Offset lithography is a printing technique which is widely used around the world. Most books, newspapers, and magazines are printed using offset lithography, and this printing technique is widely regarded as the workhorse of printing, because it is fast, efficient, cheap, and relatively easy. The “offset” in the name refers to the fact that the ink is transferred to a separate surface before being applied to the paper. The first step in offset lithography is making a plate with the image to be printed. If the image is in black and white, only a single plate is required, because the plate can simply be inked with black ink. Color images are produced using a four-color separation process, in which four different plates are made for the cyan, magenta, yellow, and key (black) inks; when the plates are printed, the colors blend together visually, creating a color image. Plates in offset lithography are entirely flat, in contrast with the textured surfaces of engraved plates and movable type. They are made by creating a film negative of the image, placing it over a photo-sensitive plate, exposing it, and then developing it. Once the plate is made, it can be mounted in a press, and the real fun begins.
Advantages:
- consistently high-image quality.
- much quicker and much easier production of printing plates.
- longer life of printing plates because they only come in contact with the printing blanket, which is much softer and less abrasive than paper.
- offset lithography gives most inexpensive production on excellent quality images in big quantities.
Dis-advantages:
- expensive cost for small quantities.
- the image quality printed with offset lithography is excellent for commercial purpose however not as good as rotogravure or photogravure printing.
A rotary printing press is a printing press in which the images to be printed are curved around a cylinder. Printing can be done on large number of substrates, including paper, cardboard, and plastic. Substrates can be sheet feed or unwound on a continuous roll through the press to be printed and further modified if required (e.g. die cut, overprint varnished, embossed). Printing presses that use continuous rolls are sometimes referred to as "web presses". Today, there are three main types of rotary presses; offset including web offset, rotogravure, and flexo (short for flexography). While the three types use cylinders to print, they vary in their method.
Web offset is a form of offset printing in which a continuous roll of paper is fed through the printing press. Pages are separated and cut to size after they have been printed. Web offset printing is used for high-volume publications such as mass-market books, magazines, newspapers, catalogs and brochures. There are two methods of web offset printing, known as heatset and coldset (or non-heatset). In the heatset process, the ink is dried rapidly by forced-air heating. In the non-heatset or coldset process, the ink dries more slowly by ordinary evaporation and absorption. Web offset printing differs from sheet-fed offset printing , in which individual pages of paper are fed into the machine. Sheet-fed offset printing is popular for small and medium-sized fixed jobs such as limited-edition books.
Rotogravure (Roto or Gravure for short) is a type of intaglio printing process; that is, it involves engraving the image onto an image carrier. In gravure printing, the image is engraved onto a cylinder because, like offset printing and flexography, it uses a rotary printing press. Once a staple of newspaper photo features, the rotogravure process is still used for commercial printing of magazines, postcards, and corrugated (cardboard) product packaging.
The rotogravure process is a direct transfer method for printing onto wood-pulp fiber based, synthetic, or laminated substrates, including:
- Films such as polyester, OPP, nylon, and PE
- Papers
- Carton board
- Aluminum foil
The modern day rotogravure printing press uses a printing cylinder which has been laser engraved with minute cells capable of retaining ink, the size and pattern of which reflect the required image. These cells are forced to transfer their ink directly onto the substrate by a combination of pressure and capillary action, so producing the printed image.
The process, also commonly called gravure printing, is used in the manufacturing of food and non-food packaging, as well as labels, wall coverings, transfer printing, and has a variety of further applications in the security printing, industrial, and tobacco segments of industry.
The layout of a gravure printing press follows an in-line arrangement where the required number of printing units is installed along a horizontal plane. In a conventional gravure printing press, each unit comprises of:
- Printing cylinder: a seamless tubular sleeve or full cylinder, made from either steel, aluminum, plastic, or composite material, which is engraved with the image to be printed
- Doctor blade: the device that removes ink from the non-engraved portions of the printing cylinder and also removes excess ink from the engraved sections
- Impression roller: a rubber covered sleeve that is mounted on a steel mandrel. Its primary purpose is to press the substrate against the printing cylinder
- Inking system: consisting of an ink pan, ink holding tank, and ink pump with supply and return ink pipes
- Drying system: consisting of a chamber which dries the ink once it is on the substrate and prior to it reaching the next printing unit. Drier capacities are determined based on the required printing speed, ink type (solvent or water based), and ink lay down volume
During the gravure printing process the printing cylinder rotates in the ink pan where the engraved cells fill with ink. As the cylinder rotates clear of the ink pan, any excess ink is removed by the doctor blade. Further around, the cylinder is brought into contact with the substrate, which is pressed against it by the rubber covered impression roller.
The pressure of the roller, along with the capillary draw of the substrate, results in the direct transfer of ink from the cells in the printing cylinder to the surface of the substrate. As the printing roller rotates back into the ink pan, the printed area of the substrate proceeds through a dryer and onto the next printing unit, which is normally a different color or may be a varnish or coating.
Precise color to color registration is made possible via automatic side and length register control systems.
For a web-fed printing press, after each color has been printed and any coatings applied, the web is 'rewound' into a finished roll.
The process offers the ability to transfer ink consistently, across a wide range of densities, and at high speeds, making it suitable for applications which require high image quality, such as publishing, packaging, labels, security print, and decorative printing.
The durable nature of the printing cylinders used makes gravure printing an ideal process for providing high quality print on very long or regularly repeating runs, delivering cost advantages over other processes.
Flexography
flexography, a form of rotary printing in which ink is applied to various surfaces by means of flexible rubber (or other elastomeric) printing plates. The inks used in flexography dry quickly by evaporation and are safe for use on wrappers that come directly in contact with foods. In flexography, the desired imagery or lettering is engraved in the form of tiny indentations, or cells, onto a flexible rubber plate by means of plastic-molding techniques. Liquid ink is flooded onto a rotating ink-metering roller while a blade inclined at a reverse angle to the direction of rotation shaves any surplus ink from the ink-metering roller. The remaining ink is rolled onto the rubber printing plate, which is affixed to a rotary letterpress cylinder, and the plate’s tiny indentations receive and hold the ink. The inked plate then transfers the image or type to paper (or some other material) that is held on an impression cylinder.
Digital printing refers to methods of printing from a digital based image directly to a variety of media. It usually refers to professional printing where small run jobs from desktop publishing and other digital sources are printed using large format and/or high volume laser or inkjet printers. Digital printing has a higher cost per page than more traditional offset printing methods but this price is usually offset by the cost saving in avoiding all the technical steps in between needed to make printing plates. It also allows for on demand printing, short turn around, and even a modification of the image (variable data) with each impression. The savings in labor and ever increasing capability of digital presses means digital printing is reaching a point where it could match or supersede offset printing technology's ability to produce larger print runs of several thousand sheets at a low price. The main differences between digital printing and traditional methods such as lithography, flexography, gravure, or letterpress are that no need to replace printing plates in digital whereas in analog printing plates are continuously replaced, resulting in a quicker and less expensive turn around time, and typically a loss of some fine-image detail by most commercial digital printing processes. The most popular methods include inkjet or laser printers that deposit pigment or toner onto a wide variety of substrates including paper, photo paper, canvas, glass, metal, marble and other substances.
Screen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink or other printable materials which can be pressed through the mesh as a sharp-edged image onto a substrate. A fill blade or squeegee is moved across the screen stencil, forcing or pumping ink into the mesh openings for transfer by capillary action during the squeegee stroke. Screen printing is also a stencil method of print making in which a design is imposed on a screen of polyester or other fine mesh, with blank areas coated with an impermeable substance, and ink is forced into the mesh openings of the mesh by the fill blade or squeegee and onto the printing surface during the squeegee stroke. It is also known as silkscreen, serigraphy, and serigraph printing. A number of screens can be used to produce a multicoloured image.
Pad printing is a printing process that can transfer a 2-D image onto a 3-D object. This is accomplished using an indirect offset (gravure) printing process that involves an image being transferred from the cliché via a silicone pad onto a substrate. Pad printing is used for printing on otherwise impossible products in many industries including medical, automotive, promotional, apparel, and electronic objects, as well as appliances, sports equipment and toys. It can also be used to deposit functional materials such as conductive inks, adhesives, dyes and lubricants. Physical changes within the ink film both on the cliché and on the pad allow it to leave the etched image area in favor of adhering to the pad, and to subsequently release from the pad in favor of adhering to the substrate. The unique properties of the silicone pad enable it to pick the image up from a flat plane and transfer it to a variety of surfaces, such as flat, cylindrical, spherical, compound angles, textures, concave, or convex surfaces.
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