Offset lithography

In short

Offset lithography is a planographic process: image and non-image areas lie in the same plane on a thin aluminium plate, separated chemically — the image area is oleophilic (ink-accepting, water-repellent) and the non-image area is hydrophilic (water-accepting, ink-repellent). The inked image is not printed directly but offset onto a rubber blanket cylinder and then onto the substrate, which is why a hard plate can print on rough paper and board. It is still the dominant process in India for books, newspapers, magazines, commercial print, folding cartons and labels, and it carries the largest share of marks in printing-technology semester papers. Printing is not a GATE paper, so it is examined through university exams and the written/technical rounds of Indian printers and converters — Manipal Technologies, Thomson Press, Repro India, Replika Press, Parksons Packaging, TCPL, ITC PSPD, Navneet, SPMCIL/India Security Press, and the pressrooms of newspaper groups.

Explained

Principle and the chemistry that makes it work

• Lithography works on selective wetting, not on relief or depth: a flat plate carries an image that prefers ink and a background that prefers water.
• Plate structure: grained and anodised aluminium (0.15–0.40 mm for sheetfed, thinner gauges on small presses) coated with a light-sensitive layer. Graining increases surface area to hold water; anodising gives a hard, hydrophilic aluminium-oxide layer.
• Non-image area is kept damp by fountain solution (water + acid/buffer + gum + wetting agent + alcohol or substitute); the water film, about 0.5–1.0 µm thick, stops ink transferring there.
• Gum arabic / synthetic gum desensitises the non-image area and protects the plate when the press stops.
• Offset (indirect) transfer: plate → blanket (rubber-faced, compressible) → substrate. Benefits: the plate never touches paper (longer plate life), the blanket conforms to the sheet surface, lower impression pressure, and the plate image is right-reading on the substrate because it is reversed twice (plate image is right-reading, blanket image is wrong-reading).
• Ink is a stiff, high-viscosity paste (tack 7–12 on an inkometer scale, viscosity in tens of Pa·s) so it can split between rollers; film on paper is only about 1–2 µm per colour.

The three cylinders, inking and dampening

1. Plate cylinder — carries the plate, clamped in the gap; has an undercut so that plate + packing sheets bring the surface to bearer level.
2. Blanket cylinder — carries the blanket (typically 1.6–2.0 mm) plus packing; the blanket's compressible layer absorbs nip variations.
3. Impression cylinder — hard, backs up the sheet; gripper-fed in sheetfed presses.
4. Nip and squeeze: a controlled interference (commonly 0.10–0.15 mm plate-to-blanket and blanket-to-impression) creates the printing pressure; too much causes dot slur, blanket smash and premature wear.
5. Inking train: ink duct (fountain) with blade and keys → ductor roller (oscillates, meters ink intermittently) → distributor and oscillating (vibrator) rollers that work ink axially → riders → 3–4 form rollers of differing diameter touching the plate. Each nip splits the film roughly in half, so a long train converts a thick duct film into a thin, uniform film on the plate.
6. Keyless inking (common on coldset newspaper presses) removes zone keys and uses a chambered system with a scraper, avoiding across-the-sheet ink imbalance.
7. Dampening systems:
   • *Conventional/intermittent:* fountain pan, pan roller, ductor, oscillator, form roller covered with molleton/paper — cheap, less precise.
   • *Continuous-feed (Dahlgren type):* metering nip plus alcohol in the solution; the dampener feeds the first ink form roller (indirect).
   • *Brush/spray/anilox-type continuous dampeners:* direct feed to the plate, fast response, fitted on most modern presses.
   • *Combined (integrated) dampening:* bridge roller lets water and ink share a path; good start-up but risks emulsification.
8. Fountain-solution control: pH about 4.8–5.5 (too low attacks the image and rollers; too high lets ink scum); conductivity typically 800–1,500 µS/cm measured against the incoming water's base value; temperature 8–12 °C in a refrigerated recirculator; isopropyl alcohol 8–12% in older Dahlgren systems, now replaced by 2–5% alcohol substitutes (glycol ethers) to cut VOC emissions.
9. Ink–water balance is the central skill: the least water that keeps the non-image area clean. Too little → scumming/toning; too much → emulsification, weak, washed-out solids, paper curl and poor drying.

Prepress, plates and platemaking

• Workflow: design → preflight → RIP (PostScript/PDF, PDF/X-1a or PDF/X-4) → colour management (ICC profiles, CMYK separation with GCR/UCR, total area coverage usually held to 300–340% on coated stock) → trapping → imposition → CTP output → plate processing → press.
• Halftoning: AM (amplitude-modulated) screens at 133–200 lpi for coated sheetfed, 85–100 lpi for newsprint; screen angles 15/75/0/45° (C/M/Y/K) to avoid moiré; FM/stochastic and hybrid screens avoid rosettes and moiré on fine work.
• CTP plate types: thermal (830 nm infra-red laser, ablation or thermal cross-linking, daylight-safe, run lengths of several hundred thousand), violet photopolymer (405 nm, fast, lower cost), and processless/chemistry-free plates that develop on press — now common in India for environmental and effluent reasons.
• Plate quality control: 1×1 pixel checkerboard and UGRA/FOGRA or GATF control wedge, step-wedge solid steps, measured dot area on plate with a plate reader (e.g. 50% file area should read close to 50% on the plate).
• Waterless offset: a silicone-rubber layer replaces the water film as the ink-repellent background; needs temperature-controlled inking (±1 °C) and special inks. Gives sharper dots, less dot gain, no fountain solution and faster make-ready; plates and inks cost more and are scratch-sensitive.

Press types and configurations

• Sheetfed offset: feeder → feed table (side and front lays for register) → grippers → 1 to 10+ printing units → optional coater/UV units → delivery. Formats in India run from small-offset (A3/A2) to B1 (720 × 1,020 mm) and VLF for cartons. Speeds 8,000–18,000 sheets/hour.
• Perfecting: printing both sides in one pass — by a turner bar/drum between units (convertible press) or a blanket-to-blanket web unit. Sheetfed perfectors need extra attention to set-off and gripper-edge register.
• Web offset — coldset (non-heatset): newspapers; ink dries by absorption into uncoated newsprint; blanket-to-blanket units; speeds up to ~45,000–80,000 copies/hour on modern towers.
• Web offset — heatset: magazines, catalogues and inserts on coated paper; a gas-fired hot-air dryer evaporates the solvent, then chill rolls set the ink; needs a silicone applicator and an incinerator/oxidiser for solvent fumes.
• Common impression cylinder (satellite/arch) presses: four plate/blanket pairs around one large impression cylinder — excellent register, used for high-quality web work.
• Constant vs variable cut-off: newspaper presses have a fixed cut-off (commonly 546–578 mm for broadsheet, varies by press); commercial web presses offer variable cut-off via interchangeable cylinders or sleeves.
• Digital offset / DI presses: imaging heads mounted on the press image the plate in position — short-run offset quality without separate platemaking.
• Offset for packaging: heavy-duty sheetfed presses with board-handling feeders and double-diameter transfer cylinders print folding cartons, while sheetfed metal-decorating presses print tinplate sheets for three-piece can bodies, ends and closures — both with inline coating and UV curing. (Formed two-piece D&I can bodies are decorated separately, by dry offset on can-decorating machines.)

Make-ready, running and process control

• Make-ready steps: load plates (register pins/CTP punch), set packing, set side/front lays and sheet size, set ink keys from the job's area-coverage data (CIP3/CIP4 JDF presetting), bring up ink and water, pull first sheets, check register and colour against the OK sheet, then run.
• Colour bar (Brunner, GATF or FOGRA) in the trim: solids, 25/50/75% tints, slur/doubling targets, grey balance patches, star target.
• Measurement: reflection densitometer (status E/T) for density and dot area; spectrophotometer with CIELAB/ΔE for standardised colour; ISO 13655 defines measurement conditions (M0–M3), ISO 5-3/5-4 density.
• Standards: ISO 12647-2 is *the* offset standard — it fixes substrate and ink colorimetry, tone value increase curves by paper type, screen ruling ranges, register and ΔE tolerances; ISO 2846-1 specifies process-ink colour; ISO 12647-7 covers digital proofs; PSO/FOGRA certification and G7 grey-balance calibration are voluntary schemes used by Indian export printers. BIS has specifications for printing and writing papers and boards; paper properties are tested by grammage (ISO 536), caliper, bursting strength, Cobb sizing, brightness and smoothness.
• Drying: oxidative polymerisation + absorption for conventional sheetfed inks (hours); UV/LED-UV and EB inks cure instantly by free-radical polymerisation; anti-set-off spray powder and interleaving control set-off in the delivery pile.
• Common faults: scumming, tinting (ink pigment dispersed in the dampening solution), piling (on blanket or plate), picking/linting of paper fibres, hickeys (dark spot with white halo from dirt/skin), ghosting (mechanical and chemical), set-off, slur (smear in one direction), doubling (second faint image from blanket/impression slip), misregister, mottle, fan-out (web/sheet widening after taking moisture).
• Safety and environment (India): the OSH&WC Code, 2020 (in force 21 November 2025, replacing the Factories Act) governs guarding of nips and cylinders, lock-out before washing, noise and solvent exposure; the other three Labour Codes also apply from the same date. Solvent-soaked wipes, used blanket wash, waste ink and plate-processor chemistry are hazardous waste under the Hazardous and Other Wastes (Management and Transboundary Movement) Rules, 2016, requiring authorised disposal; general press waste follows the Solid Waste Management Rules, 2026. Paper trim is sold to recyclers. ISO 9001:2026 is the current quality-system edition, and FSC/PEFC chain-of-custody and food-contact ink compliance (low-migration inks) are usual customer requirements in carton printing. Under GST (5%/18%/40% slabs since 22 September 2025) printed books are nil-rated while printing job work is taxed — the applicable rate depends on who supplies the paper and on the current notification.

Advantages, limitations and comparison

• Advantages: very good image quality and fine screens; low plate cost and fast platemaking; prints on a wide range of papers and boards; economical from a few thousand to millions of impressions; wide press and consumable availability in India.
• Limitations: needs ink–water balance (a skilled operator variable); start-up waste; not suited to very short runs or variable data without a DI/digital alternative; cannot print thick ink films, so metallics and heavy opaque whites are weaker than in screen; poor on non-absorbent or very uneven surfaces without special inks.

Key points

• Planographic - image and non-image on one plane, separated by ink/water preference, not by height or depth.
• Oleophilic image, hydrophilic non-image - the whole basis of lithography.
• Offset transfer - plate to blanket to substrate; protects the plate and conforms to the paper.
• Three cylinders - plate, blanket, impression; undercut plus packing sets surface height and squeeze.
• Ink–water balance - minimum water that keeps the plate clean; too much water emulsifies ink and weakens solids.
• Fountain solution - pH 4.8–5.5, conductivity typically 800–1,500 µS/cm, chilled and recirculated.
• Ink film on paper is only 1–2 µm; ink is high-tack paste that splits roughly 50:50 at each nip.
• CTP - thermal (830 nm) or violet (405 nm); processless plates avoid developer chemistry.
• Screen angles 15/75/0/45° for C/M/Y/K prevent moiré; rosette is the correct pattern.
• Dot gain / TVI - printed dot area larger than the file value, from optical and mechanical causes; measured with Murray–Davies.
• Solid ink density (SID) is set first, then TVI and grey balance are judged; density alone does not guarantee colour.
• ISO 12647-2 is the process-control standard for offset; ISO 2846-1 for ink colour, ISO 12647-7 for proofs.
• Perfecting - both sides in one pass, via turner bar (sheetfed) or blanket-to-blanket (web).
• Heatset vs coldset - dryer and chill rolls versus absorption into newsprint.
• Waterless offset - silicone background replaces water; sharper dots, lower gain, temperature-controlled inking.
• Scumming is too little water or a damaged plate; tinting is ink emulsified into the dampening solution.
• Slur vs doubling - slur is a smear from excess pressure/slip; doubling is a second faint dot from cylinder or blanket slip.
• Set-off is wet ink transferring to the back of the next sheet in the pile; controlled with spray powder and pile height.

Formulas and examples

Example 1 — cut-off and web speed. A web offset plate cylinder has a circumference (cut-off) of 600 mm and runs at 500 rev/min.

• Plate cylinder diameter D = 600/π = 190.99 mm.
• Impressions per hour = 60 × 500 = 30,000.
• Web speed V = 0.600 m × 500 = 300 m/min (18 km/h).
• Paper length for 30,000 impressions in one hour = 0.600 × 30,000 = 18,000 m.

Example 2 — paper and press time for a sheetfed job. 50,000 copies of a 4-colour poster, one up on 640 × 900 mm, 130 gsm art paper; press rated 12,000 sheets/h at 90% efficiency; make-ready 45 min; press rate ₹3,500/h; paper ₹85/kg; spoilage 5%.

• Sheets needed = 50,000 × 1.05 = 52,500 sheets.
• Mass per sheet = 0.64 × 0.90 × 130 = 74.88 g.
• Paper mass = 52,500 × 74.88 = 3,931,200 g = 3,931.2 kg; cost = 3,931.2 × 85 = ₹3,34,152.
• Run time = 50,000 ÷ (12,000 × 0.90) = 4.63 h; with make-ready 0.75 h → 5.38 h.
• Press cost = 5.38 × 3,500 ≈ ₹18,830; paper is ~95% of this bill — the usual reason spoilage control matters more than press speed.

Example 3 — dot area and TVI. A 70% cyan tint reads density 0.54 and the cyan solid reads 1.45 (paper subtracted).

• 10^−0.54 = 0.2884 → numerator = 1 − 0.2884 = 0.7116.
• 10^−1.45 = 0.03548 → denominator = 1 − 0.03548 = 0.96452.
• A = 0.7116 ÷ 0.96452 = 0.7378 → 73.8%.
• TVI = 73.8 − 70 = +3.8 percentage points, which sits inside the few-percent mid-tone deviation band ISO 12647-2 allows (the exact figure depends on the patch and paper type in the standard's table).

Example 4 — print contrast. Same cyan: solid 1.45, 75% tint 0.95.

• K = (1.45 − 0.95)/1.45 × 100 = 34.5%. If density is pushed up and the 75% patch fills in to 1.10, K = (1.45 − 1.10)/1.45 = 24.1% — proof that more density is not more detail.

Example 5 — ink consumption. 20,000 sheets of 500 × 700 mm, 4 colours, average coverage 35% of the sheet area per colour, ink film 1.2 µm, ink density 1.1 g/cm³.

• Sheet area = 0.5 × 0.7 = 0.35 m²; covered area per colour = 0.1225 m²; for 4 colours = 0.49 m².
• Volume per sheet = 0.49 × 1.2 × 10⁻⁶ = 5.88 × 10⁻⁷ m³ = 0.588 cm³.
• Mass per sheet = 0.588 × 1.1 = 0.6468 g → for 20,000 sheets = 12,936 g ≈ 12.9 kg of ink, about 3.2 kg per colour.

Example 6 — imagesetter/CTP resolution. For 175 lpi with 256 grey levels: resolution ≥ 175 × √256 = 175 × 16 = 2,800 dpi. A 2,400 dpi plate-setter gives (2,400/175)² ≈ 188 addressable cells, so about 188 grey levels — usually acceptable, but at 200 lpi it falls to 144.

Where it is used

• Books and education publishing: Manipal Technologies (Manipal), Thomson Press, Replika Press, Repro India, Navneet and S Chand print textbooks on sheetfed and web offset.
• Newspapers: every major Indian title runs coldset web offset towers with keyless inking and automatic reel splicers; roles include unit operator, reel-stand hand, shift production manager.
• Folding cartons and labels: Parksons Packaging, TCPL, ITC PSPD and Huhtamaki-type converters use B1/VLF sheetfed offset with inline UV coating for pharma, FMCG and liquor cartons.
• Security and government printing: SPMCIL units and India Security Press, Nashik, combine offset with intaglio and numbering for stamps and documents.
• Commercial and export print: brochures, annual reports and catalogues, usually ISO 12647-2/PSO or G7 aligned with FOGRA/G7 proofing for overseas customers.
• Job roles: prepress/CTP operator, colour management specialist, machine minder and press operator, quality and process-control engineer, production planner, technical service engineer for ink, plate and press suppliers.

Interview questions

• Q: Why is the process called "offset" lithography? A: Because the inked image is first transferred (offset) from the plate to a rubber blanket and only then to the substrate, instead of printing directly from plate to paper.
• Q: What makes the image and non-image areas of an offset plate behave differently? A: The image coating is oleophilic and water-repellent, while the grained, anodised aluminium background is hydrophilic and holds a thin fountain-solution film that rejects ink.
• Q: Why is a blanket used rather than printing straight from the plate? A: Its compressible rubber surface conforms to paper roughness, allows lower pressure, protects the hard plate from abrasion and so greatly extends plate life.
• Q: What is ink–water balance and how do you judge it on press? A: Running the least dampening that keeps the non-image area clean; judged by clean tints, full solid density, no tint in the water tray and a stable colour bar over the run.
• Q: What is the difference between scumming and tinting? A: Scumming is ink taking in the non-image area because of too little water or a damaged plate; tinting is a faint overall colour caused by ink pigment emulsified into the dampening solution.
• Q: What is dot gain and what are its two components? A: The printed dot reads larger than the file value; mechanical gain from pressure, blanket and ink spread, plus optical gain from light scattering inside the paper.
• Q: Why are different screen angles used for the four process colours? A: Offsetting the angles by 30° (and yellow by 15°) produces a fine rosette instead of a visible moiré pattern.
• Q: Difference between heatset and coldset web offset? A: Heatset uses a hot-air dryer and chill rolls to set solvent-based inks on coated paper; coldset has no dryer and relies on absorption into newsprint.
• Q: What does cut-off mean and what fixes it? A: The printed repeat length on a web press, equal to the plate cylinder circumference (πD); it is fixed unless the press has variable-cut-off cylinders or sleeves.
• Q: How does waterless offset differ, and what is its main operating demand? A: A silicone layer replaces the water film as the ink-repellent background; it needs tightly temperature-controlled inking because ink tack, not water, now controls where ink goes.
• Q: Which standard would you quote for offset process control, and what does it fix? A: ISO 12647-2 — substrate and ink colorimetry, tone value increase by paper type, screen ruling ranges, register and colour tolerances.
• Q: Density of your cyan solid is on target but the print looks dull and flat. What do you check? A: Tone value increase and print contrast on the 75% patch, plus grey balance — filled-in mid-tones from excess pressure, over-emulsified ink or a worn blanket flatten the image even at correct solid density.

Common mistakes

• Calling lithography a relief or intaglio process, or saying the plate image is raised — it is planographic and the separation is chemical.
• Saying the plate prints directly on paper, or getting the image reversal wrong (plate right-reading, blanket wrong-reading, print right-reading).
• Chasing colour with more ink instead of fixing water, pressure or packing; this fills in mid-tones and lowers print contrast.
• Ignoring paper subtraction when computing dot area, or using the wrong densitometer status/filter and then comparing readings across presses.
• Confusing slur with doubling, and scumming with tinting, in fault-diagnosis questions.
• Forgetting undercut and packing in cylinder calculations, or assuming cut-off can be changed on a fixed-cut-off newspaper press.
• Mixing units in production sums — gsm with kg, mm with m, impressions per hour with cylinder rpm — and quoting ink or paper quantities without the spoilage allowance.

Related topics

Flexography and gravure printing · Screen and digital printing · Prepress, colour management and CTP workflow · Printing inks and substrates · Print finishing and packaging converting · Print quality control and ISO 12647 standards