Figure 1

The Digital Eye: The Printer Interface

Stephen Romaniello is an artist and educator teaching digital art at Pima Community College in Tucson, Arizona for over 20 years. He is a certified instructor in Adobe Photoshop and the author of several books on the creative use of digital graphics software. Steve is the founder of GlobalEye systems, a company that offers training and consulting in digital graphics software and creative imaging.

Large-format inkjet printers are computer controlled printing devices that support wide substrate rolls, sometimes more than 100" wide. Printers with capacities over 100" wide are considered to be super-wide or grand format (see Figure 1). The price range of these devices is as wide as the surfaces they print on—from as little as about $20,000 to well over a half million or more.

Large-format printing has grown in popularity over the past 25 years and is now a common technology used in many sign and commercial graphics shops. Creating oversized prints of high-quality digital images on a large variety of surfaces is central to signage and commercial graphics. I thought this would be a good time to take stock of the printing equipment and technologies in use today.

 

Applications

Large-format printers are used to print signage, banners, posters, trade show graphics, wallpaper, murals, flags and textiles, fine art, billboards, backlits and vehicle wraps—not to mention electronic circuit schematics, architectural drawings, construction plans, theater backdrops and media sets (see Figure 2). Large-format prints are extensively used as high-impact advertising media. Let's look at some of the technologies being applied.

 

RGB Printers

I’ll start with the least common type of printer—the RGB printer. RGB printers don’t use ink. They are photographic printers that transfer colored light onto photosensitive paper, (see Figure 3). An RGB printer known as LightJet was originally developed by Cymbolic Sciences in the early 1990s to write to 8"×10" photographic film. These "digital originals" were indistinguishable from scanned slides and negatives, and prints had the look and feel of a traditional photograph, and offered resolutions of about 4,000 dpi.

Later Oce acquired Cymbolic Sciences and a large-format version of the LighJet was developed—along with competing brands such as the Durst Lambda. These units had the capability of writing much larger digital images directly onto photographic paper and backlit duratrans film using colored lasers. Though fairly popular among commercial graphics shops some years ago, their expense and limited substrate options have made them less popular, especially as the quality and speed of highly versatile, less expensive inkjet systems emerged.

 

Piezo & Thermal Inkjet Technology

The two most common printhead systems in today's inkjet printers employ either thermal technology or piezoelectric technology.

Piezoelectric printhead technology, which was first introduced by Epson in 1964, employs piezo-ceramic crystals at the rear of each printhead ink reservoir. When an image file is RIPed and electronic data is sent to the printheads, an electric charge excites the piezo crystals causing them to expand and contract at very specific frequencies. The pulsing of the crystals drives ink in the printhead reservoir so that a tiny droplet of ink fires out of the nozzle and onto the substrate, (see Figure 4). Piezoelectric printheads are quite durable, are used in a wide variety of inkjet printers, and can accept a wide variety of ink types.

Thermal inkjet technology—used only with aqueous-based ink systems—employs heat behind the ink reservoir in the printhead. A RIPed file sends the data to the printheads causing a pulse of heat that in turn forms a tiny bubble in the ink chamber. The bubble fires an ink droplet out of the nozzle and onto the substrate, (see Figure 5).

Both thermal- and piezo-based inkjet printers can generate professional-quality, high-resolution large-format images. Some photo print shops use archival aqueous inks that have proven to be better than the standard ink for long-term durability. Archival inks are humidity and light resistant and claim to last for as much as a century rather than fading over time. They have a protective resin coating that increases durability and an impressive color gamut keeps photos true to the original image.

 

Aqueous Ink: Dye vs. Pigment

Both thermal and piezo inkjet printheads can deposit aqueous (water-based) inks. The term "water-based" is an industry-accepted misnomer. The colorant is actually suspended in a neutral carrier solution that may be water or a soy-based liquid. Generally aqueous prints are used for interior applications, but with lamination can also be used outdoors.

Various substrates are available for aqueous printing, including many types of matte, semi-gloss and glossy papers, canvases, vinyl, and cloth. Aqueous technology requires that these materials be coated to accept and hold the ink, and to control dot gain.

Colorants in aqueous inks come in two varieties: dyes and pigments. Dyes rely on dispersed organic (plant-based) materials for their color while pigments use finely ground inorganic materials.

Dye-based inks offer the widest color gamut and produces intense colors, but prints can fade in time when exposed to ambient UV light. For this reason dye-based inks are primarily used for indoor printing. Pigment-based inks are less vivid, but offer much better resistance to fading due to UV rays. Finished aqueous prints should be laminated and encapsulated if they are to be displayed outdoors.

 

Solvent and Eco-Solvent Inks

Traditional true solvent inks consist of color pigments suspended in petroleum or a petroleum by-product such as acetone or ketone as a carrier. Due to the presence of hazardous VOCs (volatile organic compounds) emitted by true solvent inks, they are not used in the U.S. as widely as they once were.

More ecologically friendly eco-solvent and light-solvent inks are widely used today. They employ less toxic glycol esters as the carrier, and are an effective waterproof outdoor printing solution, but they produce slower drying prints. Most eco-solvent printers can be used in an office environment with minimal or tolerable odor levels, and very low VOC levels.

 

Latex Inks

Another type of water-based ink used in wide-format print production is called Latex inks. HP developed the technology as an outdoor printing alternative to light- and eco-solvent technology, and introduced it to the market in 2008. Latex inks are pigmented, water-based inks employing an aqueous-dispersed polymer (“Latex”) technology that gives these aqueous inks outdoor durability with very few environmental concerns.

HP Latex printers—Designjet series and HP Scitex printers—use internal radiant heaters and forced airflow to drive off water and cure the inks, producing dry, ready-to-use prints. Prints can be laminated immediately using cold, hot, or liquid processes. The company claims un-laminated durability of three years for outdoor applications, and up to five years for indoor displays. Printers to produce vivid, durable prints on a wide variety of coated and uncoated materials including most low-cost, eco-solvent/low-solvent compatible media.

In recent years Mimaki and Ricoh have developed similar Latex printing systems.

 

UV-Curable Inks

UV-curable inks work in an entirely different way than aqueous or solvent inks. In addition to the pigment colors, they are a chemical soup consisting mainly of monomers and oligomers and a photoinitiator catalyst that, when exposed to a strong source of UV irradiation (an arc lamp or LED-based cure lamp) become a solid. The monomers and oligomers chemically change into a solid polymer. Most wide-format UV-cure printers used in the sign industry have cure lamps mounted on the printhead and cure the deposited inks with every pass of the printhead.

UV-cure printing is viable on a huge range of substrates, and is especially useful for outdoor applications printed directly onto rigid substrates using a flatbed printer. Fully cured UV inks can be extremely durable and hard or remain quite flexible, depending on the formula used.

 

Dye Sublimation

“Dye Sub” as it is called in the vernacular, is a method for digitally printing onto textiles. In this process, special sublimation inks—under heat and pressure—sublimate and diffuse into polyester-based media and fabrics, forming a permanent bond. There are two techniques used with dye sub: direct-printing to textiles and transfer printing.

For transfer printing, a reverse image is printed onto a special transfer paper which is then placed with the intended substrate in a heat press where the inks can then be sublimated to the fabric. These printers produce very durable photo-quality continuous-tone images. Dye-sublimation printing uses polyester and polymer-coated substrates and is used to print onto apparel, and poly-coated items such as cell phone covers, plaques, ceramic containers, etc.

The heat press (clamshell/drum press), or calender unit uses a combination of time, temperature (380°-420° F.) and pressure, which vary depending on the substrate, to transfer the sublimation dyes at the molecular level onto the surface.

With direct printing, the printer deposits the dye sub inks directly onto the fabric which can then be run through a heat press—eliminating the need for a transfer process. Some direct-printing printers include a set of heated rollers (calender) that perform on-board sublimation. 

Dye sublimation produces an extremely permanent, high resolution, full color print. The prints will not crack, fade or peel under normal conditions.

 

Plotters

Plotters were the first type of printer that could print with color and render graphics and full-size engineering drawings. This technology is primarily used for producing CAD (computer aided design) and CAM (computer aided manufacturing) drawings. A plotter interprets data from a computer into line drawings on paper with one or more automated pens, (see Figure 7). The plotter can draw continuous point-to-point lines directly from a vector file. There are a number of different types of plotters the most common being:

  • Drum Plotters—draw on paper wrapped around a revolving drum to create the vertical coordinates of the drawing. The pens move in a horizontal direction and draw on substrate.
  • Flatbed Plotters—draw on paper placed on a flat surface with pens that move both horizontally and vertically.
  • Electrostatic Plotters—draw on negatively charged paper with positively charged toner.
  • Vinyl Cutters—though not actually a printer, a vinyl cutter uses plotter technology to accurately cut shapes from vinyl sheets.

 

Printing

There are many variables that need to be considered before you push the print button.

Considering the amount of material that is consumed by large-format printing and the length of time it takes to print a large-format document, it is essential to get the print right the first time, otherwise a lot of time and money can be wasted.

That about covers the types of large format printers and their characteristics and capabilities. There are many brands and sizes on the market and you should consider what system is best for your particular needs.

In this article, I’ve presented the various types of hardware and media that prints large images. Next month’s DigitalEye column will present important tips on how to configure and color manage your large-format printer and software to produce accurate, reliable color every time you print.