Anilox Roll

ANILOX ROLLER

INTRODUCTION  The anilox roll is known by several other names.  These include form roll, meter 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.

CONSTRUCTION  Special grade steel is used.  Some people believe that stain-less steel anilox rolls have hardness

   

which will give  Greater roll life  Overcome corrosion. The engraving process requires soft, malleable and homogeneous material. Anilox roll must be chrome plated to present wear hard surface and service life. Rolls are designed with removable, interchangeable journals to reduce the downtime of rolls to nearly half. A little wear causes drastic changes in the amount of that will be transferred.

 Anilox cell volumes are typically measured in units of BCM (Billion Cubic

  



Microns per square inch, a 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. As a unit of length it gives a measure of the linear size of the cells. 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; 1. Trihelical, 2. Pyramid, 3. Quadrangular, 4. Hexagonal or 5. Hexagonal Channel Screen.

1.

2.

3.

4.

5.

A Trihelical screen is an unbroken line inscribed at a 45 degree angle to the roll axis. These are primarily used in coating applications of viscous fluids. A Pyramid cell is a full inverted pyramid. They are the most popular. These are primarily used in wipe roll metering(non-doctor blade inking) systems. The total amt of ink transferred to plate is a function of cell volume and the thickness of surface ink layer. A Quadrangular or Quad cell is a truncated inverted pyramid. These have more versatile and more positive release characteristics. Used with wipe roll and doctor blade systems. A Hexagonal cell is a truncated inverted hexagonal pyramid. They have very good release characteristics and can be chrome coated and ceramic coated more efficiently. 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, coatings and adhesives. It holds more ink than a quad cell, has high release property and create low amt of hydraulic force on all types of doctoring devices.

SPECIFICATIONS  Must be constructed to withstand the pressures applied by fountain

 





roller. It should conform(match) to the anti-deflection manufacturing parameters of fountain rollers. Steel base should be made within 1 thou TIR error for conventional flexography and 0.5 thou for doctor blade flexography. The diameter of finished anilox roller(including its chrome or ceramic coating) should be exactly equal to the pitch diameter of the gear which propels it. All anilox rolls should be balanced(dynamically and statically).

DRIVE SYSTEM  Anilox roll should rotate at the same speed as the plate cylinder in a 1:1   

   

ratio with web speed. Power is delivered to impression roller. A gear train transmits the power to plate cylinder and then to the anilox roller. If the anilox roller rotates at a differential speed than plate cylinder,  Excessive plate wear,  Color density problem. The diameter of re-engraved anilox roll should match its gear’s pitch diameter to ensure speed match with the plate cylinder. Anilox roller must be capable of rotating when plate cylinder and web are stationary. A separate electric or hydraulic motor is used to run the anilox roller at all press stops if the fountain roller has a gear transmission system If the fountain roller is driven by an adjustable speed motor, then that motor idles the anilox roller through a separate gear train from the fountain roller.

POSITIONING  There are two methods of positioning the anilox roller,

1. Screw positioning system

2.

Stop positioning system

Screw positioning system utilizes 2 threaded rods(screws) or rack and pinion mechanisms, one on each side.  It move the anilox roller carriage on its notched side blocks called dove-tails.  For moving roller large distances, both screws should be capable of turning simultaneously.  For moving small(fine) distances, screws must be capable of turning individually.  Few poorly built flex presses use a chain for setting mechanism. But it is not good as mechanical gear or shaft positioning systems. A chain always have much more backlash(reaction) in it than a gear.  Usually, two threaded rods are connected with jackshaft or gear train for

making coarse adjustments.  Each threaded rod can be rotated individually to make best contact possible with plate cylinder.

 There is a disconnection clutch on the jackshaft or gear system which 

   

allows each screw to rotate independently. Chain system has a chain removal device which disconnects each side of anilox roller positioning mechanism. Chain system is less expensive than jackshaft method. But flexo printers prefer to spend more money for a better positioning system. Jack shaft system allows for fast clean up and change over times. Most flexo presses have dial readouts. It permit accurate repositioning of the anilox roller. Flexo presses have more severe vibration problem than other ptg machines. So a locking system is needed to maintain the location of screws throughout the run.

Stop positioning system employs an accurately positioned set of metal obstructions which limit the motion.  Stop is positioned on each side of the deck and are positioned with a finely threaded rod(screw).  The threaded rods should be connected by a jack shaft or other device so that the turning of one handle will make coarse settings on both stops.  A disconnect clutch allows fine adjustment individually.  Good flexo presses have a mechanical or electronic readout.  Mechanical locking devices are required on stop screws.  It is possible to re-position the anilox roller after it has been moved.  Fast rack-back is one of the advantages. A simple turning of a knob or depression of button will move the entire carriage away from the PC in few seconds.

SPECIFYING THE RIGHT ANILOX ROLL  There are three main cell characteristics important to understand including: 1. The angle of the cells (60°, 45°, 30°, etc.) 2. The carrying capacity of the cell (or cell volume) 3. The number of cells per linear inch (or line screen)

1. Screen Angle

 Today, the 60° hex pattern is recommended for most flexo printing applications

for the following reasons:  Cells can be nested more tightly together, allowing for approximately 15% more cells

in a given area.  Post areas are eliminated due to the placement of the cells during laser engraving.  Cells can be shallower while still releasing the same amount of ink because more cells

fit into a square inch area.  Channeling is avoided (as compared to the 30°) because the straight side of the cell

wall is not positioned horizontally.  The 60° hex pattern is most easily reproducible from a manufacturing standpoint.

Therefore, you will see greater consistency from roll to roll.  45° and 30° patterns may be preferable in non-printing flexo applications (such

as coatings, laminations, and specialty printing), but ordinarily are not recommended for general flexo printing applications.

2. LINE SCREEN  Line screen indicates the number of cells per linear inch on an anilox

roll, and is a major component when specifying an anilox roll.  Different printing applications require different line screen counts.

Application

Appropriate Anilox Line Screen

Appropriate Anilox Volume

Heavy line and solids

180 – 330

9 - 4 BCMs

Line and type

200 – 400

8 - 3 BCMs

Vignettes

360 – 500

3.6 - 2.8 BCMs

Process

500 - 1200

2.8 - 0.9 BCMs

 The reasoning behind this is that lower line screen anilox rolls typically have

higher volumes and carry larger amounts of ink or thicker ink films. These thick ink films do not produce the graphic quality necessary for high-end process work. Therefore, a 330 line screen anilox roll will not be adequate when printing process flexo.  On the other hand, a high line screen anilox roll (800-1200) is not capable of

laying down enough ink for a block solid job. The printer will not get enough density or coverage from such small cells.  A printer must choose the best line screen for their anilox roll to meet the needs

of the specific job he or she is printing. One helpful tool for determining the line screen needed is a banded roll.

 A banded roll contains actual bands of different line screen and volume

combinations. A banded roll test helps determine the correct line screen and volume combination for printing the thinnest ink film possible and still delivering the color strength you need.

Number of Cells per Square Inch  For 45° Cell Patterns: Line Screen X Line Screen = # of Cells per Square

Inch  Example: For a 500 Line Screen, 45° anilox roll:500 line screen X 500 line

screen = 250,000 cells per sq. inch

 For 60° and 30° Cell Patterns: (Line Screen X Line Screen) X 1.15 = # of

Cells per Square Inch  Example: For a 500 Line Screen 60° or 30° anilox roll: (500 line screen X 500

line screen) X 1.15= 287,500 cells per sq. inch.

3. Anilox cell volume  Cell volume is the capacity of the engraved surface in a square inch, expressed

in Billion Cubic Microns (BCM).  Higher volume translates to higher solid ink density, more color or a heavier

coating thickness. Lower volumes apply thinner ink films directly associated with higher print quality and process efficiency.  Thinner Ink Films Yield:  Predictable Dot Gain  Clean Printing  Smaller Dots  Clean Type  Sharp Edges  Less ink=>less waste=>less VOC=>less cost

 Anilox cell volume changes over time, due to wear, plugging and cleaning

procedures. Dirty or plugged cells are temporary and correctable to restore cell volume and color. Wear is irreversible and permanent resulting in weaker or less color due to lower volume.  Depending on the market and the substrate being printed, typical volume

ranges; Solid Coverage / Coatings: 5.0 BCM or higher Combination printing

: 2.0 to 4.0 BCM

Process printing

: 1.3 to 3.0 BCM

 A lower volume anilox roll with a 60° cell is good as more cells fit into a

square inch. However, a cell with too small of a volume will cause ink to dry before it can be transferred, unless there are sufficient drying inhibitors used. The goal is to specify an anilox roll that will deliver the color strength and densities needed with the thinnest ink film possible.  A cell whose depth is between 23% to 33% of the cell’s opening is

recommended.  For example, if the opening of a cell is 100 microns, the depth of that cell

should be between 23 and 33 microns.

 Disadvantages of deep cells (>33% depth to opening ratio):

 Ink will not release efficiently  Difficult to clean  Creates rough cell walls  Extremely difficult to make consistent and predictable engravings.

 Example of deep cell (50% depth to opening ratio)

 Disadvantages of shallow cells (‹23% depth to opening ratio):  Creates rough cell cavity  Extremely difficult to make consistent with predictable engravings  Example of shallow cell (15% depth to opening ratio)

 Advantages of bowl shaped cells (23% - 33% depth to opening ratio):  Uniform cell size and shape  Smooth glazed cell lining for efficient ink release  Shallow angle for easy ink release  Smooth, thin cell walls  Possible to make consistent and predictable engravings

 Example of bowl-shaped cell (28% depth to opening ratio)

 The volume of the inverted pyramid is given by

V = D/3[A1 + A2 + (A1A2)1/2] where D is the depth of the pyramid, A1 is the area of the opening and A2 is the area of the cell bottom.  OR

V= πx(opening)2x(depth/8)x(line screen)2x1.15 109

ANILOX ROLL WEAR  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 4050% reduction in volume depending on the screen count(no: of cells per inch or centimetre) 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.

USE AND CARE  At running speeds, never let rolls run dry.  Use proper solvents for cleaning.  When printing unit is not in use, separate rolls as soon as possible and









wash all the components especially anilox roll with brass brush and recommended solvents. Flaking chrome indicate damage to fountain roll. (A minute portion of moist ink at the very bottom of cells may penetrate the chrome if given enough time and will corrode the steel base under the surrounding chrome and cause chips of chromium to flake-off.) Transport with journals and roller face protected. Check for roller wear with 14 power magnifying glass. Wear is seen by increase in shine on roll and loss of color strength in the print.

CHROME PLATING  Most uniform protective covering.  Maximum of 0.0005” chrome thickness for flexo printing on 

  

corrugated. More thickness  Chrome will become rough.  Destroy the uniformity of carrying capacity.  Alter the releasing properties in a non-uniform way. Copper flash(0.0001”) before chrome plating is done to prevent most corrosive fluids from reaching steel. Chrome plating does not alter the capacity of the cell and its release characteristics. (figure – proper and improper chrome plating)

CERAMIC ANILOX ROLL  Ceramic is an extremely hard and dense material.  In order to abrade the surface, it requires concentrated bursts of focused

energy from a high power industrial laser.

   

Chrome vs.  Cell counts to 500 LPI 45 degree only Poor wear resistance Looses volume quickly Lower achievable volume

  Uses a knurled tool

Ceramic  Cell counts to 2300  Many patterns  Resists wear  Maintains volume  Greater cell volumes are achievable  Cost 3 times –life also 3 times.  Uses laser

PLATE DOT VS. ANILOX CELL MODELS  The relationship of your printing plate dot to anilox line screen cell directly

affects the quality of flexo print.  A low line screen printing plate (such as 55) require a lower line screen anilox

roll (such as 250 – 500).  When using a high line screen printing plate (such as 150) require a higher line

screen anilox roll (such as 600 – 1200).

% Plate Dot 1% Plate Screen (per inch)

2% 3% Minimum Anilox Line Count (per inch)

4%

55

500

330

280

250

65

550

400

330

280

85

700

500

400

360

100

900

600

500

440

110

900

660

550

500

120

1000

700

600

500

133

1200

800

660

600

150

1200

900

700

600

175

N/A

1000

900

700

200

N/A

1200

900

800

 when you are using a 100 line screen plate and trying to hold a 2% dot, the

chart above indicates that you will need a 600 line screen anilox roll. Whereas, if you’re using a 100 line screen plate, and you’re trying to hold a 4% dot, you’ll only need a 440 line screen anilox roll.  The reason for this is simple. You do not want the dot on your printing plate to

be smaller than the cell on your anilox roll. If that occurs, the dot can actually enter the anilox cell, picking up excess ink. This phenomena is referred to as "dot dipping" and can lead to very dirty printing situations.

TYPES OF ANILOX ROLLER ANILOX ROLLER CLEANING METHODS ANILOX SLEEVES