Anilox Rolls

Description

The ink-metering roll, or anilox roll, is usually chrome-plated or ceramic coated metal and is covered with tiny, engraved cells, numbering anywhere from 80 up to 1,200 per linear inch. The amount of ink that is printed on a substrate is dependent on the anilox roll cell volume. The depth of the individual cells that are engraved in it determines anilox roll cell volume. Development of high quality anilox rolls that accurately deliver thinner ink films has resulted in a pollution prevention mechanism to reduce VOC emissions.

The anilox engraving has three main variables: line count (LPI) refers to the number of cells per inch, engraving angle refers to the spatial orientation of the cells in relation to one another, and cell volume (BCM) refers to the ink carrying capacity of the cells. A fourth, and often overlooked, variable pertains to the condition of the engraved cells.

Best Management Practices & Pollution Prevention

Anilox Roll Cleaning: before and after
Anilox Roll Cleaning: before and after

Roll Selection

Proper roll selection, standardization of the roll inventory, and proper roll maintenance can reduce print process variability and uncontrolled, artificially high delivered ink density.

Cleaning Anilox Rolls

While there are a number of variables that can contribute to poor printing, a dirty, plugged anilox will always result in major print quality problems, which result in production waste. A dirty roll will have reduced ink carrying capacity, causing coverage and color density problems.

Regardless of the cleaning method used, it is important to thoroughly clean rolls as soon as possible after the pressrun. By their nature, water inks, unlike solvent inks, do not readily re-wet. Once dried cleaning is difficult, the dried ink is very abrasive and can cause damage to the ceramic or chrome coating on the anilox rolls. If the ink is allowed to dry on the surface of the anilox roll, it will require more aggressive solvent or caustic cleaners to adequately remove the ink from the cells.

Fine screen count anilox rolls are, by the nature of the much smaller cells, more susceptible to quicker plugging. They will require more frequent maintenance and are more difficult to clean.

Daily on-press cleaning will help maintain printability. At a minimum, after each run, circulate solvent or water through the inking system using the ink pumps while rolls are turning. Then the roll should be scoured. On chrome coated rolls a brass bristle brush should be used. A stainless bristle brush should be used on ceramic coated rolls. A good, daily hard scrubbing will go a long-way toward maintaining "open cells" on your rolls.

There are also mechanical systems that may be used on-press to clean anilox rolls:

  • Blast systems utilize a pressurized cleaning head with vacuum removal as the cleaning system. The cleaning head is moved, by hand, across the roll. These systems utilize compressed carbon dioxide, fine particle materials such as baking soda, or plastic beads to remove dirt and ink.
  • Chamber doctor blade units, with internal nozzles connected to a pumping system, flush the anilox cells cleaning solution which may consist of water, surfactant solution, solvent, or a caustic solution.
  • Flushing systems use pumps to flood the inking system with clean-up solution.

Deep Cleaning Anilox Rolls

For maximum maintenance of anilox cell volume, rolls should receive a thorough cleaning as-soon-as they are removed from the press. This can consist of hand cleaning with a brush as described above, or placing the roll in a machine specially designed for roll cleaning. There are several technologies used to deep clean anilox rolls, including ultrasonic, pressure, media and laser cleaning.

Ultrasonic Cleaning

Ultrasonic cleaning consists of a tank of cleaning solution (commonly a detergent or caustic bath). High efficiency cleaning occurs because sound waves, created by ultrasonic transducers, create microscopic bubbles that collapse or "implode" producing shock waves. In anilox cleaning these microscopic bubbles implode in the cells and force the ink out. The process consist of placing the into a heated tank containing the cleaning solution and is slowly rotated. This exposes all surfaces of the roll to the action created by ultrasonic transducers in the bottom of the tank. The combination of heat, chemicals and the imploding bubbles, effectively lift the dried ink off the roll.

Pressure Cleaning

There are two types of pressure cleaning: spraying a high pressure liquid directly on a roll, and soak-pressure wash immersion systems.

Direct spraying of a high pressure liquid on a roll is normally limited to narrow-web rolls because it is typically a hand operation. Cleaning is usually done in an enclosed glove-box cabinet using a heated caustic or detergent cleaning solution. A solvent would not likely be used due to the risk of fire/explosion.

A soak-pressure wash immersion system consists of a tank to hold the bath, heaters, a rotating roll-holder, pumps and filters. The pump supplies the heated liquid under pressure through the bath (spray under immersion) and into the anilox cells.

Media Cleaning

A non-abrasive material, such as sodium bicarbonate (baking soda), polyethylene beads or Carbon Dioxide (CO2) (also known as dry ice) is sprayed under pressure onto the roll. Because the cell walls can be easily damaged by the pressurized particles, care must be used. Both baking soda and polyethylene bead cleaning are usually done in an enclosed cabinet to prevent the media from becoming airborne. Baking soda is non-toxic and non-hazardous. Polyethylene beads may be reused or disposed of as polyethylene waste. It’s waste classification may change as a result of the contaminant pick-up during the cleaning process.

When using CO2, all that remains when cleaning is completed is the ink particles and waste removed from the surface. The CO2 evaporates into the atmosphere. Once the blast media is contaminated with ink and other materials removed during the cleaning process, its waste classification may change. Review of the MSDS and laboratory testing may be necessary.

Laser Cleaning

A laser can remove foreign matter from the high melting point ceramic surface of an anilox roll. Laser cleaning can remove all additives, restoring the hydrophilic properties of a ceramic roll. Cell volumes have been reported to increase three fold due to this cleaning process. The laser cleaning method removes dried inks, polymer plate materials and residue from chemicals without harming the cell structure, no matter what the line count. Laser cleaning is not recommended for chrome coated anilox rolls.

Service companies will clean the anilox rolls for flexographic printers. These companies will either come to the printing facility to clean rolls on site, or the rolls can be sent to the service company’s facility to be cleaned.

Cleaning Solutions

Solvent inks usually can be readily removed with a compatible solvent. Solvent may be used in a hand/brush/rag process on or off-press, or in a specially designed cleaning machine.

Aqueous cleaning chemicals for water-based inks rely primarily on pH to rewet the ink. If the ink is only dry, a pH of 9 will usually work. Inks that are totally dried-in, will probably require a pH of 10-11, which is very caustic and must be used carefully to avoid injury and damage to the anilox.

If used heated, the temperature of the chemical should not exceed 110oF as it can cause resins to harden beyond the norm, thereby making removal almost impossible.

Long soaking of an anilox in any solvent or chemical is not recommended because there is a degree of porosity in any roll. Prolonged soaking can damage the underlying structure of the roll, causing failure of the whole roll surface.

An alternate approach to cleaning anilox rolls of ink without risking damage, involves a measure of preventive maintenance before the ink is applied to the roll. This method consists of applying a thin coat to the roll surface protecting the ceramic (or chrome) from dried-on ink deposits. Using this approach, a clean surface remains clean, rather than cleaning a surface that has increasingly more ink deposits left behind on the roll.

For access to vendors who may supply alternative materials and equipment, see the PNEAC Vendor Directory.

Environmental Regulations

Air Emissions

"Cleaning solvents" must be stored, handled and disposed of or recycled as you would when they are used with ink.

Environmental safety and fire safety specifications for explosion proof design requirements must be considered when using cleaning equipment and caustic or solvent solutions. If the equipment is to be used along with flammable solvents, explosion proof wiring and fire safety precautions must be observed when used.

Health & Safety

Workers must be protected from particulate cleaning products as well as caustic and solvent solutions with gloves, goggles, aprons, etc. Appropriate PPE must be provided.