DIGITAL PRINTING OF TEXTILES R.B.CHAVAN Department of Textile Technology Indian Institute of Technology Hauz-Khas, New Delhi 110016 E-mail: firstname.lastname@example.org
ANALOG PRINTING CONSIST OF REPRODUCTION OF IMAGES FROM MASTER IMAGE All conventional methods of textile printing are Analog printing FLAT AND ROTARY SCREEN PRINTING, BLOCK PRINTING, ROLLER PRINTING Master image in the form of flat screen or rotary screen Block Design 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.
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.
continuous stream of ink is produced by forcing it through a narrow nozzle at a pressure of about 3x10 5 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.
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.
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.
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.
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 are shear 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
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 dyes Disperse 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 processing Post-treatments are associated with the printing operation; examples are baking, steaming and/or washing. These processes are similar to those for conventional textile prints,