Continuous Ink Jet

In industrial outlets the inks are often based on solvents such as alcohols and ketones containing the binder resins reqnired for the adherence of the colour to a particular substrate, whether this be paper, card, plastic or metal. 

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There was a logical progression of technology development from continuous to piezoelecttic ink jet. Designers of continuous ink-jet systems ensure that the ink stream breaks into drops of constant size and frequency by applying vibrational energy with piezoelecttic crystals at the natural frequency of drop formation. This overcomes the effects of any random forces from noise, vibrations, or air currents. 

The principal physical properties influencing ink performance ate surface tension and viscosity. High surface tension is desired for good droplet formation and capillary refill in dtop-on-demand ink jet. Low viscosity is desired because less energy is required to pump and eject ink. Conductivity is also an important parameter. Continuous ink-jet inks must have some conductivity to allow for charging. Low conductivity is generally preferred for impulse, particularly thermal ink jet, because excess ions can cause corrosion of the printhead. 

Inks for continuous ink-jet printers typically comprise dyes dissolved in water or solvent having salts added to make the ink conductive for electrostatic charging. Whenever waterproof printing is required, low boiling solvent inks ate used. For printers that ate used in office environments, water is used as the ink solvent. Using water-based inks, humectants may be added to inhibit drying of ink in the sump and surfactants ate added to wet the printing surface. 

The Hertz method [4] resembles the Sweet technology [4], since it also uses a continuous ink jet, the drops of which are charged by control electrodes. However, it uses charge intensity modulation instead of deflection and is especially suited for high resolution color printing. 

The impulse or drop-on-demand ink jet is a system of fundamental simplicity. It differs from continuous ink jet systems in two aspects (1) The image-forming ink droplets are not charged, so no deflection device is involved (2) Electrical signals are only used to control the moment when an individual droplet is needed. 

Generally, ink dyes for ink jet applications and writing, drawing, or marking materials are selected from food, acid, direct, sulfur, and reactive dyes. The choice of dye depends on the application and the ink used, whether it is aqueous, solvent based, or hot melt, and on the printer type continuous ink jet or drop-on-demand, piezo or thermal inkjet. 

In conductive inks for continuous ink-jet printers salts such as lithium acetate and potassium cyanate are added to adjust the specific conductivity. 

Continuous mode ink-jet systems are widely used in industrial market, mainly for product labeling of food and medicines. They have high throughput capabiUties, especially array continuous mode systems, and are best suited for high duty cycle applications. Few continuous ink-jet systems are multicolor (multifluid) but two-color systems are in use. 

The design of the print head in a continuous ink-jet system is shown in Fig. 9. In this example of a binary deflection system the drops are generated continuously by a piezoelectric device and pass between a pair of charging electrodes which, depending on the signal applied. 

FIGURE 19.66 Character printing with continuous ink jet. The deflection plate applies an analog voltage to steer the drop to the desired location unwanted droplets are undeflected and captured by the return gutter. (Source Durbeck, R.C. and Sherr, S. 1988. Hardcopy Output Devices. Academic Press, San Diego, CA. With permission.)... 

Binary charged continuous ink jet with its high droplet rate makes a simple gray scaling technique... 

Devices employing the vaporization of superheated fluid are known concurrently as thermal ink jet or bubble jet printers, the choice of name depending on the manufacturer. Since drop on demand ink jets rely on capillary refiU, their operational frequencies are much lower than for continuous ink jet devices. This stresses the importance of the compactness of the actuating system so as to achieve reasonable printing speeds via multiple nozzle printheads. The nozzles must also be precisely registered with respect to each other if systematic print artifacts are to be avoided. 

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