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High precision and high density printing

for high-precision printing, it can increase the amount of information per unit area and reduce the rose spots and moire patterns that usually exist in plate making. In addition to increasing the workload and reducing the image reproduction range, it is far superior to conventional printing in terms of printing quality. In other words, through the research on high-grade printing technology, it has become the standard of printing quality in the all digital era

1 high-precision printing

1) characteristics of high-precision printing

generally, the printed matter with a resolution of more than 300lpi is called high-precision printed matter

the biggest difference between high-precision printing and ordinary printing lies in the difference of dot enlargement rate. For example, under the accuracy of 175lpi, the point enlargement rate of 50% is about 15%; At 600lpi accuracy, the expansion rate of 50% points is close to 30%. Therefore, it is natural to adjust the conditions of plate making and color separation for high-precision printing. How to control the expansion of printing points is also an important link to determine the printing quality. In addition, relative to 175 LPI highlight point 20 μ The side length of 600lpi highlights is only 6 μ m。 How to keep 6 μ The reproduction of M dots, plate making, color separation, printing and other conditions are important

2) printing plates for high-precision printing

the commonly used printing plates can be divided into two types: the water PS version with anlimu representation and the anhydrous silicone rubber version. From the point of view of point enlargement, since the waterless printing plate is a flat intaglio plate, there is less mechanical point enlargement between the printing plate and the blanket, with excellent step reproduction performance and good adaptability to high-precision printing. However, there is still room for improvement in the development of small dots due to the solvent expansion method used to remove the silica gel layer of the unexposed part (positive pattern)

the watery PS plate is a flat relief plate, so the mechanical dot enlargement is more prominent, and it will have an adverse impact on the level reappearance under the joint action of raw resin, color masterbatch, the mixing of color masterbatch and raw materials, injection molding process, injection molding machine, mold, etc. when the influence of fountain solution on the Emulsification Characteristics of ink is touched. However, the recent 4-layer sand mesh PS version has improved its water storage performance, which can inhibit the expansion of points. Compared with the previous PS version, the playback performance has been improved

in terms of plate exposure, we must pay attention to the reproduction of the highlight part and reset the exposure amount. Compared with the normal situation, the exposure will be greatly reduced. It is also easy to cause quality problems due to dust during exposure, which requires careful operation

3) high precision printing ink

the importance of controlling the dot enlargement rate of high precision printing has been described above. High precision printing dot enlargement is also related to the ink used. Especially for water offset printing, the following points are very important

① improvement of emulsifying performance

when the ink emulsification is serious, the dot deformation is serious, the dot enlargement rate increases, and the printing quality decreases significantly. In terms of ink, we should properly absorb the fountain solution, and do not produce "blead" fault, so as to minimize the excess water. Such ink is what we expect. It is important to select the resin components and pigments that have the greatest impact on the emulsifying properties of the ink

② expand the control point by increasing the ink concentration and viscosity

in order to expand the control point from the ink composition, it is necessary to increase the proportion of pigments and maintain the required ink concentration with a thinner ink layer. It is also an effective method to improve the viscosity of ink without affecting the ink transfer

③ improve printing gloss

the thinner the ink layer, the worse the flatness, and the lower the printing gloss, which is just opposite to the above effect of increasing the ink concentration. Therefore, in order to improve the printing gloss, it is important to select the resin with good pigment wettability and the pigment with good dispersibility

④ printing conditions of high-precision prints

in order to stably carry out high-precision printing production, it is necessary to minimize the dot expansion caused by the printing machine and control the ink emulsification. One of the important issues is to adjust the temperature and humidity of the printing press, equipment and fountain solution

① stamping control

dot enlargement mainly occurs in the process of ink transfer from printing plate to blanket. Therefore, the pressure between the printing plate and the blanket is too large, which will cause the point enlargement rate to increase. It is best to minimize the pressure between the printing plate and the blanket without affecting the ink transfer. In addition, the rubber blanket with good compression performance shall be selected

② temperature control

when the equipment temperature is too high, the ink viscosity decreases, resulting in an increase in the dot enlargement rate and a change in the emulsifying suitability, which constitutes a dual connection difference. Therefore, the cooling water shall be adjusted to keep the equipment temperature at a certain level. Water free offset press is particularly prone to sticky dirt after temperature rise, which becomes an important factor affecting quality

③ control of fountain solution

when the ink volume is kept at a certain level and the fountain solution is increased, the water content of the ink will increase, resulting in the decrease of ink viscosity and ink transferability (concentration)

when printing with too much fountain solution, in order to maintain a certain ink concentration, the ink thickness must be increased, resulting in an increase in the dot enlargement rate

therefore, in order to control the excessive expansion of points, the water volume should be minimized as far as possible. In addition, in order to improve the emulsifiability of the ink, it is also important to select the fountain solution that matches the ink used

2 high density printing

1) characteristics of high density printing

compared with color printing with the highest density above 2.0, in order to maintain the hierarchical reproduction of the dark tone part, the density range must be compressed. The density of traditional color plate making is about 2.0 for black plate; Magenta version and cyan version 1.4 ~ 1.5; The yellow version is about 1.0. High density printing adjusts the color separation curve to make its density range as close as possible to the density range of color printing (2.0). This is a way to improve the three-dimensional and thick sense of printing

2) high density printing and printing plate

by implementing high-density printing, the point enlargement on the printing machine is measured, and the results are fed back to the tone curve set in the plate making process. For the printing plate, of course, it is required to reduce its dot enlargement rate. Therefore, in general, the printing plates for high-density printing and high-precision printing should have the same characteristics. For PS plates with water, printing plates with good water storage performance shall be selected. For anhydrous printing plates, the thickness of silicone layer shall be increased to improve the ink concentration

3) high density printing ink

although high density printing can immediately obtain thick prints, the disadvantage is that the point density of the main part of the halftone print becomes larger, and the transfer of the ink printed at the multicolor part becomes worse (poor overprint), resulting in obvious roughness. In particular, the thickening of the ink film promotes the turbidity and darkening of the ink. Therefore, in addition to having the same stable emulsification characteristics as high-precision printing inks and increasing their concentration, the inks used also need to meet the following points:

* improve the overprint performance of the inks

increasing the ink film thickness can improve the covering rate of the inks in the pure color parts, but the ink transfer in the multicolor parts is poor, which is easy to cause poor overprint. Therefore, it is necessary to adjust the viscosity balance between various inks without increasing the dot enlargement rate

* improve the brightness of the ink

the increase of the ink density makes the ink turbidity become prominent, especially the blue ink, which has an adverse impact on the color presentation of the multicolor parts. In order to eliminate the turbidity factor as much as possible, it is necessary to change the hue of the ink

Author: yangzhigang article source: Printing World

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