Experimental Analysis of Factors Affecting the Change of Color Screen Printing Dots

Screen printing claims to be "all items except air and water can be printed." It has such wide printing adaptability, thick printing ink layer, soft layout, and is not limited by the size, shape, and material of the printing object. It is currently irreplaceable by other printing methods. It is also for these reasons that there are a wide variety of products printed by screen printing, from smooth to rough, from paper, fabric to iron sheet, glass, and so on. With the emergence of high-quality photosensitive adhesives and inks, people hope that screen printing can print things that can only be printed with offset printing in the past, so many people directly use the color separation film of offset printing to print, but the results are always endless. Desirable, either the loss of tone is serious, the color cast is serious, or the unsightly moiré is produced. The reason for this phenomenon is because they are not very clear about the factors that affect the change of screen printing dots, so they can not be quantitatively controlled. To this end, this paper makes a simple analysis of the factors that affect the change of screen printing dots such as dot shape, screen line number, printing material, ink, exposure time, etc., and then provides a certain reference basis for color screen printing tone reproduction.

1 Influence of dot shape on dot change

First of all, use Coreldraw8 as the test tool for tone test-gray scale, its tone range is 5% -100%. The hair arranging adopts 40lpi, 60lpi, 80lpi, 100lpi screen, the screen angle is 45 degrees, and the dots are round and square. The experimental material is coated paper, Offset Paper, 300 mesh screen, screen printing ink (fluorescent pink, fluorescent yellow, fluorescent blue), offset printing ink (red, sky blue, medium yellow, magenta), use X-RITE 528 love X-ray reflection density meter and transmission density meter measure the percentage of dots. Experiment with the above experimental materials and instruments to obtain experimental data (as shown in the following tables), according to these data, the dot percentage (F0) on the film is taken as the horizontal axis, and the dot percentage (Fprint) and dot percentage on the printed matter Increment (â–³ F) is the vertical axis (the difference between the percentage of printed dots and the percentage of film dots) to draw the dot change curve (as shown in Figures 1 to 5).

Experiment 1 Use the same ink, the same line number, the same exposure time, the same printing material, and choose different dot shapes for printing (screen printing green ink, 40lpi, exposure time is 40s, coated paper, square dot and round dot).

From the data of Experiment 1 (Table 1) and the dot change curve (Figure 1), we can see that the largest expansion of square dots and round dots is 50% and 50%, respectively, because the square dots are compared with other dots at 50%. The expansion factor is the highest, because 50% of the dots are connected at the four corners. The corners of the dots are easy to cause ink clogging and increase the area of ​​the dots during printing; the circumference of the round dots is the smallest among all dots, relatively speaking, its dots expand The coefficient is small. Generally, under normal circumstances, the dots are connected around the 70% point, but once the dots are connected, the dot expansion coefficient will become larger, and the dots in the dark tone area will be blocked due to excessive ink volume during printing. , And eventually the dark part of the image loses its proper level.

Table 1 Dot percentages with different dot shapes

Figure 1 Changes in the area of ​​dots with different dot shapes

2 The influence of the number of network lines on the change of network points

Experiment 2 Use the same number of screen lines, the same exposure time, the same substrate, the same exposure time, the same substrate, the same ink type for printing (40lpi, 60lpi, 40s, square dots, coated paper, offset red ink), The measured data is shown in Table 2.

From the data of Experiment 2 (Table 2) and the dot change curve (Figure 2), it can be seen that the higher the number of screen lines, the higher the dot expansion value, and the larger the dot expansion value, which means that the clarity of the image reproduction is guaranteed. In the case of, the greater the difference between the number of color separation lines and the number of screens, the more the tone of the image can be maintained; and the less likely the interference pattern appears repeatedly, because 300 meshes are used in this experiment When using 100lpi screened dichroic film, moire has appeared on the printing plate.