High- and high-density printing plates, inks and printing conditions
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In the case of high-definition printing, the amount of information per unit area can be increased to reduce the presence of rose spots and moiré on the conventional plate making. In addition to increasing the workload and narrowing the image reproduction range, printing quality is far superior to conventional printing. In other words, by studying high-grade printing technology, it has become the standard for printing quality in the digital age.
1 high-definition printing
1) Features of high-definition printing
Generally, a print having a resolution of 300 lpi or more is referred to as a high-definition print.
The biggest difference between high-definition prints and ordinary prints is the difference in dot enlargement rates. For example, at an accuracy of 175 lpi, the 50% dot gain rate is about 15%; at 600 lpi accuracy, the 50% dot gain rate is close to 30%. Therefore, it is a matter of course to adjust the conditions for making high-definition prints for color separation. How to control the expansion of outlets on printing is also an important part of determining the quality of printing. In addition, the side length of the 600 lpi highlight portion dot is only 6 μm with respect to the side length of 20 μm of the 175 lpi highlight portion. How to maintain the reproduction of 6 μm small dots, plate making, color separation, and printing conditions are all important.
2) High-precision printed plates
The commonly used printing plates can be divided into water PS plates and anhydrous silicone rubber plates. From the point of view of the expansion of the dot, since the water-free version is a flat concave plate, the mechanical dot produced between the printing plate and the blanket is less expanded, the tone reproduction performance is excellent, and the suitability for high-definition printing is good. However, there is still room for improvement in small dot development because the development uses a solvent to expand and remove the unexposed portion (positive pattern) of the silica gel layer.
The water PS plate is a flat-convex plate, so the mechanical dot enlargement is more prominent, and under the joint effect of the dampening solution on the emulsification characteristics of the ink, it will have an adverse effect on the tone reproduction. However, the recent four-layer sand mesh PS version has improved water storage performance, which can inhibit the expansion of outlets. The reproduction performance has been improved compared to the previous PS version.
Regarding the exposure of the printing plate, it is necessary to pay attention to the reproduction of the highlight portion and reset the exposure amount. The amount of exposure will be greatly reduced compared to the usual situation. It is also easy to cause quality problems due to dust during exposure, and careful operation is required.
3) High-precision printing ink
The importance of controlling the dot gain rate of high-definition printing has been as described above. The expansion of high-precision printing dots is also related to the ink used. Especially for water offset printing, the following points are important.
1 improvement in emulsification performance.
When the ink is emulsified seriously, the dot deformation is severe, the dot enlargement rate is increased, and the printing quality is significantly reduced. As far as the ink is concerned, the dampening solution should be properly absorbed, and no "blead" failure occurs, and the amount of excess water is minimized. Such an ink is desirable. It is important to select the binder and pigment of the resin component that has the greatest influence on the ink emulsification characteristics.
2 Control the dot gain by increasing the ink concentration and viscosity.
In order to control the dot enlargement from the composition of the ink, it is necessary to increase the proportion of the pigment and maintain the desired ink concentration with a thinner ink layer. It is also an effective method to increase the viscosity of the ink without affecting the ink transferability.
3 improve the printing luster.
The thinner the ink layer, the worse the flatness and the corresponding decrease in printing gloss, which is exactly the opposite of the above effect of increasing the ink concentration. Therefore, in order to improve the printing glossiness, it is important to select a resin having good pigment wettability and a pigment having good dispersibility.
4 printing conditions for high-definition prints.
In order to stably perform high-definition printing production, it is necessary to minimize the dot enlargement caused by the printing machine and control the ink emulsification. An important issue is the adjustment of the printing pressure, the temperature and humidity of the equipment, and the fountain solution.
1 printing pressure control.
Dot enlargement mainly occurs during the transfer of ink from the printing plate to the blanket. Therefore, if the pressure between the printing plate and the blanket is too large, the expansion rate of the dot will increase. It is best to adjust the pressure between the plate and the blanket to a minimum without affecting ink transfer. In addition, you should choose a blanket with good compression properties.
2 temperature control.
When the temperature of the device is too high, the viscosity of the ink decreases, causing an increase in the dot enlargement rate and deterioration in the emulsification suitability. Therefore, the temperature of the equipment should be kept constant by the adjustment of the cooling water. After the temperature rise of the waterless offset printing machine, it is particularly prone to stickiness and become an important factor affecting the quality.
3 control of dampening solution.
When the amount of ink is kept constant and the fountain solution is increased, the water content of the ink is increased, and as a result, the viscosity of the ink and the ink transfer property (concentration) are lowered.
In the case where the dampening solution is excessively printed, in order to keep the ink concentration constant, it is necessary to increase the thickness of the ink, resulting in an increase in the dot enlargement ratio.
Therefore, in order to control the excessive expansion of the outlets, the amount of water should be minimized as much as possible. In addition, in order to improve the emulsification suitability of the ink, it is also important to select a fountain solution that matches the ink used.
2 high density printing
1) Features of high density printing
In contrast to color printing with a maximum density of 2.0 or higher, the usual plate-making color separation is required to compress the density range in order to maintain the hierarchical reproduction of the dark-tone portion. The density of traditional color plate editions is: black version 2.0 or so; magenta plate, cyan plate 1.4 to 1.5; yellow plate 1.0 or so. High-density printing adjusts the color separation curve to a density range as close as possible to the density range of color printing (2.0). This is a way to improve the three-dimensionality and thickness of the print.
2) High-density printing and printing plates
By performing high-density printing, the dot gain on the printing machine is measured, and the obtained result is fed back to the plate making process to set the tone curve. For printing plates, of course, it is required to reduce the dot enlargement rate. Therefore, in general, high-density printing plates should have the same characteristics as high-definition printing plates. Water PS version should use a plate with good water storage performance. No watermark version needs to consider increasing the thickness of the silica layer to increase the concentration of the ink.
3) High density printing ink
Although high-density printing can immediately obtain a printed product with a strong sense of solidity, the disadvantage is that the dot density of the main portion in the halftone print becomes large, and the transferability of the ink printed on the multi-color portion is deteriorated (poor overprinting) ), producing a noticeable roughness. In particular, thickening of the ink film promotes turbidity and darkening of the ink. Therefore, in addition to the same stable emulsification characteristics and higher concentration as the high-definition printing ink, the ink used needs to satisfy the following points:
* Improve the overprinting performance of ink
Increasing the thickness of the ink film can improve the hiding rate of the ink in each of the solid color portions, but the ink transfer property after printing on the complex color portion is deteriorated, and overprinting defects are likely to occur. Therefore, it is necessary to adjust the viscosity balance between the inks of the respective colors without increasing the dot enlargement ratio.
* Improve the vividness of the ink
The increase in ink density makes the ink turbidity become prominent, especially the blue ink is particularly serious, which adversely affects the coloration of the complex color portion. In order to eliminate turbidity as much as possible, it is necessary to change the hue of the ink.