Offset Lithography
Offset lithography is a process used for printing on a flat surface, using printing plates. An image is transferred to a printing plate, which can be made of a variety of materials such as metal or paper. The plate is then chemically treated so that only image areas (such as type, colours, shapes and other elements) will accept ink. Water and ink is applied to the plate. Because of the chemical treatment, ink only "sticks" to the image areas, which reject the water. Areas without images reject the ink. The plate is then rolled onto a rubber cylinder applying the inked area, and in turn the rubber cylinder (or "blanket") applies the image to the paper. The system if "offset" because the plate does not come in direct contact with the paper, which preserves the quality of the plate.
Rotogravure
Gravure Printing Process is used for long run printing with sharper, fine and clear images. The general operation involves image preparation, cylinder preparation, printing and finishing. It is a form of intaglio printing. The image is formed on a depressed or sunken surface. The image area consists of honeycomb shaped cells that are etched or engraved into a copper cylinder. As the cylinder rotates in an ink pan (bath of ink), any extra ink is wiped off the cylinder with the help of a steel doctor blade. As the substrate passes between the plate cylinder and the impression cylinder, ink is directly transferred to the substrate and image is formed.
Applications include printing magazines, greeting cards, gift-wraps, labels, flexible packaging, cartons and others.
Rotogravure Printing Machines operate at very high speeds and are used by a number of industries for printing magazines, advertising print materials, cards etc for printing job that are long running in nature and require sharper and finer prints & images on different materials including PET, PVC, PE and paper. These machines employ turret type unwinder and rewinder; automatic splicing and web discharging can also be done. To enhance thedrying performance and minimize printing defects, these machines are equipped with high speed drying systems.
Applications: Greeting Cards, magazines, advertising materials including brochures, catalogs, flexible packaging, self-adhesive labels
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Screen Printing
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. Ink is forced into the mesh openings 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
The image to be printed is created on the printing plate, normally by chemical etching. The plate is generally steel or a photopolymer material. There are other materials and methods of image creation that will be discussed later. Etch depth is approximately 25-30 microns. The thickness of the dried ink film varies between 2 and 20 microns. Compared to screen printing this is a thin film process.
The etching is filled with ink; the action of the silicone rubber printing pad picks up ink from the etching and transfers it to the object to be printed. Filling the etched portion of the plate can be done in various ways, but the mechanism of picking up the ink from the etched portion of the plate and transferring it to the object to be printed is always the same in conventional pad printing machines.
Whilst the pad moves towards the object to be printed the solvent continues to evaporate from the ink on the pad, and the outside surface of the ink becomes tacky. As the pad makes contact with the object and compresses the print surface of the pad rolls across the object and the tacky surface of the ink attaches itself to the surface of the object. The pad then lifts and the printing surface of the pad rolls away from the surface of the object and releases the ink leaving it laying on the surface. Whilst this is happening the etched portion of the plate is recharged with ink, and the pad returns to pick up another image from the plate.
The rolling motion of the pad caused by its shape is important as it squeezes out the air over the plate and the ink surface. The pad surface must come in direct contact with the surface of the ink in order to pick it up and so carry it to the component.
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Rotary Printing
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Flexographic Printing
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.
Digital Printing
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.
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Web Offset Print
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.
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