It’s been a few years since Xaar announced the development of a silicon-MEMS thin film piezo printhead to form the basis of their P4 (platform 4) product range. So has it been worth the wait? On 2 June, 2016 we got first details at the product launch and it certainly looks both impressive and competitive. Here I’ll explain why.
But first let me just explain what silicon MEMS (abbreviated to Si-MEMS) actually means and why it's being increasingly seen as the future for piezo printheads. The use of silicon as a base for developing powerful computer chips, memory and other electronic devices has revolutionised industry and our lives. These devices are made by the diffusion and coating of materials in layers at extremely high resolutions. MEMS technology is making structures in and on silicon using similar techniques. This involves etching as well as depositing materials. Over the past decade or so etching techniques have become much better, faster and cheaper, making the mass production of MEMS devices viable. The technology is used for a very wide range of applications, and one of those is in inkjet printheads. Thermal or bubble jet heads can be considered MEMS devices in that physical processing is often required in the manufacturing process, and Memjet takes the process much further by building CMOS circuitry with the actuator structure on top.
But the other term often heard in conjunction with Si-MEMS is thin film piezo, and they go hand in hand. Si-MEMS allows a printhead manufacturer to make actuator chambers and ink passageways at a finer scale than previous techniques. The material is much stiffer too, so that the walls between chambers can be made thinner. Unfortunately as the chamber dimensions are reduced, you need to make a thinner and more flexible piezo actuator layer to flex into the chamber to displace a drop from the nozzle. Until recently inkjet devices used bulk piezo, made from wafers of the material. Thick film deposition of piezo has also been used, forming layers around 20 microns thick. For the use in Si-MEMS devices much thinner coatings are used, with piezo layers 2-5 microns thick on top of flexible diaphragm layers only half that.
For Xaar, moving to Si-MEMS offers many benefits. Perhaps the biggest one is it will finally allow them to work with aqueous inks which are becoming increasingly important for commercial and packaging printing, and are of course essential for textile printing. It will also operate at much higher linear speeds than possible with Xaar’s previous designs.
Xaar 5601 GS3p0 printhead
The new printhead consists of 4 chips mounted in a Z-shaped module. Each module can print over roughly a 4.5 inch (115 mm) width at 1,200 dpi and at 120 metres per minute. Alternately the printhead can be supplied with 2 colour inks and print at 600 dpi. Inks can be either aqueous or solvent-based and the printhead can be operated in binary or greyscale mode with drop sizes ranging from 3-21 pl.
The actuators are thin film piezo configured in roof mode, with nozzles at a pitch of 300 per inch. That equates to a pitch between nozzles of 85 microns. Each row of nozzles is offset, with each pair at 1/600th of an inch and the pairs offset by 1,200 of an inch. This allow single or 2 colour printing.
Although later to the game than some of their competitors, Xaar has taken the opportunity to use the latest technology internally. For instance the connection technology used is state of the art. They have also focussed on producing a design that is easy to implement. The head has alignment features to allow easy replacement, with the final nozzle alignment taking place in software. As the head covers a wide width there are fewer ink and electrical connections reducing costs and complexity. The 'Z' shape enables printheads to be assembled in wide arrays with stitching also taken care of by software. The heads are also reasonably narrow in width, so stacking heads together for a full colour system will be compact - Xaar claims only 200 mm for 4 colours.
The printhead has been designed with a high drop velocity - a figure of 9 metres per second was quoted at the press conference but I understand higher velocities are possible. This should allow wider spacing between printhead and substrates while maintaining print quality.
Drop placement accuracy will also be high - Ramon Borrell, CTO of Xaar, stated they were achieving 3 sigma within 5 micron at 1 mm spacing.
One of the design considerations with a high density piezo printhead is thermal management. The heat from the actuators heats the ink, and if the temperature of the ink varies then so does its viscosity and therefore the drop volume. So Xaar has incorporated its high flow rate technology which will equalise and stabilise the temperature across the printhead, and which will also keep ink fresh at the nozzles and rapidly remove bubbles by passing ink right behind the nozzles.
A new feature incorporated into the printhead design compensates for manufacturing variations between nozzles. It allows the printhead to be calibrated to equalise drop volume and velocity across each printhead and all of the printheads in an array. It claims to do this dynamically, so perhaps can also be used to reduce crosstalk and maximise printhead life.
Partnership with GIS
Xaar also announced a partnership with Global Inkjet Systems who will supply drive electronics and ink supply systems for the 5601. Xaar’s overall aim is to minimise the total cost of ownership by reducing development and integration costs and time to market.
Xaar aims to be manufacturing the printhead at the end of 2016 with the first product - a textile press - appearing mid-2017. A version capable of handling UV-curable inks is due in September this year, and some manufacturers will be adopting the printhead for 3D printing with a major vendor launch in 2017. Xaar is out-sourcing the silicon MEMS manufacturing, focussing on final module assembly and testing, so won’t be investing in expensive silicon fab facilities.
So, will the Xaar 5601 printhead family succeed? It does seem to tick all the right boxes for high quality inkjet printing at speed. In terms of linear speed and print resolution it is up there with the Fujifilm Dimatix Samba and Kyocera printheads, while being easier to integrate into a process. Landa Digital claimed 300 metres/minute with Samba printheads but like others this is achieved by doubling up the printheads, which Xaar could also do. HP is running web presses at 120 metres/minute in ‘quality’ mode, but their claimed 2,400 nozzles per inch is actually 1,200 dpi with 2 nozzles in line to get 4 grey levels.
Mike Willis, Pivotal Resources