1. About the Digital Textile Printing (DTP)
The advantage of digital printing and coloring of textures, yarns, and garment includes a piece of a couple of decades, a dynamic present and likely a brilliant future. This presentation represents the birthplaces and advancement of material printing to computerized arrangements. It distinguishes a portion of the numerous makers and pioneers of these innovations and surveys their effect on the printing industry. It talks about a couple of the false begins that added to supported accomplishments of computerized printing innovations for material embellishment. It utilizes the presentations that have seen the presentation of inventive advances for material printing as street denotes that demonstrate patterns and indicate the way computerized material printing's brilliant future.
In the near future, customer-oriented mass customization will become one of the most important trends in many manufacturing industries. DTP (Digital Textile Printing) is appropriate for mass customization in the article of clothing the business. In any case, DTP isn't so appropriate for large-scale manufacturing in light of low profitability and generally high cost contrasted with customary printing forms. For commercial textile products, good strength and fastness of color are necessary. Therefore, most studies on DTP focused on the pre-and post-processing of fabric, as well as modification of additives to increase color strength and fastness, used cotton fabric and reactive dye ink to determine the optimum printing condition for each color. Challenge in, to find the optimum steaming condition including process time and temperature for reactive dyes.
Inkjet technologies are typically classified into two large classes: Continuous Ink Jet (CIJ) and Drop-on-Demand Ink Jet (DOD). In CIJ, ink is squirted through spouts at a steady speed by applying a consistent weight. The stream of ink is normally shaky and separates into beads soon after leaving the spout. The drops are left to go to the medium or diverted to a drain for distribution relying upon the picture being printed. The redirection is generally accomplished by electrically charging the drops and applying an electric field to control the direction. The name continuous' starts in the way that drops are shot out consistently.
In DOD ink stream, drops are shot out just when expected to frame the picture. The two principles drop ejector components used to produce drops are piezoelectric ink fly (PIJ) and warm ink fly (TIJ). In PIJ, the volume of an ink chamber inside the spout is immediately decreased by methods for a piezoelectric actuator, which crushes the ink bead out of the spout. In TIJ, an electrical warmer situated inside every spout is utilized to raise the temperature of the ink to the point of air pocket nucleation. The unstable development of the vapor bubble pushes the ink outside the spout.
Also Read: Digital Textile Printing Process
2. Classification of Inkjet technologies
There are mainly 2 types of inkjet technologies.
CIJ (Continuous ink jet)
DOD (Drop on demand).
Classification of Ink Jet Technology
(E MARIANO FREIRE, 2006)
2.1 Continuous inkjet
In CIJ the jet of ink generated by each nozzle breaks up into droplets shortly after exiting the nozzle. Without any other intervention, the breakup would occur randomly and would result in droplets of variable sizes. This is usually corrected by providing a periodic excitation
to the nozzle in the time domain that translates into a spatial perturbation in the jet of fluid. The combination of the jet velocity and frequency of the excitation determines the droplet size, which can be controlled to very high accuracy. In the traditional CIJ approach, a piezoelectric transducer is coupled to the print head to provide the periodic excitation. The oscillations are therefore mechanical in nature. After leaving the nozzle, the drops are electrically charged by an amount that depends on the image to be printed. The drops then pass through an electric field to cause them to deflect. There are two ways of deflecting the drops in piezoelectric-driven CIJ. In the binary deflection method, the droplets are directed either to a single pixel location in the medium or to the recirculating gutter. In the multiple-deflection method, the deflection is variable so the drops can address several pixels.
In CIJ, ink is squirted through nozzles at a constant speed by applying a constant pressure. The jet of ink is unstable and breaks into droplets as it leaves the nozzle the drops are left to go to the medium or deflected to a gutter for recirculation depending on the image being printed. The deflection is usually achieved by electrically charging the drops and applying an electric field to control the trajectory. The name `continuous' originates in the fact that drops are ejected at all times. (E MARIANO FREIRE, 2006)
2.2 Drop on demand technology
In DOD ink jet, drops are ejected only when needed to form the image. The two main drop ejector mechanisms used to generate drops are piezoelectric ink jet (PIJ) and thermal ink jet(TIJ).
In PIJ, the volume of an ink chamber inside the nozzle is quickly reduced by means of a piezoelectric actuator, which squeezes the ink droplet out of the nozzle. In TIJ, an electrical heater located inside each nozzle is used to raise the temperature of the ink to the point of bubble nucleation. The explosive expansion of the vapor bubble propels the ink outside the nozzle. (E MARIANO FREIRE, 2006)
Drop on demand
3. Digital Textile Printing Method
The digital textile printing method
Direct to Fabric (DTF)
Direct to Garment (DTG)
3.1 Direct to Fabric (DTF)
In Direct to Fabric (DTF), the printing is performed directly on a roll of fabric. Depending on the type of fabric various inks are used, which include:
Pigment ink is used for cotton fabrics
Acid dyes work primarily with nylon and silks
Reactive dyes provide best results on cotton and other viscose materials, and
Disperse dyes (sublimation) are almost exclusively used with polyester. DTF digital textile printer enables printing on cotton, silk, polyester and rayon using reactive ink, acid ink and disperse ink.
Source: Clothing, Sports Wear, Home Textile, Soft Signage. (http://www.dcc.co.in/
3.2 Direct to Garment (DTG)
Direct to garment printer by EPSON
DTG printing is a process of printing on textiles and garments using specialized or modified inkjet technology. Direct to Garment (DTG) print making the best use of the newest in print technology with smaller droplet sizes, more precise placement and ever clearer images printing onto light and dark t-shirts.
The following applications under DTG covered:
4. Pre-treatments for ink-jet printing on cotton & cellulosic fiber
Pretreatment with stentering machine
Cotton, viscose rayon and lyocell fabrics are normally jet printed with reactive dyes by the two-phase method, i.e. the fabric is pre-treated with thickener and alkali, while the ink contains the dye. The pre-treatment liquor is normally applied with the aid of a pad mangle, though it could be screen printed. In either case the fabric must be dried to about 5±7% moisture content before printing. The main constituents of the aqueous liquor are usually thickener, alkali and urea.
4.1 Pre-treatment for reactive dyes (MCT type) on cotton
100 g/L Medium viscosity sodium alginate, e.g. 6% Lamitex M5
100 g/L Urea
20±30 g/L Sodium carbonate
Pad (approx.75% pick-up) ± Dry at120°C or below*
For viscose rayon increase urea to 200 g/L and add10 g/L Lyoprint RG (Ciba)
The reason for the low drying temperature is that urea is unstable at higher temperatures and these cause high levels of fumes at the stenter exit.
4.2 Pre-treatment for silk prior to reactive ink-jet printing
10 g/L Lyoprint RG (Ciba)
100 g/L Alginate medium viscosity, e.g. 6% Lamitex M5
100 g/L Urea
30 g/L Sodium bicarbonate
Pad (approx.75% pick-up) ± dry at low temperature,100±120°C.
4.3 Pre-treatment for wool and silk prior to acid dye ink-jet printing
Pre-treatment of silk (Fabric passing the from Stenter machine)
150 g/L Guar gum thickener, e.g. Meyprogum NP 8 (8% solution)
100 g/L Urea
50 g/L Ammonium tartrate solution (1 part water to 2 parts ammonium tartrate)
Pad (approx.75% pick-up) ± dry at low temperature,100°C or below.
4.4 Pre-treatment for polyester printed with disperse dye ink
10 g/L Cibatex AR (Ciba)
100 g/L Sodium alginate medium viscosity, e.g. 6% Lamitex M5
Pad (70% pick-up) ± dry.
5. Reasons for pre-treatment
The main reasons for separating the dye ink 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 hydrolyze when alkali is present in the ink.
Chemicals in the ink cause corrosion of jet nozzles; the deleterious effect of 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 dye 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 droplet size.
The advantage of applying thickeners and chemicals separately from the dyes is that it allows the wettability and penetration properties of the fabric to be adjusted.
Post-processes include steaming, washing, and drying. Steaming is used for fixing the dye on the printed fabric. The temperature, pressure, and time should be determined for optimum results.
When the pre-treated fabric has been dried and then jet printed there is usually little need to provide a drying station to dry the print, as the printing process is so slow. By the time the fabric is batched on a roll it has dried by exposure to the warm atmosphere in the room. However, in most instances, fixation and washing will be necessary. This not only ensures that the full fastness properties of the dyes are realized but also brightens and alters the colors significantly.
Steaming is the process normally used to fix printed textiles. Reactive and acid dyes are steamed under atmospheric pressure at just over 100 Degree Centigrade. During the process, steam condenses on the fabric and is absorbed by the thickener and hygroscopic agents in the printed areas. Dyes and chemicals dissolve and form extremely concentrated dyebaths within the thickener film. As a result of the extremely low liquor ratio (approximately 1:1) 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, the steam is heated to 170±180°C at atmospheric pressure, but sometimes pressure steaming at 130±150°C is used. Pigment prints are cured using hot air in a stenter or a roller baker.
Fixation: Steaming process
After printing and steaming, washing of ink-jet printed fabric is carried out. Washing the fixed jet printed fabric is likely to be carried out on a batch-wise basis since the lengths printed are usually quite short. The process, whether batch-wise or continuous, takes place in several stages, the first of which should be a cold rinse. The reason for this is that thickener, auxiliaries and loose dye should be removed under conditions where the dye is unlikely to stain white or unprinted ground shade areas. The risk of this happening may be reduced further by the inclusion of reagents that hold the unfixed dye in the bath. For instance, the inclusion of 1 g/L sodium carbonate in the first bath when washing off acid dyes prevents protonation of amino groups on nylon, wool or silk, thus removing any potential dye sites.
Washing after printing
Washing after printing
The washing was done in three steps:
First, the fabric was cold rinsed.
The main requirement after the first cold rinse when washing off reactive dye prints is to ensure that the temperature of the hot wash reaches a minimum of 90°C, otherwise, hydrolyzed dye may not be removed.
Then the fabric was washed with hot water (at the temperature of 80 - 90°C) containing 2-3 drops of surfactant (soaping SN).
After that, the fabric was again cold rinsed.
Washing after printing