Materials: paper, ink and substrates - specification, drying and compatibility
What the exam tests
Materials questions test paper specification (grammage, bulk, grain, coating), ink type and drying route, and whether substrate, ink and process are compatible.
Paper and board
- Grammage (g/m²) is mass per area, not thickness. Caliper (µm) and bulk (cm³/g) describe thickness; two 90 g/m² sheets can differ in bulk by 30 %.
- Uncoated (offset, bond) is absorbent and takes ink quickly. Coated (gloss, silk, matt) has a pigment and binder layer: higher gloss and print contrast, but ink dries mainly by oxidation or UV, not absorption.
- Mechanical (wood-containing) pulp yellows and is weaker; chemical pulp gives strong, archival sheets. Recycled fibre affects brightness rather than strength.
- Grain direction: folds and binding must run with the grain parallel to the fold or spine, or the coating cracks.
- ISO 216 A series: A0 = 1 m² and each size halves the area; A4 = 210 × 297 mm.
Inks and drying
- Offset: paste ink, high tack, dries by oxidation and absorption, or cures by UV. Flexo and gravure: liquid, low viscosity, dry by evaporation (solvent or water) or UV. Screen: thixotropic paste, thick film. Digital: toner fused by heat, or UV/water inkjet.
- Ink = pigment + binder + vehicle + additives. Tack, viscosity and pigment loading must match both the process and the substrate's surface energy.
Worked example
A ream of A4 80 g/m², 500 sheets.
- Area of one sheet = 0.210 × 0.297 = 0.06237 m².
- Total area = 500 × 0.06237 = 31.19 m².
- Mass = 31.19 × 80 = 2,495 g ≈ 2.5 kg.
- A ream invoiced at 2.9 kg works out at 2,900 ÷ 31.19 ≈ 93 g/m², not 80 - query the delivery.
Traps that cost marks
- Using grammage as a measure of thickness.
- Folding across the grain and blaming the paper for cracking.
- Assuming coated stock absorbs ink quickly; it needs oxidation or UV.
- Confusing brightness (blue reflectance) with whiteness (overall reflectance).
- Matching a solvent ink to a substrate without checking surface energy and adhesion.