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Printing Methods

Welcome to the Printing Methods section of the Zoo Print School. This section serves to inform you of the multitude of different printing processes that may be employed to print your printed product.

The types of printing:

  • Digital Printing
  • Flexography
  • Letterpress Printing
  • Offset Printing
  • Rotogravure
  • Screen Printing

Digital Printing

Digital printing is the reproduction of digital images on physical surface, such as common or photographic paper, film, cloth, plastic, etc. It can be differentiated from litho printing in many ways, some of which are;

  • Every impression made onto the paper can be different, as opposed to making several hundred or thousand impressions of the same thing from one set of plates, as in traditional methods.
  • The Ink or Toner does not absorb into the paper, as does conventional Ink, but forms a layer on the surface.
  • It generally requires less waste in terms of chemicals used and paper wasted in set up.

Printing at home or in an office or engineering environment is subdivided into:

  • small format (up to ledger size paper sheets), as used in business offices and libraries
  • wide format (up to 3' or 914mm wide rolls of paper), as used in drafting and design establishments.

Some of the more common printing technologies are

  • line printing — where pre-formed characters are applied to the paper by lines
  • daisy wheel — where pre-formed characters are applied individually
  • dot-matrix — which produces arbitrary patterns of dots with an array of printing studs
  • heat transfer — like early fax machines or modern receipt printers that apply heat to special paper, which turns black to form the printed image
  • blueprint — and related chemical technologies
  • inkjet — including bubble-jet — where ink is sprayed onto the paper to create the desired image
  • laser — where toner consisting primarily of polymer with pigment of the desired colours is melted and applied directly to the paper to create the desired image.

Vendors typically stress the total cost to operate the equipment, involving complex calculations that include all cost factors involved in the operation as well as the capital equipment costs, amortization, etc. For the most part, toner systems beat inkjet in the long run, whereas inkjets are less expensive in the initial purchase price.

Flexography

Flexography, often abbreviated to flexo, is a method of printing most commonly used for packaging.

A flexo print is achieved by creating a mirrored master of the required image as a 3D relief in a rubber or polymer material. A measured amount of ink is deposited upon the surface of the printing plate (or printing cylinder) using an anilox roll. The print surface then rotates, contacting the print material which transfers the ink.

Flexo is so named because it was originally used as a method of printing onto corrugated cardboard, which has a very uneven surface. It is required that the printing plate surface maintain contact with the cardboard, which it does by being flexible. Also, unprinted high points on the cardboard must not get printed by ink remnants not on the plate surface, which is achieved by ensuring a sufficient depth for the non-print areas of the plate.

Originally flexo printing was very low quality. In the last 3 decades great advances have been made, including improvements to the plate material and the method of plate creation—usually photographic exposure followed by chemical etch, though also by direct laser engraving. Laser-etched anilox rolls also play a part in the improvement of print quality. Full colour picture printing now occurs, and some of the finer presses available today in combination with a skilled operator allow quality that rivals the lithographic process. One ongoing improvement has been the increasing ability to reproduce highlight tonal values, thereby providing a workaround for the very high dot gain associated with flexo print.

Flexo has an advantage over lithography in that it can use a wider range of inks and is good at printing on a variety of different materials. Flexo inks, like those used in gravure and unlike those used in lithography generally have low viscosity. This enables faster drying and, as a result, faster production; that means low cost. Printing press speeds of 450 meters per minute are regular with modern technology high end printers, like Windmoeller und Hollscher or Schiavi type. The main printing process worldwide for flexible packaging are rotogravure, for very large runs, and flexo for large and medium runs.

Flexography_Diagram

Also Known As:

  • Flexographic printing
  • Flexo
  • Surface printing

Examples:

Some typical applications for flexography are paper and plastic bags, milk cartons, disposable cups, and candy bar wrappers. Flexography printing may also be used for envelopes, labels, and newspapers.

Letterpress Printing

Letterpress printing is a term for printing text with movable type, in which the raised surface of the type is inked and then pressed against a smooth substance to obtain an image in reverse. In addition to the direct impression of inked movable type onto paper or another receptive surface, the term letterpress can also refer to the direct impression of inked media such as zinc "cuts" (plates) or linoleum blocks onto a receptive surface.

Early Chinese woodblock printing used characters or images carved in relief from before 750AD, and this form of printing was widespread throughout Eurasia as a means of printing patterns on textiles. Printing of images, first on cloth, then from about 1400 on paper was practised in Europe. In the 1400s, Johann Gutenberg (among others) is credited with the invention of movable type printing from individually-cast, reusable letters set together in a forme. This had previously been invented in Asia, but the two inventions were probably not connected. He also invented a wooden printing-press where the type surface was inked and paper laid carefully on top by hand, then slid under a padded surface and pressure applied from above by a large threaded screw. Later metal presses used a knuckle and lever arrangement instead of the screw, but the principle was the same.

With the advent of industrial mechanisation, the inking was carried out by rollers which would pass over the face of the type and move out of the way onto a separate ink-bed where they would pick up a fresh film of ink for the following sheet. Meanwhile a sheet of paper was slid against a hinged platen (see image) which was then rapidly pressed onto the type and swung back again to have the sheet removed and the next sheet inserted (during which operation the now freshly-inked rollers would run over the type again). In a fully-automated 20th century press, the paper was fed and removed by vacuum sucker grips.

Rotary presses were used for high-speed work. In the oscillating press, the forme slid under a drum around which each sheet of paper got wrapped for the impression, sliding back under the inking rollers while the paper was removed and a new sheet inserted. In a newspaper press, a papier-mâché mixture (flong) was used to make a mould of the entire forme of type, then dried and bent, and a curved metal plate cast against it. The plates were clipped to a rotating drum, and could thus print against a continuous reel of paper at the enormously high speeds required for overnight newspaper production.

As computerised typesetting and imaging replaced cast metal types, letterpress began to die out, as high-speed photographic imaging onto smooth flexible plates (lithography) became more economical. However, photopolymer plates and the invention of Ultra-Violet curing inks has helped keep rotary letterpress alive in areas like self-adhesive labels. There is also still a large amount of flexographic printing, a similar process, which uses rubber plates to print on curved or awkward surfaces, and a lesser amount of relief printing from huge wooden letters for lower-quality poster work.

Letterpress_Diagram

Also Known As:

  • Offset Letterpress
  • Relief Printing

Examples:

Mostly replaced by offset printing and other processes, letterpress printing is still used for some newspapers, books, and limited edition prints. Letterpress printing may also be used for printing business cards, letterhead, posters, and some forms.

Offset Printing

Offset printing is a widely used printing technique where the inked image is transferred (or "offset") from a plate to a rubber blanket, then to the printing surface. When used in combination with the lithographic process, which is based on the repulsion of oil and water, the offset technique employs a flat (planographic) image carrier on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a film of water, keeping the non-printing areas ink-free.

Advantages of offset printing include:

  • Consistent high image quality. Offset printing produces sharper and cleaner images and type than letterpress printing because the rubber blanket conforms to the texture of the printing surface.
  • Quick and easy production of printing plates.
  • Longer printing plate life than on direct litho presses because there is no direct contact between the plate and the printing surface.

Offset_Diagram

Also Known As:

  • Lithographic
  • Offset
  • Planographic

Examples:

Offset lithography is the most commonly used commercial printing process for the bulk of desktop publishing on paper. Offset lithography is used on both sheet-fed and web offset presses.

Rotogravure Printing

Rotogravure is a type of intaglio printing process, in that it involves engraving the image onto an image carrier. In gravure printing, the image is engraved onto a copper cylinder because, like offset and flexography, it uses a rotary printing press. The vast majority of gravure presses print on reels of paper, rather than sheets of paper. (Sheetfed gravure is a small, specialty market.) Rotary gravure presses are the fastest and widest presses in operation, printing everything from narrow labels to 12-feet-wide rolls of vinyl flooring. Additional operations may be in-line with a gravure press, such as saddle stitching facilities for magazine/brochure work.

In 1932 a George Gallup "Survey of Reader Interest in Various Sections of Sunday Newspapers to Determine the Relative Value of Rotogravure as an Advertising Medium" found that rotogravures were the most widely read sections of the paper and that advertisements there were three times more likely to be seen by readers than in any other section. The rotogravure process is still used for commercial printing of magazines, postcards, and corrugated (cardboard) product packaging.

In the latter quarter of the 19th centure, the method of image photo transfer onto carbon tissue covered with light-sensitive gelatin was discovered and was the beginning of rotogravure.

Gravure cylinders nowadays are typically engraved digitally by a diamond tipped or laser etching machine. On the gravure cylinder, the engraved image is composed of small recessed cells (or 'dots') that act as tiny wells. Their depth and size control the amount of ink that gets transferred to the substrate (paper or other material, such as plastic or foil) via a process of pressure, osmosis, and electrostatic pull. (A patented process called "Electrostatic Assist" is sometimes used to enhance ink transfer.)

A rotogravure printing press has one printing unit for each color, typically CMYK or cyan, magenta, yellow and key (printing terminology for black). The number of units vary depending on what colors are required to produce the final image. There are five basic components in each color unit: an engraved cylinder (whose circumference can change according to the layout of the job), an ink fountain, a doctor blade, an impression roller, and a dryer. While the press is in operation, the engraved cylinder is partially immersed in the ink fountain, filling the recessed cells. As the cylinder rotates, it draws ink out of the fountain with it. Acting as a squeegee, the doctor blade scrapes the cylinder before it makes contact with the paper, removing ink from the non-printing (non-recessed) areas. Next, the paper gets sandwiched between the impression roller and the gravure cylinder. This is where the ink gets transferred from the recessed cells to the paper. The purpose of the impression roller is to apply force, pressing the paper onto the gravure cylinder, ensuring even and maximum coverage of the ink. Then the paper goes through a dryer because it must be completely dry before going through the next color unit and absorbing another coat of ink.

Because gravure is capable of transferring more ink to the paper than other printing processes, gravure is noted for its remarkable density range (light to shadow) and hence is a process of choice for fine art and photography reproduction, though not typically as clean an image as that of sheet fed litho or web offset litho. Gravure is widely used for long-run magazine printing in excess of 1 million copies. Gravure's major quality shortcoming is that all images, including type and "solids," are actually printed as dots, and the screen pattern of these dots is readily visible to the naked eye. Examples of gravure work in the United States are typically long-run magazines, mail order catalogs, consumer packaging, and Sunday newspaper ad inserts.

Other application area of gravure printing is in the flexible packaging sector. A wide range of substrates such as Polyethylene, Polypropylene, Polyester, BOPP etc can be printed in the gravure press.

Rotogravure_Diagram

Also Known As:

  • Gravure

Examples:

Gravure printing is often used for high-volume printing of packaging, wallpaper, and giftwrap using fast-drying inks. Although less common, gravure printing may also be used for printing magazines, greeting cards, and high-volume advertising pieces.

Screen Printing

Screen printing is a printmaking technique that creates a sharp-edged image using a stencil. A screen print or serigraph is an image created using this technique.

A screen is made of a piece of porous, finely woven fabric (originally silk, but typically made of polyester or nylon since the 1940s) stretched over an aluminum frame. Areas of the screen are blocked off with a non-permeable material—a stencil—which is a positive of the image to be printed; that is, the open spaces are where the ink will appear.

The screen is placed on top of a piece of dry paper or fabric. Ink is placed on top of the screen, and a squeegee (rubber blade) is used to push the ink evenly into the screen openings and onto the substrate. The ink passes through the open spaces in the screen onto the paper or fabric below; the screen is lifted away and then the squeegee is pushed back across the screen, with the screen lifted, "flooding" the ink into the screen. The screen can be re-used after cleaning. If more than one color is being printed on the same surface, the ink is allowed to dry and then the process is repeated with another screen and different color of ink.

While the public thinks of garments in conjunction with screen printing, the technique is used on tens of thousands of items, including birthday cake designs, decals, clock and watch faces, and many more products. The vast majority of silk-screen printings are monochromatic.

Graphic screen printing is widely used today to create many mass or large batch produced graphics, such as posters or display stands. Full color prints can be created by printing in CMYK (cyan, magenta, yellow and black). Screen printing is often preferred over other processes such as dye sublimation or inkjet printing be

Also Known As:

  • Silkscreening
  • Serigraphy

Examples:

Screen printing can be used to print images on T-shirts, hats, CDs, DVDs, ceramics, glass, polyethylene, polypropylene, paper, metals, and wood.