Color change refers to replacing the ink color of a certain printing unit or switching from a standard primary color ink to a spot color ink. Nowadays, it is very common to use spot color inks, and sometimes a shift may require changing spot colors four or five times. Frequent cleaning of the rollers is indeed troublesome, especially when the rubber rollers are of poor quality or the spot ink color is light. Sometimes, even after several attempts of cleaning and color change, the result is still unsatisfactory, disrupting normal production. Therefore, color change work should not be ignored and requires certain techniques to make spot color printing less troublesome. Based on years of practical experience, the following points are summarized:

(1) First, one should study relevant knowledge of light color theory, grasp the principles of color subtraction and the basic rules of ink color changes. One should connect this knowledge with actual production, contemplate repeatedly, and apply it flexibly.

(2) Upon receiving a spot color printing job, roughly analyze the ink set for that spot color. Determine which primary color inks are used to mix it and in what approximate proportions. For example, orange generally uses more yellow ink and less red ink; sky blue generally uses more red ink and less blue ink; coffee color generally consists of half yellow and half red plus some black ink; lighter spot colors require a large amount of diluent.

(3) Choose an ink set with a hue close to the spot color to clean the rollers and change colors. Four-color machines are relatively common now, and the flexibility of choosing a blue set is relatively high. If the wrong ink set is chosen, it will undoubtedly be counterproductive. For instance, orange ink should never use a blue set but rather a yellow or red set, because even a little blue residue in orange will make the color dull; silver ink should never use a red set but should prioritize a black set, making color changes easier. Even if the cleaning is not perfect, it will not noticeably affect the visual effect or product quality.

(4) After selecting the ink set and before preparing to clean and change colors, analyze the hue of the spot color and pre-apply some of the other primary colors contained in the spot color onto the rollers. For example, if the spot color is metallic red, apply some yellow ink to the original red set, or if it is coffee color, apply some yellow and a small amount of black ink to the red set. Then, run the machine for a while and rinse with cleaning water. The benefit is that the ink on the rollers will already be roughly close to the spot color before cleaning, so one cleaning is usually sufficient, maybe two at most. Even if not completely clean, residual ink will not affect the printed color.

(5) Rollers should be cleaned quickly, ensuring correct pressure between rollers, between the cleaning blade rubber strip, and the transfer rollers. Also, pay special attention to cleaning both ends of the rollers longer. After a period of use, rollers tend to become olive-shaped, and the ends easily age, developing cracks and spots. Residual ink in these areas is hard to clean and can affect the printed color. When necessary, some cleaning agents may also be used, such as anti-residue water or paste cleaners, though the cost is slightly higher.

In conclusion, by relying on accumulated knowledge and experience, mastering proper working methods and techniques, and reasonably using effective auxiliaries, one can save time, effort, and money overall, achieving twice the result with half the effort.