Understanding the ink drying process and its impact on print performance

When printing onto a porous or non-porous substrate, the process of drying of the ink is a vital part that needs to be taken into account for the finished result to be a high quality print. Without this care, the result may be smeared, lacking sharpness, not as highly coloured as desired and not sufficiently robust. So, how do we prevent these problems? The answer to this is in understanding the ink drying process in detail.

Here we define drying as the process that, for a solvent or water-based ink, comes between the landing of the drop on the substrate and any final curing, fixing or post-treatment processes. The ideal drying process removes moisture and/or solvent from the surface by delivering sufficient power to the ink, but without significant substrate temperature increase. It also promotes proper adhesion of the ink to the surface and low penetration into the surface (for a porous substrate), as well as good fastness to rub and other degradation mechanisms. A key distinction needs to be made between a non-porous (or almost non-porous) substrate such as a coated paper, polymer film or metal, and porous substrates like standard paper, board and textiles. For porous substrates there is an additional process to be taken into account – the absorption of ink carrier into the substrates, which occurs over a time period of several hundred milliseconds after drop landing. The speed and extent of this process depends on the viscosity (and to an extent the surface tension) of the ink, the degree of porosity of the substrate and the amount of time elapsed before drying commences. Uncontrolled absorption leads to problems such as loss of colour (as colorant becomes buried below the surface), loss of image definition as well as wrinkling and fibre damage.

For a drying system to work effectively, it needs to supply power to the un-dried ink in order to evaporate water/solvent, ideally without heating the substrate. For a system based on infra-red incident light, this depends on the relative absorption properties of the ink and substrate. The ink absorbs light both due to the carrier and also the colorant, with both having a spectral characteristic (which for the colorant differs markedly between different colours). Alternative methods of drying depend on heating the ink by conduction or convention rather than radiation, and in addition electron beams and other methods can be used. The goal remains to introduce power into the ink carrier preferentially over the substrate, and each method has its own strengths and weaknesses in this regard.

For example, hot air dryers are relatively slow as the drying is indirect and the maximum temperatures usable are limited by substrate heating. Near infra-red radiative drying, in contrast allows very high drying speeds with much lower substrate heating due to the different absorption characteristics of ink and substrate in the near-IR region. Adphos have developed a drying process combining near-IR radiation with air flow and moisture exhaust, which is found to provide a number of benefits with high drying speed, low substrate damage and ultimately, superior image quality. We will show some case studies of the drying process in action.

I look forward to discussing this further with you in Lausanne, at the IMI Europe Inkjet Ink Development Conference.

Dr Kai Bär, CEO, Adphos