Overprint of ink
In color printing, the adhesion of the latter color ink to the ink layer of the previous color is called overprinting of the ink, also called the load transfer of the ink. The color on the color printed matter is generally through yellow, magenta and cyan with different area sizes , Black and other colors dots overlap or juxtaposed and presented. Therefore, the color reproduction effect of the printed matter not only involves the optical properties of the ink. And it is also related to the ink overprinting sequence and the amount of ink overprinting.
1. Overprint rate and its determination
Overprint rate, also known as the ink's strength, is a physical quantity that measures the degree of ink overprint. The higher the value of the overprint rate, the better the overprint effect. The overprint rate can be calculated by measuring the density of each color ink and the density value of the ink overprint. When studying the phenomenon of ink overprinting, the overprinting of the second color ink on the first color ink film layer is often the focus of discussion.
The color of color printed matter is generally made of two or three colors of three colors of yellow (Y), magenta (M), and cyan (C). When measuring the density of a single-color ink, a color filter is installed in the density meter using the principle of subtractive color method. The color of the color filter is usually complementary to the color of the measured ink. For example, the blue filter is used to measure the density of yellow ink, the green filter is used to measure the density of magenta ink, and the red filter is used to measure the density of cyan ink.
2. Ink overprint of monochrome offset press
When the first printed ink is fixed on the substrate and is close to dry, the latter color ink is printed and attached to it, which is called dry printing. The ink overlay of the monochrome offset press is carried out by wet pressing and drying. Printing.
After the ink is attached to the substrate, the original properties of the surface are changed by being covered by the ink, and the surface properties of the ink film become the main factors affecting the ink overprinting. For ink overprinting of a monochrome offset press, if the viscosity of the first color ink is greater than the viscosity of the second color ink, the second color ink can adhere well to the first color ink film. Overprint effect is good. On the contrary, if the viscosity of the second color ink is greater than the viscosity of the first color ink, the second color ink not only does not adhere well to the first color ink film. And it will also take away the first color ink, overprinting can not be achieved. It can be seen that the viscosity of the ink is very important for the overprinting of the ink.
In dry printing, if the printing interval of the two-color inks is improperly controlled, and the ink printed first has been dried before printing the ink of the next color, the ink of the ink layer is difficult to adhere to because the dried ink film has lost its viscosity. This caused the so-called "ink crystallization" phenomenon. In order to prevent the occurrence of ink crystallization, it is not advisable to add dry oil to the first color ink. Sometimes to delay the drying of the ink, it is also necessary to add an auxiliary agent to the ink to inhibit the drying of the ink.
In summary, in the actual production, in order to make the ink printing of the monochrome offset press smoothly, the ink layer printed first must have two characteristics:
(1) Maintain a certain viscosity, and the adhesion of the post-printing ink is greater than the cohesion of the post-printing ink.
(2) Maintain a certain wettability, which can be wetted by the post-printing ink and adhere to each other.
3. Multi-color offset printing machine ink overprint
When printing with a multi-color offset press, the printing interval between each color is extremely short. The ink printed in the front needs to be overprinted with new ink within a fraction of a second. The ink printed first cannot be dried before the ink of the next color is overprinted. The ink film adheres to each other in a wet state, so it is called wet printing. Overprinting of ink is carried out in a wet press wet manner.
In dry printing, the post-printed ink is transferred to the pre-printed ink that has been fixed and gradually dried. The post-printed ink is separated in the middle of the ink film, that is, half is transferred to the previous color ink film, and the other half remains in Blanket roller surface. This is not the case with wet overprinting. The latter color ink is generally split in the middle of the ink film where the two colors are superimposed, and a small amount is transferred to the wet ink film of the previous color, and most of it remains on the surface of the blanket cylinder. Obviously, the overprinting rate of wet printing is lower than that of dry printing. The ink transfer situation is far better than that of dry printing. During the overprinting process, the phenomenon is more complicated, and there are more factors that affect the ink overprinting rate. In particular, the most important factors that affect the multi-color offset press ink overprinting are ink viscosity, density, and brightness. The following uses the ink used in the four-color offset press as an example to discuss in detail.
The viscosity of the ink has a great influence on the overprint effect of wet printing. The greater the viscosity of the ink, the greater the viscosity. Therefore, the ink layer shows greater cohesion. If the viscosity of the ink printed first is lower than the viscosity of the ink printed afterwards, the ink printed afterwards will rely on its own cohesion to peel off the ink printed first, resulting in reverse overprinting. In this way, wet printing cannot be performed. Therefore, the ink used in the four-color offset press should gradually reduce the viscosity of the ink in the order of the cylinders of each unit. For yellow, magenta, cyan, and black, the viscosity relationship is: black> Cyan> Magenta> Yellow.
The density of the ink and the thickness of the ink film have a certain relationship. Generally, as the density value increases, the thickness of the ink film gradually decreases. For wet printing, the ink with a larger ink film thickness is printed in the subsequent color sequence, and the overprint effect is good.
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