Reasons and Solutions for Expanding Screen Printing Points (1)

Dot enlargement is the physical and optical properties associated with density and color intensity that halftone dot printing has. In terms of physical performance, dot enlargement increases the diameter and thickness of halftone dots. Since halftone dot printing uses clear ink, its immediate effect is to make the printed image look darker than the original. From an optical point of view, the dot enlargement will cause the coincidence of neighboring dots. The finer the structure or number of lines of the halftone dots, the more pronounced the optical enlargement effect. Using a reflection densitometer, the percentage of the dots can be measured, resulting in an expanded dot value.

Mesh is usually measured at 50% gray, because this is the converted value from the light spot to the dark spot. If the network is enlarged by 25%, it means that the area of ​​black in the dot after printing is changed from 50% to 75%. There is no need to worry about network expansion. This is a very common phenomenon. It exists in all printing processes and is not considered bad. In the halftone dot printing process, if the printing effect of each sheet can be kept consistent, dot gain can be foreseen and effectively controlled. However, how to ensure that the expanded value of outlets in the printing process remains the same is the biggest problem that silk screen printers encounter. Therefore, it is almost impossible to control the precision of dot gaining, which is very difficult. This article will do its best to help you understand the importance of consistency in controlling and maintaining the copying effect in the screen printing process, so that everyone can print more satisfactory works.

The network expansion is not equal, that is, the dot enlargement rate of different tone is different. Unlike other printing processes, screen printing also suffers from dot loss, which is the reason for the loss of color density and image distortion in halftone dot printing. There is a balance between dot loss, one-to-one replication, and dot gain for all images. This balance varies with the number of screen lines and the accuracy of the dot structure. The finer the outlets, the greater the difficulty of control. The first thing you need to do is to find the most stable halftone dot size and shape that can achieve the printing effect you require. Site limitations, ink types, screen tension, screen mesh number, and printing steps all affect the printing effect. Involving tone loss or dot gain, AM or FM halftone dot printing is more complicated.

Since the dot enlargement will affect the reproduction tone, we need to think of ways to control it. The hue is a transition from white to black or from white to a single color. The smoother the reproduction, the better the continuity of the hue. The simulation of the continuity of the original tone is the basis of the halftone reproduction. Halftone dots are enlarged, and the reproduction of tones is discounted. The tone of the copied work is not good. The normal tone value is shifted to cause the reduction of the color to be deeper than expected. The printed image looks dark and dirty. In addition, color deviations occur, especially in the third, gray, and brown lines. Finally, halftone dot enlargement has an increasing trend, that is, the greater the number of printed dots, the higher the degree of expansion will be, until a relatively stable value is achieved. As halftone dots constantly change, color instability results in color deviations during copying.

The effect of all network expansion is negative. The range of hue is compressed, which means that it is difficult to control the shadow of the image or the fine area of ​​the highlight. The shadow disappears and the image details disappear, making the highlight area in the image appear darker than actually required. After the range of hue is compressed, the contrast of the image disappears. Contrast is used to distinguish between light and dark. When the bright part of the image darkens and the shadow part disappears again, the contrast between the light and dark of the reproduced image disappears, and the details appear to be “eaten”. The image does not seem to be as clear as it should be.

The following are four reasons related to dot-scale expansion. Includes halftone dot design (shape and connection of dots), print strength, ink characteristics, and substrate surface. There may be other minor reasons, but I believe that most of the problems you have encountered can be answered in these four areas.

Halftone dot features

The first is the halftone dot shape and design. The expansion rate of the outlets is closely related to the gray level of the outlets. That is, as the gray level of outlets increases, the expansion rate of outlets also increases accordingly. Since the maximum expansion rate of the traditional halftone dots is 50%, 50% is the maximum expanded area of ​​the dots, and more than 50% of the dots become the dot of the opposite color. The sum of the two dots forms a complete 100%. In other words, an area of ​​60% of negative graphs and 40% of positive graphs have the same area. Since most of the dot gains occur at the edge of the re-dot, the larger the dot area, the higher the diffusion rate of the dot.

Next we will discuss in depth the shape of the dots. The most common dot shapes include circles, squares, ovals, or diamonds. The shape of the dot is determined based on its shape at 50% of gray scale. Because circular dots have higher magnification than all other shapes, circular dots are rarely used for screen printing. Round is usually used for offset printing. When offset printing runs at a high speed, the dots are shaped like ovals, which is the worst choice for screen printing. When the four points of contact of the dots are connected at the same time, the area of ​​the adjacent contacts forms an acute angle with the dots (less than 90 degrees). This area is filled with ink at random, causing serious dot enlargement.

If the dots are square, dot gain occurs only in the four corners during printing, and the expansion rate is usually higher than 20%. When using square dot (mainly used for single-slate printing), the four corners of the dot contact at the same time and the contact surface forms 90. At right angles, the base of the corner is quickly filled with ink, but it is not as serious as a circle.

Finally, let's take a look at the oval mesh. Elliptical dots are connected first in one direction and then in the other direction. For example, in 40% connection in one direction, the ratio increases to 60% in the other direction. According to the design, different proportions can be formed. The higher the ratio, the closer the halftone dot is to the line type. The typical contact area ratio is 45/55, 40/60, 35/65, the former one represents the main connection plane, and the latter one represents the remaining area. Square area ratios can be 50/50. The area ratio can only be known by observing 50% grayscale dots under a magnifying glass or microscope. When you get a screening film from a color separation person or other service department, you need to test this to see if it meets your requirements.

In the case of AM or FM halftone, the dot expansion is different from that of ordinary halftone printing. This is because of the different frequencies and dot sizes used. The feature of FM halftone is that the larger the dot, the easier it is to print, but the printed image has particles that appear rough. The finer the outlets, the smoother the hue, and the greater the expansion and loss of outlets. Unlike ordinary halftone printing, FM uses many dots with the same location but the same size. In the four-color dot printing, the bright part of the image is masked to cause a dot loss. We use the number of perimeters of halftone dots multiplied by the number of dots to measure the amount of dot expansion. We don't use complex mathematical models. In short, the brighter parts of the effect image that is visible to the eye are lost, and the dot enlargement rate is lost from the midtones to the shadows. This change is dramatic and the contrast is very strong. The printed lines at this time appear to be curved compared to the smooth lines printed by ordinary halftone printing methods. Observed under the microscope, the finer the halftone dot, the more severe the bending. For screen printing, the effect is best when the dot size is greater than 80 microns. The dot gain value is controlled to a minimum, and the dot mesh size of 100 to 120 microns can be optimized for the reduction effect.

Because of the same method as the AM halftone screen printing, the FM dot is square. Both methods are different from ordinary halftone printing methods. The intersection of outlets is in the form of four corners or side-by-side contact. This means that the perimeter of the outlets is shortened. If it is on the four sides, the total perimeter is 8S (where S represents the length of one side). If it is side-by-side contact, minus the two sides, the total perimeter is only 6S. This random combination of dot structures is what causes the image to produce particles. A typical example is a 50/50 tuning dot with a size of 100 microns and the dot structure is enlarged. (to be continued)