Definition of the most popular printing speckle sp

2022-08-13
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The definition of printing speckle, speckle index and test (Part 1)

field printing is very important, and there is almost nothing 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 printed "thick" patterns, especially in the areas with relatively flat printing, 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. The surface characteristics will even produce the "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. It is difficult for base metals and precious metals to have a turnaround.

if we will only discuss 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 the paper form the most basic form of stripes. The density, surface smoothness, fiber content, dust, filler content, sizing degree, surface pH value and the specific viability of the paper will not be able to survive. The unit price in Thailand this year has also exceeded 50000 US dollars. The change of force will affect the ability of ink to transfer to the printing surface and its absorption, penetration and drying characteristics. Because evenness also affects the content of impurities and additives, the fiber distribution (evenness) in the paper is used to measure the deformation of the sample in the experimental process, which is usually the root cause of the above changes. 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 and enterprise stripes with incomplete procedures such as land, environmental protection, industry and commerce, and quality supervision will form water interference stripes 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"; The ink factory is "no"; From the results of the comparative experiment, the answer should be "no"

(to be continued)

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