Halftone screen printing technology network computing method (2)

Understand halftone dot and tone range

Before evaluating dot size and gradation 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. 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 increases. This is an important basic principle, because dots will be lost below a certain size, so they cannot be reproduced in screen printing.

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.

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) ÷ lines/cm) x 1000

For example: Calculate the size of a half-tone dot of 48L/cm halftone dot - 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

For example: Calculate the size of a half-tone dot of a halftone dot of 120L/in.—the dot size=((1.1284×5 square root)÷120)×2540=53.4 microns

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. 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.

(to be continued)