The Top 4 Drying Methods for Inkjet

Inkjet is able to print large, completely variable formats at high speeds and high resolution. There is one catch: the ink needs to be dried for the product to become useful. Just as there are many types of inkjet printheads and ink, there are many types of processes for inkjet drying.

There are four main processes to dry or solidify ink. However, most printers combine several processes to create a drying solution. The pros and cons described for each individual technology may be overcome by combining with other technologies.

The focus of this discussion will be on the fundamental factors related to drying in document and packaging printing environments including absorption, phase change, radiation and evaporation. Pros and cons in industrial printing can be totally different – for example having a lot of material piled on top of the substrate can be desirable when printing electronic circuits or 3D components.

Absorption

The easiest case of drying in inkjet is absorption. The solvent (water or liquid organic compounds) is absorbed by the substrate and leaves the pigments and binders partly on the surface or soaked into the bulk of the substrate. The drier design is the simplest, as there is none – like in the typical home inkjet printer. There are some downsides however. The process requires an absorbent substrate, the absorption can take time, limits the ink coverage and solvents can accumulate in the substrate – leading to waviness for paper. With inkjet optimised substrates limitations can be mitigated, but this comes at a price (if you ever bought photo paper for your home printer you can grasp the cost difference).

Absorption is part of the process in most other forms of inkjet drying, but production printers rely more on active forms of drying than on absorption. The exception of a production printer mainly using absorption is the RISO ComColor series in which an oil-based ink is used to avoid bringing too much moisture into the paper.

Phase change

The process of ink solidifying on the substrate is referred to as phase change. In this process a liquid becomes solid on the substrate without any other curing or drying process. It sounds a bit like magic, but is akin to molten wax hitting a cooler surface and solidifying immediately. This can be accomplished for inkjet using heated inkjet heads and a heated ink supply system or with chemicals in the inks and the paper that react and cause the ink to harden. There is a bit of phase change built into many inks with solvents that are being soaked up making the droplet more gel-like. This gel consistency helps reduce spreading of the droplet.

The drying system is very simple and can be quick. Usually it requires no energy. The solidified ink sits mostly on top of the substrate and penetrates little. As the whole droplet volume is preserved, noticeable amounts of material are deposited. This can be desirable, but more often it is not as it makes the print susceptible to scratches, can lead to lower adhesion and produces a raised surface. Slower drying can improve adhesion, as can managing the temperature of ink and substrate and applying to some pressure to anchor the ink into the paper fibres. However, too much heat can cause the ink to become liquid again, leading to blocking of prints.

There are not that many devices that use phase change ink commercially. Xerox has used this technology for a long time in their Tektronix office printers and more recently in their CiPress web-presses. The Canon uses this technology in Océ ColorWave wide format printers, but not in their production printing portfolio.

Radiation

When drying ink through radiation often the term curing is used. Radiation curing incites a chemical reaction in the ink which leads to polymers in the ink cross-linking. While short pieces of polymers are liquid at room temperature, the cross-linked long polymer chains are not. Colorants are embedded in the polymer solution.

There are two types of radiation used commercially: UV and electron beam. While the principles of the drying process are the same for both, the chemicals used in the ink and the drying system vary. UV is the by far more popular type of radiation used. There are several UV lamp technologies available today with the LED and mercury lamps being the most widely used. LED UVs keep gaining share on mercury lamps for several reasons: lower temperature, lower energy consumption, more eco-friendly by avoiding mercury. Mercury lamps are still more cost efficient for wide widths and have a wider selection of photoinitiators to start the cross-linking reaction.

Electron beam curing is rarely used, although it has a number of advantages: there are no photoinitiators needed, pigments do not interfere with the drying process, there is almost no temperature increase in drying, the ink is thoroughly hardened, and the dryer can be very compact even for high speeds. There are also downsides: shielding the drier is required avoid exposing the operator to X-rays, and an inert atmosphere (nitrogen) is usually required for efficient drying. Roll-fed applications are better suited as the sheet edges in cut-sheet devices carry along oxygen into the drying zone. Crucially, the dryers are not cheap at €500k to €1 million (550k – 1.1 million USD) per unit and are only worthwhile for very productive systems. Due to the low usage of EB curing, few compatible inks are available today.

As with phase-change inks, the radiation cured inks sit on top of the substrate, however adhesion or scratch resistance is usually less of an issue. It still means more material is deposited. As soaking into the substrate is not required also non-absorbent substrates can be used.

While many wide format printers use UV inks today, and UV is the inkjet technology of choice for labels, there is limited use in production document printing.  The Konica Minolta KM-1 and Komori IS-29 B2 sheetfed inkjet presses are the most prominent UV inkjet devices.

Evaporation

The drying process used most frequently in production print is evaporation. Pigments and binders are dissolved or dispersed in a carrier liquid (water or organic) and this liquid is evaporated in the drying process – usually with a portion of the liquid being absorbed in the substrate as well. Evaporation is achieved with a combination of heat and airflow. Increasing heat, without proper airflow, would not result in evaporating more solvent into a solvent-saturated atmosphere. Too much heat can also cause only top layers of ink to dry leaving ink tacky underneath. As most of the ink is evaporated only a thin film of pigment and binders remains on top of, or somewhat soaked into, the substrate. The dilemma of evaporation  is typically to dry the ink without heating the substrate too much in order to preserve the moisture content of the substrate.

There are several technologies used to create the heat for drying. Hot air nozzles or air knives create heat and air movement at the same time. Heated rollers or belts can heat the substrate directly – but only the back of the printed sheet can be heated as the ink film is still wet on the front. This process can be supported (or even replaced) by Infrared (IR) radiation. In this case the radiation does not incite a chemical reaction as in radiation curing, but is instead used to heat the ink. Any heated surface can act as IR emitter, but there are more specialised solutions available. The NIR (Near IR) technology from Adphos emits short wave IR which is specifically absorbed by water molecules and helps evaporate water from the printed ink. Heraeus Noblelight developed a new IR element with a longer wavelength that reacts quickly, with good uniformity and being independent from the pigment type.

Most document production printers today have evaporation as the main drying process. The faster the device, and the higher the ink coverage, the higher the demands on the drying system. In general, the less absorbent a substrate is the more complicated drying becomes. Inks and driers can be optimised for certain substrate and coverage ranges but can lack performance for others. Inks and driers have improved a lot over the years and more improvements will come.

Final remarks

This article provides only the high-level factors on the different processes to dry or cure inks and on what can go wrong. Drying has been a major area of research and many of the latest inkjet improvements in print quality seen at Hunkeler innovationdays 2019 were due to improvements in drying. Additionally, drying can be a limiting factor in print speed, which is influenced by substrate and ink coverage. Again, improvements in drying can lift the system performance noticeably. However, finding the magic bullet will remain difficult and for some time we will have multiple approaches – some that work well for certain applications but less so for others. Make sure to ask your OEM about the drying options, and combinations of options, offered and how they impact speed performance, ink coverage, energy usage and environmental impact.

For a few other perspectives on drying in production inkjet, see also:

Heavy Ink Coverage and Twice the Drying Power

Solving quality problems with NIR Drying

HP Drying Options and Trade-offs

What’s the Best Drying Option? (Note Ricoh delivered some new, patented, drying options since this video was released.