Flexography is the majoy process used to print packaging material. It is used to print corrugated containers, folding cartons, multiwall sacks, paper sacks, plastic bags, milk and beverage cartons, disposable cups and other containers, lables, adhesive tapes, envelopes, newspapers, and wrappers. 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. Flexography is a form of rotary web letterpress, combining features of both letterpress and rotogravure printing, using relief plates comprised of flexible rubber or photopolymer plates and fast drying, low viscosity solvent, water-based or UV curable inks fed from an “anilox” or two roller inking system. The flexible (rubber or photopolymer) plates are mounted onto the printing cylinder with double-faced adhesive. Plates are sometimes backed with thin metal sheets and attached to the cylinder with fastening straps for close register or ink alignment. This adds additional cost to the plate and requires more makeready time, but when quality printing is critical this type of plate can make the difference. The process of printing each color on a flexo press consists of a series of four rollers:
- Ink Roller
- Meter Roller
- Plate Cylinder
- Impression Cylinder
The first roller transfers the ink from an ink pan to the meter roller or Anilox Roll, which is the second roller. The Anilox roller meters the ink to a uniform thickness onto the plate cylinder. The substrate then moves between the plate cylinder and the impression cylinder, which is the fourth roller. The impression cylinder applies pressure to the plate cylinder, thereby transferring the image onto the substrate. The web, which by now has been printed, is fed into the overhead dryer so the ink is dry before it goes to the next print unit. After the substrate has been printed with all colors the web MAY be fed through an additional overhead tunnel dryer to remove most of the residual solvents or water. The finished product is then rewound onto a roll or is fed through the cutter.
The stack press is characterized by one or more stacks of printing stations arranged vertically on either side of the press frame. Each stack has its own plate cylinder which prints one color of a multicolor impression. All stations are driven from a common gear train. Stack presses are easy to set up and can print both sides of the web in one pass. They can be integrated with winders, unwinders, cutters, creasers, and coating equipment. They are very popular for milk carton printing. A drawback of stack presses is their poor registration; the image position on every printed sheet is not as consistent as in many other printing processes.
Central impression cylinder (CIC)
Central impression cylinder (CIC), like the common impression rotary letterpress, use a single impression cylinder mounted in the press frame. Two to eight color printing stations surround the central impression cylinder. Each station consists of an ink pan, fountain roller, anilox roll, doctor blade, and plate cylinder. As the web enters the press it comes into contact with the impression cylinder and remains in contact until it leaves the press. The result is precise registration which allows CIC presses to produce very good color impressions. CIC presses are used extensively for printing flexible films.
In Line flexo printing is similar to a unit type rotary press or the stacked press except the printing stations are arranged in a horizontal line. They are all driven by a common line shaft and may be coupled to folders, cutters, and other postpress equipment. These presses are used for printing bags, corrugated board, folding boxes, and similar products.
Newspaper Flexographic Presses
A newspaper flexographic press consists of multiple printing units, each unit consisting of two printing stations arranged back-to-back in a common frame. The use of paired stations allows both sides of the web to be printed in one pass. Multiple printing stations are required to print the many pages that make up a typical newspaper. Single and double color decks, stacked units, or 4-, 5-, or 6-color units are sometimes positioned above those units where the publisher wants to provide single or multiple spot color, spot color for both sides of the web, or process color, respectively (Buonicore).
Commercial Publication Flexographic Presses
Commercial publication flexographic presses are compact high-speed presses with wide web capability that utilize dedicated 4-, 5-, or 6-color units. Typically, two four-color units are paired in one press to allow printing on both sides of the web. Publication flexographic presses generally incorporate infrared dryers to ensure drying of the waterborne ink after each side of the web is printed (Buonicore). The photomechanical plate making method begins with making an engraving. This is accomplished by exposing a metal plate through a negative and processing the exposed plate in an acid bath. The metal engraved plate is used to make a master which is molded out of bakelite board. The engraving is placed in a mold press. The mold is produced by applying heat & pressure to the mold material (bakelite board), which can be either plastic or glass, against the engraving under controlled temperature and pressure. The bakelite board fills the engraving on the metal plate. When its cooled you end up with a master mold for the plastic or rubber compound that will be pressed into the mold under pressure and elevated temperature to produce the flexible printing plate with raised areas that will transfer the ink. The second method of flexo plate making is relief plates. This utilizes a solid or liquid photopolymer. The sheet of photopolymer is exposed to light through a negative. The unexposed areas are then washed away with solvent or water wash. This is fast becoming the most common method of making plates. The process differs depending on whether solid sheets of photopolymer or liquid photopolymer are used, though the two processes are similar in general outline. In both processes the plates are made in ultraviolet exposure units. A negative of the job is placed between the photopolymer and the ultraviolet light source. The photopolymer sheet or liquid is then exposed to ultraviolet light, hardening the image area. Lastly, the plate is processed to remove the unhardened non-image area. Photopolymer plates are replacing rubber plates because they offer superior quality and performance at a lower cost. Flexographic printing plates may be made by laser engraving, which is called direct digital platemaking. In this process an image is scanned or computer generated. Then a computer-guided laser etches the image onto the printing plate.
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. The cost of Pollution control equipment for a small Flexo or gravure printer will cost approximately $400,000 (1998 estimate) for the equipment and approximately $50,000 for testing and certification. The price increases as the size and/or volume of the operation increases. 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.