DIGITAL PRINTING OF TEXTILES

R.B.CHAVANDepartment of Textile Technology

Indian Institute of TechnologyHauz-Khas, New Delhi 110016

E-mail: rbchavan@hotmail.com

Analog and digital printing

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 PRINTING

Master image in the form of flat screen or rotary screen BlockDesign Roller

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.

ESSENTIAL ELEMENTS FOR INK JET PRINTING

• Computer

• Software

• Digital printer

• Textile substrate

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 by coating or padding.

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.

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 • 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.