Small craftsmanship in flexo platemaking

Friends of the flexographic printing industry, have you considered using an anilox roller without a "cell hole"? I am afraid that most people will say: "Impossible." As we all know, the anilox roller is the core part of the flexographic printing, it is to rely on the surface of the network to transfer ink. But at the same time, we also noticed that the printing plate used in flexographic printing has no cells, and that in the ink transfer system, the platen roller is next to the anilox roller.

Since the anilox roller is covered with cells, in the same way, the field of the printing plate should also have a network of holes. These "print plate holes" may have an important influence on the future of flexographic printing.

First, smooth surface and liquid immiscible

A long time ago, people had discovered the fact that the adhesion of liquids to the surface of smooth objects was very poor. Such as water droplets on the windshield of a car, and "spattered milk droplets" on a glass tabletop, etc., are examples of fluids in our daily life that are incompatible with smooth surfaces. In the above two examples, the wetting action of the liquid on the smooth surface of the object is very uneven, but in a spherical or random pattern.

Two more examples are closer to our industry, such as the printing plates used in offset printing. We know that in offset printing, plates must first be plated in order to evenly wet the fountain solution. Edition to avoid the above problem. Similarly, a number of pockets are engraved on the surface of a gravure printing gravure cylinder, and these cells can store and transfer ink during operation of the machine.

The surface of the photopolymer printing plate is originally very smooth, and is also non-porous, but on such a surface, it is impossible for the ink to spread uniformly. To print a flat field, you must use more ink, and the required printing pressure is slightly larger than the normal pressure. In the flexographic printing process, too much ink and excessive printing pressure is exactly The root cause of most printing failures. If the cell site is also used in the field part of the plate, the need for ink volume and printing pressure can be greatly reduced, that is, there is no need for a large amount of ink and excessive printing pressure, thereby helping to improve the printed matter. the quality of.

II. Screening the field in the field

One of the methods in flexo printing to solve the problem of smooth surface of flexo is to screen the field area. This method has been used in the flexographic printing industry for at least 15 years, and has also achieved varying degrees of success. In fact, the fundamental task is to determine the best dot shape, the best cable angle, and the best number of screen lines in the field, so as to improve the shape of the printing plate.

The results of the first phase test show that although the size of the best network is generally expressed as a percentage value (for example, 96% of screen dots with 133 lines/inch screen), the actual size of the non-network area (usually "Micrometer" is more important. For the sake of brevity, we will refer to the non-dot area on the printing plate as the "printing plate hole." For example, 96% of the dot area corresponds to 4% of the cell (100-96=4). Assuming that the number of screen printing lines is 133 lines/inch, the diameter of this 4% network hole is 43 microns.

The enlightenment of these tests is that in flexo printing, it is not the “percentage value” (such as 4%) but the “micron diameter” (such as 43 microns) that is decisive for the transfer of ink. Assume that the number of screens is 133 lines/inch, and the number of lines of the anilox used is 800 lines/inch. Take 96% of the screen as an example. If 43 microns is the optimal size of the cells, the following problems will arise: : Why use 1/133 inch intervals to separate the cyberspace? Why not separate them without 1/800 inch intervals? Why do they cover only 4% of the surface of the plate? Why not cover 40% or more of the surface of the plate?

We studied the optimal number of screens for ink transfer. Through research, we have found that the number of screens in the field is actually the same as the number of screens in the anilox roller.

However, the non-dot (ground) area can take special shapes. For example, the circular dot used in the solid area is actually a transformed diamond dot. In the high frequency area, this circular dot is very small and many There is no imaging on the plate, and even if it can be imaged on some plates, it seems to be insignificant.
This effect is especially noticeable when using 16-bit or higher number of screens. If we expand the number of network points, it will bring a series of problems, just as the connection of outlets forms a channel, which often hinders the normal operation of the network.

Third, the formation of "printing plate hole" on the printing plate

By exposing the image on an imagesetter or a CTP device, it is possible to obtain a pocket in the solid area of ​​the printing plate. The user generally selects the shape of the cell in combination with other characteristics of the job.

The RIP software informs the output device what kind of cells should be formed in the field, while forming normal halftones in other parts of the same tone. The shape of the plate cavity is displayed as a positive 1-bit TIFF image that can output film or digital plates on any imaging device.

In the process of copying, from the digital file to the film and to the printing plate, the cell site constantly shifts. Since the cells in these high frequency areas consist of a few pixels, the best results can be obtained if the imagesetter has been calibrated for the flexographic printing process. The density is higher and the edges are clearer. When a film is output on an imagesetter that has been calibrated for a flexographic printing process, the reduction in the number of dots in the dots will correspondingly increase. In fact, the cells that are formed on the plate by the exposure plate will be reduced, and the expansion of the cells when the film is output can precisely compensate for this.

Many users choose to add screens in all field areas. In addition, screens and lines can also be screened. The advantages of screening the field or fine text and lines are mainly the following:

1. The density of field areas is higher.

2. The field area looks smoother.

3, reduce or eliminate the "glow."

4, the image is clearer and sharper.

5, Yin map (text) paste is even lower.

Fourth, the field area has a higher density and smoother

After screening the field in the field, the printing plate has a more adequate amount of ink, so the field area obtained is higher in density than the unscreened plates, and is evener. Practice has proved that this is indeed the case when the amount of ink is low (such as when printing halftone images) or when the amount of ink is very high (such as when printing negative lines or intaglio).

Fifth, eliminate the halo

Practice has proved that if you screen the solid area of ​​the plate or fine text and lines, you can effectively reduce or even eliminate the halo in flexographic printing.

Sixth, Yang Wen is clearer, Yin Wenwen is not sticky

After adding screens to the field or fine text and lines, the printed Yang Wen is clearer, and the Yin Wen does not stick.

Print the same image with the screened and non-screened plates. Observe the results carefully and compare the overwhelming death of the two images with the expansion of the positive text. You will find that: Under the high magnification magnifying glass observation can still find some shortcomings and deficiencies, but within the normal observation distance, the former print out the graphic effect is significantly better than the latter.

Seven, save ink

There is another advantage to point out that this is to reduce the consumption of ink. The user's practice has already confirmed this. In other words, when printing solid areas or fine text and line parts after screen printing on solid areas or fine text and lines, the printer can use a web with a lower ink transfer rate (ie, a higher number of screen lines). Roller, therefore, the amount of ink used to "cover" the substrate material is also less.

Below we analyze from a purely digital perspective. If one anilox roll with 4 million cubic centimeters of ink is replaced with an anilox roll with 3 million cubic centimeters of ink, the amount of ink transferred will be reduced by 25% and actually transferred to the substrate material. The amount of ink (that is, the amount of ink eventually consumed) may be less.

So far, there are no clear statistics on the issue of ink saving. From a business survey, this amount is still quite large. If the user's ink consumption is US$1 million, a 1% decrease in the amount of ink transferred means that US$10,000 can be saved each year; a 10% reduction in the amount of ink delivered means a savings of US$100,000. This shows that the potential for cost savings is still very large, and researchers will continue to conduct in-depth research in order to obtain more detailed information.

Eight, print color test sample

If you want to screen parts of the field or fine text and lines, depending on the number of screens, screen angles, and dot size, a total of 400 different options can be combined. With so many choices, what kind of cyberspace is best? This depends on the printing conditions.

Here's a simple way to introduce it to you. This is a sample test. This wide color patch includes 83 different printing plate hole blocks, 8 unscreened solid color blocks (for the purpose of comparison) and 7 halftone color blocks. The user prints a test sample of the color standard on the printing machine, and selects the kind of cell with the best effect from the printing result.

The printed test sample is generally evaluated visually. The user can first observe the characteristics of the color patches in the test sample within the normal viewing distance (the length of the arm), such as the smoothness of the solid area and the clarity of the text. Next, use a magnifying glass to carefully examine its characteristics, such as "halo." Some users also use colorimeters or spectrophotometers to measure color patches to determine which type of cell has the highest density value after printing.

Nine, choose and use

In the process of selecting and using the cell site, the following two basic principles can be followed: 1. Under a certain printing condition, select and use the cell site with the best replication effect. 2. Select and use the network points that have good copying effect under various printing conditions, and say that the most common (BestCommon) network points are selected.

From a purely technical point of view, despite the wide variety of printing conditions, it is always possible to have a cell with the best reproduction effect under certain specific printing conditions. Therefore, in order to determine the best cell under certain printing conditions, the test sample needs to be printed and analyzed under various printing conditions.

In addition, there is another method that is "Best Common Cell Method". That is to say, to find out which kind of nets are better for copying under various printing conditions. It has been carried out first, printing test samples under various printing conditions, and then, checking the printed color patches one by one to determine which of the printing plate cells