Caps on plastic bottles must be absolutely airtight but they should also be easy to open. To ensure these qualities, silicone-modified compounds are used for these caps.
These materials have low surface tension, however, so printing on them (especially with solvent-free food-safe inks) can only be accomplished after plasma treatment. For that reason, Openair® Plasma has been used by leading manufacturers for years to increase surface tension on bottle caps.
The plasma jets are integrated directly into modern pad printing machines that operate at high speeds.