Printing Inks and Substrates
Printing Inks and Substrates links ink formulation and paper properties to printability, drying and final durability.
What the exam tests
Expect ink ingredients, drying mechanisms, paper measurements and compatibility problems.
Core rules
- Ink has three main parts: pigment (colour), vehicle/varnish (carries pigment, binds it to the substrate) and additives (driers, waxes, anti-set-off, defoamers). Ink tack is stickiness; high tack can pick weak paper fibres.
- Drying mechanisms: absorption into porous stock, evaporation of solvent or water, oxidation of drying oils, radiation curing (UV/EB) and heat-set. UV ink cures instantly but needs a photoinitiator and UV lamp.
- Substrate measures: GSM is grams per square metre (paper weight); caliper is thickness; bulk is thickness per weight. Grain is the fibre direction; folding parallel to grain avoids cracking.
- Coated papers have a mineral/clay layer; they hold fine dots and gloss but absorb ink slowly. Uncoated papers are porous and bulky but show dot gain and show-through.
- Porosity, smoothness, brightness, opacity and pH all affect ink holdout and colour. Newsprint needs low-tack, absorbed ink; plastic film needs corona treatment or specially formulated ink.
Worked example
A 90 GSM uncoated text sheet prints a heavy solid and shows mottling and pick. Reason: high ink tack and a weak, porous surface are pulling fibres and allowing uneven absorption. Remedies: reduce ink tack, use a slightly heavier or coated stock, reduce solid area or split it into a tint, and check press speed. Increasing ink density alone worsens picking.
Traps
Confusing GSM with caliper; assuming heavier paper is always thicker; folding across grain; using UV ink without a photoinitiator; expecting gloss coated stock to absorb like newsprint.