Appvion recognizes that flexography is the most widely used print process for pre-printing direct thermal paper and film substrates. Our direct thermal products are designed to facilitate good performance in pre-printed flexo applications from the time of printing to imaging.
Direct thermal products have specialty coatings applied to the substrate surface that enable variable image function in the presence of heat. The heat applied by the thermal printer during imaging often exceeds 300⁰F while in the presence of nip pressure and shear from motion across the static print head. These dynamics present challenges to the printed substrate that printers/converters need to be aware of.
Across most basis weights, Appvion offers two main coating design platforms:
- Résiste® topcoated grades – A non-absorptive surface that requires high pigment and good wetting ink for dark colors.
- Alpha® non-topcoated grades – A very absorptive surface; watch out for solvent interaction with certain inks. They aren’t compatible with most UV overprint varnishes.
Most applications use water-based and UV flexographic inks. The amount of solvent present in water-based inks should be kept at a minimum. UV inks tend to create high drag under the print head, especially during imaging. Such drag can prove problematic, resulting in poor label tracking through the printer. To counter this, choose ink with good slip properties (like a gloss-coat overprint varnish) or use a UV-curable overprint varnish.
UV-cured inks tend to show lesser adhesion than water-based inks. The use of a water-based primer can substantially improve UV ink adhesion. Water-based inks also tend to create excess drag under the printhead during imaging. Therefore, water inks should have good heat-resistant properties and good slip properties when exposed to elevated temperatures.
Most major ink suppliers have an existing ink series suited for direct thermal applications. Start with your current ink supplier, always trial the ink and substrate in the end-user printer before launch, and modify if quality issues are encountered.