Color management of spot color inks in gravure printing

In gravure printing, spot colors are used more often. Spot color ink is generally prepared by the ink factory according to the color samples provided by the printing factory, the paper used for printing, the depth of the gravure mesh, etc. Even so, the spot color ink will produce a difference in printing hue and color swatch when it is printed on the machine.

The paper used by the printing plant is not fixed. The whiteness, smoothness, and tightness of different batches of paper also vary, and the depth of the printing plate cell will also be different. Moreover, there are more and more ink varieties, and some inks are very close in hue, which brings a lot of inconvenience to the storage and storage of ink. Therefore, gravure printing manufacturers must strengthen the color management of printing. From the determination of printed color samples to the monitoring of the color management process, it is crucial to establish an excellent color management system.

In order to solve the problems caused by the variety of spot color inks for gravure printing, the types of inks are generally classified, and several commonly used reference inks are found as base inks, which are measured and monitored using a color difference meter or a spectrophotometer.

Common inks and toning inks are red and yellow. Black, deep red, gold red, red, blue, special purple, special green, sky blue, peach rose red, special blue. Transparent yellow, orange, white ink and diluent, gold oil, etc. 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 composition of the ink, the better, so as not to affect the saturation and brightness of the ink and cause the ink to be mixed. Variable factors increase. Coarse adjustment can be directly observed after scraping the sample with the help of an ink scraper. Conditionally, a color difference meter or spectrophotometer can be used to finely adjust the main color ink of a large area when the product is sampled on the machine to determine the ink mixing ratio.

Let's take the preparation of red ink and light green ink as an example to introduce the preparation techniques of spot color ink.

1. Red ink deployment

For a big red, 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 basic ink that needs to be selected for the preparation of this color is mainly red, and the color ink is dark red, gold red, black and diluent.
In the comparative measurement, when one of ΔL *, Δa *, and Δb * is close to Z in the tristimulus value or greater than or equal to Z, the color difference is more obvious when the color is observed with the naked eye. Therefore, L * is a key chromaticity index, which determines the main position of the color in the CIEL * a * b * color space.

(1) If ΔL *> 0, and the value of ΔL * is large, it means that the color is bright, and the ink layer looks thin from the perspective of printing; from the perspective of the composition of the ink, the proportion of dark red ink in the ink is too small. In this regard, there are two ways to adjust, one is to increase the thickness of the ink layer by increasing the printing viscosity of the ink and reducing the printing speed; the other is to increase the proportion of dark red ink, then a very small amount of purple ink or The black ink can be adjusted, and the yellow phase can be adjusted with gold and red ink.

(2) If ΔL * <0 is large, it means that the color is too dark with too many ingredients. And the absolute value of ΔL * and the brightness of the color in the ink will have some influence on the saturation of the color. Therefore, while adjusting the brightness, you can add a little gold and red ink for fine adjustment, but it will not cause too much impact on the red and yellow cabinets in the color. Of course, you cannot adjust the hue alone. When adjusting the yellow phase in red ink, you can use orange ink or gold-red ink instead of yellow ink for fine-tuning.

2. The allocation of light green ink

The light green ink color is in the CIEL * a * b8 color space, L * = 81.69, a * =-11. O6, b * = 4.00. When adjusting the color, it is mainly white ink, and a small amount of special green ink is added for adjustment. The blue phase and red phase can be selected from sky blue ink, medium yellow ink or transparent yellow ink. In addition, when allocating the color, the problem of reading the cursor must also be considered. When the blue color is displayed, it is difficult to read the light mark, and when the color is yellow, the light mark is read normally. Therefore, the proportion of yellow ink can be appropriately increased to make yellowish in light green. Since the printing plant can only carry out mass production after the customer approves the first sample, the determination of the first sample becomes a very important part in color management.

In paper gravure printing, the determination of the first sample has its particularity. In particular, cigarette labels and wine labels with large batch sizes (from tens of thousands of sheets to hundreds of thousands of sheets or even millions of sheets) generally require the printing plate to have a durability of about 1 million sheets. Some printing plates can even reach 2 million to 3 million sheets if they are handled properly and kept properly. But as the number of prints increases, the printing plate net holes will gradually become shallower. The thickness of the ink layer continues to decrease, and the color saturation also decreases. Compared with the sample printed with the new version, the color will have a level difference.

To keep the color of the entire batch of products from fluctuating greatly, you can add about 20% to the ink when printing with the new version for the first time according to the printing suitability of each paper; The color saturation is slightly reduced. As the depth of the printing plate mesh becomes shallower, the proportion of diluting agent can be continuously reduced, even without adding diluent, so as to relatively increase the color density of the ink. At the same time, the viscosity of the ink can be adjusted appropriately to keep the ink transfer stable.

After the first sample standard is confirmed, the quality control department can seal and mark the off-print color samples for later processing and retention of samples. The color managers of the quality control department shall establish standard color sample files and distribute them to the process department and production workshop. Moreover, the deviations in the printing process should be immediately notified to the process department for process recipe adjustment and printing parameter adjustment, and the changed process instructions should be notified to the printing machine.
On paper and plate. After the ink formula and standard color samples are determined, the printing machine should strictly implement the following process requirements within a relatively stable time.

(1) Ink color density. You can check whether the formulated ink formula is stable by scraping the ink.

(2) Ink printing viscosity. Controlling the printing viscosity of the ink can help the ink to wet the printing plate during scraping and reduce the abrasion of the printing plate by the doctor blade.

(3) Printing machine speed. Controlling the machine speed helps the ink transfer steadily.

(4) Drying temperature. The drying temperature of the printed matter should not be too low, otherwise the ink film that is not dry and not firmly attached will stick to the cooling roller,

(5) The temperature and humidity of the environment. The temperature and humidity of the environment have a direct impact on the temperature and water content of the ink.

The printing machine should be sampled and self-checked at a fixed frequency. At the same time, for continuous operation, the quality control department should implement monitoring and measurement. Within 30 minutes, the color difference should be detected at least once. In the case of unchanged process parameters, if there is an obvious color change, the color administrator should coordinate the technologist to make on-site process adjustments, and record the changes to the printing machine. It is worth mentioning that the state of the colorimeter determines whether the test 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 of use.

The key to color management is compliance, consistency, and longevity. Paper must be used in conjunction with printing plates and inks. The physical and chemical properties of the paper determine the depth of the printing plate cell and the level of ink viscosity. Color management must first check the material inspection and carry out physical and chemical tests on its important indicators, such as paper self-degree, smoothness, tightness and other indicators, ink hue, viscosity, fineness and other indicators, the depth of the printing plate mesh, Net hole shape and other indicators. Suppliers are required to provide materials with stable quality. If the materials are to be replaced, they must be tested by the quality control department, process department, and marketing personnel, and if necessary, they should be tested on the machine. For example, each type of paper has its own printing speed, printing pressure, ink viscosity, drying temperature, etc., and some papers have different color areas and different packaging suitability. Cannot print in large quantities without customer approval. After printing out small batches of printed samples, they will be sent to customers for packaging tests, and will be mass-produced after customer approval.

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