Drop on demand

DOD inkjet machine delivers a drop of ink only when required for printing. There are two common systems available: Piezoelectric or Bubble jet.

The majority of inkjet printers use bubble jet or thermal pulse. In these machines the computer signal heats a resistor to a high temperature, which creates a vapour bubble in a volatile component in the ink and the vapour bubble causes a drop of ink to be ejected from the nozzle. The vapour bubble must then cool and collapse, allowing the ink chamber to refill from the reservior.

Cycle time is limited to approximately 10,000 drop per second and volume per drop of ink is typically 150 - 200 pico litre (1 x 10 -12 L). Thus a single thermal inkjet can deliver approximately 0.1 ml of ink per minute.

The main problem with the thermal inkjet is the high nozzle failure rate. The high temperature required for rapid drop ejection cause decomposition of ink component on the resistor, which leads to poor heat transfer on nozzle clogging. Thermal inkjet offer low cost print heads but with low reliability and slow speed. Canon wonder printing machine of this type could print fabric of 1.65m wide at a linear rate of approximately 1m/m. It uses two print head in each of eight different colours with a total of 21760 nozzles. The system could print cellulosic, polyamide, polyester with reactive, acid and disperse dyes.

In piezoelectric DOD the computer imposes an electrical potential across a piezoelectric material, which causes a contraction in the direction of the electric field and an expansion in the perpendicular direction. The expansion causes a drop of ink to be ejected. The piezo on removal of the potential return to its normal dimensions and the ink chamber is filled from an ink reservoir by capillary action. The replenishment rate can be 14000 cycles per second. The drop size is somewhat smaller, but it gives good resolution (2880 dpi). They also have greater print head life.

Continuous Inkjet

It produces a continuous stream of drop and requires a system for selecting drops that will form the image on the substrate. The continuous drop stream is created by pumping the ink under pressure to a nozzle and either letting it break into drops randomly or stimulating drop formation by creating mechanical instability in the stream. Drop selection is achieved by deflecting the desired drops for the image to the substrate and collecting the unwanted drops for recycle. Most CIJ devices for high resolution printing use mechanical stimulation to generate a uniform continuous stream of drops. This is usually achieved by subjecting the ink stream to acoustic waves generated by piezoelectric operating at just under 1000,000 cycles per second.

Drop selection in most high resolution machines is achieved by including a charge on the drop and using an electrostatic field to select drops for printing. In this system the ink is electrically conductive, as the ink passes between the charging plates.

The critical aspects of inkjet printing still lies in the hardware, because commercial machine for textile printing needs minimum speed of at least 5m/m, which requires significant advances in nozzles techniques and its numbers.

Inks

The ink used for digital printing must be of very high quality because of the environment in which they are used. They must have very low viscosity so that they can flow through very small nozzles, and should be stable at as high temperature as 300oC. They should be reliable and consistant in colour, and have good light and washing properties.

The most significant advances have been made in the ink department area. Most companies have gone to high purity acid, reactive, and disperse dyes. All of these dye based systems require pretreatment and after treatment of the fabric, such as steaming, washing, finishing etc.

The Amber machine, from Stork

Digital Printing System’s DPS 65 was developed specifically for industrial printing of decorative fabrics.

Acid Dyes

Acid dyes are used to print wool, silk and polyamide fibres. This is a small segment but has proved its importance in high quality fabrics of wool and silk. The trend for unique design on swim suits have high demand for ink jet printing on polyamide fabric. Steaming is necessary for the fixation and a separate wash off is essential for the removal of unfixed dye.

Reactive Dyes

These inks can be used to print cotton, viscose and up to some extent wool and silk. These dyes gives very good alround fastness properties due to their covalent bond formation. For their fixation alkali and heat is necessary. Alkali must be applied by a pre-treatment process as it interferes with reactive dyes and nozzle components if put in the ink itself. The fixation is done by steam or hot air fixation process. A substrate wash-off is also necessary.

Disperse Ink

Disperse dyes based inks are already introduced for transfer printing of polyester fabric. The same class of inks with selection of fastness properties fulfilling the requirements of particular needs. They are applied directly on to the substrate and fixation is done by high temperature steaming, after which washing may or may not be necessary.

Pigment Inks

The advantages of these inks are that they can be applied to many different substrates and it does not require a special pretreatment like reactive and also does not require any after wash-off.

Future of inkjet printing

Looking into the future of ink jet printing, we must overview the efforts made so far in this technique by the printing machine manufacturers and dyes/ chemical companies. In this respect we should take into account the development carried out by DuPont and Stork who have made more practical advancements in this field.

DuPont artistry technology includes a specially developed printing machine built by Vuteck Inc. The technology is based on DuPont's inks with the company's colour control and management system (CCMS). This system comprises a fully integrated production-capable digital textile printing solution, which is targeted at the wide width home furnishing sector for items such as bed covering, sheets, pillow cases and comforters, curtains, draperies and upholstery.

The system offers a combination of speed, quality, affordability as well as design flexibility and the ability to produce profitable and quick high quality short run jobs.

Artistri ink are available in eight aquous pigment colours, including cyan, light cyan, magenta, light magenta, yellow, green, orange and black.

Stork and Lectra have introduced their Amber inkjet printing machine which is capable of printing acid, pigments and disperse dyes. This machine does not require a pre-treatment nor after-treatment (except for heat fixation of pigments). Stork also offers Zircon polyester printer and the Amethyst which has roll to roll printing facility.

The future for inkjet printing technology is definitely very promising. However it is very difficult to forecast the level of market penetration they will reach in the immediate future, as speed limitation against existing methods and the industry's build-in resistance to change so far have blocked its wide spread acceptance.

However, many important suppliers of machines, chemicals, and software are confident that inkjet will have an increasing future role.

References

· Karl Siemensmeyer BASF AG TCC& ADR vol .32 No 10
· Stefanini, J.Philippe, Book of papers 1995 IC & E, AATCC.
· Tincher, Wayne, textile chemist and colorist vol .28, No 12 1996
· Noll Fred. 1st digital printing of textiles briefing .
· 12th international conference on digital printing technologies. October 1996
· AATCC review June 2001.
· AATCC review January, March, July 2003.

Printed fabric is rolled immediately on the DuPont 3210 printer