Formation and making mechanism of dot in the hotte

2022-08-14
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The formation and production mechanism of the midpoint of the printing manuscript

I. overview

the feature of the continuous tone manuscript different from the line manuscript is that the image intensity change of the continuous tone manuscript is continuous, without separation and grade. When printing, there are two ways to truly reproduce the image intensity changes on the continuous tone original: one is to use the point to obtain the halftone image to represent the image intensity level of the continuous tone original, such as relief printing, lithography and hole printing; The other is to use the holes with different depths on the plate cylinder to transfer the ink to the substrate, and the different thickness of the ink layer constitutes the depth of the imprint density, such as gravure printing. Of course, sometimes gravure printing still needs to be used. The purpose of gravure printing is to make the printing plate cylinder store the necessary ink in printing, and the produced line plays the role of supporting the doctor blade in printing, as shown in Figure 1

figure 1

printing reproduction of continuous tone originals includes color restoration, hierarchical reproduction and sharpness reproduction. All of them are realized through the transfer of points

as shown in Figure 2a, it is a monochrome continuous tone original. If there is no dot continuous tone negative without adding technology, when drying the printing plate, the light in the middle density part also reaches the printing plate through the film, so that the photosensitive film on the printing plate can be decomposed into hydrophilic blank areas, and the resulting printing plate is only divided into black and white, without intermediate tone levels. After printing, only the line original as shown in Figure 2B, also known as the block image original, can be obtained, The shades on the manuscript are missing. Only the originals such as PLA and flexible materials are processed, and the continuously adjusted originals are transformed into eye adjusted negatives. When printing, the spot part on the negatives is opaque, and the transparent part is completely transparent, so that the photosensitive film on the printing plate is fully decomposed, thus forming two parts on the printing plate: a hydrophilic and ink repellent empty white area and an ink repellent and water repellent graphic area. According to the size of the point area, the hierarchical changes on the original are reproduced, as shown in figures 2C and D. For the printing and reproduction of color continuous tone originals, it is also necessary to add processing to make each color plate become a eye tone printing plate to obtain eye tone prints

Figure 2 (A-D)

therefore, point is a tool for copying continuous tone images; Dot is the carrier of the changes in the gradation of printed matter; Dot plays the role of diluting ink diluent in printing, just like mixing color with oil in painting

second, the formation mechanism of points

in 1852, the British physicist w.h.fox.talbot took pendulum → hanging pendulum used an object similar to gauze as a screen to decompose the continuously adjustable image into large and small volume modulus: 2300 ⑶ 50 it is an important measuring tool to test whether the new product and the product meet the standard after production. The points with different 0mm and the same density of each point show different levels of depth changes through these points of different sizes, Thus, it pioneered the use of point reproduction to print monochrome or color continuous tone images

since then, the processing technology has gone through three development stages: glass screen processing, contact screen processing and electronic laser processing

1. Glass screen plus:

the glass screen (Figure 3) is marked with opaque vertical cross lines, and the edges of each square hole are clear. The screen is placed on the screen frame of the camera black box, parallel to but at a certain distance from the wet version coated with silver iodide. During the overtime, the light is projected onto the photosensitive wet plate through the hole on the screen. Due to the limitations of the color sensing performance of the photosensitive wet plate, the light forms dots of different sizes on the wet plate, so that the light and shade changes of the continuously adjusted image are transformed into dots of different sizes and equal spacing on the photosensitive wet plate. These dots will show the light and shade level of the image on the print after printing

figure 3

because the glass screen has many shortcomings, such as high cost, fragile, poor light transmittance, poor photosensitive performance of photosensitive wet plate, etc., it was later replaced by the contact screen made of photosensitive film, as shown in Figure 4

figure 4

(to be continued)

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