Printing Processes and Press Technology
The exam tests how each process transfers ink, what its consumables are, what it does best, and how a printer controls it on press. You should be able to select a process from a job description and diagnose common press faults.
Core rules
- Offset lithography: image and non-image areas share one plane; the plate is dampened, inked, offset via a blanket to the substrate. Ink–water balance is the core skill; fountain solution is held around pH 4.8–5.5 and its conductivity and alcohol content are monitored against the supplier specification. CTP plates are thermal or violet; processless plates remove chemistry.
- Flexography: photopolymer plates or sleeves, chambered doctor blade, anilox roller metering ink by cell volume (cm³/m²). High-volume anilox for solids, low-volume for fine type. Used for labels, flexible packaging, corrugated.
- Gravure: cells engraved into a copper-plated, chrome-faced cylinder; solvent inks; very long runs and high-quality packaging; cylinder cost is high, so short runs are uneconomic.
- Screen printing: mesh count in threads/cm, emulsion stencil, squeegee, off-contact gap; deposits a thick ink film for textiles, signage, glass and spot varnish.
- Digital: electrophotography (charge, expose, develop toner, fuse) or inkjet (piezo/thermal, aqueous, UV, latex). No plates, so short runs and variable data win; spot and metallic colours are simulated only.
- Press control: densitometry on the colour bar, register, cylinder and blanket packing, ink tack order, spray powder against set-off.
Worked example
A run is approved at K solid density 1.50 on the OK sheet. A pull halfway through the run reads 1.62 and the grey-balance patch has drifted magenta. Reasoning: higher density means a thicker ink film, so tone value increase rises, shadows fill in and the grey patch shifts. Action: reduce ink feed on the units concerned, re-read the colour bar, compare against the approved OK sheet under D50, and only then continue. Do not adjust register - nothing reported a register fault.
Traps that cost marks
Confusing anilox cell volume with gravure cell depth; expecting digital presses to print true Pantone matches; blaming plate exposure when the fault is over-tight packing wearing the plate; ignoring how ink–water imbalance changes dot gain mid-run.
Key points
- Offset balances ink against fountain solution at pH ~4.8–5.5.
- Flexo meters ink with an anilox roller: volume matched to image type.
- Gravure means engraved cylinders and long-run economics.
- Screen printing deposits the thickest ink film of the four.
- Digital wins on short runs and variable data, not on spot colour.
- Judge every pull against the signed OK sheet, not against memory.