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December 3, 2008 — ‘There are many advantages that UV flexo inks have over other flexo inks,” says Hunter Paterson, Product Manager for contact web cleaning technology specialist Teknek Ltd. “Colour consistency, product resistance from cross linking, no viscosity adjustment, and a option of shutting down the press without clean up, are just some of the advantages. Recent developments contributing to growth in UV flexo include, inks that are less odorous, provide higher densities and are of lower viscosity.”
While printing with UV flexo inks can be highly rewarding, it’s not without its challenges. On porous substrates for instance, there is a tendency, with a lower viscosity UV ink for the ink to dive into the substrate making it look weak. While this can be compensated for, some printers opt for a work around, using one of the higher viscosity UV flexo inks. However, this in itself can be a challenge in that the ink doesn’t flow as well as expected. In a worst case scenario the anilox roll is suddenly starved of ink, even though the pan is full. This effect, known as cavitation, causes ink starvation and may result in plate skipping.
Profitable and high quality flexographic printing depends in large measure on having clean base stock material – material free from paper dust, lint, loose fibres, material free from contaminants suspended in the press room environment, which can be attracted onto the a moving web, as a result of circulatory air and electrostatic charging. All print processes and converting processes are subject to the problem of airborne and surface borne contamination. However, the UV flexo process is particularly sensitive to contamination due to the nature of the inks viscosity and thinner ink film application. Hunter Paterson is at pains to point out that hickeys and other particulate contaminants are not washed away as they are with solvent and water-based flexo inks.
Non-stop operation is critical to cover overheads and recoup a respectable profit. Line interruption in order to clean rollers, plates and aniloxes represents an impediment to that goal. More than just a minor annoyance is the high levels of material throwaway, wasted time, and a risk of missing delivery deadlines that contaminated webs and components also cause.
“First, lets look at the difficulties associated with keeping a web clean. Transporting a web through a processing machine such as a flexographic press, though it may just as likely be a coater, a sheeter or a slitter, causes a boundary layer of air to form,” explains Hunter Paterson.
Continuing he says: “This boundary layer occurs as a result of the directional movement of a flat surface (in this instance a moving web) and the friction that is created between the surface of the material and the air. As the web accelerates, surrounding air is drawn along and becomes entrained with the moving web. In essence this layer of air acts as a suction to hold particles of dirt and other debris fast. It is impossible to remove contaminants effectively without somehow penetrating this boundary layer.”
Contact and separation of the web (rollers, etc) compounds the problem still further as static is generated and static acts like a magnet to dust and dirt and holds the contaminants even tighter. The faster the web speed, the more difficult it becomes to resolve the problem.
While there are various methods for cleaning a web, the ability to penetrate the boundary layer of air and remove particles down to the micron level is the preserve of the contact cleaning principle adopted by Teknek Ltd, providers of ultra-narrow, narrow, mid and wide web width cleaning technology.
Hunter Paterson acknowledges that whereas once a customer would tend to ignore minor blemishes, increasingly that is no longer the case as more and more customers, and their applications require micron level contaminant removal.
Tekenk’s contact cleaning system concept involves the use of an elastomer roller with a high energy surface grab which runs in contact with the web, penetrating the boundary air layer to remove particles down to the micron level. The roller lifts the contaminants, transferring them to a special reverse wound pre-sheeted adhesive roller where the contaminants become permanently trapped. When the outermost adhesive sheeted layer becomes saturated it is simply removed exposing the next underlying adhesive layer ready for use. And so the cleaning cycle continues. An anti-static system ensures contaminants are not re-attracted to the cleaned moving web.
By integrating a system such as Teknek’s NWP (Narrow Web Platform) inline with a process such as UV flexo label printing, quality of output remains at the highest level, material waste is kept to acceptable levels, and downtime stoppages are minimised or eliminated.
Further details visit www.teknek.com
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