By Rob Bosman, Senior Sales Manager at Contiweb
Whether your customers are big brands or increasingly discerning consumers, you’ll know there are no excuses for a drop in quality because a product is produced digitally – be it a book a direct mailer or a personalised catalogue. Customers are no longer willing to compromise because a product is produced on-demand, personalised, and certainly not for an exclusive limited run.
If the quality of your digital press line is such that you’re able to switch jobs over from an offset press line then there are substantial benefits to be exploited; maximising your share of the market and meeting tight turnarounds whilst utilising your digital press line to its fullest and getting a quicker ROI. Boosting quality can also lead to new business opportunities, broadening product lines or entering entirely new market segments.
As digital print strives to match offset quality for all the reasons above, there are various factors to consider in order to achieve that end. One such issue affecting quality is the loss of moisture.
With the use of water-based inks in digital inkjet printing, printed matter must be dried before being finished inline or rolled for efficient storage before the next stage of production. Using drum dryers, infrared, hot air – or a combination of these techniques – air temperatures rise considerably. Whilst printed areas with good ink coverage protect the paper underneath from rising temperatures, unprinted parts are not protected and can reach temperatures as high as 120 ˚C (248°F), at which point a lot of moisture is lost – and result in large differences in moisture content across the paper between the printed and unprinted areas. With some digital printers, ink setting requires even higher temperatures, up to 150 ° C, which dries out the entire product, printed or unprinted.
Consequently, with loss of moisture, the quality of printed matter suffers in various ways.
When paper is dehydrated it becomes brittle making it much more likely to break when folded and causing staples to break out more easily. Split paper and missing staples before a product has even shipped don’t make a premium consumer experience.
When paper dries out, over time it will naturally re-absorb moisture from the environment around it to come to an equilibrium with the atmosphere. This is because the fibres in it are hygroscopic – they will release moisture into a dry environment or absorb moisture from a damp environment. When this occurs in an uncontrolled manner, it can affect the quality of a product in a variety of ways.
For instance, we’ve all seen books and booklets where the covers don’t fit properly. The signatures are often dried after printing, but the covers are not because they are printed on sheetfed presses without dryers, so the signatures reabsorb moisture at a later stage – after they’ve been cut and assembled. The signatures expand as they take on moisture, more in the width direction than the length (because paper fibres thicken more than they lengthen), whilst the covers don’t. Ultimately the cover no longer fits neatly on the product.
The flatness of signatures is also affected in a similar way. As the edges of the paper are exposed to the atmosphere, they reabsorb moisture and thicken – distorting the edges and causing a wavy effect and making it difficult for signatures to lie flat.
Curling control is another issue in digital inkjet printing. In this case the edges of the printed paper curl up or down – in general, they curl towards the side where the most water is applied. This occurs because the paper is printed first on one side and dried, and then printed on the other side and dried. Curling can be avoided by applying moisture on the top and bottom sides of the paper independently according to their differing needs.
Remoistening paper on the press in a controlled, precise manner – particularly with the ability to add moisture to each side of the paper independently – eradicates these issues, maintaining the premium quality of products. Contiweb’s Digital Fluid Applicator (DFA) – based on the popular Contiweb Fluid Applicator for heatset web offset applications – is an innovative and user-friendly system for remoistening. Using tap water and a surfactant to intelligently and very precisely remoisten the paper on digital web presses, it can also use silicone as an added option, decidedly advantageous if inline finishing is being employed. The DFA is compatible with digital web presses from all well-known manufacturers, such as HP, Kodak, Canon, Océ, Screen and Ricoh.