Digital Inkjet Printing:
Digital inkjet printing is one of the most modern ways of printing textile fabrics. This method can be used for most of the commercially available fabrics. In this method, a printing pattern can be directly printed from the computer onto the fabric with an inkjet printer, without any need for making printing screens or engraved rollers. The design-to-print lead time is minimum in digital inkjet printing and complex designs of photographic quality can be promptly printed. However, as compared to rotary screen printing, the productivity of inkjet printing is very low. Hence, the method is mostly used for very short production runs or for printing smaller articles such as flags, banners, etc.
Inkjet printing is a non‐contact printing system because the print head does not touch the substrate surface. The dyes to be printed are in the form of inks rather than pastes, and the viscosity of the inks is low compared to other printing systems. Ink jet printing involves the deposition of small jets (as ink droplets) of a colored ink on the substrate. The jet of ink is controlled by a computer as required by the digital image. The quality of the image being printed is defined by the resolution of printing by ‘dots per inch’ (dpi).
Rather than the traditional primary colors red, green and blue used in dyeing, inkjet printing is based upon the colors cyan, yellow, magenta and black, known as CYMK. Although there are four main colors, the color palette is quite restrictive, but ‘spot’ colors can be added by the printer since print heads offer the capability of dispensing 6, 8 or even 12 colors during printing.
The quality of an inkjet printing is very much influenced by the physico-chemical properties of printing ink. Here lies the importance of inkjet dyes. Dyes for Inkjet ink are a bit different from the usual dyes that are available for fibers. This is because of the fact that they need to have high purity, and do contain small salt and inorganic matters. Dye inks are prepared by dissolving of the liquid colored dyes into a fluid carrier. This makes the dyes easy to apply. When it is applied to a paper, the dyes are absorbed very uniformly so they reflect light very evenly. As the printing is a high precision job the inkjet dyes need to have superior quality in terms of colors, physical properties, and stability. Generally direct, reactive, and acid dyes are used as dyes for Inkjet Ink. Inkjet dyes can also come in the form of powders besides the usual liquids.
Inkjet printing on textiles was originally used for printing designs on carpets; however it has become much more popular for printing on textiles for apparel wear. As print runs per design become shorter, ink jet printing is fast become a commercially viable method of printing textiles, since the cost of expensive inks is offset by the elimination of the need for screens per colorway of a design as is required in flat and rotary screen printing.
Properties Required of Ink-Jet Dyes:
The two major classes of colorants, namely dyes and pigments. The choice of dye depends upon the ink used, whether it is aqueous, solvent or hot-melt and on the type of printer (thermal or non thermal). However irrespective of the solvent system, all ink-jet dyes have to satisfy a number of stringent criteria.
Though Black is the predominant color, for full color printing the three subtractive primary colors of yellow, magenta and cyan are also required. The key parameters are the absorption maxima (peak wavelength), and the shape of the absorption curve, in particular its broadness and the presence (or absence) of unwanted secondary absorption. Ideally, bright dyes (which have narrow absorption curves) are required in order to produce a comprehensive balanced color gamut because bright dyes can be made duller (e.g. by adding a shading color such as a black), but dull dyes cannot be made bright.
2. Color Strength:
The color strength of the dye should be as high as possible for several reasons. One reason is to enable prints having high optical densities to be produced, Other one is the increased flexibility possible in the ink formulation. The ink staprecipitation and also kogation on thermal ink-jet systems.
A dye for ink-jet should have as high solubility as possible to minimize any tendency for the dye to crystallize and cause problems such as nozzle blockage. Generally greater the number of sulfonic acid groups per molecule, the greater the water solubility. Carboxylic acid groups, especially in the form of salts, also confer water solubility on dyes.
4. Electrolytes / Metals:
Anion such as sulfate and especially chloride are undesirable in ink-jet inks due to the corrosion problem they cause to the metal print heads. Certain metal cations particularly divalent cations such as calcium, need to be removed, since these can precipitate with certain anions such as sulfate and with dye itself so the permissible amount is 100 ppm. Which is generally accomplished by dialysis or ultra filtration.
5. Light Fastness:
The final printed document must have reasonable light fastness (resistance to fading by light) if it is to serve any useful purpose. However it is well-known fact that in general dyes have much poorer light fastness than pigments.
6. Water Fastness:
The final print should be resistance to water so that if the paper gets wet or is rubbed by moist fingers, smudging does not occur. On the one hand high solubility is required for the ink, but on the other hand, water insolubility is required once the dye is on the paper. Hence a compromise position has to be reached to adequate solubility and adequate water fastness. One of the promising approaches to achieving high water fastness is by using the concept of differential solubility.
7. Smear Fastness:
This is effectively resistance to smear when using a highlighter pen. Such pens usually contain aqueous inks and can cause a dye to smear by facilitating the formation of water soluble species. Again dyes from solvent or wax / resin inks are less prone to smear than those from aqueous inks.
The shade or hue appears the same irrespective of the substrate on which it is printed. The shade or hue appears the same irrespective of the substrate on which it is printed. This can be achieving science the substrate varies enormously in physical properties. For instance, paper varies in texture, adsorption, additives, and pH, and ideally the dye should be insensitive to this differences.
Because of the increased awareness of Environmental health and safely, in recent years. It has been made imperative that screening test for mutagenicity (in bacteria) is the Amens test, and it is normally required that dyes should be Amens negative, i.e. not cause mutations in bacteria. Therefore, ink-jet dyes should ideally be Amens negative.
10. Thermal stability:
If the dye is required for thermal ink-jet system such as canon’s bubble jet or Hewlett Packard’s Think Jet, then the extra parameter of thermal stability or kogation fastness is required. Since the temperatures involved are as high as 300°C, then dyes stable to these conditions are required. Good kogation is when these deposits are absent or very low, whereas poor koagtion is when the deposits substantial. Poor kogation produce by two factors. The first is the presence of inorganic impurities such as iron, copper and silicon. The second is the degradation of the dye itself Solving the second cause of kogation, namely dye degradation, has proved more difficult.
Aqueous Ink-Jet Dyes:
Colorants comprising the first generation of inkjet dyes were selected for their vividness (high chroma), good aqueous solubility, and the stable inks derived there from which gave a reliable printing performance Inkjet printers have become considerably more sophisticated over the years, and demand for dyes and inks capable of producing very high print quality has grown.
Ink formulation research has complimented dye development and has been aimed towards achieving the following print properties:
- Good optical densities
- No feathering
- Minimal black to color bleed
- Uniform and controlled drop spreading
- Good water fastness
- Rapid dry time
- Smear resistance
- Media compatibility
Solvent based inks find application in continuous printers in the industrial segment where they are used to print bar codes, batch numbers, sell-by dates and other such information on packaging materials. The used of solvent based inks rather than aqueous inks affords a faster dry time and makes the inks more suitable for printing hydrophobic or non-porous substrates. Traditionally, ketonic solvents like methyl ethyl ketone are used but these are being replaced by less flammable alcohol. Solvent dyes are usually used as colorants and the predominant color is black. A typical example of a black dye used in this area is the trisazo dye. Another dye used in solvent inks is CI Solvent Black 35. This dye exhibits high light fastness.
These inks are solid at room temperature but become fluid when heated at 60-125°C. They are fired using a piezo printer. The ink vehicle includes Cig-24 fatty carboxylic acids and alcohol. Dyes that are soluble in the vehicle tend to be hydrophobic, an example of which is the modified xanthenes magenta.
Types of Inkjet Printer:
There are two types of ink-jet printer. The coarse resolution type and Stork true cote within these two types are further sub groups. It is in this area of fine resolution that there has been the most recent research activity.
Coarse Inkjet Printer:
These are normally based on valve technology and have essentially found use only in the carpet industry. There are two main commercial available systems. The Millitron system use an array of jets with continuous streams of dye liquid which can be deflected by a controlled air jet. The chromo jet uses computer-activated on/off valve systems to control the flow of the dye liquid. The use of electrochemical valves which are computer controlled to open and close rapidly so that liquid is fired in a succession of short pulses. The contribution of BTTG in Manchester on the type of technology but with increased resolution.
Stroke Tru Color inkjet Printer:
The Stroke Tru Color printer is a development based on the continuous stream technology (using the binary method). Essentially a dye formulation is pumped at constant pressure through nozzle, 14.4in diameter. The continuous stream is brolen up into droplets by modulation 625kMz, meaning that 625000 drooplets of colorant are formed each second . Because of the number of droplets per color in any one pixel area can vary between zero and 15, 16 color level possible. On each pixel of pattern which allows the production of smooth continuous tones. This contrast with drop on demand printing, which has to relay on dither patterns to naked eye, instead of the smooth shade afforded by continuous stream method.
Pre and Post Treatments in Inkjet Printing:
Printing with reactive and acid dye inks generally involves pre and post treatment in order for the dyestuff to fix onto the fabric. This is a multi- step process with a substantial degree of complexity.
Reasons for Pre Treatments:
The main reasons for separating the dyes from thickeners and other chemicals and applying them separately to the fabric are as follows.
- ‘All In’ inks are less stable and have lower storage stability, e.g. reactive dyes are more likely to hydrolyse when alkali is present in the ink.
- Chemical in the ink cause corrosion of jet nozzle; the detrimental effect of the sodium chloride on steel surfaces is well known, for instance; inks for use in ‘charged drop’ continuous printers should have low electrical conductivity.
- Thickeners in the ink often do not have the desired rheological properties.
- Some chemicals can be utilized in pre treated fabric but would cause stability problems in the ink e.g. sodium carbonate as alkali for reactive de fixation is acceptable on the fabric but not in the ink.
- The presence of large amounts of salts in aqueous inks reduces the solubility of the dyes; concentrated inks are required in jet printing due to the small droplets size.
- When pre treated fabric has been dried and then jet printed there is usually little need to provide a drying station to dry the print.
Steaming is the process normally used to fix printed textile. Reactive and acid dyes are steamed under atmospheric pressure at just over 100ºC. During the process steam condensed on the fabric and is absorbed by the thickeners and hygroscopic agents in the printed areas. Dyes and chemicals dissolves and form extremely concentrated dyebath within the thickener film. As the result of extremely low liquor ratio fixation is much more rapid than in exhaustion dyeing. High temperature steam is necessary for the fixation of disperse dyes on polyester. The Tg of polyester in steam is lower than it is in dry air, and fixation is more efficient. Usually steam is heated to 170-180ºC at atmospheric pressure, but sometimes pressure steaming at 130-150ºC is used. Pigment prints are cured hot air in a stenter or a roller baker.
Advantages of the Digital Inkjet Printing:
When consider as a whole, it can be said that digital inkjet printing above all saves time. It speeds up the process between design and industrial production in an almost dramatic way in an emergency, sampling for shade-Prototypes. Design variation ate possible overnight
There is also considerable cost reduction in these areas the cost savings are between 50% and 90%. Ink jet printing enables the production of luxury goods (designer fashion etc), custom made textile and the economical production of short runs and small batches. And yet, I the course of technical progress, the profitability ceiling of short production runs is fast moving upwards and could very soon reach a level where the average production quantities could be a interesting proposition.
A further advantage of high speed printing is that for the first time ‘Just In Time” production and delivery are possible. This opens up entirely new business opportunities and at all level reduces the stock level and fashion related risks.
In addition the digital production is more environmentally friendly as far as effluents; energy consumption and waste are concerned. Because of software aided optimization processes, there is hardly any textile waste and it is possible to make an exact calculation of the amount of the colorant to be applied.
In the end we can conclude that, inkjet printing is one of the state of art technique available in terms of quality, productivity and assured reproducibility in spite of initial high investment. Further research has to be focused for the betterment of technology.
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.