Surface quality is essential
If there is one key variable that is often overlooked in the printing process, it’s the quality of the substrate, or to be more precise, the quality of the substrate surface. While important across all types of substrates, paper and board are easier to manage – it’s the films and foils, where there is no absorption, that can prove tricky.
Corona for good adhesion
Corona treatment is the process that modifies the surface tension of the top layer of substrate to allow ink or lacquer to adhere to it, and it is vital that the substrate web is as clean as possible with all contamination removed. Any particles left on the surface will result in imperfections in the final print quality. With time costing money and today’s complex and expensive substrates requiring careful handling, no flexo house wants to engage in reprints to keep its customers happy.
Clean before corona treating
Critical to the process of preparing the web is to clean it before corona treatment and this is always the recommended sequence of events. Cleaning after corona treating can significantly reduce the dyne level – the capability of the substrate to afford secure ink adherence without the risk of smudging.
Identify the problem
Converters are repeatedly reporting issues with different substrates that have been contact cleaned. While the standard industry practice is effective at removing particles down to 1-micron in size, it can also lower the surface energy (dyne level) and affect the final print quality.
On closer inspection, it was found the issue related to two features: first, that in many cases the web was being cleaned after surface treatment; and second, the poor quality of the particle transfer roller was depositing a high level of silicone on the web. The result was that either a more powerful and costly corona treater was required to combat the silicone, or that the production speed had to be reduced to obtain the correct corona dosage needed to ensure the quality of print required – either way there were serious cost implications.
Coordinated technology
What’s required is a web cleaning system that works in tandem with the surface treater. The web cleaner can be single- or double-sided and needs to be capable of being mounted in various locations to suit any press configuration and web orientation. An active anti-static system is needed to prevent any possibility of re-contamination after the cleaning process has finished.
Ideally, the web cleaner should be linked to the corona generator to eliminate the need for any additional operating system. This improves production efficiency and reduces operator stress because the web cleaner control and maintenance is fully integrated with the corona system via the same module. This means that one screen provides all the information required for both web cleaning and surface treatment.
In summary
The quality of the substrate surface is often overlooked until it becomes a major issue in the print process, by which time it has become an expensive oversight. By careful planning and utilising coordinated technology, converters are assured that whatever other issue they may have, a dirty web is not one!
Keen to know more? Read this article by our CSO Kevin McKell!
