Ink for shrink

6 May 2008



Shrinkable UV flexo inks are a reliable partner on narrow web presses for printing shrink sleeve laminate, says Niklas Olsson


Shrink sleeves are a true phenomenon in the modern packaging world – a real innovation that has transformed the face of the retail shelves. They offer 360 degrees pack decoration and can be applied to virtually any shape of package. They can also provide differential shrinkage over different areas of the container. Ink technology, which is a totally different chemistry, has also been challenged by the shrink sleeving concept.

Gravure, originally the prime choice of printing process for shrink sleeves, offers exceptional ink density coupled with high print speeds. This combination aptly met the needs of the original developers of shrink sleeves: the vertically integrated companies who extruded the film itself and offered sleeve print and specialist application equipment.

There are many sleeve applications that can be met by today's narrow web print processes, which are also used on unsupported films for flexible packaging such as pouches and toothpaste tubes. Narrow web printing is particularly appropriate for small to medium length print runs, which is a growing market for shrink sleeves. Currently, the largest installed narrow web print technology base is UV cured flexo. So there has been a distinct imperative to extend the capabilities of this versatile process into shrink sleeve print.

The challenges

UV inks for shrink sleeves have to combine two conflicting performance parameters: delivering an ink film that not only cures at high speeds, but also shrinks as required, as well as providing strong adhesion to the film substrate with good scratch resistance. The most challenging requirement of all is for the ink to be able to cope with varying degrees of film shrinkage across the same container.

Many different shrink films are used as sleeve substrates today, including PVC, PET, OPP, OPS, and even PLA. Different substrates have different surface slip characteristics called coefficient of friction (COF), whose needs have to be met by inks without the use of additional primer coats in order to achieve the required adhesion to substrate.

Over and above these special demands, the prime concern of inks is to provide eye-catching, high colour strength graphics that give a product first class retail shelf standout.

An all-purpose shrink sleeve ink for UV cured flexo was the ‘holy grail' that narrow web converters needed to give them access to the high value shrink sleeve market, at all levels of shrinkage, without any compromise on quality, to simplify ink ordering and minimise ink inventory. Those goals can now be met. Earlier UV flexo inks for shrink films offered only success in low shrink (less than 25 per cent) and medium shrink (25-50 per cent) applications.

Broadly speaking, these new formulations are free radical curing inks which deliver the best possible shrink ability coupled with very high colour strength, high cure speed, and excellent on-press and print performance across a wide range of film substrates. Partnering these inks with an opaque white ink with very high surface slip (low COF) eliminates the need for primers or over varnishing for print protection, whether the print is destined for the front or the reverse of the sleeve. The scratch resistance, even at levels of shrinkage greater than 70 per cent, is excellent.

Today’s UV flexo shrink sleeve inks offer considerable improvements over gravure and solvent based flexo inks in terms of fine, tight register graphics, with lower dot gain. There are no fuzzy or slightly blurred images and they give zero image distortion with retained image crispness when shrunk to the required size.

An ‘ideal’ set of parameters for the narrow web converter using UV flexo inks for sleeve laminate might be as follows.

Firstly, high opacity is essential. Opacity is generally associated with a thick ink layer but that can be difficult to cure and can cause poor shrink performance. On the pack, heavy ink layers can also result in an unattractive 'relief' effect.

Experience shows that using two base layers of opaque white ink, with the white printed in dots, will give good opacity but care should be taken to use as little white ink as possible in areas that will experience the very highest shrink.

The application process involves slipping a tube of printed film over the top of the pack to be sleeved. The sleeved container then passes along a conveyor into a heated shrink tunnel which may use IR, hot air, or steam, or a combination of these, to provide the different levels of warmth needed to create required shrinkage levels. The inks used have to be able to withstand these additional processes, while shrinking with the film to deliver the prescribed 'morphed' image.

In tests on the application of sleeves we conducted with leading application equipment suppliers, at speeds of up to 22,500 bottles per hour, UV flexo inks performed well. On-pack performance of the inks in terms of dimensional stability was stable, even over the long term.

All in all, shrinkable UV flexo inks are proving to be a reliable partner on narrow web presses for printing shrink sleeve laminate. As today's label printers look to make better use of the exceptional capacity of their advanced, high speed UV flexo presses, shrink sleeves represent a growing opportunity for increased productivity – and the ability to promote narrow web print's high quality capabilities in a new and profitable arena. n

Niklas Olsson is the global brand manager for Flint Group Narrow Web


Niklas Olsson, global brand manager, Flint Group Narrow Web Niklas Olsson Ink for shrink

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