By Clemens de Vroome, Senior Development Engineer at Contiweb
Curling is a common phenomenon in inkjet printing, in which the edges of the paper curl. It is an annoying problem, because although inkjet is considerably faster and more economical for short runs, customers expect print quality that is on a par with offset. In this blog Clemens de Vroome, Senior Development Engineer at Contiweb, zooms in on the curling problem. He discusses how the problem arises and also how you can solve it.
Today's print buyers order on demand, want fast delivery times and at the same time expect high quality print. With high-end inkjet presses, you can produce printed matter that is almost of the same quality as offset, even though the composition of the inks used is completely different. Offset inks consist for the most part of oil and resin. Inkjet inks consist mainly of water. In relation to the phenomenon of curling, this makes a big difference.
Water in inkjet inks can cause curling
The application of water to paper (e.g. by printing with inkjet inks) sometimes leads to the problem we call curling. This is where much more ink is applied to one side of the paper than to the other. The fibres, on the surface of the side where there is water, will expand, while the fibres on the other side of the paper do not do so at first. As a result, the paper curls. Initially it curls away from the water. After a while, the water flows through the paper, to the other side. The paper then becomes flatter. But, as the water spreads further, it curls towards the other side. Drying has no positive effect on this process. In fact, the high temperatures can cause other damage. We wrote about this in an earlier blog. Curling can only be "repaired" by applying a very precise amount of moisture on both sides. This can be done by using the Digital Fluid Applicator (DFA) developed by Contiweb.
Applying moisture to the paper
Decisive for the "repair" of the paper is the amount of moisture applied. We use as a starting point 2% of the paper grammage, equally divided over both sides. For example, if a type of paper weighs 80 g/m², you would add 0.8 g/m² of moisture on both sides. However, paper types react differently and the operator must judge how the recommended amount will affect the quality of the product. Using the display on the Contiweb DFA, you can easily and very precisely adjust the amount until the best result is achieved.
If there is too little moisture, the curling problem will remain. If there is too much moisture, wrinkles (cockling) will appear on the paper. These wrinkles usually have a diameter of 0.5 - 1 cm and appear in random places on the paper, causing the stack of paper to no longer lie neatly together and also to become more bulky. Of course, you don't want that and you have to reduce the moisture yield to remove this problem.
Curling occurs at every grammage. The higher the grammage, the more water you have to add to remove the curl. However, there is a limit to this. For very heavy paper, for example 250 g/m², according to our rule you should add 2% water (i.e. 2.5 g/m² per side) to the paper. That is a lot for paper and the paper is likely to cockle. So there are limits to the application of moisture.
The Contiweb DFA in practice
In practice, many printers use a limited number of paper types. Usually 4 or 5 types. When installing the Contiweb DFA, it is a matter of testing these types. Our 2% rule is taken as a starting point to then arrive at the optimal setting per paper type. The ink coverage, which of course fluctuates greatly with digital printing, hardly plays a role in this. As a result, one setting per paper type can be used in almost all cases.
The Contiweb DFA is an innovative and user-friendly system that uses tap water and a surfactant to intelligently remoisten the paper. Installation of the machine takes only 2 to 3 days. The Contiweb DFA takes up limited space and is compatible with web inkjet printing presses of all brands.
Last year Vistaprint installed a new web inkjet pressline dedicated to printing flyers. Like the rest of the business, the jobs on the pressline were extremely varied. It soon became apparent that there was negative curling of some flyers, causing jams later on in the finishing equipment.
In this case study you will read how Contiweb stabilised the negative curl that Vistaprint were getting as a results of the wide variation of jobs.
The Contiweb Digital Fluid Applicator (DFA) is a standalone remoistening applicator for digital web presses, developed to significantly improve the quality of digitally printed products. Remoistening overcomes common quality issues after drying, such as waviness and curling.
Silicone application can also be added as an option, decidedly advantageous if inline finishing is being employed.