Applications:
Flexography is the major process used to print packaging materials. Flexography is used to print corrugated containers, folding cartons, multiwall sacks, paper sacks, plastic bags, milk and beverage cartons, disposable cups and containers, labels, adhesive tapes, envelopes, newspapers, and wrappers (candy and food).
Flexographic presses are capable of producing good quality impressions on many different substrates and is the least expensive and simplest of the printing processes used for decorating and packaging printing. The use of flexographic printing presses is on the rise. There are two primary reasons for this: 1) it is a relatively simple operation; and 2) it is easily adapted to the use of water-based inks. The widespread use of water-based inks in flexographic printing means a large reduction in VOC emission compared to the heatset web or gravure printing processes.
Process Overview
In the typical flexo printing sequence, the substrate is fed into the press from a roll. The image is printed as substrate is pulled through a series of stations, or print units. Each print unit is printing a single color. As with Gravure and Lithographic printing, the various tones and shading are achieved by overlaying the 4 basic shades of ink. These are magenta, cyan, yellow and black.
The major unit operations in a flexographic printing operation are:
• Image preparation
• Platemaking
• Printing
• Finishing
Below is a process flow diagram for flexographic printing:
Image PreparationImage preparation begins with camera-ready (mechanical) art/copy or electronically produced art supplied by the customer. Images are captured for printing by camera, scanner or computer. Components of the image are manually assembled and positioned in a printing flat when a camera is used. This process is called stripping. When art/copy is scanned or digitally captured the image is assembled by the computer with special software. A simple proof (brown print) is prepared to check for position and accuracy. When color is involved, a color proof is submitted to the customer for approval.
Flexographic Plate Making
Flexographic and letterpress plates are made using the same basic technologies utilizing a relief type plate. Both technologies employ plates with raised images (relief) and only the raised images come in contact with the substrate during printing. Flexographic plates are made of a flexible material, such as plastic, rubber or UV sensitive polymer (photopolymer), so that it can be attached to a roller or cylinder for ink application. There are three primary methods of making flexographic plates; photomechanical, photochemical and laser engraved plates.
Flexographic Printing Presses
The five types of printing presses used for flexographic printing are the stack type, central impression cylinder (CIC), in-line, newspaper unit, and dedicated 4-, 5-, or 6-color unit commercial publication flexographic presses. All five types employ a plate cylinder, a metering cylinder known as the anilox roll that applies ink to the plate, and an ink pan. Some presses use a third roller as a fountain roller and, in some cases, a doctor blade for improved ink distribution.
Flexographic Inks
These inks consist of colorants, which may be pigments and soluble dyes along with a binder and various solvents. Both Solvent based and water based inks commonly contain various types alcohol as the primary solvent or drier. Alcohol rapidly dries through evaporation and contributes to VOC emissions. The inks may also contain glycol ether and/or ammonia which facilitates drying.
Types:
• Water Based
• Solvent Based
• U.V. Curable
Water based flexo inks dry through evaporation and absorption on paper. This evaporation requires a greater amount of fuel or energy to dry the ink. Coated papers may be used to control the absorption through the paper. Due to the speed of the presses and volume of inks consumed daily a pollution control system may be necessary, especially if the printer is using solvent based inks. If the product allows, the printer may avoid pollution control equipment if they convert to water based inks or UV curable inks.
UV flexo inks are commonly used for top coats and lacquers and are responsible for many improvements in image quality of flexographic printing. The use of UV curable colored inks is rising within the flexographic printing industry, but product concerns and equipment investment are obstacles. Note, water based or UV curable inks may not be an option for some printers due to the substrate being printed or design of the product.
Finishing
After printing, the substrate may run through a number of operations to be "finished" and ready for shipment to the customer. Finishing may include operations such as coating, cutting, folding and binding.
Off set Lithography
The function of the original stone printing surface is now served by thin aluminum plates, although other materials, such as stainless steel and plastic, can also be used. The plates are wrapped around the circumference of the printing cylinder and make direct contact with the rubber blanket cylinder. Rubber rollers carry ink and water to the plate surface. The ink is transferred first to the blanket cylinder and then to the paper.
Lithographic plates are the least expensive printing surfaces available today, and this fact has
contributed greatly to the success of the process. Aluminum plate materials have a thin surface coating of light-sensitive material, such as a photopolymer, that undergoes a solubility change when exposed to an intense source of blue and ultraviolet light. Images are transferred to the surface by exposing the plate through a film positive or negative (see Photography). Some materials can be exposed directly, as in a graphic-arts camera or by a computer-controlled laser beam, thereby eliminating the expense of film and speeding up the platemaking process.
Modern offset lithographic presses range in size from small sheet-fed duplicators—used for small, single-color jobs such as brochures and newsletters—to massive web presses capable of printing millions of copies of magazines, catalogs, mailing pieces, and packaging materials in full color. No other process has such a broad range of applications.
Rotogravure printing
Gravure, also called rotogravure, is a high-volume printing process employing an ink transfer mechanism that is fundamentally different from that of relief printing. The printing surface is a polished metal cylinder covered with an array of tiny recesses, or cells (as many as 50,000 per sq in), that constitute the images to be printed. The cylinder, which can be 2.5 m (8 ft) or more in length, is partially immersed in a reservoir of solvent-based fluid ink. As the cylinder rotates, it is bathed in ink. A steel blade called a doctor blade running the entire length of the cylinder wipes the ink from the polished surface, leaving ink only in the cells. The ink is then transferred immediately to a moving web of paper forced against the cylinder under great pressure.
Gravure cylinders are constructed of steel with a thin surface layer of electroplated copper. The copper can be either chemically etched or electronically engraved to form the cells that will transfer ink. Once the cells have been created, the cylinder is electroplated with a thin layer of chromium to produce a hard surface for the doctor blade. Each cell transfers a tiny spot of ink to the paper. The cells can be made to vary in depth from one part of a cylinder to another, causing the darkness of the resulting ink spots to vary also. This enables gravure to print a wide range of gray tones and thus to render excellent reproductions of photographic originals.
Color printing is accomplished by using separate printing cylinders for the cyan, magenta, yellow, and black inks. Each cylinder is housed in a separate printing unit. The web is transported by rollers from unit to unit and can reach speeds of close to 900 m (3000 ft) per minute. After each color is printed, the web passes through a dryer, where the solvent base of the ink is evaporated. The solvent is either reclaimed or burned to produce energy. Some gravure printers have begun to use water-based inks. This trend is likely to continue because of health and environmental threats posed by the use of hydrocarbon-based solvents.
The expense of manufacturing a set of gravure cylinders has restricted its use to long-run jobs (millions of reproductions). Mass-circulation monthly magazines, mail-order catalogs, and packaging are natural markets for the process. Gravure is also used to reproduce a variety of textures and patterns on decorative materials. Computer-controlled electronic engraving machines have reduced the time required to prepare a set of cylinders, but they are still far more expensive than lithographic printing surfaces.
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