Digital Press Technology-Part 1: Electrophotography

Digital presses have been designed to replicate the print quality and substrate versatility of offset, flexographic, and rotogravure printing presses —without the time and expense of creating plates for each color. Digital presses are used to print marketing collateral, booklets, books, photo books, calendars, tickets, short-run magazines, direct mail, folding cartons, flexible packaging, labels, posters, and point-of-sale signs.

Digital press technology makes it possible to output full-color prints that can be customized or personalized during production. The presses don’t have to be stopped to load new plates for each new job.

For a long time, the term “digital press” primarily described digital print systems that use toners instead of inks. Today, some digital press manufacturers use the term “digital printing press” to encompass both electrophotographic (toner-based) systems and production (single-pass) inkjet systems.

In this post, we’ll review inks, substrates, finishing, and other elements of the electrophotographic process. In the next post, we’ll review some of the high-speed production inkjet systems that compete with offset and flexographic printing.

Electrophotography (EP), also known as Xerography, uses electrostatic charges, toner, and light. After a photoconductive drum is positively charged, a negative of the image is beamed onto the drum with either a laser or LEDs. The beamed image cancels the charge on the drum, leaving a positively charged replica of the original image.

A negatively charged toner is attracted to the positive image on the drum, and the toner is then attracted to the paper, also positively charged. A final fusing stage uses heat and pressure, pressure alone, or light to permanently adhere the toner to the substrate.

Liquid electrophotography (LEP) is a “thermal offset” process created by Indigo to use a liquid toner. An electrostatically charged Photo Imaging Plate (PIP) mounted on the imaging cylinder is scanned by laser diodes controlled by the RIP (raster image processing) software. Binary Ink Developer units for each ink color prepare a thin film of electrically charged ElectroInk on the roller surface. When a BID roller for each color engages with the PIP cylinder, the electrical fields between the PIP and BID attract the ink paste to the image area and repel it from the non-image areas. A heated blanket melts the pigment-carrying particles within the ElectroInk and blends them into a smooth film of ink The inked areas on the heated blanket solidify when transferred to the non-heated substrate held by the impression cylinder.

Types of Toner-Based Presses

Dry-toner digital presses were adapted from the color copier technologies used by companies such as Xerox, Canon, Rioch, and Kodak. color-copiers from Canon, Xerox, Ricoh, Kodak.

The term “digital offset press” was first used by Indigo in the early 1990s. In 2001, HP acquired Indigo with the goal of leading the commercial printing industry into the digital era. Today, different models of HP Indigo digital presses have been built to replace or complement commercial sheetfed, commercial web, and packaging and label presses.

Digital presses that use dry toners are popular in lower-volume print shops, including in-plants, copy shops, and start-up printing companies. Sign shops, wide-format businesses, design studios, and screen-printing businesses have purchased different makes and models of electrophotographic digital presses to offer short runs of marketing collateral along with signs, photo prints, or display graphics.    

Monochrome presses use black toners to print documents such as instruction booklets, pamphlets, financial statements, manuals, and books.

Color presses have up to 7 ink stations. In addition to cyan, magenta, yellow, and black, presses can apply white, clear, light cyan and light magenta, light, light black, and fluorescent colors. Some toners have properties that can help financial institutions detect counterfeit checks or event venues spot counterfeit tickets.

Cut-sheet presses start with standard-sized sheets of papers or paperboard that can be easily folded or cut to create popular sizes of postcards, direct-mail pieces, or folding cartons.

Continuous feed presses can print on rolls of paper or film. Some continuous feed presses are roll-to-roll printers. Other models convert the printed rolls into cut sheets during the process.

Types of Toners

Dry toners consist of pigments embedded inside polymer beads. The fusing phase of the electrophotographic process melts the polymer beads to the surface of the paper.  

Liquid toners developed by HP contain electrically charged ink particles dispersed in a liquid that evaporates when heated. According to HP, the use of particles as small as 1 to 2 microns dispersed in the liquid carrier allows for higher resolution, uniform gloss, sharp image edges, and very thin image layers that ensure that the imaged area of the page has a texture similar to that of the non-imaged area.

Substrates

Toner-based presses can print on the enormous variety of papers, paperboards, films, shrink-sleeves films, synthetic papers, and pressure-sensitive label stocks used to print all types of documents, folding cartons and labels.

To improve adhesion on materials that haven’t been optimized for Indigo printing, some HP Indigo production digital presses include integrated priming or corona treatment systems. These systems enable print shops to use the same types of “off-the-shelf” materials used for offset or flexo presses.

Prepress

Unlike analog-digital printing systems that require the use of plate making systems, digital presses use software to process data from the digitally created print file into instructions that the digital device requires to lay down ink or toner.

A digital press uses a digital front end (DFE) to convert data from PDF or Postscript files into print-ready files that have the correct fonts, image resolutions, colors, page impositions, variable data, and halftone screening,

The type of DFE used with the digital press depends on the volume of data the server will process and whether the press will be used to produce publications, photographs, labels, and packaging, or marketing collateral.

EFI introduced the first “color servers” for color copiers in 1991 and partnered with Canon, Ricoh, and Xerox to develop digital front ends for their first EP digital presses. HP developed their own SmartStream servers to help streamline the job set-up, color matching and variable-data capabilities of HP Indigo presses.

Finishing

Digital presses can be equipped with inline finishing systems that convert the printed sheets into booklets, books, or ready-to-mail documents. Interface ports can be used to connect finishing equipment for binding, folding, cutting, die-cutting, coating, hole-punching, and laminating.

HP offers finishing modules that can be added to digital presses to create: direct mail and billing statements; marketing collateral such as data sheets and point-of-sale signs; photo prints, greeting cards, and perfect-bound photo books; and saddle-stitched magazines and perfect-bound books.

How Technology Has Advanced

Because fused toners form a thin, smooth plastic layer on the surface of the paper, pages printed with electrophotography often feel different than papers printed on offset presses.

Advances in toners and transfer technology have improved the look, feel, and adhesion of prints output on toner-based digital presses. Better control of pigment particle size has also led to greater image quality.

Some HP ElectroInks for packaging presses complies with the low-migration ink standards required for safety in food packaging. HP also offers an ink-mixing service to create spot colors needed for packaging, labels, and brand marketing.

Integrated priming systems or advanced toner transfer systems make it possible to print on specialty materials such as clear films, magnetics, and textured media.

Digital front ends have become incredibly powerful in managing prepress and handling production data for multiple jobs. HP Mosaic software for SmartStream makes it possible to automatically generate and print millions of unique reproducible graphic designs for labels, mailers, packaging, and other printed products.

Cloud-based print management systems enable print-shop owners to continuously monitor and upgrade the performance of their digital presses and automate file submission and preflighting processes.

Follow Us on Social Media

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. Our next post discusses the production of inkjet printers.
Follow Ordant on LinkedIn, Facebook, or Twitter to learn when the next post in this series has been published.