Color Control Techniques for Spot Color Printing (Part 2)

2. Effect of surface treatment on color

The surface treatment methods of packaging products mainly include coating (bright film, matt film), coating (bright oil, matt oil, UV varnish) and so on. After the surface treatment of the printed products, there will be varying degrees of hue and color density changes. These changes are divided into physical changes and chemical changes. The physical change is mainly reflected in the specular reflection and diffuse reflection on the surface of the product, which has a certain influence on the color density. When covering the bright film, cover light oil and UV oil, the color density increases; when covering the matt film, cover the matte oil, the color density decreases. The chemical change mainly comes from the plastic film, UV base oil, and various organic solvents contained in the UV oil, which will change the color of the printing ink layer.

3. Effect of lighter on color

Absorption agent is a paste-like colorless and transparent substance, which mainly plays a role of diluting color in spot color printing. The amount of lighter added is different, and the influence on hue is also different, especially the blue color is the most obvious.

In the process of formulating spot-color inks, the leveling force and pressure generated by the uniform ink meter and the ink-extracting instrument are smaller than those on the printing press. There is no need to add a lighter when printing, but only use a lighter to create a uniform color ink display card. In the case of the same color density, there is a color difference between the color card and the printed product. This is because the addition of the diluting agent changes the distribution of the pigment in the ink and causes the ink to absorb, refract, and reflect light. As a result of the change, there is a color difference, which is caused by the difference in the system.

4. The influence of the dry density difference

For printed products that have just been printed, the ink is still wet and has a density difference when dry. The phenomenon that the wet color density is greater than the dry color density is referred to as a dry retreat density phenomenon. This is because the ink layer just printed has a certain leveling property, so the surface reflection is dominated by specular reflection, which looks bright and shiny. When the ink layer is in a dry state, the reflection of the surface is dominated by diffuse reflection, and the color is naturally dull and darker than when it was printed.

Since the dry color density is generally measured after printing for 30 to 60 minutes after drying, this results in difficulty in measurement and control of spot color density.

A densitometer with a polarizer device can eliminate light from the surface of the ink layer due to specular reflection, and the measured wet color density is very close to the dry color density so that the measured density value is not affected by the wet and dryness of the ink layer. For coated paper, the measured density difference is 0.05 to 0.15, and the non-coated paper has a measured density difference of 0.1 to 0.2. The color difference is also different for different colors, with the smallest yellow difference, the largest black color, and the blue and red colors in between. Therefore, when measured with such a densitometer, the measured value should be higher than the density value of the standard color sample, so as to play the role of control.

5. Effect of system differences

The process of making a color card with a uniform ink meter and an ink displayer is a "dry printing" process. No water is involved, and printing is a "wet printing" process. A wetting liquid participates in the printing process. Therefore, the ink must be generated in the offset printing. The emulsification phenomenon of the water-in-oil emulsified ink changes the distribution state of the pigment particles in the ink layer, and inevitably produces a color difference, and the printed product also appears to be dark in color and not vivid.

In addition, the stability of the ink used for the deployment of spot colors, the thickness of the ink layer, the accuracy of the weighing ink, the new and old differences in the ink supply area of ​​the printing machine, the speed of the printing press, and the amount of water during printing will also have different effects on the color difference. Impact.

Third, the control of spot colors

In summary, in order to ensure that the color difference of the same batch or different batch of products meets the national standards and customer requirements, we have carried out such control on spot colors during the printing process.

1. Make a color card

Firstly, according to the color standard provided by the customer, the proportion of spot color ink is given by the computer color matching system; the ink sample is then recalled, and the ink sample of different density is displayed by the uniform ink meter and the ink exhibition instrument; then according to national standards (or Customer) to the range of requirements for color difference, using spectrophotometer to determine the standard, shallow limit, deep limit, production of printing standard color card (color difference exceeds the standard need to be further amended). Half of the color card is an ordinary color sample, and the other half is a surface-treated color sample. This is for the convenience of quality inspection, as shown in the drawing.

2. Verify spot colors

Considering that paper is the main factor influencing the color difference, each time before printing, the actual printing paper should be used to "show" the color samples, and micro-correction should be made against the color cards to eliminate the influence of the paper.

3. Printing control

The printer uses a printing standard color card to control the thickness of the special color ink layer, and assists in measuring the main density and Bk values ​​of the color with a densitometer to overcome the difference in ink dry and wet color density.

In short, in packaging and printing, there are various reasons for spot colors causing chromatic aberration. It is necessary to analyze specific problems in practical production, solve problems, minimize deviations to the minimum, and produce package printing products that satisfy customers. .


Source: Greater China Graphic Arts Network