by Debbie Thorp, business development director, Global Inkjet Systems Ltd.
Decorating objects using inkjet technology is not a new application – we’ve seen printers on the market for several years. The small-format, semi-desktop flatbed scanning devices that print such promotional items as USB memory sticks, lighters and key chains have been successful, but have tended to be limited to objects of a certain size and height. Now the market has expanded considerably with production-capable systems – larger flatbed devices customized to accommodate taller objects, and single-pass units often integrated into manufacturing lines – printing anything from industrial saw blades to perfume bottles.
The drivers for the adoption of digital technology and the growth of product decoration using inkjet are well known: cost-effective short runs; the ability for every image on each object to be different, offering new marketing opportunities; and potentially significant reduction in overall costs, as so many consumables required for screen printing or pad printing are no longer needed. Although inkjet may not be the technology of choice for long runs with the same image, for many other jobs the arguments for inkjet are increasingly compelling.
Recently, we’ve seen the growth of semi-production-capable systems for product decoration and also the introduction of systems that print onto tubes and even conical shapes. In addition, systems have been introduced for printing onto cylinders, tubes and even tubs that may start to challenge traditional analog technologies in terms of productivity, plus having all the added benefits of digital technology.
Flat and semi-flat surfaces
Printhead reliability has improved dramatically in the past 10 years, and inkjet technology has become much more accepted in industrial applications. It now is proven that inkjet can be reliable in heavy-duty 24/7 manufacturing environments, and development has advanced in inks, system design, software and electronics to attain this level of reliability. However, maintaining accurate drop placement on a range of surfaces still can present challenges.
Product decoration often requires inks to adhere to a very wide range of substrates – wood, glass, metals and many types of plastics. Different products may require different surface pre-treatments or primers before inkjet printing, or they may require coating after printing. Compatibility with other parts of the manufacturing line, post processing and handling, ink migration, recyclability and other regulatory requirements all add to the challenge.
However, innovative ink development and careful process development have opened up many new applications to inkjet technology, and new systems have been developed specifically for these opportunities. In the flatbed arena, companies have created custom systems to print onto larger objects – the ITW Transtech Inspiration flatbed system can print onto products up to 10 inches high. In single pass, a profusion of new product introductions include the Cyan Tec Cyjet, Colour Print engines from Industrial Inkjet and the Mach 1 from Innovative Digital Systems.
Overcoming limitations
Inkjet printheads are designed to print onto flat surfaces, and throw distance can affect print quality dramatically. Drops only jet a few millimeters and decelerate quickly, so printheads are best positioned as close to the surface as possible – but in industrial applications, this may not always be feasible. Larger drops can jet further, but smaller drops improve graphical image quality. The further away a printhead must be from the surface, the greater the potential for jet straightness to be an issue, along with the sharpness of images and text. Depending on the printhead and drop size, the maximum throw distance is approximately five millimeters.
Throw distance is the sort of challenge that, for example, printing onto a washing machine control panel presents. The substrate may be flat in some print areas, but the plastic mold may have parts that protrude and require that the printhead be positioned higher than optimum to avoid damaging the nozzle plate. Much depends on the type of image being printed – small text and fine barcodes typically are less forgiving than some graphics.
Ink development has come a long way already, but regulatory requirements still bar inkjet from some large markets, such as on toys and when ink is in direct contact with food. However, the introduction of low migration inks with specific high reactive monomers and diffusion-hindered photoinitiators, compliant with legislative mandates, have enabled inkjet to enter some of these previously inaccessible markets. Systems now are available that print directly onto PET beverage bottles, such as the InnoPrint from KHS and the Decotype from Krones. Other inkjet systems available print directly onto medical bags, syringes and blister foils.
Curved surface printing
Devices such as the 9150 from Dubuit and those mentioned earlier represent a growing number of new systems that can print onto curved surfaces. Most of the new systems print onto cylinders or tubes, which do not require image compensation. Cut a tube lengthwise and lay it flat and you have a rectangle or square – essentially the printers are wrapping a “flat” image around the cylinder.
However, the physical characteristics of the printhead create new challenges. Three key issues exist: printhead symmetry, distance between nozzle rows and the number of nozzle rows. Symmetrical orientation of the object under the printhead is important. If a printhead has two rows of nozzles, the object should be oriented so the rows are symmetric on either side of the center line of the tube. The narrower the distance between the nozzle rows, the better. Although more rows of nozzles in large printheads may increase productivity, the wider the printhead, the harder it is to print with high quality onto narrow tubes. Konica Minolta 1024 and 1024i and Xaar 1002 printheads are used in many of the latest tube printing systems. These are printheads with a small number of rows, a narrow gap between the rows and a compact, narrow overall printhead width, i.e. physical characteristics that contribute to obtaining good print quality.
Printing onto conical objects adds greater complexity and challenges for inkjet. Cut a cone down its length, lay it flat and you have what looks like a section of an old vinyl record. Image compensation is required to wrap the image around the cone, and the system also has to contend with resolution changes as the cone rotates under the printhead, so screening complexity and the need to compensate for drop density increase. Without correction, these errors can result in unacceptable image quality.
Global Inkjet Systems (GIS) has developed software that corrects nozzle misalignment, dot gain and density changes, generating a fully compensated image with minimal screening artifacts. The technology works excellently on conical objects but also is ideal for more complex objects with discontinuous curves, such as tubs. Tubs are a mixture of flat and curved surfaces and require complex corrections during printing – often pixel to pixel. The GIS multi-dimensional nozzle, density and screener correction technology can be adjusted to each surface type as required.
What’s next?
As direct product decoration becomes mainstream and more machine vendors enter the market, the pressure will be on further innovation in ink technology and process development to open access to more applications. Certain sections of the market will aim for increasing productivity; others will target compact, smaller systems. Print quality always will be critical, and the ability to print taller images onto cones and tubs highly desirable. One thing is clear – direct product decoration of a wide variety of objects is a growing opportunity for printers to expand their businesses, and the range of systems offered to tempt them will surely increase.
Debbie Thorp worked in digital printing for more than 20 years, including 10 years in senior positions at specialist digital printing consultancy companies such as BIS Strategic Decisions, IT Strategies and CAP Ventures. She joined Xaar as head of group marketing in the late 1990s and then moved on to be director of business development at AIP Ltd. More recently, Thorp was vice president EMEA operations at iTi. In early 2010, Thorp was appointed business development director at Global Inkjet Systems Ltd, a leading developer of software drivers and electronics. She has a BA degree in modern languages from Manchester University and an MBA from Durham University Business School.
Article reprinted with permission from Specialty Graphic Imaging Association (SGIA), an association supporting the leaders of the digital and screen printing community. For more information, visit www.sgia.org.
