Control of dot size during printing

The change of dot size during the printing process is the most important factor affecting the printing quality. Bright tone refers to the bright tone on the screen, generally expressed by the area of ​​the dots, and the small dots of the bright tone refers to the transition from the absolute dots to the dots on the image that can be printed, and the color tone begins to change. If it is not printed, the image is bright and stiff; if it should not be printed, the image is bright and gray, both affect the three-dimensional, spatial and texture of the image. Therefore, the minimum dots for printing should be strictly controlled. However, since the dots are independently colored, the dot area is normally different by 1%, and the hue will be affected, so it is difficult to control.

Dot increase is divided into dot geometric increase and dot optical increase. The geometric increase of the dots refers to the amount that the actual dotted area of ​​the dots in the unit area on the substrate is larger than the corresponding area of ​​the film dots. It is the phenomenon that the dot size expands under the force. The optical increase of the dots refers to the fact that the visual perception of the dots by the paper is a point larger than the actual geometric area. The extra amount of this part is called optical increase.

The main factors affecting the geometric increase of the dots are as follows:

â‘  Ink layer thickness. The geometric increase is caused by the expansion of the ink around the dots under the pressure during the normal printing process. The thicker the ink layer, the more serious the dot increase.

â‘¡ The nature of the ink. The greater the fluidity of the ink, the greater the possibility of dot gain.

â‘¢The sliding amount of the stamping surface. There is an inevitable slippage on the embossing surface of lithographic printing. The sliding causes the deformation of the dots to increase. When ordinary blankets and soft linings are used, the embossing surface slides more and the dots increase more. When the radius ratio with the rubber roller does not meet the rolling ratio, the greater the difference from the synchronous rolling, the greater the sliding amount of the embossed surface and the more the dot increase.

â‘£Printing pressure. The greater the pressure, the more the ink spreads, and the more the dots increase.

⑤ The printing plate is worn. In the bright tone part of the printing plate and text, the area coverage of the dot is small, and the hydrophilic area is larger than the lipophilic area, which is beneficial to the expansion of the wetting liquid. Once the dot base of the high-profile part is worn, it is immediately invaded by the wetting fluid and becomes the basis of the hydrophilic and oleophobic, making the high-profile dots smaller or incomplete. In the dark tone part of the printing plate, the coverage of the dot area is large, and the oleophilic area is larger than the hydrophilic area, which is conducive to the expansion of the ink. After the sand on the blank part around the dot is worn, it is easy to be spread by the ink and increase the dot. And even confusing.
â‘¥ The effect of paper type on the increase of dot geometry. In theory, under the same printing conditions, the dot increase of different papers is the same, but in actual production, when non-coating is used, the amount of ink supply must be increased due to the poor smoothness of the non-coated paper and good ink absorption. , Increase the thickness of the ink layer on the paper to achieve sufficient saturation; the printing pressure must be increased to make the paper ink fully contact, and sufficient ink is transferred to the paper surface; the viscosity of the ink must be reduced to use the fluidity of the ink , To obtain the effect of uniform ink color. The ink with low viscosity and large amount of ink is transferred to the PS plate, and then transferred to the blanket under the action of relatively greater printing pressure. When it finally reaches the paper, it will cause a larger dot area increase.

⑦Increase in dot deformation caused by various printing failures.

In actual production, as long as we are fully prepared, understand and master the characteristics and printing requirements of the printed parts before printing, and do a more detailed operation, it will reduce the amount of deformation of the dots. 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 dots, according to the printing requirements of the prints, first check the elasticity of the rubber blanket of the rubber cylinder of the machine. Whether it is 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 dots on the printing plate are solid, whether there is any deformation, etc., adjust the printing ink, and control the amount of ink in the printing, so that Will greatly reduce the amount of dot deformation, so as to ensure the correct restoration of tone, and ensure the color reproduction of printed products.

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