Detailed offset printing error - color difference

Color difference

Color difference is also called color deviation, which is one of the typical quality defects in the printing process. During the production process, red, yellow, or partial blue color that does not match the print and proof color often occurs, or the color failure of the pattern screen after overprinting occurs. The light affects the quality of the product, which will severely cause the entire batch of products to be scrapped. The cause of color cast and color mixing failures can be summarized as follows:

(1) Yellow, magenta, and cyan dots caused by improper plate making operations are too dark or too shallow. Here mainly refers to printing plate drying process, the intensity of the printing plate light source, the spectrum does not match the photosensitive characteristics of the printing plate, or the exposure time is improper, the PH value of the developer and the length of the development time are not properly grasped, etc., all may cause the printing plate outlets to be too deep. Or too shallow.
(2) Improper ink color matching or three primary color inks do not match. That is not the same brand, the same type of ink.
(3) The arrangement of the color sequence is unreasonable, mainly referring to the arrangement of the color sequence of a multicolor offset printing press.
(4) There is a serious color shift in the printed paper itself.
(5) The phenomenon of color mixing (usually seen in semi-satellite offset printing machines) is caused by serious dirt on both sides of the printing plate.
(6) The ink roller wash is not clean when changing colors.
(7) The paper feed is abnormal.
(8) Change of version is not timely.
(9) The signer is not responsible.
(10) Ink and ink are not balanced.

For the above reasons, offset press operators must pay attention to the following aspects.

(1) When correcting the ink color, the ink color must conform to the proofs in order to open the print and pursue the output one-sidedly. Often, the ink begins to print at a larger distance from the print sample, which will inevitably result in a larger color difference between the front and back printing products. The correct operation should be: After the printing plate is calibrated, use the printing paper to correct the ink color. The proof printing sample must basically meet the printing sample before printing.

(2) In most cases, the signer must sign the sample within a short time to reduce the occurrence of color aberration.

(3) Grasp the ink balance properly, paying particular attention to the PH value of the wetting fluid. Under the same ink condition, small water and large water will also affect the darkness of the color. The principle of water consumption is the following: Use the smallest amount of liquid supply.

(4) According to the quality of the paper, the frequency of cleaning the blanket is decided. If the quality of the paper is poor, and the blanket is not cleaned in time, the color difference will also be caused.

(5) Ensure that the paper is fed properly, and each paper feed pause will cause the product to become darker. Therefore, it is necessary to develop good habits, that is, to remove 6 to 8 products each time after the paper feed is paused, so as not to mix in the finished product and cause chromatic aberration.

(6) If a long version of a product is found in a printing plate, it should be replaced promptly. The reduced ability of the flower plate to absorb the ink will also cause the product to be imprinted with a dark color and form a color difference.

For positive and negative printed products, we must pay attention to the positive and negative aspects of the product. We must always compare the ink with the reversed product to prevent the deeper printing or the weaker the printed product from affecting the quality of the product. For cross-page puzzle products, arrange as many prints on the same machine as possible, the same shift, otherwise it will cause serious quality accidents.

[Case 8-2]
A factory prints a batch of calendars for a certain unit. There is a rectangular bar at the unit's signature, which is a light green spot color. When the four-color printing was finished, when the light green spot color was printed, it was found that the gap was very far from the original color sample. After the captain analyzed, he thought it was the reason for ink distribution. After the identification, the ink distribution was normal. The ink roller in the cyan printing machine was machine-washed several times and failed to eliminate darkness. After the printing of the blue ink, the individual ink rollers and the ink roller are aged. In severe cases, they become orange peels. The fault disappeared after replacing the new ink roller.

[Expert Tips]: Some light spot color inks must be well cleaned ink roller, for high-precision products, in order to ensure non-color cast, the ink roller must be hand-washed.

Source: Ke Yin Network

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