Continuous inkjet systems operate by forcing pressurized ink in a cylinder through nozzles in a continuous stream. Nozzle diameters range from 3 to 0.5 mils; the smallest nozzles may require up to 600 psi to eject ink. The ink stream is erratic and breaks up into individual droplets either naturally or by some applied stimulus such as ultrasonic vibrations. Electrostatic deflection is used to control the droplets, which either reach the page in a desired pattern or are deflected towards a "gutter" or "catcher" (see Figure 26.1). Deflection can be binary or variable.
Continuous inkjet printers recycle or discard unused ink, which can account for up to 98% of the droplets produced. Although preferable in terms of supply cost, recovery is greatly increased due to the need for pumps and ink purity protection measures (such as filters and solvent balance controls).
Figure 26.1 Continuous inkjet printing system. (From Sweet, RG et al. US Patent 3,373,437.)
Continuous inkjet systems are capable of very high-speed printing, with drop frequencies sometimes exceeding 100 kHz. At these speeds, it's difficult to control individual droplets: they tend to coalesce into larger droplets which can cause print quality issues. However, ideally (i.e. when controlled), this tendency can improve image quality: since each printed pixel may consist of multiple droplets, the pixel size can be controlled, resulting in a halftone feature. High-resolution halftones, however, significantly slow down continuous inkjet systems.
