Introduction to the basics of printing network
Abstract: Introduction to printing machinery knowledge - characteristics - classification chapter: the article mainly introduces an overview of the printing dot, dot formation mechanism, dot size, dot deformation causes and other related knowledge points.
Introduction to the basics of printing network
The feature that distinguishes continuous tone originals from line originals is that the image intensity changes in continuous tone originals are continuous, non-interval and non-hierarchical. In printing, to truly reproduce the image intensity changes on the continuous tone original, there are two methods: one is to use the dot to obtain a halftone image to show the image intensity levels of the continuous tone original, such as letterpress, lithography and hole printing; the other is the use of different depths of the cave on the plate cylinder, the ink is transferred to the substrate, the thickness of the ink layer to constitute the depth of the density of the print, such as Gravure printing. Sometimes, of course, gravure printing also still makes use of dots. The purpose of gravure printing plus the screen is to make the plate cylinder in the printing of the necessary ink reserves, the production of the screen line in the printing to support the role of the squeegee, 1 shown.
The printing reproduction of continuous tone originals includes color reproduction, level reproduction and clarity reproduction. All three are achieved through the transfer of dots.
2a is a monochrome continuous tone original manuscript, if not after screening technology, no dot continuous tone Yang figure negative, in the sun printing plate, the middle density parts also have light through the soft film to reach the printing plate, so that the photographic film on the printing plate see light decomposition to form a hydrophilic blank area, thus obtaining the printing plate is only black and white, and no intermediate tone level, after printing can only get 2b lines original manuscript, also known as the original block image The original is lost, the original on the intensity of the light level. Only the original manuscript for the screening process, the continuous tone of the original manuscript into the mesh tone of the negative, the plate, the negative on the dot part is not translucent, the transparent part of all translucent, so that the plate on the photographic film layer fully decomposed, thus forming a hydrophilic ink repellent blank area and pro-ink repellent graphic area on the printing plate two parts. Depending on the size of the dot area, the reproduction of the original on the level of change, 2c and d shown. For the printing reproduction of color continuous tone originals, it is also necessary to go through the screening process so that each color plate becomes a mesh tone printing plate, to get mesh tone printing products.
Therefore, the dot is a tool to replicate the continuous tone of the image; dot is a carrier of the changes in the thickness of the print; dot in printing plays a role in diluting the role of ink diluent, as if painting with oil blending colors.
二、 the formation mechanism of the network
In 1852, the British physicist W. H. Fox TalBot used an object similar to gauze as a screen to decompose the continuous tone image into dots of different sizes and the same density per dot, and through these dots of different sizes to show different levels of shades, thus pioneering the use of dots to reproduce the printing of monochrome or color continuous tone images.
Since then, screening technology has gone through three stages of development glass mesh screen screening, contact mesh screen screening and electronic laser screening.
1．Glass mesh screen plus mesh:
The glass screen (Figure 3) is etched with opaque vertical crosses, and each square mesh has a clear edge. The screen is placed on the screen frame of the camera dark box, and the wet plate coated with silver iodide is kept parallel but separated by a certain distance. When screening, light is projected through the mesh holes on the screen onto the photographic wet plate, due to the limitations of the color-sensitive performance of the wet plate, the light is sensitized on the wet plate to form dots of different sizes, so that the light and dark changes in the continuous tone of the image into a photographic wet plate of varying sizes, equally spaced dots, these dots will be presented after printing the image on the print light and dark levels.
As the glass screen screening has many shortcomings, such as: high cost of glass screen, fragile, poor light transmission, light-sensitive wet version of the poor light-sensitive performance, etc., later replaced by a contact screen made of light-sensitive soft film, 4 shown.
2．Contact screen screening:
Contact screen and light-sensitive soft film attached together, placed in the camera dark box on the photographic frame (contact screen "contact" three words from this), exposure, light through the screen on the light-transmitting holes in the photographic film exposure imaging. As the screen on the edge of the net hole vignetting, so that the exposure point in the middle of the exposure, exposure to the surrounding area gradually reduced, and the light-sensitive properties of the photographic film, thus forming a varying size, equal spacing of the dot, 5 shows.
3、Electronic laser screening:
Whether glass screen screening or contact screen screening, are done manually; and each color version of the screen angle are different, the workload is large; screen type is limited, the application of special dots is limited.
With the development of electronic color separation machine, the emergence of the dot computer so that electronic color separation can be based on the intensity of the scanning input light, the digital signal obtained will be sent to the dot computer of the electronic color separation machine image output recording system, and through the comparison loop, the dot computer calculates the size of the dot, together with the input of the dot shape, dot angle, the number of lines added to the network address instruction control laser recording system control signal and added to the The optical modulator, so as to control the working state of each photoelectric modulator, and finally the light signal controlled by the modulator in the electronic color separation machine output roller light-sensitive soft film scanned out one in line with the original requirements and design requirements of the production of dots (Figure 7).
Advantages of electronic laser screening:
(1) The dot is solid, high density, good precision and clear dot edge.
(2) Each mesh-tuned dot is composed of many tiny dots. The number of these dots can be determined by the resolution of the output system, such as laser phototypesetting machine, and therefore, can meet the high definition, rich layers, good reproducibility of fine level image reproduction. 8 shows a 50% mesh tone dot with 40 sub-dots, the output resolution of the laser phototypesetting machine is 1270 dots / inch.
(3) can produce different shapes of dots at different orders, easy to control the dot expansion. In each color version can get different mesh angle, different screening line number of dots, easy to control the quality of printing.
Printed images of changes in the intensity of the level is reflected by the amount of reflected light to reflect the light on the paper, the inking area to absorb light, the blank area to reflect light to the human eye. Large dot inking area on the print, absorbing more light, reflecting less light, optical density is low, making people feel gray, that is, dark or low tone area; small dot blank area is large, absorbing less light, reflecting more light, optical density is high, making people feel bright, that is, bright tone area or high tone area. Medium-sized dot area is the middle tone area or middle tone area. A variety of different sizes of dots, according to the different proportions of light absorption and light reflection, the performance of a variety of different levels of tone (Figure 9).
Usually, people use the dot coverage to indicate the size of the dot. Dot coverage is the percentage of the area occupied by the dot in the unit area, so also called the dot percentage. In China, it is customary to "into" to indicate the size of the network (Figure 10). In general:
Field network coverage of 100%, that is, 10 into the network; 95%-99% of the network is called a small white dot; 1%-5% of the network is called a small black dot, also known as the very high light area; 5%-10% of the network is called the high light area; 10%-30% of the dot is called the bright tone area; 30%-60% of the dot is called the middle tone area; 70%-95% of the dot is called the dark tone area.
Printing, if the dark tone area can show 95% and more of the network, it is a fine product; if 90% of the network printed into the field, but 85% of the network is still clear, it is a better product; if 80% and more than the network are printed into the field, can only be called general products; if 70% of the network is also generally no two with the field, the product is not qualified. These are the requirements of the darker part of the print.
Based on the human eye is more sensitive to a bright tone part than to the dark tone part, so the bright tone part of the printing reproduction requirements are higher. Generally, super-fine products require printing 0.5% of the dot; fine products require printing 1%-4% of the dot; just 5% of the dot can be printed is a better product; if you can only distinguish 10% of the dot for general products. The finer the dot printed in the bright tone area, the better.
2．Deformation of network point:
The ideal situation is: when the original is screened to obtain the mesh tone printing plate, the light and dark levels on the original are accurately converted into the corresponding size of the dot coverage; when the printing plate is inked to obtain the print, the size of the ink dot can fully reproduce the image tone level, and the dot as the image carrier in the whole process requires neither expansion nor reduction.
However, in practice, the dot, when going through a series of processes such as screening, sheeting, copying, developing, solarization, printing, etc., is bound to be accompanied by an enlargement or reduction of the area percentage, which affects the reproduction of the image tone level, the composition of the color and the guarantee of clarity (Figure 11).
Dot deformation is divided into visual deformation, optical deformation and mechanical deformation.
The so-called visual deformation refers to a circle of white paper from the light to the edge of the dot, only 10% of the white light is reflected to the human eye, the remaining 90% of the white light is absorbed, we feel a circle of gray, and thus visually feel the expansion of the dot (Figure 12). Visual dot enlargement is objective and equal for all prints and all dots.
The so-called optical deformation refers to the scattering of light through the base of the soft film when copying and drying, so that the dot in the transfer of the deformation. When a negative image is copied into a positive image, the dot is enlarged and the small white dot is lost; when a positive image is printed, the dot is reduced and the small black dot is lost. This is also difficult to avoid the traditional plate making.
The so-called mechanical deformation refers to the printing process, due to printing pressure caused by the ink droplets to the blank part of the extrusion; or due to the compression of the blanket deformation caused by the network in the transfer of the slip; or due to the use of fountain solution, resulting in the spread of ink droplets around; or due to paper deformation, resulting in radial and axial extension of the network. Mechanical deformation caused by the dot deformation to dot expansion, and, machine printing can not avoid a certain amount of dot expansion.
In the actual printing process, the operation of the irregularities will exacerbate the deformation of the network. Such as exposure time and development time is insufficient or excessive; developer concentration is too large or too small; printing workshop temperature and humidity is too high or too low; printing pressure is too large or too small; field printing ink layer is too thick; rubber blanket compression deformation is too much; fountain solution with ink and PH value does not meet the requirements and so on.
Statistics show that: under the same circumstances, the amount of dot deformation and dot circumference is proportional. Therefore, (1) the percentage of each dot, the number of 50% of the dot perimeter is the largest, that is, 50% of the expansion rate of the dot at the top of the dot coverage. (2) The perimeter of the square dot is greater than the circular dot and other shapes of the dot, that is, the expansion rate of the square dot is greater than the circular dot and other dots. Therefore, print quality inspection is commonly used when 50% of the square dot to determine the expansion of the network. (3) ink and all paper combinations, dot expansion value with the roughness of the paper surface increases year-on-year, the dot expansion value of newsprint most people, coated paper dot expansion value than offset paper is small. (4) cyan version of the expanded value of the network than the other three color version of the expanded value of the network of about 3% a 4%.
In practice, on the one hand, we must strictly operate the specifications and various values of control, on the other hand, can also borrow the adjustment of the scale level curve to properly compensate for the deformation of the network.
Printed images of the reproduction of the tone level depends on the quality of the dot reproduction. The dot edge is not complete, it can not complete the dot percentage of expressiveness; dot printing is not real, it can not absorb enough light, dark tone is not black; dot expansion, dark tone increase, light tone midtone decrease, the print is dark show "boring"; dot reduction, light tone increase, dark tone is not enough, the print appears " flat floating"; ignore the middle tone, highlight the bright tone and dark tone, although the print "up" but "empty"; highlight the middle tone, ignore the bright tone and dark tone, the print "light ".
The ideal image reproduction requires the original and the density of the print to meet the 1:1 correspondence.
But in practice, this ideal level of reproduction curve is impossible to achieve, and sometimes may not need. Because (1) the print is a product of visual effect, to meet certain requirements of people subjectively, 14 shown to enhance the artistic appreciation of the print through the enhancement of the fine details of the antique;
(2) from the original to the print of the various image transfer process efficiency will be subject to a variety of objective conditions, such as the color-sensitive properties of the soft film, plate quality, exposure conditions, development effects, printing pressure, ink characteristics and paper characteristics. Therefore, the density range of the original manuscript is generally greater than the density range of the printed copy, such as the density of the original manuscript is generally 0.05 a 3.0 (or even 4.0), the density contrast can be 2.95 and greater, while the maximum density range of the current stage of printing is 0.00-1.8, the density contrast is 1.8013. Based on this, in the printing reproduction process, the original manuscript must be compressed to fit the level of the requirements of the printed product.
The problem is that the compression of the order level will inevitably cause the level and level, lost, the image effect is not good. Therefore, in practice, depending on the type of original, customer requirements, and reproduction process, the level curve should be flexibly adjusted to emphasize the main part, lose the secondary part, highlight the subject, and ensure the overall reproduction effect of the image.