12 Digital Printing

S. Karpagam Chinnammal

1. Introduction

Printing is the method of applying colour to a textile substrate in a specific patterns or designs. There are different methods used for printing and are quite varied from primitivemethods like block printing to sophisticated automatic machinery. The journey has been from brushes, blocks, batiks to the rollers, screens (flat and rotary) and digital.Non conventional printing is a new concept developed recently.These kinds of printing methods include the usage of modern machinery and skilled labour. The most recent is digital printing on textiles.It provides a tool to the designer where uninhibited thinking for design creation is possible, which can be finally transferred on to the fabric. Digital printing is a technology that has gained reputation in the last couple of decades. The demand for digital textile printing for apparel and home furnishing is rapidly growing .

2. Learning Objective

• To gain knowledge about digital printing and its method
• To be aware of the advantages and disadvantages of digital printing
• To be acquainted with the future scope of digital printing

3. What is digital printing?

Digital printing is the process of creating prints designed and generated from a computer as opposed to analogue printing which require printing screens. It is a non contact method of substrate decoration which produces images, colours and patterns using computer driven inkjet nozzles. The design is transferred directly from the” monitor” to the textile. The design to be printed sits in the computer in the form of digital data that can be converted repeatedly, with or without any variation, into a print. The print can be obtained by connecting computer to a printer

.The RIP (Raster Image Processor) software instructs the printer on when and how much drops to be placed on the fabric to create the colour design. The print design is created by proper positioning of drops.

Unlike conventional printing in digital printing colourants are mixed on the fabric during the printing process. Digital textile printing is an amazing alternative to replicate original artworks with the right blend of classic and elegant view. Inkjet printing gives the printer a very high degree of flexibility when compared to traditional screen printing. This technology offers quick changes in colour and design, faster and cost effective production. Further it reduces waste and pollutants significantly. Digital printing processes, mainly based on inkjet systems, are making inroads into the textile printing sector which is presently dominated by rotary screen printing.

3.1.The elements of digital printing

The essential elements of this technology are the substrate, pre and post treatments, ink, print heads, fabric feeding system or printer and Software. Getting all this right is the key to success

3.2.Digital printing process

Designs can be created digitally with almost any graphic design software (Photoshop, coreldraw or Illustrator). Alternatively, existing artwork or photographs can be scanned and then digitally manipulated to make a pattern. Usually designs are created as a seamless pattern that is repeated across the fabric. You can also create a design that fills an entire yard without repeating,

During the printing process, the fabric is fed through the printer using rollers and ink is applied to the surface in the form of thousands of tiny droplets. The fabric is then finished using heat and/or steam to cure the ink (some inks also require washing and drying).

3.3.Printing machine

Digital printing machines can be classified on the basis of type of pass as follows

1.  Scan pass

2.  Single pass

In the scanning head method both the print head moves in the width direction as the fabric is gradually advanced. The other is line head printing, also called as single pass method where the print heads are stationary and the fabric moves. By this method the faster speeds have been achieved like in some machines you have an array of fixed heads across the width of the machine.

3.4.Inkjet technology

Among the various approaches for digital printing, ink jet has gained a very important place in the field of novel printing techniques. This technique is more like reprographic industry than textile printing as it uses toners, inks and printers rather than dyes, pigments , print paste and screens.

In digital inkjet printing, printheads containing banks of fine nozzles, fire fine droplets of individual coloured inks on to pretreated fabrics. The print design is created digitally and the ink droplets are mixed together on the fabric surface to create final colour , so called spot colour.

The print design concept is manipulated digitally and the set up time to produce short runs, sampling or proofing is dramatically decreased. Dyes are supplied in catridges and are ready for immediate use. Compared to conventional method consumption of energy and water is very low.

3.5Principle of inkjet printing

The basic principle is directing tiny droplets of ink from a nozzle onto the printing substrate. General feature of droplet formation is computer control of droplet position on substrate in response to high frequency digital electronic signals. It involves application of controlled pressure on the ink in the catridge; as it flows through the printing nozzle the ink is broken into droplets.

The print head is one of the important parts of a digital printing machine it can be said print head is the heart of the digital printer and software its brain.Print head determines the speed of production and overall print quality.

3.6.Classification of inkjet printers

They are classified as

1. Coarse resolution type – it is Based on valve control technology with resolution of 40Dpi
2. Fine resolution type-It has resolution upto 300 dpi. The significant features of this inkjet printers are the printing heads and nozzles. Numerous nozzles are used for each colour. These nozzles are 10 to 1000µ in diameter. It can be subdivided into
3. Continuous stream printing: Drops are continuously produced and selectively printed
4. Drop on demand: Drops are produced when required and fires this on to the substrate

3.7.Digital inks /jet inks

Digital printing techniques make new demands on the colourants and formulations used. In screen printing colours are applied in the form of high viscosity pastes. Inkjet printing requires very thin ink formulations, and the colours or dyes used for inkjet printing should have small particle size as over sized particles block the jets. The average particle size of the inks must therefore be tiny compared to the nozzle orifices and then only smooth ink flow can be guaranteed. The Choice of auxillary system is crucial for the stability of ink formulations. Basically inkjet printing ink is classified into two types

1. Water based
2. Solvent based

Water constitutes the best vehicle for jet inks because of low viscocity, conductivity, odour, and cost and since it is a good solvent for most dyes. Main disadvantage is evaporation. As water evaporates salts and organic dyes may precipitate , increasing the probability that they will clog the nozzle.

To reduce evaporation and prevent drying , ethylene glycol is normally added as humectants.

The ink must be able to dry quickly after printing, but not during printing process or inflight. In order to produce well defined print inks main elements are viscosity, surface tension, density must be monitored. If viscosity is too high droplets will not form, if too low velocity will not be uniform. If density is high, velocity will drop below operational level. Careful selection of thickening is necessary for correct viscosity.

Substrate type determines the colourant selection. For textile fabrics of cotton, reactive dye and for nylon or wool acid dyes are used

A typical formulation will consist of a suitable solvent, colourant, thickener, bactericide, pH buffer and sometimes an antioxidant.

The physical property desired for inkjet printing can be summarized as

• Purity- it will minimize clogging of print heads
• Particle size-As pigments are coarse and insoluble in nature, they must be finely ground to obtain sub micron size
• Viscosity-low viscosity is necessary to allow easy flow through the nozzle
• Dye-high affinity for the substrate and good fastness
• Jet-steady ejection of a homogeneous droplet reproducible in size, speed and direction.
• The functional properties required are machine compatibility, stable drop formation, no particle contamination, no nozzle blockage

Types of inks dye

• Dye based and
• Pigments based-these are insoluble particles applied as fine particulate dispersions.

There are four types of dyestuffs available for commercial use as digital inks:

Fiber reactive inks: Cellulosic and protein fibres can be printed with fiber reactive inks. Reactive colours are bright, posses good light fastness. Because the dye chemically reacts with fiber it exhibits excellent washfastness as well. Fibre reactive inks require fabric that is pretreated for printing. Once printed the fabric must be steamed and washed.

Acid inks:These are anionic dyes .They are solube and used for coluring nylon, silk, wool and leather. Acid inks give very bright colours and have good lightfastness , making them suitable for outdoor flags. Acid dyes also react with the fibres, giving very good wash fastness. The fabric must be pretreated with materials to facilitate their fixation. Post processing involves steaming and washing .

Disperse inks: These have very low solubility in water but are solvent soluble.They are applied as finely dispersed aqueous inkjet inks to the media. They are limited to being used on polyester. The colours are bright but not as bright as acids and reactives. They sublime when heated to very high temperatures. Once they become gaseous , they are absorbed by the polyester fibres, the dye condenses and becomes physically trapped inside, thus giving very good resistance to laundering. Excellent resistance of disperse dyes to fading in sunlight make them appropriate for the toughest outdoor applications including lawn furniture and upholstery.

Pigment inks: Are the best choice for printing textiles. They tend to have excellent light fastness and can be used on all fibres. Fixation is very simple, involving only heat or UVcuring. However, pigment inks must contain a binder to glue the pigments onto the fibre. The Latest development is a pigment ink that can print on all types of fabrics.

Features of new pigment inkjet printing are they are suitable for all fabrics, rich in colour, brilliant with sharp outlines and are wash resistant

3.8.Selection of fabric

Selection of fabric plays an important role in print quality. For durability reasons synthetic materials like nylon and polyester must be selected for outdoor usage like flags and banners. Other fabrics may be used for apparel and furnishing

3.9.Fabric pretreatment for inkjet printing

In Digital textile printing, pretreatment and post treatment are required to fix the colourants onto fabric and yield improved colour appearance and durability. Pretreatment chemicals cannot be included in the ink formulation because they affect physical properties of the ink making it unsuitable for jetting from the print head.

Surface hairs lower the print quality and must be removed by singeing , impurities must be removed and fabric is made absorbent and white by desizing, scouring and bleaching. Depth of colour can be improved by mercerization process.

Due to purity reasons, auxiliaries that help in dye fixation , colourfastness and sharp prints normally used in printing paste formulation cannot be added to inkjet inks.Hence prior to printing , the fabric is coated or padded with chemicals necessary to fix the dyes specific to the ink being used, except pigment inks which requires no pre treatment. Chemical solution range from simple to very complex . simple formulations use soda ash, alginate or urea, while complex ones can be made of combination of polymers, inorganic particulates , cationic agents and softners.

The pad liquor for reactive dye contains alginate for penetration, urea for better, colour depth, and alkali for better fixation

For pretreatment of wool and silk fabrics an acid releasing agent such as ammonium sulphate and moisture absorbing agent such as urea should be included with some thickeners in the pretreated solution which can be padded or coated on fibres. A receiving layer on the surface of the fabric is then formed which cannot only prevent the spreading of inkjets on the fabric but can also improve the moisture regained from steaming, helping the fibres to swell and make the dye penetrate into the fibre more rapidly.

• For basic fabric pretreatment, solution includes
• Antimigrants: To prevent migration of ink and prevent bleeding.
• Acids/alkalis-To support reaction of ink
• Urea/glycols: To increase moisture content of fabric giving high and  even fixation of  inks
• Pretreatment conditions: The samples are pretreated with cationic substances or with a pretreatment paste containing binder in addition to thickener- the fabrics are padded in thecationic agent for 3 min to gain a 90 % pick up then dried at room temperature. The pretreated fabrics are then printed with the pigment ink by inkjet printer and fixed at 150 °C for 5 min

Pigment printing needs no pre or post treatment saving production time and reducing water pollution

3.10.Fixation and development of print

Post treatment of the digitally printed fabric is of supreme importance . Unless the print is properly fixed colorfastness would be a question mark

Based on the type of dye used in ink formulation, fixation method such as curing , chemical padding as in conventional printing could be adopted. For example for reactive inks cotton is steamed at around 100° C or 8-10 minutes or thermofixed at 140-150° C for 5 min or steamed at high temperature for 2-4 min at 120° C and washed As with reactive dyes acid dyes are steamed in the fixation step. Temperature is same but time is doubled Pigments need only curing at 325-350 °F for 30 -90 sec Disperse: transfer of dye sublimation from paper to polyester is done at 380-410° F for 30-90 sec

4.Advantages

Digital printing has ‘n’ number of advantages.

• Attractive and complex design
• Unlimited colour and repeat size
• Clarity , fine line prints can be achieved with great precision
• Since designers use photoshop, illustrator and digital photograpghy, the new printing method acts as a tool in sync with such digital technologies. Designers are able to get photographs printed exactly as they are.
• HD print can be obtained which is of great importance for designers these days
• Easy sampling
• Short runs (Reduced process steps) and quick delivery
• Fast and frequent design changes is possible. It is possible for designer to have last minute changes to their designs
• Small batches are as cost effective as large ones.
• Repeat orders are easy to process
• Non contact printing hence image distortion is eliminated
• No paste preparation and no waste as all colour goes onto the fibre
• It is a clean process as there is no preparation of equipment or colours since the software controls the printing process
• Customized and personalized goods at a competitive price.
• Overall lower cost
• Right quantity produced in right time thus reducing environmental hazards caused by over production
• Eco friendly with no liquid effluent, uses less hazardous chemicals and produces less waste
• Less water, energy and ink usage . Hence Sustainability improved.
• Sophisticated colour matching and calibration technology , producing accurate colour matching
• Digital printing combines design concepts like shadow, shimmer, translucent, vibration, reflection, blurring,layering and superimposing to textile print designs. Applying these concepts creates new trends and design movements, which can be avant-garde or very commercial
• Independent designers are able to create custom textile prints with less effort, and without having to depend so heavily on textile designers and large textile mills.
• Digital textile printing allows textiles to become more creative , innovative and commercially viable
• Ability to print anything you see on a computer screen

   5. Disadvantages

• Banding is a problem created by the print head movement over the substrate.If head is not properly aligned or surface is uneven a slight horizontal band or line of unprinted area occurs
• A misfire occurs when the inkjet nozzle fails to send a drop of ink and results in a small unprinted area
• Another problem is nozzle clogging. Pattern may lose some colours.
• Surface static charge on the surface may push or pull the ink particles away from the intended position causing over spray or fogging
• It requires a specially prepared and coated substrates for ink absorbency
• Digital Inks and toners are limited in capacity and carry high price tag
• Textiles unlike paper vary in fibre content, weight, thickness, ink absorbency and yarn size

6.    Future scopes

• The estimated potential of total printing today is around 30,000 million square meters and this is expected to grow at a rate of 1.5%-2 % per year. At the same time, digital printing which currently has a share of around 1% is expected to grow at a rate of 25%. This is a clear indicator of the importance of this new technology.
• The digital textile printing market is forecast to grow at an average annual growth rate of 20% in value terms between 2014-2019.It is estimated the total Digital textile printing out put will rise to 1.6 billion sq.m annually by 2019
• One area for expansion is customization, whereby a consumer is able to personalize a product by choosing a design
• In order to improve speed and production, the one pass method is gaining importance. Speed could also be increased by increasing the number of nozzles per colour or per head .An increase in length of the print head will also help increase printing speed.

• Common application includes apparel, home furnishing, soft signage and flags. Development of new applications for this technology will contribute to development and growth of this technology. Two key application drivers for this include

1.  Digital printing of automotive textiles (car seat cover, interior carpet , seat belt webbing) and
2. Deposition of functional fluids onto textiles using inkjet technology-Is a finishing as this process , it uses auxiliary chemicals to change the functionality of a textile substrate rather than simply creating coloured design as in regular digital printing.These finishing techniques improve or add properties to the textile.The functional components that could be coated or deposited are quite varied eg antibiotics, moisture absorbents, water repellents, antistatic agents, UV absorbents etc

Conclusion

Digital printing is an innovative way to print anything on fabric, breaking the limitations faced with the conventional printing methods. Digital printing is a relatively new style in the field of textile printing and has immense potential to fulfill the demands of customers, designers and processors. In order to harness its full potential designers should be able to break free from their conventional mould in terms of creative freedom and think unconventional designs. overall , the digital textile printing allows textiles to become more creative , innovative and commercially viable. It is undoubtedly the most upcoming technology for value addition of textiles. With further advances in software, printer and ink technologies, the inkjet printing of textiles will increase in importance in future

REFERENCE

1. Introduction to digital printing, Dr.Sanjiv Kamat and Shyam Phadke, Colourage july 2014 p-30-32
2. Digital printing 2: The history Dr.Sanjiv Kamat and Shyam Phadke, Colourage August 2014
3. Digital printing 3: Print head –the heart of digital printer Dr.Sanjiv Kamat and Shyam phadke, Colourage    September 2014
4. Deboijyoti Ganguly and Prithwiraj Mal, Non conventional printing process on textile material, Asian dyer, June –July 2013 p.54-57
5. R.V.Adivarekar, S.S. Biranje and N.S.Khurana, Inkjet printing: A futuristic textile printing technology, Asian dyer, April-May 2012, p.29-32
6. M.M.Marie, Y H El Hamaky D Maamoun , D.Ferky and S,M,A,Abbas, Treatment of fabrics for inkjet printing with pigment inks, Indiaqn Textile Journal, April 2012 p 41-46