Printing is a technique of image reproduction. What is the image then? An image can be understood as an object rendered using color differences. For example, primitive humans use iron ore powder to draw pictures on rocks. The concept of images refers not only to photographs but also to all kinds of paintings and texts. Therefore, we can understand printing specifically as a technique that uses colors (such as ink) to reproduce images on printed materials (such as paper). This kind of replication process is done by printing plates in traditional printing methods. The characteristics of the plate are that the image area can accept and transfer ink, while the non-image part cannot accept ink. Printing is one of the four great inventions in ancient China. The invention of printing is an extension of the seal, dating back at least as far back as China's Sui Dynasty.
Plates, inks and printed materials are the three elements of printing. According to the method of printing plates for transferring images, we can classify traditional printing into four categories: letterpress, lithography, gravure and screen printing. The principle of letterpress printing is that the graphic part of the printing plate is convex, and the graphic part is not concave. The raised portion contacts the ink transfer ink roller during operation to receive the ink and transfer it to the print; the non-graphic portion cannot accept the ink and thus cannot transfer the ink. Therefore, the ink receiving portion and the inkless portion are formed on the object to be printed, and the image is formed by the contrast that they form on the object to be printed. Similarly, lithography uses the principle of mutual repulsion of oil and water to form a oleophilic graphic portion composed of resin and a hydrophilic non-graphic portion exposed without resin on the surface of an almost planar printing plate. The lipophilic portion accepts and transfers the ink, while the hydrophilic portion accepts moisture and repels the ink. The gravure printing plate is partially concave and can accept the ink, and the non-graphic ink on the printing plate surface is removed by a doctor blade or other means to form graphic and non-graphic parts. Screen printing is the use of missing printing principle. The graphic part of the printing plate composed of silk screen is hollowed out. The ink can be printed on the printed object through the hollow part through the pressure of the squeegee to form the ink image.
The ink image on the printing plate must complete the transfer of the ink image through the contact of the printing plate with the object to be printed, as well as the pressure inevitably existing during the contact process. Therefore, pressure is also an important factor in traditional printing methods.
In addition to the above-mentioned direct printing method in which the printing plate is directly brought into contact with the printed material to complete the transfer of the ink image, there is another way in which the printing plate does not directly contact the printed material, but the transfer of the ink image is performed through the transfer body. Indirect printing on the printed matter. Our common offset printing is an indirect printing method. Pad printing used for printing on irregular objects is also an indirect printing method. In addition, according to the printing plate movement during printing can also be divided into flat pressure and rotary printing; According to the transmission of printed materials can be divided into a single sheet and web printing; According to the number of a printable color can be distinguished Into monochrome and multi-color printing. There are also ways to distinguish print types based on different prints and different inks. This classification method is generally only used in the special printing, the basic printing principle is the above four categories.
Traditional printing is a mechanical copying method, while modern digital printing employs computer image processing techniques to control image copying techniques. The types of printers we often see are the output devices for such images. Modern digital printing technology also originates from digital printing technology. Laser and inkjet are common.
From the above description, we can know that the traditional printing method uses the ink image transferred by the printing plate to copy the image on the printed matter. Under this method of copying, the image is formed by "with" ink and "no" ink on the print. Therefore, the image formed by this method can only show two states: "have" or "nothing." That is, it can only be used to represent a single-bit image, such as a single-color text and a graphic composed of a solid-colored line-colored block, etc., and it is difficult to express a continuous-tone changing image having a rich level as in a photographic print.
The Munsell Color Theory believes that the various components that make up the color in the same area, regardless of hue, saturation, or brightness, will change color as long as one of them has a different proportion. So people found a way to express changes in the color level. Although the traditional printing method cannot change the composition of the ink image, it is also possible to change the level of the ink image in the unit area, and it is also possible to change the level of the monochromatic ink image. This is the halftone image replication technology. The following figure is a continuous adjustment of the gray ladder rule using the copy of the network.
From the bottom of the figure above, we can see the structural features of the image composed of dots. The size of the dot can be changed, but the dot density of the dot ink does not change, and each dot occupies the same area. That is, the frequency of occurrence of the network point is unchanged, and the amplitude of the network point changes with the change of the level. This is the so-called AM network. Of course, an image composed of dots does not always look like a continuously adjusted image. Because the hierarchical changes of the dot image are hierarchical rather than continuous. It's as if a person climbs a stairway. It does not follow a continuously changing slope, but climbs up a stairway in an equidistance. So we also call this level change the tone, or halftone. The meaning of halftone is that there should be a level between each adjacent tone. However, halftones in the middle of the copying of the image using the dot structure are lost, and only half of the hue is obtained, regardless of how many levels you use.
Under normal circumstances, as long as the image is reproduced with a gradation of 100 or more, the change in the image level appears continuous to the human eye. However, too small a tone will cause a slight loss, so the reproduction scale of the standard image is mostly at 256 levels. Secondly, because the outlets are too large to be seen directly by the human eye, the area occupied by each outlet is generally controlled within the normal range of the human eye. We usually use how many dots per inch can be arranged, that is lpi (line/inch) to represent the density of dots. This line is roughly around 150 lpi. Of course, the specific number of printing lines also involves paper and printing conditions. Fine products generally use 175 lpi; newspapers are printed at 100 to 120 lpi; corrugated carton printing and screen printing are roughly around 60 lpi. In addition, the angle of the dot arrangement is also a factor that influences the observation. In general, the 45° arrangement is considered to be the most beautiful, so the default angle of the dot arrangement for monochrome printing is mostly 45°. In addition to the usual circular points, the shape of the dots is also square, oval, diamond, and so on. The various types of outlets have their own advantages and disadvantages. When selecting, specific analysis should be made according to specific conditions.
We first come to know the printing of black and white images. The so-called black and white, in fact, refers to the ink that the printing ink left on the object to be printed. The contrast with the color of the object to be printed is not necessarily black and white. Or it can be called monochrome printing. We generally call R, G, B, or C, M, and Y colors in the image color as a color image, and the image colors that do not have the above hue are called non-color images. That is, there is no hue and saturation in the non-color image, only lightness. The reproduction of black and white images is based on this principle. Full understanding of the law of copying black and white images helps us to further understand the rules of copying color images.
In the process of transferring the ink on the upper part of the printing plate to the object to be printed, due to various reasons, such as the printing pressure inevitably existing when the printing plate comes into contact with the object to be printed and transferred, the transfer printing medium such as a blanket is in The deformation caused by the contact between the printing plate and the printing material, the absorption and penetration of the ink by the printing material, and the optical phenomenon of the ink. Finally, the ink transferred to the printing object is relative to the printing plate or the printing plate. The graphic part of the original film is deformed. This phenomenon is manifested in the dot-tone printing is the expansion of outlets. The rule is generally that regardless of the size of the outlets, the outlets are represented by the equidistant expansion of the edges. Reflected in the change in the image tone is the image of the high light area consisting of small points and dark areas composed of large dots, the density of the ink does not increase much, and the intermediate tone, especially at the 50% dot network dot increase value is the largest. We generally use two methods to express the law of dot expansion: First, according to the actual test data, from the high light small point to the largest dark tone large point, a curve is arranged on the coordinates, that is, the curve method; second is to take its network point The average value of the change. The following figure shows a 10% to 90% circular dot and its enlargement (peripheral shadow). Among them, from 50% of the outlets from the circle to the square, and take place with the adjacent network phenomenon. The shape of the dots in the future is represented by a combination of four adjacent dots.
Under normal circumstances, dot gain values ​​in offset printing average 8% to 12% in coated paper, 20% in offset paper, and more than 30% in newsprint. However, in the same type of offset printing presses, the same copperplate printing paper is used. The dot expansion value of the hard-liner and air-cushion blanket is 8%~12%; the neutral lining is more than 15%; and the soft lining is more than 20%. In the same way, the dot gain value changes due to improper control of printing pressure, ink volume, and ink balance during printing. The phenomenon of dot enlargement is mainly reflected in three aspects in printing control. One is the minimum point of highlights, because 1% or 2% of the small dots in the printing are actually not printed, while 2% or 3% of the small dots may jump to 5% or 7% due to the effect of dot gain. The second is the dark tone of the big idea. Due to the existence of network expansion, more than 85% of outlets have become 100% on the ground (that is, regions that are not composed of outlets). Third, since the value of the network expansion at 50% is the largest and the transition from a circular network to a square network at the same time, the tone of the tone may change significantly. The presence of these conditions shifts the entire reproduction tone curve to a dark tone during the printing process. There are no white spots (absolute nets) at high light spots, and dark spots are dotted with dots, making the rich levels contained in the dark tone not be displayed correctly. The overlap of dark spots at the high light spot and the dark spot at the dark spot are the main printing troubles in printing. The appearance of this situation indicates that the entire image copy is lost at the level of the highlight area or the shadow tone area, and the copy curve also shifts. The following figure 1 shows the appearance of high-light opacity and shadowing and paste at the same time in the image network. Figure 2 below shows the network structure of normal replication.
Correctly copied images should not be screened even in the brightest areas; the darkest places should also have anti-white features. Another factor that causes the image-reproduction curve to shift toward mid-darkness is because human vision does not feel the same for the highlights and shadows of the image. The hierarchical tones reproduced in equal density do not appear to be evenly distributed in human eyes. Therefore, the image must be brightened during the normal copying process. Although this will result in loss of the high light level, it will allow the dark tone level to be fully represented. Although the phenomenon of dot enlargement and the offset of medium-level shadow tone are mainly manifested in the printing stage, overcoming these problems is not a solution to the printing process. The fundamental method to solve these problems is to correctly set the GAMMA value of the white field black field and the copy curve of the image copy according to the actual printing situation in the prepress production stage, especially in the image scanning stage, and enlarge the dots displayed in the printing in advance Be small. Put the problems that arise in printing before printing. This is the basic principle of data standardized print control.
Color images are usually printed using four colors of cyan (C), magenta (M), yellow (Y), and black (K). The principle of color synthesis is subtractive color. We know that the three primary colors of the subtractive method or the three primary colors are cyan, magenta, and yellow. However, the three theoretical colors of the three primary colors used in printing have the same theoretical three primary colors. Especially magenta and blue. For example, three-color inks used in color mesh printing, there are mainly two kinds of products based on Japanese standards or European standards. Among the three standard inks of Japan, the hue of the magenta ink is approximately C23, M69 based on the C, M, and Y ratios; C63, Y29 are blue; C3, M4, and Y94 are yellow. Obviously, only yellow is ideal in three colors. The following figure shows the three primary colors R, G, and B of the additive method and the three complementary colors of Y, C, and M produced by the blending method; the three primary colors of C, M, and Y below the subtractive method.