Single pass industrial inkjet printing: How difficult can it be?

At DRUPA 2016 it was clear that single pass inkjet printing had moved into the mainstream of graphics printing. Single pass printing has also become dominant in the printing of ceramic tiles. The challenge now is to bring inkjet printing into other industrial processes: vinyl tiles, flexible packaging, decorative laminate, cardboard boxes, fabrics, carpets etc.

The conventional analogue printing processes for industrial applications are technologically mature and highly refined. Inkjet printing may offer advantages of flexibility and low set-up costs, and may meet the quality expectations for analogue printing, but it is not easy to match analogue printing processes in terms of speed, cost and reliability.

Traditional wisdom has it that inkjet printing allows much lower setup costs than analogue printing, so that smaller production runs become possible. This in turn leads to reduced stock holding and greater production flexibility, so that for small to medium production runs, the higher cost per print does not matter. However, for certain applications, there can be additional advantages of inkjet which help speed up its adoption. For example, with ceramic tile printing, the non-contact nature of inkjet printing allows thinner tiles to be used, reducing shipping costs as well as eliminating breakages in manufacturing. The adoption of inkjet printing in industrial applications will be based not on the cost of printing, but on its impact on the entire value chain of the product.

From a superficial level, the task of making a single pass inkjet printer is simple:

  • hold the printheads
  • over a moving substrate
  • supply them with ink
  • fire out the ink to make an image
  • dry or cure the ink

However, the Devil is in the detail, and achieving these requirements to the precision and scale necessary is difficult and expensive. The fact that a small desktop inkjet printer can be bought for $50 might give the impression that inkjet technology is simple. However, it must not be forgotten that over the years several billion dollars have been spent on development of this technology.

Industrial analogue printing machines are considerably larger than conventional inkjet machines, so bringing inkjet up to industrial scales is an immediate challenge to the system designer. Print quality expectations require consistency of droplet placement to a few tens of microns, but print speed expectations require that this accuracy be achieved at linear printing speeds in excess of 3 m/s over a printable area several metres wide.

The first challenge is simply to hold the printheads in the correct position with respect to each other and to the substrate. Stability of positioning needs to be held to order 10 µm and any initial adjustments may need to be applied to hundreds of printheads. Moreover, any printhead should be easy to replace and re-align. All this must be achieved in an “industrial” environment rather than under temperature-controlled laboratory conditions.

The moving substrate is the next hurdle. With analogue printing, the ink is transferred by direct physical contact so there is good coupling between the printing cylinder and the substrate. With inkjet, there is no direct contact, so the scope for velocity ripple and “swimming” of the substrate from side to side is much greater. On a web-based system, some of these effects can be reduced by using a common impression drum, but this does not eliminate all sources of error which can affect the quality of printing.

Supplying printheads with ink at the correct temperature and pressure, adequately degassed and filtered, does not appear to be too difficult. However, it must be remembered that any particular ink may be printing from all printheads, or not printing at all. So the temperature and pressure and degassing levels must be maintained from zero ink flow to maximum ink flow, and the transition between these two extremes may be virtually instantaneous. 

As for supplying data to the printheads, a 1.6m wide, 4-colour printer, operating at 2.5m/s web speed with a resolution of 1200 by 600 dpi requires data at a rate of about 18Gbit/s. The data has to be corrected to account for missing or deviating nozzles and for imperfections in the drop mass across each printhead: this requires several server-grade PCs. Moreover, the detection of print imperfections needs to be automated, calling for more cameras, or contact images sensors, together with another rack full of server-grade PCs.

The final part of printing is to dry or cure the ink. For the system described in the previous paragraph, several hundred kilowatts of power are needed to dry a water-based ink. This power needs to be applied in such a way as to avoid over-drying the substrate or boiling the ink.

The final quality of a print is determined not just by the accuracy to which the printer can set down the ink droplets but also by the behaviour of the ink on the substrate. Factors such as ink laydown order, ink-ink interactions, ink-substrate interactions, substrate coatings (pre-coat and post-coat), plasma treatment, inter-colour drying all come in to play. It is also essential that the inkjet-printed output is compatible with the rest of the production process (e.g. the Melamine process for decorative laminate applications). We refer to this collection of activities and requirements as the “print process”. The print process determines the layout of the printing machine and certain properties of the inks used. Hence any industrial printer development will require a small and flexible “process development” printer to tune the ink formulation, and the coating, printing and drying configurations. Until a print process is defined it is not possible to design a full-scale printer. If you design the printer first, it might be the wrong printer!

It should be clear from the above discussions that development of a large scale single pass industrial printer is a project costing tens of millions of Dollars/Euros/Pounds requiring close cooperation between suppliers of substrates, coatings, inks, printing hardware, drying hardware and web transport systems. In summary, developing a large single pass industrial inkjet print process, and associated printer, is not a trivial task. Anybody embarking on such a project needs to be aware of the costs and complexities involved. Each application will be different and, although common printer parts can be used, the machine configuration will be driven by the requirements of the print process. It can sometimes be a frustrating and difficult area to work in, but it is interesting and rewarding too.

You can hear more from Nick Campbell of Inca Digital Printers at the IMI Europe Inkjet Engineering Conference in Lausanne, Switzerland on 14 March 2017.