Inkjet Technology
There are a number of different inkjet technologies in use. The basis of all of them is that tiny ink droplets are ejected from a printhead and projected onto a paper. To increase print speed, a printhead now consists of many nozzles – up to about 3,000 per ink channel on contemporary printers. The technique making the ink droplets eject differs with various print-ers and printer makers (most printer manufacturers use a single technique in all machines).
The main techniques are:
* Continuous flow inkjet printers
The following methods are also called drop-on-demand, as they eject a droplet only when needed on the paper:
* Thermal inkjet printers (e. g., used by most HP and Canon printers). Canon calls this technique bubble-jet.
* Piezoelectric inkjet printers (e. g., used by most Epson printers)
Continuous Flow Inkjet
This technique was developed by IBM in the 1970s. With continuous ink flow systems, a continuous stream of charged ink droplets is produced. Those droplets intended to print fly straight onto the paper, while undesired droplets are electronically deflected into a gutter for recirculation. This is the oldest inkjet technology and is used for high-speed production lines. The complex ink-circulation system makes these printers costly in mainte-nance. They can be very fast compared with the typical drop-on-demand type printers.
Piezo Inkjet
Certain kinds of crystals expand or contract when subjected to an electrical charge. This piezoelectric effect is used in certain inkjet printers. To eject a droplet, a voltage is applied to the crystal in the print head, the crystal deflects inward, forcing a droplet out of the nozzle. The returning deflection pulls fresh ink from the reservoir, and the cycle repeats. A print head consists of many of these miniature jets (nozzles), and the system allows variations in the size of droplets to produce a finer pattern and smoother color gradients.
Thermal Inkjet
With thermal inkjet printing – also called bubble-jet printing – there is a resistor in the print head chamber. When it is heated by a short pulse of electrical current, a vapor bubble forms in the chamber increasing pressure. This pressure forces an ink droplet out of the nozzle. Then the bubble collapses and draws in more ink from the reservoir. For the next droplet, the cycle repeats. This technique is used by some HP printers (e . g . HP Designjet 30), as well as by most Canon printers (e . g. Canon i9900, W6200). Alsosome wide-format printers like the HP Designjet 130 and the ENCAD Novajet 1000i use thermal print heads. The technique may be used with dye-based, as well as pigment-based inks, however, it does require an ink suited for thermal inkjet printing (with a low boiling point). The life-cycle of these printers is a bit shorter than that of piezo-based print heads, but theproduction cost is lower.
Droplet Size
Along with increasing printer resolution, the size of the individual ink droplets has decreased. A smaller droplet allows production of a finer raster of dots on the paper. Today, photo printers use a droplet size down to 1–5 picoliters (1 picoliter is 0.000 000 000 001 liter or 1 × 10 -12 liter), thus allowinga single pixel (a raster cell) of the image to be built by many tiny dots, achieving a fine raster (e . g . 360 ppi or even 600 ppi) with a broad range of tonalvalues.
With some photo inkjet printers (e . g . , with piezo-based ones) the droplet size can vary. For dark colors – especially colors of a primary ink color – larger droplet sizes are used. This allows increasing print speed. Forlight colors, a smaller droplet size and wider droplet spacing is used.
Printer Resolution
When inkjet printers first arrived, a resolution of 150 dots per inch was considered good. Today, resolutions of 2,400 dpi, 4,800 dpi, 5,800 dpi, and even 9,600 dpi are normal for photo printers. Please do not be mislead by these advertising claims, as they refer to resolution only in one direction, e.g., horizontal. Canon’s i9950 photo printer has a maximum resolution of 4,800 x 2,400 dpi, where the higher resolution is seen in the horizontal direction (achieved by the horizontally moving print head) and the lower resolution in the vertical direction. Here, the increments are determined by the step motor that moves the paper. The same is true for most inkjet printers.
Before you invest your money in one of these high-resolution printers, consider carefully whether you actually need the maximum resolution advertised by these printers. To take advantage of a manufacturer’s maximum resolution, you need a paper or other printing medium that can accommodate the fine pattern of ink droplets, so the ink will not bleed noticeably into the open area. The paper (or its coating) must absorb ink very quickly to keep it localized. Ensure your paper can accommodate the resolution you wish to use.
We have found that a horizontal resolution of 1,400 dpi or 2,800 dpi, both of which are typical for Epson printers, is sufficient, even for fine art prints. For higher resolutions, the print speed decreases dramatically andink usage increases, neither of which result in noticeably better image quality. Of course, print speed and ink usage can vary among different printers, even from the same manufactures.
Number of Inks
While inkjet printers originally used a single black ink, three more inks (CMY) were added to produce what we now take for granted as the norm: CMYK. All of today’s photo inkjet printers use at least six inks: CcMmYK (c = light cyan, m = light magenta).
To enhance the color gamut of these printers, more inks are often used.
Epson’s Stylus Photo R1800, for instance, uses Red and Blue (and, option- Stylus Photo R1800,for instance, uses Red and Blue (and, option- R1800, for instance, uses Red and Blue (and, option- for instance, uses Red and Blue (and, optional ly, a gloss optimizer) but has no Light Cyan nor Light Magenta. Epson’s R2400 uses three shades of black inks: a Photo Black (or alternat ively Mat te Black), a Light Black, as well as a Light Light Black, in addition to the CcMmYK, thus totaling eight inks. This latter collection of inks produces neutral black-and-white prints with very fine tonal gradients.The color gamut and print permanence of today’s professional and semi-professional inkjet prints now surpasses that of traditional silverhalide photographic prints, although there are some weak spots: e.g., saturated blue and red. For this reason, the number of different inks used in a fine art printer will likely increase to 11, with the addition of R, B and a gloss optimizer.
In 2006, Canon announced it s “imagePROGRAF iPF5000”. This printer accommodates 12 single inks: CcMmYK plus RGB plus Light Black (Canon calls it Grey) and Light Light Black (which Canon calls Photo Gray). For Black there is a Photo B and a Matte B (all in the printer at the same time). This 17" printer uses pigment inks.
Practically, though, the number of inks that may be used is limited by the size and weight of theprint heads, and the cost of the numerous ink cartridges.
Type of Inks
Two different types of inks are mainly used in today’s normal inkjet printers:
*dye-based
*pigment-based
There are, however, several other types of inks on the market, e. g., inks with an oil base that are primarily for large-scale printing and weatherized and/or ruggedized prints for outdoor use. Other inks are formulated for printing on fabric, and are usually solvent based. Some printers use hybrid inks: dyebased for colors, and pigment-based for black to achieve a dark, deep black, a technique used by some Canon printers.
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