Printing Processes and Press Operations
This section covers Printing Processes and Press Operations, the domain that tests how offset, gravure, flexography, screen and digital presses form and transfer images, and how a press is made ready and run.
Core rules and concepts
- Offset lithography uses a planographic plate, dampening solution and ink; non-image areas accept water and reject ink. Ink-water balance is critical.
- The offset image path is plate to blanket to paper, so the plate reads right-reading and the blanket reverses it.
- Gravure uses an engraved cylinder and doctor blade; ink sits in cells and transfers directly, giving dense images and long runs.
- Flexography uses a relief photopolymer plate and anilox roller to meter ink; it suits labels, cartons and flexible film.
- Screen printing forces ink through a mesh stencil with a squeegee; it gives thick ink films on textiles, glass and signage.
- Press operations include feeding, registration, inking, dampening, impression, drying and delivery; make-ready precedes the run.
Worked example
A job needs 2,00,000 four-colour cartons on coated board with photographic images. Reasoning: the long run justifies offset plate and make-ready cost; sheetfed offset gives high image quality on board. A 400-copy short run would go digital to avoid plate set-up. A flexible-film version would use flexography because its plates and inks suit non-absorbent webs.
Traps
Do not say offset prints directly from plate to paper; do not run too much water, which causes emulsification and poor drying; too little water causes scumming; do not confuse anilox with blanket; do not ignore registration when diagnosing colour fringes.