Effect of printing speed on ink transfer
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Ink is the basic element of the printed image. The thickness of the ink directly affects the layer, color, and definition of the image. Therefore, it is very important to control the transfer of the ink. In general, the total ink transfer rate of offset printing is relatively low, about 38%. Paper, ink suitability, plate, blanket suitability, ink balance, plate, printing pressure and other factors will affect the amount of ink transfer and uniformity. From the performance of the printing press, the printing speed can be easily adjusted within a large range, and the printing pressure cannot automatically follow the change according to a function relationship with the change of the speed. Therefore, in actual production, it is always customary to fix the printing pressure first, 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 ink quantity, printing speed and printing pressure in production.
First, the experiment
In order to study the influence of printing speed on ink transfer amount and ink uniformity, the following test strip was designed: a 40 cm × lcm field, placed in the mouth of the mouth, used to measure the density of the ground and the uniformity of ink distribution in the axial direction of the drum. . Two 20-step scales, which are incremented by 5%, are placed on both sides of the image to measure the increase value of the dots at each level and calculate the relative contrast. The outlets used are circular amplitude modulation dots, and the number of screen lines is l75LPl. In the experiment, SBK-B iodine gallium lamp printing machine is used for printing, the lamp power is 3500 (TH4200)W, manual development, and the levels on the scale are clearly defined. Using the DigiDens T6CR color reflection densitometer, the level error of each level is controlled within ±1%.
The paper is 80g/m2 double-sided coated paper, which is printed by WIN500 monochrome offset printing machine. The inking system is arranged in ten-roll Z-shape. The inking parameters are: accumulation coefficient Kj=3.41; uniform ink coefficient Ky=2.49; inking coefficient Kg = 0.92. By observing the reflection of the printing plate during printing, the amount of water is controlled by the dark ash. The printing pressure is controlled by adjusting the roller wheelbase and observing the width of the ink mark on the blanket cylinder to judge the pressure between the three rollers. When the pressure is normal, the ink roller width of the blanket cylinder should be 4±0.5mm, and the pressure is stabilized during the printing process. . In order to measure the effect of printing speed on ink transfer, printing was performed at 1, 5, and 9 (×1000 RPH), respectively.
Experiment l Ink supply control: 1 adjust the ink fountain key to control the ink output, set the ink roller to take the number of teeth to l, start the plate with 1000RPH to the field density of 1.75, and after printing 100 sheets, from 100 proofs Randomly extracted 10 for the analysis of the measurements. 2 Print at 5000 RPH for 10 minutes, take samples every 2 minutes, take a total of 10 sheets. 3 Print at 9000 RPH for 10 minutes, take samples every 2 minutes, and take a total of l0 sheets.
Experiment 2 Ink supply control: 1 Adjust the ink cartridge key to control the ink output, set the ink roller to move the number to 5, start the plate with 9000RPH to the solid density of l.7, and after printing 100 sheets, from 100 proofs Ten sheets were randomly selected for measurement analysis. 2 Print at 5000 RPH for 10 minutes, take samples every 2 minutes, take a total of l0 sheets. 3 Print at 1000RPH for 10 minutes, take samples every 2 minutes, take a total of 10 sheets.
The extracted samples were measured for the corresponding solid density and dot values using a DigiDens T6CR color reflection densitometer, and the relative contrast K value was calculated. The K value was calculated using the average solid density and the average density value of the dots of 75%.
Second, experimental analysis
1. Field density analysis
The solid color block is actually a layer of ink film with a certain thickness. The solid density is determined by the properties of the ink pigment and the binder to absorb light, and is also affected by the thickness of the ink layer and the state of the ink film. In printing, once the variety of ink is determined, the factors affecting the density of the ink in the ink are the thickness of the ink film and the state of the conjunctiva. The quality of the print is controlled by controlling the thickness of the ink film during printing. Measure the density value of the 40 cm × lcm field test strip at the mouth of the sample, take a test point every 2 cm, and measure the average of 10 samples to be listed in Table 1.Table 1 Field homogeneity of the experiment
The experiment uses the inking system of the printing machine. The pressure between the rollers is generated by the self-weight of the roller. The pressure between the rollers cannot be adjusted according to the change of the printing speed. When the printing speed is increased, the pressure between the rolls is substantially constant, and the time of contact between the rolls and the rolls is reduced, resulting in a decrease in the ink transfer rate and a decrease in the ink-dispensing performance. Table 1 compares the solid density values at three printing speeds of l000RPH, 5000RPH, and 9000RPH. The dispersion is different, and the standard deviations are 0.018, 0.032, and 0.045, respectively. It shows that the printing speed not only affects the transfer rate of the ink, but also significantly affects the ink distribution performance. The greater the printing speed, the worse the leveling performance. The inking system of this experimental printer is a typical short ink path, and the uniform ink coefficient is Ky=2.49. Compared with the large offset printing machine, due to the short ink path, the uniform ink performance is not ideal, and the higher printing speed is not suitable. . From the data in Table 1, the similar situation is also seen in the case of large ink supply. Some studies have also shown that even if the printing machine is in a stable state, there will be a density change of about 0.02; the structural difference of the ink transfer device will cause a density change of about 0.12 between the mouth and the tip.
By comparing the gradation density values of the three printing speeds in Fig. 1, the faster the printing speed, the smaller the actual density of the printing product in the case of the same ink supply amount. As the printing speed increases, the time between the printing plate and the blanket, the blanket and the paper is reduced, and the amount of ink transferred to the surface of the substrate is also reduced, and the density is reduced, resulting in the ink color of the print. shallow. Figure 2 shows that the normal ink supply at 9000 RPH at the printing speed is significantly larger for the printing speed of 1000 RPH. Under the 9000RPH printing speed, the ink supply amount of 1.68 density is obtained. At the low printing speed of 1000RPH, the corresponding solid density reaches 2.13, resulting in the combination of the middle and dark levels. When 5000RPH printing speed, the combination is from the 6th mesh point. At the beginning, at 1000RPH printing speed, the merger starts at the 4th mesh point. It shows that the printing speed has a huge impact on ink transfer.
2. Network analysis
Offset printing uses dots as the basic element of the image, and changes in the dots necessarily affect the changes in the tone, color and image sharpness of the image on the printed matter. The dot gain value refers to the difference between the dot area of a certain portion of the printed matter and the dot area on the corresponding color separation sheet. Due to the double reflection effect and pressure extrusion of the ink, the dot gain is inevitable. However, the increase value of the dot should be controlled within a certain range. Once the range is exceeded, the quality of the printed product will be seriously degraded, such as the loss of the tone, the density, the color change, etc., and the increase is abnormal. According to the expansion value of 50% outlets specified in GB7705-87, the fine products should be in the range of 8% to 20%, and the general products should be in the range of 10% to 25%.
From Figure 3 and Figure 4, the printing speed has an obvious influence on the increase of the dot value. Under high-speed printing, the amount of ink transfer is small, and the contact time between the inking roller and the printing plate, the printing plate and the blanket, the blanket and the paper is also reduced, and the degree of extrusion of the ink is also small, and the comprehensive result is that the dot is enlarged. The value is small, especially in the mid-high light area, which is very obvious. The dot gain value at the low ink speed shown in Figure 4 is as high as 50% at 50%, which is obviously not in compliance with the printing quality requirements. Since the layer range of the original is generally larger than the range that can be copied by printing, the larger the amount of ink transfer in theory, the thicker the ink layer and the higher the density, and the print can achieve a larger level. However, the amount of ink transfer is increased, which will inevitably lead to excessive value increase of the dot, merging, tone, color and image clarity. When it is serious, the back will be dirty, and the print will appear. The situation of flying ink under the speed, seriously affecting the quality of printed matter. In production, by controlling the printing contrast K value, it is possible to control the solid density and the dot gain value more reasonably. Table 2 shows the print contrast K values for the six cases in the two experiments. It can be seen that the printing speed affects the print contrast K value by affecting the amount of ink transfer.
The data of Experiment 1 shows that as the printing speed increases, the amount of ink transfer decreases, and the printing contrast value decreases. In this case, the ink supply amount can be increased to increase the printing contrast value, but the ink uniformity of the machine should be closely monitored to avoid poor uniformity at high speed. The data of Experiment 2 shows that under the condition of high ink volume and low printing speed, the dark tone level in the printed products has been seriously merged.
Third, the conclusion
Printing speed has a significant impact on ink transfer. In actual production, the ink supply amount and printing speed should be reasonably determined according to the ink delivery performance of the printing machine. It is not advisable to change the printing speed frequently during the printing process, nor is it suitable to adjust other printing conditions frequently, such as the amount of ink supply, the amount of water supplied, and the printing pressure. Higher printing speed can greatly improve production efficiency, but it will also reduce the quality uniformity of printed products. At the same time, it will introduce some unstable factors, such as paper feeding failure, positioning failure, overprinting accuracy, flying ink, paper pulling. Wait. At the same time, we should pay attention to the ink inking parameters of the printing machine. Although the ink mixing parameters obtained by various models are similar, the difference in ink path length is different, and the ink mixing effect is not the same. In order to obtain stable ink transfer, it should be provided. The ink volume, printing speed and printing machine ink performance parameters are organically combined.