Corona treatment - the preparation of PE and PP materials for printing
Many printed materials, such as for example PE and PP, has a chemically inert and non-porous surface with low surface tension.
Due to this, no bonds are created here with colours, protective coats or adhesives. In order for the adhesive or the colour to adhere to the foil, it is necessary to pre-treat the foil, for example with corona treatment. Materials such as polyethylene, polypropylene, nylon, vinyl, PVC, PET, metal-coated surfaces, foil, but also paper and cardboard are usually treated using this method.
Corona charging needs to be set up according to the thickness of the foil, and some foils a charged several times. In flexo printing, a material is used with the adhesive already applied, which also only adheres to the foil because it was previously corona charged. Before printing, the tape is charged again from the other side, where the adhesive was not applied, and it is printed.
In case of any printing, here, too, problems may occur, when one charging, for example, overrides another and either the adhesive does not adhere to the take, or the second charging before printing is too weak and the adhesive pulls down the entire print.
In case of sandwich printing, a similar problem may occur, when the voltage disappears from the foil between the printing and the subsequent application of adhesive and the adhesive does not adhere well.
The strength of the charge of course depends on the thickness of the foil, and hard though we may try, on an area of several thousand square metres, minimal variations in thickness may appear during production. These variations cannot be measured by a mere eye, but a tiny variation always appears during printing.