Paper gravure color management skills

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. Although it is possible to contact the ink factory and make adjustments, this is not a long-term solution after all. Because the paper used by the printing factory is not very fixed, the whiteness, smoothness, and tightness of different batches of paper are also different, and the depth of the printing plate's network will 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.

The selection of base ink

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: sky blue, pink, yellow, black, dark red, gold red, rose red, red, blue, special purple, green, blue, clear yellow, orange, white ink, and thinner , transfer oil and so on. After the technologist finds out the base ink needed, it is necessary to pre-adjust the hue of each color ink before the product is printed, and try to find the closest spot color ink. The less the ink composition, the better, so as not to affect the ink. Saturation, brightness, and variable factors that result in formulated inks increase. 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.

Coloring techniques

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. The deployment of red ink

For a large red, it is in the CIEL*a*b* color space, L*=40.80, a*=59.80, 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 the value of 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 apparent. 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 ΔL* value is large, it means that the color is bright, and the ink layer is thin from the perspective 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, and the absolute value of ΔL* is large, the color is dark and there are too many pigment components in the ink.

When adjusting the brightness, there will be some effect on the color saturation. Therefore, while adjusting the brightness, slightly red gold can be finely tuned, but it can not make too much influence on the red and yellow phases 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. The deployment of light green ink

Light green ink In the CIEL*a*b* color space, L* = 81.69, a* = -11.06, 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 adjusted by choosing sky blue ink and medium yellow ink or transparent yellow ink. 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.

The determination of the first sample and standard sample sealing and distribution

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 printing plate network will gradually become lighter, the thickness of the ink layer will continue to decrease, and the color saturation will also decrease. Compared with the new type of printing, the color will produce a grade difference.

In order to maintain the color fluctuation of the whole batch of products, according to the printing adaptability of each type of paper, adding 20% ​​of diluting agent to the ink during the first printing of the new edition will adjust the viscosity moderately, so that the color can be directly Slightly lower saturation. 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. The color administrator of the quality control department should establish a standard color sample file folder and distribute it 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.

Printing station process management and color difference detection

After the determination of paper and printing plates, ink formulations, and standard color samples, 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 printing viscosity of the ink helps the ink to wet the printing plate when the ink is scraped, and reduces the wear of the plate by the squeegee.
(3) Printing speed. Controlling the speed helps stabilize the ink transfer.
(4) Drying temperature. The drying temperature of the print should not be too low, otherwise the ink film that is not dried and adhered to it 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 should conduct sampling self-tests at certain frequencies. At the same time, for the continuously operating machines, the quality control department should monitor and measure them. At least one color difference should be detected within 30 minutes. 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 detected data is accurate and reliable. Therefore, it is necessary to ensure that several important indicators of the colorimeter such as fatigue characteristics, stability, repeatability, reproducibility, and accuracy meet the requirements for use.

Establish inspection system for printing plates, paper and ink

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 perform physical and chemical tests on important indicators such as the whiteness, 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 kind of paper has its own appropriate printing speed, printing pressure, ink viscosity, drying temperature, etc., and some paper color areas are not the same, the 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.

In short, color management is a systems science, and every aspect is crucial. Training an excellent color manager and continuously maintaining a quality control system is of utmost importance.

Qingdao Ren Yin Design Co., Ltd. Liu Jian