ink jet printers

The continuous ink jet method is used commercially for marking and coding of products and packages. The idea was first patented in 1867, by Lord Kelvin and the first commercial model was introduced in 1951 by Siemens.

In continuous ink jet technology, a high-pressure pump directs liquid ink from a reservoir through a gunbody and a microscopic nozzle, creating a continuous stream of ink droplets. A piezoelectric crystal creates an acoustic wave as it vibrates within the gunbody and causes the stream of liquid to break into droplets at regular intervals - 64,000 to 165,000 drops per second may be achieved. The ink droplets are subjected to an electrostatic field created by a charging electrode as they form, the field varied according to the degree of drop deflection desired. This results in a controlled, variable electrostatic charge on each droplet. Charged droplets are separated by one or more uncharged “guard droplets” to minimize electrostatic repulsion between neighboring droplets.

The charged droplets pass through an electrostatic field and are directed (deflected) by electrostatic deflection plates to print on the receptor material (substrate), or allowed to continue on undeflected to a collection gutter for re-use. The more highly charged droplets are deflected to a greater degree. Only a few percent of the droplets are actually used to print, the majority being recycled.

Continuous ink jet is one of the oldest ink jet technologies in use and is fairly mature. One of its advantages is the very high velocity (~50 m/s) of the ink droplets, which allows for a relatively long distance between print head and substrate. Another advantage is freedom from nozzle clogging as the jet is always in use, therefore allowing volatile solvents such as ketones and alcohols to be employed, giving the ink the ability "bite" into the substrate and dry quickly.

The ink system requires active solvent regulation in order to accommodate for solvent evaporation during the time of flight (time between nozzle ejection and gutter recycling) and from the venting process whereby air that is drawn into the gutter along with the unused drops is vented from the reservoir. Viscosity is monitored and a solvent (or solvent blend) is added in order to counteract the described solvent loss.

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