- January 31, 2005, Dr. Richard M. Podhajny, Ph.D., Contributing Editor
T he concept of forming droplets of liquid in a stream was studied by Lord Kelvin and Lord Rayleigh and other technologists as early as the late 1800s. However, it was not until 1951 that it was patented for the first practical ink jet device based on Rayleigh’s concept.
By the early 1960s, Dr. Sweet of Stanford Univ. demonstrated a method of breaking an ink stream into droplets of equal size that could be spaced. At the same time, the drops could be charged and deflected to form an image pattern. This was the earliest version of the continuous ink jet process concept (CIJ) that was soon to be commercialized by A.B. Dick and Mead during the late ’60s.
The 1970s saw the development of drop-on-demand (DOD) ink jet systems. These early systems lacked reliability and suffered from nozzle clogging and image quality consistency.
However, in 1979 Cannon invented a DOD method based on generating a vapor bubble using a small heater located near the nozzle. This system soon was termed the "bubble jet" system. The bubble jet printhead proved to be simple and reliable and could be produced at low cost with high nozzle packing density.
A similar technology was developed by Hewlett-Packard. It was commercialized in 1984 and the term "thermal ink jet" was coined. Thermal ink jet and "bubble jet" dominate the low-end color printer market. One limitation of this technology is that the jet inks must be thermally resistant; even small amounts of impurities can degrade the heating elements.
In addition to the thermal or bubble technologies, a competing DOD ink jet technology is based on piezo crystals. In this technology, the piezo material causes the ink volume to change in the pressure chamber to generate a pressure wave that propagates the liquid drop at the nozzle.
The piezo crystal expands and contracts in response to an electrical pulse. Inks do not require the high temperature resistance used in the thermal or bubble DOD systems, nor the conductive components required in the CIJ systems.
Between 1993 and 1997, Epson introduced piezoelectric ink jet printers that were competitive with low-cost thermal or bubble systems. In this hot melt jet ink technology, the ink is a solid that can be melted and applied as a molten liquid. On contact with the substrate, the ink solidifies. These inks are based on a solid wax vehicle.
Water-based jet inks often are used in home- or office-type ink jet printers. Most DOD ink jet systems are water-based, whereas CIJ systems typically are solvent-based. Today, both water- and solvent-based inks can be formulated with pigments for good physical and lightfast properties.
Adhesion to film, metal, and glass is still the clear advantage of the solvent-based ink formulations. Since these primarily are CIJ, the CIJ ink printing is favored in many difficult packaging applications.
The introduction and development of UV jet ink systems very well may bridge this difference, as the application of UV jet inks can provide good adhesion characteristics on difficult substrates. The low VOCs and high press drying speeds offer excellent productivity enhancement for many packaging applications. The downside of this product is the lack of FDA status for sensitive food packaging printing requirements.
The ink jet industry growth favors DOD technologies due to greater short-run printing applications. One of the current trends in ink jet development is to increase the number of nozzles while simplifying the ink jet formulations. Pigmented systems clearly dominate the higher-performance package printing demands.
The marriage of jet ink technology and UV curing appears to be a natural for many applications that require speed, high quality graphics, and product resistance. At the most recent Drupa, high quality piezo UV ink jet capabilities were demonstrated at a resolution of 600 dpi, but printing speeds still are below package printing expectations. However, use of UV jet inks is poised for rapid growth over the next decade.
Competition among CIJ, DOD, and hot melt jet technologies will continue as unique applications are commercialized for each.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 267/695-7717; firstname.lastname@example.org.