Mist, missing nozzles and condensation – Challenges for high speed inkjet printing

It’s easy to focus on inks and printheads when thinking about inkjet presses as they are essential components and go a long way to determining performance and applications. When I bump into contacts next week at Drupa 2016 we’ll be swapping notes on what printheads are being used or who’s ink. This tends to mask all the other vital sub-systems and issues that need to be optimised to achieve a reliable and stable press.

Those who know me, or who have attended the Inkjet Academy course that I jointly present with Dr Alan Hudd, know that I’ve been a great advocate of taking ink supply and nozzle maintenance seriously during the concept phase of developing a new product. They are certainly key components of desk-top products, and their careful design and optimisation can lead to reliable industrial presses too.

The use of inkjet for high-speed printing applications – commercial and packaging printing in particular – has required the development of page-wide fixed array printheads. Printing at high-speed with tens of thousands of nozzles leads to some other problems too, and I’m going to explore three of them in this blog.

Are you wondering how I know about these problems? They don’t get mentioned in public most of the time. Do I hang around in bars close to manufacturers R&D labs hoping to overhear discussions at the end of a hard day’s development? No, nothing that exciting. I read patent applications.

Ink misting
Printhead manufacturers and ink formulators try and optimise the jetting process between them, often not an easy partnership as they almost always work for different companies. But no matter how elegant the drop formation process might appear, using an optical drop monitoring system, there will be microscopic drop fragments created at the point where the tail of the drop breaks away from the nozzle. These very small, often sub-micron-sized fragments of ink, have little forward momentum and tend to drift with the air currents around the printhead. In high-speed printing the substrate passes underneath the printheads, drawing air along with it. The turbulence this creates distributes the ink mist throughout the rest of the machine.

Aqueous inks are the choice for many of the new presses under development. But these require extensive dryers within the press. At very high speeds it may be necessary to heat the web before, or immediately after, printing. Driving off the water carrier can improve the print quality by preventing excessive bleed and absorption into the substrate. But it also leads to the creation of large amounts of water vapour. Just like your kitchen windows when you are cooking, this water vapour will condense on anywhere cold enough; like the printheads and mountings within the press. And as it builds up it will drip onto the substrate and ruin the beautiful print you have worked so hard to create.

Drop detection and missing/misaligned nozzle correction
When you have tens of thousands of nozzles, you have the problem that the failure of just one of them could conceivably affect the high print quality demanded by these applications. There are some great technologies for hiding any artifact caused by a missing or misaligned nozzle – for instance by increasing the dot size each side of the missing nozzle, or substituting a different colour. But you can only play these tricks if you know which nozzles are causing the problems. Technology is therefore being developed to either check the drops in flight, or the printed image, to look for these defects. Taking into account the number of nozzles involved and the print speed this is not a trivial issue to overcome.

So when you see the impressive new inkjet presses at Drupa 2016 think of the difficulties that the engineers have had to tackle. Some of problems are not very glamorous, but they all require satisfactory solutions to provide reliable systems.

Mike Willis, Pivotal Resources

If you want to learn more about these issues, a section of the Inkjet Academy course at the upcoming IMI Europe Inkjet Summer School on 20-24th June 2016, presented by Mike Willis himself, deals with these issues. They will also be covered in relation to digital textile printing systems in the Digital Textile Printing course, also at the Inkjet Summer School.