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