Sean Smyth investigates the latest developments in printheads and their properties to help you achieve success.

In conversation with the marketing director of a leading inkjet equipment manufacturer I asked why they would not say which printheads are used in a recently released machine. ‘Because we don’t want low-cost competitors, made in China, claiming that their equipment is equivalent to ours just because they use the same heads,’ came the reply.

This goes to illustrate that, while there is more to a successful inkjet print system than the printheads, they remain the critical component in a press. There is much choice in labels and packaging, with: continuous heads from Kodak; thermal heads from HP and Memjet; and piezo from Epson, Fujifilm Dimatix, Konica Minolta, Kyocera, Ricoh, SII Printek (Seiko), Toshiba Tec and Xaar. They all work in different ways, handling the ink and ensuring consistent print at ever-higher speed.

Continuous heads push ink through the nozzle under pressure, this stream breaks into drops and there are various mechanisms that remove non-printing drops. Drop-on-demand (DoD) heads have an ink reservoir that moves ink into a chamber behind a nozzle plate, drops are ejected onto the substrate through the nozzle and then fresh ink recharges the chamber. There may be several thousand nozzles in a head, in some cases ink circulates within the head behind the nozzle plate. Keeping a flow of fresh ink across the head boosts reliability by avoiding ink drying in the head, while removing air bubbles and debris. This was pioneered by Xaar and now most piezo suppliers offer the capability. Nozzles immediately recover from blockages, reducing routine nozzle cleaning meaning heads are always ready to jet, eliminating priming or purging and before starting printing.

Head developments have improved resolution and speed over many years, with 1200dpi native resolution printheads capable of printing at speeds up to 300m/min available. Fujifilm Dimatix is reporting a growing number of label and packaging customers for its Samba heads. They are a silicon MEMS (microelectromechanical systems) construction, offering 1200 native dpi with 2pl drops and continuous ink recirculation. Each head contains 2048 nozzles in a rhomboid shape, designed to stack into wide arrays with no overlap in a compact arrangement.

Bailey Smith, senior vice president responsible for sales, marketing and business development at Fujifilm Dimatix, believes that it is the head technology that is opening packaging as an inkjet market. ‘The packaging market is moving very quickly into inkjet printing. The key advantages of inkjet printing, including speed, scalability and cost of ownership, have been returning on investment for years in other industrial markets.

With the growth in adoption of high productivity 1200dpi print heads such as Fujifilm’s Samba, a significant portion of the high image quality packaging markets are now becoming accessible to inkjet technology.

‘Attributes such as print speed, resolution, productivity and head life are key drivers to a successful product execution. We are very proud of the machine providers who have developed successful printers using our printheads, and look forward to working with them to bring the benefits of inkjet to other segments of packaging as well.’

Enhanced capability

Ranges are continually updated. As the manufacturers gain experience in production, so the quality and reliability improve, along with the yield of saleable heads. Manufacturing methods are developing, with silicon MEMS techniques, borrowed from chip manufacturing in the semiconductor industry, becoming widespread to improve the consistency of ever smaller components with thin film piezo production enabling ever smaller drops to be printed. New techniques allow very thin films, a couple of microns deep, of the ceramic piezo material to be deposited, allowing very small ink chambers to be constructed. Some providers, for example SII Printek with the RC1536 series, use an isolated channel structure to avoid potential interference rising from a shared wall. As piezo heads can generate up to 100,000 drops per second from each nozzle any design feature that stabilises the size, direction and velocity of a drop is advantageous, as well as mechanisms that reduce blockages and downtime.

However, despite the best endeavours of head manufacturers there will inevitably be nozzle failures and the head design can make a significant difference to the final appearance of the print. This is particularly important in single-pass printing. HP has built redundancy into its thermal heads, so the failure of a nozzle does not result in a defect.

HDNA (high definition nozzle architecture) printheads contain 21,120 nozzles across the 108mm print width. These are arranged into rows and if there is a failure then other nozzles in front or behind can print in that area resulting in no visible artefact in the print. Vision systems detect problems that are compensated in real time. HP builds in 8x redundancy in its HDNA heads. In other systems with one or two lines of nozzles, larger drops can be printed from nozzles adjacent to a failure, masking the impact and avoiding the dreaded white-out lines.

In April this year Ricoh launched the MH5320/5340/5320 Type A printheads, higher specification versions of the well-established Gen5 heads. They allow smaller 5pl drops with improved jetting accuracy, while performance is boosted to a maximum 50kHz meaning increased productivity. Ricoh says the new models feature enhanced bonding technology to improve durability, for example with aqueous ink the service life of the MH5320 is more than double that of the MH5421 that preceded it. The 1280 nozzles are arranged in four rows, each spaced at 150dpi to give a native 600dpi resolution with the capability of compensating for a faulty nozzle.

 New applications

In some cases, new categories of head are launched, such as the Memjet DuraLink head technology that will open many new high-performance packaging applications with pigmented inks.

Patti Smith, vice president of business development and marketing at Kodak’s Enterprise Inkjet Systems division, knows the importance of printheads in delivering new print systems. Brands are increasingly challenged by consumer demands and the trends affecting their labels and packaging.

More SKUs, smaller brand name introductions and private labels are creating the need for faster turnaround with great image quality, at a reasonable price. She says, ‘Inkjet printing technologies have improved significantly, narrowing the gap with conventional print, which originally was challenged with compromised image quality and came at a much higher price. Image quality, colour management and cost of print have been at the forefront of customer requirements and among the promises of Kodak’s Stream and Ultrastream inkjet technology. Kodak’s Ultrastream next generation technology, first introduced in a prototype press at drupe 2016, is now delivering on those promises with commercialised product solutions launching at drupa 2020 and beyond.’

Kodak’s proprietary continuous inkjet Stream technology has proved successful in high volume commercial print at litho-like quality at high speed. This has been extended into packaging with the Uteco Sapphire Evo for flexible packaging,

Prosper 6000S for folding cartons, and Prosper Plus for imprinting. The new Ultrastream heads will take this forward with developments into corrugated, flexibles, cartons and labels.

John Corrall, managing director at Industrial Inkjet, which sells Konica Minolta printheads has identified a potential niche in pulp packaging, an area he believes, is going to take off as a sustainable alternative to plastic. Adding colour and interest to the plain appearance is currently through a label or printed sleeve. Several inkjet coding and marking systems can print text, logos or barcodes directly onto the pack, at low print quality. This print is generally informative rather than promotional, but as pulp and solid fibre becomes more mainstream this will change with inline full-colour direct-to-carton inkjet print systems, for the top, bottom and sides of the box.

The choice of printhead is important to make this application work, as Mr Corrall explains. ‘Printheads need to operate at almost any angle as pack sides generally slope, but the main challenge is variable ink throw distance. Often pulp packs are stepped, to boost physical strength resulting in a gap of 5mm or more. So the printhead won’t deliver tiny ink drops. A good compromise between small drops for high print quality, and large drops that will ‘fly’ a long way, is around 14pl. Production environment have a lot of dust, humidity and heat. So the printheads need to be tough. Our experience on our first install into this market has been that the current KM1024iM printheads work very well.’

The specification of heads is not the sole performance indicator. Integrators and equipment suppliers consider mechanical and electronic properties, not to mention ink compatibilities. Operational life is important as heads can cost thousands of pounds and replacements will be expensive, with machine downtime to be factored in.

While head manufacturers may not agree, there is no ‘best’ printhead for use in labels and packaging, because the requirements are so varied. High resolution graphics and text benefit from small ink drops to print fine detail and smooth gradations, but larger drops are preferable to deliver a good solid opaque white or a high-build effect varnish. When choosing a press or print system the overall performance is the key determinant for the converter, and this is certainly dependent on the choice of printhead.