Continuous Inkjet Printing — Recirculating Inkjet Printing
Principle of Continuous Inkjet Technology
The principle of inkjet printing is to use a piezoelectric driving device to apply a fixed frequency and pressure to the ink inside the printhead, causing it to be continuously ejected. A vibration signal from an oscillator stimulates the ink stream to break into droplets, thereby controlling the size and spacing of the ink droplets.
The print signal generated by the character generator and analog modulator controls the charging electrode, forming both charged and uncharged ink droplets. A deflection electrode then alters the trajectory of the ink droplets, directing those required for printing onto the paper to form characters or graphic records. Ink droplets that are not needed for printing are collected and returned through a conduit.
For the deflection electrodes, some systems use two pairs of mutually perpendicular deflection electrodes to achieve two-dimensional deflection positioning of the ink droplets; other systems employ multidimensional control of the deflection electrodes, known as multidimensional deflection. This continuously recirculating inkjet system generates high-speed ink droplets, resulting in high printing speed. It can use ordinary paper and produce high-quality results on various printing media. It also facilitates color printing.
Continuous Recirculating Inkjet Printing
Continuous recirculating inkjet printing involves a printhead that ejects a continuous stream of ink droplets. Some droplets are selectively directed onto the substrate, while the remaining droplets are returned to the system for recirculation. This technology primarily achieves printing by controlling the presence or absence of an electric field: if a dot requires ink, no electric field is applied; otherwise, an electric field is applied to deflect the droplet, and the unused droplets are recycled through a recovery system.
This technology is widely used in high-speed marking, coding, and addressing applications, and is also suitable for large-format printing.
1. Continuous Deflection Inkjet Printing
Liquid ink passes through a small circular nozzle under pressure, generating a continuous ink stream at high frequency. The stream is then broken into individual ink droplets, which are electrostatically charged. Under the control of image information, the droplets are either directed onto the substrate or deflected and collected for recycling.
Due to the high velocity of the ink stream, it breaks into fine droplets. The droplet size and frequency depend on the surface tension of the liquid ink, the applied pressure, and the diameter of the nozzle orifice.
The landing position of the droplets is controlled by the deflection electrodes. The degree of deflection depends on the voltage level applied to the electrodes. There are two types of deflection:
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Fixed deflection (binary deflection system): When the voltage amplitude applied to the control electrode remains constant, the droplet deflection distance is fixed.
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Multi-level deflection (multi-value deflection system): When multiple voltage amplitudes are applied, the droplet deflection distance varies accordingly.
2. Continuous Non-Deflection Inkjet Printing
In this method, the deflected (charged) droplets are recovered, while the non-deflected droplets travel straight to form text or graphics. Because printing can occur only at fixed positions, this system requires multiple nozzles for printing or relies on moving the substrate during printing to complete the overall image formation process.
3. Electrostatic Breakup Inkjet Printing
Ink is continuously ejected from the nozzle, but the nozzle diameter is extremely small (10–15 μm). The ink stream spontaneously breaks into very fine droplets without requiring vibration or electrostatic stimulation.
Droplets required for printing pass through a shutter plate and directly strike the substrate. Droplets not needed for printing pass through an electrode ring, where they are induced with a large electrostatic charge, causing them to split again into an ink mist and lose directional control. These droplets are then blocked by the shutter plate and collected for recycling.