Printing Secrets from Print Acumen

Lithographic Printing

For fine illustration work there is no better printing process

Sometimes referred to as offsite or litho printing, Lithographic printing is well suited for printing text, solid colour and fine illustration work.

Typical products printed utilising the offset printing process include:

• General business stationery
• Books  & magazines
• Business Forms Financial and Legal Documents
• Sales & marketing brochures and folders

Lithography is an "offset" printing technique. Ink is not applied directly from the printing plate (or cylinder) to the stock as it is in gravure, flexography and letterpress. Ink is applied to the printing plate to form the "image" (that is, what is to be printed) and then transferred to a rubber blanket. The image on the blanket is then transferred to the stock (typically paper or boards) to produce the printed product.

On sheet-fed presses, the stock is fed into the press one sheet at a time at a very high speed. Web fed presses print on a continuous roll of stock, or web, which is later cut to size. There is a total of 3 types of offset printing: non-heatset sheetfed, heatset, and non-heatset web offset. The difference between heatset and non-heatset is primarily dependent on the type of ink and how it is dried.

Offset Lithographic Printing Process

All offset presses have three printing cylinders, as well as the inking and dampening systems. The plate cylinder, the blanket cylinder and the impression cylinder.Lithography uses a planographic plate, a type of plate on which the image areas are neither raised nor indented (depressed) in relation to the non-image areas. Instead the image and non-image areas, both on essentially the same plane of the printing plate, are defined by deferring physiochemical properties.

Lithography is based on the principal that oil and water do not mix (hydrophilic and hydrophobic process). Lithographic plates undergo chemical treatment that render the image area of the plate oleophilic (oil-loving) and therefore ink-receptive and the non-image area hydrophilic (water-loving).

During printing, fountain (dampening) solution, which consists primarily of water with small quantities of isopropyl alcohol and other additives to lower surface tension and control pH, is first applied in a thin layer to the printing plate and migrates to the hydrophilic non-image areas of the printing plate. Ink is then applied to the plate and migrates to the oleophilic image areas. Since the ink and water essentially do not mix, the fountain solution prevents ink from migrating to the non-image areas of the plate.

As the plate cylinder rotates, the plate comes in contact with the dampening rollers first. The dampening rollers wet the plate so the non-printing areas repel ink. Then the inking rollers transfer ink to the dampened plate, where ink only adheres to the image areas. The inked image is transferred to the rubber blanket, and the stock is printed as it passes between the blanket and impression cylinder.

There are three basic lithographic press designs: unit-design, common impression cylinder design, and blanket-to-blanket design. The unit-design press is a self-contained printing station consisting of a plate cylinder, a blanket cylinder, and an impression cylinder.

Two or more stations may be joined to perform multi-colour printing. A common impression cylinder press consists of two or more sets of plate and blanket cylinders sharing a common impression cylinder. This allows two or more colours to be printed at a single station. A blanket-to-blanket press consists of two sets of plate and blanket cylinders without an impression cylinder. The paper is printed on both sides simultaneously as it passes between the two blanket cylinders (Field).

The major unit operations in a lithographic printing operation include:

• Image preparation Processing printing plates Printing Finishing
• Image Preparation of Lithographic Printing Plates

Image preparation begins with camera-ready (mechanical) art/copy or electronically produced art supplied by the customer. Images are captured for printing by camera, scanner or computer. Components of the image are manually assembled and positioned in a printing flat when a camera is used. This process is called stripping. When art/copy is scanned or digitally captured the image is assembled by the computer with special software.

A simple proof is prepared to check for position and accuracy. When colour is involved, a colour proof is submitted to the customer for approval.

Processing of Lithographic Printing Plates

There are a number of different types of plates used in lithographic printing but the most common type used by the vast majority of lithographic printers is the "presensitised" plate.

Offset Lithographic Plate Making

Silver halide plates use photosensitive coatings similar to photographic film, except that the silver halide emulsions are slower and for colour reproduction are coated on anodized aluminum. The processing solutions contain silver which must be recovered with the proper equipment before being discharged to the sewer. Silver halide plates are used when using film and metal based silver halide plates are used in computer-to-plate systems.

Offset Lithographic Inks:

• Petroleum Based
• Vegetable Oil Based
• UV & EB Curable
•  Heatset

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