FEATURE

Lasers make their mark

Nick Morris reads up on the latest advances in the printing and graphics industry

Photonic devices in general, and lasers in particular, are ubiquitous in the printing industry – from the most humble laser printer in a home office, to the printing presses of major daily newspapers, the laser is a truly enabling technology. Many industry experts believe this to be a mature market. However, there are many exciting developments and advances being made in the field.

Flexography is a form of printing that is widely used in the production of packaging materials and newspapers. In flexographic printing, ink is supplied from a reservoir to an anilox roller that then delivers ink evenly to the printing plate. The printing plate is made from a flexible rubber or photopolymer material, containing raised areas that transfer ink to the piece being printed. The flexibility of the printing plate, together with the large height of its raised areas, makes this a popular method for printing on uneven surfaces such as corrugated cardboard.

Flexo is becoming increasingly popular for several reasons: it is a relatively simple process, and technical advances continue to improve output quality; it is also easily adapted to the use of water-based, rather than oil-based, inks, which makes it more environmentally friendly than other processes, such as lithographic printing.

Traditionally, lasers have been used to engrave the repetitive pattern of small inkwells used on the ceramic anilox rolls. Exposing the photopolymer sheet to ultraviolet light through a negative produced the printing plates. This masked exposure causes selective hardening of the photopolymer. Material in the unexposed areas is then removed with a solvent or water wash. But lasers are now also being used in a computer-to-plate (CTP) process for production of the flexo printing plates themselves. Laser CTP systems for flexo deliver many benefits over other printing technologies. Specifically, they eliminate all the time, expense and chemicals related to the process of creating, developing and correcting film. This advantage becomes particularly important in time-sensitive printing applications, such as newspaper printing.

A highly intense UV light source is required in order to achieve sufficient exposure of the photopolymer in a reasonable amount of time. Until a few years ago, no laser could deliver this power level in a beam that could be focused easily. But that situation changed with the introduction of a new type of laser technology – the mode-locked, diode-pumped, solid-state laser. Moreover, the resonator architecture of this type of laser produces a small diameter, low M2 beam, making it well suited to direct-write applications. These lasers produce high power output at a very high repetition rate – some approaching 80MHz – that appears as a continuous wave for most applications. In the latest CTP systems for flexography, the plate for exposure is mounted on a flat bed that moves in one axis. A polygon mirror is used to raster scan the laser beam across the plate, while the plate is translated in the other direction. The image is created by external modulation of the laser intensity. One example of this technology is the Coherent Paladin, which generates 8W of power. The peak power of this quasi-continuous wave laser allows it to be efficiently frequency-tripled from 1064nm to 355nm. Current systems utilise the highest-power Paladin laser available in order to maximise the writing speed.

A new application area for printing technology that has opened up recently is printing conductive tracks directly onto a substrate. Traditional methods of producing micro-fine wiring tracks has been carried out using mask etching, where a laser is used to remove materials from a semiconductor substrate. However, Conductive Inkjet Technologies has developed a method for using inkjet-printing methods to print electrical connections onto substrates directly, greatly reducing the cost of producing fine electronic devices and connections.

Firstly a layer of catalytic material is printed onto the surface of the substrate. A strong ultraviolet light is then projected onto the catalytic surface to cure it. Once cured, the printing surface is immersed in a solution of metal ions. The ions selectively attach to the catalytic layer; the metal wires grow through autocatalytic deposition. This method can create features down to 50 microns in size. Combining the inkjet printing method with laser ablation can create even smaller features. Ultimately, this combination of methods is only limited by the laser wavelength and optics used.

Opto-electronic devices are not only being used for direct printing, but also for quality assurance and quality control areas of the printing and graphics industry. ICI Imagedata asked AstraNet Systems to design and install an on-line monitoring system to check production of ribbons for dye sublimation printing. Three Astra UV/VIS Spectrometers now provide high-speed measurement of the total optical density (TOD) of each coloured dye panel applied to a moving web in the production process.

ICI Imagedata is one of only a few manufacturers of ribbons for thermal dye sublimation printing, which carry a repeating sequence of yellow, magenta and cyan dye panels. AstraNet designed a quality control solution using three fibre-optic spectrometers working in the ultraviolet and visible portions of the spectrum. Fibres deliver light from a tungsten lamp to one side of the moving web, and collection fibres carry transmitted light from the other side to the three computer-controlled spectrometers. A stepper motor provides transverse movement of the monitoring system so that the whole ribbon is checked over a 10-minute cycle. Product parameters, such as the sequence of colours, panel length and absorbance specifications, are input to the software program and if averaged absorbance values fall outside specified limits, an alarm is triggered.

Before installing the new system ICI Imagedata relied on visual inspection of the moving ribbon, combined with spot measurements on random samples on completion of each three-kilometre roll. Now, using the AstraNet spectrometers, quality is continually monitored at the end of the production line across the whole width of the web. Mike Mills, managing director of AstraNet, explains the specific challenges of the project: ‘This was the first time we had designed a system using multiple spectrometers linked to a single PC. The PC also had to be networked to allow managers to monitor and analyse quality information remotely, without having to enter the controlled environment of the manufacturing area. Even though there are only milliseconds in which to measure each dye panel on the fast-moving web, that is enough time to complete several full wavelength scans. Using USB2 allows fast data transfer between the spectrometers and the PC, another key requirement for this set-up. ICI Imagedata now has a monitoring system that delivers greatly enhanced quality control for its dye sublimation printing ribbons.’

Printing is one of the ‘staples’ of the photonics industry, and while there is a demand for cheap, more efficient and more reliable technologies there will always be a demand for better photonics products.