In the actual production of printing, the biggest printing defect caused by poor overprinting is exposed white. According to practical experience, the main causes of multi-color overprinting in color image printing are ink viscosity, printing pressure, printing speed, paper surface strength, printing distance time, etc. Any of these factors are not well controlled, which will affect the ultimate color printed matter. The printing quality is very easy to cause dew failure.
This article is based on the above factors that affect the effect of overprinting, to study and discuss the law of multicolor overprinting in color printing. In order to achieve the above purpose, this article focuses on performing overprinting experiments from factors that affect the effect of overprinting, such as changing ink viscosity, printing pressure, and printing speed, and analyzes the relationship between the experimental results and overprinting, and also analyzes other factors that cause printing, such as paper surface function , Overprint time distance, printing color sequence, etc., and finally get the rule of multi-color overprint in color printing.
1. The effect of ink viscosity on overprinting
1. Ink viscosity
It has a great influence on the effect of wet overprinting. The viscosity of the ink is large, and the ink layer shows a large 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 to transfer the first ink Sticking away, resulting in "reverse overprinting" causing color mixing. Therefore, it is very important to control the viscosity.
2. Experiment of the effect of ink viscosity on overprinting effect
Experimental equipment: S-1000 water-based four-color printing and slotting machine produced by Changsheng Industry Co., Ltd., 4 # viscosity test cup
Experimental agent: thickener, thinner, leveling agent
Experimental steps: â‘ Overprint black on blue, the viscosity of blue ink is 11.3s, the viscosity of black ink is 13s, the initial printing speed is 40 rpm, the printing pressure is 3.6mm, and the substrate used is facial paper Type B corrugated paper, which is white cardboard, has an initial whiteness of 10% under the premise of printing.
â‘¡ Change the viscosity of black ink, keep other initial premises unchanged, print, observe the dew and record the data.
â‘¢ Through the experimental data in the table, draw a graph of the relationship between ink viscosity and overprint.
Second, the impact of printing pressure on overprinting
1. Printing pressure
The printing process is, in a sense, a process to resolve the contradiction between ink and paper. 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. If the printing pressure is too large or too small, it will lead to a reduction in print quality. Especially when the pressure is too great, it brings more disadvantages. It often causes graphic distortion, local accumulation of ink, paper roughening, expanded dots on the imprint, and unclear printed product levels. On the contrary, if the printing pressure is too small, it will also cause many failures such as incomplete transfer of printed graphics, false dots, and more significant defect exposure. Therefore, in the printing process, it is very pivotal to control the printing pressure within a more appropriate range.
2. Experiment of the influence of printing pressure on overprinting
Experimental equipment: S-1000 water-based four-color printing and slotting machine produced by Changsheng Industry Co., Ltd.
Experimental steps: â‘ Overprint black on blue, the viscosity of blue ink is 11.3s, the viscosity of black ink is 13s, the initial printing speed is 40 rpm, the printing pressure is 3.6mm, the substrate used is facial Type B corrugated paper, which is white cardboard, has an initial whiteness of 10% under the premise of printing.
â‘¡ Change the printing pressure, keep other initial premises unchanged for printing, observe the whiteness, and record the data.
â‘¢ Through the experimental data in the table, plot the relationship between printing pressure and overprint.
3. The effect of printing speed on overprinting
1. Printing speed
Judging from the function of the printing machine, the printing speed can be conveniently adjusted in a larger range, but the printing pressure cannot automatically follow the change according to a certain functional relationship with the speed change. Therefore, the actual production is always used to fix the printing pressure first, and then adjust the amount of ink to adapt to changes in printing speed. Studying the effect of printing speed on the transfer of ink has certain guiding significance for the adjustment of the amount of water ink, printing speed, printing pressure, etc. in production.
2. Experiment of the effect of printing speed on overprinting
Experimental equipment: S-1000 waterborne four-color printing slotting machine and stopwatch produced by Changsheng Industry Co., Ltd.
Experimental steps: â‘ Overprint black on blue, the viscosity of blue ink is 11.3s, the viscosity of black ink is 13s, the initial printing speed is 40 rpm, the printing pressure is 3.6mm, and the substrate used is facial paper Type B corrugated paper, which is white cardboard, has an initial whiteness of 10% under the premise of printing.
â‘¡ Change the printing speed, keep other initial premises unchanged and print, observe the dew and record the data.
â‘¢ Through the experimental data in Table 3, plot the relationship between printing speed and overprint.
4. Analysis of other factors
1. Ink film thickness and color sequence
As the density of the ink film increases, the thickness of the ink film also gradually increases, and printing is performed in the order in which the thickness of the ink film increases in sequence, and the overprint effect is the best. The final printing of the ink with high brightness can make the whole picture bright and bright; and the density is the lowest, as the ink with thicker color for the outline of the picture should be printed afterwards. Multi-color printing adopts the cyan → product → yellow color sequence that gradually increases the thickness of the ink film, which is beneficial to improving the effect of multi-color printed matter overprinting.
2. Overprint time distance
When two-color overprinting is required, the first printed color must have a certain drying time, otherwise, the next color printing will cause sticking, dot expansion and other failures. Therefore, dry overprinting should control the time distance of overprinting. If the time distance is too long, the first printing color is dried quickly, and the overprinting has not started to overprint, then the post-printing ink cannot be attached to the pre-printing ink, forming ink spots; If the time distance is too short, overprinting after printing is too late to dry, the dry overprinting of the ink cannot be completed well.
3. The surface function of the paper
â‘ Ink absorption of paper. The ink absorption of paper is the performance of fiber properties, reflecting the interweaving conditions between fibers, the nature and content of fillers and gums, etc. Its essence is the capillary adsorption of pores between fibers to the ink binder. The paper's absorption of ink is related to the pressure of the printing machine, the right and wrong of the printing time, the viscosity of the ink and the structure of the paper itself. Paper with a lot of lignin has high ink absorption, such as newsprint with less filler and glue, good ink absorption, and the pressure can be easily adjusted to reduce the pressure appropriately, which can increase the printing plate's durability. However, if the paper is too slack and the fiber gap is too large, when the ink viscosity is large, it will produce "brushing" phenomenon; if the ink viscosity is low, the binder will quickly penetrate into the paper fibers, and "through-printing" phenomenon. Therefore, it is necessary to properly adjust the viscosity of the ink to avoid the ink from falling off after the ink binder penetrates and bleeds, which will cause the product to lose luster and affect the printing quality.
â‘¡Elastoplasticity and smoothness of paper. The elasticity of the paper refers to the degree to which the paper can recover after the printing pressure disappears. The plasticity of the paper refers to the degree of deformation caused by the printing pressure.
The elasticity of the paper affects the ink absorption function; the paper has a good plastic machine to ensure the elasticity of the paper, which directly affects the printing pressure and the amount of ink supply. Depending on the nature of the paper, different manipulations should be applied. The printing pressure of paper with higher elasticity should be higher, and the ink supply can be smaller; on the contrary, the printing pressure of paper with lower elasticity should be lower, and the ink supply can be increased. The smoothness of the paper has a great influence on the overprinting effect. The smoothness is high. The graphics and text of the layout after overprinting can be well expressed, especially the small dots can be restored well when the dots are overprinted. Trapped, it is easy to make the dot printing unclear, and the level of the printing plate is not clear.
V. Conclusion
In summary, there are many and complex factors that affect the overprinting effect of color printing. Any one factor that is not well controlled will affect the quality of the final printing. Based on the above discussion and experiments, we have summarized the following multi-color overprinting rules for color printing:
â‘ The viscosity of the ink should be reduced in sequence, and the difference between the viscosity of the two stacked inks is generally 4s. â‘¡According to the difference of the viscosity of the ink and the surface properties of the paper, the printing speed should be properly controlled. Generally, it should be controlled at 80-120 rpm. â‘¢ According to the viscosity of the ink and the different surface functions of the paper, control the printing pressure, and without affecting the expansion of the dot, when the ink and the selected paper are better, the printing pressure should be increased.
OSFE INDUSTRIAL CO.,LTD , http://www.1758cookware.com