Author: Dr. Mark Bale, Director, DoDxAct, Cambridge, UK
Industrial inkjet printing encompasses a surprising variety of applications, from the application of colour through to the additive manufacture of a range of functional devices. The diverse range of chemistries being used has meant historically that piezoelectric printheads have dominated. This had a knock-on effect of the flexibility to design the driving signal to the printheads to adapt the use of printheads for different needs. In this blog we discuss both the importance and limitations of waveform as a tool for optimising inkjet solutions.
Our discussion is aimed at new entrants as well as people with some inkjet experience, so let’s start by considering what a waveform is, and what adjustments are possible. The easiest way to picture a printhead is as a volume in a box (chamber) with an inlet (the restrictor) and an outlet (nozzle). The chamber is squeezed by a piezoelectric element to create a pumping action of filling the box from the inlet and pumping (jetting) it out the nozzle. Pulse timings are designed to match the flow in/out according the fluid property and the size of the box and the nozzle. For this reason, waveforms are specific to the head design and to different inks. Each squeeze of the box has the potential to form a drop under the right conditions as shown schematically below based on a Kyocera print head. For more info on waveforms, see ImageXpert’s blog on the topic (ref1).
If we fire the head so that many pulses are made very close together in time, then we can create bigger drops. This is often called greyscale, since it allows the printer to produce different amount of colour to be deposited at each pixel position which for graphics arts printing, improves grey level reproduction. It also requires the image data to the head to be programmed in bit levels, using a greyscale image rather than a binary on/off image. For a more complete description on greyscale we point you in the direction of the ImageXpert blog (ref2). For industrial applications, the author prefers to call the method multi-pulsing, since it is both more literal and more generally applicable to non-colour applications. The simple graphic below demonstrates the reason for using small drops, what pulses can be used, with examples from a Dimatix SG1024 head.
The power of the waveform can be demonstrated easily with a picture of the drop formation of the same ink with an appropriate, but non-optimised timing compared to an optimised one, as shown below using a Ricoh GH2220 head. The improvement in nozzle consistency, and reduction in satellites are seen in the bottom dropwatcher images and result in the better print quality shown in the micrographs. There are also correlated improvements in nozzle reliability (not shown). All this was made possible by re-designing the pulse timing from first principles using a dropwatcher, technology that is commercially available from IMI speakers and Inkjet Innovation Academy course providers ImageXpert, Kruss, and Meteor Inkjet.
As a result, the main use of dropwatcher and waveform tuning hardware/software is to make small modifications to what are otherwise quite standard approaches recommended by the printhead manufacturers. Though no doubt valuable, these optimisations are just the start of what understanding waveforms can make possible. In our lab we used multi-pulsing waveforms in small-drop printheads as a method for simulating the expected process performance of different drop sizes at varied DPIs (Ref3,4).
For industrial print the implementation of highly tailored waveforms can turn a potentially expensive printer re-design (based on choosing another head entirely, for example), into a simple re-targeting of the existing head to suite the new needs. So-called “print-to-shape” applications are a good example of this, where the throw distance involved can mean that smaller drops produced by common higher-resolution heads never reach their target (Ref5). The dropwatcher photos below compare the difference between a “standard” 3-pulse and a novel 6-pulse waveforms created to deposit larger ink volumes using a Ricoh Gen5 head.
The increased drop sizes >>40pL for a head with native volume <10pL can be only achieved without faceplate wetting with careful design, otherwise the stability of ink meniscus in the nozzle is disrupted. This causes faceplate wetting, which is a common failure mode, especially when ability to optimise the ink/fluid rheology for the head may be constrained by application needs. The only trade-off to such multi-pulsing is that print speed and/or resolution will become limited due to the length of the pulse train (in time), which limits frequency. In industrial applications, however, printing at ~100kHz+ to obtain substrate speeds >100 m/min is an objective of only the most demanding roll-to-roll applications, like corrugated packaging.
NOTE: Dr. Bale will discuss this topic in more detail during his presentation at IMI’s Inkjet Conference 2020 on February 12-13, 2020 in Tempe, Arizona (www.imiconf.com) where measurement comparison between drop sizes and on different printheads will be presented along with some further print examples.
Dr. Bale is also the course leader for IMI’s Inkjet Innovation Academy Course Inkjet Inks: Materials and Applications on February 10-11, 2020 in Tempe, Arizona (https://www.imiconf.com/ijiaii-feb2020).
To ask questions about this topic or to schedule a meeting with Dr. Bale during IMI’s Inkjet Innovation Academy on February 10-11, 2020 or Inkjet Conference 2020 on February 12-13, 2020 – please email him mark@dodxact.com
References
3. M. Bale, IS&T, Sept 2019, pp Presentation available on request
4. M. Bale, InPrint, Nov 2019, pp Presentation available on request
5. E. Kempeneers (TTEC), IJC, Oct 2019, pp19-21
About the Author
Dr. Mark Bale, Founder & Director, DoDxAct, Somerset, UK
After working many years for a leading ink company, Dr. Mark Bale founded DoDxAct Ltd, an inkjet technology consultancy in 2017. Based in Somerset UK, DoDxAct offers bespoke training and practical assistance in support for all aspects of inkjet R&D from ink formulation and manufacture through jetting & process integration to final application optimization. Working with start-ups to large companies with global reputations, his inkjet applications experience takes in production inkjet, wide-format graphics, labels & packaging, decorative surfaces, print-to-shape, electronics manufacturing, product coding, and 3D printing.
Dr. Bale earned his undergraduate degree and PhD in Physics from the University of Birmingham UK and is a published author of academic papers, patents, and online content on topics ranging from microfabrication, OELD devices to inkjet printing.