Corona treatment is a well-proven and simple way to achieve good adhesion of inks, additives and lacquers on a variety of substrates. But while most converters know of its importance, many see it only as an auxiliary product – a box that is mounted near the unwinder on the press, and fail to appreciate what it can do to improve productivity.
Learning to understand how it works, and more importantly how to get it to perform to optimum effect will have a major impact on the quality of the final printed product.
The key factor is achieving the correct level of treatment. Often when operators encounter an adhesion problem their normal reaction is to turn up the power on the Corona treater, but this is not always the right thing to do. In fact some materials are sensitive and can become over-treated or damaged with too much power. To get the right level of treatment, converters need to consider the substrate and the dyne level that is required for the ink, glue, lacquer, or coating that is being used.
So how does a converter know what dyne level is required? Once again this can vary, based on the application, but all good ink and adhesive suppliers will be able to recommend the correct levels for their products.
The effect of Corona will also vary according to the type of substrate. Some materials are very receptive to Corona treatment, whereas others, such as PP, BOPP, and OPP are more difficult to treat.
The amount of additive will also change the effectiveness of the treatment. Additives migrating to the surface will require an increased watt density. In simple terms, the higher the slip additives, the higher the power the material will require.
Taking all these points into consideration, Vetaphone recommends converters check the surface energy of each substrate before processing. This is done using a dyne pen after the material has been treated. If the level is insufficient, the Corona treater should be adjusted to a higher setting, and the dyne test repeated until the required adhesion is achieved. Other factors to consider are the web width, speed, and the number of sides to be treated. If any of these are changed, then the setting of the Corona treater must be adjusted accordingly.
On some substrates it can be impossible to reach the dyne level required with regular Corona. If this is the case, Plasma treatment may be the solution, and Vetaphone will be pleased to advise and supply the technology required.