Selecting pigments for printing inks requires matching the pigment’s particle size distribution, oil absorption, and solvent compatibility to the specific print process. Offset lithography demands high-color-strength pigments that remain transparent in the thin ink film (1–2 μm). Gravure printing requires pigments ground to sub-micron particle size with D50 < 0.1 μm to prevent doctor blade streaks. Flexographic inks need pigments with fast solvent release and low viscosity contribution. Screen printing inks demand high pigment loading and opacity for thick-film coverage. The table below provides a process-by-process selection reference.

Print Process Selection Table

Pigment Selection by Print Process

Offset Lithography — Transparency Is King

Offset printing applies ink films of only 1–2 μm thickness. In this thin film, pigment transparency and color strength are paramount — an opaque pigment at this film thickness produces a washed-out, low-chroma print. PR57:1 (Lithol Rubine, calcium lake) is the dominant magenta pigment for offset printing worldwide, prized for its high transparency, exceptional color strength, and true process magenta hue. PY12 and PY13 (diarylide yellows) are the standard yellows for offset process sets — PY12 for coldset and general purpose, PY13 for heatset applications requiring higher transparency and heat stability. PB15:3 serves as the universal cyan for all offset processes.

Flexographic Printing — Speed and Solvent Compatibility

Flexographic printing runs at 100–400 m/min with extremely low ink viscosities (0.5–3 Poise). Pigments must disperse easily in low-viscosity vehicles without contributing excessive viscosity, must release solvent rapidly after impression to prevent blocking on the rewind, and must resist bleed into adjacent colors at the print nip. For water-based flexo, PY83 (disazo yellow HR-02 grade) offers the best price-performance combination with good pH stability and high color strength at low loading. For solvent flexo, PR48:2 (calcium azo lake) is the standard magenta — cost-effective with good solvent resistance, though lightfastness is limited to grade 4. For UV flexo, shift to high-performance organics (PR254, PR122, PY150) to maintain color integrity through the UV cure cycle.

Gravure Printing — Particle Size Is Everything

Gravure printing is uniquely demanding on pigment particle size. The engraved cylinder transfers ink from cells typically 30–60 μm deep, and the doctor blade removes excess ink from the non-image area. Pigment particles larger than about 1 μm become trapped between the doctor blade and the cylinder surface, creating visible streaks (called “doctor blade lines”) that ruin the print. For publication gravure, pigments must be micronized to D50 < 0.1 μm with D99 < 0.5 μm — a particle size distribution one order of magnitude finer than offset-grade pigments. Our micronized gravure series (suffix “-GR”) delivers PR57:1, PY12, PB15:3, and PG7 ground to gravure specifications with controlled crystal form and surface treatment for rapid solvent wetting.

Frequently Asked Questions

Q: Why must offset ink pigments be transparent?

Because offset lithography prints ink films of only 1–2 μm thickness — approximately 1/50th the thickness of a human hair. In films this thin, opaque pigments like TiO₂ or chrome yellows (PY34) scatter light before it can penetrate through the film, reflect off the substrate, and return to the eye. The result is a pale, chalky, low-chroma print. Transparent pigments like PR57:1, PY12, PB15:3, and PG7 allow light to transit the full film thickness twice (inbound + outbound), effectively doubling the optical path and producing the high saturation required for four-color process printing. This is a fundamental physical limitation of offset lithography; no formulation adjustment can compensate for the wrong pigment transparency class.

Q: Which yellow pigment works across offset, flexo, and gravure?

No single yellow pigment works optimally across all three processes, but different grades of PY12 (diarylide yellow AAA) come closest to universal applicability. In offset, standard PY12 provides good transparency and color strength at process loading (8–12% in paste ink). In flexo, PY12 in a surface-treated grade designed for low-viscosity vehicles provides adequate color development at 10–15% loading. In gravure, micronized PY12 (D50 < 0.1 μm) must be used to prevent doctor blade streaking. The chemical identity is the same, but the physical form (particle size, surface treatment, crystal morphology) must be tuned for each process. For a truly process-optimized alternative, PY83 (disazo, higher MW than PY12) provides better heat stability and migration resistance, making it preferred for packaging flexo and gravure, while PY12 remains the offset standard.

Q: How does pigment particle size affect gravure print quality?

Pigment particle size affects gravure print quality through three mechanisms. First and most critically: doctor blade streaking. The doctor blade in gravure presses against the engraved cylinder with a contact pressure of 0.5–2.0 N/cm. Pigment particles larger than approximately 1 μm act as micro-wedges between the blade and cylinder, creating visible streaks (doctor blade lines) that render the print unsellable. This is the most common gravure pigment failure mode and demands D99 < 0.5 μm. Second: color strength development. Sub-micron particles provide higher specific surface area, increasing color strength per unit mass — critical because gravure inks are very low viscosity (0.3–1.0 Poise) with limited pigment loading capacity. Third: gloss and transparency. Fine particle size (<0.1 μm D50) produces superior gloss and transparency in the dried print film. The trade-off is that over-grinding can damage crystal structure, reducing chroma and weather resistance, so micronization must be carefully controlled.