Ink is a combination of pigment, resin, solvents, and additives mixed together to form a fluid or paste capable of printing from a printing press. Every ink system is built around three core components:

• Colorants — provide optical properties and fastness
• Binders (polymers, resins & liquid components) — control printability and drying
• Additives — deliver specific performance properties

Types of Printing Ink

Printing ink splits into two broad families:

Paste Ink — used for Letterpress, Offset, Metal, and Silk printing

Liquid Ink — used for Flexographic and Rotogravure printing

Paste Ink / Offset

Common applications include business cards, brochures, packaging boxes, labels, and commercial print (CMYK process printing). Key pigment selection criteria for paste/offset inks:

• Oil absorption
• Transparency
• Flow
• Gloss
• Acid and alkali resistance
• Heat stability

Liquid Ink

Liquid ink splits further by printing process:

Flexographic — Water-based or Alcohol-based systems (commonly used for paper products, plastic bags, and woven sack bags)

Rotogravure — split into:

• Publication (Maleic) inks
• Flexible Packaging inks — using resin systems like Polyamide, Vinyl, NC, CIPP, and PU

Flexible Packaging: Resin Selection

Flexible packaging inks are further categorized by Reverse Printing (printed on the inside of a laminate — bond strength & transparency preferred) and Surface Printing (printed on the outer surface — needs good scratch, nail, rub, and deep-freeze resistance, with gloss being very important).

Sample Ink Formulations

Liquid Ink – Gravure (Polyamide system)

• Pigment: 9 parts
• Polyamide: 70 parts
• Toluene: 16 parts
• N-Butanol: 3 parts
• Luwax A.F-32: 1 part
• Dispersing & Wetting Agent: 1 part

Flexographic Ink (Water-based)

• Joncryl 671 varnish: 30 parts
• Pigment: 30 parts
• Water: 30 parts
• Disperse BYK193: 1.5 parts
• Leveling agent: 0.5 parts
• Mono Ethylene Glycol: 3 parts
• Antifungal agent: 0.4 parts
• Water: 4.6 parts

Flexographic Ink (Solvent-based)

• Pigment: 9 parts
• Polyamide: 80 parts
• Iso Propyl Alcohol: 7 parts
• N-Butanol: 3 parts
• Luwax A.F-32: 1 part

Key Properties of Ink

• Viscosity/Flow — affected by pigment-resin interaction, moisture, pigment-pigment interaction, solvent drying speed, and temperature
• Transparency — depends on printing type and requirement
• Migration/Retort — critical for food packaging; tested at 130°C for 30 minutes on nylon film
• Color Strength — visual/instrumental comparison against standard
• Print Density — measured with a density meter; dot gain affects final image quality
• Gloss — measured optically at 20°, 60°, and 90° angles using a glossmeter
• Fineness of Grind — assessed with a grindometer (paste inks) or under microscope (liquid inks)
• Resolubility — tested by applying ink on metal plate or PET film and checking solvent reactivation

Souring Test

Used to check the effect of moisture on PU/Polyamide resin systems — 0.1%, 0.5%, and 1% water is added to the final ink, and viscosity, color, opacity & gloss are checked on the same day and after 1 week.

Soap Resistance Test

Used for inks in soap wrapper printing — a 10% soap jelly is prepared, ink panel strips are placed on it, and staining is observed after 24 hours.

Common Printing Problems & Root Causes

Source: Technical Training – Basic Introduction to Inks, Prashant Nigade / Sudatech, Oct 2019