Halftone screen printing technology

When using trichromatic printing, stencil control is especially important. Since the inks have a certain degree of transparency (especially UV inks), and the deviation of the stencil leads to a change in the thickness of the printing ink film, the printing color is significantly shifted.

This color shift can be described by the absorption of light by the ink film (ie optical density). The optical density can be measured using a densitometer to measure the printed color. Measurement results are displayed in logarithmic form. It can also be said that this figure represents the ratio of the light absorbed by the "absolute" white material to the light absorbed by the test ink.

"Normal ink volume" is a term used to describe the correct optical density printed with a particular ink and substrate. By using a densitometer to measure prints that are more accurate (colors appear visually correct), the results can be used as a reference for printing or proofing under the same ink and substrate conditions. Using this method, you can perceive the change in color and adjust it instantly to ensure that the print quality is maintained within acceptable latitude.

The color we feel is related to the thickness of the ink layer in addition to the size of the printed dot. Mesh opening size, mesh diameter, stencil thickness, ink type, and ink viscosity all have an effect on ink layer thickness.

We know that the wet thickness of the ink layer 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 cubic centimeters per square meter forms a wet ink layer thickness of about 20 microns. The ink layer thickness can be simply calculated by dividing the color value by the mesh area according to the following method: ((20 cm 3 ÷ (100 cm × 100 cm)) × 10,000 μm/cm) = 20 μm.

The stencil film thickness also has an effect on the thickness of the ink layer, and the thickness must be increased according to the screen color value. Therefore, using a stencil with a thickness of 10 μm on a screen, a theoretical color value of 20 cm 3 /m 2 results in a wet ink film thickness of approximately 30 μm, which is 10 μm more than the above calculation. In order to print fine halftones, the choice of a screen fabric with a low theoretical color value is just as important as the use of a thin stencil film.

Before setting up the quality control parameters, it is also necessary to measure the characteristics of the stencil, such as thickness, surface roughness, and the like. Once a precise, acceptable value based on printing has been determined, a standard screen making process should be established to ensure that the same template characteristics can be obtained consistently.

Under the condition that screen printing does not exist on the same basis, many companies have established production standards and their deviations without considering their respective conditions. Due to the absence of objective quality control conditions, it has caused many screen shops to produce inaccurate and unpredictable printing colors in printing. In fact, this is completely avoidable because the screen printing process can be controlled by a series of quantifiable parameters. You can get these parameters by measurement and get the desired color reproduction standard.

In addition, it can help you to obtain accurate and consistent colors, help you to improve the overall quality of the printing, and enhance the stability of product reproduction in printing. When setting up standards, the following parameters should be considered:
1. Printing dots and tonal range
2. Ink thickness
3. Optical density of printed colors
4. Dot gains and losses
5. Ink overprint

The first two parameters are mainly affected by the quality of the silk fabric. Choosing the right mesh diameter is just as important as using the correct silk fabric and mesh count. In addition, in order to ensure the consistency of the print quality standards, you should learn how to calculate the deviation of these parameters.
Other parameters are affected by some measurable factors in screen making and printing. In the following periods, the above five parameters will be briefly described.

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 standards, the most basic variables to consider include the following:

* Printing dot size and tone range
* Ink thickness
* Optical density of printed colors
* Dot increase and dot loss
* Ink overprint control

The first two variables are mainly affected 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. (image 3)

The dot size can be calculated based on 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, the dot size = ((1.1284 x F square root) x lines/cm) x 1000 Example: Calculate the size of a 48 L/cm halftone dot with 5% dot size--- - Dot size = ((1.1284 × 5 square root) ÷ 48) × 1000 = 52.6 μm
2. When the number of halftone lines is given in inches, the dot size = ((1.1284×F square root) ÷ lines/inch) × 2540 Example: Calculate the size of a halftone dot of 120L/in. - Dot size = ((1.1284 × 5 square root) ÷ 120) × 2540 = 53.4 μm

The mesh fiber diameter relative to the mesh width dimension also affects the print performance 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 screen supplier provides mesh-specific (Mo) dimension data for a particular fabric, this information can be used to calculate the approximate lateral mesh diameter using one of the following formulas:
a. If the mesh number is given in centimeters (Mc/cm), the transverse fiber diameter = (10,000 ÷ Mc/cm)-Mo
b. If the mesh number is given in inches (Mc/in.), the transverse fiber diameter = (10,000 x 2.54 ÷ Mc/in.)-Mo

For example, if you want to calculate the actual diameter of the fiber, a 305 wire/inch, a low stretch wire mesh with a normal diameter of 31 microns and a mesh size (Mo) of 48 microns, the formula will be expressed in the form: transverse 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 mesh size = 2 × (mesh width + transverse fiber diameter)

In all cases, the printable dark mesh 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 printed dark tone dots = 100-(π×100%×((Mo+Thd)×Lc)÷2)^2)

For example, suppose it is known to print the maximum and minimum gradation values ​​that can be reproduced on a 305 silk/inch fabric with a halftone value of 85 lines/inch. 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%)
In the following, the approximate value is substituted into the formula of the maximum gradation value: the maximum gradation value = 100-(π × 100% × ((0.083 × 3.35) ÷ 2) ^ 2) = 93.9% (≈ 94%)

The 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.

Image size and viewing distance affect the number of halftone screen lines and gradation ranges 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 printing dot size