Definition of printed markings, marking index and test method
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Field printing is critical, and there is hardly anything more eye-catching than a rough field. The roughness formed by bright spots and dark spots in the printed field is called a streak. The factors that cause this can be ink, paper, printing conditions or a combination of these.
First, what is the marking
Zebra refers to unintended, varying light reflections that occur in a uniform hue of printed areas, in a vague cloud, grain or other regular pattern. Generally, no markings are seen in text printing, and it is difficult to find in printing "thick" patterns, but particularly in areas where printing is relatively flat, such as in the sky or other uniform background.
Depicting the streaks as uneven reflections seems to be concise, but does not reflect (indicate) the complex process of light reflection and the uneven reflection of light from the human eye.
The first consideration is the light reflection of the printed surface. When we look at a flat, dense print, we think that light is reflected from the surface. But this is not the real situation. The surface of any object, including metal, plastic or paper, is rough, rich in potholes and small holes. The primary role of fillers such as paints and inks is to fill a portion of the potholes. When part of the light is incident on the surface of the sample, part of it is absorbed by the substrate material and converted into heat. The other light is reflected on the surface to form a smooth surface, and a part of the light passes through the pattern. In fact, some of the light reaches our eyes after passing through the ink, paper, and other objects behind the paper.
Ink and paper density or reflectivity and even changes in the background material will produce a noticeable pattern of markings. The geometric shape and physical properties of the reflected light on the paper also have a significant effect on the marking phenomenon. We generally believe that the paper and printed surface are diffuse sources that reflect light in all directions regardless of the angle of incidence. But the surface of any object cannot achieve complete diffuse reflection. Both the paper and the printed pattern diffusely reflect the incident light and have a mirror reflection that reflects a portion of the light at a foreseeable angle. Generally, the incident angle is the same as the reflection angle and the direction is opposite. The small undulations on the surface of the paper, wrinkles, marks and other surface characteristics can significantly affect the reflection of light, showing a varying reflectivity. The surface characteristics may even produce a "corner reflection source" effect, and in a particularly pronounced area, the density of light is magnified. Thus, many factors can form streaking, many of which are not directly related to the printing suitability of the printed material for the ink.
Second, the types of printed markings and the reasons for their formation
If we limit the effects of changes in reflectivity only on the printed surface, there are still many variations in the printing process that can form streaks. Paper, ink, printing methods, and other printing problems can form streaks, and in most cases are the result of a combination of factors. The uniformity and surface characteristics of the paper form the most basic form of the marking. The density, surface smoothness, fiber content, dust, filler content, sizing degree, surface pH value and paper specific activity will affect the ink to the printing surface. The ability to transfer and its absorption, penetration and drying characteristics. Since the uniformity also affects the content of impurities and additives, the internal fiber distribution (meanity) of the paper is usually the source of the above changes, and poor uniformity adversely affects the diffuse reflection coefficient and opacity.
In 1994, Sandreuter defined three types of markings produced by the (offset) printing process.
1. The back peeling streaks are caused by the deterioration of the transfer ability of the ink from the blanket to the paper. The type factor is that the ink is unstable or the solvent/grease is too detached from the pigment. Any change in ink transfer rate will form a streak. The backside imprinting usually appears in the first ink, which is also caused when an inappropriate pH and viscosity value of the ink is selected.
2. Water Interference Patterns Water interference streaks are formed when the paper does not absorb the carrier of the ink and reduces the amount of ink transferred. The cause is generally due to excessive water, incorrect ink formulation, incorrect carrier mixing or too high alcohol content.
3. Wet-bonded ink markings When the order of viscosity and pH between each ink is chosen incorrectly, the second printing will form a marking after printing. However, these inks do not appear to be streaked when printed separately.
Finally, streaking can occur due to plate mounting errors, the elasticity of the printing roller, the drive gear marks, the grooves, and the like. The ink transfer function is irregular, but there is a tendency to act only on certain inks, and the random process of the speckles generated by the surface characteristics of the paper has an effect on all the inks, and the change due to printing is usually predictably It happens regularly, which helps to limit it to a certain frequency range.
Third, visible print markings
Due to the definition of markings and the aforementioned markings, it is difficult to objectively obtain a visible level of marking.
There is another point to discuss: should obvious mechanical factors such as wrinkles, wrinkles, pinholes, etc. be considered as markings?
The answer is “yes” from the final printing result or the printer's position; considering the quality of the raw materials, the paper mill's answer is “yes”; the ink factory is “no”; from the comparison of experimental results, the answer should be “no”. .
Fourth, the determination of markings
From an image analysis diagram, in what we think of the phenomenon of markings, at least by two independent factors, the process acts in a non-linear way at the same time, and finally gives us the feeling of marking. Based on our definition of speckle, suppose we divide the printed image into a dark region (lower than the average reflectance) and a bright region (above the average for the luminosity), and the zebra is the amount of change between the two sets of images. For the sake of clarity, we identify those areas of the image that are brighter or darker than the average background area as "spots." Sometimes these areas are also referred to as smearing, agglomeration, low-quantity areas (fibers, flocs), and the like.
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 on the image and its surrounding area. The size and contrast of these spots, we call the "particle morphology" of the striated print.
We noticed that when the brightness is lowered, our feeling of the markings or the severity of the markings is also reduced. If we extend this trend further, assuming that the contrast between the bright spots and the dark spots is reduced sufficiently, or the size of the spots is reduced sufficiently small, we do not feel the appearance of the streaks. Similarly, it is assumed that the area of the plaque is large enough to wrap around the entire printed area so that our eyes do not feel that they are segmented, and we will not call it a streak in this case. The actual situation is the same - all printing areas have produced a certain level of markings, but because of the small area of the grain or low contrast, we do not feel the appearance of markings. Therefore, when the contrast is low, the size of the bright area and the spot is not important. In other words, when the area of the spot is small, the eye feels that the spot is integrated with the printed content of the whole piece. For example, a halftone pattern is considered a brighter color. Therefore, the size and contrast of the spot are direct factors affecting human vision.
Fifth, Test method for determining speckle
The Dutch IGT company developed a method for determining the markings using the printability tester produced. The peeling and wet-sweet wet-resistance test was performed on the AIG2-5 printability tester, and the peeling test was also performed on the CI type printability tester. The wet pull temperature resistance test has been discussed in another article.
principle
The paper to be measured is printed with a special speckle measurement ink. When the first method is used, the ink on the printed test strip is transferred to a clean printing plate. The test was transferred four times, each time using a clean printing plate.
Due to the irregularities of the ink absorbed by the paper, the ink that is not completely absorbed is transferred to the printing plate. With this method, the performance of the markings is enhanced, making the evaluation of the results easier.
The second method is to print an inked printing disc onto five clean strips, during which the printing disc does not need to be inked. This method is quicker but the test accuracy is poor.
the first method
A 50 mm blanket printing disk was inked with a zebra measuring ink by a conventional method. The test strip was printed on AIC 2-5. No padding is used for printing. The speed is 0.2m/s and the pressure is 500N. After printing for 10 seconds, another 50 mm blanket printing disc was used to print again at 500 N. The ink on the strip is transferred to the printing tray. Print again on the next cleaned printing plate every 10 seconds. Stripped 4 times in total.
The test can also be carried out on C1 with a width of 35 mm and a pressure of 450 N. Not all types of paper and board are suitable for testing on C1, and the results are not comparable to those obtained on AIC 2-5. Please note.
The amount of ink used is selected according to the type of paper and paperboard, but it is generally recommended to use 1cm3 for AIC2-5 and 0.3cm3 for C1.
The second method
A 50 mm printing plate was inked and then printed on AIC 2-5 at 0.2 m/s, 625 N. After 10 seconds, change a piece of paper and reprint it. Use a clean strip of paper for 5 times, 10 seconds apart. If the C1 type is used, the pressure is set to 450N, and the ink is 0.3 cm3 on the AIC2-5 and 0.5 cm3 on the C1.
Evaluation of test results
The method of evaluation is generally compared to prefabricated spline prints that represent a certain print quality.
This method of evaluation can only be used by the user. Data cannot be exchanged with other units unless the comparison sample can be unified or the zebra index table is used.
The plaque index is based on two conditions that affect the assessment of the marking: the perimeter of the bright and dark spots and their density contrast. The area and contrast of the spot are large and easy to observe. With the scanner, the effective perimeter of the bright dark spot can be measured and expressed, and the density change can be calculated as Coefficient, Of Variables.
Through the above two variables, the speckle index is obtained:
At high coefficient of variation (large contrast) and small effective circumference (large spots), the speckle index increased, which is consistent with the results of the visual assessment.
The relationship between the quality of the print and the parameters associated with the patch index can be expressed in the following table:
MI=COV/(SP)1/2
COV SP MI
Good print quality small big small
Less marking
General print quality small small in
Large spot
Low contrast
Some markings
General print quality big big in
Small spots
High contrast
Some markings
Poor print quality big small big
Large spot
High contrast
Severe marking
Table 1 expected speckle index
Speckle analysis is not blindly tested on just a few samples. It is important to clearly set goals and how to achieve them.