Flexographic Press Working Process
1. Plate Making
The first step of plate development uses light sensitive polymer. A film negative is placed over the plate, which is exposed to ultra-violet light. The polymer hardens where light passes through the film. The remaining polymer has the consistency of chewed gum. It is washed away in a tank of either water or solvent. Brushes scrub the plate to facilitate the “washout” process. The process can differ depending on whether solid sheets of photopolymer or liquid photopolymer are used, but the principle is still same. The second method used a computer guided laser to etche the image onto the printing plate. The third method is to go through a molding process. The first step is to create a metal plate out of the negative of our initial image through an exposition process (followed by an acid bath). This metal plate is relief is then used in the second step to create the mold that could be in bakelite board or even glass or plastic, through a first molding process. Once cooled, this master mold will press the rubber or plastic compound which is under both controlled temperature and pressure, through a second molding process to create the printing plate.
2. Printing
A flexographic print is made by creating a positive mirrored master of the required image as a 3D relief in a rubber or polymer material. Flexographic plates can be created with analog and digital plate making processes. The image areas are raised above the non image areas on the rubber or polymer plate. The ink is transferred from the ink roll which is partially immerged in the ink tank. Then it transfers to the anilox roll or meter roll whose texture holds a specific amount of ink since it’s covered with thousands of small wells or cups that enable it to meter ink to the printing plate in a uniformed thickness evenly and quickly, the number of cells per linear inch can vary according to the type of printing job and the quality required. To avoid getting a final product with a smudgy or lumpy look, it must be ensued that the amount of ink on the printing plate is not excessive. This is achived by using a scraper, called a doctor blade. The doctor blade removes the excess ink from ailox roller before inking the printing plate. The substrate is finally sandwiched between the plate and the impression cylinder to transfer the image.
3. Flexographic Printing Inks
The nature and demands of printing process and the application of the printed product determine the fundamental properties required of flexographic inks. Measuring the physical properties of inks and understanding how these are affected by the choice of ingredients is a large part of ink technology. Formulation of inks requires a detailed knowledge of the physical and chemical properties of the raw materials composing the inks, and how these ingredients affect or react with each other as asll as with the environment. Flexographic printing inks are primarily formulated to remain compatible with the wide variety of substrates used in the process. Each formulation component individually fulfuls a special function and the proportion and composition will vary according to the substrate.
Normally there are 4 types of inks that can be used in flexographic press: solvent-based inks, water-based inks, EB (Electron Beam) curing inks and UV ultraviolet curing inks.
4. Applications
Flexographic has an advantage over lithography in that it can use a wider range of inks, water based rather than pil based inks and is good at printing on a variety of different materials like plastic, foil, acetate film, brown paper, and other materials used in packaging. Typical products printed using flexographic include brown corrugated boxes, flexoible packaging including retail and shopping bags, food and hygiene bags and sacks, milk and beverage cartons, flexiable plastics, self adhesive labels, disposable cups and containers, envelopes and wallpaper. A number of newspapers now eschew the more common offset lithography process in favour of flexo. Flexographic inks, like those used in gravure and unlike those used in lithography, generally have a low viscosity. This enables faster drying and, as a result, faster production, which results in lower costs.
Print Series Team
14 March, 2012
