Ink jet priting

The term ink jet is used to descibe a number of relatedtechnologies in which very fine droplets of ink are propelled form a print headthroughtair on to a supstrate to from a printed image. All the technologies share the following characteristics:

Non-contact. Moving or delicate substrates can be printed succsessfully because only ink droplest touch the surface.

Variable data. Messages can be continually varied so that times, dates, batch codes and similar can be changed on-line and in real time.

Speed . depending upon the technology, itmes moving at any speed up to about 5 m/s can be printed – sufficent for most production procresses.

As in all things there are other technologies which may suit particular applications but, for sheer breadth of capabilitry, ink jet is hard to beat.

The knowledge required to construvt an ink jet printer is not new. In fact the ability to break up streams of liquid into droplest, induce a charge and then deflect them was developed during the 19th century. Similarly, piezo electric materials have been around for some time and are in common use in, for example, sonar transducers and cigarette lighter.

However, the catalysts thet brought about the development of ink jet tecnologies were the:
. need to date and batch code products ( valve & continuous jet printers )
. requirement for Pc printers ( impulse jet printers )
. availability of the microprocessor.

Starting in the 1960s a number of weird and wonderful technologies emerged, flowered and withered away. The survivors today fall into three main types: valve jet, impulse jet and continuous jet.

These three main technologies tend to overlap, with each having its own strengths and weaknesses which, to a large extent, determine the applications for which they are best suited.

The intention of this paper is to explain each method and help customers select the most appropriate for their application.

VALVE JET

This method is the easiest to implement and has, over the past two decades, found its major application in the area of outer carton and tray overprinting.

Basicly a unit comprises an ink system containing ink at a fairly low pressure, an electronics cabinet and a printhead attacked to both by a flexible conduit.

Ink from the system is fed to the printhead where it is supplied to nozzles (generally 7 or 16 nozzles with a typicak diameter of 200um) via simple open / close valves. When a drop of ink is required the electronics open the relevant valve and the ink is ejected by pressure. This is sdown schematically in Diagram 1 below:



Velve jet system are easy to consturct, in that they are mechanically simple and the selection between competing manufacturers will often be made by comparing the user interface ( i.e. how easy it is to programme), print capability / flexibility / quality and the range of inks acailable.

Print quailty is often variable because ink remains in the nozzle until used, which can cause blockages if it dries in a channel that is only used infrequently. The systems work best with water-based inks and on absorbent substrates. A number of manufacturers produce inks for non-porous surfaces which are designed to dry at a fster rate than water-based inks but again blockages can occus and drying times are still relatively slow, of the order about 30 seconds.

Overall, these system work well provided that print quality is not an issue and regular cleaning is carried out. Althought the initial cost is low, the cost of ownership over one or two years is high and as a result this technology is increasingly being displaced by impulse jet.

Impulse Jet

Impulse jet printing splits into two main technology types – piezo and bubblejet – bur the implementation of these technologies varies widely.

Impulse jet printheads were originally developed for the office printing market where they are now well accepted and produce excellent results.

Although impulse jet printing is apparently simple in concept, it is worth noting that the orginal patents were filed in the 1970s and, despite the tremendous technical resources deployed by companies such as Canon and Hewlett Packardi it was not until the 1990s that reliable and cheap products really hit the market. So, the technoloy is not quite as easy as it looks and care needs to be taken when assessing the implications of taking a product designed to print on to clean paper at a fixed print distance in the office, into the less conrollable operating conditions of a factory environment.