Color management of spot color inks in gravure printing

In gravure printing, spot colors are used more often. The spot color ink is generally prepared by an ink factory according to a color sample provided by a printing factory, a paper used for printing, and a depth of a gravure net cavern. Even so, the spot color ink will have a difference in the hue and color sample when the spot color ink is printed on the machine.

The paper used by the printing plant is not fixed, and the whiteness, smoothness, and tightness of different batches of paper are also different, and the depth of the printing plate's network points will also be different. Moreover, there are more and more kinds of inks at present, and the hue of some inks is very close, which brings a lot of inconvenience to the storage and storage of inks. Therefore, gravure printing manufacturers must strengthen the color management of printing. From the determination of printing color samples to the monitoring of the color management process, how to establish an excellent color management system is crucial.

In order to solve the problems caused by the wide variety of intaglio printing spot color inks, the types of inks are generally categorized, several common reference inks are used as base inks, and colorimeters or spectrophotometers are used for measurement and monitoring.

Common inks and toning inks are red and yellow. Black, dark red, gold red, blue light, special purple, green, sky blue, peach red, blue. Transparent yellow, orange, white ink and dilute agent, gold transfer oil and so on. The technologist needs to pre-adjust the hue of each color ink before the product is printed to find the closest spot color ink, and the less the ink composition, the better, so as not to affect the ink saturation, brightness and cause the ink to be formulated. Increased variables. Coarse adjustment can be directly observed by means of scraping with a scraper, and conditions can be adjusted by means of a colorimeter or spectrophotometer to finely adjust the main color ink of a large area when the product is proofed on the machine to determine the ratio of ink preparation.

The following uses the deployment of red ink and light green ink as an example to introduce the deployment techniques of spot color inks.

1. Red ink deployment

For a large red, it is in the CIEL*a*b* color space, L*=40.80, a*=5 9.80, b*=42.11. From this, we generally know its position in the color space. The base ink to be selected for this color is mainly red, and the color ink is dark red, gold red, black and diluting agent.
In the comparison measurement, when one of ΔL*, Δa*, and Δb* is close to Z in the tristimulus value or is greater than or equal to Z, the color is observed with the naked eye, and the chromatic aberration is more noticeable. Therefore, L* is a key chroma index, which determines the main position of the color in the CIEL*a*b* color space.
(1) If ΔL*>0, and the ΔL* value is large, it means that the color is bright, and the ink layer appears to be thin from the viewpoint of printing; from the ink composition, the proportion of dark red ink in the ink is less. In this regard, two ways can be used to adjust, one is to increase the ink layer thickness by increasing the printing viscosity of the ink, reducing the printing speed; the other is to increase the proportion of deep red ink, then a very small amount of purple ink can be used or Black ink to adjust, the yellow phase can be adjusted with gold and red ink.

(2) If ΔL*<0 is large, it means that there are too many dark color components. And ΔL* absolute value and the color in the ink adjusts the brightness, which has some effect on the color saturation. Therefore, while adjusting the brightness, you can slightly add a little gold red ink for fine-tuning, but do not have too much effect on the red and yellow cabinets in the color. Of course, it is not possible to adjust the hue alone. When adjusting the yellow phase in the red ink, orange ink or gold ink may be used instead of yellow ink for fine adjustment.

2. Light green ink deployment

Light green ink in the CIEL*a*b8 color space, L*=81.69, a*=-11. O6, b*=4.00. When the color is formulated, it is mainly white ink, plus a small amount of special green ink adjustment. The blue and red phases can be selected by sky blue ink and medium yellow ink or transparent yellow ink to adjust. In addition, when the color is deployed, the problem of reading the cursor must also be considered. When the blue phase is dark, the light mark is difficult to read, and when the light is yellowish, the light mark is read normally. Therefore, the proportion of yellow ink may be appropriately increased to give yellowish light green. Since the printing factory can only mass-produce after the customer approves the first sample, the determination of the first sample becomes a very important part of color management.

In paper gravure printing, its particularity is determined for the first sample. In particular, large quantities (from tens of thousands to hundreds of thousands of large sheets, or even millions of large sheets) of cigarette labels and labels generally require that the printing plate has an imprint resistance of about 1 million sheets. Some printing plates can reach 2 million to 3 million sheets even if they are properly handled and properly kept. However, with the increase in the number of printing, the plate mesh will gradually become shallow. As the thickness of the ink layer continues to decrease and the color saturation also decreases, the color will produce a grade difference compared to the proofs printed with the new version.

In order to maintain the color fluctuation of the whole batch of products, according to the printing adaptability of each kind of paper, add 20% of the ink in the first printing of the new edition; medium light agent, adjust the viscosity moderately, so that it can directly The saturation of the color is slightly reduced. As the depth of the plate base becomes shallower, the proportion of the diluting agent can be continuously reduced, and no diluting agent can be added to relatively increase the color concentration of the ink. At the same time, the ink viscosity can be properly adjusted to maintain stable ink transfer.

After the first sample is confirmed, the quality control department can seal the samples and mark the color samples of the next machine for processing and storage of samples. Color managers of the quality control department should create standard color sample files and distribute them to the process department and production workshop. In addition, the deviations in the printing process should be immediately notified to the processing department to adjust the process recipe and adjust the printing parameters, and notify the printing press of the changed process instructions.

On paper and plates. After the ink formulation and standard color samples are determined, the printing press should strictly implement the following process requirements within a relatively stable period of time.

(1) Ink color density. The ink formulation can be checked for stability by squeegeeing.
(2) ink printing viscosity. Controlling the ink's discretionary printing viscosity helps the ink wet the printing plate when the ink is scraped, and reduces the wear of the printing plate by the squeegee.
(3) Printing speed. Controlling the speed helps stabilize the ink transfer.
(4) Drying temperature. The drying temperature of the printed matter should not be too low, otherwise the ink film that has not dried out and adhered unstably will adhere to the cooling roller.
(5) The temperature and humidity of the environment. The temperature and humidity of the environment have a direct effect on the temperature and moisture content of the ink.

Printing presses shall conduct sampling self-testing at a predetermined frequency, and at the same time, the continuous operation shall be carried out at the discretion of the machine, and the quality control department shall conduct surveillance and measurement. Within 30 minutes, color difference should be detected at least once. In the case of unchanged process parameters, if there is a clear color change, the color manager should coordinate the technologist to make on-site process adjustments, and record the changes made to the printer station. It is worth mentioning that the state of the colorimeter determines whether the data is accurate and reliable. Therefore, it is necessary to ensure that several important indexes of the colorimeter such as fatigue characteristics, stability, repeatability, reproducibility, and accuracy meet the requirements for use.

The key to color management is compliance, consistency, and longevity. Paper is used with plates and inks. The physical and chemical properties of the paper determine the depth of the plate and the viscosity of the ink. Color management should first test the material well and carry out physical and chemical tests on its important indicators, such as the degree of self-degree, smoothness, and tightness of the paper, the hue, viscosity, and fineness of the ink, and the depth of the printing plate. Net shape and other indicators. The supplier is required to provide stable quality materials. If it is necessary to replace the materials, the quality control department, the process department, and the marketing personnel are required to conduct the test and if necessary, go to the test. For example, each type of paper has its own cabinet-adapted printing speed, printing pressure, ink viscosity, drying temperature, etc., and some paper color areas are not the same, and packaging is also different. Can not print in large quantities without customer approval. After printing small batches of printed samples, they are given to customers for testing in packaging and mass production is required after acceptance by the customer.

Methods for controlling the color quality of printing In the printing process, constraints on the quality of printed images such as paper, ink, and printing plates are imposed. When these factors have been fixed, how can the color quality of the printed image be controlled when the image is reproduced on a printing press?

1. According to the upper body production situation, adjust the printing pressure

According to the print image reproduction quality requirements, paper thickness, printing plate and other conditions to adjust the printing pressure, so that the image network points to get the best reproduction.

2. Reasonably arrange the color sequence

At present, multi-color printing generally uses ink, cyan, magenta, and yellow or cyan, magenta, yellow, and black sequences. This color sequence makes color reproduction easier and more accurate. The thickness of the ink layer is arranged from small to large. For example, when black, cyan, magenta, and yellow are used in offset printing, the thickness of the ink layer is: black ink is 0.8 μm, cyan ink is 0.9 μm, magenta ink is 1. 0μm, yellow ink is 1.1μm; ink viscosity is arranged from large to small, such as black ink 1188Pa, green ink 488Pa, magenta ink is 477Pa, yellow ink is 147Pa. When the ink viscosity does not meet the alignment requirements, the viscosity of the ink can be adjusted in advance.

3. Calibration printing machine

The printing press is adjusted according to the proof sheet and the predetermined printing conditions so that the printing image effect is consistent with the print sheet.

Reprinted from: Qilu Graphic Arts Network