Abstract:
Customers’ demands are now changing rapidly. Conventional printers are not capable to response to these frequent changes. Also, textile wet processing like printing, dyeing, creates huge bunch of effluents, which are hazardous for the ecosystem and even for the human being. Digital textile printing will help the manufacturers to respond to the changes of customers’ demand and to reduce the amount of effluent. Digital textile printing is not the competitor of traditional printing system but it can be installed as a complementary opportunity of conventional printing for printing houses along with traditional printing. The main issue related to digital printing is wash fastness of the printed substrate. This very issue is now resolved by the introduction of compatible ink chemistry for particular substrate. Digital textile printing is economical for nearly mass production as the production speed is relatively lower than conventional printing machine. Basically, it is appropriate for customized product and frequent design changes to meet the rapidly changing customer choice. The intended purpose of the project is to create consent about digital printing.
Keywords: Ink jet printing; color management; Ink chemistry; Textile substrate, Mass customization, sublimation inkjet printing.
1. Prologue:
Almost all sorts of machinery in the manufacturing industries are now equipped with sophisticated but automated technology, which is the consequence of modernization of technology. In the textile printing sector one of the prominent advancement is the innovation of digital textile printing technology. From the first simple block printing to precise metal engraving, then photographic technique used to engrave flatbed screens, rotary cylinders and copper rollers. Now we are only at the beginning of the digital age of printing and this new technology will be just as important as the arrival of the jacquard loom long ago. It will change the mode of textile production and distribution and it will give the designer an area of exploration that has never been explored. [13]
Digital textile printing is very much desirable technology to print textile substrate. The idea comes from the well known paper printing, which is done through the aid of computer. The requirements of processing in digital textile printing are less than conventional printing. Digital textile printing is quite a new technology to adapt for Bangladesh but it will not be difficult to install this very technology. Normally people consider this technology as what it cannot do instead of what it can do. [12]
Now-a-days markets are changing rapidly and the only way to survive in this unstable market is to follow customers’ needs. However, conventional printing cannot cope with this ever changing market. Digital printing can satisfy the customers’ demand without increasing costs and waste. [9 ]
2. Direct Ink jet printing technology:
Ink jet technology enables the discharge of liquid ink to a substrate whereby only the ink droplets make contact with the substrate. It is a non impact printing method. Theories of ink jet printing were developed by Lord Rayleigh in the late nineteenth century but the improvement of technology itself did not start until the late 1950s to 1960s. Ink jet printing requires three basic elements, all of which need to coincide well in order to produce admissible output. These elements are the print head, the ink, and the medium. [1]
Ink jet technologies are basically classified in two broad classes: Continuous ink jet (CIJ) and Drop on demand (DoD) ink jet. In CIJ, ink is forced out through nozzles at constant pressure. The jet of ink is naturally unstable and breaks up into droplets after leaving the nozzle. The drops are allowed to fall to the medium or deflected to the gutter. The distraction is usually done by electronically charging the drops and applying an electric field to control the trajectory.
In DOD ink jet, drops are poured only when needed to form the image. There are two main drop ejector mechanisms used to generate drops, one is piezoelectric ink jet the other one is thermal ink jet. In PIJ, the volume of an ink chamber inside the nozzle is rapidly reduced by means of piezoelectric actuator, which forces the ink out of the nozzle. In TIJ, an electric heater placed inside each nozzle is used to raise the temperature of the ink to the point of bubble nucleation. [1]
2.1 Profiles of some digital textile printers:
Dupont Artistri 2020 [4]
- $185K
- 16 Seiko printheads, 8 per gantry
- 8 color pigment, acid, reactive, disperse dye
- 10 mm clearance
- Adhesive blanket
- 1.8 m max print width
- Color control/workflow
Reggiani DReAM [4]
- Aprion Magic PIJ
- 7-512 nozzle heads/color
- 600 dpi
- $750K 6 colors
- 155 cm print width
- 150 m2/hr speed (35kHz)
- Fiber reactive & acid dye sets
- Hot air blower dyer
- Blanket washing
Osiris [4]
- $3 million
- Image CIJ
- Over 1000 linear meters/hr
2.2 Process flow chart of direct ink jet printing [5]
3. Indirect ink jet printing:
3.1 Heat transfer printing:
Heat transfer printing is as sustainable as screen printing, but offers more details. Heat transfer causes heat to create a bond between the resin and fabric, locking the ink between the two layers, but has a slightly plastic feel after the printing has been done. In this method logos are usually printed onto the T-shirts very economically. It would be an effective means for apparel industries to add value to the finished products as well as the appearance of the product will be spectacular. [5]
Fig 1: Heat transfer printing [5]
3.2. Sublimation ink jet printing:
It is an alternative to heat transfer printing technology. It performs better than transfer printing. Once the sublimation takes place the image is permanently bonded with the fabric. The printed surface will be smooth to touch. It is the major technology used in digital textile printing, although the process is limited to synthetic fabric, but controlled processing will allow printing natural fabric. [5] In case of printing on natural fabric, the temperature of sublimation should be compatible with the target substrate.
Sublimation uses disperse dye, heat-activated inks that change when heat is applied and have the ability to bond with polyester or acrylic fibers. Consistent standard temperature is important in the production process, because, otherwise, temperature variation will result in color variation in printed surface. [5]
3.3 Sublimation ink jet printing process flow chart: [5]
4. Fabric preparation:
Fabric preparation for optimum print quality is required in digital textile printing. Scouring or bleaching is used to remove natural impurities to have clean white surface for the application of color. There is another way to process the fabric by coating onto the fabric before printing to confirm that when the droplets of ink come in contact with the surface of the cloth they do not spread. The coating basically comprises of alginate thickener for reactive dyes and carbohydrate-based or synthetic thickener for acid or disperse dye.[5]
5. Color Management system:
Perception of color is subjective, so it is difficult to categorize. The CAD screen, printers, scanner and digital camera all use the additive RGB model as a fundamental for displaying color, but textile printers use CMYK subtractive model. It is tough to transfer one model to another one. Additive color is formed by the combination of light wave length and subtractive color is produced by mixing color particles. Basic colors for additive model are Red, Green, Blue and for subtractive model are Cyan, Magenta, Yellow, Black. [5]
Color management is the technology related to the correct interpretation and rendering of color information. Matching colors from design to the digital printer is critical to the customer. It is also important that colors are matched to the screen printing process. Screen printers will visually look at the printed textile sample and mix different base colors of ink to obtain the final color. This is done in a `color kitchen’ similar to mixing paint colors. A typical rotary screen printer can have from 1 to 12 spot colors and from 1 to 12 screens. Each color is mixed individually and is called a colorway. To change a colorway, any one or more of the spot colors change, but will print the same pattern. The color gamut is very large due to the relatively high number of base colors available. [3]
In a digital textile printer the colors are mixed or dithered directly onto the fabric, and almost all systems use the four base colors CYMK. Some systems, such as the 2020, use up to eight colors. The color information is in a digital file and must be converted to the correct color by mixing the colors available in the printer.
The most common color data format for printers is L*a*b*. L* is the lightness ranging from 0 (dark) to 100 (light). The a* value defines the colors of a red ± green axis and b* defines the yellow ± blue axis. Using a spectrophotometer to measure the colors in L*a*b* space, a printer’s color gamut can be determined and a lookup table can be created. The lookup table tells the printer what colors to mix or dither to create the required color. This is measured against a color standard such as a CIELAB reference. The user can print a `color book’ for a visual representation of the color gamut. [14]
One of the most difficult challenges is to match the process color of the printer to the spot color of the screen system. Digital printers are not economical for printing long run lengths. After sampling and short runs are complete, a large production run may need to be done on a rotary screen printer. If the color gamut of the digital printer is outside the available spot colors or vice versa, a color match may be difficult to achieve. [6]
Textile screen printing is primarily a spot color process and most inkjet printers, such as the 2020, utilize process color. These two approaches differ in that the colorants used to color the textile are premixed in the case of spot color, and mixed on the fabric in the case of process color. Process color printing is generally composed of black, cyan, magenta and yellow inks that are mixed in varying proportions by jetting droplets onto the fabric to create a variety of colors. The color gamut achievable by mixing only four colors of the same chemistry is far less than the colors obtainable in spot color. Spot color printing uses a set of `mother’ colors numbering between four and 12. In an attempt to produce more colors with process color printing, either dilute four-color process inks or up to eight different colors are used. While this provides improvements, it still does not reach the combination of color correctness and functionality of spot color printing in color-critical applications. [6]
6. Dye –fiber chemistry:
6.1. Water based ink:
There are several types of inkjet inks for textile printing, depending on kinds of fabric and dyes. In each combination various dyes and textiles have been developed and improved to satisfy advancing practical requirements. The principles of dyeing of polymer are exemplified in the figure below in which the inter-molecular interaction and chemical bonding between dye molecule and polymer in fiber are shown. Acid and direct dyes are bound by ionic bond, reactive dyes on the cotton are bound by covalent bond, disperse dyes are bound by Vander Waals forces and hydrogen bonding. The printing processes give required chemical and physical conditions for these bindings. Process conditions promote absorption on the fiber surface, diffusion into the fiber, and dissolving in the fiber polymer or formation of chemical bond.
Fig 2: Fiber and dye interaction
In the case of inkjet printing on textiles these post-processes, including instruments for fixing, can be applied, but main differences between desktop inkjet printers and textile printers are kinds of dye, their concentrations, and physical properties. The reliability and printing characteristics of the formulation are not inherent properties but depend on print head and process architecture. Ink formulation and properties must be tested and tuned on each printing system. [3]
Fig 3: Dye absorption on printing and color strength (k/s) of printed fabric. [3]
6.2 Pigment based ink:
Textile pigment inks with emulsion-based textile binders for ink jet printing is extremely challenging due to ink stability and jetting reliability (drying and nozzle clogging) issues, especially for low viscosity print-heads. Pigment inks for digital textile printing present different challenges than dye-based textile inks. Pigment inks contain 50 to 150 nm crystalline particles as a colloidal system while dye inks are uniform dye molecule solutions. The permanence properties such as wash and crock come from acid (on the dye molecule) and base (on the fabric) interaction for acid dyes, from the dye solubilization in the polyester fibers for dispersed dyes and chemical reaction between the dye and the fabric for the reactive dyes. On the other hand, for pigment inks, the acid ± base interaction and/or chemical reaction between pigment particle and the fabric, if any, are simply not enough and a soft, low glass transition temperature (Tg) polymeric binder is required to achieve adequate permanence properties such as wash and crock. Pigment based ink is perfect for all sorts of textiles, especially for cotton and polyester. Quick drying up of the binders builds a thin membrane and blocks the nozzle opening. As a consequence, the flow of ink gets hindered that may cause production break down. Right choice of binder could be the perfect solution.
Table 1: Fastness of pigment printed fabric. [11]
6.3 Dye based ink:
Different types of dye molecules are used to formulate dye based ink, which is usually to print particular type of fabric. For instance, Acid dye based ink is used to print silk, Nylon, Wool. In this case pretreatment is done by acid donor and as post-treatment, washing and steaming is carried out. Disperse dye based ink is used for polyester fabric. Thickener is used as pretreatment agent and post-treatment is done steaming and washing at high temperature. Reactive dye based ink is used for cotton and rayon fabric printing. Alkali is used as pretreatment agent and post-treatment is done by steaming and washing. [10]
7. Raster image processor (RIP):
The software frequently used for digital textile printers, is known as a RIP-Raster Image Processor. It is used to convert RGB data from the displayed image into the printer, and drives large format printers and to print larger volumes of ink and parameters such as the number of repeats to print out and the total length of fabric entered in this stage. The printer driver software is also used to set variables such as print speed, number of passes for the print head and how much ink will be laid down. [8]
8. Areas of application of Digital textile printing:
Digital printing has wide range of area to cover up such as sampling, strike-off, mass customization but the use depends on the intention of the manufacturer. In digital printing sampling cost and time is negligible. Sample or pattern can be prepared for screen printing through digital technology to reduce costs and time.
Strike off is a way of producing a sample of a single, salable item for markets such as luxury, entertainment, or special events. With digital printing strike off can be done easily to response the instant demand of market.
Mass customization means the responsiveness to the individual customers’ demand. This opportunity allows the increase in variety of products without corresponding increase in costs. [9]
9. Graphical presentation and interpretation:
9.1 Cost per square meter verses print run in square meter: 
Fig 4: Print cost comparison with print run length. [4]
In the above graph, it is clearly seen that the costs associated with ink jet printing is always almost constant irrespective of print run length, but in case of screen printing, which is the most preferable means for printing, the costs are very high at the short run length and for long run length these costs are lower than ink jet printing. Instead of higher costs for long run length ink jet printing is preferable to have sheer perfection in printing.
9.2 Drop diameter verses time:
Fig 5: Ink drop impaction process with time [7]
In the depicted graph, the relation between the state of diameter of ink drop and time is shown. It is shown that the diameter of ink is changed as the time goes on. At the equilibrium state of the diameter gets its permanent shape in the fabric to emerge as constituents of printed image.
9.3 Investment in machine against performance:
Fig 6: Performance according with investment on printers. [6]
Most low range printers use Epson print head technology and Mimaki is the leader in this range with the TX-1, TX-2 and TX-3. Other, Epson print head- based textile printers are offered by Mutoh and Roland. There are Konica printers in this range that have been sold primarily by Stork. Konica’s printers use their own print head technology, which is based on a Xaar license. These low-end textile printers are typically wide format printers modified for printing on fabric. The primary applications are sampling due to the low speed. Prices in this arena are under $100,000.
The DuPont ArtistriTM 2020 printer is in the mid-range category in both speed and cost. The Robustelli Monna LisaTM printer, based on Epson print- heads, is also in this range. Konica-Minolta has recently launched a new printer, the Nassenger V, using their own print head technology. Printers in the mid- range category are used for sampling and short run production. Prices for printers in this range vary from $185,000 for the ArtistriTM 2020 to approximately $250,000 for the Robustelli Monna Lisa.
At the high end is the Reggiani DReAM printer. Performance is up to 150 square meters per hour. The DReAM utilizes Aprion (Scitex Vision) printhead technology. It is capable of printing up to six process colors. The primary application is short run production. The DReAM printer price is over $500,000. [6]
10. Impact of ink jet printing:
Ink jet printing contributes in various ways, for instance design, work flow, environment etc. Unlike conventional printer, digital textile printing has some great advantages. In digital printing scope of creativity is wider than conventional printing, so, quick response to the customers demand will be easier. These advantages are the significant signs to install digital textile printing in Bangladesh. Though there is some sort of drawbacks but those are negligible. The benefits of digital printing are listed below.
- Reduced production costs
- Efficient use of consumables
- No requirement to produce new screens
- Minimal set-up costs –short runs are economical
- Cost per print same for 1, 10, 100, 1000, 10,000
- Printing on readymade garments, even jackets, sweaters possible
- Possibility of printing over the stitch and uneven surface of the
- substrate
- Storing the design in memory for longer period to use again, in
- case, needed
- No time for set-up –printer is always printing
- Faster response
- Print on demand
- Just-in-time customization/personalization
- Much quicker introduction of new designs
Though there are lots of positive signs of digital textile printing, it is still impossible to install this very technology for mass production. The vital obstacle is the consistency of production cost regardless of the production volume, whereas in screen printing cost rapidly decreases in screen printing as the production volume gets bigger. So, digital printing should be used with screen printing in the same printing house to get the best cost effectiveness and greater quality product. [10]
11. Issues with ink jet printing:
Digital textile printing is still used for sampling on the other hand it is not used bulk production. As this technology is not in mass production stage, the issues related with this technology is not visible.
One of the prominent issues is “banding”. Usually, the print head moves to and fro over the fabric in single line and the substrate is dragged under the printing head and the image is transferred on to surface. So, slight interruption of the dragging speed will cost in narrow strips, banding, of unprinted surface. Fortunately, banding could be reduced using redundant printing head. [9]
Another type of problem associated with printing head is misfiring or clogging. In both cases some pixels could be missing or partially colored. This problem is similar as “stick-in” problem in the rotary screen printing, but in case of digital textile printing it can easily be removed. Here, redundancy of nozzle could be used as the solution of the problem as well.
Fabric handling is an area of mechanical failure of digital printing. Deformation of dimensional stability during printing will eventually cause greater loss and image distortion. Proper and fault free tensioning device will lessen the problem.
Color repetition is another issue, which is one of the greatest obstacles of digital printing. Production of similar color from printer to printer is almost impossible. Difference in quality of ink from manufacturer to manufacture, from cartridge to cartridge, and physical properties of substrate is the main reason behind this color variations.[9]
Finally, digital printing at the moment is feasible for short run printing, as the price of ink is excessive and printing speed is limited. Instead of above mentioned issues, it could be possible to install digital printing in Bangladesh. But this attempt will require huge attention in creating particular consumer market, where access to internet and other available information will be providing to the consumer in quick succession. Recognition of particular consumer market and proper managerial approach could be the considerable ways to persuade for digital printing installation in Bangladesh.
12. Epilogue:
Digital textile printing technology has come a long way since the time it was first introduced a couple of decades ago. Initially driven by the need of simplifying optimizing the sampling process, it has evolved a new technology with several areas of application in mass customization and luxury market. It could be used as one of the prominent means of decoration for designer product.
This technology could not be helpful alone, but in addition with conventional printing it would give the best cost effectiveness practice for manufacturer. Low volume and event oriented customized product can be printed by this technology because printing cost for low volume is less than the printing cost in conventional printing.
In Bangladesh textile engineers have very few knowledge about digital printing. This project will help them have some technical knowledge as well as scope of digital printing in textile printing industry. Redecoration of printing house with the combination of digital and conventional printing machine is the intended purpose of this project. Bangladesh needs to grab the opportunity of technological advancement in textile field such as digital printing. Adaptation of this very technology along with conventional printing will eventually lead us to financial and esthetic benefits.
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