% dot area, while for general products, it is 3%-5% dot area.
Concept and Requirements of Gradation and Clarity of Printed Matter: Gradation refers to the brightness levels perceptible to the naked eye within the density range that can be reproduced, while clarity refers to the tone differences between adjacent details. To achieve clear gradation in printed matter, it is necessary for the levels of highlights, midtones, and shadows to be clearly defined. This requires high-quality originals, good color separation quality, accurate plate specifications, copy specifications, and exposure specifications, as well as accurate printing registration. For example, the K value of the yellow plate for fine printing should be controlled between 0.25 and 0.35, and for magenta, cyan, and black plates, it should be controlled between 0.35 and 0.45. For general products, the yellow plate K value should be controlled between 0.20 and 0.30, and for magenta, cyan, and black plates, it should be controlled between 0.30 and 0.40. This ensures a better presentation of the hierarchical sense and clarity of the printed matter.
Requirements for Dot Angle, Enlargement Value, and Overprinting Quality: High-quality printed matter requires clear dots, accurate angles, and no phenomena of moiré or rosette. The enlargement value of 50% dots for fine products should be controlled between 10% and 20%, while for general printed matter, it should be controlled between 10% and 25%. Dot overprinting must be accurate, and the contour and position of the color image in multicolor overprinting should be relatively accurate, with an allowable error for fine printing of ≤0.10 mm and for general printing of ≤0.20 mm. This is a basic requirement for the inspection of halftone screen printed matter.
Requirements for Color and Appearance Quality of Printed Matter: Achieving color reproduction in halftone overprinting requires the realization of a gray balance. The color of printed matter should match the original and have a realistic, natural, and coordinated appearance. The allowable error for the field density of different sheets from the same batch is: cyan and magenta ≤0.15, black ≤0.20, yellow ≤0.10. To ensure color reproduction quality, careful control is required in terms of original quality, color separation quality, ink quality, and operational process technology. The appearance quality of printed matter mainly refers to the visual characteristics of the product, such as ink stains, spots, plate sticking, plate flower, plate scratch, excessively dark or light ink, smudging, accurate printing registration, dimensional errors, and consistent plate tone for adjacent pages. For halftone screen printed matter, the allowable error for plate registration of fine products is ≤0.5 mm, and for general products, it is ≤1.0 mm. The overall appearance of the printed matter must be clean, without obvious dirt, and the text on the plate must be complete, clear, and accurately positioned. These are basic requirements for the inspection of halftone screen printed matter.
II. Key Elements in the Quality Testing of Halftone Screen Printed Matter
Detection of Dot Enlargement Value: The dot enlargement value refers to the difference between the clear dot area on the printed matter and the corresponding effective dot area on the original plate. Under the action of printing pressure, the dot will inevitably enlarge to some extent, regardless of whether the pressure is normal. The quality of the printed image in terms of gradation, levels, clarity, and color is mainly determined by the process effect of the combination and overprinting of dots in different areas of the four-color plates. Clearly, accurate dot arrangement, precise overprinting, and standardized dot area enlargement within the allowed range will result in good reproduction of the image's hierarchy, clarity, and color effects. Conversely, poor control of dot area enlargement will lead to inferior printing process effects.
The calculation formula for detecting the dot enlargement value is as follows:
Control of Field Density Value: The field density value on the printed sheet increases with the increase in ink layer thickness. When the ink layer thickness reaches a certain value, the field density of the ink will reach a maximum limit. Therefore, controlling the field density value is equivalent to controlling the ink layer thickness and dot area enlargement value, which also means controlling the image's gradation value. Different types of paper, different printing pressures, different cushion characteristics, and different inks will result in different field density values. Therefore, it is crucial to accurately measure and control the field density value of printed matter to ensure good control of the printing process's product quality.
In conclusion, only by correctly understanding and mastering the items and elements of print quality inspection and through careful control in terms of process technology can the quality of printed products be reliably guaranteed. Scientifically and reasonably inspecting printed matter correctly is fundamental to improving the technological quality level of halftone screen printed products and meeting the usage needs of customers.