In screen printing, silk screen is one of the main raw materials. When printing thin lines or four-color dot images, the relationship between the screen and the dots must be taken into consideration. This is because the existence of the screen makes the dots of the high-light part of the printed product be disturbed by the mesh and the dots are deformed or lost; the dots of the dark-modulated part may fall in the middle of the mesh hole, and the adhesion is not secure due to insufficient support. Loss of level. The screen parameters mainly include: the material type of the screen, the color of the screen, the weaving method of the screen, and the geometric parameters of the screen. Since the first three parameters are easily selected in the production practice, the following mainly discusses the geometric parameters of the screen. The ok.
1. Calculation of dot size
The number of screen dots is L, and D is the dot diameter (um). The dot area (u m2) with the dot area ratio of H% is: S1=(l/L)2×H%×l03
The area of ​​the circle is: S2=π(D/2)2
Let Sl = S2, then: D = 1.1284 × 1000 (2.1)
In multi-tone tone printing, the Euclidean dots are used more often, that is, in the range of higher or lower tone values, the dots are circular, and in the area ratio of 50% is a square.
2. Calculation of mesh geometry parameters
1 mesh number n: number of wires per inch or per cm, n as shown.
2 Wire diameter d: indicates the thickness of the wire. For the screen with the same number of meshes, the thickness of the mesh is divided into the following four different levels: S, M, T, and HD. The mesh gradually grows thicker from S to HD.
3 Opening OP: Means the distance between the two meshes in the screen, calculated from the number of meshes and the wire diameter. OP=1/nd
4 Opening rate A0: The percentage of the total area that the screen leaked. A0(%) = OP2(OP+d)2
5 Mesh thickness Ft: Means the distance between the surface of the screen and the bottom surface. The empirical expression of the screen thickness in actual production is Ft≈1.82d.
3. The relationship between screen resolution and screen parameters
Screen resolution refers to the ability to accurately reproduce the fine lines and dot sizes on the substrate. According to equation (2.1), the circular dot diameter of the screen dot line is 40 μm/cm, the dot dot area ratio is 1%, and the diameter of the circular dot dot is 28 um. . Obviously, due to the barrier of the mesh to the dot, this size dot cannot be reproduced. The resolution of screen printing is influenced by the mesh (mesh number, wire diameter and weaving method), photosensitive material, ink (fineness, viscosity, etc.), squeegee parameters (pressure, angle, speed), mesh distance, etc., but The influence of the screen is decisive. Since the screen is measurable, people are committed to establishing a mathematical relationship between screen and resolution.
At present, the relationship between screen parameters and screen printing resolution proposed at home and abroad is summed up in four formulas:
Dmin= (l+nd)/n (2.2)
Dmin=2(d+OP) (2.3)
Dmin=2d+OP (2.4)
Dmin=3d (2.5)
The minimum size of the complete mesh; n is the mesh size; d is the wire diameter; 0P is the opening. In order to analyze which of the above four formulae is most suitable for expressing the screen printing resolution, the concept of missing area ratio of dots is introduced here. Leakage Area Rate (Ar): The ratio of the area of ​​ink leaked from the dot to the corresponding dot area. The value is related to the position of the dot on the screen. Figure 2 shows a point relationship between dots and screens obtained from the above four formulas. Since they contain four knots, they are considered to be the dots with the lowest missed area ratio. The dot blots on the points are complete, so other points are not a problem.
To facilitate the intuitive comparison of different resolution calculation formulas, we now use silk screen as an example. The dmin calculated from the above four formulas and the missing area ratio (Ar) are listed in Table 1:
The two basic parameters of the screen, n and d, affect the resolution. The higher the number of meshes, the finer the mesh, and the smaller the interference to the mesh points, the more able to reproduce small mesh points. Equation (2.5) considers only the parameter d; and equation (2.3) is actually equivalent to dmin = 2n-1 (which is independent of d), ie, the formula does not take into account the influence of the wire diameter of the screen on the dot. Therefore, the above two formulas are logically inadequate. Judging from the leakage area ratio shown in the table, equation (2.4) is the smallest, so the imprinting defects are likely to occur, and its applicability needs to be experimentally verified.
4. The choice of screen
From the above theoretical analysis, it can be seen that the screen mesh must first be selected to ensure the integrity of the bright-tuned network, and at the same time, the reproduction of the dark-tone network must be taken into account. However, due to the increase in the number of dots, the level ratio of 90% or more is actually on the ground, and 90% of the darkest dot size is equivalent to 10% of the bright dot size. Take the 40L/cm manuscript for example. ) Know that the dot diameter is 90um. From the viewpoint of the eye/line ratio, a 40L/cm original should be selected to have a mesh of at least 120 mesh/cm, otherwise moire may occur. From the attached table 1, it can be seen that the opening of the screen of 120 meshes/cm or more is a maximum of 52 μm, and the diagonal distance thereof is 74 μm. As long as the diameter of the dark screen dots is larger than the diagonal line of the opening, dark screen dots can be ensured. Get enough support without losing it. To sum up, as long as the screens that can ensure the integrity of the bright-tuned dot-spots can meet the reproduction of the dark-spotted dots, only the reproduction of the bright-tuned dots should be considered when selecting the screen. Therefore, the selection of screens can be performed as follows:
1 First, the minimum dot diameter of the printing is determined based on the number of screen lines of the positive picture and the number of dots of the bright tone.
2 Second, according to the minimum dot diameter D, determine the required mesh diameter and screen mesh number. When purchasing the screen, ask the supplier for the screen parameter specification, which can be calculated by equation (2.2) to determine which screens can meet the minimum dot reproduction requirements.
3 Finally, according to the ink layer thickness and the actual production requirements to select the same mesh number of different levels of screen.