Color difference, also known as color shift, is one of the typical quality defects in the printing process. During the production process, reddish, yellowish, or blueish phenomena often occur in the color of the printed matter and the proof sample, or the color mixing failure after the overprint of the pattern screen The lighter ones will affect the product quality, and the serious ones will make the whole batch of products scrapped, causing color deviation and color mixing failure can be summarized as the following reasons:
(1) The dots of one color of yellow, magenta and cyan are too dark or too light caused by the non-standard plate-making operation. Here mainly refers to the printing plate printing process, the printing plate light source intensity, the spectrum does not match the printing plate photosensitive characteristics, or the exposure time is improper, the developer PH value and the development time are improperly mastered, etc., may cause the printing plate dots to be too deep Or too shallow.
(2) Improper color adjustment of ink or mismatch of three primary colors ink. That is not the same brand and same type of ink.
(3) The color sequence arrangement is unreasonable, mainly referring to the color sequence arrangement of the multi-color offset printing machine.
(4) The printed paper itself has a serious color shift phenomenon.
(5) The blank space on both sides of the printing plate is heavily soiled, causing color mixing (commonly seen in semi-satellite offset presses).
(6) When changing colors, the ink roller is not clean.
(7) Paper feeding is abnormal.
(8) The version change is not timely.
(9) The signer is not responsible.
(10) Ink and ink are unbalanced.
For the above reasons, offset press operators must pay attention to the following aspects.
(1) When correcting the ink color, the ink color must be basically consistent with the sample before printing can be started. One-sided pursuit of output, often starts printing when the ink color is far away from the printed sample. The correct operation should be: after the printing plate is calibrated, the used printing paper is used to correct the ink color, and the trial printing sample must be basically in accordance with the secondary printing sample before normal printing.
(2) Under normal circumstances, the signer must sign the sample within a short time to reduce the occurrence of color difference.
(3) Correctly control the balance of water and ink, pay special attention to the PH value of the wetting fluid. Under the same ink volume, large water and small water will also affect the shade of the color. The principle of water consumption is not to be dirty. Use the smallest fluid supply.
(4) Depending on the quality of the paper, the frequency of cleaning the blanket is determined. If the quality of the paper is poor, and the blanket is not cleaned in time, it will also cause chromatic aberration.
(5) Ensure that the paper feed is normal. Each time the paper feed stops, it will cause the sheet product to be deep. Therefore, it is necessary to develop a good habit, that is, remove 6 to 8 sheets of product each time the paper is stopped before printing, so as not to be mixed in the finished product and cause color difference.
(6) If the long version of the product is found in the printing plate, it should be replaced in time. Decreased adsorption capacity of the printing plate on the ink will also cause the darkening of the product imprint and the formation of color difference.
For the products printed on the front and back sides, we must pay attention to the shades of the front and back sides, and often compare the ink color of the products on the back side to prevent the deeper or darker printing from affecting the quality of the product. For the products of cross-page puzzles, arrange them on the same machine and print in the same shift as much as possible, otherwise, it will cause serious quality accidents.
[Case 8-2]
A factory printed a batch of calendars for a unit. There is a rectangular bar in the unit's attribution, which is a light green spot color. When printing the light green spot color after the four-color printing was completed, it was found that it was far away from the original color sample. After the captain's analysis, it was considered to be the reason for ink matching, and after identification, the ink matching was normal. The ink roller during cyan printing was machine washed several times, and the dark color phenomenon could not be ruled out. After printing the blue ink, the individual ink leveling roller and ink roller are aging, and in severe cases, orange peel. The fault disappeared after replacing the new ink roller.
[Expert Tip]: Some light spot color inks must be well cleaned. For high-definition products, in order to ensure no color deviation, the ink rollers must be hand washed.
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