Printing Processes

"Print - It’s everywhere! It’s on your coffee table; in your freezer; on the bumper of your car. It can be found on your walls, your doorstep, and on your clothes. It’s on your mail, in your wallet, and most often in your hands." The commercial printing industry is one of the largest industries in the United States. According to 2009 data from the Printing Industry of America, the printing industry employs 909,179 people among 33,565 establishments with annual sales totaling over $140.7 billion in annual shipments.

While the industry accounts for a significant portion of the nations' total volume of goods and services, it also represents the largest conglomeration of small businesses in the domestic manufacturing sector. Seventy nine percent of the plants in the industry employ 19 people or less (PIA 1999 Report to Congress). Most firms in the industry serve local or regional markets, though some printers and many publishers reach national and international markets (USIO 1992).

The industry is dominated by five separate and distinct processes, lithography, letterpress, flexography, gravure, and screen printing. However some of the newer plate-less technologies are beginning to take hold in the market. Based on 1997 sales figures lithography accounted 68.5% of the market; screen 9.0%; flexographic 6.4%; quick printing 5.7%; gravure 5.4%; letterpress 4.5%, and digital printing 0.6%. The market share is drastically changing as indicated by comparing 1990 sales figures with these current figures. In 1990 the market share was broken down to lithography 47%, gravure 19%; flexography 17%; letterpress 11%; and screen printing 3%. (1999 US Economic Census Report)

Print Technology	1990	1997
Lithography	47 %	68.5 %
Gravure	19 %	5.4 %
Screen	3 %	9.0 %
Flexography	17 %	6.4 %
Quick Printing	N/A	5.7 %
Letterpress	11 %	4.5 %
Digital	N/A	0.6 %

The introduction of plateless printing processes are beginning to significantly impact the printing industry. Based on 1991 projections the plateless technologies include electronic printing such as xerography and laser printing; ink jet printing; magnetography; thermal printing; ion deposition printing; direct charge deposition printing; and the Mead Cucolor Photocapusle process.

While most printing facilities utilize primarily one process or type of printing press, it is not uncommon to see multiple processes or types of printing presses at a printing facility. For example a newspaper publishing company may be utilizing both offset lithographic printing presses as well as flexographic printing presses. At many smaller printing facilities which print a variety of products such as business cards, stationary, advertisements, etc. it is not uncommon to find both offset lithographic printing presses as well as letterpress printing.

Ref. PIA’s 1996 Report to Congress

Ref. EPA’s Use Cluster Profile, EPA 744-R-94-003, June 1994

Ref. 1999 US Economic Census report

General Overview of Printing Process

The five major printing processes are distinguished by the method of image transfer and by the general type of image carrier employed. Depending upon the process, the printed image is transferred to the substrate either directly or indirectly. In direct printing the image is transferred directly from the image carrier to the substrate, examples of direct printing are gravure, flexography, screen printing and letterpress printing processes. In indirect, or offset, printing, the image is first transferred from the image carrier to the blanket cylinder and then to the substrate. Lithography, currently the dominant printing technology, is an indirect (offset) process.

Image carriers (or plates) can generally be classified as one of four types: relief, planographic, intaglio, or screen. In relief printing, the image or printing area is raised above the nonimage areas. Of the five major printing processes, those relying on relief printing are letterpress and flexography. In planographic printing, the image and nonimage areas are on the same plane. The image and nonimage areas are defined by differing physiochemical properties. Lithography is a planographic process. In the intaglio process, the nonprinting area is at a common surface level with the substrate while the printing area, consisting of minute etched or engraved wells of differing depth and/or size, is recessed. Gravure is an intaglio process. In the screen process (also known as porous printing), the image is transferred to the substrate by pushing ink through a porous mesh which carries the pictorial or typographic image.

Each printing process can be divided into three major steps: prepress, press, and postpress.

Prepress operations encompass that series of steps during which the idea for a printed image is converted into an image carrier such as a plate, cylinder, or screen. Prepress operations include composition and typesetting, graphic arts photography, image assembly, and image carrier preparation. Press refers to actual printing operations. Postpress primarily involves the assembly of printed materials and consists of binding and finishing operations.

Within each process, a variety of chemicals are used, depending on the types of operation involved. Prepress operations typically involve photoprocessing chemicals and solutions. Inks and cleaning solvents are the major types of chemicals used during press operations. Depending on the finishing work required, postpress operations can use large amounts of adhesives. This is especially true where the production of books and directories is involved. Of all the chemicals used in a typical printing plant, inks and organic cleaning solvents are the categories used in the largest quantities. Many of the chemicals used in the printing industry are potential hazards to human health and the environment.

Prepress Operations

Introduction

Prepress consists of those operations required to convert the original idea, such as a photo or sketch, for a printed image into a printing plate or other image carrier. Prepress steps include composition and typesetting, graphic arts photography, image assembly, color separation, and image carrier preparation. With the exception of image carrier preparation, the prepress process is similar for the five major printing processes. Plateless process do most of the prepress steps using a computer.

Typesetting and Composition

During composition, text, photographs and artwork are assembled to produce a "rough layout" of the desired printed image. The rough layout is a detailed guide used in the preparation of the paste-up or camera ready copy from which an image carrier can be produced.

Traditionally, rough layouts and pasteups were composed by hand using: drafting boards; light tables; various paste-up tools such as technical pens, rulers, and cutting tools; and adhesives. The text used in the paste-up was typeset and printed mechanically. However, composition has changed dramatically with the advent of computers. Both type and artwork can be generated and edited using computers. Computer systems can be equipped with both optical character recognition and photographic image scanners and digitizers so that pretyped material and photographic images can easily be incorporated into the document being composed. With the systems now available, the computer can directly drive the typesetting and image carrier preparation processes once the page or entire document is laid out and ready for printing.

Typesetting operations assemble the type characters into pages. There are a number of methods of typesetting including manual assembly of pieces of metal type (letterpress), mechanical assembly of lines of type, and phototypesetting. Until the 1950s, the majority of typesetting was performed using the Linotype machine which produces a "slug" or line of type from molten metal. Similar machines produced single characters of type. Today phototypesetting devices have almost completely replaced manual and mechanical methods of typesetting.

Phototypesetting devices, first demonstrated in the late nineteenth century, were introduced commercially in the early 1950s. They rapidly overtook the Linotype and similar machines in importance. In phototypesetting, individual type characters or symbols are exposed onto photographic film or paper. In early mechanical phototypesetting units, entire fonts of characters were stored as negatives on film. In the later generations of computer-driven phototypesetters, the image is generated electronically, and, in the latest generation of units, a laser is used to project the image onto the photographic film or paper. Phototypesetting produces high contrast, high resolution images ideal for printing purposes. Other computer driven output devices, which include strike-on, line, ink-jet, and laser printers are used extensively in-plant printing applications.