In actual printing production, the biggest printing defect caused by poor overprinting is exposed. According to practical experience, the main causes of color image printing in multi-color overprint white are ink viscosity, printing pressure, printing speed, surface strength of paper, printing interval time, etc. Any one of the factors is not well controlled, will affect the printing quality of the final color print, easy to cause white failure.
Based on the above factors that affect the effect of overprint, this paper studies and discusses the rule of multi-color overprint in color printing. In order to achieve the above purpose, this paper focuses on the overprinting experiment from the factors that affect the overprinting effect, such as changing ink viscosity, printing pressure and printing speed, and analyzes the relationship between the experimental results and the overprinting whitening, and analyzes other factors that cause the printing whitening, such as paper surface performance, overprinting time interval, printing color sequence, and finally obtains the law of multi-color overprinting in color printing.
First, the impact of ink viscosity on overprinting
1. Viscosity of ink
It has a great impact on the wet overprinting effect, the viscosity of the ink is large, the ink layer shows a larger cohesion, if the viscosity of the first printing ink is lower than the viscosity of the post-printing ink, the post-printing ink will rely on its own cohesion, the first printing ink sticky away, resulting in "reverse overprinting" resulting in color mixing. So it is very important to control the viscosity.
2. Experiment on the influence of ink viscosity on overprint effect
Experimental equipment: S-1000 water-based four-color printing slotting machine and 4# viscosity test cup produced by Changsheng Industrial Co., LTD
Experimental agents: thickening agent, diluent, leveling agent
Experimental steps: ① Black is overprinted on blue, the ink viscosity of blue is 11.3s, the ink viscosity of black is 13s, the initial printing speed is 40 RPM /min, the printing pressure is 3.6mm, the substrate is B-type corrugated paper with white cardboard, and the initial exposure under this printing condition is 10%.
② Change the viscosity of the black ink, keep other initial conditions unchanged for printing, observe the exposure, and record the data as shown in Table 1.
③ According to the experimental data in Table 1, the relationship between ink viscosity and overprinting is drawn, as shown in Figure 1.
From the relationship curve in Figure 1, we can see that with the gradual increase of viscosity, the percentage of whitening gradually decreases within the range of 13 ~ 15s. However, when the viscosity increases to more than 15s, the phenomenon of whitening is further aggravated with the gradual increase of viscosity. When the viscosity increases to a certain extent, the base ink is glued by the post-printing ink. Therefore, the viscosity should be controlled at about 4s difference with the base color ink, and the ink viscosity can not be endlessly increased.
Second, the impact of printing pressure on overprinting
1. Printing pressure
The process of printing, in a sense, is the process of solving the contradiction between ink and paper. To transfer the ink on the printing plate to the substrate, a certain force must be applied, that is, the so-called printing pressure. It plays an important role in printing.
The control of printing pressure is not easy. Too much or too little printing pressure will lead to a reduction in the quality of the printed product. Especially, the pressure is too large, which brings more disadvantages, which often causes graphic distortion, local accumulation of ink, paper hair, the expansion of the dot of imprinting, and the unclear level of printing. On the contrary, the printing pressure is too small, it will also cause the transfer of printed graphics is not complete, the network is not real, and the defect exposure is more obvious. Therefore, in the printing process, it is very critical to control the printing pressure within a more appropriate range.
2. Experiment on the influence of printing pressure on overprinting
Experimental equipment: S-1000 water-based four-color printing slotting machine produced by Changsheng Industrial Co., LTD
Experimental steps: ① Black is overprinted on blue, the ink viscosity of blue is 11.3s, the ink viscosity of black is 13s, the initial printing speed is 40 RPM /min, the printing pressure is 3.6mm, the substrate is B-type corrugated paper with white cardboard, and the initial exposure under this printing condition is 10%.
② Change the printing pressure, keep other initial conditions unchanged for printing, observe the exposure, and record the data as shown in Table 2.
③ According to the experimental data in Table 2, the relationship between printing pressure and overprinting is drawn, as shown in Figure 2 below.
FIG. 2 Relation curve between printing pressure and overprinting
As can be seen from Figure 2, in the beginning of a certain range, with the increase of printing pressure, the percentage of exposure is constantly decreasing, but when the printing pressure continues to increase, the percentage of exposure also begins to increase, which indicates that the printing pressure is too small is not conducive to ink transfer, but when the pressure is too large, it is not conducive to ink transfer.
Third, the impact of printing speed on overprinting
1. Printing speed
From the performance of the printing press, the printing speed can be easily adjusted in a large range, and the printing pressure can not automatically follow the change in a functional relationship with the change in speed. Therefore, in actual production, it is always customary to fix the printing pressure first, and then adjust the ink quantity to adapt to the change of printing speed. The study of the influence of printing speed on ink transfer has a certain guiding significance for the adjustment of ink quantity, printing speed and printing pressure in production.
2. Experiment on the influence of printing speed on overprint
Experimental equipment: S-1000 water-based four-color printing slotting machine and stopwatch produced by Changsheng Industrial Co., LTD
Experimental steps: ① Black is overprinted on blue, the ink viscosity of blue is 11.3s, the ink viscosity of black is 13s, the initial printing speed is 40 RPM /min, the printing pressure is 3.6mm, the substrate is B-type corrugated paper with white cardboard, and the initial exposure under this printing condition is 10%.
② Change the printing speed, keep other initial conditions unchanged for printing, observe the exposure, and record the data as shown in Table 3.
Table 3 Overprint exposure at different printing speeds
③ According to the experimental data in Table 3, the relationship between printing speed and overprinting is drawn, as shown in Figure 3 below.
FIG. 3 Relation curve between printing speed and overprinting
As can be seen from Figure 3, with the continuous increase of printing speed, the percentage of overprint exposure is gradually increasing, that is to say, the phenomenon of overprint exposure is gradually increasing. Therefore, in the actual printing production, high printing speed is a negative factor for the overprint effect, but in order to improve the production efficiency of enterprises, we have to work hard to improve the printing speed. In actual production, we can only make up for the impact of high printing speed on overprint quality by controlling other influencing factors.
4. Analysis of other factors
1. Ink film thickness and color sequence
With the increase of the density of the ink film, the thickness of the ink film is also gradually increased, according to the order of the increase of the ink film thickness for printing, the best overprint effect. The final printing of high brightness ink can make the whole picture bright and bright; The lowest concentration, as the picture outline with a stronger color ink, should be printed after. Multi-color printing adopts the color sequence of blue → product → yellow, which gradually increases the thickness of the ink film, which is beneficial to improve the overprinting effect of multi-color printed matter.
2. Overprint interval
When carrying out two-color overprinting, the color printed first is required to have a certain drying time, otherwise, when printing the next color, there will be sticky, dot expansion and other failures.
Therefore, dry overprinting to control the overprinting time interval, if the time interval is too long, the first printing color quickly dry, and then the printing color has not begun to overprint, then the post-printing ink can not be attached to the first printing ink, forming ink spots; If the time interval is too short, the first printing color has no time to dry before overprinting, and the dry overprinting of the ink can not be completed well.
3. Surface properties of paper
① Ink absorption of paper. The ink absorption of paper is the performance of fiber properties, is a reflection of the interweaving situation between fibers, the nature and content of fillers and rubber materials, and its essence is the capillary adsorption of the pores between fibers on the ink binder. The absorption of ink by paper is related to the pressure of the printing machine, the length of the embossing time, the viscosity of the ink and the structure of the paper itself. Paper with more lignin has large ink absorption, such as news paper with less filler and adhesive, good ink absorption, easy to adjust the pressure, can properly reduce the pressure, so as to increase the printability of the printing plate. However, if the paper is too slack, the fiber gap is too large, when the ink viscosity is large, it will produce "hair" phenomenon; If the viscosity of the ink is small, the binder will quickly penetrate into the paper fiber, resulting in the phenomenon of "through printing". Therefore, it is necessary to adjust the ink viscosity properly to avoid the ink falling off the surface after the ink bonding material penetrates, making the product tarnish and affecting the printing quality.
② Elastoplasticity and smoothness of paper. The elasticity of paper refers to the degree to which the paper can recover after the printing pressure has disappeared. The plasticity of paper refers to the degree of deformation produced under the action of printing pressure. The elasticity of paper affects the ink absorption performance. The paper has good plasticity to ensure the elasticity of the paper, which directly affects the printing pressure and the amount of ink supply. Different operations should be performed according to the different properties of the paper. The printing pressure of the paper with greater elasticity should be larger, and the ink supply can be smaller; On the contrary, the printing pressure of the paper with low elasticity should be smaller, and the ink supply can be increased. The smoothness of the paper has a great impact on the overprinting effect, the smoothness is high, and the graphics of the overprinting page can be well performed, especially when the dot is overprinted, the small dot can be well restored, and the paper with low smoothness is easy to make the dot printing unclear and the plate layout unclear due to the depression on the surface of the paper.
V. Conclusion
To sum up, the factors that affect the overprint effect of color printing are more and more complex, and any factor that is not well controlled will affect the quality of the final printing. Based on the above discussion and experiment, we summarized the following color printing multi-color overprinting law:
① The viscosity of the ink should be reduced in turn, and the viscosity difference of the two overlapping inks is generally 4s. According to the viscosity of the ink and the surface properties of the paper are different, a reasonable grasp of the printing speed. Generally, it should be controlled at 80-120 RPM /min. ③ According to the viscosity of the ink and the different surface properties of the paper, control the printing pressure.
The printing process is an extremely complex process. In this process, with the transfer of ink, a variety of problems often occur. These problems are closely related to the quality of printed matter. In multi-color printing, if the overprinting of various colors of inks is not well controlled, the color of the printed matter will change, resulting in the quality of the printed matter. Therefore, we must analyze and study the overprinting of ink.
