Blocking — ink transferring from the printed surface to the back of the next layer after winding or stacking — is one of those defects that passes inline inspection and fails later, at lamination or pouch-making. By the time it’s discovered, the entire roll may be compromised. Two scenarios dominate: blocking under heat and pressure in storage (especially during humid seasons), and blocking discovered at the next process step — composite lamination, slitting, or bag-making.
Seven Root Causes
- Excessive rewind tension. The tighter the roll, the higher the interlayer pressure — and the more ink-to-back contact force.
- Incomplete ink drying. Solvent retained in the ink film continues to evaporate after winding, softening both surfaces.
- Impure solvents. Contaminated ethyl acetate, ethanol, or other solvents introduce slow-evaporating fractions that extend drying time and increase residual solvent.
- Solvent blend mismatch to press conditions. The evaporation profile doesn’t match the press speed and dryer temperature — the ink leaves the last dryer wetter than it should.
- Unstable dryer performance. Temperature fluctuations or mismatched supply/exhaust airflow leave the ink film under-dried.
- Heavy ink coverage in the plate design. Excessive overprinting of multiple colors, or heavy metallic ink areas, put more wet ink mass onto the substrate than the drying system can handle.
- Post-print storage. Rolls stored horizontally under weight, or in high-temperature warehouses, apply sustained pressure and heat to the ink film.
Countermeasures
- Rewind tension: Set by substrate and ink type. The hand-press test: the finished roll should feel firm but not hard when pressed with the thumb. For transparent inks, do not use the rewind pressure roller — let the web wind under its own tension only.
- Ink drying: Specify fast-drying ink formulations for your product structure. If spare print stations are available downstream, run the web through them with cold air — no ink, just cooling airflow — to accelerate solvent release before winding.
- Solvent quality: Test every incoming solvent batch for purity. Store solvents in cool, ventilated areas — not in humid zones. Don’t draw solvent in areas where moisture condensation is possible.
- Solvent formulation: Match the true-solvent/diluent ratio and evaporation rate to the ink resin type and the press speed. Viscosity and ink film thickness also affect blocking — thicker films retain more solvent.
- Dryer maintenance: Verify temperature stability and ensure the exhaust airflow rate matches or exceeds the supply rate — positive pressure in the dryer traps solvent in the film.
- Storage: Never stack rolls horizontally. Store vertically or hang them. Maintain warehouse ventilation and avoid high-temperature storage areas.
Special Case: Color Bleed Transfer
Certain pigments — particularly violet and a small number of other colors — can transfer from the printed surface to the back of an adjacent bag even without heat or pressure. This isn’t blocking; it’s pigment migration through the ink film and into the contacting surface. It occurs most commonly with surface-print inks used on the reverse side of PE garment bags. The solution is application-specific: understand the final packaging format and storage conditions, and qualify the ink-pigment-substrate combination through testing before production release.
References
- Wikipedia: Solvent: Solvent chemistry, evaporation rates, and purity specifications relevant to ink drying and residual solvent in flexible packaging.
- Wikipedia: Flexible Packaging: Overview of flexible packaging printing and converting including post-print processing and storage considerations.
- Wikipedia: Rotogravure: Gravure printing technology including drying systems, ink delivery, and rewind tension control.
- Wikipedia: Pigment: Pigment chemistry and migration mechanisms relevant to color transfer and bleed in printed packaging films.
- Flexible Packaging Association (FPA): Industry resource covering printing defects, quality troubleshooting, and storage best practices.