Flexography or flexo printing is characterized as a relief process. The image areas on the compressible plate are raised above the surrounding non-image areas. An ink metering cylinder called an anilox roll applies ink to the raised areas. The plate is then moved into light contact with the substrate to transfer the image. The minimal pressure during image transfer allows printing on material, such as corrugated board, that may be adversely affected by impression pressure.
Flexographic printing technology today
Flexo printing units in use today consist of three basic types: the two roll unit, the two roll unit with a doctor blade, and the dual doctor ink chamber system. Two roll units are usually found on older flexographic presses, and on narrow web presses. Narrow web presses equipped for process colors often use the two roll unit with a doctor blade, and more modern wide web presses use the dual doctor ink chamber system. Each type of flexographic press uses an anilox roll. The surface of every anilox roll is engraved with a pattern of tiny cells, so small they can only be seen under magnification. The size and number of these cells determine how much ink will be delivered to the image areas of the plate, and ultimately to the substrate.
An anilox roll is either copper engraved and then chrome-plated, or ceramic coated steel with a laser engraved cell surface. Anilox rolls are carefully selected for specific types of printing, substrates, and customer requirements. Often the flexographic printer will perform test runs to determine the ideal anilox for producing the desired ink distribution for halftones, spot color, and solids. The design of the flexographic printing unit enables press manufacturers to build presses in any one of three configurations: the stack press, the inline press (including corrugated presses), and the common impression cylinder press. Each configuration can be equipped with any of the basic printing units, depending upon the needs of the flexographic printer.
Flexographic Ink Delivery System
On a two-roll flexographic printing unit, the rubber covered fountain roll rotates in a fluid ink bath, dragging ink from the pan to cells of the anilox roll. The soft rubber fountain roll is held in tight contact with the anilox roll. As the anilox rotates past the nip point, the fountain roll wipes excess ink from non-cell areas. Once past the nip point, each cell is filled with ink, and a measured, repeatable amount of ink is available to the printing plate. The metered anilox roll is moved into light kiss contact with the image areas of the plate, and the plate cylinder is moved into kiss contact with the substrate to transfer the image.
The steel impression cylinder supports the substrate. When a thin metal or polyethylene doctor blade is used with a two-roll unit, the nip point between the fountain and the anilox roll is opened to allow ink to flood the anilox and fill the cells. The doctor blade comes into contact with the anilox to clear excess ink from non-cell areas. With a dual doctor ink chamber, the fountain roll and inking pan can be eliminated; ink is delivered directly to the anilox through an enclosed chamber.
Characteristics of Anilox Rolls
Cell Per Inch : Range: 140 to 1200 CPI. As cell count increases, ink delivered to plate decreases. As line screen resolution increases, CPI should also increase.
Cell Volume :Range: 1.8 to 17 BCM (Billion Cubic Microns per square inch of cells). As CPI increases, cell volume decreases.
Cell Angle :Typical anilox cell angles are 30°, 45°, and 60°. A 60° angle allows for more complete ink transfer, and is the preferred cell angle. The screen angle of the printing plate and the cell angle can combine to cause a moiré pattern, even with one color halftones. Moiré is avoided by angling separation screens.
|Application||Substrate||Cells Per Inch||Cell Volume|
|line art||corrugated board||200-280||7-8.5 BCM|
|halftones at 65 lpi||corrugated board||360-400||4.0-5.5 BCM|
|4/c halftones at 133 lpi||polyethylene bags||600-900||1.8-2.0 BCM|
Flexo printing Press Configurations :
Common Impression Cylinder (CIC) Press
• 4-8 color units
• Limited to one-sided printing
• Ideal press for hairline register at high speeds on stretchable films
• Longer make-ready times required because printing units are more difficult to access
• Up to 12 color units
• Can print two sides with the aid of a turn-bar
• Used for printing thick substrates such as paperboard
• Not recommended for printing thin packaging film
• Often used inline with other converting operations such as lamination, rotary and flatbed die cutting, and side seal bag converting.
Light-sensitive photopolymer is supplied in either solid sheets or in a thick liquid state. The image area of the plate is exposed through a film negative. Liquid photopolymer, about the consistency of honey, solidifies when exposed to ultraviolet light. After exposure, the nonimage area is removed by processing.
Laser Ablated Plates:
Some platemaking machines can transfer images directly from the computer to the plate, a process known as direct to- plate that voids the production of film. Some photopolymer plates can be directly imaged by lasers and then conventionally processed. A design roll is a cylinder covered with rubber or photopolymer and molded or imaged by a laser. Design rolls can provide a true continuous repeat with a continuous background color. Laser ablated plates must be nested to hide the plate seam for the appearance of continuous repeat.
As the soft plate wraps around the cylinder, it can elongate, stretching images, halftones, and text across the curve dimension. Without compensation for plate elongation, images will not print as designed. In the example, if no compensation is applied, the sun image is printed as an oval and the vertical lines on each side are lengthened.
Distortion is usually performed within the flexographic software application or at the RIP stage before the film is imaged. The amount of distortion depends on the thickness of the plate and the mounting tape used to fasten it, and on the circumference of the cylinder (the repeat length). In general, thicker plates and shorter repeat lengths increase the elongation. Special flexographic software can compensate for plate elongation by adding distortion, using a basic formula.
This is the overview of a basic flexo printing technology.