DIGITAL PRINTING OF TEXTILES

R.B.CHAVANDepartment of Textile Technology

Indian Institute of TechnologyHauz-Khas, New Delhi 110016

E-mail: rbchavan@hotmail.com

ANALOG PRINTINGCONSIST OF REPRODUCTION OF IMAGES FROM MASTER IMAGE All conventional methods of textile printing are Analog printingFLAT AND ROTARY SCREEN PRINTING, BLOCK PRINTING, ROLLER PRINTINGMaster image in the form of flat screen or rotary screen BlockDesign Roller

DIGITAL PRINTING

ANALOG PRINTING

Conventional textile printing techniques

Master image present on block or screen is reproduced onto textile in the form of print.

DIGITALPRINTING

Design is in the form of electronic file in a computer.

The computer is linked to a suitable machine e.g. inkjet printer

Design is printed onto paper or fabric in the form of analog image with the help of coloured microscopic dots.

MASTER IMAGE

• MASTER DESIGN

Single colour Multi colour

COLOUR SEPARATION

Manual, Through computer with the help of suitable software

Master image on screen

Printing operations

Screen washing

DIGITAL PRINTING

• Design in the form of electronic file on computer

• Computer is linked to printing machine (Ink jet printer)

• Conversion of electronic design data in the form of analog image with the help of dots or pixels.

• No need of design screens, block or design rollers.

• Also known as printing without screens or plate less printing

STEPS

• Master design

• Scanning

• Transfer of design data to ink jet printer with the help of software

• Printing onto surface (Non contact printing)

Textile printing technology trends

Present share of Rotary sc. Printing 60%

Trends in global printing market

Textile printing is fashion dependent.

The fashion seasons are becoming shorter resulting in 5-6 fashion forecasts in a year.

Customers are demanding great variety of colours and unique designs.

Due to these reasons there has to be quick sampling and quick order turnaround. The chances of repeat orders are becoming rare.

In addition to this average run lengths are rapidly dropping.

Thus the world of textile printing is rapidly changing. Globalization, quick response and ecology aiming at waste minimization and reduced environment pollution impose substantial demands on the different components of the printing process.

Trends in global printing market

In short: these demands have common denominators: flexibility and versatility.

In order to meet such market demands there must be a technology which will facilitate Mass Customization.

It is a new concept of production which specializes in short runs as little as one unit in which the customer dictates exactly his/her requirements.

It aims of producing unlimited designs of customer’s choice.

Trends in global printing market

Digital printing technology supports the present industrial trends: short runs at economical cost, quick delivery, exclusive unique designs and personalized textiles. Digital printing can also contribute to the ‘green image’ of textiles; the ecological impact is clearly lower compared to conventional printing. Digital printing is already applied for sampling, short runs and mass customized apparel. It allows the user to bypass the extremely time consuming and expensive screen making process, providing the opportunity for quick changes to colour or design elements The conventional printing requires 6-7 weeks whereas the digital printing requires about 2.5 weeks delivering the final printed products In addition to this the change over from one colour scheme to other and from one design to another is also much simpler and less time consuming in case of digital printing.

Time to introduce a new product

INKJET TECHNOLOGIES• PRINCIPLE

• Directing small droplets (usually between 50 and 60 microns in diameter), of ink from a nozzle onto printing substrate.

• The droplets can have different colors. • Droplets are combined together on the substrate to create photo-quality images. • Positioning of droplet is controlled by high frequency digital signals from

computer. • Droplet formation involves application of a controlled pressure on liquid ink in

its reservoir as it streams into the printing nozzles the ink stream is broken into droplets.

• Different technologies for application of pressure on liquid ink.

INKJET TECHNOLOGIES

• two main technologies

• Both technologies use large number of nozzles (10-100 Micron dia,) for each colour (CMYK)

• Between 1000 to million drops processed per second.

•Depth of shade is controlled by number of drops applied on substrate.

CONTINUOUS JET

• continuous stream of ink is produced by forcing it through a narrow nozzle at a pressure of about 3x105 Pa.

• The resulting high velocity breaks the ink stream into droplets. • The size and (number) of droplets produced depends largely on • the surface tension/viscosity of the liquid ink, • the nozzle diameter and • the applied pressure. • Around 100,000 droplets per second leave the ink chamber, • although with very high performance printers upto 625,000 drops/

second can be ejected.• Directional control over the droplets is obtained by

selectively inducing an electrostatic charge on them• as they leave the nozzle. The charged droplets then

pass through a set of like-charged plates which repel and deflect the droplets either to the required position on the substrate and the uncharged droplet to the gutter for recycle or vice versa.

SINGLE JET SYSTEM

CONTINUOUS JET

• Drop volume larger than DOD inkjet system.• Volume of ink delivered per unit time larger than

DOD systems.• Both features most suited for fabric printing.• Due to continuous ink stream, no nozzle clogging.• Disadvantages• Complex system, Print heads manufactured

manually. • Each nozzle has its own pump.• Expensive system• Electrically conductive inks essential.• Low resolution comparee to DOD systems.

Hertz technology

• It is a form of single jet continuous ink jet that uses extremely small nozzles,

• producing very small droplets. • form a more controlled image. • The system offers higher print quality. • The disadvantages are that the fine jets are• prone to clogging. • Further, present systems are very slow and

can take up to one hour to produce an A4 format in four-colour print.

Multiple jet system

• Differ from single jet system.• Ink drops are given variable charge that gives different

deflection as the drops pass through deflection plates. • This allows multiple positioning of ink drops (up to 30) on the

substrate to be printed from a single jet. • The uncharged drops go to the gutter for recycle.

•

AIRJET DEFLECTION

Ink droplets are deflected by an air stream.

The system has been in commercial use for a number of years for example, the Milliken Millitron.

It is most suitable for relatively low definition (20-30 dpi) and

high ink volume applications such as for carpet printing.

Advantages and disadvantages

• Advantages• ability to cover a larger band width print area with one

pass• Higher productivity compared to single jet system• long print head life over thermal or piezo drop on

demand printers, • Disadvantages• High initial cost of the system• Low resolution• Extremely low viscosity ink (3-6 CP) and electrical

conductivity.• In general, the initial high cost of CIJ heads currently

prohibit their use for low volume applications.

Drop-on-demand technology

1.Thermal (or bubble-jet)

2 Piezo drop on demand.

pressure applied to the ink reservoir is not continuous, but is only intermittently applied when a droplet is needed

Bubble jet printer:

(a) bubble jet chamber; (b) bubble formation

Bubble jet or Thermal jet principle

Used by manufacturers such as Canon and Hewlett Packard, this method is commonly referred to as bubble jet. In a thermal inkjet printer, tiny resistors create heat, this heat

vaporizes ink to create a bubble. As the bubble expands, some of the ink is pushed out of a

nozzle onto the paper. When the bubble "pops" (collapses), a vacuum is created. This pulls more ink into the print head from the cartridge. A typical bubble jet print head has 300 or 600 tiny nozzles, and

all of them can fire a droplet simultaneously

Thermal ink jet or bubble jet technology

Side shooter thermal inkjet Rear shooter thermal inkjet

Thermal printers are well suited for low-volume printing.

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. The high temperatures cause often decomposition of ink components, which leads to nozzle clogging.

Therefore, only thermally stable inks can be used.

Poor quality production results are possible.

Finally, compared to piezo-systems the droplet size is larger resulting in a lower resolution.

Bubble jet

• Bublle jet print device prints • cellulosic fibers with fiber reactive dyes, • synthetics with disperse dyes, • and nylon and protein fibers with acid dyes. • These printed fabrics require conventional post processing

including steaming and washing. • Canon extended the life of its print heads from 8 to 14 and then to 130

hours of continuous operation, which it guarantees. • It achieved this with the addition of a head cleaning mechanism and the

reformulation and refining of its inks. • The Canon Bubble Jet Textile Printer delivers 360 dpi resolution, 8 color

capacity, over 1,000 nozzles• prints up to 1.65 meter widths at the rate of 1 linear meter per minute. • The high force associated with droplet ejection from Bubblejet heads

provides the advantage of fabric penetration and the disadvantage of increased ink splatter.

• Some coarser, deeper pile fabrics benefit from its advantage while hiding the splatter, while tightly woven fine fiber fabrics reveal splatter.

Advantages and disadvantges of thermal jet printers

• low equipment cost • Very large installed base • Aqueous inks • The development resources and guarantees of giants

such as Hewlett Packard, Canon and Lexmark.• Disadvantages • Limited head life due to and resistor failure • Limitations of their low viscosity inks. • Only aqueous inks suitable• Binder ink system not suitable. Due to ink heating

binder polymerization.

MAJOR PLAYERS IN INKJET PRINTERS

• Thermal (bubble jet) DOD • Canon, Hewlett Packard, Lexmark (formerly-IBM),

Xerox (recent) • Piezo DOD • Aprion, Brother

Epson, Hitachi-Koki (formerly Data products), Konica,Spectra,Tektronix,Trident,XaarXerox.

• Binary continuous • Domino,Scitex,Stork,Toxot.• Multi-deflection continuous • Jemtex, Linx, Imaje/Toxot, Marconi (formerly

VideoJet), Willett

Piezo drop on demand technology

This is one of the simplest ways of generating drops on demand.

It makes use of the piezoelectric effect in which small electronic impulses delivered to suitable crystalline materials (transducer) causes them to expand.

This transducer, incorporated in the ink chamber, enables pressure pulses to be created in the ink.

Droplets are generated intermittently according to the electronic signals received.

Piezo Drop on demand technology

Several variations exist of the basic mechanism by which crystals of Lead-Zirconium-Titanate (PZT) (transducer) turn electrical signals into mechanical pressure pulses to produce ink droplets.

These areshear mode, bend mode, push mode, squeeze mode and hybrid or “coupled” mode.

A number of patented designs and manufacturers exploit these various mechanisms to differing effect.

ADVANTAGES

These printers have the advantage of

much greater print head life than the thermal-based systems (100 times).

Piezo inkjet printers are now in use in a number of printers for textile substrates. A number of companies (e.g. Konica, Mimaki, Epson) are producing and developing wide format piezo printers for printing wide width fabrics.

Advantages inherently linked to piezo-systems are:

• Ink formulation: as the inks are not heated, the formulation can be less critical and hence the inks are less expensive

Well suited for high-volume printing (industrial)

• Reliability is built-in through design

• Uses a wide range of ink formulations (solvent, water, UV curable)

• Produces high resolution by using small drop size

• Capable of using binder containing pigment inks

ESSENTIAL ELEMENTS FOR INK JET PRINTING

ESSENTIAL ELEMENTS FOR INK JET PRINTING • COMPUTER

• Computer system essential for data processing

• CPU Pentium IV 600Hz x2 or above • OS Windows NT4SP4 • Memory 256 MB or above • Hard disk 9.1 GB or above • CD-ROM 40 x or above  • Color display (Monitor) 17 inch 

SOFTWARE

• Raster Image Processing (RIP)

• Colour Management System

• Convert scanned design into electronic signals.

• Allows editing of design

• Give command to the inkjet printer for faithful printing of scanned/edited design.

DIGITAL PRINTER

• Mainly DOD printers used for textile printing• Choice• Thermal or bubble jet printer• Piezo printer• Piezo preferred • wide choice inks• Reliability• Durable print heads

TEXTILE SUBSTRATE

• In digital world known as “ media “• Fabric to be printed

FABRIC PRE-TREATMENT

• Conventional printing• Bleached fabric without any pre-treatment is used

for conventional printing• Chemicals and auxiliaries necessary for print

fixation like thickener, urea, alkali, acid, defoaming agents etc. are incorporated in the print paste

• Viscous print paste. No danger of print spreading • Digital printing• Printing chemicals/auxiliaries can not in

incorporated in printing ink.• They are incorporated in fabric in the form of fabric

pre-treatment.

FABRIC PRE-TREATMENT

• Such pre-treatments help to maximize the absorbency and reactivity of the textile substrate towards the inks.

• Minimize ink spreading to prevent loss of definition and colour intensity.

• Many patented and proprietary formulations exist,• ranging from simple formulations of soda ash,

alginate and urea • to more sophisticated combinations of cationic

agents, softeners, polymers and inorganic particulates such as fumed silica.

• Many of these have been aimed at fashion fabrics such as cotton, silk, nylon and wool.

• 3P InkJet Textiles (Germany) is marketing pretreated fabrics ready for inkjet printing.

Fabric pre-coat machine

Fabric Feeding System

Fabric feeding

Fabric Exit

ensures perfect registration and alignment throughout, even for delicate and unstable fabrics such as knits or fine silks. If required, this machine may also pre-heat, dry or set the printed fabric, before finally rolling-up the output smoothly and with even tension.

Ichinose-unit: conveyor belt and dryer

Ichinose uses a conveyor belt to transport and align the textile substrate.

The conveyor belt carrying the fabric gently moves ahead for inkjet printing operation. The print head nozzles are set up right above the carrier belt, and the cloth printed with the inks sprayed from the head nozzles. This can prevent the inks from bleeding onto the cloth. After printing operation the cloth at the exit end is released from the conveyor belt. The conveyor belt can be cleaned whenever necessary

Inkjet inks

The inks comprising of pigments or dyestuffs of high purity must be milled to very fine particle size and particle size distribution.

Inkjet inks must be formulated with precise viscosities, consistent surface tension, specific electrical conductivity and temperature response characteristics, and long shelf life without settling or mould-growth.

Other important parameter is colour build up on substrate.

The colorants must have very high strength and high chroma to achieve a broad colour gamut with a minimum number and amount of deposited colorant.

In addition, further properties such as adequate wash-, light- and rub-fastness are necessary.

Reactive and acid dyes

From the outset, suppliers of textile inkjet inks were quick to offer products based on reactive and acid dyestuffs.

Reactive dyes are particularly suited to cotton, viscose and other cellulosic materials,

whereas acid dyes are used for wool, silk and nylon.

Both are fully water soluble and relatively easy to formulate for a wide range of inkjet heads, especially the widely installed thermal drop on demand jet types.

Pigment colours and disperse dyesDisperse dyes (for polyester and nylon) and pigments present a more difficult set of problems for ink maker.

Both exist in water as dispersion of small particles.

These inks must be prepared with high degree of expertise so that the particles will not settle or agglomerate (flocculate).

The particle size must have an average of 0.5 micrometer and the particle size distribution must be very narrow with more than 99% of the particles smaller than 1 micrometer in order to avoid clogging of the nozzles

The major outstanding problem with their use in inkjet systems is how best to formulate and apply the resins which are required to bond the pigment particles to the fabric surface.

Several different approaches, from spraying resin through a separate jet head to screen printing binder over an inkjet printed colour have been suggested.

In the long run, improved resin binder technology seems likely to prevail, allowing trouble- free formulation and printing from a single inkjet head for each colour.

Spot colours versus process colours

The inks used in Conventional printing systems are known as spot colours.

This means the required shade is prepared by mixing appropriate colours before printing.

Although it is a skilled job, it allows matching the desired shade as closely as possible.

The inks used in inkjet printing are known as process colours.

The desired shade is produced on fabric itself during printing operation.

This is achieved by super imposing the dots of limited number of colours essentially, cyan, magenta, yellow and black (CMYK).

In some cases additional colours may be used to increase the colour gamut.

Fabric post processingPost-treatments are associated with the printing operation; examples are baking, steaming and/or washing.

These processes are similar to those for conventional textile prints,

Slow adoption of inkjet printing

• The existing speeds adequate for sample printing but not for bulk production

• Availability of printing inks at reasonable cost• Colour matching problems in flat colours• Reproducibility of results from one printer to

another printer.• Migration of manufacturing capacity to Asia where

labour intensive processes prevail.• Main stream textile printers are geared to low cost

mass production business model and long response time

• Niche market has to be build up from scratch• Educating the consumers about the potentialities of

digital printing.

Future Vision• Sampling: This is the traditional application area and

this may be expected to continue with modest growth.

• Bulk production for batches less than 1000 metres. • This is the vision of many • Mass-customization: The creation of new niche

markets for small-medium batches of printed textiles for specific customers.

• It may be possible that garment makers decide to buy a digital printer and attach it to a laser cutting table.

• After printing, the fabric could be cut single ply using a computerized system and then converted to made-ups.

Future Vision

• Major inkjet manufacturers are working to resolve the issue of production speed and it is hoped that inkjet printers will be available with a speed to compete with rotary screen printing.

• The ITMA 2003 exhibition in Birmingham, UK, was a significant milestone for digital printing,

• 27 companies offering textile digital printing equipment.

• Many of the machines shown were said to print at over 50 m2 per hour,

• Reggiani printer was said to print at 150m2/hour.

• However this far less than rotary screen printing (3600 meters/hour)

Future Vision

• The other possibility is that inkjet printing technology may be used as weaving technology

• where printers may have large number of inkjet printers like looms to carry out the printing production.

• In Bangkok a printing unit has 25 Stork Sapphire machines run much like a traditional weaving department.

Conclusion

• Digital printing provides an opportunity to meet the present day market trends of mass customization.

• It has established as an acceptable technology for sample production.

• Among other technology problems speed of printing is the main hurdle in commercialization of technology.

• Attempts are being made to achieve commercially acceptable printing speeds.

• Till then the practice of combination of digital printing for sampling and rotary screen printing for production will continue.

• What now seems certain is that there is sufficient industrial investment and commitment by manufacturers to ensure that commercial ink jet textile printing will become a reality.