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Secret of Ink Design
CIJ inks
• Key properties– Flow– Jet Stability– Surface Tension– Conductivity– Stability
Ink Properties CIJ
• Ink viscosity
– Each commercial printer configured differently– Values range from 2.8 to 6 cP– Viscosity at nozzle is very different
Flow Properties
Ink Properties CIJ
• Largely influenced by polymeric binders• Molecular weight determines viscoelastic effects • High molecular weight causes erratic satellite formation • Satellite & Ligament formation critical
Jet Stability
Ink Properties CIJ
• Largely influenced by solvent base• Interfacial energy between polymer & solvent
important– controls cleanliness of droplet (PQ)
Surface Tension Properties
Ink Properties CIJ
• Charging & Phasing– needs to be accurately controlled– source of many ink failures
• Salts added to provide ink conductivity– typically 500 - 1000 micro siemens per cm
• Recirculating sensor– conductivity used to activate ink return mechanism
Conductivity
Ink Properties CIJ
• Corrosion– especially lower cost
components in printheads
• Nozzle crusting
• Ageing stability– purity of flow through
nozzle – no chemical change
after 1 month at 60oC• Chemical compatibility
– purity of colorant– salt, polymer and
colorant interactions– solubility over a
temperature range 5oC to 60oC
Stability
Jet Break up Images
Ideal droplet formationreliable ligament control
Poor droplet formationerratic and long ligaments
Difficult droplet formationHighly visco-elastic jet
Strobed optical image capture of a continuous inkjet jet travelling at 27m/s creating 64,000 drops per second with each drop approximately 1.5 nano litres
Ink Formulations CIJ• Solvent and Water based inkjet inks
• Binder / Polymers
• Colorants– Dyes– Pigments
• Conductivity salts- Formulation specific
• Additives
Types of CIJ Ink Formulations• Solvent-based inkjet inks
– solvents provide fast dry times– Range of polymers available for adhesion on
non porous substrates – Flammable and VOC issues limit growth to the
applications • Water-based inkjet inks
– VOC constraints using solvent-based inks driving new development
• UV curable inkjet inks– Capital cost and use of uv light limits use with
cij systems
Piezo DOD inks
• Industrial and office systems– Trident, Dataproducts, OTT– Xaar, Spectra– Epson
• Key properties– Flow– Jet Stability– Surface Tension– Stability
Ink Properties Piezo DOD
• Viscosity properties
– Flow rate properties through print head is critical– High rates of flow, especially at high repetition
rates affect: • air• creep back of ink backwards in the nozzle• wetting of the nozzle plate
Flow
Ink Properties Piezo DOD
• Much lower shear rates at nozzle • Dynamic viscosity influences drop creation• Polymer molecular weight influences print quality
– drop speed and ligament length – throw distance dependent
Jet Stability
Drop Ejection
• High molecular conjugated polymer.• Optimise waveform for every fluid. Molecular weight has
a significant bearing on jetting performance.
– High mol. wt (<r> = 209nm)• Highly viscoelastic• Long ligaments
– Lower mol.wt (<r> = 91nm)
•Uniform droplet formation
Droplet formation….. a simplistic view
a. meniscus at rest
b. meniscus disturbed by pressure wave
c. jet formation process initiated
d. surface tension creates droplet generation
Droplet formation….. a simplistic view
e. Surface tension and viscoelastic forces balance jet break up process
f. Droplet and satellite forms with jet retreating towards the meniscus
g. nozzle plate wetting
h. reliability
i. de-priming /starvation
Jet Break up Images
Typical jet stability, droplet and ligament formation of ink from a DOD print head at high repetition rate
Ink Properties Piezo DOD
• Corrosion– Printhead components susceptible to
corrosion– Chloride salts < 100 ppm
• Nozzle crusting– solubility of components at nozzle– purity of colorant
Stability
Ink Formulations Piezo DOD
• Solvents generally higher glycols– Newer generation inks based on more
aggressive solvents• Colorants
– Dyes and pigments• Binders/Polymers• Additives
– Generally few additives used
Types of DOD Ink Formulations
• Solvent-based inkjet inks- Trend towards solvent based systems for
faster drying non porous media• Water-based inkjet inks
– Mainly used in office based systems eg. Epson
– Limited use in shared wall technology due to need to passivate heads
• Oil-based inks– Used in shared wall and industrial systems– Suitable for porous media only
Oil Based IJ
• Used in numerous applications for printing on absorbent substrates
• High durability images• Excellent operability and rapid dry time• Good line speeds can be achieved for production printing
• Stable, small particle size pigment dispersions in a low viscosity, non-volatile oil.
• Excellent colloidal stability through use of polymeric hyperdispersants.
Oil Based Ink
Types of DOD Ink Formulations
• Phase change inkjet inks– Clean and environmentally the best inkjet technique– Control of print quality on porous and non porous
media is excellent– Extremely poor adhesion and durability has limited
growth• UV curable inkjet inks
– Some commercial uv curable inks for industrial systems appearing
– Important patents restrict use of some key materials– Opportunity for non porous media
100% UV Cure
• Excellent print performance across range of non-porous substrates: plastics
• High durability images• Excellent operability – no
volatile components in ink• No VOCs• Good line speeds can be
achieved for production printing
IJ Print Cure
Ink Properties TIJ
• Typically low viscosity• Simple rheological systems
– do not usually incorporate polymers due to drive voltage limitations
– special applications include low molecular weight materials (oligomers)
– limits ability to be used for demanding applications
Viscosity
Ink Properties TIJ
• Purity of colorants critical– degradation products during operation leave hard
residues adjacent to heating element– Affects print quality
• Filtered to less than 0.2 microns– pigment particles around 100 nanometres
• Ink subjected to rapid thermal shock– bubble created and collapses many times per second– ink temperature reaches about 350C
Stability
Takeaway Messages
• Ink jet inks not simply low viscosity or diluted fluid• Reliability can be engineered by:
– Purity of components– Avoidance of unstable particles, gels and viscoelastic
components