The technical parameters and selection principle Of Cigarette Boxes Of Hotstamping roller are discussed from the practical point of view

Sep 19, 2023

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Today flexographic printing is in a leading position in the development of today's printing industry in the world.

The new progress of roller technology has become a decisive factor to promote the rapid development of flexo printing process, especially because of the emergence of laser engraving roller, which makes the flexo printing industrial production can almost keep pace with the gravure and offset printing industrial production, and the roller with ceramic surface improves the transfer performance of the ink layer and improves the quality of the ink layer, which is the result of the adoption of new technology.

The roller should be able to deliver ink to the plate quantitatively according to a predetermined standard, and the ink layer should be as thin and uniform as possible. The stability of the ink layer will restrict the printing quality of the image. In the process of transfer, the thinner the ink layer is, the easier the printing adjustment is, and the expansion rate of the dot is also smaller. The uniformity of the ink layer helps to improve the constancy of the ink transfer amount.

First, the uniformity of ink layer

Why is it possible to obtain an excellent uniform ink layer after using a laser-carved roller? The main reason is that it can reduce the gap between the mesh, in the traditional mechanical engraving, is the use of metal movement method to form the mesh, and in laser engraving, it is the laser beam of the ceramic gasification after the formation of the mesh. It is because of this fundamental difference that the laser engraving roller brings the advantages of strict control. First, it can significantly reduce the gap between the holes, that is, the thickness of the mesh wall, and second, it can carve out a higher density of mesh holes in the same true line distance. Based on the above two factors, the ink layer transmitted by the mesh hole has a higher uniformity.

Laser engraving roller is how to make the transfer of ink layer thin? The thickness of the ink layer depends on the volume size of the mesh hole, such as a 300 line 5.50BCM volume of the mesh roller and an 800 line 2.00BCM volume of the mesh roller compared, obviously 800 line of the mesh roller due to its smaller capacity, so its transfer of the ink layer is thinner, the use of laser engraving can obtain a smaller capacity than mechanical engraving of the mesh hole. Thus forming the advantage of thinner ink layer. The thinner the ink layer, the more you can control the printing effect of the highlight dot.

Second, the specification of engraving

Laser engraving comes in three sizes: 30° hexagon, 45° diamond and 60° hexagon. After polishing the surface of the ceramic roller, a circular pit with a parabolic bottom pit is burned by a rounded laser beam on the ceramic surface, and the diameter of the laser beam can be adjusted according to the number of lines of the mesh or the size of the mesh hole, no matter how adjusted, the beam always remains circular.

The geometry of the mesh depends on the arrangement position between the mesh holes, such as the diamond-shaped pattern is arranged by four mesh holes around the center of a hole, and the hexagonal pattern is arranged by six mesh holes around a net hole. Where the carved standard mesh will form a regular diamond shape or regular hexagonal. Once the engraving is wrong, the gap between the mesh holes will increase, and the geometry of the mesh will be irregular.

Hexagonal mesh is better than diamond shape, for two reasons, first, there is a large mesh wall area around the diamond shaped mesh hole. Obviously, the ink layer it transmits is not very uniform. Second, in order to achieve the same volume, the diamond-shaped net has to be carved deeper, which we can learn from nature, where the honeycomb of the bee is made up of broad holes of hexahedra, and living things instinctively know that only in this arrangement can more bees be stored. So the hexagon is the most efficient geometry, it makes the most use of space.

According to mathematical calculations, in the same area, the hexagonal occupies 15% more space than the diamond shape. Therefore, the hexagonal mesh hole with 15% more space on the mesh roller means that it can reduce the depth of the hole by 15% without affecting its original capacity, and it is obvious that the depth of the hexagonal mesh hole is shallower than the diamond shape.

Third, the ink release and cleaning of the net hole

What if the roller has been able to transfer the ink layer is thin and uniform, but the speed of ink release is not fast enough, especially in high-speed printing, or the bottom of the mesh can not be cleaned? Obviously, this is due to the depth of the net hole is too large, and the problem of insufficient ink transfer is caused by the difficulty of ink cleaning.

Of course, the net is too shallow is not without problems, too shallow can not print the due effect. For example, the depth of the mesh hole is 4 microns, and the error is 1 micron, which means that the error between the capacity and the depth is 25%. For a printer with a quality concept, this deviation is unacceptable. And when the depth of the net is too shallow, it will also bring the ink easy to dry out. Therefore, for laser engraving roller should choose the best engraving depth, must not be extreme.

How to determine the optimal hole depth? The depth of the net must also be considered in relation to the size of the opening of the net. If we compare the soup bowl with the test tube, although they have the same capacity, the bowl is faster than the test tube when the liquid in the container is poured out, not only because the depth of the bowl is smaller, it is also related to the width of the bowl opening. The situation is similar on the mesh roller.

According to further research, the optimal depth/opening width ratio d/o of the mesh hole should be between 23 and 33%. It means that the depth of a mesh hole with an opening width of 100 microns should be between 23 and 33 microns. The three-dimensional geometry formed according to this proportional relationship can make the mesh hole have good ink release and cleaning properties. The shade area on the engraving conversion table shows the relationship between depth and opening degree. For example, when using a net hole with 400 lines and a capacity of 2.6BCM, the d/o ratio on the conversion table is 18%. If the number of mesh lines is changed to 550 lines, then its d/o ratio becomes very pure and gets the best results. On the contrary, if the 550 lines 4.5BCM capacity mesh holes in the table with a d/o ratio of 40% are changed into 400 lines, it will maintain the best d/o ratio, so as to obtain the most ideal use effect.

Fourth, the number of traditional mesh lines

In the previous introduction, we emphasized the mesh capacity on the mesh roller, the best ink layer quality and the best mesh geometry, whereas in the past, the emphasis was only on the number of mesh lines. In FIG. 4, three types of laser-carved roller mesh holes are shown. They all have the same mesh number, capacity, and configuration, but their characteristics are different. The top of a kind of net hole depth is shallow, the net wall is thin, so its ink release performance is good, easy to clean, the formed ink layer is also very uniform; The bottom one, the deepest net hole, the network wall is thicker, so the cleaning is poor, the formed ink layer is not uniform. These differences lead to differences in printing quality.

In the laser engraving roller, the quality of the ceramic itself is one of the key factors affecting the quality of the roller. The ceramic surface should be hard and smooth, the porosity as small as possible, the hardness of the ceramic will affect the printability, and the flatness of the surface, whether before or after carving is of great significance. It will affect the consistency of the mesh hole carved on the surface of the roller before engraving, and will affect the consistency of the thickness of the ink layer after engraving; The less the porosity of the ceramic surface, the more the corrosion effect of solvent on the roller body can be reduced.

Five, turtle pattern

On the laser engraving roller, another important aspect to be considered is the relationship between the Angle of the mesh and the Angle of the mesh on the printing plate, which will be related to the formation of the moire on the print. The mesh on the laser engraving roller is available in three angles: 30°, 45°, 60°. The advantage of 30° and 60° hexagonal mesh is that regardless of the Angle of the printing plate, the risk of turtle pattern can be avoided. The reason is that the hexagonal mesh corresponds to the arrangement of the two axes on the plate with the arrangement of three axes, while the 45° mesh roller corresponds to the plate with two axes.

The choice of the number of mesh lines depends on the printing method and the quality of the ink, because they determine the capacity of the mesh holes and the corresponding mesh line number. For example, in order to facilitate the control of the highlight of the printing of the network cable version, the ink layer needs to be thinner, which means to reduce the capacity of the mesh hole, so you have to choose a finer mesh line number. For the printed parts, thicker mesh should be selected to obtain more ink capacity, so as to form a thick ink layer. On some specific printing pieces, the effect of high-quality ink is better than the effect of ink volume, and it will also affect the selection of mesh lines.

In the United States, when the film is printed on the network plate, the capacity of the network hole is generally controlled in the range of 1.0 to 3.3BCM/VPSI, and then converted into the best engraving parameters. If the above d/o ratio is used, the number of mesh lines should be selected between 1200 and 400LPI.

Color screen printing, should be based on the strength of the ink, the thickness of the plate of the network cable and the percentage of the dot to be copied (size) and other factors to choose the best roller mesh line number. For example, an 800LIP mesh roller can adapt to the reproduction requirements of high light dots on the printing plate of 85 ~ 150LPI, and the traditional 4 times the mesh line ratio of the roller/plate is only suitable for medium tone printing pieces and not for the reproduction of high light small dots.

6. Cleaning and maintenance

When talking about the roller, it all involves the difficulty of cleaning and maintenance expansion. Whether the laser roller can always maintain a constant ink release is related to the reproduction effect of fine images. Laser engraving roller itself is a rather expensive investment, once the mesh is damaged or blocked, it can not print good printing, to update is not a small loss.

The general concern in cleaning is what means to effectively clean the roller, and to prevent the ink from drying on the surface of the roller is the best way to maintain the roller. The key to cleaning is to thoroughly remove pigment particles and resin from the net before the ink dries, and once the ink dries in the net, it will reduce the capacity of the net. To clean the dry ink, there are two common methods, solvent burning method and ultrasonic method. The former is the most commonly used traditional method, this method works well, but the trouble is that the roller must be removed from the machine and burned immediately or immediately when the ink no longer flows. In many packaging and printing plants, it is customary to clean with wipes or other wiping materials, which can only clean the surface part of the mesh wall and the pit part is still out of contact. When cleaning by combustion method, the mesh roller should be continuously turned to facilitate the thorough burning of ink particles. The stainless steel brush is used to clean the ceramic surface, and the copper brush is used to scrub the chrome-plated roller surface. The two can not be mixed, otherwise, the stainless steel brush will brush the chromium layer, and the copper brush will settle the copper chips on the ceramic surface, and even mixed with the ink. Some factories use lye to clean the dry ink in the pit, although this non-standard method is easy to popularize, but because the lye is easy to corrode the roll body or further worsen the surface damage, it is not recommended to use.

The use of ultrasonic cleaning roller is very effective, but it should be checked frequently in cleaning to avoid excessive impact and damage to the ceramic surface. The ultrasonic cleaning time must be strictly controlled, the ultrasonic cleaning time of the fine mesh roller should be between 2 and 5 minutes, and the cleaning time of the coarse mesh roller can be slightly extended. Another thing to note is that the operation must follow the operating procedures of the ultrasonic equipment manufacturer and establish regular monitoring measures.

The maintenance of roller also includes storage, use and periodic identification.

The use of the plant should be based on the following guidelines to establish the corresponding cleaning and maintenance rules and regulations:

1. Use the appropriate brush to remove the wet ink in time.

2. When scrubbing, use a rotating brush.

3. Special person should be responsible for cleaning and operation.

4. Use protective cover when storing.

5. The storage of roller should be carefully selected.

6. Regularly carry out in-factory self-examination and inventory statistics of the roller.

7. Educate and train operators regularly.

With the continuous promotion of flexo printing process, if you want to compete for the market with gravure and offset printing process, you must closely cooperate with ink factories, plate manufacturers and prepress system suppliers to meet the needs of printing factories, only in this way can you achieve greater development of your own.