Indian Journal of Fibre & Textile Research Vol. 30, March 2005, pp. 99-113

Review Article

Advances in ink-jet printing technology of textiles

S K Malik", Savita Kadian & Sushil Kumar

Department of Texti les, The Technological Institute of Textile & Sciences, Bhiwani 127021, India

Received 11 February 2004; accepted 21 April 2004

A brief account of various paths used in the development of ink-jet printing for textiles has been reported. Thi s technology has achieved considerable success till the production stage and a lot of R&D work is being done to improve it further. It is a clean and environment-friendly process that reduces printing duration and costs by increasing productivity of pre-production in printing process. The potential applications of ink-jet technology emerged in the past few years have also been presented.

Keywords: Digital Printing, Ink-jet printing, Textile printing IPC Code: Int. C1 7 B41J27/00; D06P3/00; D06P7/00

1 Introduction The art of textile printing is probably as old as the

civilization itself. Excavations revealed wooden printing blocks in Egypt at around 400-600 A.D. India excelled in printing at around 13th and 15th centuries. Then new age of printing grew rapidly in U.K (Lancashire). As a major breakthrough, Peter Zimmer of Austria introduced rotary screen printing machine in 1963 (ref. 1). Before that, the majority of printers used to follow a totally manual approach to the production of textile prints from initial stage to bulk production. Now worldwide, 60% of the fabric printing is done by rotary screen printing, 18% by flat bed and 22% by conventional screen printing methods . In 1980, CAD was introduced in textile printing. The adoption of CAD has lead to better quality and flexibility in design development. At present, textile printing covers a huge market proportion , i.e. 28 billion square yards (23,000 km2

)

of materials are printed worldwide each year. In spite of its developments, the dominant rotary

screen printing method has several limitations, like color and pattern changes require long process set-up time, screen preparation is slow and expensive, and the screens have relatively short lives and require considerable storage space even when not being used.

Thus, a new technology for fabric printing was needed which can permit frequent style and color change with minimum downtime for chancre over.

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"To whom al l the correspondance should be add ressed. Phone: 09416087919; Fax: +91 - 1664-243728; E-ma il: maliksk88@yahoo.co.in

The revolutionary printing technique 'Ink jet printing ' has the potential of meeting these requirements2

.3 .

2 Development Path The mechanism by which a liquid stream breaks up

into droplets was described by Lord Rayleigh in 1878. In 1951, Elmqvist of Seimens patented the first practical Rayleigh break-up ink-jet device. This invention led to the introduction of Mingograph, one of the first commercial ink-jet chart recorders for analog voltage signals. In the early 1960s, Dr Sweet of Stanford University demonstrated that by applying a pressure wave pattern to an orifice, the ink stream could be broken into droplets of uniform size and spacing. When the drop break-off mechanism is controlled, an electric charge could be impressed on the drops selectively and reliably as they are formed out of the continuous ink stream. The charged drops when pass through the electric field are deflected into a gutter for recirculation, and the uncharged drops could fly directly onto the media to form an image. This printing process is known as a continuous ink jet. By late 1960s, Sweet's inventions led to the introduction of A. B. Dick VideoJet and the Mead DIJIT products. In 1970s, IBM licensed the technology and launched a massive development program to adapt continuous ink-jet technology for their computers printers. The IBM 4640 ink-jet printer was introduced in 1976 as a word processing hardcopy-output peripheral application. In ] 977, Zoltan and Kyser and Sears were among the pioneer inventors of the drop-on-demand ink-jet systems. In 1984, Hewlett-Packard commercialized the Thermal

100 INDIAN J. FIBRE TEXT. RES., MARCH 2005

ink-jet printer. It was the first successful low cost ink­jet printer based on the bubble jet principle. Hewlett­Packard named the technology thermal ink jet. All this research was done for the printing of paper. The first ink-jet printer for textiles (TruColour) was launched by Stork Brabant By at ITMA 1991 in Hanover2. The key players now in the development of this technology for textiles are Stork, Toxot, Seiren , Zimmer and Reggiani Machine S.p.A (ref.4).

3 Technology Ink-jet printing is a non-contact technology where

micro droplets of liquid are ejected through microjets to impact a substrate at a precise location to create an image. Its functional principle is that the pattern is drafted or transferred from any digital model or by scanner from standard photographic models or drawings to the fabric via an ink-jet printer. A digital photograph just taken can be directly entered and printeds

,6. The essential elements of a textile ink-jet printing system are:

(i) An assembly of one or more ink-jet print heads, which generate the streams of microscopic ink droplets and aim them to their target. There are many types of ink-jet head technology in the market today and innumerable further sub-variants , each with distincti ve economic/ technical advantages and limitations for different fabrics and application -

(ii) A machine or system which feeds and presents the fabric to the traversing ink-jet heads and ensures perfect registration and alignment throughout, even for delicate and unstable fabrics , such as knits or fine si lks. If required, thi s machine may also pre-heat and dry or set the printed fabric before filially rolling-up the output smoothly and even with tension.

(iii) Post-treatments associated with the printing operation, e.g. baking, steaming and/or washing. These processes are similar to those used for conventional textile prints, except that the process is undertaken with a much smaller batch size, typically a few tens of metre or even individual sample length. Critically, such processing steps must not negate the quick response benefits of digital textile printing which show its economic and market attractiveness.

(iv) Software including printer drivers , raster image processing (RIP) and color management systems to convert computer-based designs into the electronic signal s which control the scanning ink-jet head and machine. These systems can also ensure faithful and reproducible results with different batches of fabric, and provide a total interface with the other

components of the digital design, sampling and production environment.

(v) Ink-jet inks comprising pigments and/or dyestuffs, which need to be milled and filtered to much finer tolerances than for conventional screen or roller printing. Ink-jet inks must be formulated with precise viscosities, consistent surface tension, specific electrical conductivity and temperature response characteristics, and long shelf life without settling or mildew growth7.

(vi) Textile substrates which generally need some pretreatments or special preparation to ensure proper take-up and absorption (without excessive wicking) provide adequate adhesion and/or reactivity and be compatible with any post-treatments or conditions of use.

The ink-jet printing technology may be divided into various basic technologies (Fig. 1)

3.1 Coarse Resolution Type The coarse resolution type is based on valve control

technology and is used in the carpet industry . The resolution of all these jet printers is relatively coarse, reaching a maximum of 40 dpi (dots per inch), which is unacceptable in the textile fabric printing. Millitron system and Chromo jet (Zimmer, Austria) are the two mai n commercially available sys tems. Millitron system uses an anay of jets with continuous stream of dye liquids, which can be deflected by a controlled air jet, whereas Chromo jet uses computer activated on/off valve systems to control the flow of liquids.

3.2 Fine Resolution Type

The area of fi e resolution has attracted most recent research activities. This type is based on following two basic technologies:

3.2.1 Continllolls Stream Printing In continuous stream printing, which is also called

the synchronous droplet ejection technique, droplets are continuously produced and selectively charged for printing of designs.

In this system, ink is forced at a high pressure (Fig. 2) through a small nozzle (15 /lm diameter), providing a resolution of up to 2880 dpi . The jet spacing is 12 needles/inch. Continuous stream printers have a high rate of droplet ejection (around 50,000-1,00,000 droplets/second). The emerging stream of ink is broken into small droplets by vibration. After creation, the droplets need to be selectively controlled so that the images can be formed. To do this, the droplets are selectively charged and deflected while passing through high voltage plates. A variable

MALIK et al.: ADVANCES IN INK-JET PRINTING TECHNOLOGY OF TEXTILES 101

Elmjet Scitex Image

Videojet Diconix Domino Amjet Linx

Hewlett-Packard Canon Olivell i Xerox

Iris Graphics

Hitachi

Lex mark .--------"T-L-----.---------, Squeeze Tube

Siemens Gould

Tektronix Sharp Epson On Target Tech.

Data products Epson Trident

Spectra Xaar Nu-Kole Brother Microfab Tech. Philips Topaz Tech.

Fig. I-Ink-jet technologies map

Piezo crystal T

0000000000000 ( I ()qoi I 0 = 0

Deflect~n 0 Plates Gutter

Substrate

Head Assembly Data

Charge Electrodes

Fig. 2-Principle of continuous ink-jet technology (Binary method)

electric charge is imparted to the droplets by placing a charged electrode near the point of jet breakup. The charged droplets are then deflected when they subsequently pass through an electric field created by applying high voltage between a pair of electrode plates. The cost of continuous ink-jet heads is about $5,000 per head .

There are four possible methods of obtaining a design, namely multilevel, binary, hertz and microdot. In multilevel method, the charged droplets are deflected onto the substrate in a predetermined manner and the uncharged droplets are collected in feed tank and recycled (Fig. 3). This method is also known as Raster-scan method.

102 INDIAN J. FIBRE TEXT. RES., MARCH 2005

Tj'Ti ll Charge

electrode signal

[}~ ... ~, L-----1 ; 0--- Charge electrode

Gutter ~=====.J

--_t 16 Raster

Fig. 3-Principle of continuoLis stream ink-jet printing (Raster­scan method)

In binary method, uncharged droplets are used for printing while the charged droplets are collected in the feed tank.

Both these systems are based on methods developed by Sweet in 1964. The binary method was further developed by Hertz and his group at Lund University to other new methods8

- ' 3 named as Hertz method and Microdot method.

Commercial printers based on continuous stream technology are given below:

Stork TruColour Jet Pril1ter

This printer is based on the Hertz continuous stream technology (binary method). It is designed for sample printing on 100% cotton fabric . Here, dye formulation is pumped at a constant pressure through a nozzle of 14.4 /lm diameter. The continuous stream is broken up into droplets by modulation at 625 kHz (6,25,000 droplets/s). The droplet train then enters an electrical field between two 1500 V plates and the charged droplets are deflected, picked up and fed back into the colour container for recycling. Uncharged droplets pass through the deflection plates in a group of up to 15 and reach the substrate in the desired pattern in an area covering only 0.1 mm2 or one pixel, producing smooth continuous tones.

The three substractive primaries plus black are used to produce the needed shades via overlap printing. The most important feature of TruColour jet printer is the incorporation of high purity Procion P (reactive dyes) into the ink formulations.

Storks (charged drop) TruColour 4000 is a well­established printer and has been promoted as a part of an integrated design/pre-print assessmentlbulk­printing formulation system. The other printers of this series are: Storks TruColour 4001 and Storks TruColour 4002. The Storks TruColour 4001 has a print format of 1020x690 mm and the Storks TruColour 4002 can even print 1850 x 1020 mm. To facilitate handling of samples of this size, the front of the Storks TruColour 4002 is equipped with a substrate loader that can hold a roll of cloth. Both these printers are suitable for printing on cloth and paper. The distance from the print head to the substrate can be adjusted to 8 mm. The specifications of Storks TruColour printer are given in Table 1.

Amethyst Jet Pril1ter

This printer was also introduced by Stork. It is a true digital production printer with maximum printing width of 1.60 m. Amethyst ink-jet printer is a multilevel deflected technology operated unit. This system is designed for sample printing on cotton, viscose and silk. The main specifications of Amethyst jet printer are given in Table 1.

Toxot Jet Printer

The technology of this printer is based on the electrostatic deflection of color droplets from a continuous stream. These printers are based on small print module, printing 1.34 inches (34 mm) in width using light jets. Print resolution is 180 dpi and continuous printing speed is 21.8 yards (20 m) per minute. The Toxot technology is used for textile applications from sample preparation to short run production.

The interesting feature is that UV curable binders have been developed for ToxotlImage inks that allow complete fixation of the colors on textile substrates without a thermal post cure or after scour-wash-dry sequence I4

•15

•

Chromotex Printer

Zimmer, the pioneer in ink-jet printing for carpet, recently introduced the Chromotex SPM, a true colour printer with a printing speed of 1.5 mlmin. Multilevel charged drop print engine developed by Jemtex, which uses only spot colours, is said to produce larger drops than the other machines but yeild the desired definition of 100 or 125 dpi. The Chromotex uses a continuous flow system and prints through 80 /l diameter nozzle which stands 60 mm above the moving apron. The liklihood of clogging is low. The

MALIK et al.: ADVANCES IN INK-JET PRINTING TECHNOLOGY OF TEXTILES 103

Table 1- Specifications of Storks TruColour, Amethyst, Zircon and Amber machines

Specification Storks TruColour Amethyst Zircon Amber

Printing method Continuous ink jet, Continuous ink jet, Piezo, drop-on-demand Piezo, drop-on-demand binary method multi-level deflected

Printing head 8 8 8 7

Printing speed Hi gh, m% - I 16 10 4.6 Hi gh precision, m2/h - 8 3.5 1.8

Printing width, m 1.60 1.60 1.60 1.60

Co lor reproduction 16.7 million colors 16.7 million colors 16.7 million co lors 16.7 million colors (256 gradati ons)

Dimension 1.020xO.690 m2 (TC 4001), - 4.04 (w) x 1.30 (h) x -2.86 (w) x 1.24 (h) x -2.43 (w) x 1.1 6 (h) x 1.850x 1.020 m2 (TC 4002) 0.80 (d) m3 0.73 (d) m3 0.58 (d) m3

Ink Reactive dyestuff ink for Reactive dyestuff ink for Di sperse dyestuff ink for Reactive dyestuff ink for natural fibres natural fi bres polyester natural fibres and acid

dyes for polyamides

Computer Pentium n, 450 MHz, Pentium II , 450 MHz, Pentium II, 450 MHz, Pentium II , 450 MH z, 10 GB hard disk 10 GB hard di sk ECP Port 10 GB hard disk 10 GB hard disk ECP Port

Software Stork software or Microsoft Stork software or Stork software or Stork software or Windows JTR pl atform Microsoft Windows JTR Microsoft Windows JTR Microsoft Windows JTR

platform

volume of ink delivered (1400 picoliter) is up to 10 times greater than that in other systems with smaller nozzle.

Recently, Zimmer planned to introduce the Chromotex PM, a true production model capable of printing 100 m2/h using 48 jets per colour ' 6.

3.2.2 Drop-on-demand Printing

The majority of activity in ink-jet printing today is in the drop-on-demand method. It is an asynchronous technique in which individual droplets are ejected in response to electrical impulses. This technology produces an ink droplet only when required and fires this on to substrate as shovm in Fig. 4. This approach eliminates the complexity of drop charging and deflection hardware as well as the inherent unreliability of the ink recirculation systems required for the continuous ink-jet technology . These systems operate at lower droplet production rates than the continuous stream systems as the maximum ejection rate is about 25,000 droplets/s/nozzle.

These systems use solenoid valves to control the flow of ink to an air stream that carries the drops to the substrate. Resolution in newly developed machines of this type is claimed to be 2880 dpi and almost continuous tone can be achieved by controlling the amount of ink at each spot in the image. These machines are limited to special textile printing

platform platform

Pressure wave ")0

~'6"":; 0

i~

Data pulse train

Substrate

Ink supply

Fig. 4- Principle of drop-on-demand technology

applications such as printing billboards, banners, draperies, carpets, wall hangings, etc.

In this group, there are four technologies­piezoelectric, thermal excitation, electrostatic and acoustic- of ejecting a droplet from a small reservoir behind the nozzle. Most, if not all, of the drop-on­demand ink-jet printers use either thermal or piezoelectric principle. Both the electrostatic ink-jet and acoustic ink-jet methods are still in the development stage with many patents and few commercial products available.

In piezoelectric printers (Figs 5 and 6), the computer imposes an electrical potential across a piezoelectric (ceramics) material, which causes contraction in the direction of electric field and expansion in the perpendicular direction. This deformation of the piezoceramic material causes the

104 INDIAN J. FIBRE TEXT. RES., MARCH 2005

Cylindrical piezo-electric

transduGer

v

Ink. cavity

Ink.

Nozzle

Substrate

Fig. 5- Principle of piezoelectric drop-on-demand ink-jet printing

Fig . 6- The basic configuration of a piezoelectric print head

ink volume change in the pressure chamber to generate a pressure wave that propagates toward the nozzle. This acoustic pressure wave overcomes the viscous pressure loss in a small nozzle and the surface tension force from ink meniscus so that an ink drop begins to form at the nozzle. When the drop is formed, the pressure must be sufficient to expel the droplet toward a recording media. The basic pressure requirement is shown in Fig. 7. The piezo, on removal of the potential, returns to its normal dimensions and the ink chamber is filled from an ink reservoir by capillary action. The cycle time of the piezo-based printers is limited by the ink replenishment rate and can be somewhat higher (14,000 cycles/s) than the thermal inkjet but drop volumes are usually somewhat smaller (as low as a picolitre) . The small drop size allows the piezo-based printers to produce very high resolution prints (2880 dpi is commercially avai lable). These printers also have the advantage of much greater (up to 100 times) print head life than the thermal based systems. The cost of piezoelectric heads ranges from about $30 to $3000.

In case of piezoelectric ink jet, depending on the piezoceramic deformation mode, the technology can be classified into four Inain types, namely squeeze, bend, push and shear. A squeeze-mode ink jet can be

V~ous

preswre r~se «0. 1 aun)

Dynamic pn:ssurt ofliq~li(\

(- 0.5 aIm)

Fig. 7-The basic pressure requirement for ejecting an ink droplet

Diaphragm .... Piezo ...----';JooRlL

ceramic

Fig. 8-A bend-mode piezoelectric ink-jet design

Diaphmgm

Fig. 9- A push-mode piezoelectric in k-jet design

designed with a thin tube of piezoceramic surrounding a glass nozzle. In a typical bend-mode design (Fig. 8), the piezoceramic plates are bonded to the diaphragm, forming an array of bilaminar electromechanical transducers used to eject the ink droplets. In a push­mode design (Fig. 9), as the piezoceramic rods expand, they push against ink to eject the droplets. In a shear mode print head, the electric field is designed to be perpendicular to the polarization of the piezodriver (Fig. 10). The shear action deforms the piezoplates against ink to eject the droplets 17- 19.

Thermal excitation is also known as 'bubble jet' or thermally activated ink-jet technology. It is the most successful method in the market today. Over 85 % of all ink-jet heads manufactured are thermal DOD because these are economical to manufacture but have comparatively short life. Depending on its configuration, a thermal ink jet can be a roof shooter (Fig. 11) with an orifice located on top of the heater, or a side shooter (Fig. 12) with an orifice on a side located nearby the heater. In these printers, the

MALIK et aL.: ADVANCES IN INK-JET PRINTING TECHNOLOGY OF TEXTILES 105

Fig. lO- A shear-mode piezoelectric ink-jet design

Heater

Ink

Pressure chamber

Fig. ll - A roof-shooter thermal ink-j et

Ink

Pressure chamber

I-!eater

Oriffce

J

Fig. 12-A side-shooter thermal ink-jet

computer signal heats a resistor to a high temperature (>350°C), which creates a vapour bubble in a volatile component in the ink. This vapour bubble causes a drop of ink to be ejected from the nozzle. The vapour bubble then cools and co llapses, allowing the ink chamber to refill from a reservoir. The whole process of bubble formation and collapse takes place in less than 10 ~lS. The ink then refill s back into the chamber and the process is ready to begi n again. Depending on the channel geometry and ink 's physical properties, the ink refill time varies from 80~s to 200 ~s. This process is shown in Fig. 13 . Fig. 14 also shows the same process by plotting the parameters including electrical pulse, temperature, pressure and bubble vol ume against time. Cycle time is limited to approximately 12,000 drops/s and volume per drop of ink is typically 150-200 picolitre. Thus, a single

Fig. J3-Drop formation process of thermal ink-jet

Nucleation --.

---.. ~ Cavitation TIME

Fig. 14--Pressure, temperature, and bubble volume change during a drop formation cycle of thermal ink-jet

thermal inkjet can deliver approximately 0.1 ml of ink per minute. The major problem with thermal inkjet is the high nozzle failure rate. The high temperatures required for rapid drop ejection cause decomposition of ink components on the resistor, which leads to poor heat transfer and/or nozzle clogging. The major advantage of thermal ink-jet technology is the low cost of nozzle fabrication . Thus, the thermal ink-jets offer low cost print heads but suffer from reliability and slow speed. The cost of thermal ink-jet heads is about $20.

Electrostatic technology has not yet achieved wide spread commercial use. In this technology, the electrically conducting ink is subjected to an electrical potential between the nozzle and the valving eiectrode. The potential causes a drop of ink to be pulled from the nozzle and to move towards the

106 INDIAN J. FIBRE TEXT. RES., MARCH 2005

valving electrode, which is immediately behind the substrate being printed. The volume of ink delivered is directly related to the width of potential pulse so that a very large gray scale is possible.

In thermal or piezo or any other drop-on-demand ink-jet method, one of the most critical components in a print head design is its nozzle. Nozzle geometry, such as diameter and thickness, directly affects drop volume, velocity and trajectory angle. Variations in the manufacturing process of a nozzle plate can significantly reduce the resulting print quality. Image banding is a common result from an out-of­specification nozzle plate. Various nozzle geometries designed for the ink-jet print heads are shown in Fig. 15 .

The important difference between DOD and continuous stream systems is that with the continuous stream system more than one drop can be directed at any pixel location . In some continuous systems, up to 15 drops of each of the four inks (black, cyan, magenta and yeIlow) can be directed onto any pixel. With the DOD machine jet printer, only one drop per pixel can be achieved. The use of a matrix of drops to form a super pixel, sometimes referred to as a dither pattern, produces half tones . Fig. 16 compares half tone production by a continuous stream jet (binary method) and the ordered dither pattern of a DOD printer ' 7-

22.

Commercial printers based on drop-on-demand technology are given below:

Zircon Printer

Stork also introduced Zircon that enables roll-to­roll printing of polyester fabrics on substrates of up to 1.60 m width with a resolution of 360 dpi. With Zircon, it is possible to produce the finished fabric from designs, which are directly supplied from the CAD system.

The printer uses a pre-treated polyester substrate and heat is applied after printing to fix the colour. In operation, it uses eight basic colours from which all designes are built up. Through these, a high percentage of shades from the complete colour spectrum can be achieved. It gives good results when pale pastle tints are used. The specifications of Zircon jet printers are given in Table 1.

Amber Jet Printer

Stork also introduced Amber, the ultimate entry level printer, for continuous digital printing on natural fibre fabrics with 720 dpi. Amber ink-jet printer is a piezo technology operated unit23

,24. The main

Cylindrical orifice Convergent orifice (Tektronix Sharp) (HP. Data products)

~t963 Tapered orifice Tapered with cylindrical

(Canon) exit orifice (Seiko-epson)

~~-Triangle orifice

(xerox) Square orifice

(IBM)

Fig. IS- Various types o f ink-jet print head nozzle designs

Dither method True halftone

Fig. 16-Comparison of conventional dither of half-tone production (via DOD printers) and true half-tone production (via binary method continuous stream)

specifications of Amber jet printer are gIven In

Table 1.

DreAM Printer

Reggiani Machine S.p.A of Bergamo recently presented this new high performance digital textile printing system after a series of test trials . This system is capable of digitally printing up to 150 m2/h of woven fabric at working width of 160 cm. The machine is designed for fabric lengths from 100 to 800 running metres, a field reserved to date for screen-printing. The piezoelectric printing heads, developed by this company, make possible a resolution of 600 dpi in textile printing and clear image presentation of enormous variety with "flying pattern changes" at a high production speed.

The high performance six-color digital printing system is equipped with printing heads each with 512 individual nozzles and the "Multiple Array Graphic Inkjet Color" technology, which can deliver up to

MALIK et al.: ADVANCES IN INK-JET PRINTING TECHNOLOGY OF TEXTILES 107

25000 color droplets per second per nozzle. The distance between printing head and fabric is continuously adjustable for all textile fabrics . Continuous dye feed with colors being changed during operation is a special feature. Provision is made for extending the machine for 8 colors .

A medium production output range of about 200 m2/h has been achieved and the efforts are being made to achieve 400-800 m2/h . The break-even point between digital and screen printing is currently be~ween 600 and 1000 running metre fabric length, depending on the number of screens employed. The new digital industrial printing machine, a joint development by Reggiani Machine Aprion Digital and Ciba Speciality Chemicals, is equipped with a roll-to-roll material feed system. The fabric is precisely positioned by a newly developed "Dynaplast" system and fed through to the printing position. The system includes an integral drying and fixation system and an adapted fabric take-up system complete with fabric inspection.

Sieren is another largest ink-jet fabric printing company in Japan . They are very secret about their digital printer project, only describing it as an exceptional production system. Unconfirmed reports from Japanese sources have mentioned its capabilities of printing on cotton, silk, polyester and polyamide using reactive and disperse dyes. It appears that the system uses DOD heads and one related patent describes a 180 dpi print resolution. The company has already started supply to the market with digitally printed fabrics2s

•26

.

DuPont Jet Printer 3210

The DuPont Artistri textile pnntmg solution comprises following three components :

(i) The DuPont ink-jet 3210 eight colour ink- jet printer, manufactured for DuPont by Vutek, features a new Nova Q print head for water-based inks from Spectra. It is the first aqueous print head that Vutek has built in any of its units. The 3210 unit for textile printing is based on Vutek's 3360 superwide printer. The printing speed is 30 m2/h with width of 3.2 m (126 inches). The width of fabric rolls to be printed is 3.05 m (120 inches). One of the technology breakthroughs thought to be significant in the engineering of 3210 printer is an online heating process, allowing inks to be printed without post processing requirements. The approximate cost of DuPont ink jet 3210 printer is $650,000.

(ii) The Artistri 3210 pigmented water-based inks for textile printing-Eight inks (cyan, magenta,

yellow, black, light cyan, light magenta, orange and green) have already been developed by DuPont.

(iii) The Artistri color control and management software (Artistri CCMS)-This colour management system is designed to interpret most common textile CAD files , including separated files used for screen engraving, and prepare colours for eight colour process textile printing. Artistry CCMS is designed to create process colours that will match the existing palette of 12-14 screen printing support colours that dominate textile designs today27.

Crystal Jet Printer

The CaIcomp Topaz Crystal Jet print heads are a hybrid or coupled piezo technology which combines shear mode with bend/normal technology to squeeze and push ink through nozzles. Three PZT side walls and roof collapse in on the ink channel to eject droplets. They use separated rather than shared walls and can generate a number of different drop sizes per pixel for 12 gray levels to be expanded to 16. They consist of 256 nozzles per print head with one print head per color. This produces 180 dpi with one pass, 360 dpi with two passes and 720 dpi with 4 passes . These are relatively fast and robust print heads, which can use a range of ink types . They have the advantages of relatively fast processing speeds and the ability to produce variable droplet sizes and gray levels with higher viscosity inks than thermal inkjet. They own the disadvantage of a short track record28

.

4 Printing Ink Inks are the central factor for efficiency and

economy of ink-jet printing. The quality of the ink is also responsible for the long-term stability of the prints. The inks used in various ink-jet printers may be classified as either dye- based or pigment-based. A dye is colorant, which is dissolved in the carrier medium, whereas a pigment is colorant that is insoluble in carrier medium but can be dispersed or suspended in the form of small particles, often stabilized against flocculation and settled by the use of dispersing agents. Commonly used carrier media include H20, mixture of H20 and 0rganic co-solvents and organic solvents such as hydrocarbons, esters, ketones, etc. To ensure a fault-free printing, following factors of ink should be controlled:

Viscosity

Printing quality and efficiency depend very much upon viscosity of the ink used. To check the influence of ink viscosity on the print quality, a model ink

108 INDIAN 1. FIBRE TEXT. RES ., MARCH 2005

sample was prepared using 20 gil of a Reactive Black dye with a salt-free formula. Polyethylene glycol (PEG) was added in increasing amounts and the viscosity of the ink was measured. The print quality was checked on a printer with 1.2 x 3 m2 print. The results are not found to be satisfactory at the beginning; low viscosity ink does not work on the printer. The results improve with increasing viscosity but get worse when the viscosity is increased above 5 mPas. The best results are obtained when viscosity is 2.3-4.4 mPas.

SUI/ace Tension

Surface tension plays an important role during the formation of a droplet. The surface tension of commercial inks varies between 2ImN/m and 48 mN/m. These values do not show whether high or low surface tension is necessary for best printing results. To test the effect of surface tension on print quality , a model ink with 2% Reactive Black and increasing amounts of a branched non-ionic surfactant was used . Decreas ing the surface tension of the ink from 78 mN/m to a constant value of 28 mN/m with 1% surfactant shows no improvement in print quality. A 5% surfactant concentration shows improvement in print quality while a ] 0% concentration gives good prints .

COl/ductivity

Printing ink should have a conductivity value between 6mS/cm and 12 mS/cm. To investigate the effect of conductivity on print quality, increasing amount of salt (NaCl) was added to ink containing 2% Reactive Black and 10% PEG. The results show that with the increase in NaCI concentration from 0% to 2.5% the conductivity increases from 5 mS/cm to 33 mS/cm. At 4% of NaCl, the conductivity further increases up to 48 mS/cm but gradual color deviation starts.

Miscellaneous Substances

If there is any dissolved gas in the ink, it forms bubbles like those formed while opening a bottle of

mineral water. These gas bubbles interrupt the flow of ink and block the nozzles. It is, therefore, necessary to de-aerate the ink using ultrasonic treatment before filling the ink tanks .

According to the system of pnnt1l1g to be employed, the particle size distribution should be carefully adjusted «1 ~lm) with regard to very high purity, minimized organic elements, safe pH and good chemical stability. In pigment printing systems, the particle size di stribution should be <1 !l (refs 29, 30).

Ink COil/position

The ink-jet ink, which provides an image having an improved wash fastness, comprises water, an anionic water-soluble dye and -0.1-10% (owm) hardener. The hardeners employed in the composition of ink fall into following different categories:

• Formaldehyde and compounds that contain two or more aldehyde functional groups .

• Blocked hardners such as substances that contain blocked aldehyde functional groups.

• Active olefinic compounds having two or more olefinic bonds.

A humectant may also be employed in the ink-jet composition to prevent the ink from drying out of crusting of orifices of the print heads? !

Various essential components of a water-based ink are given in Table 2.

5 Developments in Inks For a jet print to be comparable to conventional

textile print produced by screen or roller printing, the ink formulation must make use of the same dye chemistry. Various inks developed specially for digital printing are:

• Cibacron RAC-Ciba Speciality Chemicals, Basel, presented the 'Cibacron RAC' reactive dye inks specially for the "DreAM" system and for woven fabrics made from cotton, viscose, modal and Lyocell. Other new dyestuff groups developed include

Component

Deioni zed water

Water so luble solvent

Dye or pigment

Surfactant

Table 2- Water-based ink-jet ink composition32

Function Concentration, %

Biocide

Buffer

Other additives

Aqueous carrier medium

Humectant, viscosity control

Provides colour

Wetting, penetrating

Prevents biological growth

Control s the pH of ink

Chelating agent, defoamer, solubilizer, etc.

60-90

5-30

1-10

0.1-10

0.05-1.0

0.1-0.5

> I

MALIK et al.: ADV ANCES IN INK-JET PRINTING TECHNOLOGY OF TEXTILES 109

'Lanaset RAe' acid dyes for high fashion products and silk and polyamide/Lycra blends, 'Irgaphor RAe' pigment dyes for the home textile sector and in principle for all fibre types and 'Terasil RAe' disperse dyes for polyester fabric.

• Bafixan inks-These are disperse dyes produced by the BASF Performance Chemicals for Textiles. Bafixan compri ses of a range of six colors for printing polyester. These inks can either be printed on paper first for subsequent heat transfer, or printed directly on polyester (in this case using Luprejet HD). These are distinguished by excellent running properties when used in conjunction with piezo printing units (360 dpi and 720 dpi).

• Helizarin inks- This range of pigmented inks developed by BASF comprises ten colors and provides a broad color scope in case of cotton and its blends with the brilliance of colors. These pigment inks of high brilliance are specially selected to meet the requirements for textile industry . These inks are suitable for application in the piezo technology (360 dpi and 720 dpi). Besides cotton, almost all substrates are suitable for the pigmented systems (using Luprejet HD) .

• Procion P based inks- Stork and Zeneca Colors have worked closely to develop extremely pure version of certain Procion P dyes and to incorporate these dyes into formulations that satisfy the stringent requirements of the stork TruColor jet printer. The reactive group used in the Procion P dyes range is the monochloro-s-triazinyl group, which reacts with cellulose under hot alkaline conditions to produce a covalent dye-fibre bond, producing prints of excellent fastness. 33.34

6 Fabric Preparation for Ink-jet Printing As many of the first generation textile ink-j et

printers have been devehped by' modification of machines used in the wide-format graphics industry, they have relatively rudimentary capabilities for fabric handling. Unstable fabrics such as knits and lighter weight wovens frequently need to be supported by a stiffening binder or temporary lamination to a support paper. This is acceptable for sampling and proofing applications only. Therefore, the machine vendors are increasingly focusing their attention on improved fabric feed and take-up mechanisms as well as devices such as adhesive printing belts (fitted with washers to prevent a build-up of printed-through

7 . ink) .

The demand of fabric preparation for ink-jet printing is same as for traditional printing and its

success also depends upon proper preparation. Pretreatment provides a reactive surface for ink-jet printing and prevents undue penetration or spread. The fabric sample should be properly desized, scoured and bleached. Singeing not only improves the quality of prints but also the process reproducibility. In case of unsinged fabric, the projecting hairs can block the path or cause mixing of colors . As the color yield has limitations in ink-jet printing, the use of mercerized cotton material is advantageous. Zeneca Colors developed an auxilliary Zetex enhancer SJP that can enhance the coloring ability of dye. Without enhancer SJP, the shades obtai ned are lighter because comparatively small amount of dyes is applied. When this pretreatment agent is padded along with other chemicals prior to dye, good color yield can be obtained on wide range of cellulosic and protein fibres. Polyester and polyester/cellulose blends can be treated with a hydrophilic enhancing agent (Matexil Enhancer PET) that acts as a receiving layer for the reactive dye formulations. Otherwise, on polyester there is no fixation of the reactive dye.

Owing to their low viscosity, inks have a much greater tendency to run on coarse cloth during digital textile printing as compared to conventional printing paste. BASF has developed a new product (Luprejet HD) for textile pretreatment that claims to solve thi s problem by a controlled absorption of ink droplets on the fabric. The excess liquid in the form of droplets no longer spreads on the textile surface, but is transported directly into the fabric. The contours of the print are thus maintained. If required, it is possible to work with a higher resolution. Fig. 17 shows the defined ink absorption. Luprejet HD combines the following properties for improving the print appearance on fabric : clear contours (no running of ink), increased color intensity and brilliance, no loss of textile properties (e.g. soft handle and drape), and good through print (if required, e.g . for flags and banners).35

7 Printing of Fabrics Printing with reaCtive dye and acid dye inks

involve pre- and post-treatments to fix the dyestuff

• Untreated textil e

. '.-... Treated with • LupreJet HD , ...

Fig. 17-Diagram showing the operating principle of Luprejet HD

110 INDIAN J. FIBRE TEXT. RES., MARCH 2005

onto the fabric. This is a multi-step process with a substantial degree of complexity. However, printing with pigment or disperse dye inks is usually a simple single-step process.

7.1 Single-step Printing The preparation of inks containIng insoluble or

dispersed components such as disperse dyes or pigments is not as easy . However, these systems provide a more simplified approach to ink-jet printing. With pigment and disperse dye based inks, it is possible to cover all textiles with a workable coloration solution and these are relatively simple to apply.

Pigment Inks

Now-a-days, pigment pnntIng (in conventional printing) accounts for almost half of all printed textiles and is therefore an important coloration group. The colored pigment is fixed to substrate using a binder system. In screen-printing, this binder is put along with printing paste. In ink-jet printing, it must be ap'plied either in the ink by separate nozzle system or after ink-jet printing. The advantage of these inks is that they can be applied to many different substrates and the application route, which does not employ a pretreatment or a washing process, is shorter than that of reactive-based inks.

Disperse Dye Inks

Disperse dyes are the main pnntIng system for PET. Transfer inks are based on a special type of disperse dyes that can be applied onto paper and then transfened to textiles using a heat press. These inks can also be printed directly onto substrates. High temperature steam is generally used here to fix the dye.

7.2 Multi-step Printing

Process involving reactive dye and acid dye inks generally requires pre- and post-treatments to fix the dyes onto the fabric and hence substantial degree of complexity is there.

Reactive Dye Inks

Inks based on reactive dyes can be used to print cotton or viscose and to some extent wool and silk. In order to achieve their full reaction with the fibre polymer, alkali treatment and heat application are required. Because of the stringent purity requirements and the conductivity specifications required by

continuous stream ink-jet printers, the conventional printing chemicals such as alkali, urea and sodium alginate thickener cannot be incorporated into the ink formulation. Hence, the sequence given in Fig. 18 is adopted. The alkali must be applied by a pretreatment process as it interferes with reactive dyes and nozzle component, if put in the ink itself. The heat is applied after printing by a steam or hot air fixation process. A separate high temperature wash process is must to wash off any unfixed reactive dye and to ensure optimum fastness.

Acid Dye Inks

Acid dye inks are used to print wool, silk and polyamide fabrics. Although having a small sector, acid dye inks are still quite important for ink-jet printing as many high quality designs are printed on luxurious fabrics such as wool and silk. A pretreatment is generally necessary to prevent wicking of ink on the fabric. A post-treatment such as steaming is necessary to get fixation and a separate wash-off process ensures removal of unfixed dye36-4o.

8 Colour Depth in Ink-jet Printing It is not possible to realize the same color depth in

digital printing as in screen printing. A comparative account of color depth in both digital and screen

T Matexil Enhancer SJP 200 parts / 1000 Sodium Bicarbonate 25 parts / 1000 Sodium Alginate 150 parts /1000

~ Controlled conditions

..--__ -'-~ __ ~ Using Procion dye formulation And

jet printer

On completion of printing

Atmospheric steam conditions (102 DC, 8 min)

Fig. 18-Process route for jet-printing of mercerized cotton fabrics with reactive ink

MALIK et al.: ADV ANCES IN INK-JET PRINTING TECHNOLOGY OF TEXTILES III

25~------------~~ ~----------------.

20

~ 15

5

Depth of colou r in screen printing

(a)

Depth of colour in digital printing

(b) Depth of colour in screen printing

Depth of colour in digital printing

o+-~-.--.--,--.-~ ~~--,-~--'--'--1 o 2 4 6 8 10 12 0 2 4 6 8 10 12

Dye cone., %

Fig. 19-Compari son of color depth of screen printing and di gital prin ting [(a) Reactive Black, and (b) React ive Turquoise]

printing is shown in Fig. 19. The maximum dyestuff concentration on a fabric obtained by using co mmercial printing technique is about 10%. In screen printing, the color uptake is 100 g/m2 while the maximum amount of ink that can be applied with the current piezo technology is 20 g/m2. This corresponds to a color depth of 2 % prin ting pas te in screen printing. This means that in screen printing, five times more dyestuff is printed onto the fabri c than in digital printing. Increased color depth in digital printing is only poss ible by either increas ing the diameter of nozzles of the printer or the pattern has to be printed repeatedly . The former method proves very expensive

1'1 hi' . . 4 1 47 W 11 e t e atter IS time consumll1g '-.

9 Control of Printing Ink-jet prin ting faces the problem of impregnating a

relatively large volume of fabric from only minute bubbles of ink. To achieve best results following

points should be considered carefull y: (i) the ink should be thin enough to avoid clogging of nozzles but not so free flo wing that it spreads too far across the fabric and the desig n looses its definition , (ii) thorough pretreatment should be g iven to the substrate

as it provides a receptive surface for ink-jet printing and prevents undue penetration or spreading, (iii) application of any type of thickener as a coating material on the substrate reduces spreading; (iv) ink should be of good compatibility characteristics because the technology also has to work on a wide range of materials (synthetic or natura l); surface can be stretchable, flexible, highly porous and textured, (v) inks should have good fas tness towards light, water and perspiration and should be able to

withstand subsequent fini shing operations, (vi) ink should also withstand heavy wear, abrasion and dry cleaning, and (vii) the print should look good and the ink should not alter the hand of the fabric43

-46

.

10 Advantages Following are the advantages of digital printing:

• Digital printing requires minimal press setup and has multi-color registration built-in to its system . This eliminates many of the front-end time consuming processes and permits quick response and just-in time print delivery.

• Digital processes can vary every print "on-the­fly", i.e. while production printing, providing variable data, personalization and customization.

• Digital printing technologies are non-contact printing, which permits printing of substrates without touching or disturbing them. This eliminates image distortion encountered in some analog processes such as screen printing . It also does not require as aggressive substrate hold down methods which can distort or damage some substrates.

• Digital technologies can do print proofing­sample and short runs are more cost effectively than in analog methods. Digital color printing offers a range of color processes, including 3-color process (CYM), 4-color process (CYMK), and 5, 6, 7 and 8 extended gamut color options in addition to some spot colors. The match growing marke t demands for full color.

• Most digital print processi ng requires less or no color overlap or trapping.

• Digital printing does not use film masters, stenc ils, screens or plates. It requires much less space for archiving text and images than analog printing methods.

• Generally, digital printing uses chemicals, produces less waste and negative environ mental impact technologies.

less hazardous results in less than analog

• Digital printing employs sophisticated color matching and calibration technology to produce accurate process color matching.

• Digital web printers can print images limited only by the width of fabric and the length of the bolt or ro ll. They can print panoramas and are not restricted to repeat patterns.

• Drastic time reduction in sampl e printing. • This technology can work on wide range of

materials and allows the printing of single garment pieces.

• Pattern variation can be created and evaluated on the PC screen, and design varieties can be seen on P.c. screen .

• The finest color gradations can also be produced with this technology.

112 INDIAN 1. FIBRE TEXT. RES. , MARCH 2005

• It is possible to match prints to the highest quality levels and with maximum color fidelity and completely new design possibilities.

• Border to border printing is possible. • It reduces the stock keeping of raw materials and

labour cost.

• It considerably reduces the cost of printing of smaller lots; the cost saving is between 50% and 90%.

• Just in time production and delivery are possible.

• Up to 70% of color material can be saved as compared with conventional textile printing.

• Up to 50% of energy saving can be done47. 48 .

11 Limitations • Discharge and resist effects on deep shades of

dyeing are not possible by present ink-jet technology. • The present systems work with 20 g/m2 inks,

limiting the color depth of prints. On the other hand, from screen printing one can achieve five times more depth of shade.

• Digital printing may cost more per copy than other printing for longer print runs.

• Digital inks and toners are limited in capacity and carry high price tags.

• There is very high temperature close to the resister and hence damage of ink may take place.

• Clogging of nozzle may occur due to evaporation.

• Continuous loss of heat transmission causes stray reduction in ink drop volume and velocity.

• This is a new technology, which requires investment for training as well as equipment.

12 Precautions • The ink must not dry up in the nozzles, which

have diameters down to hundredth of millimeters, as nozzles blockage would immediately result 10

.,stripping faults. • Colour viscosity and surface tension should be

adjusted to the production conditions.

• Continuous dye feed must also be guaranteed. This could not be offered or could only conditionally be offered on machines up till now.

• Another important problem is uniform fabric feed during printing as textile fabrics are not so uniformly flat as paper. It is necessary to set the printing heads to operate with fabrics of uneven thickness.

• The penetrating ink must not cause the fabric to arch49

•5o

.

13 Conclusions Ink-jet printing has -opened new windows of

opportunity for textile printing industry to grow and flourish in the future. This revolutionary printing technology gives high quality textile prints with unlimited colour variations and no repeat restrictions. It has a wide range of potential applications. Therefore, it has attracted major research activities all

. over the world and as a result faster printing heads are coming in the market with increasing resolution.

There is sufficient industrial investment and commitment by manufacturers and now it seems certain that the commercial ink-jet printing will, over the times, become a reality.

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