A collection of practical questions and answers from flexible packaging production, covering solvent selection, ink testing, corona treatment levels, surface vs. reverse printing, composite structures, and troubleshooting.
Ink Solvent Selection: 10 Criteria
- Match the press speed and dryer capacity.
- Compatible with the substrate — no corrosion of the plate material.
- Good solvency, free of mechanical impurities, free acids/alkalis, and water.
- Appropriate evaporation rate for the process.
- Chemically inert toward ink components.
- Low odor, low toxicity — food-grade for packaging; no residual solvent odor on the finished product.
- Storage-stable — no decomposition or moisture absorption.
- Cost-effective with reliable supply.
Solvent evaporation is influenced by blend ratio (mixed solvents don’t evaporate at the arithmetic average of their components — molecules interact), resin content (dissolved resin slows evaporation by binding solvent molecules), pigment particle size (finer particles create larger surface area and slow evaporation), and ambient temperature and airflow (higher temperature and better ventilation accelerate drying).
Ink Testing: 9 Parameters
Standard ink test protocol: color, viscosity (flow cup), absolute viscosity (poise/centipoise by falling-ball or capillary viscometer), tack, drying speed, color strength, grind fineness, flow, and chemical resistance (acid, alkali, ethanol).
Common Solvents by Chemistry
- Hydrocarbons: Toluene
- Esters: Methyl acetate, ethyl acetate, n-butyl acetate
- Ketones: Acetone, MEK (methyl ethyl ketone)
- Alcohols: Isopropanol, isobutanol, n-butanol, ethanol
Corona Treatment Levels by Film
| Film | Untreated (dyne/cm) | Treated (dyne/cm) | Minimum for Printing |
|---|---|---|---|
| OPP | 30 | 38–42 | 36 |
| CPP | 30 | 38–40 | 36 |
| PET | 38–40 | 48 (set point) | — |
| ONY (BOPA) | 38–40 | 40 (set point) | — |
Corona treatment decays over time — expect a 2–3 dyne/cm drop after 6 months of warehouse storage, faster in hot, humid conditions. Treated film stored beyond 6 months should be re-tested before use. For PET and ONY, corona treatment is now standard practice for packaging grades — even though PET’s untreated surface energy is nominally adequate, treated film provides more consistent ink adhesion.
Surface Printing vs. Reverse Printing
Surface printing places the ink on the outside of the package — exposed to abrasion, scratching, and heat. Surface inks must be formulated for wear resistance and thermal durability. Applications include: non-transparent substrates (aluminum foil, paper), single-layer structures (where reverse printing would put ink in food contact), matte-finish requirements, difficult-to-print substrates (ONY, PE), and in-mold labeling.
Reverse printing sandwiches the ink between the outer film and the heat-seal layer, where it’s protected and viewed through the film — producing high gloss and clean appearance. This is the dominant method in flexible packaging. Reverse inks must bond strongly to both the substrate and the laminating adhesive with good internal cohesion.
Composite Structures: Quick Reference
- Retort pouch (4-layer): PET 12μm / PA6 12μm / aluminum foil 9μm / CPP 70μm, or PET 12μm / Ny 12μm / PVDC or EVAL 10–12μm / CPP 70μm
- Instant noodle oil/sauce pack: PET 12μm / PA6 12μm / CPP 70μm — dry lamination, vacuum-fill, retort sterilization
- Noodle lid stock: PET / reverse print / PU adhesive / paper / PU adhesive / aluminum foil / hot-melt EVA. Paper porosity demands higher adhesive coat weight — use reverse-roll coating rather than gravure cylinder application. Flame-treat the aluminum foil to improve bond strength.
Troubleshooting Notes
Color fading during the run: Check for water ingress, dust contamination, or incompatible diluent in the ink. These cause gelation — the gelled ink traps pigment in sediment, reducing the pigment concentration in the circulating ink and producing progressively weaker color.
Powder adhesion inside metallized pouches: Either static (add humidity, steam, static eliminators, conductive lubricant on all rollers) or hygroscopic product (keep air dry).
Jagged tear notches on pouch packs: Dull knife blade or incorrect clearance/angle between fixed and moving blades.
References
- Wikipedia: Solvent: Solvent chemistry, evaporation rates, and solvent selection criteria for printing ink formulations.
- Wikipedia: Corona Treatment: Surface energy modification of polymer films including dyne level measurement and treatment decay over time.
- Wikipedia: Flexible Packaging: Overview of flexible packaging structures, printing methods, and lamination technologies.
- Wikipedia: Retort Pouch: Multi-layer retort pouch construction, material requirements, and sterilization conditions.
- Flexible Packaging Association (FPA): Industry resource covering flexible packaging materials, processes, and quality standards.