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Offset printing

                                                                                       

   Offset printing is a commonly used printing technique in which 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. Ink rollers transfer ink to the image areas of the image carrier, while a water roller applies a water-based film to the non-image areas.

Sheetfed offset printing

    Sheetfed offset printing presses are the most cost-effective, particularly for large runs. It’s also ideal when you need large quantities and/or flexible colour, paper and sizing options.

speedmaster CD 74

With sheetfed offset printing, sheets of paper are fed into the printing press to be printed on. Some sheetfed press contain what’s called a “perfector cylinder” and this enables the pressman to flip the sheet of paper so both sides can be printed on in one pass.
  Sheetfed presses can print from 4,000 to 18,000 sheets per hour. The printed products it produces are enormous and cover most items printed, with the exception of certain types of packaging and long runs of magazine publications.
Presses can be grouped into three size ranges. The first is described as “small offset” and prints on paper up to 14-x-17 inches. Many instant printers and small local printers use these presses, and the products produced consist of business cards, stationery, flyers, and short-run business forms.
The second category includes presses that print on paper up to 25-x-38 inches. Most print two or more colors and are used by medium to large commercial printers for printing brochures and medium runs (5,000-20,000) of general color work.

sheet-fed press - process

The last category is the largest sheetfed press and is capable of printing on paper up to 49-x-74 inches or larger. These presses are often used for long runs (up to 100,000) of color work.
Sheetfed offset printing is capable of producing high-quality printed products on a range of light and heavy-weight stocks. Color variation is a common problem and is often caused by fluctuations in the ink and water balance. Sheet-to-sheet registration and overall image quality is usually superior to web offset printing. Modern sheetfed presses have sophisticated electronic controls for adjusting color and register. This technology will often shorten set-up time (makeready) and reduce printed waste, and, in a competitive market, the result is an economical product.

Web-fed offset

Web-fed refers to the use of rolls (or "webs") of paper supplied to the printing press. Offset web printing is generally used for runs in excess of five or ten thousand impressions. Typical examples of web printing include newspapers, newspaper inserts or ads, magazines, direct mail, catalogs, and books. Web-fed presses are divided into two general classes: coldset (or non-heatset) and heatset offset web presses, the difference being how the inks dry. Cold web offset printing dries through absorption into the paper, while heatset utilizes drying lamps or heaters to cure or "set" the inks. Heatset presses can print on both coated (slick) and uncoated papers, while coldset presses are restricted to uncoated paper stock, such as newsprint. Some coldset web presses can be fitted with heat dryers, or ultraviolet lamps (for use with UV-curing inks), thus enabling a newspaper press to print color pages heatset and black & white pages coldset.
Web offset presses are beneficial in long run printing jobs, typically press runs that exceed ten or twenty thousand impressions. Speed is a determining factor when considering the completion time for press production; some web presses print at speeds of 3,000 feet (915 meters) per minute or faster. In addition to the benefits of speed and quick completion, some web presses have the inline ability to cut, perforate, and fold.

Heatset web offset

This subset of web offset printing uses inks which dry by evaporation in a dryer typically positioned just after the printing units; it is typically done on coated papers, where the ink stays largely on the surface, and gives a glossy high contrast print image after the drying. As the paper leaves the dryer too hot for the folding and cutting that are typically downstream procedures, a set of "chill rolls" positioned after the dryer lowers the paper temperature and sets the ink. The speed at which the ink dries is a function of dryer temperature and length of time the paper is exposed to this temperature. This type of printing is typically used for magazines, catalogs, inserts, and other medium-to-high volume, medium-to-high quality production runs.

Coldset web offset

This is also a subset of web offset printing, typically used for lower quality print output. It is typical of newspaper production. In this process, the ink dries by absorption into the underlying paper. A typical coldset configuration is often a series of vertically arranged print units and peripherals. As newspapers seek new markets, which often imply higher quality (more gloss, more contrast), they may add a heatset tower (with a dryer) or use UV (ultraviolet) based inks which "cure" on the surface by polymerisation rather than by evaporation or absorption.