3. digital printing technologiesPresentation Transcript
DIGITAL PRINTING TECHNOLOGIES R.B.Chavan Department Of Textile Technology Indian Institute of Technology Hauz-Khas, New Delhi 110016
Digital printing technologies
Various technologies for digital printing of paper.
Inkjet printing most popular both for paper and fabric printing.
Directing small droplets (usually between 50 and 60 microns in diameter), of ink from a nozzle onto printing substrate.
The droplets can have different colors.
Droplets are combined together on the substrate to create photo-quality images.
Positioning of droplet is controlled by high frequency digital signals from computer.
Droplet formation involves application of a controlled pressure on liquid ink in its reservoir as it streams into the printing nozzles the ink stream is broken into droplets.
Different technologies for application of pressure on liquid ink.
two main technologies
Both technologies use large number of nozzles (10-100 Micron dia,) for each colour (CMYK)
Between 1000 to million drops processed per second.
Depth of shade is controlled by number of drops applied on substrate.
continuous stream of ink is produced by forcing it through a nozzle at a pressure of about 3x10 5 Pa.
The resulting high velocity breaks the ink stream into droplets.
The size and (number) of droplets produced depends largely on
the surface tension/viscosity of the liquid ink,
the nozzle diameter and
the applied pressure.
Around 100,000 droplets per second leave the ink chamber,
although with very high performance printers upto 625,000 drops/ second can be ejected.
Directional control over the droplets is obtained by selectively inducing an electrostatic charge on them
as they leave the nozzle. The charged droplets then pass through a set of like-charged plates which repel and deflect the droplets either to the required position on the substrate and the uncharged droplet to the gutter for recycle or vice versa.
SINGLE JET SYSTEM
Drop volume larger than DOD inkjet system.
Volume of ink delivered per unit time larger than DOD systems.
It is a form of single jet continuous ink jet that uses extremely small nozzles,
producing very small droplets.
form a more controlled image.
The system offers higher print quality.
The disadvantages are that the fine jets are
prone to clogging.
Further, present systems are very slow and can take up to one hour to produce an A4 format in four-colour print.
Multiple jet system
Differ from single jet system.
Ink drops are given variable charge that gives different deflection as the drops pass through deflection plates.
This allows multiple positioning of ink drops (up to 30) on the substrate to be printed from a single jet.
The uncharged drops go to the gutter for recycle.
Advantages and disadvantages
ability to cover a larger band width print area with one pass
Higher productivity compared to single jet system
long print head life over thermal or piezo drop on demand printers,
High initial cost of the system
Extremely low viscosity ink (3-6 CP) and electrical conductivity.
In general, the initial high cost of CIJ heads currently prohibit their use for low volume applications .
Milliken Millitron Air jet deflection
Ink drops are deflected on the substrate by computer controlled air stream.
Bursts of air from the nozzles are used on computer demand to deflect colour dorplets out of the stream and onto the moving substrate.
The system has been in commercial use for carpet printing for a number of years. (Milliken Millitron)
The initial Millitron system had 8-colour capability with 10 nozzles/in for each colour
No attempts to merge primaries to produce a variety of shades is attempted and the newer versions of the Millitron technology incorporate an expanded range of colours and jets.
Air jet deflection
Air jet deflection
This type of inkjet used to print carpets better than analog technologies due to its ability to vary dye delivery pressure so as to penetrate color into different carpet pile thickness.
Due to their larger orifice and drop size, carpet printers deposit a larger volume of ink with somewhat higher viscosity than other inkjets. ( 100 to 400 cps against 1-30 cps)
Since most carpet yarns are coarser than apparel yarns, carpet inkjet output with coarse resolution in the 20 to 30 dpi range do not encounter customer resistance, because the coarseness of carpet inkjet printing matches that of the carpet yarn.
These printers have proven successful because they operate at profitable production speeds (about 20 running meters per minute, with print resolutions acceptable to the market.
Their disadvantages are their low resolution and that they generate waste ink because system is based on spot colours and not on process colours.
The disadvantages of the Millitron system are cleanup times required on colour change over and the lack of fine print definition
Not Suitable for textile printing.
Drop on Demand (DOD) Technology
The difference between DOD and CIJ
In CIJ the pressure applied on ink reservoir is continuous
In DOD the pressure applied is non-continuous
The pressure is applied only when a droplet is needed. (Drop on demand)
The pressure is applied in response to digital electronic signals from the imaging computer.
Entire colour goes directly onto the fabric: no waste".
cost effectiveness of short runs.
In the valve jet system, an array of 7–9 jets are aligned in vertical formation.
The valves open on response to digital electronic signals and allow a droplet of ink to be propelled, under pressure, onto the substrate.
the print quality is crude and of low resolution. i.e. low DPI
Suitable for printing carpets.
Piezo print-head technology
This technology uses Piezo crystal
A Piezo ceramic crystal is located at the back of the ink reservoir of each nozzle .
small electronic impulses delivered to suitable crystalline materials (Transducer) causes them to expand.
crystals of Lead-Zirconium-Titanate (PZT) or ceramics are used as transducer.
On removal of electronic impulse the contraction of crystal takes place.
Crystal expansion will apply pressure on ink to eject the drop from nozzle.
Droplets are generated intermittently according to the electronic
drop volumes are usually (150 picoliters).
The small drop size allows the piezo-based printers to produce very high- resolution prints (1440 dpi is commercially available).
Piezo-print engines are now in use in a number of printers for textile substrates
Piezo crystal technology A typical construction comprises an array of nozzles, each with its own piezoelectric crystal. Since no deflection of droplets is needed, guttering and recirculation are not required, which simplifies the design and construction of the printer.
Principle of drop-on-demand and piezoelectric ink jet printer
TYPES OF PIEZO PRINTHEADS
Several variations exist of the basic mechanism by which crystals of Lead-Zirconium-Titanate (PZT) (transducer) turn electrical signals into mechanical pressure pulses to produce ink droplets.
hybrid or “coupled” mode.
A number of patented designs and manufacturers exploit these various mechanisms to differing effect.
Electric charge causes a shearing action in the distortion of the PZT piezoplates against the ink causing ink drops to eject from the nozzle
PIEZO SHEAR MODE
The shear mode action makes it possible to achieve tightly packed assembly of many jets in a printhead with just one piece of piezoelectric plate.
Spectra, Xaar, Brother,and Olympus.
Wide ink choice of 20-25 cp viscosity range.
Manufacturers: Tektronix, Sharp, Epson. The advantages of these heads are that they form very accurate circular droplets, and they are mass produced and low cost. Their disadvantage lies in the requirement of extremely low viscosity inks of about 1.6 to 3 cp.
MANUFACTURERS:Dataproducts, Trident and Epson It is a relatively high energy process. ejects droplets which are more elongated They are relatively robust and can use inks in the 10 to 20 cP range.
Seimens successfully marketed this technology for office printing. In this mode, electrical charge deforms a piezoceramic tube so that it squeezes the ink within the tube forcing it out through the nozzle end.
Combines shear mode with bend mode technology to squeeze and push ink through its nozzles. Relatively fast and robust printheads. Use a range of ink types. Relatively fast processing speeds and the ability to produce variable droplet sizes with higher viscosity inks than thermal inkjet.
Advantages and disadvantages of Piezo electric printers
Greater print head life than the thermal-based systems (100 times). A number of companies (e.g. Konica, Mimaki, Epson) are producing and developing wide format piezo printers for printing wide width fabrics.
Ink formulation: as the inks are not heated, the formulation can be less critical and hence the inks are less expensive.
most piezo designs are able to jet higher viscosity inks of wide range of formulations based on solvent, water, UV curable, pigment binders and even thermoplastic (phase-change) or heat-sensitive inks.
Despite their higher cost, piezo heads are generally more reliable and better suited to higher volume industrial printing applications than thermal heads.
Advantages and disadvantages of Piezo electric printers
Certain piezo heads can achieve high resolution using very small droplet sizes, down even to 2 or 3 picolitres. Other types produce much larger droplet sizes, more suitable for achieving good colour saturation on thicker textiles.
speed and cost.
New configurations of these heads are producing throughput which is much greater than that of comparable thermal inkjet printers
Themal ink jet or Bubble jet technology
This technology was the first of the drop on demand.
The technique boils the water content of the ink and the resulting steam pressure forces a droplet of ink out of the nozzle.
The bubble jet printer (Canon) uses a small heating element to create pressure droplets on demand within the ink reservoir.
The droplet is then directed onto the substrate according to the design.
volume per drop of ink is typically 150 to 200 picoliters. Thus, a single thermal ink jet can deliver approximately O.I ml of ink per minute.