Definition index and test method of speckle in the

2022-08-21
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The definition index and test method of speckle in field printing

field printing is very important, and almost nothing is more eye-catching than a rough field. The roughness formed by bright and dark spots in the printing field is called speckle. The factors that cause this phenomenon may be ink, paper, printing conditions or a combination of these conditions

I. what is speckle

speckle refers to the unexpected and changing light reflection generated in a printing area with uniform tone, which is in the form of fuzzy cloud, granular or other regular patterns. Generally, there are no stripes in the text printing, and it is difficult to find them in the thick printing patterns, especially in the flat printing areas, such as the sky or other backgrounds with uniform colors

it seems concise enough to describe speckle as uneven reflection, but it cannot reflect (indicate) the complex process of light reflection and human eyes' perception of uneven light reflection

the first consideration is the light reflection of the printing surface. When we observe a flat and dense print, we think that light is reflected from the surface. But this is not true. The surface of any object, including metal, plastic or paper, is rough and full of pits and holes. The primary function of fillers such as paint and ink is to fill some potholes. When part of the light enters the surface of the sample, some of it is absorbed by the substrate and converted into heat. Other light is reflected on the surface for many times to form a smooth surface, and some of it passes through the paper sample. In fact, some light passes through ink, paper and other objects behind the paper before reaching our eyes

the density or reflectivity of ink and paper, and even the change of background material will produce obvious speckle patterns. The geometric shape and physical properties of the reflected light on the paper will also have a significant impact on the speckle phenomenon. We generally believe that the paper and printing surface are diffuse reflection sources, which reflect light in all directions, regardless of the incident angle. But the surface of any object cannot achieve complete diffuse reflection. Both paper and printed patterns diffuse the reflected light, which has the function of mirror reflection, and reflects part of the light at a predictable angle. Generally, the incident angle is the same as the reflection angle, and the direction is opposite. Small fluctuations, wrinkles, marks and other surface characteristics on the paper surface will significantly affect the reflection of light, showing varying reflectivity. Surface characteristics can even produce horn reflection source effect. In particularly obvious areas, the density of light is enlarged. It can be seen that many factors can form speckle phenomenon, many of which are not directly related to the printing adaptability of printing materials to inks

second, the types of printing stripes and the reasons for their formation

if we limit our discussion to the impact of changes in the reflectivity of the printing surface, there will still be many changing factors in the printing process that can form stripes. Paper, ink, printing method and other printing problems will form stripes, which are the result of many factors in most cases. The evenness and surface characteristics of paper form the most basic form of stripes. The changes of density, surface smoothness, fiber content, dust, filler content, sizing degree, surface pH value and specific vitality of paper will affect the ability of ink transfer to the printing surface and its absorption, penetration and drying characteristics. Because evenness also affects the content of impurities and additives, the change of fiber distribution (evenness) in the paper is usually the root cause of the above changes, and poor evenness will have a negative impact on the diffuse reflection coefficient and opacity

Sandreuter defined three kinds of stripes produced by (offset) printing process in 1994

1. The peeling speckle on the back is caused by the poor transfer ability of the ink from the blanket to the paper. The factor of type is that the ink is unstable or the solvent/grease separates from the pigment too quickly. Any change in ink transfer rate will form stripes. Overprint markings on the back usually appear in the first printing color, which will also be caused when the inappropriate pH value and viscosity value of the ink are selected

2. water interference stripes will be formed when the paper does not absorb the ink carrier and reduces the amount of ink transfer. The causes are generally due to excessive water use, wrong ink formula, incorrect mixing of carrier or high alcohol content

3. wet stick ink speckle when the order of viscosity and pH value between each ink is not selected, the speckle will be formed after the second printing. However, when these inks are printed separately, there will be no speckle

finally, due to the wrong installation of the printing plate, the elasticity of the printing roller, the driving gear marks, grooves and other factors, there will also be speckle phenomenon. The ink transfer function is irregular, but it tends to work only on some inks. The random process of speckle produced by the surface characteristics of paper will affect all inks. The changes caused by printing usually occur predictably and regularly, which helps to limit it to a certain frequency range

III. visible printing stripes

due to the definition of stripes and the aforementioned factors that can cause stripes, it is difficult for us to objectively obtain a visual grade of stripes

another point needs to be discussed: should obvious mechanical factors such as wrinkle marks, wrinkles, pinholes, etc. be regarded as stripes

answer yes from the final printing result or the position of the printer; Considering the quality of raw materials, the answer of the paper mill is yes; And the ink factory is no; From the results of the comparative experiment, the answer should be No

IV. determination of speckle

from an image analysis diagram, the phenomenon of speckle we think is affected by at least two independent factors, and its process seems to work simultaneously in a nonlinear way, and finally gives us the feeling of speckle. Based on our definition of speckle, assuming that we divide the printed image into dark areas (lower than the average reflectivity) and bright areas (higher than the average emissivity), speckle is the amount of change between these two groups of images. For the sake of clarity, we identify those areas in the image that are brighter or darker than the average background area as spots. Sometimes these areas are also called smearing, caking, low quantitative areas (of fibers and flocs), etc

in any case, these spots can be described in two ways: size and reflectivity. We can feel the difference between bright spots and dark spots and their surrounding areas from the image. The size and contrast of these spots are called the particle morphology of speckle pattern

we notice that when the brightness decreases, our sense of speckle, or the severity of speckle, also decreases. For example, if we extend this trend further in the maintenance method of the main machine of the universal testing machine, assuming that the contrast of bright spots and dark spots is reduced to low enough, or the size of spots is reduced to small enough, we will not feel the appearance of spots. Similarly, assuming that the area of the spots is large enough to wrap around the entire printing area, so that our eyes do not feel that they are partitioned, we will not call it speckle. This is the actual situation - all printing areas have a certain level of speckle, but because of the small area or low contrast, we can't feel the appearance of speckle. Therefore, when the pair is relatively low, the size of the bright area and spots is not important. In other words, when the area of the spots is small, the eyes feel that the spots are integrated with the whole printed content. For example, a mid tone pattern is considered a brighter color. Therefore, the size and contrast of spots are the direct factors that affect human vision

v. test methods for determining speckle

Dutch IGT company has developed some methods for determining speckle by using the printing suitability instrument produced by it. The peel speckle test and wet roughening wet resistance speckle test are carried out on the aig2-5 printability tester, and the peel speckle test can also be realized on the CI printability tester. Wet drawing temperature resistance test has been specially discussed in another article

principle

print the paper to be measured with special speckle measurement inking. When using the first method, transfer the ink on the printed test strip to a clean printing plate. In the test, four transfers were made, each time with a clean printing disc

due to the irregularity of paper synthetic fiber replacing 20000 tons of imported products to absorb ink, the ink that has not been completely absorbed will be transferred to the printing disc. With this method, the performance effect of speckle is strengthened, making the result evaluation easier

the second method is to print one inked printing disc on five clean strips of paper, during which the printing disc does not need to be inked again. This method is faster, but the test accuracy is poor

the first method

the 50mm blanket printing disc is inked with speckle measuring ink by conventional methods. Print the test paper on aic2-5. No padding is used for printing. The speed is 0.2m/s, and the pressure is 500N. After printing for 10 seconds, use another clean 50mm blanket printing disc to print again at 500N. The ink on the paper will be transferred to the printing plate. Every 10 seconds, print on the next clean printing disc. A total of 4 times of stripping

the test can also be carried out on C1. The width of the printing disc is 35mm and the pressure is 450N. Not all types of paper and paperboard are suitable for C1 test, and the results cannot be compared with those obtained on aic2-5. Please pay attention

the amount of ink used is selected according to the type of paper and paperboard, but it is generally recommended to use 1cm3 when using aic2-5 and 0.3cm3 when using C1

the second method

inking 50mm printing disc, and then printing on aic2-5 at 0.2m/s, 625n. After 10 seconds, change a piece of paper and reprint it. Print it five times with clean paper, with an interval of 10 seconds each time. If the C1 type is used, the pressure is set to 450N, and the printing ink is 0.3cm3 on aic2-5 and 0.5cm3 on C1 with an image cloth printing disc

evaluation of test results

the evaluation method is generally compared with prefabricated spline printing strips that represent a certain printing quality

this evaluation method can only be used by the user. Unless the comparison samples can be unified or the speckle index table can be adopted, data cannot be exchanged with other units

speckle index is based on two change conditions that affect the evaluation of speckle: the perimeter of bright spots and dark spots and their density contrast. The larger the area and contrast of the spot, the easier to observe. Using the scanner, the specificperimeter of bright and dark spots can be measured and expressed, and the density change can be calculated by the variable coefficient at the same time. The Asia Pacific Innovation Park Phase 2 project

(coefficient, ofvariables) has been started

through the above two variables, the speckle index is obtained:

when the variable coefficient is high (large contrast) and the effective perimeter is small (large spots), the speckle index increases, which is consistent with the result of visual evaluation when checking whether the oil hole of the buffer is blocked

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