Halftone screen printing quality control

Although some factories establish standards and tolerances to guide production in a specific production process, the same standards do not exist for the screen printing industry. As a result, some screen printing plants try to print halftone products without objective quality control. The result is not enough precision to reach the desired color. However, this is not the reason, because the screen printing process is controlled by a quantifiable range of variation that can be refined and used to perfect its own color reproduction standards. In addition to helping you achieve accurate color on a constant basis, setting up quality control standards can also help manufacturers improve printing quality overall, increase reproduction stability throughout the printing process, and ensure repeatability of the same or similar jobs. In the process of improving the standard, the most basic variables to consider include the following: * Print dot size and tone range * Ink thickness * Optical density of printed colors * Dot gain and dot loss * Ink overprint control first two The variables are mainly influenced by the screen fabric used, and you will see that choosing the right fiber diameter is as important as using the correct fabric and mesh number. You can also learn latitude calculation methods related to these variables to ensure consistency with quality standards. The remaining variables are affected by other measurable features of screen production and printing. You will find how to better evaluate these variables. Understanding Halftone Dots and Tone Ranges Before evaluating dot size and tone range, you need to understand the tricks and screen parameters that affect them. The best starting point is halftone video. Halftoning is defined by the number of lines and the range of gradations. The line number refers to the number of dots per inch or centimeter (line/inch, or line/cm). The higher the number of lines, the more dots per measurement unit and the better the image resolution. The gradation range is determined by halftone dot sizes used to represent varying degrees of image density or ink range. For copying, the image is decomposed into different sized yin and yang dots to represent brighter and darker areas. Each dot size represents the percentage of coverage from 0 to 100% (the ratio of printing to non-printing areas). A full-tone dot size for a particular halftone line produces a halftone tone range. This range includes highlights, midtones, and dark lines. For the 5 to 50% of the tone, the use of positive printing dots, continued growth, second-order tone from 51-95% negative graph outlets continued to decrease. (Figure 1) In screen printing, dots less than 5% and greater than 95% are usually discarded. Note that as the number of halftone screens increases, the dot size also increases (Table 1). This is an important basic principle, because dots will be lost below a certain size, so they cannot be reproduced in screen printing. As shown in Fig. 2, the minimum high gloss dot size that can be uniformly printed is limited by the mesh fiber diameter. Because it is not guaranteed that the dot ink will fall in the open area of ​​the mesh, when the high light dot is equal to or less than the fiber diameter, it cannot be printed. The ability to print dark spots is also affected by the width of the open cells. When the dark tone dot is smaller than the mesh width, the template area containing the dark tone dot dot will not adhere to the dot hole, and the dot dot will not be printed. (Figure 3) The dot size can be calculated for a specific gradation value (F) based on the number of halftone screen lines. Simply use the following formula: 1. When the number of halftone lines is given in centimeters, dot size = ((1.1284 x F square root) x lines/cm) x 1000 Example: Calculate a 48 L/cm halftone dot 5 % dot size - dot size = ((1.1284 x 5 square root) ÷ 48) x 1000 = 52.6 microns 2. When the number of halftone lines is given in inches, dot size = ((1.1284 x F square root) ÷ line number / inch) × 2540 For example: Calculate the size of a halftone dot of halftone dot of 120L/in. 5% dot size --- dot size = ((1.1284 × 5 square root) ÷ 120) × 2540 = 53.4 μm mesh The fiber diameter relative to the mesh width dimension also affects the printability of the image. However, the fiber diameters listed in the technical data sheets of most wire mesh manufacturers are normal values ​​and it represents the measured values ​​before the fiber weaving. During weaving and finishing, the circular cross section of the fiber deforms into a flat, elliptical shape, and the fiber diameter increases along the plane of the screen (Figure 4). For the purposes of this article, I will refer to this wider fiber diameter as the lateral fiber diameter. If the wire mesh supplier provides the mesh with the relevant mesh (Mo) size data, this information can be used to calculate the approximate cross mesh diameter using one of the following formulas: a. If the mesh number is given in centimeters (Mc /cm), transverse fiber diameter = (10,000 ÷ Mc/cm)-Mo b. If the mesh number is given in inches (Mc/in.), transverse fiber diameter = (10,000 x 2.54 ÷ Mc/in .)-Mo For example, if you want to calculate the actual diameter of the fiber, 305 filaments per inch, a low elongation wire with a normal diameter of 31 microns and a mesh size (Mo) of 48 microns, the formula will be expressed as follows: Fiber diameter = (10,000 x 2.54 ÷ 305) - 48 = 35.3 or 35 microns. With wire mesh selected, the ratio of the mesh pores to the transverse fiber diameter should be as high as possible. Screen fabrics with meshes that are much larger than the transverse fiber diameters have less screen interference and more ink flow than those with smaller meshes and thicker fibers. Therefore, they are more suitable for printing small outlets. However, some work requirements may limit the fabric, where the mesh width is less than or equal to the transverse fiber diameter. Regardless of the environment, the minimum highlight height for a given fabric can be calculated. 1. When the mesh is larger than the transverse fiber diameter, the minimum mesh size = mesh width + transverse fiber diameter 2. When the mesh is equal to the fiber diameter, the minimum mesh size = (2× mesh width) + the transverse fiber diameter 3. When the mesh is smaller than the fiber diameter, the minimum dot size = 2 x (cell width + transverse fiber diameter) In all cases, the printable dark mesh dot must be equal to or greater than the mesh width + fiber diameter. Since the finest fiber diameter of the screen fabric that dominates today is about 30 microns, the minimum printable spot height will be 85 microns or greater. The small dot size has an impact on the print quality and the consistency between printing and printing. Table 2 gives the range of mesh number and minimum highlight and dark mesh size within the fiber diameter. Once the minimum highlight and dark spot sizes supported by the screen fabric have been determined, the maximum and minimum tone values ​​that the main object will generate under a given number of halftone screen lines can be calculated. The following formula can be used, where Lc = the number of halftone screen lines, Mo = blank area, Thd = transverse fiber diameter: 1. The minimum tone value for printing highlight dots = π × 100% × (printable dot size × Lc) ÷2)^2 2. The maximum gradation value of the printed dark tone dot=100-(π×100%×((Mo+Thd)×Lc)÷2)^2) For example, suppose that 85 lines/inch is to be known The halftone values ​​print the maximum and minimum gradation values ​​that can be reproduced on a 305 silk/inch fabric. Because in the previous example, the manufacturer provided a mesh size of 48 microns and a normal fiber diameter of 31 microns. First, the transverse fiber diameter (Thd) was calculated as previously described and was approximately equal to 35 microns. Next, determine the minimum dot size. Since the mesh is larger than the fiber diameter, the dot size is equal to the sum of the mesh and the lateral fiber diameter (35 microns + 48 microns = 83 microns). Note that this value also represents the minimum dark spot size. Finally, these values ​​are inserted into the formula of the minimum gradation value in order, and the set of note converts all values ​​according to the unit (in millimeters here): Minor gradation value=π×100%×(0.083×3.346)÷2)^2= 6.06% (≈6%) Next, substituting the approximate value into the formula of the maximum gradation value: the maximum gradation value = 100-(π × 100% × ((0.083 × 3.35) ÷ 2) ^ 2) = 93.9% (≈ 94% ) Print resolution for a given application depends primarily on image size and viewing distance. Table 3 lists the combinations of ideal halftone screen lines and tone ranges for different image sizes and viewing distances. Use this table to select the halftone and mesh combinations suitable for printing. Table 3 Image size and viewing distance affect the number of halftone screen lines and the range of adjustments that are appropriate for a particular application. This table gives the ideal combination of line and gradation ranges for screen printing images. Ink deposition thickness In addition to the print dot size, the color of the printed image is also affected by the ink film thickness. Mesh size, fiber diameter, stencil thickness, and ink type and viscosity all have an effect on ink film thickness. We know that the wet thickness of ink deposition is equal to the theoretical color value of the screen fabric, which is usually provided by the manufacturer. For example, a theoretical color value of 20 cm 3 /m 2 will result in a wet ink film thickness of approximately 20 μm. The ink film thickness can be calculated simply by dividing the color value by the mesh area in the following way: ((20 cm3(100 cm x 100 cm)) x 10,000 μm/cm) = 20 μm. However, the stencil film thickness also has an effect on the deposition of the ink, and the thickness must be increased in accordance with the screen color value. Thus, using a stencil having a thickness of 10 microns on a screen, a theoretical color value of 20 cubic centimeters per square meter would result in a wet ink film thickness of approximately 30 microns thick. For better printing of halftones, the selection of screen fabrics with low theoretical color values ​​is just as important as the use of thin stencil films. Before setting up the quality parameters, it is also necessary to measure the stencil parameters such as thickness and surface roughness. Once it is determined that an accurate, print-based, acceptable value is determined, the screen making process is set to ensure that the same template parameters can be obtained consistently. Optical density Stencil control is particularly important when working with three primary colors. Because these inks are transparent to some extent (especially UV inks), variations in the stencil can result in changes in the thickness of the ink film of the printed ink and, consequently, in the color shift of the printed color. This color shift can be described by the light absorption of the ink film, which is called the optical density. The densitometer can be used to measure the optical density of printed colors. The density meter realistically measures the density of the ink in logarithmic form. Simply stated, this value describes the ratio of light absorbed by the "standard white" material to light absorbed by the measurement material. "Normal Inking" is an optical density used to describe the correct printing on a particular ink/material combination. By recording the densitometer measurements of prints, one can realize normal inking - meaning a normal reproduction of the color vision - so that when using the same ink/material combination, there is a basis for the printing. In this way, changes can be detected and the print can be kept within an acceptable latitude. Dot gains and loss dot size changes may be the most common - and difficult to accurately - leading to inaccurate colors. Any change in the dot before or during the production process can have a devastating effect on the color quality of the image. For this reason, the dot size of the entire production process is monitored and any deviations found are noticed and corrected. During screen exposure, the dot size increases or decreases when a halftone dot is transferred to the template. Unexpected enlargement or reduction of dot size between these films and the final print is often referred to as dot gain and dot loss, as described in FIG. Note that the loss of high-light outlets is often referred to as sharpening, while the loss of outlets is often referred to as slush. In most cases, the enlargement and loss are usually caused by the following reasons: * The ink film thickness caused by the incorrect selection of the screen fabric (the ratio of the mesh size of the selected fabric to the fiber diameter is incorrect) is incorrect. * Improper stencil making processes, including low vacuum pressure during exposure, overexposure, and insufficient stencil development (eroded after exposure). * Incorrect roller settings, including excessive pressure and speed. * Incorrect ink stickiness. * Surface characteristics of the material To ensure accurate reproduction of the halftones, it is necessary to measure and normalize acceptable tolerances for dot gain and loss. Films with halftone patterns are very useful for determining whether a particular halftone/mesh combination can achieve an acceptable range of gradation. However, they need to be connected with a density meter in order to measure the degree of increase or decrease in the expected dot. Densitometer will provide information about halftones