Continuous Inkjet Imaging Technology — Deflected Continuous Inkjet | Array Inkjet | Modulated Inkjet
Continuous Inkjet Imaging Technology — Deflected Continuous Inkjet
The application of the basic principle of continuous inkjet (CIJ) printing dates back more than one hundred years. The earliest example involved using a single nozzle to eject droplets, which were then magnetically deflected onto a moving surface to form rough dot-matrix images. A single-nozzle printhead can be used on various substrates, whether flat or three-dimensional. This printing method is mainly applied for high-speed printing of simple information on products, such as printing production dates on assembly lines.
Deflected continuous inkjet can be divided into:
binary deflected continuous inkjet and multi-level deflected continuous inkjet.
Binary Deflected Continuous Inkjet Printing
The basic principle of continuous inkjet printing is shown below. A fine liquid stream ejected from a reservoir through a small nozzle is subjected to high-frequency vibration, causing it to break up into a uniform and stable stream of ink droplets. The vibration source used in CIJ nozzles is a piezoelectric crystal capable of generating thousands of droplets per second.
The flight path of the emitted droplets can be controlled in a specific manner to guide them onto designated areas of the substrate surface, thereby forming an image. After each droplet separates from the nozzle, it passes through a charging electrode.
Based on image data control, droplets are selectively charged or uncharged. Droplets corresponding to image areas remain uncharged, while droplets corresponding to non-image areas are electrically charged. The charged droplets are deflected in a subsequent electric field (deflector, plate-type capacitor) and returned to a collection system, whereas the uncharged droplets reach the paper and form the image.
The size and spacing of the droplets mainly depend on the nozzle diameter, liquid viscosity and surface tension, as well as the excitation frequency.
▲ Binary deflection inkjet principle
Multi-Level Deflected Continuous Inkjet System
In a binary deflected CIJ system, droplets are charged using a constant voltage at the charging electrode, resulting in only two states—charged or uncharged (binary).
In multi-level deflected inkjet systems, droplets can receive different levels of electrical charge within the charging system. As a result, droplets are deflected by varying amounts between the deflection plates according to their charge intensity, as shown below.
This makes it possible to use a single nozzle to image a short line segment—for example, by deflecting droplets into 16 discrete positions. When drawing lines in this manner, the line height depends on the distance between the printhead and the substrate surface. Increasing the distance reduces resolution but increases the drawable height.
The resolution in the printing direction is jointly determined by the printing speed and the droplet frequency.
▲ Multi-value deflection inkjet principle
In principle, a two-dimensional deflected inkjet system is feasible. In such a case, the substrate remains stationary while the deflection mechanism prints two-dimensional characters in two directions.
However, using a single-nozzle inkjet device imposes limitations on image complexity and size. While it is sufficient for simple, low-resolution text, it cannot print large-format or highly complex images
Continuous Inkjet Imaging Technology — Array Inkjet
In continuous array inkjet printing, the printhead consists of multiple nozzles arranged in an array configuration. Each nozzle emits a continuous stream of ink, and every individual droplet within the stream can be independently controlled. In practice, inkjet printing is accomplished by the combination of two electronic subsystems: one printhead responsible for ejecting fine ink droplets through a single row of small orifices, and another unit functioning as a charging device to control the deflection of the ink stream.
For example, a single row of micro-orifices can be etched onto a metal plate. The horizontal distribution density of these orifices determines the resolution of the inkjet printing system. The ink inside the ink chamber is broken into a sequence of fine droplets through the resonance vibration of a piezoelectric crystal. Each nozzle continuously emits an ink stream, and each droplet within that stream is independently controlled by the charging device. The droplet size and spacing are uniform, enabling resolutions up to 300 dpi, with 8 grayscale levels per dot.
The oscillation frequency of the piezoelectric crystal determines the precise rate of droplet formation. At a frequency of 100 kHz, each printhead can generate approximately 100,000 droplets per second.
The typical length of an array is about 4.25 inches, with approximately 240 nozzles per inch, totaling over 1,000 nozzles. In practical applications, if a wider print width is required, multiple nozzle arrays can be assembled together to output standard-width sheet-fed prints.
The droplet ejection speed of the system is extremely high, allowing printing speeds exceeding 500 sheets per minute. Therefore, high-speed continuous array inkjet printheads are widely used in sheet-fed printing presses, web presses, collators, folding machines, and other inline or offline equipment.
However, compared with electrostatic imaging printing systems, the resolution of continuous array inkjet printing machines is relatively lower. Image quality may also be reduced due to ink splashing and feathering on paper. Additionally, the requirement for ink circulation pipelines and high-pressure pumps limits the lateral density of nozzle arrangement, making it difficult for such systems to exceed a resolution of 300 dpi.
Continuous Inkjet Imaging Technology — Area-Modulated Inkjet
Continuous area-modulated inkjet printing is a variation of the continuous ink method. It adopts an area-modulated droplet control approach, in which multiple droplet streams are directed toward the same printing point to produce a reproduction effect similar to gravure halftone cells.
Because this method uses area-modulated droplet control technology, it delivers excellent imaging results that approach photographic quality. It is suitable for high-quality color image output. However, the printing speed is relatively slower.
This type of inkjet printing equipment can achieve a maximum output format of 760 mm × 1010 mm, which is larger than the A1 paper size.