Production of metal anilox rollers

In all modern printing processes, the past neglected flexo printing has shown great potential for development in the narrow and wide range of printing. The entire supply industry has succeeded by continuously improving the methods and has achieved high and reproducible print quality by working together to form a more acceptable approach in terms of "flexible plate standardization". As a result, printing presses, metal anilox rollers, inks, printing plates, materials, and all attendant processes must all be integrated at a high level, such as platemaking, printing technology, and quality control.

This article provides some background information about the flexo rollers. The role of the metal anilox roll in flexographic printing is ink transfer, which is why they are so important in the print quality control chain.

If there is enough technical knowledge and practical experience, the options for purchasing will be even greater. In the past, different manufacturers of metal anilox rollers tried their best to recommend their own products. There is no set of standard specifications. For users, they will feel a little confused when purchasing. The main purpose of this article is to eliminate doubts on this issue through the main production process of metal anilox rollers.

Important criteria

The three most important factors in the production of metal anilox rollers are:

* Material selection and structure

* Ceramic coating or plasma treatment

* Ink hole laser etching

Material selection and structure

The flexographic printing industry continues to face a wide variety of rapidly changing needs, including short-run print jobs, and must therefore focus on reducing the “non-productive time” and preparation time for job replacement. This includes the replacement of materials, plates, inks, and metal anilox rollers. Depending on the combination of printing units and the structure of the printing press, the selection of the printing unit may have some difficulty more or less. And only a handful of narrow web presses are equipped with integrated lifting devices. In wide web presses, the situation is very different. Therefore, in narrow web printing, the printing worker must lift the metal anilox roller to install it. For most narrower metal anilox roller narrow prints, the weight problem is not significant. However, for solid-state stainless steel anilox rolls, the process is quite stringent. In general, metal anilox rolls are not properly protected. A typical example is the cracking of the ceramic coating on the end of the roll: the roll is difficult to move, but there is not enough protection.

All advanced metal anilox roller manufacturers offer hollow, lightweight drums. Aluminum alloy and stainless steel are commonly used materials for roller core production. The journal is made of stainless steel.

What are the advantages and disadvantages of these materials? Because of its low hardness, aluminum is very light and easy to mold. In order to ensure it has sufficient durability and stability, it is required that it must be minimized in size, especially for the thickness of the roller core. An aluminum core and a stainless steel journal are a mixture of two materials with different expansion/contraction characteristics. In general, these two materials must be glued together very carefully to ensure that the rollers run concentrically.

The stainless steel core is thinner than the aluminum core. However, in order to ensure the internal balance of the stainless steel core, the inside and the outside of the stainless steel roller to be assembled are generally processed at the same time to achieve the required diameter. This precise process ensures the regular wall thickness and concentricity of the rollers. The weight is reduced by the thin wall method. But at the very least, stainless steel journals are an important part of throwing away the weight of the roller core material. Drilling can be used to reduce the weight of sophisticated and sophisticated roller journals. The journals of the same material as the roller wall are combined with each Other by means of a heating and fitting method to form a very precise and permanent connection. Even in the case of high temperatures, no problem occurs, and the adhesive connection is at a high temperature. It is easy to have problems. In order to prevent the ink or solvent from penetrating into the ceramic coating, it must be protected with a chromium nickel coating. When treating with very corrosive solvents with very low pH, the entire roll must be coated to prevent the base material from being attacked. In general, erosion occurs at the crack of the ceramic coating and stops at the surface of the drum.

Roller surface "blasting"

This is the most important operation before coating. The air contains very regular particles and is sprayed onto the surface of the roller under high pressure using special equipment. When properly performed, dirt on the surface of the roller to be coated is removed; since the oxides affect the adhesion of the ceramic coating, they are also removed; this results in a very fine surface that provides good ceramic coating. The adhesion characteristics.

To achieve these goals, the "blasting process" must meet the following criteria:

* The blasting equipment must be adjusted to be exactly parallel to the axis of the roller;

* The sandblasting device must be constantly updated to prevent dirt and ensure stable operation;

* Roughness measurement and visual control to detect minimal surface irregularities.

* The maximum time between sandblasting and coating must be determined based on the materials used. This time must be as short as possible to prevent reoxidation of the surface.

The oxidation rate of aluminum alloys is much faster than that of stainless steel, so the process is also quite difficult. Nickel or high-grade stainless steel coatings are correspondingly much better.

Ceramic coating

The next step after “blasting” is the ceramic coating of the rolls, which is very important for the quality of the metal anilox roll later.

Plasma is a well-known and widely used ionized gas mixture. Chromium oxide is added to dissolve and accelerate its mixing with the rotating drum surface. The spraying device moves along the length of the roller until the desired coating thickness is obtained. It must be noted that to a certain degree the coated rolls cannot be cooled too fast, otherwise the adhesion of the coating will be reduced and the homogeneity of the final coating will be considerably affected. Once the rolls cool down, a single coat can be seen. High temperature does not have any effect on stainless steel tubes. This is totally different from aluminum material. In general, the material is not constant in temperature. Also, the expansion rate of the material during cooling can cause shrinkage problems.

The quality of the plasma treatment process itself determines the final morphology (porosity, hardness, structure) of a complete coating. There are two things to consider:

* The kinetic energy of ceramic particles impacting the roller;

* Thermal energy of ceramic particles.

The computer control of the workstation designed by Simec can complete the entire coating process and can electronically measure, record and balance the gas flow rate, the plasma itself, and the energy supply. Unlike the previous plasma system, this precise production process ensures a stable supply of thermal energy.

Special attention should be paid to ceramic powder. In standard chromium oxide powders, the particle size range is 5-50 microns. Even if other parameters remain unchanged, such large differences can lead to huge irregularities in the coating. For example, when small particles (5 microns) have evaporated, the large particles (50 microns) are only partially melted. Thin coatings are generally more accurate and more uniform. A large difference in particle size can lead to inaccuracies in laser etching.

Taking all of these factors into consideration, the Simec system accurately detects chromium oxide powder under experimental conditions to ensure the necessary quality levels and chemical needs. The test process is synchronized with the production process and the results are printed out for production services.

It has been demonstrated that chromium oxide is the best surface material for laser etching of metal anilox rolls. Plasma coated chromium oxide is extremely durable due to its extremely high hardness and high density. If the ceramic coating process is very meticulous, the coating will be very hard and almost impossible to break. These features create a high quality surface so that the laser etching process can be performed precisely to produce the desired mesh.

Laser etching using CO2 and YAG (illustration)

In general, the laser etching unit is computer-controlled and its stored data can be restored at any time to ensure high reproduction performance. The intensity of the laser beam is determined by the depth of the etching. And focus on the surface of the roller through a specific optical device. The diameter D, focal length f, and wavelength λ of the radiative power determine the minimum dot size S generated by the laser.

The minimum dot size is very important for the number of high screen lines and the subsequent better mesh. Previous laser systems could not etch exceptionally good screen lines. The operating CO2 laser wavelength is operated at approximately 10 microns and is used in a limited range of network lines.

With the advent of a new generation of laser-YAG lasers, this limitation has ceased to exist. Because wavelengths as short as 1 micron enable smaller dot sizes. It is therefore conceivable that the shorter the laser wavelength, the higher the resolution. The latest YAG laser system is equipped with a "Multi-Hit" device, which allows it to etch the mesh in a series of finer steps. The final mesh wall is finer and the mesh profile is steeper.

The mesh formed by the CO2 laser is generated by stronger pulses. The laser beam hitting the ceramic coating is partially reflected and partially absorbed by the material. This leads to two different processes:

* melting;

* Volatilization of the ceramic coating.

One characteristic of the CO2 laser is that the fused ceramics accumulate on the walls of the mesh and must be specially trimmed before removal.

Etching with a YAG laser is mainly a volatilization process without melting. This provides a smoother, more uniform etching effect for high quality printing.

This does not mean that the YAG laser is the only one used in the future. Their low energy is not suitable for producing rough metal anilox rolls with a correspondingly high number of cells. Therefore, it is not economical to produce such an anilox roll with a YAG laser. The minimum limit of the YAG laser is 160-200L/cm, but it should be mainly used for particularly high cable production.

The diamond polisher will also make final special modifications to the roller to remove the slightest unevenness. In this way, each roller can be used immediately without the need for a previous running-in process. Every metal anilox roll needs precise control during production, diameter, focus, etch depth and theoretical mesh size as well as production quantity, sequence, customer and production data on the anilox roll (it accompanies the entire transfer process) , have a clear mark.