Common cleaning procedures for anilox rollers – an overview

Cleaning procedures for anilox rollers

The clogging of the cells of laser-engraved ceramic anilox rollers with contaminants and dried ink impairs the printing quality. This means that contaminated rollers need to be cleaned as soon as possible after printing. Different procedures are available for this purpose.

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Some of these procedures have a certain aggressiveness to achieve a satisfactory cleaning result but must at the same time not damage the ceramic layer.

By Ansgar Wessendorf and Sebastian Reisig

 

Contents

Cleaning of anilox rollers – Method 1: Chemical cleaning procedure

Method 2: Blasting procedure

Method 3: Ultrasound cleaning

Method 4: Laser procedure

Conclusion

 

General considerations about cleaning anilox rollers

Every procedure to clean must be capable of penetrating the microscopic cells in the ceramic layer of a roll down to the bottom of the cells in order to remove the dried ink and lacquer residues. It is important that the walls of the cells and the ceramic layer are not damaged.

In practice, chemical procedures, blasting processes (sodium carbonate, plastic pellets), the ultrasound method, and increasingly, laser technology are commonly used for cleaning anilox rollers. For all of these procedures, the duration from the time of contamination of the anilox roller until its basic cleaning is problematic. The older the ink and lacquer residues, the more difficult they are to remove, or the more incompletely they are removed due to their reactive binding agents.

Cleaning of anilox rollers –
Method 1: Chemical cleaning procedure

The chemical cleaning method is often based on alkaline cleaning substances in connection with highboiling solvents. The solvent molecules separate the binding agent from the ink leaving a mixture of pigment particles. This separation process is extremely fast.

The alkaline substances (e.g. caustic soda) then affect the individual ink components (e.g. binding agent) and dissolve the heterogeneous substance mixture which is where flocculation takes place. This reaction is slower and thus takes more time. For this reason, it is important to let the alkaline cleaning agent absorb the ink remains for a certain period. The penetration of the cleaning agent into the ink is improved and the contact between the ink and the cleaning agent by using wetting surface-active agents with a low solvent content.

For the cleaning procedure to be successful, the ink system and solvent are of utmost importance. If the solvents correspond to those used in the printing ink, they are in all likelihood suited for the cleaning process. For solvent inks, these are nonpolar solvents (e.g. ester); for water-based inks, these are water-based cleaning agents. Often, however, solvents or solvent mixtures with different polar components achieve an improved cleaning result. Heating the cleaning liquid supports the cleaning procedure.

With the various compositions of the ceramic, the interfacial tensions vary as well, which influences the wettability of the cleaning agent on the anilox roller. However, the texture of the ceramic surface must also be taken into consideration. The rougher and more porous it is, the better is the adherence of the ink but the more difficult is the cleaning. Furthermore, an excessively porous or damaged surface may have the effect that the cleaning liquid penetrates the area underneath the ceramic layer, proceeds further up to the metallic base material and causes corrosion there. For older anilox rollers in particular, there is an increased risk in this respect.

However, corrosion rarely occurs in practice. Although it cannot be excluded that the protective layer on the ceramic base material (e.g. nickel layer) has not been applied correctly. The corrosion is increasingly encouraged by acid rather than lye. This means that the nickel and/or aluminium layer underneath the ceramic blisters and destroys the surface of the anilox roller.

Furthermore, it may also be the case that the blister spreads evenly and thus modifies the circumference of the anilox roller. Due to the increased diameter, a higher doctor blade pressure than originally set on the machine occurs, which then destroys the doctor blade when printing.

When cleaning with chemical liquid agents in closed systems, stains on the surface occur frequently, this may have an unfavourable effect on the ink transfer characteristics of the anilox roller. Such stains occur if the cleaning agent drops from the protective hood and the side walls of the system onto the surface of the anilox roller. Therefore, it is important to remove any residual cleaning agent after the cleaning process has been completed. Controlling the cleaning process is often difficult as it is carried out within a sealed system in which the anilox roller is blasted with the heated cleaning liquid (60-80°C) under high pressure.

Additionally, when using chemical cleaning agents, the safety provisions (compliance with the limit values for volatile organic compounds (VOC), protective gloves, protective goggles etc.) and the regulations with respect to environmental protection and disposal must be observed. For instance, cleaning agents consisting of alkaline substances in connection with solvents must not be disposed of via the public sewage system. Cleaning agent producers offer a series of environmentally friendly solutions,which, however, can show great differences in their cleaning effects.

Cleaning of anilox rollers –
Method 2: 
Blasting procedure 

The blasting method is a mechanical, dry cleaning procedure in which, by means of a nozzle and relatively low pressure (2.5-3.5 bar) e.g. the white powder sodium carbonate (NaHCO3) is blasted onto the contaminated anilox roller. Here, NaHCO3 is blown onto the anilox roller using a spray nozzle. Due to the acceleration, very small NaHCO3 particles are blasted onto the anilox roller surface, which are then crushed by the sharp-edged blades and are thus able to penetrate the recesses of the cells. This does not damage the anilox roller since the hardness of the grains is only half the hardness of the ceramic surface.

Due to the modification of the rotational speed of the anilox roller and the axial movement of the nozzle, the cleaning quality can be set depending on the degree of contamination. The cleaning period takes approx. 40-60 minutes.

The blasting method is a mechanical, dry cleaning procedure in which, by means of a nozzle and relatively low pressure (2.5-3.5 bar) e.g. the white powder sodium carbonate (NaHCO3) is blasted onto the contaminated anilox roller. Here, NaHCO3 is blown onto the anilox roller using a spray nozzle. Due to the acceleration, very small NaHCO3 particles are blasted onto the anilox roller surface, which are then crushed by the sharp-edged blades and are thus able to penetrate the recesses of the cells. This does not damage the anilox roller since the hardness of the grains is only half the hardness of the ceramic surface.

After cleaning with sodium carbonate, white traces of the nebulised cleaning agent are visible on the roller surface and an extremely small amount remains in the cells, which normally does not have a detrimental effect on the print. Nevertheless, remaining NaHCO3-residues should simply be removed with water and a cleaning cloth. In addition the degree of hardness of the tap water must not be too high, otherwise, chalk may deposit in the cells. Therefore, for subsequent cleaning, using distilled water is recommended.

This blasting procedure system requires little space and is characterised by its easy handling. To a large extent, the use of sodium carbonate does not pose any risk for human health and the environment and it can be disposed of in the domestic waste. The granulate contaminated with ink is filtered out so that the non-contaminated powder can be reused for the next cleaning process. Due to the impact with the anilox roller, the particles are destroyed, which reduces their cleaning effect compared to the original powder. Usually, the granulate is used up after two cleaning runs.

Soft, recyclable plastic pellets are another blasting agent for cleaning ceramic anilox rollers. As is the case with NaHCO3, the system is completely encapsulated during the cleaning process here as well. The pellets consist of polyethylene and are applied to the anilox roller surface by means of a nozzle at approx. 4 bar. Upon impact with the surface, the plastic pellets first deform but then return to their original form. Deposits are removed from the surface and evacuated together with the blasting agent. A magnetic field is also created, which removes the metallic particles.

Subsequently, the contaminants are separated from the blasting agent so that the clean pellets can be reused multiple times. With this procedure, the cleaning quality depends on the form of the cells and the number of lines/cm. The polyethylene pellets are available in different degrees of fineness.

Cleaning of anilox rollers –
Method 3: 
Ultrasound cleaning

With the combination of ultrasound and a cleaning agent, the anilox roller is thoroughly cleaned of ink and paint residues and thus an excellent cleaning effect is achieved. When correctly implemented this technology does not damage. With continuous filtering, the cleaning liquid can be used for a relatively long period of time without the necessity to change it frequently. Depending on the size of the anilox rollers, an ultrasound cleaning system requires less space and is easy to install and operate.

For creating ultrasound, the system requires a container with cleaning liquid which can be heated, an oscillator system consisting of one or several piezoceramic plate oscillators, and a generator. This procedure is particularly suitable for cleaning anilox rollers with high cell rulings.

The anilox roller to be cleaned is rotated in the cleaning liquid where it is completely or partially immersed in the diluted caustic soda (10%). During the ultrasound cleaning process, sound waves create a cavitation during which microscopic gas bubbles occur which are subject to pressure and implode upon impact with the roller surface. During the implosion of the gas bubbles, extreme but locally limited pressure and temperature peaks occur which dissolve or destroy solid ink particles in connection with the chemicals.

The ultrasound frequency has an opposite effect on the cavitation intensity. When reducing the frequency, the blister size and the intensity decline. As a rule of thumb, a minimum frequency of approx. 40 kHz (40,000 oscillations/sec) is recommended in order to guarantee sufficient intensity and bubble formation, which will completely remove the deposits in the cells.

The roller must continuously rotate in the cleaning liquid. This way, roughening of the anilox roller surface due to partially excessive sound intensities is prevented. Furthermore, with increasing distance from the sound source, the soundwaves lose their efficacy and the ultrasound can only have an irregular effect. For this reason, the defined distance of the sound source to the anilox roller and the frequency setting must be accurately adapted to each other.

For a good cleaning result with ultrasound, the cleaning liquid must also be heated to 50-65 °C (better: 80 °C) while the roller must rotate in the bath for approx. 20–30 minutes. The anilox heated roller must first cool down to room temperature after cleaning, which prevents its immediate use in printing production.

With ultrasound cleaning, pressures of approx. 1500 bar may occur, which can damage the anilox roller surface. For instance, cracks may extend further so that the cleaning liquid can further penetrate the roller down to its substructure and cause corrosion. Damaged and leaking front sides and ends of
a roller may also be causes. Additionally, with high rulings, the fine cell land areas may be damaged
if the anilox roller is exposed to ultrasound cleaning for an excessive period.

Cleaning of anilox rollers –
Method 4: 
Laser procedure

The principle of laser cleaning is based on the fact that a very short focussed laser pulse has a high-intensity impact on the ceramic surface of the anilox roller which absorbs the energy of the laser beam. Here, what is known as “cold” evaporation of the contamination layer and foreign particles occurs. In particular, when using metallic material, the surface is hardly heated due to light reflection.

The laser method is therefore a gentle cleaning procedure providing the pulse frequency and scan width parameters of the laser beam and the forward feed and rotational speed of the laser are correctly set and/or adapted to each other.

In most cases, a diodepumped solid-state laser is used, which is usually operated at a power of approx. 300 W and wave lengths of 1064 nm. By means of the variable settings of the laser light pulse frequency, the laser can be used for a broad range of materials. The shorter the laser pulse (or the higher the pulse frequency), the greater is the laser output. Thus, the heat input into the material is greater or lower respectively.

With the variable settings of the rotational speed, different roller diameters can be balanced without increasing the energy input. By doing so, a thermal overload of the anilox roller is avoided. On the other hand, by changing the rotational speed, the energy supply can be increased. Here, there is a risk that the roller is partially thermally overloaded. Additionally, the setting of an incorrect rotational speed is indicated by small horizontal stripes on the anilox roller surface.

In particular with heavily contaminated anilox rollers, the correct advance feed speed is important since this parameter also influences the energy input into the anilox roller.

The outlet of the laser beam (scan width) can be set variably so that the energy is distributed over a greater or smaller area. Naturally, this influences the removal rate.

The laser cleaning of an anilox roller with a roll width of approx. 1300 mm takes approx. 45 minutes with an advance feed speed of 30 mm/min. After a short cooling-down period, the roller is ready for operation. For difficult to clean substances, such as emulsion paints and 2K inks as well as for high rulings, the process has achieved good cleaning results. Under these conditions, other cleaning procedures face their limits.

The “dry” laser procedure does not comprise any chemicals and no subsequent cleaning of the anilox rollers is required. A suction unit is required for removing contaminants and ink residues. The system operator must wear suitable protective goggles for protection against the laser beams. Additionally, the procedure is suitable for inline cleaning, e.g. for cleaning difficult to access anilox rollers in the direct printing of corrugated cardboard.

For many potential users, one disadvantage of the laser procedure is the high investment costs. Furthermore, the correct setting of the laser towards the anilox rollers with different rulings and the substances to be removed (emulsion paints, solvent inks, 2K or UV paints etc.) requires a relatively long time, much experience and a lot of know-how. However, once this ground work has been implemented, the cleaning results are extremely satisfying.

Conclusion

The costs of procurement of a ceramic anilox roller is several thousand Euro. In order to guarantee a long lifetime, regular and efficient cleaning is required. In the course of the printing process, wearing of the printing forme, the dissolving of fibrous material occurs, which is then transferred to the anilox roller via the printing plate and clogs the cells, or the ink dries in the cells.

For removing such contamination, a basic cleaning procedure which is implemented outside of the printing press is required. Here, various cleaning procedures are available, which all have advantages and disadvantages.

Comments

When we share articles on other channels, we sometimes get comments we want to include for our readers on the blog.

I’ve never tested an anilox roller in the field that wasn’t out of spec due to its condition. Everyone needs to use the theory “an ounce of prevention is worth a pound of cure”. When you don’t apply the proper amount of coating and you stick a job because the ink migrated through the coating your film is too thin.
Greg Falkenstein, Digital Imaging Specialist at Prisco Digital a division of Prisco

Source

DFTA [German association for flexographic printing] research report: „Untersuchung der Wirksamkeit, Nachhaltigkeit und Gefahren von Rasterwalzen-Reinigungsverfahren im Flexodruck“, Dietmar Wolf.

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