More on Flexo Sleeves

More on Flexo Sleeves
The special  flexo sleeve market has been making great strides over the last few years. However, boasting credentials such as the ability to print on almost any type of substrate, including plastic, metallic films, cellophane, and paper, and use a wider range of inks, including water based rather than oil based ink, it isn’t really surprising that this method has become extremely popular, particularly within the food packaging industry where printing on non-porous substrates is required. Damien Leterrier, Trelleborg’s sales and development manager for its Axcyl flexo sleeve product, discusses 

Since 1990, great advances have been made to the quality of flexo printing presses, printing plates and printing inks. This has mainly been in response to a growing market demand for shorter print runs, quicker changeover between jobs, and variable print sizes. 

These developments in pre-press, anilox rollers, digital plate technology and sleeve-dedicated printing presses has meant that the quality achieved on a modern flexo press equals that of any other commonly used method of printing, taking flexo printing into areas that would have once been unthinkable.

Amongst the enormous technological breakthroughs there have been in this field it would be easy to overlook the huge advances made in sleeve construction. The development of sleeves, which enable the next few jobs to be pre-mounted while the press is still running, has been crucial to enabling flexo printers achieve quick changeovers and therefore handle short runs and design alterations cost effectively. They also provide a straightforward way of storing plates for repeat orders.

Sleeve Construction 
Today’s flexo sleeves are highly engineered and there are a wide range of options from a growing number of suppliers. Press manufacturers do not produce sleeves, leaving this to consumable companies specialising in the technology. It imperative that printers keep a close eye on developments and maintain a good understanding of the differences between products and types or they could miss out on opportunities to improve productivity or save money. 

For example, sleeves of various thicknesses enable the reproduction of different repeat lengths when combined with an adaptor or bridge sleeve that fits on to the press shaft, upon which the plate mounting sleeve can subsequently be added. The use of such bridge sleeves enables the printer to reduce capital expenditure by using thinner sleeves for plate mounting, which fit the array of bridge sleeves designed for the specific press. The recent introduction of carbon fibre adaptors, which have excellent rigidity and vibration filtration properties, allow high speed production of the most demanding print designs. 

Except for corrugated post-print applications, sleeves are used on all forms of flexo presses. They are generally built with a ‘sandwich structure’ concept that incorporates two key elements - the mounting layer and the repeat build-up layer. The former expands under a pressure of six bar, enabling the sleeve to be mounted and removed from the press shaft, whilst the build-up layer enables several repeat lengths to be obtained from one single inner diameter.

Sleeve Specification
Sleeves are made out of a variety of materials, so their properties differ greatly. The majority fall into the cheaper end of the cost spectrum. This includes glass fibre and polyurethane foam based products. These are light, but generally unstable over long periods of time, particularly with their tendency to absorb moisture/solvents from the air during storage. In addition, polyurethane foams can be very elastic, with limited vibration filtration properties.
To obtain sleeves with good stability over long periods, printers should select products at the higher end of the price scale which are based on expanded, non-woven fabrics embedded into a resin system. Leading manufacturers have developed sleeves which feature a mounting layer and are composed of a specially formulated polymer designed to filter vibration from the impression nip, while the build-up layer is made of epoxy resin with a honeycomb structure. This provides stability and rigidity, resulting in low dot gain and the ability to print jobs at speeds where the level of press vibration would otherwise adversely affect print quality. 

Sleeve Protection
Most sleeve damage occurs during handling and any ‘protection features’ provided by the manufacturer are worth looking at closely. For instance, a shock-absorbing face will enable the sleeve to last longer, while the incorporation of a cutting guide prevents excessive knife cuts on the outer surface. With good housekeeping, sleeves should be reusable for at least five years.
Another issue that continues to crop up across the sector is the installation of automatic sleeve cleaning systems without previously checking that the sleeve material is compatible with such equipment. Too often the cleaning solvents used on machines penetrate the sleeve ends if these are not sealed during manufacture, resulting in permanent sleeve deformation and poor register.

Quick registration of the sleeve to the shaft is normally achieved via pre-cut slots, but if these are not properly reinforced the press operator may well find that pre-register, as defined via the sleeve slot, does not remain accurate because of premature wear. 

Rather surprisingly, it is not unknown for printers to incorrectly specify sleeve dimensions, either to match the shaft or the plates. This information is worth double checking prior to sleeves. In addition, the varying thickness of plates means that not all plates can be used with all sleeves. 

The ideal sleeve and adaptor for the printer should combine lightness with strength. Handling is very important considering the frequency of changeovers, so lightweight materials that incorporate higher stiffness and dynamic response (to minimize vibration during printing) are the ideal solution. These desirable features account for the rise in popularity of carbon fibre adaptors. 

A Sound Solution
By carefully selecting the correct sleeve solution for the job and the printer, print managers are set to maximise efficiencies and cost over the long run, especially during these difficult economic times. 

And with leading manufacturers offering high performance, reliable sleeve solutions which involve creating a honeycomb ‘tube’ system with variable diameters, so that plates can be mounted off-line before sliding the assembly on the shaft of the press, the future for flexo looks even more positive. 

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