This article is based on these factors that affect the effect of overprinting, and researches and discusses the law of multicolor overprinting in color printing. In order to achieve the above objectives, this article focuses on the overprinting experiment by changing the factors such as ink viscosity, printing pressure and printing speed that affect the overprinting effect, and analyzes the relationship between the experimental results and the overprinting whiteness, and analyzes other factors that cause the printing whiteness, such as the paper surface performance. , Overprinting interval, printing color sequence, etc. Finally, the law of multi-color overprinting in color printing is obtained.
First, the effect of ink viscosity on the overprint
1. The viscosity of the ink
It has a great influence on the wet overprinting effect. The viscosity of the ink is large, and the ink layer shows a greater cohesive force. If the viscosity of the ink to be printed first is lower than the viscosity of the ink after printing, the ink after printing will rely on its own cohesion, and the ink to be printed first Stick away, resulting in "reverse overprint" causing color mixing. So controlling the viscosity is very important.
2. The effect of ink viscosity on the overprint effect
Experimental equipment: S-1000 water-based four-color printing slotting machine and 4# viscosity testing cup produced by Changsheng Industry Co., Ltd.
Experimental Pharmacy: Tackifiers, thinners, leveling agents
Experimental procedure: 1 Superimpose black on blue, blue ink viscosity is 11.3s, black ink viscosity is 13s, initial printing speed is 40r/min, printing pressure is 3.6mm, substrate used is white paper. Jammed B-type corrugated paper, the initial whitening under this printing condition was 10%.
2 Change the viscosity of the black ink, keep the other initial conditions unchanged, print, observe the whitening condition, and record the data as shown in Table 1.
3 According to the experimental data in Table 1, draw the ink viscosity and overprint relationship curve, as shown in Figure 1.
From the relationship curve of Figure 1, we see that as the viscosity increases gradually, the percentage of whiteness gradually decreases in the range of 13 to 15 seconds, but as the viscosity increases to more than 15 seconds, the viscosity increases with the gradual increase. The whitening phenomenon is further aggravated. When the viscosity is increased to a certain degree, the bottom ink is adhered to the ink after printing. Therefore, the viscosity should be controlled to be about 4s different from the background ink, and the ink viscosity cannot be increased endlessly.
Second, the impact of printing pressure on the overprint
1. Printing pressure
The printing process, in a sense, is the process of resolving the conflict between ink and paper. In order to transfer the ink on the printing plate to the substrate, a certain force must be applied, the so-called printing pressure. It has an important role in printing.
Printing pressure control is not easy. Excessive or too low printing pressure can lead to a decrease in the quality of the printed product. In particular, the pressure is too great, which brings more disadvantages. It often causes graphic distortion, ink accumulation, paper pulling, enlarged dot prints, and unclear levels of printed matter. On the contrary, if the printing pressure is too low, it may cause many failures such as the incomplete transfer of printing and graphic images, the lack of network points, and the more obvious defects. Therefore, it is very important to control the printing pressure within a proper range during the printing process.
2. Effect of printing pressure on overprint
Experimental equipment: S-1000 water-based four-color printing slot machine produced by Changsheng Industrial Co., Ltd.
Experimental procedure: 1 Superimpose black on blue, blue ink viscosity is 11.3s, black ink viscosity is 13s, initial printing speed is 40r/min, printing pressure is 3.6mm, substrate used is white paper. Jammed B-type corrugated paper, the initial whitening under this printing condition was 10%.
2 Change the printing pressure, keep the other initial conditions unchanged, print, observe the whitening condition, and record the data in Table 2 below.
3 According to the experimental data in Table 2, plot the relationship between printing pressure and overprint, as shown in Figure 2.
Figure 2 Print pressure and overprint relationship curve
From Figure 2, we can see that in a certain range at the beginning, with the increase of the printing pressure, the percentage of whiteness is constantly declining, but when the printing pressure continues to increase, the percentage of whiteness begins to increase, which means that the printing pressure is too high. Small is not conducive to ink transfer, but when the pressure is too high, it is also not conducive to ink transfer.
Third, the impact of printing speed on the overprint
1. Printing speed
From the point of view of the performance of the printing press, the printing speed can be easily adjusted within a relatively large range, and the printing pressure cannot follow the change automatically according to a certain functional relationship with the speed change. Therefore, the actual production is always used to first fix the printing pressure, and then adjust the amount of ink to adapt to changes in printing speed. Studying the influence of printing speed on ink transfer has certain guiding significance for the adjustment of the amount of ink and water in production, printing speed, and printing pressure.
2. Effect of printing speed on overprint
Experimental equipment: S-1000 water-based four-color printing slot machine and stopwatch produced by Changsheng Industry Co., Ltd.
Experimental procedure: 1 Superimpose black on blue, blue ink viscosity is 11.3s, black ink viscosity is 13s, initial printing speed is 40r/min, printing pressure is 3.6mm, substrate used is white paper. Jammed B-type corrugated paper, the initial whitening under this printing condition was 10%.
2 Change the printing speed, keep the other initial conditions unchanged, print, observe the whitening condition, and record the data as shown in Table 3.
Table 3 Overprinting at different printing speeds
3 According to the experimental data in Table 3, plot the relationship between printing speed and overprint, as shown in Figure 3 below.
Figure 3 Printing speed and overprint relationship curve
From figure 3, we can see that as the printing speed continues to increase, the percentage of overprinting is gradually increasing, which means that the overprinting phenomenon gradually increases. Therefore, in the actual printing production, the high printing speed is overprinted. The effect is an unfavorable factor, but in order to increase the production efficiency of enterprises, we have to work hard to increase the printing speed. Therefore, in actual production, we can only compensate for the impact of high printing speed on the quality of overprint by controlling other influencing factors.
Fourth, other factors analysis
1. Ink film thickness and color sequence
As the density of the ink film increases, the thickness of the ink film also gradually increases. Printing is performed in the order of increasing thickness of the ink film, and the overprinting effect is best. The high-brightness ink is printed at the end, which makes the entire screen bright and bright, while the ink with the lowest density is used as the ink for the outline of the screen. Multi-color printing adopts the order of blue, yellow and yellow, which gradually increases the thickness of the ink film, which is advantageous for improving the overprint effect of multicolor prints.
2. Overprint interval
In the case of two-color overprinting, it is required that the color to be printed first has a certain drying time. Otherwise, when the next color printing is performed, problems such as stickiness and dot gain may occur.
Therefore, dry overprinting should control the time interval of overprinting. If the time interval is too long, the first printing color is rapidly dried, and then the printing ink has not yet begun to be overprinted, then the latter printing ink cannot adhere to the first printing ink to form ink spots; If the time interval is too short, the color printing is too late to print, and the dry-printing of the ink cannot be completed.
3. Surface properties of paper
1 Paper absorbency. The ink absorption property of paper is the performance of fiber properties. It is a reflection of the interweaving condition of fibers, the nature and content of fillers and rubber compounds, and its essence is the capillary adsorption effect 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 stamping time, the viscosity of the ink, and the structure of the paper itself. The paper containing more lignin has high ink absorbency, such as newsprint with fillers and rubber materials, good ink absorption, and the pressure is easily adjusted, and the pressure can be appropriately reduced, thereby increasing the printing durability of the printing plate. However, if the paper is too loose, the fiber gap is too large. When the viscosity of the ink is large, a phenomenon of “pulling hair†occurs. When the viscosity of the ink is small, the linking material quickly penetrates into the paper fibers, resulting in “cross-printing†phenomenon. Therefore, the viscosity of the ink needs to be properly adjusted to prevent the surface ink from falling off after the penetration of the ink binder material, causing the product to lose luster and affecting the printing quality.
2 paper elasticity and smoothness. The elasticity of paper refers to the extent 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 pressure of printing. The elasticity of the paper affects the ink absorption performance; the paper has good plastic properties to ensure the elasticity of the paper, which directly affects the printing pressure and the amount of ink supply. Different operations should be performed depending on the nature of the paper. Slightly elastic paper printing pressure should be greater, the amount of ink can be smaller; conversely, the printing pressure of small elastic paper should be smaller, the amount of ink can be increased. The smoothness of the paper has a great influence on the overprinting effect, and the smoothness is high. After the overprinting, the graphic image of the layout can be well expressed. Especially when the dots are overprinted, the small dots can be well restored, and the smoothness of the paper is low due to the presence of flaws on the surface of the paper. Sinking, easy to make the network printing is not clear, the layout of the printing plate is not clear.
V. Conclusion
In summary, the factors influencing the color printing overprint effect are more and more complicated. The poor control of any one factor will affect the final printing quality. Based on the above discussion and experiments, we have summarized the following rules for multicolor printing in color printing:
1 ink viscosity should be reduced in order, the difference between the viscosity of two-phase ink is generally 4s. 2 According to the viscosity of the ink and the surface properties of the paper, reasonably control the printing speed. Should generally 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 complicated process. In the process, various problems often occur accompanied with the transfer of the ink. These problems are closely related to the quality of the printed matter. In multi-color printing, if the overprint of each color ink is not well controlled, the color of the printed matter will be changed, resulting in the quality problem of the printed matter. Therefore, we must analyze and study the overprint condition of the ink.
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