310 - Digital printing technologies

Digital Printmaking Technologies

Common technologies used to produce digital prints include inkjet, electrophotography (dry toner and liquid toner), thermal transfer (mass transfer and dye sublimation transfer), and laser imaging (digital photo printing) on photographic paper.

INKJET

Inkjet is a popular technology based upon the ejection of small drops of fluid by an actuator that is controlled by a digital computer system. Once ejected from a print head orifice (nozzle), the droplets pass through the air to a printing medium, typically paper, on which they form spots or dots. By controlling both the actuator and the relative position of the medium, an array of spots is produced on the medium to form a pattern. With the appropriate ink droplet sizes, ink colours, and ink-receiving medium, an image is created. Today, image quality and permanence can be produced in commercially available inkjet systems with performance exceeding traditional photographic technology. This is by far the most popular type of printer on the market today, and they are now as fast as laser pinters, and can be used to print large posters and a variety of advertiosng work. I have a small A4 printer of this type at home and is adequate for the jobs required for it to perform. I have only had personal experience of this type of printer for photographs.

Piezo, Thermal, and Continuous-Flow Inkjet Technologies
These three are the most common inkjet technologies employed. For the digital printmaker they are usually combined with water-based (acqueous) inks, either pigmented or dye-based. The differences among them are the actuator technologies:

Piezo [or piezoelectric crystal actuator based on-demand inkjet technology, in the full term] uses a small crystal that bends when current is applied to it to produce the actuation effect. This "piezo effect" is based on a very small movement of the crystal and subsequent compression of the ink in an ink chamber. The result is the ejection of a very small droplet of ink from a small orifice in the inkjet print head. When properly designed, this system can generate very small, reproducible droplets of ink that can produce outstanding results when the droplets are properly positioned on well-designed inkjet media. Since the actuating pulses can be varied, resulting in more or less bending, a variety of droplet volumes can be produced. This leads to the ability to achieve a digital gray scale and to further improve the image-quality capability of the technology.

Thermal [thermally activated actuator based inkjet technology, in the full term] uses heat to create a very small bubble of superheated vapour to push a small droplet of ink from an inkjet print head orifice. This is the most widely used inkjet technology in the world and can provide very good print quality when properly implemented. It is a more robust technology than piezo since the energy used to eject the droplet makes thermal less susceptible to clogging of the orifice. Also, bubble formation in the ink is not encouraged by the process as it is in piezo; a very small bubble will cause a piezo printer to fail to eject a droplet.

In the early days, the heating and shock experienced in the actuation process limited the ink formulation latitude for thermal. New advances in this technology allow water-based ink designers the freedom to develop inks that do not limit the image quality or durability of the print output. As with piezo technology, final drop volume may be varied to provide grayscale rendering.

Continuous [or Continuous Flow inkjet printing technology, in the full term] is used for digital printmaking exclusively with the IRIS/IXIA inkjet printers (the IXIA has replaced the IRIS). It produces a stream of droplets all identical in size, and the electronics of the system chooses which ones will hit the medium and make a spot and which ones will not. Continuous flow has provided excellent image quality but at a very high cost in both the initial investment, reliability, and speed, as well as image permanence. Pigmented inks cannot be used with this technology. It is no longer a preferred technology.

ELECTROPHOTOGRAPHY

Electrophotography is based upon the deposition of either dry powder or liquid toner onto a photoreceptive surface on which a charge is produced, usually by light from a laser or other similar point light source. The toner is subsequently transferred either to a blanket then to paper or directly to the paper and then fused to form the desired image. Dry toner technology is limited in the size of the particles that can be used. Since these particles are relatively large, this technology generally suffers from insufficiently good image quality to satisfy the digital printmaker. Liquid toner, however, can use very fine particles that allow for production of near-photographic quality output. Today, liquid toner image quality and permanence can meet the needs of some digital printmaking applications. Liquid electrophotography's advantage is higher printing speed as compared to inkjet printing or thermal transfer printing but at a very steep price.

THERMAL TRANSER

Thermal transfer is based upon using heat to transfer either a colored coating containing a binder and colorant or dye molecules (by sublimation or diffusion) from a thin carrier film to a receiver sheet that contains a receptive surface coating. The heat may be provided by an array of nibs (similar to those in a thermal fax machine) that are activated by a computer when a colour spot is desired, or by a laser that heats the carrier film causing the transfer to occur. Good-quality colour prints can be produced by this technology, which is frequently used in photo kiosks for printing either digital or scanned photo reproduction prints.

DIGITAL PHOTO PRINTING (LASER IMAGING)

Digital Photo Printing (laser imaging) is based on traditional photographic silver-halide photographic print paper that is imaged by a laser or similar point light source to provide a full-colour photographic image. This technology is gaining popularity in both wide-format photo printing and for digital photo finishing, both in minilabs and in production labs. Laser imaging produces true photographic prints and can be very productive in larger volume applications; however, it requires photo paper as a receptor, which limits its media flexibility. The market ofr colour laser printers has grown considerably over the lats few years. They are however costly to buy and operate and not really within reach of the ametuer or semi-professional photographer.