Quality control in the most popular replication pr

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Quality control in the reproduction process

section 1 print quality and print image quality

first, the concept of print quality

print is a kind of goods and works of art that rely on visual arbitration. When people comment on the quality of printed matter, they always involuntarily associate three factors: aesthetics, technology, consistency and so on. This way of thinking is to consider human visual psychological factors and physical factors in reproduction engineering, that is, not only the commodity value or artistic level of printed matter, but also the impact of printing technology itself on the quality of printed matter. However, practice has proved that the technology of evaluating the quality of printed matter from the perspective of commodity value or art is not perfect. Such evaluation often cannot reliably express the reproduction quality characteristics of printed matter. Only from the perspective of printing technology, can we correctly evaluate the quality of printed matter and unify differences. This view has been endorsed by most experts at home and abroad

a · C · zettlemeyer and others once defined "the quality of printed matter"; The quality of printed matter is the comprehensive effect of various appearance characteristics of printed matter. From the perspective of printing technology, the so-called appearance characteristics of printed matter is a relatively broad concept, which has different connotations for different types of printing products

for lines or field prints, it should be required that the ink color is thick, uniform, glossy, the text is not flowery, the definition is high, the overprint accuracy is good, there is no through printing, excessive back projection, no back rubbing, etc

for color dot printing, it should be required that the gradation and color reproduction are faithful to the original, the ink color is uniform, the gloss is good, the dot is not deformed, the overprint is accurate, and there are no ghosting, through printing, various bars, back sticky dirt and mechanical traces

the comprehensive effect of these appearance characteristics above reflects the comprehensive quality of printing. In the evaluation of printing quality, various appearance characteristics can be used as the basis of comprehensive quality evaluation, and of course, they can also be used as the fundamental content and requirements of printing quality management

second, the concept of print image quality

the above concept of print quality has rich connotation and extension. For example, for books, binding quality should also be included in the scope of "print quality". However, it is often inconvenient and unnecessary to consider the quality of printed matter so widely from the perspective of printing image reproduction technology. G · w · Jorgensen and others pointed out that the above definition of print quality is not accurate enough. From the perspective of reproduction technology, they defined print quality as "fidelity to original reproduction". This definition method is convenient for the research of printing reproduction process and the evaluation of the quality of each stage of printing reproduction. Based on this consideration, this book distinguishes the quality of printed matter and the quality of printed image into two different concepts, that is, the quality of printed image is defined as "the fidelity of printed image to the reproduction of the original"

compared with the definition of print quality, the definition of print image quality narrows the scope of discussion. In this way, the print image can be regarded as a set of information of a certain amount of single pixels with light, dark and color changes on a two-dimensional or three-dimensional picture (note that words can also be treated as graphic information, so words can also be regarded as images). At best, the quality of printing image includes two aspects: image quality and text quality. Now, the characteristic parameters expressing image quality and text quality are described as follows. These quality characteristic parameters can be used not only as evaluation parameters in quality evaluation, but also as target parameters in quality management

1. Image quality characteristic parameters. It can be divided into the following four types: tone and color reproduction, image resolution, moire and other fault graphics and surface characteristics. The following is a description in this order

gradation and color reproduction refers to the gradation balance of the printed reproduction image and the color appearance corresponding to the original. For black-and-white reproduction, the density correspondence between the original and the replica is usually used to represent the gradient reproduction (replication curve). For color reproduction, hue, saturation and lightness values are more practical

the gradation and color reproduction ability of printed images are not only affected by the inherent characteristics of the ink used, printing materials and actual printing methods, but also often restricted by economic aspects. For example, in multicolor printing, high reproduction quality can be achieved by using high fidelity printing technology, but that will be at the cost of increasing costs. Therefore, for the printed matter with the picture as the theme, the so-called best reproduction of tone and color is to synthesize the requirements of the original theme within the various constraints and capacity limits of the printing device to produce the process and technology that most people think is a high-quality printed image when the gap between the left and right of the connecting part is greater than 0.5mm. The problem of optimal gradation and color reproduction will be described in more detail later

the problem of image resolution in optimal reproduction includes two aspects: resolution and sharpness. The resolution of printing image mainly depends on the number of EYELINES, but the number of EYELINES is restricted by the printing material and printing method. The number of EYELINES that human eyes can distinguish can reach 250 lines per inch, but in actual production, the highest number of lines is not always used. In addition, the resolution is also affected by the change of registration. Sharpness refers to the contrast on the edge of the tone. On the color separation machine, the sharpness of the image can be adjusted by electronic enhancement method. However, China is willing and able to work closely with Mongolia through China. People still don't know what the best level of clarity is. If you enhance too much, images such as landscapes or portraits will look inconsistent with reality, but images such as fabrics and mechanical products can improve performance and appeal

moire, bar, granular. Water stains and ink spots will affect the uniformity of image appearance. In point images, some moire patterns (such as rose shaped) are normal, but when the eye angle deviates, bad moire patterns will be produced. There are many factors that affect the graininess of the image. The smoothness of the paper and the sand mesh thickness of the printing plate are related to the graininess of the image. From a technical point of view, in addition to moire and particle patterns, people can make most other spots and fault patterns that cause nonuniformity close to another zero

the surface characteristics of printed images include gloss, texture and smoothness. The requirements for gloss depend on the nature of the original and the end use of the printed image. Generally speaking, when copying photographic originals, it is better to use high gloss paper. In actual printing, it is sometimes necessary to use bright oil to enhance the gloss of the theme image. High gloss will reduce the light scattering of the surface, thus enhancing the color saturation and darkness. However, when using high gloss paper to copy watercolor or pencil drawings, the effect is not very good. Using non coated paper or matte coated paper can produce better reproduction effect. The texture of the paper will damage the image to some extent, and the use of textured paper to copy photographic originals should generally be avoided. However, when using non coated paper to copy art works, the original texture of the paper will make the printed matter feel closer to the original

2. Character quality characteristic parameters. The definition of best text quality is very clear. They must be free from the following physical defects: ink blocking, broken characters, white dots, unclear edges, redundant ink marks, etc

the density of text image should be very high. In fact, the density of text image is limited by the thickness of printable ink layer. On coated paper, the maximum density of black ink is about 1.40 ~ 1.50; On non coated paper, the maximum density of black ink is 1.00 ~ 1.10

at that time, its market value will exceed 800million. The width of strokes and fonts should be consistent with the original fonts drawn by designers. The stroke and width of a font are also affected by the thickness of the ink layer. When the ink layer is thick, the deformation will be relatively large. Under a certain ink layer thickness, the deformation produced by small size words is much more obvious than that produced by large size words. In order to obtain the best reproduction effect, the change of stroke width should be kept within 5% of the specification set by the font designer or manufacturer; The character size should be kept within 0.025 ~ + 0.050mm of the original specification

Author: Liu Shichang article source: printing quality quantity and quality detection and control Chapter III quality control in reproduction process

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