Back to school - Inkjet

Back to school

Back to school
Image Reports
Ever wondered exactly how inkjet technology works? Nessan Cleary went back to school with the IMI Inkjet Academy.
In recent years inkjet has become a mainstream technology in the printing and sign making industries. But thereǃÙs a lot more to inkjet than just printing. The ability to jet a fluid through a head and to place it very precisely in exactly the right spot has also proven useful in other areas, including the manufacture of printed circuit boards, and even of human organs. In a bid to find out more about inkjet, we turned to the Inkjet Academy, and booked a place on the Theory of Inkjet Technology course. This is a one and a half day course covering the basics of inkjet technology, what it is, how it works, and what inkjet can be used for. It's open to anyone, and draws people from printers to manufacturers, mostly from the graphics industry, but with some people coming from other fields. It runs twice yearly, in both the States and Europe. Other courses include system design, ink manufacturing and the practice of inkjet technology. All of these are organised by the Information Management Institute, and its affiliate, IMI Europe, run by inkjet consultant Mike Willis, one of the original founders of Xaar.
The Theory of Inkjet Technology is led by Willis and Dr Ian Rees, from Xennia. The course is split into a number of short lectures, which together offer a fairly thorough exploration of all the main issues relating to inkjet technology. It includes a description of the different types of printhead technology, from continuous to drop on demand, how each works, and the relative merits of each. We discussed differences between thermal and piezo drop on demand technology. Thermal has proven highly cost effective for use in SOHO applications, whereas piezo is more prevalent in industrial applications. This is mainly because piezo offers more control of each individual drop, and allows for faster printing and longer life heads, but is too costly for low-end printers. Piezo printheads rely on materials such as PZT ceramic (lead zirconium titanate) that can be distorted when an electrical field is applied. This distortion can be used to change the shape of an ink chamber within a printhead, literally forcing ink through the nozzle by compressing the chamber. Thermal inkjet on the other hand relies on heating some of the ink within the ink chamber so that it vaporises, forming a bubble which leads to a pressure wave within the channel, forcing ink through the nozzle. In both systems the trick is to create enough pressure to force some ink through the nozzle, but for that pressure to subside so that instead of a continuous stream of ink being forced through the nozzle, some of the ink is drawn back to the nozzle, causing an individual drop to form. The main issue for both systems is that as each drop of ink breaks off from the stream of ink, it leaves a tail, or meniscus, which snaps back towards the nozzle, and which can ultimately clog the nozzle, unless controlled properly.
We also looked at the various methods of greyscale printing in which variable size dots give an apparently higher resolution. There are several greyscale methods, including digital halftoning, in which many small dots are used together to build up an image, but this can be expensive to achieve. Other methods include using drops of the same size, but varying the number of drops, so that a small number of drops lead to a small dot, and more drops lead to larger dots. This slows down the printing rate because of the need to fire multiple drops in the same place. Another method involves firing the same size ink drops but using inks with different densities, which requires extra ink tanks and nozzles. Alternatively, itǃÙs possible to vary the size of the ink drops by varying the pressure firing the ink drops, which in turn requires more complex printhead designs. The course went on to look at the ways of generating pressure in a printhead to fire a drop of ink, and to prevent splatter from unwanted satellite ink drops being fired alongside the main drop. We went through the major types of inks.
Thermal inkjet inks are by far the simplest, consisting of 90 per cent water, plus colorant and additives. Piezo inks are slightly more complex, being around 60 per cent water, with on average, 10 per cent colorant, 16 per cent binder and 14 per cent additives, which can include such things as a defoaming agent, or special binders for printing to glass. We then looked at some of the issues involved in creating ink formulas, including the purity of flow of the ink, the viscoelasticity, which ensures that the tail of the potential drop breaks off and snaps back to actually form the drop, and the physical properties of the different chemicals used including how they age, and whether or not they will corrode the components of the print head. Other issues included balancing the quality of each individual drop, for example by increasing the molecular weight of the polymers for less splatter, but which can slow down the printing speeds.
The discussion also included a look at some practical solutions, including a HP patent in which each of the CMYK inks were tuned to react against each other, therefore reducing the amount of bleed, and which would be particularly useful for printing onto cheaper, uncoated substrates. A separate part of the course looked at different inkjet media, and how ink and media combinations are designed to work together. For this reason many printer vendors insist that their ink and media are used for best results, although increasingly inks for home use are designed to work with a wide range of media. The media affects the bleed and feathering of the inks, as well as the drying time, the wet fastness, light fastness, and paper handling characteristics such as curl. Coatings on paper help control the drying time, by controlling the absorption and evaporation of the ink carrier, whilst keeping the colorant near to the surface. The course also touched on UV curing technology, explaining the advantages and disadvantages of this type of inkjet. It touched on the different UV ink options, and the various materials used to make UV ink formulations.
It also looked at the curing process, how different types of UV curing lamps work and some of the challenges facing UV curable inks. However, the Inkjet Academy also runs a separate course on UV curable inks, side by side with the Theory of Inkjet Technology. Not surprisingly the course was heavy on theory, but was well presented so that I never felt out of my depth, despite failing O-level chemistry. It was extremely well organised, with plenty of opportunity for delegates to mingle and chat. It came with a thick folder of PowerPoint slides, and by the time IǃÙd covered these in notes from the lecture I ended up with quite a decent reference manual on how inkjet technology works.