Water-Based Flexo Inks for Corrugated Post-Print

Executive summary

Corrugated post-print is the largest single application of water-based flexo ink, and one of the hardest to run well: a rough, absorbent, variable substrate, wide webs, high speeds, large solids next to fine bar codes, and a pressroom that is rarely air-conditioned. This paper sets out the science behind the UniCraft range — pigment strength, binder balance, pH and viscosity control, drying and rub resistance — and the process discipline that turns a good ink into consistent boxes.

1. The substrate sets the rules

Kraft and test liners absorb ink at rates that vary with basis weight, sizing and moisture, and their brown colour subtracts from every shade printed on them. An ink for corrugated must therefore hold out on the surface long enough to form a colour layer, dry by absorption and evaporation within a metre of the print station, and cover the liner colour with the minimum film — because the film weight a corrugator can carry is limited by the board’s tendency to wash-board and by the drying capacity of a press that often has no dryer at all.

2. Colour strength and hold-out

UniCraft inks are built on high-strength pigment dispersions milled below 0.5 µm and carried in a resin-bearing vehicle that holds the pigment at the surface of the liner. Strength lets the printer run the shallow, fine aniloxes (3 – 6 BCM) that give sharp bar codes and clean type, while still reaching density on solids; hold-out keeps the colour on top of the fibre. White-top liners and coated liners are printed with the same inks at reduced strength through extender addition.

3. Binder balance

The binder is a blend of alkali-soluble acrylic resin and acrylic emulsion polymer. The resin gives resolubility on the anilox and plate, gloss and pigment wetting; the emulsion gives rub resistance and water resistance once dry. UniCraft grades shift the ratio: high-resolubility grades for long runs and stop-start presses; high-rub grades, with additional wax dispersion, for shipping cases that are palletised and strapped; and water-resistant grades for produce, seafood and frozen-food boxes that meet condensation and ice.

4. pH and viscosity: the two dials

Water-based flexo inks are stable only in a narrow window. The amine that keeps the resin in solution evaporates during the run, so pH drifts down, viscosity rises, resolubility falls and the ink begins to dry in the anilox cells. The remedies are to measure pH every hour, to add the pH stabiliser (a slow-evaporating amine) rather than ammonia, which leaves as fast as it is added, and to control viscosity with water in small additions with re-measurement. Target pH 8.5 – 9.2; viscosity 18 – 25 s (Zahn 2 / Ford 4) depending on the press and anilox. Temperature changes both: an ink that is on specification at 25 °C is thinner and prints weaker at 35 °C.

Figure 1 – pH drift on press: stabiliser versus ammonia versus no correction (illustrative).

5. Drying and rub

Drying on corrugated is mostly absorption, which is why print quality falls on hard-sized or coated liners that absorb slowly. Where the press has between-colour dryers, airflow matters more than temperature. Rub resistance develops as the emulsion polymer coalesces and the wax migrates to the surface; it improves over the first hours after printing and should be tested at 24 hours to the customer’s standard (typically 50 – 100 strokes, dry, no visible transfer). For wet rub — produce boxes hydro-cooled or iced — water-resistant grades with cross-linking binders are specified and verified by wet-rub test after immersion.

Figure 2 – Rub resistance develops over 24 hours as the emulsion coalesces and wax migrates (illustrative).

6. Foam, plates and cleanliness

Foaming is the recurring nuisance of water-based flexo: entrained air weakens colour, causes pinholes and fills the doctor-blade chamber. The causes are pump cavitation, high return flow and contamination; the remedies are correct ink-level and return-line design first, and a compatible defoamer (silicone-free for coated liners that will be over-varnished) second. Plates should be photopolymer grades rated for water-based inks; aniloxes must be cleaned before ink dries in the cells, with an alkaline anilox cleaner, and inspected by cell-volume measurement rather than by eye.

7. Colour management on brown board

Brand colours on corrugated are matched on the actual liner, not on white paper, and the tolerance is agreed as a ΔE on that liner. UniCraft base colours are formulated to blend with predictable strength so that a matching system can build any shade from a fixed base set; extender adjusts strength without changing hue. Printers using ink kitchens or dispensers receive base-strength data and mixing curves for the range.

8. The UniCraft range

Standard, high-rub, water-resistant and high-resolubility grades in a full base range with process set, extenders, pH stabiliser, defoamer, wax additive and retarder, supplied press-ready with technical data sheets covering anilox selection, pH and viscosity windows, drying and rub data.

Key takeaways

Colour strength and hold-out let the printer run fine aniloxes and thin films on absorbent liner.

pH is the dial that keeps water-based ink alive on press; use a stabiliser, not ammonia.

Rub resistance is a 24-hour property; test it then, wet and dry as the box requires.

Foam and dried-in aniloxes are process problems with process solutions.

References: ISO 2846-5 (flexo colour); ASTM D5264 (Sutherland rub); FEFCO testing methods; ASTM D4212 (Zahn cup).