Offset Inks · The Complete Guide

Offset ink is a highly viscous, paste-like material, far thicker than water, thicker even than honey, that is precisely formulated to split cleanly between rollers, transfer efficiently from roller to plate to blanket to substrate, and then dry on the printed surface without spreading, smearing, or losing colour fidelity.

The ink transfer chain in offset printing is: ink duct → distribution rollers → form rollers → plate → blanket → substrate. At each transfer point, the ink film splits, approximately half transfers forward and half remains behind. By the time ink reaches the substrate, the film is only 1–3 microns thick, one to three thousandths of a millimetre. This extraordinarily thin film must carry enough pigment to produce a visually dense colour. This is why ink pigment concentration and rheology (flow properties) are so precisely engineered.

Because offset ink is oil-based, it exploits the fundamental lithographic principle, oil repels water. The dampened non-image areas of the plate repel the ink. The image areas, which are oleophilic (oil-attracting), accept it. The ink stays exactly where it should be, on the image.

Soya-based and vegetable inks

Traditional offset inks use petroleum-derived mineral oils as the vehicle solvent. Soya-based inks replace a significant proportion of the mineral oil with soybean oil, a renewable vegetable source. The environmental benefit is real but modest: soya oil reduces VOC emissions from the vehicle and the ink is more easily de-inked from recycled paper. The print performance of well-formulated soya inks is equivalent to mineral oil inks for most applications.

• Soya inks dry slightly more slowly than mineral oil inks, drying time must be accounted for in production planning
• Soya inks may soften slightly at higher temperatures, relevant for packaging going through hot-fill processes
• Not inherently food-safe, soya origin does not make an ink food-contact compliant. Food safety is determined by the specific pigment and vehicle chemistry, not the oil source

Ink rheology describes how an ink flows and behaves under stress. In offset printing, the two most practically important rheological properties are tack and viscosity. They are related but distinct, and both must be correctly specified for the press speed, substrate, and printing sequence.

Tack · the splitting force

Tack is the resistance of the ink to splitting. When the ink film between two rollers splits, or when the blanket separates from the substrate, the force required to split the ink film is its tack. High-tack ink is harder to split, it requires more force. Low-tack ink splits easily.

• Tack is measured in Inkometer units (also called tack units), typically in the range of 8–18 for process offset inks
• Tack must be higher than the surface strength of the substrate, if the force required to split the ink film exceeds the bonding strength of the paper surface fibres or coating, the paper surface tears (picking)
• In multicolour printing, inks must be printed in descending tack order, the first colour down (typically black or cyan) has the highest tack, the last colour has the lowest. This is essential for correct ink trapping, see below
• Tack increases as press speed increases and as ink temperature decreases. A fast press or a cool press room requires lower-tack inks to prevent picking.
• Adding reducing oil or tack reducer reduces tack. Adding varnish or body gum increases it.

Viscosity · the flow resistance

Viscosity is the resistance of the ink to flow, its thickness. High-viscosity ink is stiffer and moves more slowly through the ink train. Low-viscosity ink flows freely. In offset printing, viscosity determines how the ink distributes across the roller train and how cleanly it feeds from the duct.

• Offset ink viscosity is typically measured with a Laray viscometer, expressed in Pa·s (pascal-seconds). Process ink viscosity: typically 40–100 Pa·s
• Too high: ink does not feed freely from the duct, causes uneven distribution, leads to starvation in heavy coverage areas
• Too low: ink runs freely, difficult to control, can cause dot spread and excessive emulsification into the fountain solution
• Viscosity decreases as temperature increases, hot press rollers and a hot press room both reduce ink viscosity. Presses running in air-conditioned rooms have more stable ink viscosity than presses in open press rooms during Indian summer.

In multicolour offset printing, all four CMYK colours are printed in sequence, one colour unit after another, on the same press pass. The order in which the inks are applied determines whether they trap correctly on top of each other and whether the paper surface survives the printing process without picking or tearing.

Why this sequence · and what happens when it is wrong

The standard sequence K-C-M-Y (or C-M-Y-K with black last, depending on the press and job) follows the principle of descending tack. Each subsequent ink has lower tack than the one before it.

• The first ink down (highest tack) adheres directly to the substrate surface, its high tack allows it to bond strongly to the fibres or coating
• The second ink must trap onto the first. If the second ink has higher tack than the first, it will pull the first ink back off the substrate, causing back-trap mottle (a patchy, uneven appearance in overprint areas)
• Each subsequent ink has lower tack, it traps onto the previous colour without disturbing it
• Yellow is always last because it has the lowest acceptable tack of the four process colours and because a slight pull-back of yellow (the palest colour) is less visible than pull-back of black or cyan

Ink trapping is the ability of a wet ink to adhere to and print over a previously printed wet ink layer. In multicolour offset printing where all four colours print in rapid succession (fractions of a second between units), the second, third, and fourth inks must trap onto still-wet previous inks. Trapping is never 100%, a second ink always traps onto a wet first ink less efficiently than it would onto a dry surface. The degree of trapping determines the accuracy of colour in overprint areas.

Wet trapping vs dry trapping

• Wet trapping, printing onto a wet ink (the standard multicolour offset situation). Trapping efficiency is typically 85–95% for a correctly sequenced, correctly tacky ink set
• Dry trapping, printing onto a fully dried previous ink layer (as in two-pass printing). Trapping efficiency approaches 100% because the surface is consistent and stable. Used for jobs that cannot be adequately reproduced in a single wet-trap pass.

How to measure trapping

Trapping is measured from the printed colour bar using a densitometer or spectrophotometer. The standard method (Preucil formula) calculates trapping as a percentage:

• Measure density of the overprint (second ink on top of first ink)
• Measure density of the first ink alone and the second ink alone
• Apply the Preucil formula: Trapping % = (Density of overprint − Density of first ink) ÷ Density of second ink × 100
• Target: 85–100% trapping. Below 75%: investigate tack sequence and emulsification

Total ink coverage (TIC), also called total area coverage (TAC), is the sum of all four CMYK percentages at any single point in the design. A pixel that is 100% Cyan + 80% Magenta + 60% Yellow + 80% Black has a TIC of 320%.

TIC must be limited because excessive ink causes: slow or incomplete drying, setoff (the ink is too thick to dry quickly enough), ink trapping problems (too much ink prevents the next colour from adhering), show-through on the reverse of the substrate, and in packaging, failure to meet food safety migration limits for total ink deposited.

How TIC is controlled

TIC is controlled at the pre-press stage, not on press. The two pre-press tools for TIC management are:

• UCR (Under Colour Removal), replaces equal amounts of CMY in neutral (grey) areas with black, reducing total ink while maintaining neutral colour balance
• GCR (Grey Component Replacement), a more aggressive version of UCR that replaces grey components throughout the colour space (not just neutral areas) with black. GCR reduces TIC significantly and improves press stability by shifting colour balance control to a single ink (black) rather than three inks
• Both UCR and GCR are set in the ICC profile or in the RIP, they are pre-press decisions made before the job is plated

Oxidative polymerisation · conventional offset ink drying

Conventional offset inks dry by oxidative polymerisation, a chemical reaction between the linseed or soya oil vehicle and atmospheric oxygen. The oxygen crosslinks the oil molecules into a solid polymer network, converting the liquid ink film into a dry, hard surface. This process is catalysed by the metal drier salts in the ink.

• Drying time at standard conditions (23°C, 50% RH): 4–8 hours for full surface cure on coated paper. Full through-cure takes 12–24 hours.
• Factors that slow drying: low temperature, high humidity, fountain solution pH below 4.5 (deactivates driers), excessive water emulsification in the ink, heavy ink coverage (thick films dry more slowly), printing on coated stock (less oxygen penetration through the coating than on uncoated)
• Factors that accelerate drying: IR drying units in the press delivery, higher temperature, lower humidity, higher drier content in the ink

UV curing · instant cure by photoinitiation

UV offset inks contain no drying oils. Instead, they contain monomers, oligomers, and photoinitiators that react when exposed to UV light (200–400nm wavelength). The UV lamp provides the energy to trigger radical polymerisation, the ink cures from liquid to solid in a fraction of a second as it passes under the lamp.

• Instant cure, no drying time. Sheets can be handled immediately after the UV lamp.
• No setoff risk, the ink is fully solid when it reaches the delivery pile. No anti-setoff powder is theoretically needed, though a small quantity is often used for pile stacking.
• Superior scuff and chemical resistance compared to conventional inks
• More expensive than conventional, approximately 3–5× the ink cost
• Require UV-compatible blankets and UV-rated rollers, standard nitrile rubber is incompatible with UV monomers (see Printing Blankets guide)
• LED UV vs mercury arc UV: LED UV uses narrow-spectrum LEDs (typically 365–395nm) and has lower energy consumption and longer lamp life. Mercury arc lamps produce a broader UV spectrum. Ink formulation must match the lamp type.

Metallic inks

Metallic inks contain finely ground metallic pigment, typically aluminium for silver, and aluminium with a gold-coloured coating for gold. The metallic particles align flat on the ink surface after printing, creating a reflective effect. Unlike hot foil, metallic ink creates a diffuse metallic sheen rather than a mirror finish.

• Not a substitute for hot foil when a true mirror metallic effect is required, but economical for large-area metallic coverage where foil would be cost-prohibitive
• Metallic inks are abrasive, they accelerate wear on rollers, blankets, and plates. Do not run metallic ink for extended periods without checking roller condition.
• Drying can be slower than process inks, metallic pigment particles reduce oxygen penetration to the vehicle beneath. Allow extra drying time before finishing.
• Metallic inks printed over CMYK: the metallic ink must go last (in an additional unit or offline), metallic pigment disrupts ink transfer if it is in the middle of the colour sequence

Fluorescent inks

Fluorescent inks contain pigments that absorb UV light (including daylight) and re-emit it as visible light of a specific colour, creating a brightness that exceeds what ordinary reflective pigments can achieve. Commonly used in promotional printing, packaging for children's products, safety labelling, and wherever eye-catching colour is the objective.

• Not lightfast, fluorescent pigments fade significantly with UV exposure. Not suitable for any application requiring long-term colour stability (outdoor advertising, archival documents)
• Cannot be matched from CMYK, fluorescent colours are outside the CMYK gamut. They must be specified as a spot colour and printed from a dedicated ink unit
• Density should be verified after printing, fluorescent inks can appear brighter under one light source than another (metamerism)

Opaque white

A highly pigmented white ink used to create an opaque white layer on coloured, metallic, or transparent substrates. Standard CMYK white (absence of ink) only works on white paper, on any other substrate, a printed white element requires opaque white ink.

• Very high pigment loading (titanium dioxide), significantly more viscous and less transparent than process inks
• Must be printed first (before CMYK) when used as a base for overprinting, or last when used as a highlight. Sequence is job-specific.
• Coverage is rarely fully opaque in a single pass on very dark substrates, two passes of opaque white may be required for maximum opacity
• Drying is slower than process inks, allow additional drying time or use UV white for immediate cure

Pantone (PMS) spot colours

Pantone Matching System colours are pre-mixed inks formulated to a specific published standard, each Pantone number is a defined pigment mixture. They are used when a precise brand colour must be reproduced more accurately than CMYK process printing can achieve.

• Printed from a dedicated ink unit, replacing one CMYK unit or adding a fifth or sixth unit on a press with additional capacity
• Tack must be adjusted relative to the process inks if printing in the same pass, the Pantone ink's position in the sequence determines the required tack
• Always mix from fresh ink for each job, pre-mixed Pantone inks can separate or change in consistency if stored improperly

When offset ink is printed on food packaging, whether primary (direct food contact), secondary (outer packaging), or adjacent (packaging where the printed surface could contact food through migration), the ink must comply with food safety regulations. In India, the relevant authority is FSSAI (Food Safety and Standards Authority of India). For export, EU Regulation 10/2011 and Swiss Ordinance SR 817.023.21 are the most commonly referenced international standards.

The key food safety concepts

• Migration, the transfer of ink components (monomers, pigment particles, solvents, photoinitiators) from the printed surface through the packaging material into the food. Migration must be below regulated limits, typically 10 mg/dm² overall migration and 0.01 mg/kg specific migration for individual substances in EU regulations
• Set-off migration, if the printed outer surface of one carton contacts the food-contact inner surface of another during storage, ink components can transfer even without direct print-to-food contact. This is why high-migration-risk inks cannot be used on the outer surface of food cartons, even when the food contact is on the reverse
• Functional barrier, a layer within the packaging structure (foil, specific plastic layers) that prevents migration from the outer printed surface to the food-contact inner surface. If a functional barrier is present and verified, less stringent ink requirements apply to the outer layer
• UV photoinitiators, a specific food safety concern with UV inks. Some photoinitiators (particularly benzophenone derivatives) are known migrants with specific regulatory restrictions. Food-grade UV inks use photoinitiator systems with verified low migration potential. Standard UV inks are often not acceptable for food packaging.