Anilox technology – meeting the demands of HD flexo

Anilox roller production at Tech Cell Ltd

To meet the needs of printers working with high resolution plates, anilox manufacturers have been pushing conventional 60 degree hexagonal engraving to the limit of performance, while at the same time developing new engraving structures. By David Atkinson.

With the latest generation of digitally imaged flexo plates, it is now possible to achieve 10 micron highlight dots on the plate, and this has created a challenge for anilox roller manufacturers. Conventional wisdom is that the anilox cell opening should be less than the diameter of the smallest plate dot in order to prevent dot dipping.

An anilox roller with a screen ruling of 1000 l/cm or 2540 l/inch has individual cells 10 microns across, including the cell wall, which is just enough to prevent a 10 micron dot on the plate dipping into the cell of the anilox. It is possible to produce rollers engraved at this high line count, but for various reasons they are of little practical value in the pressroom.

Meeting the challenges

The main challenge for anilox suppliers when manufacturing very fine screen rollers is being able to produce a roller that can also transfer enough ink volume to achieve acceptable solid colour density values. This is more difficult to achieve in the wide web market than the narrow web market due to higher press speeds and the nature of the ink system.

There are also challenges with regard to the density and porosity of the ceramic coating. Voids are created in the ceramic during the plasma spraying process as the ceramic layer is built up. The greater the size and number of voids, the higher the porosity of the ceramic. When the size of the engraved cell approaches the size of the voids, it becomes impossible to produce a consistent engraving. In order to produce a uniform engraving at very high screen rulings, it is necessary to reduce the porosity – and hence the voids – of the ceramic to less than 1%.

The anilox manufacturers have been meeting these challenges by improving ceramic coating technology and working with fibre optic lasers, the latest of which are able to generate 500W of power. With fibre optic lasers, it is possible to experiment with the variables to produce a wide range of cell shapes and engraving angles. Other reported benefits gained from the use of the latest high power fibre optic lasers include cells with stronger walls and smoother interiors that can improve ink release by up to 15% when compared with equivalent YAG rollers. The rollers are also said to be more resistant to wear and scoring.

The Emerald engraving developed by Tech Cell Ltd is a 60 degree engraving with the cells elongated in the circumferential direction to allow for better ink flow. With this cell structure, dependent on the original cell specifications to be matched, it is possible to increase the line count by up to 30% without sacrificing volume. The length of the cell is increased to a maximum of twice its width, which is said to reduce the risk of the plate screen flooding.

Owner, Danny Goodwin, commented, ‘The Emerald screen is now overtaking the company’s standard 60 degree screen on high definition work. The extra release of ink either UV or water-based from the shallow cell profile allows a higher anilox screen count to be engraved and improves the cleaning process.’

The company has been experimenting with a chemical hardener used in the aerospace industry and introduced into the ceramic after coating and grinding to strengthen the cell walls and improve scratch resistance.

Apex Group of Companies spent three years developing the GTT roller, which has an open ‘slalom ink channel geometry’. The company has a large number of these rollers at installations across the globe. The channels are engraved into a hybrid ceramic consisting of 80% chromium oxide and 20% titanium oxide to lower surface energy and produce a hard high density coating with less than 1% porosity. Instead of pulsed laser beams a constant beam laser engraving process is used to engrave the channels as this reduces ceramic roughness and recast.

The channel geometry eliminates about 60% of the land area present on the cell walls of a conventional 60 degree engraving, while the nature of the ceramic and the engraving process is said to improve ink release, ink laydown and roller longevity. A number of ink, plate and software suppliers to the industry joined Apex to create a flexo Pantone simulation guide for release at drupa to prove that GTT technology offers a wider colour gamut than conventional 60 degree rollers.

At Cheshire Anilox Technology, they use a TriplexPro 200 plasma coating system in conjunction with the latest 500W fibre optic laser technology with custom pulse. The coating system produces hard low porosity ceramic coatings and the engraving technology offers the means to produce unlimited cell designs. This has enabled the company to produce new high line screen engravings with improved ink releases properties.

The company said that low porosity ceramic is paramount to achieving consistent results and that new advances in coating technology are helping to make ultrafine engravings possible. Its ProFlo features a new improved cell profile that produces a more consistent ink laydown and offers as much as 15% additional ink release compared with conventional engravings. The latest screen, the MaxFloUV, has been specially developed to work with the new generation high strength UV inks. This has been tested with leading label printers and is available with engravings of up to 2000 l/inch.

The most recent anilox supplier to enter the market place, Anilox Laser Technology, has installed a fibre optic ultra laser system that is capable of engraving rollers at 4.7 metres face length. The laser has the latest software installed along with a multi burn system. A Triplex ll plasma system is used to produce a ceramic coating with a porosity of less than 1% and hardness in excess of 1600 Vickers.

The company has successfully produced a high volume 60 degree hexagonal engraving at 1200 l/inch with a volume of 5.4 cm3/m2 for use with UV inks on label stock. It is currently producing a set of 1500 l/inch 60 degree rollers that will be used in conjunction with HD technology on a Gidue press in France, and are also working on some new cell structures that are in the testing phase.

In 2011, Pamarco Global Graphics invested in 500W multi-hit laser technology with new screening software. The company has developed its own bitmap file for the new laser and used it to evaluate the key engraving variables. This has enabled it to improve on the EFlo design by increasing the inner dimensions of the individual cells and producing a better cell wall shape. The enhanced cell structure of its PHD product offers improved compatibility with HD digital plate technology. Using this technology, the company has produced 1200 l/inch 2.2 BCM (3.3 cm3/m2) EFlo HD rollers for an F&K press that, as a result of better ink transfer, have achieved higher colour density values than rollers engraved at 800 l/inch 2.4 BCM (3.6 cm3/m2) with a traditional 60 degree engraving.

Harper Corporation has globally extended its XLT technology with nine new ALE Hercules laser engraving systems, five of which are 500W models. Its ceramic coating has an average porosity of 0.5%, an upper limit of 1% and is super-finished by surface grinding prior to engraving. With 500W technology, the company is not only able to achieve high line screen counts but also better cell profiles and tops to the cell walls on conventional 60 degree engravings. This has enabled it to expand its engraving charts to beyond what was previously possible.

Having conducted trials with other cell geometries, the company believes that the 60 degree hexagonal is the most proven and consistent engraving available. In January 2012, the company received Esko certification in HD flexo for 60 degree XLT engravings ranging from 900 l/inch to 1600 l/inch.

The printer’s perspective

In general, the higher the anilox line screen, the thinner the cell walls tend to be, and as a result, these rollers have less resistance to wear. This shortens useful roller life, which means that anilox rollers with very fine screen rulings will probably need refurbishing more frequently. However, if a printer can rationalise anilox inventory, for example by using the process ink set as the main ink set on the majority of jobs, then this will help to offset the increased refurbishment cost.

High resolution rollers are by definition fairly low volume rollers where a relatively small amount of wear can have a significant effect on roller volume and hence colour. This means that the printer is likely to have to start ‘chasing colour’ sooner. It is much more important with rollers of this nature that the printer uses minimum doctor blade pressure and controls anilox inventory. Inventory can be controlled quite effectively by means of volume measurement, which is used to ensure that replacement rollers meet the same specification as the original rollers, and so that wear and the effectiveness of cleaning can be monitored.

On the subject of cleaning, it may also be necessary to review the cleaning process as some methods will probably not be well suited to very fine engraving structures.

Conclusion

With the aid of multi-pulsing fibre optic lasers, the anilox industry is well placed to meet the latest demands from flexo printers wishing to adopt high definition flexo and the challenge this presents. Anilox manufacturers have developed rollers with better ceramic properties and used the latest generation of lasers to generate new engraving structures and finer engravings that are able to offer enough support to the elements on the plate, while at the same time providing sufficient engraved volume and effective enough ink release to produce acceptable solid colour density values.

While these developments have been driven by the need to meet the demands of high resolution flexo, they have also had a positive impact on the accuracy, repeatability and integrity of rollers in general.

The 60 degree engraving is still by far the most common engraving. While the practical limit of resolution has probably been reached as far as high resolution plates are concerned, anilox manufacturers are now capable of producing 60 degree engravings with finer screen rulings, higher volumes, better ink release and better wear properties than used to be possible with the YAG laser.

The versatility of the fibre optic laser means that it is now possible to produce a wider range of engraving structures than ever before and in the high-speed wide-web sector in particular, it is likely that alternative engraving structures will become more common.