inkjet printers are supplied using various technologies and are used for small or large character marking, depending. Continuous inkjet is used primarily in industry, while DOD, or drop-on-demand, printer can be configured as a piezo, pressure valve or bubble jet printer.
Using continuous inkjet, printed characters are made up of individual ink drops. The feed motion of the product is used to spray the numbers or letters one after the other onto the product. This is a non-contact process, i.e. without contact with the product. Today, this method of marking is used most often in the market to print expiry dates, like those found on yoghurt pots or drinks bottles.
Breaking up into individual drops
The inkjet comes out of the printhead through a nozzle and is broken up using a piezoelectric crystal located behind the nozzle. An ultrasonic signal of a fixed frequency is generated and breaks the inkjet into tiny drops. Depending on the voltage, the inkjet is changed from a solid stream into ink drops that separate from the stream and then break up into tiny individual drops.
Recycling of unused ink
When they have left the charge tunnel, each ink drop then passes a deflection plate. The positively charged deflection plate attracts the negatively charged ink drop and deflects it from its original path. The degree of deflection depends on the level of negative charge on the ink drop: the higher the charge, the greater the deflection. Uncharged drops continue their straight trajectory to the return block and are then fed back to the ink module for reuse.
From drop to text
Characters are made up of a matrix comprising vertical strokes of ink drops. For example, five vertical strokes, each with a height of up to seven drops, form a character in a 5 x 7 matrix. The level of the charge on an ink drop determines its position in the vertical stroke. The movement of the printing surface provides horizontal alignment of the strokes, the timing of which is controlled automatically by the printer.
With drop-on-demand (DOD), instead of an inkjet, only the drops that are required for printing are produced.
With bubble jet, water-based ink is used. The water is heated until bubbles form, and the ink is forced through the nozzle.
With piezo printers, an electrical voltage is generated, causing the piezo crystals to expand, forcing the ink through a fine nozzle. The drop volume is regulated via an electric pulse.
Pressure valve printers are only used in industrial applications. The valves, which are integrated into the nozzles, only open to allow drops to leave the nozzle.
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