INK JET PRINTING

Navanath Pingle, Laxmikant Jawale, Prashant Vispute.
Department of Fibres and Textile Processing Technology
University Institute of Chemical Technology
University Of Mumbai, India

Abstract

Whilst, the India has abolished the textile quota regime of textiles, one has to face tremendous amount of competition from producers of all over the world. One has to be very productive, economical, quality conscious, environment friendly with assured reproducibility. Modern technologies are going to play very major role in this. Ink jet printing is one such promising technique, which will be a critical tool in the hands of production units. In this article attempt is made to familiarize ink jet printing.

Introduction
Drastic changes are taking place in the textile printing; quick response, globalization and ecology impose substantial demands on the different components of the printing process. They force the textile printers to focus on: shortening and flexibility of the pre-print process; printing right first and the next time, reducing down time of the printing machine, short run production, and stock risks. Consumers are demanding a greater variety of colour and design. They want fabrics that express their individuality in their environments and in the clothes that they wear. Today's ecological stringent demands ask for eco processes minimizing waste of raw materials and pollution of the environment. In short these demands have common denominations: flexibility and versatility. Conventional printing lacks the above demands. The answer lies in digital printing. The jet-printing technology has yet to make the leap into mainstream. Although digital printing is already claimed as the most significant technological advancement to the textile printing industry in over 30 years, the adaptation of these new systems has been much slower than many had anticipated or expected, due to specific technological limitations (hardware, print head or nozzle, ink classes). These hurdles must be overcome for Commercial exploitation of these new technologies.

INKJET TECHNOLOGY

Ink-jet printing involves squinting droplets of ink, which make contact with substrate image. Ink-Jet can be divided into two major technology types: Continuous and Drop-on-Demand. Each of these subdivided further as shown in figure:


Fig 1. TYPES OF INK JET PRINTING

l.Continuous Ink-Jet Printing (CIJ):
As the name implies, in Continuous Ink-Jet systems a continuous stream of ink droplets is ejected from nozzle on to the substrate. Two designs are possible in the designing of this method. In the first design, charged ink droplets are deflected on to the paper to form the image and the uncharged droplets are collected in gutter. This is called as raster scan continuous Ink-Jet method. In the second design, the uncharged ink droplets form the image and the charged ink droplets are deflected into the gutter. This is the Binary Continuous Ink-Jet system. While in Hertz technology, fine mist of the uncharged droplets form the images. Hertz Technology is more suited to color printing than the previous two methods.1
Continuous Ink Jet printing is further divided into two main types viz; Piezoelectric and Thermal Excitation.

Fig 2. Principle of continuous stream ink-jet printing (raster scan method/the Sweet system)

A) Piezo-eleetric Technology
Piezo-electricity is a phenomenon of producing electricity by application of pressure on a crystal, which is capable of conducting electricity through it.4 This method has been suitably exploited for ink jet printing technology. Piezo ink jet printing relies on different principles for the expulsion of ink from the cartridge nozzles. In this technology, an electrical charge is applied to the cartridge nozzles which excites a small piezo crystal that is inside. As the piezoelectric crystals are stimulated, the crystals change shape and squeeze the ink chamber. This action is similar to the action of squeezing an oilcan, and forcefully expels the ink from the nozzle.

(a) Continuous ink jet-binary deflection. (left)
(b) Continuous ink jet- multiple deflection. (middle)
(c) Continuous ink jet - Hertz method (right)

After leaving the nozzle, the drops are electically charged by an amount that depends on the image to be printed. The drops then pass through an electric field to cause then to deflect. There are two ways of deflecting the drops in piezoelectric- driven Continuous Ink Jet. In the binary deflection method droplets are directed to a single pixel location in the medium or to the recirculation gutter. In the multiple deflection method the deflection is variable so the drops can address several pixels. These two concepts are illustrated in the Fig.3 (a) and (b).

In the Hertz method, the amount of ink deposited per pixel is variable. This is achieved by generating very small drops at the speed of 40m/s with excitation frequencies of over 1 MHz (see Fig 3 (c)). The drops not intended to reach the medium are charged ad deflected to the gutter. The printing drops are given the smaller charge to prevent them in merging in flight.

Since the piezoelectric process does not utilize heat, the cartridge life of these printers is greatly expanded, cartridges should last a minimum of one year under heavy usage. Piezoelectric print heads can use a wider range of inks than thermal inkjet printers because the heat is removed from the process. This means that solvent-based ink systems and pigmented-ink formulations will be more readily available, which increases the development capabilities for better inks in the future. Although piezo is currently the lesser-utilized technology, many experts predict that the long-term development of ink jet print devices will use the piezo technology because of the greater through-put speeds offered and the wider latitude with the types of inks that can be developed 6.

3. DROP ON DEMAND (DOD)
Drop-on-Demand, or impulse, inkjet systems differ in two major respects from continuous Ink-Jet systems. First, all the ink droplets are used to form the image none are wasted. Ink droplets are ejected only where a dot is required on the substrate, i.e. they are produced "on demand ". Secondly, the droplets are not charged. Hence there is no deflection involved. Drop on Demand Ink Jet systems may be subdivided into two broad types, namely piezo and thermal (or bubble jet). In DOD printing a significant proportion of the solid ink resides on the surface of the paper giving the print an embossed feel.(2,3,4)

Principle of Drop On Demand printing (5)

BUBBLE JET PRINTING
Bubble-Jet or thermal technology is a well-known technology. The technology relies on a thermal pulse to generate the ink drop. This technology was the first of the drop on demand. The technique boils the water content of the ink and the resulting steam pressure forces a droplet of ink out of the nozzle. In these engines, the computer signal heats a resistor to a high temperature(>360 C) which creates a vapor bubble in a volatile component in the ink, the vapor bubble expands and exits the nozzle followed by a contraction of the bubble causing a drop of ink to be ejected on the textile substrate. The vapqur bubble must then cool and collapse allowing the ink chamber to refill from a reservoir. Cycle time is limited to approximately 10,000 drops per second and the volume per drop of ink is typically 150 to 200 Pico liters.

The main advantage of the thermal ink jet technology is the low cost of nozzle fabrication. It is made using the mass production technique based on the integrated circuits. The system restricts the use of binder containing pigment inks. The major problem with the thermal ink jet is the high nozzle and resistor failure rate resulting from rapid thermal cycling. As the heater to boil the water has to work in a semi-explosive way, the temperature can rise up to 360 C, which can cause the nozzle to burn out. The high temperatures cause often decomposition of ink components, which leads to poor heat transfer and / or nozzle clogging. Therefore only thermal stable inks can be used. These defects are unpredictable.

Principle of Bubble jet printing.