Secret of Ink Design
CIJ inks Key properties Flow Jet Stability Surface Tension Conductivity Stability
Ink Properties CIJ Flow Properties Ink viscosity Each commercial printer configured differently Values range from 2.8 to 6 cp Viscosity at nozzle is very different
Ink Properties CIJ Jet Stability Largely influenced by polymeric binders Molecular weight determines viscoelastic effects High molecular weight causes erratic satellite formation Satellite & Ligament formation critical
Ink Properties CIJ Surface Tension Properties Largely influenced by solvent base Interfacial energy between polymer & solvent important controls cleanliness of droplet (PQ)
Ink Properties CIJ Conductivity 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
Ink Properties CIJ Stability Corrosion especially lower cost components in printheads Nozzle crusting Ageing stability purity of flow through nozzle no chemical change after 1 month at 60 o C Chemical compatibility purity of colorant salt, polymer and colorant interactions solubility over a temperature range 5 o C to 60 o C
Jet Break up Images 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 Ideal droplet formation reliable ligament control Poor droplet formation erratic and long ligaments Difficult droplet formation Highly visco-elastic jet
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 Flow 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
Ink Properties Piezo DOD Jet Stability 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
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 Stability Corrosion Printhead components susceptible to corrosion Chloride salts < 100 ppm Nozzle crusting solubility of components at nozzle purity of colorant
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 Oil Based Ink Stable, small particle size pigment dispersions in a low viscosity, non-volatile oil. Excellent colloidal stability through use of polymeric hyperdispersants.
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 Viscosity 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
Ink Properties TIJ Stability 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
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