In the past three posts, we have covered the fundamentals of three of the most widely used analog processes. Before exploring digital printing processes in our next few posts, let’s review commercial offset printing, screen printing, and flexography and discuss two other specialized analog printing processes: rotogravure and pad printing.
When you understand the basics of analog printing processes, you will have a better perspective on why so many different types of digital printing devices now exist.
Offset printing is a multi-step process that uses oil-based inks and a series of rotating cylinders. After the ink and a thin film of water is applied to imaged plates mounted on the “plate cylinders,” the plates transfer the images to rubber-wrapped blanket cylinders. The blanket cylinders then press the inks onto the substrate that is fed through the printer on impression cylinders. The thin film of water applied to the plate cylinder repels the ink in the non-printed areas of the plate so that only the inked portion of the image is transferred to the blanket cylinder and the paper.
Offset printing is used to produce documents, manuals, catalogs, magazines, books, posters, art prints, stationery, business cards, and some types of packaging.
Screen printing uses squeegees to push paste-like inks through the openings in a prepared mesh screen. The ink is only deposited in areas in which the mesh openings haven’t been blocked by the hardening of a photosensitive emulsion layer.
Screen printing can use a wide range of inks and print on almost any substrate imaginable, including papers, films, textiles, plastics, metals, wood, wallpapers, and manufactured garments, parts, and objects.
Flexographic printing transfers ink from recessed areas of engraved plates (“anilox rollers”) to photopolymer plates that have been etched to create raised text and lettering. Only the raised areas of the plate are inked. The inked photopolymer plates are mounted on cylinders and transfer the text and graphics onto rolls of paper, films, foils, and paperboards.
Flexographic printers can use a variety of water, solvent, or energy-curable inks depending on the type of substrate being printed. Flexographic printing is a high-speed printing method that is commonly used for high volumes of different types of packages and labels.
Rotogravure (also known as Gravure) is a high-speed, industrial printing process. Engraved cylinders with recessed cells are partially immersed in an ink tray. As the cylinder rotates through the tray, the cells fill with ink. Before the rotating cylinder makes contact with the web of paper, a doctor blade scrapes the cylinder’s surface to remove ink from non-recessed areas of the cylinder. Rotogravure is used to print high-quality images on millions of copies of catalogs, magazines, consumer packages, or newspaper inserts.
Pad Printing uses a flexible silicone pad to transfer 2D images from a flat, engraved steel plate that has been filled with ink to a three-dimensional object. After the pad picks up the image from the plate, the solvents evaporate, and the machine presses the tacky ink into the contours of the part. The process is commonly used to print plastic, glass, and metal parts used in automotive, medical or electronic devices or any rigid part that requires a logo or marking (e.g. bottle caps). Less costly photopolymer plates can be used instead of steel plates for shorter runs.
Printing Technology Has Always Evolved
Modern automated screen printing evolved from manual “silk screen” printing techniques that have been traced back to the Song Dynasty in China (960 - 1270 A.D.). The other four modern printing processes evolved from earlier forms of planographic, intaglio, and relief printing processes.
Planographic processes transfer the image from a smooth flat plate. Offset lithography evolved from lithography, a process in which ink was applied to a grease-treated image on a flat printing surface. Because ink and oil don’t mix, the inks were repelled from the grease-treated areas of the image.
Intaglio processes use etched or engraved plates to hold the inks in recessed areas of the plate. In industrial printing, gravure printing is a form of intaglio printing. Art prints are known as “etchings” were also created with engraved plates on a manual intaglio press.
Relief processes use raised lettering and graphics to directly imprint designs on the paper or surface. The flexographic relief printing process evolved from the centuries-old letterpress process in which plates of raised type and images pick up ink from roller and transfer it to the paper.
Artisanal print shops still use a variety of silkscreen, intaglio, planographic and relief printing process to make small runs of art reproductions, posters, invitations, and specialized prints. Many customers prefer the simple, hand-crafted, vintage looks of historic printing processes to the full-color, special-effect photographic looks that automated digital printing systems can deliver.
Digital Printing Processes
All five of the analog printing processes require the creation and precise alignment of separate plates for each color used in a print. Digital printing processes eliminate the time, labor, and expense of creating and mounting printing plates.
The first digital printing devices printed black ink on paper. Today, multiple digital printing processes have been developed to replicate and expand the capabilities of offset, screen, flexographic, rotogravure, and pad printing.
A key advantage of digital printing is the relatively low cost of producing just a few copies of full-color prints. While the first digital printing systems were relatively slow compared to analog methods, significant progress continues is being made.
Many print shops today use a combination of digital printing and analog printing systems to serve whatever range of capabilities their targeted customers might require.
In the few posts, we’ll cover different types of digital printing including wide-format inkjet, production inkjet, electrophotography (toner-based digital presses), and dye-sublimation. We’ll also talk about specialized printers for grand-format printing, textile printing, photo printing, and direct-to-garment, direct-to-shape, and direct-to-object printing.
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This post is part of a series of posts we are publishing to help you understand the many types of analog and digital printing processes used by all types and sizes of print service providers. We’ll conclude this series by reviewing the strengths and limitations of various processes and discussing various pricing models.