Inkjet ink filtration – Process overview and filtration in the lab

Understanding the basics of filtration is essential to make the right choice of filter media for different types of inkjet ink. Whether the ink is a dye-based aqueous ink for office printing or a pigmented UV curable ink, the right choice of filtration solution needs to be made to meet the requirements of inkjet ink formulation and manufacturing.

Inkjet is a filtration-intensive application, both during ink manufacture and operation of the printer. With drop sizes less than 10 picolitres in many cases, and printhead nozzles smaller than 20 microns, filtration is implemented at less than one tenth of the nozzle size (2 microns), and in the case of dye-based inks significantly smaller than this. The very high jetting frequencies and shear rates inside the printhead can expose instabilities in the formulation, leading to the formation of precipitates, gels or agglomerates, particularly where the ink contains curable materials and/or pigments. It is vital to ensure no particulates reach the nozzles, as a single failure can lead to the replacement of an entire printhead at significant cost.

Inkjet ink cleanliness is critical to ensure the correct operation of an inkjet printer. Properly filtered ink will assure printer output quality and long running periods between purge cycles, reducing costs from wasted ink and media as well as production downtime. Filtration is used at different stages in the life of the ink to achieve superior cleanliness: inks are typically filtered at manufacture, to remove contamination and agglomerates, at package filling for contamination removal, and in the ink path of the printer itself.

With the wide variety of inkjet ink chemistries in use, several different filtration technologies need to be used to give optimum results. For a typical dye-based ink, multiple stages of filtration are used after mixing of the colorant with the carrier, and again after addition of additives and dilution. The objectives are to remove any insoluble elements, particles, environmental contaminants and biological materials. The filter choice is critical, especially where multi-stage filtration is used. For pigmented inks, multiple-stage filtration is typically used after creation of the dispersion, and again after addition of additives and dilution. Here the main objectives are removal of any oversized or agglomerated pigments from the dispersion, as well as any oversized particles and contaminants from other processes.

The available filter technologies have different applications, advantages and disadvantages. Membrane, depth and hybrid filter types will be discussed, with the main considerations for filter choice being the required filtration level, whether dispersion classification is required, the importance of gel retention, chemical compatibility with the ink materials, flux rate, as well as required cost and quality. There are also practical considerations to be taken into account in implementing a filtration regime, with the final goal being the best quality and cost per litre of finished ink.

My presentation at the IMI Europe Inkjet Ink Development Conference will update you on the state of the art filtration technology available for inkjet ink filtration for various ink types. It will also guide you through the process of laboratory filtration to help you reduce costs and understand the different issues that can be resolved when using an optimised filtration process.

Michael Mehler
Key Account Manager
Pall GmbH