The anilox roll is known by several other names. These include form roll, meter roll, knurled roll, engraved roll, ink applicator roll and ink-transfer roll. The name anilox is a throwback to the times when the process was called aniline printing. Its construction is similar to a gravure cylinder in that it has a series of engraved cells. Like gravure cylinders they may be mechanically or laser engraved.
The anilox roll may be either chrome plated or ceramic-coated. Laser engraved rolls are generally ceramic-coated prior to engraving. The cells in a mechanically coated anilox are in the form of an inverted pyramid. The volume of the inverted pyramid is given by V = h/3[At + Ab + (AtAb)1/2] where h is the depth of the pyramid, At is the area of the opening and Ab is the area of the cell bottom.
Anilox cell volumes are typically measured in units of BCM (Billion Cubic Microns per square inch, a truly bastardized unit obtained by multiplying the volume of a single cell by the screen count in line per inch squared). A more sensible unit is microns (µ3/µ2). As a unit of length it gives a measure of the the linear size of the cells. The conversion is Cell volume/Area(µ) = 1.55BCM Since the opening is the base of the inverted pyramid, it is the part most sensitive to wear.
Since the region around the opening contains the greatest volume per change in depth, a little wear can cause a significant change in the amount of ink transferred. The cell structures may one of five types;
A) Trihelical, B) Pyramid, C) Quadrangular, D) Hexagonal or E) Hexagonal Channel Screen.
A) A Trihelical screen is an unbroken line inscribed at a 45° angle to the roll axis. These are primarily used in coating applications of viscous fluids.
B) A Pyramid cell is a full inverted pyramid. These are primarily used in wipe roll metering systems.
C) A Quadrangular or Quad cell is a truncated inverted pyramid. These are very versatile when used with wipe roll and doctor blade systems.
D) A Hexagonal cell is an truncated inverted hexagonal pyramid. These can be packed very efficiently in two dimensions and have very good release characteristics.
E) The Hexagonal channel screen is a deep hexagonal cell with a shallow vertical channel that links each cell. This provides a means for even pickup and release of heavy body inks.
This screen gives the most even laydown of ink. The anilox roll is changing more rapidly than any other single component of a flexo press. They are made from a special grade of seamless steel tubing treated for engraving. As stated earlier, anilox wear can have a profound effect on the volume of ink transferred. For example, a 20% reduction in depth of a pyramid cell can lead to a 40-50% reduction in volume depending on the screen count and cutting angle. Sometimes pigment particles collect at the bottom of the cells.
Its been estimated that the approximately the bottom third of the cell does not release its ink. So when a cell is worn down, the actual ink deposited may be only 40% of the designed amount. The wear of the anilox surface is compounded by the use of doctor blades. The doctor blade removes excess ink from the surface of the anilox and provides better control of ink transfer to the plate cylinder. This is particularly important in printing process colors and halftones.
The “reverse angle” doctor blades used with anilox rolls is very efficient, literally shaving the excess ink from the roll. The theoretical ideal angle is 30°. The TIR of the anilox must not exceed .0005" in order to maintain proper blade pressure. Newer chamber blade systems employ double doctor blades, one on the up side and one on the down side. These have provided a tremendous boost to flexo print quality.
There are several varieties of anilox rolls to choose from. Smooth Ceramic - The longest wearing and least expensive anilox rolls. Do not allow controlled metering of ink because of smooth finish. Usually employs flame sprayed Aluminum or Chromium oxide.
Textured Ceramic - Long wearing like smooth ceramic, but uses a coarser ceramic powder. The surface roughness provides the mechanism of ink transfer.
Ceramic-Laser Engraved - Longest wearing engraved metering rolls. Process is becoming less expensive. Available in screen counts up to 500 lpi. Identical to smooth ceramic except that cells are burned into the ceramic using a CO2 laser. A wide variety of cell patterns is available and the cells deliver a repeatable metered volume of ink.
Stainless Steel or Nickel/Copper - Flame sprayed to a smooth finish similar to ceramic. Chrome plated for longer wear.
Engraved and Chrome - Most common engraved anilox roll for high-quality print results. Fabricated from high grade seamless steel. Can be reengraved when worn. Chrome plated to protect the surface.
Engraved and Ceramic - An engraved roll flame sprayed with fine ceramic powders of aluminum or chromium oxide. Coating is very fine, about .0005" in thickness. Usually a coarser screen than for chroming.
Forerunner of laser-engraved ceramic rolls.
Electronic engraving has come to flexo from the gravure industry. The advantages are
Screens up to 600 lpi are available, but the angles of the cell walls and cell configuration cannot be varied much because of diamond surface shape.