Gravure printing is the workhorse of flexible plastic packaging 鈥?and picking the right ink makes the difference between a print job that runs smoothly and one that gums up the press. Here’s a practical breakdown of the four ink categories used on plastic film.
1. Gravure Surface (Table-Printing) Ink
Gravure surface ink 鈥?also called table-printing ink 鈥?is formulated primarily for PE, PP, OPP, and NY (nylon) films. It can be used for laminated flexible packaging too, as long as the print layout doesn’t have large solid color blocks. After lamination, the composite structure passes cold-wind rewinding tests with acceptable bond strength and residual odor levels.
The ink consists of polyamide resin binder, pigments, and additives, wet-ground and filtered. The polyamide resin softening point is critical 鈥?it should land around 121掳C. Too low and the printed film blocks (sticks together) during the rainy season. Too high and the ink freezes in winter, requiring heated mixing to re-liquefy.
Dilution solvents: toluene, isopropanol (fast-drying); xylene, butanol (slow-drying). A small addition of ester solvents can adjust viscosity. When ink dries too fast 鈥?causing dot loss in fine text and halftones 鈥?adding butanol improves reproduction clarity. But overdo it and the ink won’t dry at all, causing blocking especially in humid weather.
2. Gravure Lamination (Reverse-Printing) Ink
Lamination ink 鈥?also called reverse-printing or composite ink 鈥?is printed on the inside surface of the substrate. The binder is chlorinated polypropylene resin, which delivers strong adhesion to OPP, PET, and NY films. After lamination, the ink layer is sandwiched inside the composite structure, making this the ink of choice for premium flexible packaging.
Critical rule: never add alcohol solvents to lamination ink 鈥?the ink will degrade. Ink manufacturers formulate two variants: fast-drying for high-speed presses and slower-drying for low-speed machines. On a slow press, fast-drying ink gums up the gravure cells. On a fast press, slow-drying ink leaves the web wet at the rewind. Match the ink to the press speed.
Dilution solvents: MEK (methyl ethyl ketone), toluene, ethyl acetate (fast-drying); butanone (slow-drying).
3. Retort-Resistant Ink
For retort-sterilized flexible packaging or printed aluminum foil, standard inks won’t survive the process. Use two-component retort-grade gravure ink or aluminum foil-specific ink. Without the right ink, the printed layer thins, discolors, and loses definition after steam sterilization. The packaging structure also needs high-temperature-resistant materials and two-component retort-grade lamination adhesive to form a complete system.
4. Surface Tension and Corona Treatment
Gravure ink has a surface tension around 36 dynes/cm. PE and OPP films naturally sit at about 32 dynes/cm 鈥?too low for ink adhesion. The film must undergo corona discharge treatment before printing to disrupt the polymer surface and raise surface tension to 38 dynes/cm or above. At that level, ink bonds to the film strongly enough that rubbing and tape-peel tests don’t lift the print.
5. Environmental Trends: Low-Toxicity and Water-Based Inks
Conventional gravure inks rely on chlorinated polymers (with ozone depletion concerns) and large volumes of toluene solvent 鈥?both serious worker health and safety hazards. The industry direction is clear: manufacturers need to accelerate development of low-toxicity, non-toxic alcohol-soluble and water-soluble gravure inks for plastic films.
In paper-plastic composite rolls (used for pharmaceutical self-packaging machines and tea bag packaging) and liquid packaging film, flexographic alcohol-soluble inks are already in use 鈥?the solvent system is alcohols and esters with low toxicity, achieving odor-free and benzene-free compliance.
6. Ink Selection Summary
Choosing gravure ink for plastic packaging means weighing all these factors:
- Press type and performance characteristics
- Operator skill level
- Substrate material 鈥?PE, PP, OPP, PET, NY, or aluminum foil
- Press running speed
- Drying system capability
- Color match to the original proof
- Post-print processing requirements
- End-use application of the finished package
Different packaging structures and specifications demand different inks. The right ink on the right substrate, printed with care 鈥?that’s how you produce flexible packaging that looks sharp, performs reliably, and keeps the customer coming back.
References
- Wikipedia: Rotogravure 鈥?Gravure printing process fundamentals including ink transfer and cylinder engraving
- Wikipedia: Polyamide 鈥?Properties of polyamide resins used as binders in surface gravure inks
- Wikipedia: Chlorinated Polypropylene 鈥?Chlorinated PP resin chemistry and its role in lamination ink adhesion
- Wikipedia: Corona Treatment 鈥?Surface activation technology for improving ink adhesion on plastic films
- ISO 12647-4 鈥?Gravure Printing Process Control: International standard for process control in publication and packaging gravure