Talking about the Determination of the Process Parameters of Wire Mesh
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In screen printing, the screen is one of the main raw materials. When printing thin lines or four-color dot images, consider the relationship between the screen and the dots. This is because the presence of the screen causes the dots of the highlight portion of the printed product to be deformed or lost by the interference of the mesh; the dots of the dark-adjusted portion may fall in the center of the mesh, and the adhesion is not strong due to insufficient support. Causes a loss of hierarchy. The wire mesh parameters mainly include: the material type of the wire mesh, the color of the wire mesh, the weaving method of the wire mesh and the geometric parameters of the wire mesh. Since the first three parameters are easy to select in the production practice, the following mainly discusses the wire mesh geometric parameters. Ok.
1. Calculation of dot size
The circular dot with the number of screen lines is L, and D is the diameter of the dot (u m), then the dot area (u m2) of the dot area ratio 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 gradation printing, Euclidean dots are used more, that is, in the range of higher or lower gradation values, the dots are circular, and the square is 50% at an area ratio of 50%.
2. Calculation of wire mesh geometric parameters
Figure 1 is a schematic diagram of the geometrical parameters of the screen.
1.1 mesh number n: number of wires per inch or per centimeter, n as shown.
1.2 wire diameter d: indicates the thickness of the wire. For the mesh of the same mesh, the thickness of the mesh is divided into the following four different levels: S, M, T, HD, and the mesh gradually thickens from S to HD.
1.3 opening OP: indicates the distance between two mesh wires in the screen, calculated by the number of meshes and the wire diameter. OP=1/n-d
1.4 aperture ratio A0: indicates the area of the screen leakage area as a percentage of the total area. A0(%)= OP2(OP+d)2
1.5 screen thickness Ft: represents the distance between the surface of the screen and the bottom surface. The empirical expression for the thickness of the screen in actual production is Ft ≈ 1.82d.
3. The relationship between screen resolution and screen parameters
Screen printing resolution refers to the ability to correctly replicate the fine lines and dot sizes on the substrate. According to the formula (2.1), the number of screen lines is 40L/cm, and the diameter of the dot with a dot area of 1% is 28um. The diameter of the screen with the smallest diameter of the mesh is 420S, and the diameter is 27um. . Obviously, the mesh of this size cannot be reproduced due to the barrier of the mesh to the dots. The screen resolution is affected by the screen (mesh, wire diameter and weaving method), photosensitive material, ink (fineness and viscosity, etc.), squeegee parameters (pressure, angle, speed), mesh distance, etc., but The effect of the screen is decisive. Since the screen is measurable, people are working to establish a mathematical relationship between the screen and the resolution.
At present, the relationship between the screen parameters and the screen printing resolution proposed at home and abroad is mainly composed of 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 dot; n is the mesh size; d is the wire diameter; 0P is the opening. In order to analyze which of the above four formulas is most suitable for expressing the screen printing resolution, the concept of the missing area ratio of the dots is introduced here. Leak area ratio (Ar): The ratio of the ink leakage area of the dot to the corresponding dot area, the value of which is related to the position of the dot on the screen. Figure 2 shows a point relationship between the dot and the screen obtained by the above four formulas. Since they contain four nets, they are considered to be the one with the smallest area of the missing area. The dot on the dot is complete, so the other dots are not a problem.
In order to facilitate the intuitive comparison of different resolution calculation formulas, the screen is taken as an example. The dmin is calculated by the above four formulas, and the area ratio of the print (Ar) is listed in Table 1:
Both basic parameters n and d of the screen affect the resolution. The higher the mesh number, the finer the mesh, and the smaller the interference to the dot, the more the smaller dots can be reproduced. Equation (2.5) only considers the parameter d; while equation (2.3) is actually equivalent to dmin=2n-1 (independent of d), ie the equation does not take into account the effect of the mesh diameter of the screen on the dot. Therefore, the above two equations are logically insufficient. From the point of view of the area ratio of the printed areas shown in the table, the formula (2.4) is the smallest, so the defect of the imprint is easy to occur, and its applicability needs to be experimentally verified.
4. Screen selection
It can be seen from the above theoretical analysis that the selection of the screen must first ensure the integrity of the bright-tone dots, and also the reproduction of the dark-tone dots. However, due to the increase of the dot size, the area ratio of 90% or more is in the field, and the 90% dark tone dot size is equivalent to 10% of the bright dot size. For example, the 40L/cm original is used. ) know that its dot diameter is 90um. From the point of view of the eye/line ratio, a 40L/cm original should have a mesh of at least 120 mesh/cm, otherwise a moiré will appear. As can be seen from the attached table 1, the opening of the screen above 120 mesh/cm is 52um maximum, and the diagonal distance is 74um. As long as the diameter of the dark-adjusted dot is larger than the diagonal distance of the opening, the dark-adjusted dot can be ensured. Get enough support without losing it. In summary, as long as the screen that can ensure the integrity of the bright dot can satisfy the dark-tone dot reproduction, only the reproduction of the bright dot is needed when selecting the screen. Therefore, the selection of the screen can be carried out as follows:
4.1 First, the minimum dot diameter of the print is determined according to the number of screen lines of the positive picture and the number of dots of the bright dot.
4.2 Next, according to the minimum dot diameter D, the required mesh diameter and mesh number are determined. When purchasing the wire mesh, ask the supplier for the wire mesh parameter specification. You can calculate which screen can meet the minimum dot copying requirement by formula (2.2).
4.3 Finally, according to the thickness of the ink layer and the actual production requirements, the screen of different grades under the same mesh number is selected.