Control of dot size during printing
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The change in dot size during the printing process is the most important factor affecting print quality. Bright tone refers to the bright tone on the screen, which is generally expressed by the area of the dot. The bright dot is the transition from the absolute point to the dot and can be printed, and the color tone begins to change. If the print is not printed, the image will be bright and blunt; if it is not printed, the image will be bright and flat, both of which will affect the three-dimensional, spatial and texture of the image, so the minimum dot size of the print must be strictly controlled. However, since the dots are independently colored, the dot area differs by 1% in normal time, and the color tone has an influence, so it is difficult to control.
The dot gain is divided into the dot geometry increase and the dot dot optical increase. The geometric increase of the dot refers to the fact that the actual ink area of the dot in the unit area on the substrate is larger than the corresponding area of the film dot, and the dot size is expanded under the force. The optical increase of the dot means that the visual perception of the eye to the dot is a point larger than the actual geometric area due to the light seepage phenomenon of the paper, and the excess amount of this part is called optical enlargement.
The main factors affecting the geometrical increase of the dot are as follows.
1 ink layer thickness. The geometric increase is caused by the expansion of the ink around the dots under the action of pressure during the normal printing process. The thicker the ink layer, the more serious the dot gain.
2 properties of the ink. The greater the fluidity of the ink, the greater the likelihood that the dots will increase.
3 The amount of sliding of the embossed surface. The embossed surface of the lithographic printing has unavoidable sliding, and the sliding causes the deformation of the dot to increase. When the ordinary blanket or the soft lining is used, the embossing surface has a large sliding amount, and the dot is enlarged, and the lining plate cylinder is lining up. When the ratio of the radius of the rubber roller does not match the rolling ratio, the larger the difference from the synchronous rolling, the larger the sliding amount of the embossing surface, and the more the dot increases.
4 printing pressure. The greater the pressure, the more the ink spreads, and the more serious the dot gain.
5 plates are worn. The bright part of the printing plate, the area coverage of the dot is small, and the hydrophilic area is larger than the lipophilic area, which is conducive to the expansion of the dampening solution. Once the base of the high-profile part is worn, it is immediately encroached by the wetting fluid, which becomes the basis of hydrophilic oleophobicity, making the high-profile dots become finer or incomplete. The dark adjustment part of the printing plate, the area coverage of the dot is large, and the oleophilic area is larger than the hydrophilic area, which is conducive to the expansion of the ink. After the blank part of the mesh around the dot is worn, it is easy to be spread by the ink, so that the dot is enlarged. And even produce a paste.
6 The influence of the paper type on the geometrical increase of the dot. In theory, when the printing conditions are the same, the dot gain of different papers is the same, but in the actual production, when the non-coating is used, the non-coating paper has poor smoothness and good ink absorption, so it is necessary to increase the ink supply. Increase the thickness of the ink layer on the paper to achieve sufficient saturation; the printing pressure must be increased to allow the paper to fully contact, and sufficient ink to be transferred to the paper; the viscosity of the ink must be reduced to utilize the fluidity of the ink. , to obtain a uniform effect of the ink color. The ink with low viscosity and large ink volume is transferred to the PS plate, and is transferred to the blanket under relatively large printing pressure. When it reaches the paper, it will cause a larger dot area to increase.
7 The deformation of the dot caused by various printing failures increases.
In actual production, as long as we prepare the work well, understand and master the characteristics of the printing and printing requirements before printing, and do more detailed operation, it will reduce the deformation of the dot. In the offset printing process, the ink transfer is completed in three steps. The first step is the wetting and inking of the printing plate, the second step is to transfer the ink of the printing plate to the blanket, and the third step is to remove the ink from the rubber. The cloth is transferred to the paper. In these steps, the dots are prone to change in size and shape. Therefore, when printing the printed dots, according to the printing requirements of the printing, first check the rubber blanket tightness of the machine blanket cylinder. Is it suitable, whether the surface of the blanket is clean and smooth, whether the pressure of each impression cylinder is moderate, use a magnifying glass to check whether the dot on the printing plate is real, whether there is any deformation, etc., adjust the printing ink, and control the amount of ink in the printing, so that The amount of deformation of the dot will be greatly reduced, so that the correct restoration of the tone can be ensured, and the color reproduction of the printed product can be guaranteed.