Digital spot varnish for print sheet finishing

In conjunction with Steinemann Technology AG and Schmid Rhyner AG, the Institute for Print Technology developed an innovative varnishing system for finishing print sheets for packaging. The project resulted in the launch of a rapid digital UV spot varnish process for precise partial coating and effect generation for the graphic arts industry. The process allows for the use of different substrates such as paper, cardboard and plastics. The project was co-financed by the CTl. Up to now, conventional varnishing has been done mainly using flexo, gravure and screen printing. These technologies all require a printing plate that has to be produced in a separate time-consuming, cost-intensive process before the print job can be carried out. Another drawback is the production downtime needed to install and align the printing plate in the coating machine.

The new digital spot varnish process is based on inkjet printing technology and does not require the creation of a printing plate, achieving significant cost advantages by eliminating the associated process steps. Instead, the artwork to be coated is transferred electronically as an image file directly to the coating unit and is immediately ready for use. As soon as the print job is running, the print sheets are loaded from the feeder into the press and fed to the sheet aligner, where sensors measure sheet alignment and position. To achieve a high degree of registration accuracy, the print image is corrected and aligned to the measured sheet position using rapid image processing. The sheets pass at high speed under the print bar on which the inkjet printheads are precisely mounted. The aligned print image is printed in a single pass using the single-pass process, whereby the coating is jetted onto the surface from thousands of printing jets.

The transparent varnish specially developed by Schmid Rhyner AG is cured with UV light and can be printed without any pretreatment of the print sheets. A post-treatment developed and patented in the project removes ‘pinhole’ or imperfections in the surface of the varnish layer. Finally, it is fixed in the UV curing unit and the finished sheet is ejected.

In order to be able to research the new printing process from scratch, the institute developed a lab test printer consisting of an inkjet module, ink supply, ‘pinhole’-killer, UV curing and sheet transport. In addition, a DropWatcher was set up to optimise the formation of droplets at the inkjet nozzles and to evaluate suitable inkjet printheads. During process development, Schmid Rhyner AG also made further improvements to the new UV spot varnish and launched a new product range.

Result

Steinemann Technology AG scaled the process to a sheet format of 108 x 78 cm and produced prototypes. The pilot series was successfully put into operation at customers’ premises.