Printing processes and principles: image carriers, ink transfer and process choice
What the exam tests
The examiner checks that you can name each process's image carrier, explain how ink is metered and transferred, and match a job to the right process by run length, substrate and quality. Marks are lost on process confusion far more than on definitions.
The five families
- Offset lithography – planographic: image and non-image share one plane on the aluminium plate. Separation is chemical; the oleophilic image takes ink, the hydrophilic non-image takes fountain solution. Ink goes plate → blanket → substrate. Ink film about 1–2 µm, paper 60–350 g/m², runs from about 1,000 to over a million.
- Flexography – relief: a raised photopolymer or rubber image on a plate or sleeve. A ceramic anilox with laser-engraved cells, helped by a doctor blade, meters a low-viscosity ink. Anilox cell volume (cm³/m²) sets colour strength: change the roller and the density changes. Packaging film, corrugated, labels.
- Gravure – intaglio: the image sits in cells below the cylinder surface, filled by immersion, wiped by a doctor blade, released by capillary action. Engraving cost is high, so it suits very long runs and premium catalogues or packaging.
- Screen printing – a stencil on a mesh blocks open areas; the squeegee forces paste through. Deposits of 10–100 µm give opaque white on dark or curved glass, textiles and PCBs.
- Digital – electrophotography (toner, charged photoconductor, fuser) or inkjet. No image carrier per job, so variable data and short runs are economic.
Cost logic
Offset front-loads make-ready, plates and start-up waste but has a low cost per impression. Digital has almost no make-ready and a flat click cost. The crossover quantity, not the process reputation, decides the tender.
Worked example
A 4-colour label job of 5,000 labels. Offset: €180 make-ready plus €0.012 per label. Digital: no make-ready, €0.075 per label.
- Offset: 180 + (5,000 × 0.012) = 180 + 60 = €240 → €0.048 per label.
- Digital: 5,000 × 0.075 = €375 → €0.075 per label.
- Crossover: 180 + 0.012Q = 0.075Q → 180 = 0.063Q → Q ≈ 2,857. Below about 2,860 labels digital wins; above it, offset.
Traps that cost marks
- Describing offset as a raised image (that is flexo), or calling gravure planographic.
- Confusing flexo and gravure because both use cylinders: only gravure is intaglio.
- Ignoring the anilox when explaining why a flexo colour prints weak or strong.
- Claiming screen printing cannot hold fine detail, or that digital is always cheaper.