Flexo Direct Plate Making

As far back as the 1995 International Printing and Papermaking Fair, computer-to-platemaking technology had already begun to enter the production of flexo platemaking. DuPont Cyrel set up a group of digital image experts to meet the ever-increasing demand for direct plate-making knowledge. At present, due to the continuous expansion of demand, computer-to-plate technology has made remarkable achievements in flexo printing in image processing, digital proofing, data processing, and plate making.

In a pure CTP-based workflow, the production of negative film has been replaced by direct imaging on the plate. The removal of this intermediate part of the film minimizes the loss of quality and other disadvantages associated with simulated contact exposure of the photosensitive resin plate material (dust particles, vacuum suction problems, incorrect selection of exposure time, etc.). In practical production, it has been proven that the advantages of the CTP production process are much better than we expected.

The advantages of CTP are many, and here are two of the most important ones. The first is to compare the appearance of a digital flexo plate with that of a traditional plate. It is found that the surface of the former plate is smoother and the various elements of the printing are separated from the dots. The small dots on the ground to the fine high-light area are almost completely on one plane. In traditional flexographic printing plates, the fine dots in the highlights are usually more prominent than others. Even after the revision, the tips of these dots are several microns higher than the solid parts on the same plate, which means that during the printing process, Sites that have been completely printed on the field before the highlight area were crushed, resulting in more serious network expansion. In contrast, CTP plates provide more consistent printing characteristics, effectively reducing dot gains.

In addition, the size of all image areas on the CTP plate was reduced by a small amount during imaging, which indicates that each line, each field area, especially the dot, is reduced by several microns, effectively compensating the printing process. The dot expansion. Since this size reduction process has been automatically integrated into actual production, there is no negative impact on prepress workflow and image quality. In the traditional version, both of these are unavoidably affected because the size reduction is applied as an intermediate process to the film.

Flexo dots are extremely important, especially those made by the smallest dot. In these places, there may be some ghosting that affects the quality of the flexographic plates. This is mainly due to the way the modern prepress system processes image data, regardless of whether it is a digital process or a traditional workflow. So its solution Can be applied to the two flexo plates production process.

The photosensitive resin flexographic image is imaged with the help of a negative-image photosensitive mask. When a traditional plate is exposed it is a negative film and in the CTP it is a mask. In the screened overcast images, the finest dot is the smallest light-transmitting hole, which logically allows a minimum amount of ultraviolet light to pass through to complete the photopolymerization reaction. If the required UV light intensity is not strong enough, only a part of the photopolymerization reaction occurs because the dot size is too small, and the dot is destroyed in the process of developing with the water-based solvent, resulting in the so-called "dilapidated dots" on the printing plate. . Whether or not such outlets can be printed depends on their height, and there is a certain degree of randomness, which affects the quality of the overall image.

In the past, people have tried various methods to solve this problem, including limiting the minimum dot size between 1% and 3%, but with little success. The CTP technology can help to find the cause of the failure to a large extent and provide solutions.

The Cyrel Digital Platesetter (manufactured by Barco Graphics) is the most commonly used flexographic platesetter currently equipped with a data previewer (Viewer), a software tool not commonly found in film imaging equipment. In the past, when the dot appeared on the negative image was very small, one could not determine whether this was the cause of the data or the reason after the film development process because the data for exposure could not be viewed at all. Now, with the help of this observation tool, it was only possible for people to realize for the first time that it was entirely possible to determine the data for digital pre-printed RIP films that were supposed to be printed on prepress. The output of outlets would be much lower than the minimum required. .

According to the well-known imaging theory, the resolution of the image data should be twice the final number of lines. In other words, if you want to print 48 lines/cm, the image data must have 96 lines/cm resolution. The commonly used image resolution is 300 dpi (points per inch), which is equivalent to about 120 lines/cm, and can therefore be made into a 60 line/cm screen.

If the resolution of the image is too different from the above-mentioned 2:1 rule, for example, if the resolution of the image data is too rough, the image quality of the printed matter may be too low, such as obvious jaggies. At the same time, the above theory also shows that if you make the resolution of the image very high, it is difficult to improve the quality of the image (because the halftone screen may not show a wealth of details), and also have to deal with and The situation of storing large amounts of data files, but in recent years has been a great change, mainly due to the continuous improvement of computer and network performance and the rapid decline in the price of storage media, so the resolution is also greater than before The improvement of the image quality of printed products has also been correspondingly improved. The resolution of images used in flexo platemaking has generally been superior to the above mentioned 2:1 principle in the past and present.

However, it has been found that in contradiction with the foregoing, the use of high resolution originals also has an effect on the image quality. This is related to the applied flexo platemaking method, such as the tone range plus net-yin pictures. What's worse is that even if you strictly follow the 2:1 ratio principle and have been calibrated accurately, it is still possible to produce dot fragments in the screening process.

Experiments show that even if the dot debris is in the ideal state, it will be generated. Assuming we strictly control the ratio of 2:1, both the screen points of the image file in all directions are twice as wide as the screening screen (for the sake of simplicity, we only study the case of one color). The center of the dot to be replicated should be exactly at the point where the four pixels meet, so that each dot created by the screening is formed by 2 x 2 pixels, if the tonal values ​​of these four quadrants are significantly different --Depending on the position of the pixel - the shape of the dot may be deformed to a considerable degree.

This imaginary dot-pixel structure increases the accuracy of the details of the screened images and is therefore widely accepted. However, when images need to be screened, the problem arises considering the minimum dot value. Judging whether the tonal value after screening is higher or lower than the critical value of the image not only depends on this group of outlets, but also from the perspective of each outlet quadrant, that is, the four that constitute one outlet. section. This kind of judgment has nothing to do with the possibility of producing broken mesh.

The following negative sign constellations usually occur: Only one of the four quadrants happens to reach the minimum tone value set by the operator. Dots are generated at this value, and the hue values ​​of other pixels are lower than this value. Let's assume that, for example, the minimum value is set to 2%. Screening equipment uses this specified quadrant when screening this part, which means that it takes 1/4 percent of the 2% dots. Since the other three pixels in the digital file are below this critical value, the screening device does not consider their effect.

Despite the operator's explicit settings, RIP will also produce a tone value that is too low (this is likely to appear on the plate with a broken dot). In this example we are still very lucky. If the ratio of images to screens is increased, such as 3:1, then each dot will consist of (3×3=9) quadrants, so theoretically it is possible to generate 1/9 meshes with the lowest expected size. . At this time, if the center of the dot is not exactly in the geometric center of the image pixel, but rather to one side, the result will be worse, and this situation often occurs again. This will lead to the emergence of “satellite” outlets with lower hiding power. The shape and size of the outlets will be random. This will have a greater impact on the quality of the image.

At the same time, this explains why the use of high-resolution originals has affected the quality of flexo printing. This is especially true when printing directly on corrugated paper. For example, when printing a 300 dpi image, screens are screened with a 60 lpi (24 l/cm) screen.

The first is to adjust the resolution of the designed image to match the number of screen lines of the final printed image so that the resolution of the image phase is approximately twice the number of screen lines.

The second step is to modify the image data on the prepress workstation. It can be done with the help of image processing software (such as Adobe Photoshop). The purpose of this is to ensure that the value of the hue at or below the ideal minimum value is 0. Or the lowest threshold. They all rely on the location of the pixels, so they need to be solved according to different situations.