Inkjet printers

Generally, inkjet printers can be divided into wide-format inkjet printers for business use and narrow-format inkjet printers for general use. Their fundamental mechanisms are the same regardless of the size of the printer. The phenomenon of injection of liquid from a syringe is used as the principle of the inkjet. The head part of an inkjet printer, equivalent to the needle of a syringe, has many nozzles, and ink is ejected from each nozzle to draw images on recording media such as paper. Ink droplets ejected from the head of one of the newest printers are 4 picoliters (1 picoliter = 1/1 trillion liter), and 100 micrometers or less in diameter. Recently, sizes as small as several dozen micrometers have been reached. In the following section, we introduce the types of the head section, which is the core of an inkjet printer, and the types of ink used.

-Inkjet printer heads

All inkjet printers eject ink, but the mechanism used differs from one manufacturer to another. The following is a brief explanation of inkjet printer heads excerpted from manuals.

- Types of inkjet printer heads

Printer heads usually employ either of two systems for ink

drop formation, that is, the continuous method or the drop-on-demand method. The method adopted depends on the method of generation of ink-ejection energy. In the continuous method, electric field deflectors continuously produce ink droplets, the required number of which are guided onto the recording media. In the drop-on-demand method, ink droplets are ejected only when required. The drop-on-demand method is subdivided into the following three types: (1) the electromechanical transfer method, by which deformation of piezoelectric elements is used to eject the ink droplets; (2) the electric thermal transfer method, by which bubbles generated by heat cause ink droplets to be ejected; and (3) the electrostatic attraction method, by which electrostatic force of attraction is used to eject ink droplets. (Figure)

-Continuous method

Printers utilizing the continuous method continuously create ink droplets by applying pressure at 50 to 100 kHz with a piezoelectric vibrator. There exist two different methods in use for control of ink droplets. Printers utilizing the first method are the charge control type. In this type, each ink droplet is electrically charged by the charge electrode as shown in Figure 1 below, and the directions of the ink droplets are controlled by deflecting electrodes. The other method is the pressure vibration type, developed by Hertz in 1965. In this type, ink droplets given a very high pressure of 30 kg/cm2 are ejected from nozzles as shown in Figure 2 below. If no control voltage is applied, ink droplets are ejected straight onto the recording media. If voltage is applied, electrostatic repulsion occurs between ink droplets, which disperse, and thus no recording is made. The former type enables high-speed printing, and the latter multi-gradation printing.
These two methods are used only by business-use printers. (Figure 1) (Figure 2)

Figure 1 Figure 2

-Drop-on-demand method
In terms of recording speed, the drop-on-demand method is somewhat inferior to the continuous method. However, the drop-on-demand method is used more frequently because of its simple structure, which allows easy downsizing. In the drop-on-demand method, ink droplets are ejected only onto the area where the image is desired, and consumption of ink is thus smaller than under the continuous method. The drop-on-demand method is subdivided into the following three methods according to ink droplet ejection method:
(1) the electromechanical transfer method using piezoelectric elements, used in printers made by Epson and other makers;
(2) the electric thermal transfer method, using thermal energy, represented by Hewlett-Packard and Canon printers; and
(3) the electrostatic attraction method, using electrostatic attraction (information on the commercial application of this method is not available).

Electromechanical transfer method
In the electromechanical transfer method, piezoelectric elements deformed by electrical pulses cause ink droplets to be ejected. Deformation of piezoelectric elements puts pressure on the ink chamber through the intermediary of flexible substances, which generates ink ejection energy. Figure 3 shows the principle of the electromechanical transfer method.
Figure 3
Electric thermal transfer method
In the electric thermal transfer method, bubbles are generated in the ink chamber, and the expansive force of the bubbles causes ink droplets to be ejected. The ink is rapidly heated and vaporized by the heater in the ink chamber until bubbles are generated. The pressure of bubbles generates the required ink ejection energy. Figure 4 shows the principle of the electric thermal transfer method.
Figure 4
Electrostatic attraction method
In the electrostatic attraction method, slight pressure is put on the ink chamber to form a meniscus of an ink drop at the nozzle, and electrostatic attraction force is applied to eject the ink droplets.


-Materials and sizes of printer heads
Materials and sizes of inkjet printer heads differ as manufacturers apply in-house know-how to their products. To generate high-quality images, the ink droplets ejected must be small and controlled by a large number of nozzles. The factors that determine the size of an ink droplet are the shape, material, and processing method of the nozzle. In recent years, micro-nozzles with a diameter of between 10 to 30 micrometers have been produced thanks to the advance of laser processing technologies for electrocast nickel, polyimide, etc.

-Inkjet printer ink

Various characteristics are required for ink used for inkjet printers so that it can be ejected in the form of droplets from nozzles with a diameter of less than 100 micrometers. Mechanical requirements:
- Droplets of the same size are formed
- No clogging of the printer head
- No deterioration in quality Usage requirements:
- High optical density
- Fade-resistant
- Quick-drying
There are two types of inks for inkjet printers: solvent-based inks and water-soluble inks. Nowadays, water-soluble inks are more frequently used for inkjet printers. Water-soluble inks are further classified into dye-based inks and pigment-based inks. Generally, water-soluble dye-based inks are used for small inkjet printers for personal use.

-Water-soluble inks

One of the reasons for the widespread use of inkjet printers is that they can produce high-quality images thanks to advanced high-definition technology. Development of inks is crucial for stable ejection, without which creation of high-quality images is impossible. In particular, water-soluble inks can develop excellent colors and generate high-quality printed materials. This type of ink consists of water, colorant, penetrant, antidrying agent, pH adjuster, preservative, fungicide, etc. There are two types of colorants: dyes and pigments. Currently, water-soluble dye-based inks are mainly used because they possess higher reliability in terms of nozzle-clogging of inkjet printer heads. However, water-soluble dye-based inks run easily, and lack water- and light-fastness. Commercial application of water-soluble pigment based inks has been delayed due to problems with dispersion in water and nozzle-clogging. For wide-format inkjet printers, however, they are currently extensively used, taking advantage of their light- and water-fastness. In recent years, image quality using pigment-based ink, which was previously evaluated as inferior to the use of dye-based inks, has improved dramatically.

Water-soluble dye-based inks Water-soluble pigment-based inks
"Characteristic" "Characteristic"
Colorants are dissolved in the vehicle Colorants are dispersed in the vehicle
"Advantages" "Advantages"
Excellent colors
Wide color reproduction range
High optical density
High light-fastness
High water-fastness
Do not run easily
Almost no change in color
"Disadvantages" "Disadvantages"
Inferior in water-fastness
Inferior in light-fastness
Run easily
Colors tend to deteriorate
Low optical density
Inferior in colors
Inferior in image quality
Tend to cause clogging


-Solvent-based inks

Water-soluble inks use water as the vehicle of colorants. In contrast, solvent-based inks use solvents as the vehicle of colorants. By using solvents, nonaqueous colorants can be used, water-fastness and storage stability of printed materials are improved, and the drying time after printing can be reduced. However, there are problems such as scope of color available due to limitation of colorants, and environmental issues. This type of ink generally consists of solvent, colorant, penetrant, antidrying agent, pH adjuster, preservative, fungicide, etc. Pigments are generally used as colorants.

-Other types: Solid inks for inkjet printing

This is a solid inkjet printing system under which melted solid inks are used. Accordingly, no solvents are used and no drying procedure is required after printing. This method enables high-speed printing on any type of media that allows printing by ink. The composition of this type of ink is similar to that of plastic paint. Solid inks use a chemical compound that has characteristics similar to the wax in candles. This type of ink consists of a wax component, colorant, viscosity modifier, plasticizer, stabilizer, etc. Dyes and organic pigments are generally used as colorants.



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