Reasons for Color Differences in Ink Printing of Packaging Boxes
In order to improve the design and quality of printed materials, printing factories often use special inks when printing trademarks, packaging, and promotional materials to ensure color consistency and meet customer requirements. Next, we will discuss the reasons for color differences in special ink printing.
1、Printing process
1. Ink layer thickness and ink balance
The control of ink hue is mainly achieved by controlling the thickness of the ink layer. Changes in Ink Layer Thickness 0.1 μ M will cause a significant impact on the color difference between the left and right △ E=1.5-4.5NBS. The reproduction of ink color in offset printing is closely related to the balance of ink and wash. At the same ink level, water levels can affect the depth of color. It is important to correctly grasp the balance of ink and wash, and pay special attention to the pH value of the wetting solution. The principle of water usage is to be as small as possible and not dirty.
2. Printing pressure
The magnitude of printing pressure has a significant impact on the transfer of ink from printing to paper. When the printing pressure is insufficient, the ink transfer is insufficient and not suitable for printing; When the printing pressure is too high, the ink transfer rate not only does not increase due to the diffusion of printing ink to the blank areas outside the graphics, but also brings other drawbacks, making it unsuitable for printing. Only within an appropriate printing pressure range can thick, clear images, rich stages, and good color reproduction printed products be printed.
3. Printing speed
The increase in printing speed will shorten the contact time between printing surfaces, resulting in blurred dots, reduced ink transfer rate, and decreased printing quality. In order to ensure sufficient contact between the printing surface and improve printing quality, it is necessary to control the printing speed under a certain printing pressure to obtain a stable ink layer thickness.
4. Dry regression density difference
For the same paper, the newly printed sample has a higher density, and after a period of time, as the ink dryness and smoothness decrease, the density value will decrease. Due to the different density values of the ink layer before and after drying, the colors presented in the printed images are also different. The newly printed ink is still wet and differs from the dry state. This is because the newly printed ink layer has a certain degree of leveling, so the surface reflection is mainly mirror reflection, which looks bright and has good luster. When the ink layer is in a dry state, the surface reflection is mainly diffuse, and the color is naturally darker than when it was just printed. Therefore, the ink color of the general printing sample is slightly darker than that of the standard sample, but the depth should be controlled by data. In the sample, measure the color density value of the newly printed color, and in the printing, refer to these density values to print, which can make the ink color close to consistency. A density meter with a polarized color filter can eliminate the light generated by mirror reflection, and the measured humidity is very close to the dry color density value, while the measured ink layer dry humidity value is very small.
2、 Surface decoration of printed materials
Surface decoration is the appropriate processing of printing surfaces to improve their light resistance, heat resistance, water resistance, wear resistance, folding, and chemical resistance; Increase luster and artistic sense; Protect and beautify printing, enhance printing value.
The surface decoration methods of packaging products mainly include polishing (printing varnish, coating varnish, matte oil, UV varnish, water-based varnish, etc.), wax, coating film (glossy film, matte film), concave convex printing, hot stamping, etc. After these surface treatments, printed materials will have varying degrees of color and saturation changes, namely differences in density. These changes can be divided into physical and chemical changes. The physical changes are mainly reflected on the surface of the product, increasing its specular and diffuse reflection capabilities. Generally speaking, when using composite film, coated varnish, and UV varnish, the color density increases; When coated with matte film and matte oil, the color density decreases. The chemical changes mainly come from the organic solvents contained in the laminating adhesive and water-based UV varnish, which combine with the ink particles on the printed material, changing the color of the ink layer.
3、 Printing materials
1. Paper whiteness
The factors that affect the whiteness of paper include:
① Whiteness of pulp: the most important influencing factor.
② Dyes: The selection of dyes should be near the main wavelength for measuring whiteness, without affecting their reflectivity, that is, the whiteness of the paper.
③ Filler: depends on its type and grade.
④ Coating: The whiteness of coated printing paper is determined by the whiteness of the original paper, the whiteness of the coating, and the amount of coating.
Paper with different whiteness and a certain color (such as some paper that is blue or yellow) has different effects on the color of the printing ink layer. Therefore, in actual production, it is advisable to choose paper with the same whiteness for printing to reduce the impact of paper whiteness on printing colors.
2. The smoothness and gloss of paper
From the perspective of its impact on the quality of printed materials, there is a very direct relationship between the glossiness of paper and the glossiness of printed materials. Regardless of the type of ink used, the glossiness of printed materials will increase with the increase of the glossiness of the paper itself. The glossiness of paper is directly related to its inking efficiency. When the paper has the same ink film thickness, high gloss paper can achieve higher printing density. Therefore, the gloss of printed materials is closely related to the gloss of the paper itself.
3. Acceptance and absorption of paper ink
Ink acceptance performance refers to the ability of a paper surface to immediately receive and transfer ink during the printing process. It is closely related to three aspects of paper: the ability of the paper surface to receive printing ink; The ability of paper surface to absorb certain ink components; The ability to fix and retain a uniform ink film on the surface of paper. The absorption performance of ink by paper occurs over a long period of time from the contact between ink and paper to the complete solidification of paper, which is related to the absorption of low viscosity components in ink by paper pores and the penetration of certain components in ink into paper.
Printing pressure, printing time, ink viscosity, and paper capillary radius all affect the paper's ability to absorb ink. When the same ink is printed on paper with different absorbency under the same conditions, there will be different printing densities. The surface of paper has pore structures formed by various heterogeneous and complex structural elements. In order to achieve good uniformity and smoothness on the surface of the paper, it is usually necessary to apply different thicknesses of paint on the surface of the paper. The properties and thickness of the coating determine the ink absorption capacity of the paper surface. Different absorption abilities result in different colors of the printing ink layer.
4、 The properties of ink
The final effect of printed materials is closely related to the physicochemical properties of ink, and the composition of ink determines its properties:
1. The size and dispersion degree of pigment particles
Although the role of ink film capillary is an important factor in forming printing gloss, the apparent size of pigment particles in a dispersed state is more important, directly determining the state of ink film capillary.
Therefore, the pigment particles are small and well dispersed, which is conducive to forming a smooth ink film and improving the gloss of printed materials.
2. The viscosity of the connecting material and the pigment content
According to the principle of interface chemistry, the permeation rate of capillaries is opposite to the viscosity of liquids, that is, as the viscosity increases, the permeation rate decreases. The capillary network structure formed between ink pigment particles is an important aspect that determines the gloss of printed products. During printing, the ink is pressed into larger paper holes as a whole; After printing, the connecting material begins to separate from the ink and seep into smaller paper holes. Therefore, the size of the ink film capillary determines the separation of the connecting material. The effect of capillary retention of connecting material is much greater than the printing pressure of pressing the connecting material into the paper hole.
3. Drying time
Rapid drying of ink film on paper can reduce the amount of connecting materials entering the paper holes, thereby improving the gloss and smoothness of the ink film. The high gloss of the ink film means that the mirror reflection on the surface is relatively high, allowing the human eye to see that the white light reflected on the surface is much lower than that of the low gloss ink film in most cases, resulting in higher color saturation. Therefore, high gloss printing looks brighter and fuller. The printing ink layer of most special inks is semi transparent or opaque. When choosing an ink formula, the impact of the ink layer on special printing should also be considered based on the actual situation.
The above are the reasons for the color difference in special ink printing. There are many factors that can cause color difference in special ink printing, but mainly include material properties, ink properties, printing conditions, and printing surface decoration. These factors can affect the color reproduction in the special printing process. Xinwei is engaged in the research and production of new materials such as grain coatings, grain art inks, rubber oils, water droplet oils, pearl point oils, and orange peel oils.
