Inkjet technology
Inkjet technology has become a household word through its presence on the consumer desktop as a low cost, reliable, relatively quick and convenient method of printing digital files. Although inkjet technology has been around since the 1950s in speciality printing, the impact of the technology in a wide range of industrial applications is only now becoming clear.
Simple in theory. Complex in practice
In theory, inkjet technology is simple - a printhead ejects a pattern of tiny drops of ink onto a substrate without actually touching it. Dots using different coloured inks are combined together to create photo-quality images.
In practice however, successful implementation of the technology is very complex. The ejected dots are smaller than the diameter of a human hair (70 microns), and they need to be positioned very precisely to achieve resolutions as fine as 1440x1440 dots per inch (dpi). This precision requires multi-disciplinary skills; a combination of careful design, implementation and operation across physics, fluid mechanics, chemistry and engineering.
Industrial inkjet printing
Inkjet technology is increasingly viewed as more than just a printing or marking technique. It can also be used to apply coatings, to deposit precise amounts of functional materials, and even to build micro- or macro-structures. The potential benefits of using inkjet technology are significant and include the reduction of manufacturing costs, provision of higher quality output, conversion of processes from analogue to digital, reduction in inventory, printing onto very large, very small, fragile or non-flat substrates, reduction of waste, mass customisation, faster prototyping and the implementation of just-in-time manufacture.
Industrial inkjet printing essentially means using inkjet technology to print or deposit materials as part of the manufacturing process of a product on a production line. The principle remains similar to the inkjet printer on your desktop but the scale is very different in terms of machine size and speed, and also in the variety of fluids that need to be deposited. The introduction of industrial inkjet technology into manufacturing environments has the potential to make a revolutionary step-change to existing capabilities with huge commercial benefits. While all inkjet technologies can fundamentally be described as the digitally controlled ejection of drops of fluid from a printhead onto a substrate, this is accomplished in a variety of ways.
Inkjet technology for industrial printing
Industrial inkjet printing systems, and the industrial inkjet printheads they are based on, are broadly classified as either continuous (CIJ) or drop on demand (DOD), with variants within each classification. As the name implies, continuous inkjet technology ejects drops continuously from the printhead. These drops are then either directed to the substrate as printing drops or to a collector for recirculation and re-use. Drop on demand technology ejects drops from the printhead only when required.
Continuous inkjet printing (CIJ)
Continuous inkjet printing (CIJ) is primarily used for coding and marking of products and packages. In this technology, a pump directs fluid from a reservoir to one or more small nozzles, which eject a continuous stream of drops at high frequency (in the range of roughly 50 kHz to 175 kHz) using a vibrating piezoelectric crystal. The drops pass through a set of electrodes which impart a charge onto each drop. The charged drops then pass a deflection plate which uses an electrostatic field to select drops that are to be printed and drops to be collected and returned for re-use. With multi-level CIJ, the printed drops are deflected into several positions onto the substrate, while with binary CIJ it is the un-deflected drops that are printed. The high drop ejection frequency of CIJ gives a capability for very high speed inkjet printing, suitable for such applications as the date coding of beverage cans. An additional benefit of CIJ is the high drop velocity (of the order of 50 m/s), which allows for relatively large distances between the printhead and the substrate.
Historically, CIJ has enjoyed an advantage over other inkjet technologies in its ability to use inks based on volatile solvents, allowing for rapid drying and aiding adhesion on many substrates. The disadvantages of the technology include relatively low print resolution, very high maintenance requirements and a perception that CIJ is a dirty and environmentally unfriendly technology due to the use of large volumes of volatile solvent-based fluids. Additionally, the requirement that the printed fluid be electrically chargeable limits the applicability of the technique.
Drop on demand inkjet printing (DOD)
Drop on demand (DOD) is a broad classification of inkjet printing technology where drops are ejected from the printhead only when required. In general, the drops are formed by the creation of a pressure pulse within the printhead. The particular method used to generate this pressure pulse creates the primary subcategories within DOD, namely 'thermal' and 'piezo'.
Thermal inkjet technology (TIJ) is most used in consumer desktop printers but is also making some inroads into certain industrial inkjet applications. In this technology, drops are formed by rapidly heating a resistive element in a small chamber containing the ink. The temperature of the resistive element rises to 350-400ºC, causing a thin film of ink above the heater to vaporise into a rapidly expanding bubble, causing a pressure pulse that forces a drop of ink through the nozzle. Ejection of the drop leaves a void in the chamber, which is then filled by replacement fluid in preparation for creation of the next drop.
The advantages of thermal inkjet technology include the potential for very small drop sizes and high nozzle density. High nozzle density leads to compact devices, lower printhead costs and the potential for high native print resolution. The disadvantages of the technology are primarily related to limitations of the fluids which can be used. Not only does the fluid have to contain a material that can be vaporised (usually meaning an aqueous or part-aqueous solution) but must withstand the effects of ultra high temperatures. With a poorly designed fluid, these high temperatures can cause a hard coating to form on the resistive element (kogation) which then reduces its efficiency and ultimately the life of the printhead. Also, the high temperature can damage the functionality of the fluid due to the high temperatures reached (as is the case with certain biological fluids and polymers).
Piezoelectric (piezo) drop on demand inkjet technology is currently used for most existing and emerging industrial inkjet applications. In this technology, a piezoelectric crystal (usually lead zirconium titanate) undergoes distortion when an electric field is applied. This distortion is used to create a pressure pulse in the ink chamber, which causes a drop to be ejected from the nozzle. There are many variations of piezo inkjet architectures including tube, edge, face, moving wall and piston, which use different configurations of the piezo crystal and the nozzle.
The advantages of piezo inkjet technology include the ability to jet a very wide variety of fluids in a highly controllable manner and the good reliability and long life of the printheads. The main disadvantage is the relatively high cost for the printheads, which limits the applicability of this technology in low cost applications.
In summary, industrial printing remains a very promising area of growth for inkjet technology, with applications such as ceramics, textile, packaging and product decoration showing excellent benefits and market acceptance to varying degrees. All of these applications use piezo DOD technology almost exclusively, and this is expected to continue for the reasons outlined above.
Tim Phillips, Catenary Solutions