Compressible substructures have an increasing role to play in allowing flexo to meet the demands for high quality print in packaging, write Dr Roland Heinz and Dr Klaus Wachtmann, of tesa
Packaging demand rises constantly. Apart from its most important roles of protecting its contents and providing for ease of handling, it also increasingly has to communicate to the consumer. In addition, pack economics play an important part in design, as marketing experts from both brand owners and retailers set ever higher standards of printed packaging for the same or even a lower price.
Reasons for the rising significance of packaging’s communicative potential are obvious – more and more products are sold directly from the shelf to the self-service customer in retailers and DIY stores. An average supermarket has a range of about 15,000 products. Studies have shown that the average supermarket shopper passes around 300 articles per minute. Somewhere approaching half of their purchases are impulse purchases. There is no doubt that packaging design can be seen as a ‘mini-commercial’, right at the point of sale.
This means that printing quality is gaining in importance in the overall packaging design and content of the message carried. And it is the ever advancing technology of flexo print and the various components that come together in the process that defines the quality necessary to meet the ever rising requirements of the packaging market.
Key terms here are gearless printing machines, improved anilox rollers and new colour systems. Compressible substructures, too, are gaining in significance.
Printing quality is dictated to a large extent by the kind of compressible substructure used. This substructure can be realised by using either compressible adhesive tapes or compressible foam sleeves. The structure of a soft, foam coated sleeve provides the compressible substructure desired. Such sleeves are mostly equipped with an upper layer made of compressible material.
The sleeve’s compressibility must match that of the imprint. In this system, the printing plate is fixed to the sleeve using a non compressible film tape (see figure 1). Here, the tape’s adhesive properties should be matched to the surface of the sleeve and the back of the photopolymer plate. Tapes most suitable for this application usually have sides with widely varying adhesive powers.
In addition to compressible sleeves, foam adhesive tapes are often used. The latter define printing quality mainly by the type and hardness of the foam used; in particular the cell structure of the foam used becomes vitally important (see figure 2). For pure screen printing, a softer compressible undercut is recommended to reduce dot gain. With combination printing – screen and solid – a harder compressible undercut is recommended to attain the necessary area coverage.
In addition to its hardness, the compressible substructure’s resilience is important, especially when long runs are involved. The foam structure should still possess its full resilience after some hundreds of thousands of imprints, in order to guarantee consistent quality for the whole print run.
For some time now, economic aspects have been gaining in importance when defining the individual components of flexo printing. Here, too, compressible substructures can play an important role. Easier handling can make established working processes more efficient.
Here, the whole process of mounting, printing and demounting should be considered from the point of view of the sleeve user. The first step in the process foresees a simple repositioning of the plate and airtight fixing. The liner of the tape is especially important here, as its structure allows initial air bubbles to be smoothed out. The printing plates should also be mounted in such a way that edge lifting during printing is impossible, as this immediately means loss of quality. Simple, damage-free demounting of adhesive tape and sleeve are thus very important, as in practice ever smaller job sizes means the increasing re-use of sleeves.
For this reason, compressible sub- structures (sleeve or cylinder and tape) should be seen as an integral part of the components, printing cylinder, plate and material to be imprinted. This is why there are foam tapes on the market that are designed to be suitable for sleeves or steel cylinders. Due to their adhesive systems, they are specially configured for sticking photopolymer plates onto either on hard polyurethane sleeves or steel cylinders (figure 3).