Gravure blocking — the print surface feels dry to the touch, but after winding or stacking, the ink transfers to the back of the adjacent layer — is one of those defects that passes inline inspection and only reveals itself days later, when the finished roll or pouches are pulled from storage. Peak season for blocking is summer, but heat alone doesn’t cause it. Four factors interact: residual solvent, binder chemistry, film quality, and static.
1. Residual Solvent: The Primary Cause
If the solvent isn’t fully evaporated before winding, the residual heat in the roll continues to drive solvent out of the ink film — but now the film layers are pressed together. The escaping solvent softens the ink on both surfaces and creates the adhesion. The solution isn’t just more heat; it’s solvent selection matched to press conditions.
Evaporation rates at 30°C (lower seconds = faster evaporation):
| Solvent | Evaporation Time (seconds at 30°C) |
|---|---|
| Ethyl acetate | 18 |
| Toluene | 31 |
| Ethanol | 47 |
| Isopropanol | 55 |
| Xylene | 70 |
| Butyl acetate | 70 |
| Butanol | 147 |
A mixed solvent system outperforms any single solvent for gravure. A standard three-part blend — xylene, ethanol, and isopropanol at equal 1:1:1 ratios — provides balanced evaporation across the full drying curve. If drying is too fast and the ink skins over in the cells, add a small amount of butanol to slow it down. Butanol also improves gloss; the tradeoff is that excess butanol will cause blocking, so keep the addition minimal. If drying is too slow, increase the fast-solvent fraction.
After printing, wind the roll loosely or rack the sheets in wire baskets for continued air circulation before slitting or pouch-making. After pouch conversion, pack the finished product lightly and stand it vertically in cartons to minimize inter-layer pressure.
2. Ink Binder Melting Point
Some gravure ink binders have low softening points. When shop temperature and humidity spike — typical summer conditions — the binder softens enough to tack to the adjacent film surface under winding tension. The fix: if possible, air-condition the pressroom to 18–21°C with relative humidity below 65%. If climate control isn’t available, switch to an ink formulated with a higher-softening-point binder for summer production.
2. Film Grade
Film produced from non-packaging-grade resin, or packaging film with insufficient antiblock (opening agent) in the formulation, promotes blocking independent of the ink. The film itself is sticky. The only fix is to change the film — there’s no press-side adjustment that compensates for missing antiblock.
3. Static Electricity
Static charge makes film layers cling to each other mechanically, independent of any chemical adhesion. The cling compresses the ink surface and amplifies any tendency toward blocking. The countermeasure: incorporate antistatic agent into the film formulation, and ensure static elimination equipment on the press is functional.
References
- Wikipedia: Rotogravure: Comprehensive overview of gravure printing including ink systems, solvent evaporation, and drying mechanics relevant to residual-solvent blocking.
- Wikipedia: Solvent: Fundamentals of solvent chemistry including evaporation rate, vapor pressure, and the role of mixed-solvent systems in printing ink drying behavior.
- Wikipedia: Static Electricity: Mechanisms of static charge generation on polymer webs and the relationship between static cling and blocking in roll-wound flexible packaging.
- ISO 12647-4:2014 — Gravure Process Control: International standard for print process control including drying parameters and ink film properties relevant to blocking prevention.
- Flexible Packaging Association (FPA): Industry resource covering gravure printing troubleshooting, ink management, and quality best practices for flexible packaging converters.