Any professional equipment and technology cannot guarantee that the printed sheet and the original color are consistent. Offset copying can only create conditions as much as possible in reducing color difference. In addition to the plate making quality meeting the process requirements, we need to find a solution to this problem from the offset printing process and raw material selection.
1. Paper whiteness and absorbency
① Whiteness of paper
Whether the color of the paper is pure white is the basis for bright printing colors. Completely pure white paper can reflect nearly all shades of light. On the contrary, gray and black paper will absorb part of the color light and cause the loss of hue, lightness and purity of the imprint. From the analysis of color purity alone, the printed sheet printed with pictures, except for the spatial mixing of dots of various colors to form people's visual impression The color of the paper surface without dots (or graphics) actually participates in this kind of space mixing. The gray paper seems to add a small amount of black to the ink of each color, so it is invisible from the vision. Highly pure color ink, which shows a dark and cloudy color. Practice has proved that if other conditions are exactly the same, the degree of vividness of the imprints printed on paper with different whiteness is completely different.
From the theoretical analysis of the brightness contrast of the color alone, if only black text is printed on white paper. Due to the contrast between black and white, the black characters are darker and the white paper is whiter, forming a sharp contrast. It can be inferred from this that the whiteness of the paper used as the basis of printing has a direct relationship with the vividness of the imprint.
In the same way, the reddish paper makes the imprint slightly inclined to red; the cyan paper makes the imprint slightly inclined to green, which needs to be estimated when using color.
②Absorptivity of paper
For paper with high absorptivity, the imprint cannot form a rich enough ink layer on the paper surface, and the excessive binder penetrates into the paper, making the imprint dry; the paper with large capillary pores will allow some pigments to penetrate into the paper together with the binder, and the natural print Color saturation suffers. That is, the color cannot be fully presented on the surface of the paper, the "color" is reduced in terms of visual effect, the printed matter is dull, and the color reproduction is not bright and full.
2. Ink color matching and rolling, ensuring bright colors during coating
We all know that the saturation of a color refers to the purity of the color. To make the colors of various ink colors vivid, we must pay great attention to the selection of ink colors. Because there are many types and colors of offset printing inks, in addition to the relatively pure color of the standard three-primary inks, there are also different colors in each color of general ink colors. For example, yellows include: lemon yellow, transparent yellow, medium yellow, deep Yellow, withered yellow; blue includes: peacock blue, sky blue, medium blue, dark blue, royal blue, and light blue; red includes: withered red, golden red, bright red, deep red, rose red, light red; even black ink also has It feels yellowish, blueish, and reddish, and some ink colors have a certain unique hue. For example, when printing lush landscape forests in early spring, light yellow should generally be used, which can make the printed leaves green and lovely. If dark yellow ink is used, which contains red flavor, red is the complementary color of green, and the printed leaves will be burnt black and turbid, which is just like ink. It can't reflect the breath of early spring, let alone bright.
There are many similar considerations. In ink color matching, we must pay attention to the application of color theory, especially the deployment of complex color inks. To ensure bright colors, it is necessary to avoid the appearance of complementary relationships.
When ink color is applied, pay attention to the cleaning of the ink fountain and ink roller, that is, when changing colors, prevent color shift caused by the ink of the ink fountain, ink roller and water roller being stained with one ink color, especially for dark colors. , black, blue to yellow, if the cleaning work is not careful to cause color shift, it will inevitably bring color difference to the printed matter.
Two-color machines and multi-color machines should pay attention to the color mixing phenomenon between the front and back colors, especially the two-color combination and the four-color combination of offset printing machines that use an impression cylinder. More attention should be paid to the color mixing between color groups. , and the changed color must not meet the bright quality requirements.
3. Full copy of outlets
Dot is the most basic unit of color reproduction, its quality is directly related to the quality of color reproduction, the expansion of dots, the expanded color will have to reduce the thickness of the ink layer, which makes the low-key and its bottom part not thick, and the color is saturated If the intensity is weakened, the inherent hue and characteristics of this color cannot be brought into play, so the complexion of the printed characters is distorted, and the printed landscape is dim.
4. The coloring power of the ink and the influence of emulsification and dilution
In addition to the special case where the color needs to be diluted, the saturation of the printing ink may be reduced due to factors such as the low tinting power of the color ink, or the inappropriate addition of many auxiliary materials to dilute the ink during the process, or the large amount of emulsification of the ink. Imprinted color will not get bright effect.
Due to the dilution of the ink or the addition of auxiliary materials that inhibit drying and the large emulsification value, it will dry slowly when printed on the paper, and the permeability will be strong. The penetration time will make the imprinted ink layer not thick, which is also the reason why the color of the printed product is not bright.
5. The influence of the thickness of the imprinted ink layer
If the above-mentioned conditions are good and the depth of the pattern is in line with the original, then whether the thickness of the imprinted ink layer is just full, this is the key factor for whether the color is in line with the original, especially whether it is bright or not, which is especially important when issuing printing samples. important. Because of the thickness of the ink layer of each color, if it deviates from the original original, it will inevitably distort the hue of the superimposed picture. Therefore, the ink volume control of the first color as the basic color must be very cautious. If the foundation is not well laid, it will be difficult to remedy the colors in the future. In order to make the hue close to the original, the base color ink is less, and the color needs to be reduced in the future, so that the resulting printed matter is not bright at all, but can only make the picture old, dark, flat, or even seriously distorted.
Conversely, if the ink layer of the basic color is printed too thick, the colors in the future can only be thicker. If it is not thicker, the hue will lose balance. If it is flat, the printed pattern will be dull, thick, and unclear, and the effect of bright colors will not be mentioned.
At present, the customary color used in the industry tends to be bright. The ink layer is thick, according to the terminology of printing: the color of the screen should "jump". "Jump" means that the imprinted ink color should be "stacked" thick, which is for some products: such as prints, which may meet the needs of the original manuscript. But it is worth considering for most natural color photo originals or drawings. What's more, too much emphasis on the thickness of the "pile" will cause many difficulties in printing. Therefore, this habit should be used flexibly and adapted to local conditions. But this is not only a problem of printing technology, but also involves the color habits of plate making, especially the habits of customers, that is, the evaluation of product quality requirements.
The Colorimeter will provide accurate measurement data and can provide customers with product quality assurance based on numbers.
