| PRINCIPLES OF DIGITAL PRINTING
ZUND UVJET FLATBED INKJET PRINTING MACHINE COURTESY OF BP GRAPHICS LIMITED
THERMAL TRANSFER PROCESS
As the name suggests the thermal transfer process uses a thermal array to transfer colour that is a mixture of resins and pigments or dyes from a carrier ribbon onto the substrate.
A simple application of the thermal transfer process is single colour printing of bar codes where edge definition and density of colour is important along with reasonable speed. The two transfer processes are thermal wax and thermal dye diffusion.
Thermal wax transfer is the simplest but it does not give the range colour that is possible from dye diffusion.
A thermal wax transfer printed image adheres to the substrate using heat activated adhesives within the formulation of the colour.
Dye diffusion is achieved by the dyes in the colour being heated by the elements in the array and turning to a gas which penetrates the substrate and colours it when it returns to its solid state.
The substrate has to be receptive, normally a polyester (or similar polymer) coating. In both cases the substrate has to be flat because any irregularities that will not be overcome by the pressure applied will not take the print. The dye transfer process is known as sublimation.
INK JET PROCESSES
There are two types of ink jet printing. Continuous Ink Jet and Drop on Demand Ink Jet.
CONTINUOUS INK JET
Ink is pumped through an orifice and then given an electrical charge that is then used to produce a deflection as it passes through deflection plates. The dots deflected are re-circulated whilst those not deflected carry onto the substrate. Using this method it is possible to create simple designs on a moving substrate.
CONTINUOUS ARRAY INK JET
The quality of print obtained by continuous ink jet is a limiting factor. To improve this, nozzles are lined up in a row. It is even possible to put two rows together. There would be 240 per inch on each row and each orifice would have a discrete electrode that can deflect any or all of the droplets exiting from the orifice. The droplets are emerging at between 50,000 and 100,000 droplets per second. The very high rate that these droplets are produced mean that the rate of print can be as high as 500 feet per minute. This results in an image quality that is slightly less than the lower end laser printers.
DROP ON DEMAND INK JET PROCESSES
These are the processes that are the most suitable for use in the market to which this section is addressed. In these processes, ink is ejected from very small orifices directly onto the substrate to be printed.
There are two ejection mechanisms heat and piezo. Heat is the most common but piezo gives the opportunity to use the widest range of materials in the ink formulation.
THERMAL INK JET
For thermal ink jet the ink system needs to be water based. The ink passes from a reservoir into the ink chamber. A heating element heats the ink rapidly and forms a bubble of water vapour. This bubble in the chamber forces the ink through the orifice and it forms a droplet the droplet impacts on the substrate. The chamber has then to be refilled before the next droplet of ink can be ejected. This refilling process linked to the heating and cooling of the heating element means that the process is relatively slow.
PIEZOELECTRIC INK JET
Instead of using a heating element to create a bubble in the ink it is also possible to alter the size of the ink jet chamber and effectively squeeze a droplet of ink out through the orifice. The chamber is constructed with one or more of the walls containing PZT (Lead Zirconate Titanates).
The big advantage of piezo technology is the range of materials that can be used as the printing medium.
The Technical Paper is in PDF format and may use quicktime animations
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THERMAL TRANSFER PROCESS
CONTINUOUS ARRAY INK JET
CONTINUOUS INK JET
THERMAL INK JET
PIEZOELECTRIC INK JET
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