LESS CAN BE MORE WITH PIEZO INKJET IN CODING AND MARKING

Jason Remnant has been involved with inkjet for just over 30 years. On the anniversary of his third year at Ricoh, he was asked to write this piece on coding and marking (C&M). Remnant worked for and with various C&M companies all over the world and has seen a lot of change. He previously believed that it was the least exciting and stimulating of all the inkjet applications. However, Remnant’s thoughts on this topic have changed of late.

C&M is the placement of alpha-numeric messages and machine-readable codes onto products and their packaging. C&M typically takes place at the end of the production process or line. The most familiar and basic one is the best before or use by date. The global 2024 market alone was valued at over €5.24 billion ($6 billion) and is supported by multiple printing technologies to produce the codes.

Traditional inkjet-printing applications, such as home desktop or office printers, require relatively small print gaps. These gaps can be less than a couple of millimetres between inkjet printer and printing surface. As such, inkjet devices have not had to work too hard to send an ink droplet over this short distance. C&M applications require printing on uneven or bowed surfaces of different products. As a result, their packaging demands the ability to print with much larger and varied gaps. A larger distance also reduces the chance of possible impact to the printer causing potential print problems or, at worst, production line stoppage. 

“The MH3820 was designed and developed with C&M in mind”

Continuous inkjet (CIJ) is an inkjet technology that has led with its ability to print over large gaps – 10–50 mm. Along with its speed, it is the most common and well adopted C&M inkjet technology. The technology achieves this by using one jet to produce ‘large’, high-speed inkjet droplets, but at a cost of print height and resolution.

Traditional piezo-inkjet printing (PIJ) over a print gap of a few millimetres is well served by droplet speeds of 6–7m/second. However, as an ink droplet travels from the printhead to the substrate it slows down in the air. Therefore, the bigger the gap the slower they become. By increasing the demand to a gap of 10mm or more, many piezo-inkjet drops lose the required speed and momentum. The drop trajectories get displaced by the airflow from the moving substrate and the drops do not arrive where they should on the print surface.

Figures 1 and 2 show how the text and graphics become distorted and blurred and the barcode readability dramatically reduces, between 2mm and 10mm of print gaps. Prints were produced with a Ricoh MH5320 printhead with 7pl drops at 600x600dpi, using Ricoh plant-based ink at a linear speed of 25m/min.

Over the last few years Ricoh has been researching how its PIJ technology can meet the challenges of C&M applications and possibly resolve some of the shortcomings of its CIJ inkjet sibling.

One of Ricoh’s core strengths is its pumping power – the ability to eject uniform high-speed droplets of ink out of the firing nozzles. Ricoh achieves this through its extremely efficient and powerful bi-pitch laminated piezo actuators. These actuators are responsible for jetting the ink from an array of nozzles. 

Ricoh’s piezo technology allows the company to double the droplet speed to 10–12m/s, providing the ability to print well over 10mm print distances. Ricoh can now approach CIJ capabilities, but with smaller drops much closer together giving higher print resolution. 

The misconception and understanding of print resolution and print quality is one of the biggest problems Remnant has seen and heard about in his inkjet career. Unfortunately, it is even more apparent in C&M. This is especially true where the industry has come from working in the printing of dot matrix ‘lines’ to the super high resolution offered by thermal inkjet solutions. As such, higher resolution must be better. However, this is not always the case, especially where larger print gaps are involved.

Working with high resolution and large print gaps presents several challenges that can reduce print quality in C&M applications. Higher resolution printheads typically have smaller nozzles to produce the small droplets. The nozzles are close to each other and there are many of them. 

Smaller nozzles can block and be affected by debris more easily than larger nozzles. Smaller droplets have less mass and are more easily influenced by air turbulence from their neighbouring drops. Airflow from between the inkjet printer and substrate can easily interfere with the droplets causing print quality defects. Statistically more nozzles means more chance of them not performing. There is a reason why CIJ, with its single nozzle, has been so successful. However, there may be a happy medium where the ‘optimum’ resolution of a piezo printhead might provide a nice overlap with some CIJ capabilities.

“One of Ricoh’s core strengths is its pumping power”

Piezo and thermal inkjet printheads come in a wide range of styles and specifications that might be up to this challenge. Quite often it may be tempting to go for the highest resolution printheads on the market. However, the ‘print mode’ – how the droplets are printed – has a significant impact on print quality.

In the printed piezo inkjet examples, the same amount of ink has been printed in two different modes, same printhead, ink, printing speed and print gap of 10mm.

Figure 3 shows the impact from the high print gap when printed at a high resolution of 600x600dpi and with small 7pl drops. Figure 4 is printed with all the same conditions, but with a lower native resolution 150x600dpi (300dpi) and 28pl drops.

These figures clearly show that the higher resolution, smaller drop print mode has fared less well than the lower resolution larger drop print mode. When the droplets fail to land where they should, ink coverage reduces, the print becomes lighter and defects are more easily seen. Lines become wobbly, less sharp and print quality is reduced – not increased – from the adoption of higher printing resolution.

There is of course still more to this optimisation that can further improve high-throw printing performance. But the fact remains that less resolution can, in some scenarios, yield more print quality.

Ricoh has taken the time to understand the challenges and requirements of how its inkjet printhead devices can support C&M. To this point, the company has announced the launch of a new printhead to its existing range of C&M printheads. The MH3820 was designed and developed with C&M in mind. It optimises specification and performance to achieve the needs of the application. More information can be found on Ricoh’s newest printhead at Industrial Inkjet Heads | Ricoh Europe. 

Images and test prints courtesy of Oliver Broom, Ricoh Printing Solutions Europe