Printing Materials: Paper, Ink and Substrates
This section covers Printing Materials: Paper, Ink and Substrates, the domain that tests paper properties, ink composition and how substrate choice affects printability, drying and finished quality.
Core rules and concepts
- GSM is mass in grams per square metre; it is not thickness. Caliper is thickness, bulk is caliper per unit GSM, and ream weight depends on sheet area and GSM.
- Paper properties include brightness, opacity, whiteness, smoothness, porosity, moisture content and grain direction.
- Coated papers have a mineral coating for smoothness and gloss; uncoated papers are more absorbent and suit text and writing.
- Ink contains pigment, vehicle, solvent or water, and additives. Drying may be by absorption, oxidation, evaporation, radiation or penetration.
- Ink adhesion depends on substrate surface energy; untreated polyethylene and polypropylene need corona treatment or suitable inks.
- Paper grain should run parallel to the fold for folding and parallel to the spine for perfect binding.
Worked example
Find the mass of 500 sheets of 70 cm × 100 cm paper at 120 GSM. Area of one sheet = 0.70 × 1.00 = 0.70 m². Area of 500 sheets = 350 m². Mass = 350 × 120 = 42,000 g = 42 kg. This is the ream weight for that sheet size, not a universal ream weight.
Traps
Do not treat GSM as caliper; do not compare ream weights of different sheet sizes; do not assume coated paper always prints better; do not ignore grain direction; do not blame ink for poor adhesion on an untreated plastic film.