Session 4 – Processes & applications
Session Chair – Tim Phillips
Understanding the ink drying process and its impact on print performance
Director Business Development, adphos
Michael (a late replacement for the advertised speaker Dr Kai Bäer) described the drying process in inkjet printing, from the landing of the drop on the substrate to the final fixing and any post-processing. A major distinction needs to be made between printing onto porous and non-porous substrates. For uncoated paper for example, the drop spreading and penetration process can take more than 500 ms and lead to significant swelling of the paper fibres. Michael also reviewed existing drying technologies, including hot air, hot drum/plate + hot air, mid IR + hot air and the adphos near IR system with integrated air, with the advantages and disadvantages of each described. For techniques using incident IR, near IR has an advantage as many paper and film substrates are transparent to it, whereas water is highly absorbing, allowing fast drying with lower energy consumption and less substrate heating.
Inkjet ink filtration – Process overview and filtration in the lab
Key Account Manager, Pall
Michael described how inkjet is a filtration-intensive process, as many inks contain fine particles or other potentially unstable materials, and the inkjet system contains small orifices and demands ejection of very small drops. Filtration is required to ensure ink quality in production, promote stability in storage and to allow high performance and printhead protection during printing. There are many potential ink issues that require filtration, including gel formation, agglomeration, oversized particulates, ink instability, insoluble residues from impure dyes, bacterial growth, dust and bubbles. Different types of filtration are used at different stages of ink formulation and manufacture, and a distinction is also made for different ink types. Michael outlined some typical processes for ink manufacture, and the filtration types used at each stage. For dye-based inks the objective is to remove insoluble elements, oversized materials, contaminants and bacteria. For pigmented inks additionally the objectives are to remove oversized and agglomerated particles and to further classify the dispersion from the mill. Michael also described an ink ‘cleanliness factor’ as a useful way to measure and specify ink quality.
Ink characterisation – Why, what and when
Head Business Unit Décor, Printing Inks Technology
In a change to the advertised talk, Alexander outlined his experience of ink characterization as part of his company’s initiative to develop new inkjet inks for décor applications. He outlined the required types of characterisation, what they are used for, and the equipment required. Alexander also described when these are used during the creation, formulation and manufacture of an ink.
Simulations of inkjet drop formation in complex rheological fluids - can rheology improve jetting performance?
Dr Oliver Harlen
University of Leeds
Oliver explained his simulations of inkjet drop formation. The printability of Newtonian fluids limited by the Ohnesorge number, which relates the surface tension, density and viscosity of a fluid, with a printable window between around 0.1 and 1. Simulations showed good agreement with experimental results for a model fluid. In real systems non-Newtonian effects are important due to very high shear rates and complex fluids including particulates and polymers. A more complex model of a fluid has a shear-dependent viscosity, with the high shear viscosity applying to jetting and a lower shear rate applying to ligament behavior. Oliver showed how shear-thinning can be used to get satellite-free printing at higher speeds than can be achieved with Newtonian fluids. Viscoelasticity due to the presence of polymer chains can also be modelled and compared with experiment, again showing good agreement.
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Tim Phillips & Kirsty Inman, IMI Europe