(1) Principle of color inspection
Prints give people the most obvious feeling, that is, their colors, vividness, and authenticity of reflection. In addition to the structure of the pattern and the level of printing, almost all are expressed by color. And because there are many printing factories, printing a certain product every month requires the color of the ink to be consistent, otherwise it will cause the color of the printed matter to be inconsistent and so on. Therefore, the inspection of ink color and hue is very important.
There are two main methods for detecting ink color: one is the instrument inspection method; it is measured by instruments such as reflection densitometer and colorimeter (also known as color meter, color difference meter). Another conventional inspection method is to apply ink to the base material for comparison and judgment. The coating methods on the base material are:
a. Scraper scraping method;
b. Ink roller coating method and wire rod method;
c. The method of printing with a printability test machine.
Conventional inspection method is a method of parallel observation and comparison of standard (quasi-) and test (test) samples with human eyes. This method is relatively simple, but it cannot be represented by numbers. The tristimulus value can be obtained by instrument method and the main wavelength, purity and brightness in the CIE system or the hue saturation and brightness in the Munsell system can be calculated, and it can also be expressed by color difference data through calculation; the so-called Color difference refers to a method in which the difference in perception between two colors is expressed in digital form. Generally, the relationship between them is expressed in NBS (abbreviation of the National Bureau of Standards) color difference.
Introduced here is the conventional inspection method, namely the scrape comparison method. Because this method is relatively simple and accurate in the production of ink; internationally, this method is also commonly used.
(2) Tools and materials
a. Minor ink knife;
b. Squeegee;
c. Scrape paper (or printed with black horizontal stripes, 11 cm long and 6.5 cm wide).
(3) Inspection method
a. Take a small amount of the standard sample and the sample with a small adjustment ink, place them on the glass plate, mix it evenly, then use a small adjustment ink knife to apply a small amount of the standard sample to the upper left of the scraping paper, and then apply a small amount of the sample At the upper right, the two should be adjacent and not connected.
b. Use a squeegee to scrape the ink into a thin layer on the sample paper from top to bottom. After scraping to 3 ~ 5 cm, gradually place the squeegee flat (that is, the angle with the paper becomes smaller) while reducing The small pressure causes the ink to form a thick ink layer on the paper. Figure 1
c. Examine the difference in the background color, face color and ink color of the sample and the standard sample.
(4) Precautions
a. Scraping is one of the important basic skills of ink manufacturers and inspectors. Dilute and high-flow inks are easy to scrape, thick (thick) and high-viscosity inks are not easy to scrape, such as poor ink quality, excessive The thick and short inks are scratched out of shape (this is often the case with engraved gravure inks), which makes them ugly.
b. When the blade of the scraper is sharp, the layer of ink scraped on the paper is thin, otherwise, the layer of scraped ink is thick.
c. When scraping thin ink, the angle of the knife can be smaller to ensure that the scraped ink layer is thicker. When scraping thick sticky ink, the angle of the knife should be larger to make the ink left on the paper thinner. ,
d. The quality of the paper has a great influence on the scraping. It is generally more suitable to use marketable paper with uniform fibers and smooth surface, or high-grade paper. The color of the paper should be neutral white with no fluorescence and no watermark.
According to the situation in China, the size of the scraped sample paper is about 6.5 × 10.5 cm. It is best to print a black line with a width of about 0.4 cm at a position 4 cm above the lower end of the paper, so as to observe the transparency or hiding power of the comparison ink.
Internationally, the custom of the United States is to place the standard sample on the right and the sample on the left, which is different from ours. Considering the ease and convenience, we can think that our method is more reasonable.
Most of the size of the scraping paper in the world is about 2.5X19 cm, which is about 3.5 times larger than that of our country.
e. Detection. The content of comparing colors includes three aspects: background color, face color and ink color.
When judging the background color, the scraping paper should be turned away from the light source and compared by perspective.
In our country, the background color generally refers to the color of the scratched paper facing away from the light source and observed through perspective. Face color refers to the color of thin layer ink observed under normal conditions. Check the face color, black and colored light can be carried out at a light incidence angle of 35-45 °. Ink color refers to the color of thick layer ink. All inspections should be performed within 30 minutes of scraping.
Internationally, background color generally refers to the color of a thin layer of ink printed on paper. The face color refers to the color reflected by the thick layer of ink; the face color and ink color are meanings of words.
f. Review. When comparing colors, it should generally be done under the light in the north to avoid the illusion of using different light sources, because the work of comparing colors requires stricter light.
g. Solvent-based inks can also be compared using this method for scraping, but they can also be compared using scraping with a screw rod.
h. Lithographic inks and letterpress inks focus on the inspection of face and background colors; mesh inks and gravure inks focus on the inspection of surface and ink colors.
i. In order to check the color more accurately, you can use the printing suitability test execution (such as IGT printing suitability testing machine and RI-1 type printing suitability testing machine) to compare and judge after printing.
j. Offset printing is mostly multi-color register printing, so the ink requires sufficient transparency, so that when two or three colors are stacked together, the color reduction effect can be achieved. For example, printing the blue version on the yellow version is required to display green, otherwise it will lose the printing effect. To be able to show green, the blue ink should be transparent enough to allow yellow to pass through blue.
k. The transparency of the ink depends on the difference between the refractive index of the pigment and the binder. The smaller the difference, the more transparent the ink.
1. Ink made with high-dispersity pigments has greater transparency and more vivid colors. Multi-color overprinting with high-transparency inks is not restricted by color sequence. The ink made of chrome yellow can only be printed as the first color because of its poor transparency. Therefore, it is important for printing overprint to check the transparency of the ink.
m. The transparency of the ink can be expressed by the transmittance of a certain thin layer of ink. But now ink factories all use the scraping method to compare the brightness and darkness of the black stripes in the scraping paper.
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