Printing processes and press operation
Press questions ask you to match a process to a job and then keep it in control: ink-water balance on offset, anilox volume in flexo, mesh and stencil in screen, fusing and drum life in digital. Learn the measurable targets and the fault-to-cause links.
Offset lithography
- Image areas are oleophilic and non-image areas hydrophilic; the dampening solution keeps the plate clean while ink transfers plate to blanket to substrate under pressure set by packing and squeeze.
- Make-ready order: mount plates, set inking and dampening, register, then bring density up to target on the control bar before pulling the first OK sheet.
- Typical faults: scumming from dampening or plate problems, hickeys from paper dust, picking when ink tack exceeds the paper surface strength, ghosting from ink feed or roller layout, set-off in the delivery pile.
Other processes
- Flexo: a chambered doctor blade meters ink into the anilox; cell volume in cm3/m2 and transfer efficiency decide the wet film, and plate mounting tape thickness sets impression.
- Screen: mesh count in threads/cm, stencil thickness and squeegee hardness and angle control the deposit; snap-off distance governs edge sharpness.
- Digital toner: charging, development and fusing; paper must be conditioned because moisture and surface affect toner adhesion and curl.
Quality control
- Measure solids, tone value increase and grey balance on the printed control strip with a densitometer or spectrophotometer and compare with the approved printing condition, never with the monitor.
- Register tolerance is typically around 0.1 mm for text work; check at the tail of the sheet where fan-out is greatest.
Worked example
An anilox of 4.0 cm3/m2 transfers 55% of its volume, giving 2.2 cm3/m2 wet, which is a film of about 2.2 micrometres. If the job needs a 2.6 micrometre film, the required volume is 2.6/0.55 = 4.7 cm3/m2, so choose a higher-volume anilox rather than increasing impression pressure, which would only squash the dots.
Traps that cost marks
- Reading solid ink density as if it measured dot gain; a solid can be on target while the 50% patch is 6% heavy.
- Chasing register before the sheet has stabilised, or adjusting tension on one unit only.
- Using a single density target for coated and uncoated stock.