Measure Screen Printing Quality Control Based on Main Variables (1)

Although many printing companies have begun to implement data management and standardized production management and achieved certain results, the quality control status of the screen printing industry has not changed in an essential way, especially for companies involved in high-end screen printing. The standard guides the production, and the result is often such quality problems. One of the biggest headaches is how to ensure accurate color reproduction and reproduction of the tone.

Screen printing belongs to a variable-variable printing system, but it is still feasible to stabilize the printing quality by controlling deviations of various variables in the printing process. For tone screen printing, the challenging task is to reproduce the color and gradation of the manuscript, and the data control of color and gradation reproduction reflects the technical level of the printing process. Implementing data control and standardization of variable control as an important means of technical management, in addition to helping you obtain accurate color and level reproduction, is also conducive to stabilizing the printing quality within the company.

The most basic variables in the screen printing process include dot size and tone range, ink layer thickness, optical density of the color, dot gain and dot loss, and ink overprint. The first two variables are mainly affected by the screen type and the number of screen printing screens, among which the selection of the correct wire diameter, screen material and mesh number is of great significance. The latter three variables are affected by screen printing and printing. The effect of measurable parameters.

A printing outlet and tone range

The image dot size and tone range are usually defined and described by the number of lines and dots. The number of lines refers to the number of lines per inch or centimeter (line/inch, or line/cm). The higher the number of lines, the more dots each measurement unit accommodates, and the finer the image; the tone is the method of expressing the degree of lightness and darkness by using different dot density to represent the degree of light and shade of the image. The range between the minimum density and the maximum density in an image is called the range of tonality, and another meaning of tone is the level of the original.

For a dot generated under a specific number of screen lines, an image region with a depth similar to that of the original can be generated within a certain range. This range includes highlight, midtone, and dark tone dots. In the screen printing industry, it is customary to use 5% to 50% of the tone to use the positive picture network evaluation price; 51% to 95% of the tone tone uses the negative picture network evaluation price. In addition, the dot diameter also decreases as the number of screen lines increases. In screen printing, the gradation range of the image can only replicate the range of 5% to 95%. In addition, it is determined based on the number of screens.

Unlike other printing, the minimum height of the light spot that can be reproduced by screen printing is also limited by the screen wire diameter. Because the printing process does not guarantee that the ink just lands on the open area of ​​the screen, if the diameter of the high-light dot is equal to or less than the screen wire diameter, such dots cannot be copied. The duplication of the dots is related to the aperture of the screen. When the diameter of the dark mesh point is smaller than the mesh aperture, the dark mesh dot is represented by a negative image, so that the photosensitive paste containing the dark mesh dots will not be transferred to the surface of the screen, that is, such dots have been lost. It is impossible to get a copy.

â– 1. Dot size

Based on the above principle, we can calculate the dot size of a given gradation value under the specified number of screen lines. Assuming a gradation value of F, the dot size calculation method is:

1) Screened screen lines are expressed in lines/cm. Screen size=(1.1284 F)1/2 line/cm*1000
Example: Calculate the dot size of 5% of screen lines/cm with a screen number of dots Dot size = (1.1284 x 5) 1/2 ÷ 48 x 1000 = 49.5 μm

2) The number of screen lines is expressed in lines/inch. Dot size = (1.1284 × F) 1/2 ÷ lines/inch × 2540
Example: Calculating the number of screen lines is 120 lines/inch, 5% dot size Dot size = (1.1284 x 5) 1/2 ÷ 120 x 2540 = 53.4 μm

â– 2. The ratio of screen wire diameter to aperture size

The ratio of screen wire diameter to aperture size also affects the reproduction of the image. It is worth mentioning that the screen wire diameter given by most screen manufacturer's technical data is only a static value and represents the measured value of the material before weaving. In the process of weaving and applying tension to the stretch net, the circular cross section of the wire mesh becomes a flat elliptical shape, and the diameter of the wire mesh increases along the plane direction of the screen, and the actual diameter of the wire should be increased. The wire path we call the braided wire path. Obviously, the braided wire diameter is the final parameter that affects the printing effect.

1) Weaving wire diameter Wire mesh suppliers generally do not provide weaving wire diameter parameters, but we can approximate the screen weaving wire diameter based on the screen diameter (Mo).
1 Wire mesh number in mesh/cm: braided wire diameter = (10000 mesh/cm) - M0
2 mesh number in mesh/inch: braided wire diameter=(10000×2.54 mesh/inch)-M0
Example: Calculate the braided wire diameter of a low-stretch wire mesh with a mesh size of 305 mesh/inch, a static wire diameter of 31 μm, and a wire mesh diameter (M0) of 48 μm, then:
Woven wire diameter = (10000 × 2.54 ÷ 305) - 48 = 35 μm
Obviously, the braided wire diameter is 4 μm larger than the supplier's static wire diameter. Therefore, when selecting a screen, the ratio of the screen aperture to the braided wire diameter should be as high as possible. A screen with a large ratio has better copying ability and better ink permeability, which is important for copying small dots.

2) High-gloss dot (small dot) size In some workplaces, too high ratios need to be limited. For example, we can only use screens with a screen size smaller than or equal to the screen wire diameter. This situation requires that the screens can replicate. High light outlets.
1 The mesh diameter is larger than the diameter of the braided wire: high-light dot size = mesh aperture + braided wire diameter 2 mesh aperture is equal to the braided wire diameter: high-light dot size = 2× mesh aperture + braided wire diameter 3 mesh aperture is smaller than braided wire Diameter: High-light dot size = 2 × (mesh aperture + braided wire diameter)

3) Dark Mesh Size Regardless of the circumstances, the printable dark spot must be equal to or greater than the sum of the screen aperture and the screen wire diameter. Since the diameter of the finest fiber currently used to make silk screens is about 30 μm, the minimum height of the printable highlights is 85 μm. The small size of the dots is not only difficult to reproduce but also detrimental to the stability of the print quality. Table 2 shows the screens with different wire diameters and apertures, and the minimum highlights and dark screen points that can be reproduced when the number of screen meshes is different.

â– 3. Network area

The minimum highlight points and dark tone points determine the range of tones that can be reproduced by the used screen. The following calculation formula helps us to calculate this range:

Where Lc = number of screens, M0 = mesh aperture, Thd = braid diameter.

1) The minimum gradation value for printing highlight dots = π × 100% x (printable dot size × Lc)

2) The maximum gradation value of printed dark tone dots = 100-[π×100%×(Mo+Thd)×Lc÷

Example: Calculate the maximal and high light halftone value on a 305-mesh, 48-μm-aperture, 31-μm-net static screen with 85-line/inch screens.

1 Calculation of braided wire diameter (Thd), approximately equal to 35μm;

2 Determine the height of the highlight mesh. Since the screen aperture is larger than the screen wire diameter, the screen size is equal to the sum of the screen aperture and the braided wire diameter (35 μm + 48 μm = 83 μm). Note that this value also represents the maximum dark mesh size.

3 Bring these values ​​into the formula of the minimum gradation value, that is: the minimum gradation value = (π × 100% × 0.083 × 3.35 ÷ 2) 2 ≈ 6%

4 Substituting the approximate value into the largest gradation value Formula: Maximum gradation value = 100 - (π × 100% × 0.083 × 3.35 ÷ 2) 2 ≈ 94%

Although the specific number of screen lines and the range of gradations that can be reproduced by a particular screen are fixed, the actual effects that the human eye can experience may be completely different due to the size of the image area and the distance of observation. Table 3 lists the hierarchical effects produced by the number of screen lines at different image sizes and viewing distances. When the image area is small and the observation distance is short, when the dot is small, it is not easy to distinguish; when the image area is large and the observation distance is very long, the lower number of screen lines is selected. The lower number of screen lines produces larger dots, which are not easily distinguished even if they are far away.

(to be continued)