Field printing is critical, and there is hardly anything more dazzling than a rough field. The roughness formed by the bright spots and dark spots in the printed field is called speckle. The factors that may cause this phenomenon may be ink, paper, printing conditions, or a combination of these conditions.
1. What is speckle
Speckle refers to unintended, varying light reflections that occur in a printed area with a uniform tone, appearing as a cloudy cloud, grain, or other regular pattern. Marking is generally not seen in text printing, and it is also difficult to find in printing "heavier" patterns, and it is particularly noticeable in areas where the printing is gentle, such as the sky or other backgrounds with uniform tones.
It seems simple enough to describe speckle as uneven reflection, but it cannot reflect (indicate) the complex process of light reflection and the uneven light reflection that the human eye feels.
The first thing to consider is the light reflection on the printed surface. When we observe a flat and dense print, we think that the light is reflected from the surface. But this is not true. The surface of any object, including metal, plastic, or paper, is rough, full of potholes and small holes. The primary function of fillers such as paints and inks is to fill some potholes. When part of the light enters the surface of the sample, part of it is absorbed by the backing material and converted into heat. Other light is reflected multiple times to form a smooth surface on the surface, and part of it passes through the paper pattern. In fact, some light reaches our eyes after passing through ink, paper, and other objects behind the paper.
Variations in ink and paper density or reflectance, and even background material, will produce a distinct speckle pattern. The geometry and physical properties of the reflected light on the paper will also have a significant effect on the speckle phenomenon. We generally think that paper and printed surfaces are diffuse reflection sources, reflecting light in all directions, regardless of the angle of incidence. But the surface of any object cannot achieve complete diffuse reflection. Both paper and printed patterns diffusely reflect incident light and have a mirror reflection effect, reflecting part of the light at a predictable angle. Generally, the angle of incidence is the same as the angle of reflection and the direction is opposite. The slight undulations, wrinkles, marks and other surface characteristics of the paper surface will obviously affect the reflection of light and show varying reflectance. The surface characteristics may even produce a "corner reflection source" effect. In particularly obvious areas, the light density is magnified. It can be seen that many factors can form speckle phenomenon, many of which are not directly related to the printability of the printing material to the ink.
Second, the types of printing spots and the reasons for their formation
If we will discuss only the impact of changes in the reflectivity of the printed surface, then there will still be many changing factors in the printing process that can form speckles. Paper, ink, printing methods, and other printing problems all form speckles, 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 stripes, and the changes in density, surface smoothness, fiber content, dust, filler content, sizing degree, surface PH value and specific viability of the paper will affect the printing surface Ability to transfer and its absorption, penetration and drying characteristics. Because the uniformity also affects the content of impurities and additives, the fiber distribution (uniformity) within the paper is usually the source of the above changes. Poor uniformity will negatively affect the diffuse reflection coefficient and opacity.
Sandreuter defined three types of markings produced by the (offset) printing process in 1994.
1. The back peeling marks are caused by the poor 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 separated from the pigment too quickly. Any change in the ink transfer rate will cause speckles. The overprinted streaks on the back usually appear in the first printing color, which can also cause this kind of streaks when the inappropriate ink PH and viscosity values ​​are selected.
2. Water interference marks When the paper does not absorb the ink carrier and reduces the amount of ink transfer, water interference marks are formed. The causes are generally due to excessive water use, ink formulation errors, incorrect vehicle mixing or too high alcohol content.
3. Wet sticky ink markings When the order of viscosity and PH value between each ink is not selected properly, markings will form after the second printing. But when these inks are printed alone, there will be no markings.
Finally, due to the wrong installation of the printing plate, the elasticity of the printing roller, the trace of the driving gear, the change of the groove and other factors, the streaking phenomenon will also occur. Ink transfer function is irregular, but there is a tendency to only work on certain inks, and the random process of marking on the surface characteristics of the paper will affect all inks. The changes caused by printing are usually predictable. It happens regularly, which helps to limit it to a certain frequency range.
3. Visible printing marks
Due to the definition of speckle and the aforementioned factors that can cause speckle, it is difficult for us to objectively obtain a visual grade of speckle.
There is another point to discuss: should obvious mechanical factors such as wrinkle marks, wrinkles, pinholes, etc. be considered as markings?
From the final printing result or the standpoint of the printer, the answer is "yes"; considering the quality of the raw materials, the paper mill's answer is "yes"; and the ink mill is "no"; from the comparison experiment results, the answer should be "no" .
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Fourth, the determination of markings
From an image analysis chart, in what we think of the phenomenon of speckle, it is affected by at least two independent factors. The process seems to work simultaneously in a non-linear 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 reflectance) and bright areas (higher than the average emissivity), the speckle is the amount of change between these two groups of images. For the sake of clarity, we regard 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-quantitative areas (of fibers, floc), 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 “grain morphology†of the speckle print.
We noticed that when the brightness is reduced, our feeling of speckle or the severity of the speckle is also reduced. If we extend this trend further, assuming that the contrast between bright spots and dark spots is reduced to a sufficiently low value, or the size of the spots is reduced to a sufficiently small size, we will not feel the appearance of speckles. Similarly, assuming that the area of ​​the spots is large enough to surround the entire printed area, so that our eyes do not feel that they are blocky, then we will not call them spots in this case. The actual situation is like this-all printing areas have a certain level of speckles, but due to the small area of ​​the pattern or the low contrast, we do not feel the appearance of speckles. Therefore, when the contrast is low, the size of the bright area and the spot are not important. In other words, when the area of ​​the spot is small, the eye feels that the spot is integrated with the entire printed content. For example, a midtone pattern is considered a brighter color. Therefore, the size and contrast of spots are the direct factors that affect human vision.
V. Test method for determining speckle
The Dutch IGT company has developed some methods to determine speckle by using the printability tester produced. The peeling stripe test and wet-pulling resistance test are carried out on the AIG2-5 printing suitability tester, and the peeling stripe test is also available on the CI printing suitability tester. The wet hair temperature resistance test has been discussed in a separate article.
principle
The paper to be measured is printed with special speckle measurement ink. When using the first measurement method, 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 paper absorption of ink, ink that has not been completely absorbed will be 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 ink-printing disc on five clean paper strips respectively, during which the printing disc does not need to be re-inked. This method is faster but the test accuracy is poor.
the first method
Using a conventional method, the 50mm blanket printing plate was inked with the speckle measurement ink. Print the test paper on AIC2-5. No padding is used during printing. Speed ​​0.2m / s, pressure 500N. After printing for 10 seconds, another clean 50mm blanket printing plate was printed again at 500N. The ink on the paper strip will be transferred to the printing plate. Every 10 seconds, print on the next clean printing plate. Stripped a total of 4 times.
The test can also be carried out on C1, the width of the printing disc is 35mm, and the pressure is 450N. The quantity is not suitable for all types of paper and paperboard to be tested on C1, and the results cannot be compared with the results obtained on AIC2-5. Please note.
Choose the amount of ink used according to the type of paper and cardboard, but it is generally recommended to use 1cm3 when using AIC2-5, and 0.3cm3 when using C1.
The second method
Ink is printed on a 50mm printing plate, and then printed on AIC2-5 at 0.2m / s, 625N. After 10 seconds, change another piece of paper and print again. Use clean paper to print 5 times, with an interval of 10 seconds each time. If the C1 type is used, the pressure is set to 450N, and a leather-like printing plate is used to print 0.3cm3 on AIC2-5 with 0.5cm3 on C1.
Evaluation of test results
The evaluation methods are generally compared with prefabricated spline prints that represent a certain printing quality.
This evaluation method can only be used by the user. Unless the comparison proofs can be unified or the speckle index table can be used, data cannot be exchanged with other units.
The speckle index is based on two changing conditions that affect the assessment of speckle: the perimeter of bright and dark spots and their density contrast. Larger areas and contrasts make it easier to observe. Using a scanner, the effective perimeter of the bright and dark spots can be determined and expressed, and the density change can be calculated with variable coefficients (Coefficient, OfVariables).
Through the above two variables, the speckle index is obtained:
At high coefficients of variation (large contrast) and small effective circumference (large spots), the speckle index increases, which is consistent with the results of visual assessment.
The relationship between the quality of printing and the parameters related to the speckle index can be expressed by the following table:
MI = COV / (SP) 1/2
COV SP MI
Good printing quality, less streaks, small, large, small
General printing quality specks large contrast low some spots small small medium
General printing quality, small spots, high contrast, some spots, large, large, medium
Poor print quality speckle large contrast high severe speckle large small large
Table 1 Expected speckle index
Speckle analysis is not just blindly testing on a few samples, it is very important to set goals clearly and how to achieve them.
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