Screen and digital printing

In short

Screen printing (serigraphy) is a stencil process in which ink is pushed through the open areas of a tensioned mesh onto the substrate by a squeegee, giving the thickest ink film of any commercial process. Digital printing is a family of non-impact, plateless processes — mainly inkjet and electrophotography (toner) — in which the image is created from a digital file for every single impression, so every copy can differ. Together they cover everything offset, flexo and gravure cannot: short runs, personalised items, textiles, garments, glass, ceramics, signage, labels on demand and three-dimensional or thick substrates. They are examined in university papers on printing processes and in the printing-technology sections of competitive tests, and are the fastest-growing employment area in Indian print — packaging converters, label houses, textile and garment units, signage firms and photo-book/transactional printers all run them.

Explained

Screen printing: principle, screen and stencil

• Principle: a mesh stretched on a frame carries a stencil; ink is flooded over the mesh and a squeegee stroke forces it through the open mesh onto the substrate below. It is a through-printing (porous-plate) process, unlike planographic offset, relief flexo or intaglio gravure.
• Mesh: woven monofilament polyester (nylon for curved work, stainless steel for electronics). Specified as threads per cm (or per inch) with a thread-diameter code: S (small), T (medium), HD (heavy). A 120-34 mesh means 120 threads/cm of 34 µm thread.
• Mesh selection rule of thumb: coarse mesh (24-43 threads/cm) for heavy textile and glitter inks; medium (61-90) for general graphics; fine (120-180) for halftones and fine electronic patterns. Finer mesh = thinner deposit, finer detail.
• Mesh opening and % open area decide ink volume; the theoretical ink volume of a screen is quoted in cm³/m².
• Frame: wood (cheap, warps), aluminium (standard) or self-tensioning. Mesh tension measured in N/cm by a tension meter — typically 18-25 N/cm for polyester graphics screens; low or uneven tension causes misregister and smearing.
• Stencil types: (i) hand-cut / knife-cut film for simple blocks; (ii) direct photo emulsion coated on the mesh with a scoop coater, dried, exposed through a positive, washed out; (iii) indirect (transfer) film exposed and developed separately, then adhered; (iv) direct-indirect (capillary) film — capillary film laminated to wet mesh, giving a flat, controlled stencil thickness.
• Emulsions: diazo (cheap, long exposure), SBQ photopolymer (fast exposure, pre-sensitised) and dual-cure (photopolymer + diazo, best for solvent and UV inks).
• Exposure and development steps: degrease and roughen mesh → coat emulsion (wet-on-wet, print side first) → dry in dark cabinet → expose with metal-halide/UV LED through film positive or CTS (computer-to-screen) imaging → wash out unexposed emulsion → dry → block pinholes → tape frame.
• Exposure control: an exposure calculator or step wedge (e.g. 21-step) fixes the correct time; Rz value (stencil surface roughness) and EOM (emulsion over mesh) thickness, typically 10-25% of mesh thickness, control edge definition and deposit.

Screen printing: machines, squeegee and ink

• Machine types: manual hand table and hinged frame; clamshell; flat-bed semi-automatic (vacuum table, moving carriage); cylinder press (flat screen, rotating impression cylinder — high speed for posters and plastic sheets); carousel / oval multi-colour garment machines (6-12 stations); rotary screen press with a seamless thin nickel cylinder and internal squeegee, used in-line for textiles, labels and wallpaper at high speed.
• Squeegee: polyurethane blade, hardness 55-90 Shore A (65-75 typical). Profiles: square, square-bevel, round, double-bevel. Variables are angle (about 75° to the screen gives thinner deposit; lower angle gives more), pressure, speed and blade sharpness.
• Off-contact distance: a small gap (0.5-2 mm) between screen and substrate so the mesh peels away cleanly behind the squeegee; essential for sharp prints and to avoid smearing.
• Flood bar returns ink across the screen between strokes without printing.
• Inks by drying mechanism: solvent-evaporation (plastisol for textiles is heat-fused, not evaporated), water-based, UV-curable (100% solids, instant cure, no solvent loss), and two-pack catalysed inks for glass and metal.
• Curing: jet/hot-air dryers and conveyor tunnels; plastisol fuses at about 150-160 °C substrate temperature; UV inks cure under mercury-vapour or UV-LED lamps measured in mJ/cm².
• Ink film thickness is the big differentiator: roughly 6-50 µm wet (and up to 100+ µm for heavy textile deposits) against 1-2 µm for offset — giving opacity on dark substrates, high-build and tactile effects, and functional layers (conductive silver, dielectrics).
• Halftone screen printing: possible but limited — use fine mesh, screen ruling usually no more than about one-fourth to one-fifth of mesh threads/cm to avoid moiré between halftone dot grid and mesh; angles are rotated and a 7.5-22.5° offset from mesh direction is used.
• Advantages: any substrate (paper, board, textile, glass, metal, wood, PCB, cylindrical bottles), any size, thick opaque deposit, very low plate/tooling cost, works on thick and rigid materials.
• Limitations: slow, poor fine-detail and halftone reproduction, high ink consumption, labour-intensive, solvent/VOC and screen-washing effluent issues, registration harder on multi-colour work.

Digital printing: definition, families and workflow

• Definition: a plateless, non-impact process in which a digital file drives the imaging device directly, so the image can change from impression to impression — hence variable data printing (VDP) and print on demand (POD).
• Two dominant families: electrophotography (toner/xerography) and inkjet; niche families include thermal transfer / dye sublimation, ionography, magnetography, and elcography (largely historical), plus HP Indigo's liquid electrophotography (ElectroInk) which sits between toner and offset in appearance.
• Workflow: file (PDF/X) → preflight → imposition → RIP (raster image processor: interprets PostScript/PDF, applies colour management and screening, produces the bitmap) → engine → inline finishing. JDF carries job tickets; web-to-print portals feed jobs directly from customers.
• Screening in digital: AM (clustered dot), FM/stochastic (error diffusion) and hybrid; multi-level (greyscale) heads print several drop sizes per pixel.
• Colour: CMYK plus light cyan/light magenta, orange/green/violet or white, clear and metallic for extended gamut; ICC profiles, device-link profiles and G7 / ISO 12647-8 calibration for validation printing.
• Resolution: addressability in dpi (e.g. 600×600, 1,200×1,200); native head resolution plus multi-pass printing gives apparent resolution. Drop volumes in picolitres (pl) — 1-10 pl typical for graphics, 20-80 pl for industrial/single-pass.

Electrophotography (toner printing) step by step

1. Charging: a corona or charge roller lays a uniform charge on the photoconductor drum/belt (organic photoconductor, OPC).
2. Exposure (imaging): a laser scanning unit or LED bar discharges the areas to be imaged — forming a latent electrostatic image.
3. Development: charged toner particles (dry: 5-8 µm polyester/styrene-acrylate with pigment and charge-control agent; or liquid toner ~1-2 µm) are attracted to the latent image.
4. Transfer: toner is transferred to paper directly or via an intermediate blanket/belt (ITB), aided by a transfer corona or bias roller.
5. Fusing: heat and pressure (typically 150-200 °C) melt the toner into the sheet; radiant, roller or belt fusers. Liquid toner uses heated blanket transfer and needs little paper heat.
6. Cleaning and erase: residual toner removed; drum discharged for the next cycle.
7. Characteristics: excellent for 100-5,000 sheet runs, uncoated and coated stock, instant dry, good text; limitations are gloss differential, toner cracking on folds, substrate weight/thickness limits and a visibly different surface from offset.

Inkjet: types, heads and curing

• Continuous inkjet (CIJ): a pressurised stream breaks into drops which are charged and deflected; unused drops are recirculated. Used in coding and marking (batch numbers, MRP, expiry on packaging) and in high-speed single-pass presses.
• Drop-on-demand (DOD):
  • Thermal (bubble-jet): a micro-heater vaporises ink to eject a drop — cheap heads, mostly aqueous inks, head often part of the cartridge.
  • Piezoelectric: a PZT element flexes to eject the drop (bend, push, shear or squeeze mode). Long-life, handles UV, solvent, aqueous and latex inks, grey-scale capable — the industrial standard (Epson, Kyocera, Ricoh, Fujifilm Dimatix, Xaar, Toshiba Tec, Konica Minolta).
  • Electrostatic and acoustic DOD are rare.
• Architecture: scanning (multi-pass) carriage for wide format versus single-pass page-wide arrays for production speed; stitching of heads and nozzle redundancy matter in single-pass.
• Inks: aqueous dye (photo, low light-fastness), aqueous pigment, UV and UV-LED curable, solvent/eco-solvent, latex (resin), dye-sublimation (disperse dyes for polyester), and DTG/DTF for garments.
• Pre-treatment and priming: bonding agents or primers for offset coated stock; corona treatment for plastics; cotton garments pre-treated for white pigment DTG.
• Common defects: nozzle-out streaking (banding), satellite drops, coalescence/mottle, bleed and wicking, head strikes on cockled stock, colour-to-colour latency (drying time between passes).
• Advantages: no plates, no make-ready, true variable data, economical very short runs, prints on 3D/irregular and large formats, low waste.
• Limitations: higher cost per copy on long runs, limited gamut/opacity against screen for spot colours, substrate restrictions and pre-treatment cost, nozzle maintenance, lower speed than web offset.

Comparison, cost crossover and economics

• Cost model: digital has near-zero fixed cost and a high, flat cost per copy; conventional processes have high fixed cost (plates, make-ready, waste) and a low variable cost. The break-even (crossover) quantity is where the two total-cost lines cross — below it digital wins, above it offset/flexo wins. The crossover has moved steadily up with each generation of digital press.
• Screen vs digital for the same job: screen is chosen for opacity, durability, large flood colours and non-flat objects; digital for photographic detail, many colours, personalisation and no tooling. Hybrid working is common — digital CMYK plus a screen-printed spot white or metallic.
• Quality metrics used in both: solid ink density (densitometer), ΔE colour difference (CIELAB, CIEDE2000), dot gain/tone value increase, registration, and durability tests (crock/rub, abrasion, wash fastness, adhesion by cross-hatch tape test).

Indian standards, law and practice

• BIS printing standards: Indian Standards exist for printing inks, printing papers and boards, and for screen-printing mesh and textile printing; several older IS numbers in this area have been revised or withdrawn, so quote the current edition from the BIS catalogue rather than from memory. For process control most Indian commercial printers work to ISO 12647 (offset/other process control), ISO 12647-8 for validation (digital) printing, ISO 15311 for print quality requirements of digital print, and ISO/ICC colour management rather than a national printing standard.
• Quality and environment management: ISO 9001:2026 is the current quality-management edition; ISO 14001 for environment; both voluntary certifications.
• Labour and safety: the four Labour Codes in force from 21 November 2025 — the Code on Wages, the Industrial Relations Code, the Occupational Safety, Health and Working Conditions (OSH&WC) Code 2020 and the Code on Social Security — replaced the former Factories Act, Minimum Wages Act, Industrial Disputes Act and BOCW Act. A screen or digital print unit is governed by the OSH&WC Code for registration, hazardous-process duties, ventilation, PPE and working hours — important where solvents, UV lamps and toner dust are handled.
• Solvents and VOCs: hazardous waste (solvent-laden rags, waste ink, used emulsion remover, spent developer) must go to authorised recyclers under the Hazardous and Other Wastes (Management and Transboundary Movement) Rules; non-hazardous print waste falls under the Solid Waste Management Rules, 2026 (in force from 1 April 2026, replacing the 2016 Rules). State Pollution Control Board consent to establish and operate is required.
• Plastic packaging EPR: screen- and digitally-printed plastic packaging comes under the Plastic Waste Management Rules and EPR registration on the CPCB portal; printing ink on packaging must not use specified heavy metals.
• Food-contact printing: inks for food packaging must comply with FSSAI packaging regulations and are usually specified as low-migration UV or water-based; Swiss Ordinance / EuPIA exclusion lists are followed contractually by exporters.
• GST: printing services and printed material attract GST at the slabs in force since 22 September 2025 (5% / 18% / 40%); job-work printing and the sale of printed matter are classified differently, so the applicable rate depends on whether the content and the physical inputs are supplied by the printer or the customer — confirm the current rate notification for the specific service code.
• UV lamp safety: UV-C and ozone from mercury lamps require extraction; UV-LED avoids ozone. Eye and skin protection is a statutory duty of the occupier under the OSH&WC Code.

Key points

• Screen printing - a stencil on a tensioned mesh; ink forced through open areas by a squeegee; the only major process that prints a thick, opaque film on almost any substrate.
• Mesh count - threads per cm with thread diameter; finer mesh gives finer detail and a thinner ink deposit.
• Mesh tension - measured in N/cm (about 18-25 for graphics polyester); low tension causes misregister, smear and inconsistent deposit.
• EOM - emulsion over mesh thickness, usually 10-25% of mesh thickness; controls edge sharpness and deposit.
• Off-contact - 0.5-2 mm gap letting the mesh snap back behind the squeegee, preventing smearing.
• Squeegee variables - hardness (55-90 Shore A), angle, pressure, speed, profile; steeper angle and harder blade give a thinner deposit.
• Rotary screen - seamless nickel cylinder with internal squeegee, continuous and fast; used for textiles, labels and wallpaper.
• Ink film thickness - screen 6-50 µm (more for high-build) versus offset 1-2 µm, gravure 1-8 µm; the reason screen is used for opacity and functional layers.
• Screen moiré - interference between halftone dot grid and mesh; limit screen ruling to roughly a quarter of mesh threads/cm and offset the angle.
• Digital printing - plateless, non-impact, image regenerated for every impression; enables variable data and print on demand.
• Electrophotography steps - charge, expose, develop, transfer, fuse, clean (six stages on a photoconductor).
• Inkjet classes - continuous (CIJ, charged and deflected drops, recirculated) and drop-on-demand (thermal and piezoelectric).
• Single-pass vs multi-pass - page-wide fixed arrays for production speed; scanning carriage for wide format and lower cost.
• RIP - raster image processor: interprets the PDF, applies colour management and screening, outputs the bitmap for the engine.
• Break-even quantity - the run length where digital's flat per-copy cost equals conventional's fixed plus low variable cost.
• Drop volume - in picolitres; smaller drops and greyscale heads give smoother tones and higher apparent resolution.
• Quality metrics - solid ink density, ΔE (CIEDE2000), tone value increase, registration; ISO 12647-8 and ISO 15311 for digital print.
• Regulatory frame - OSH&WC Code 2020 (in force 21 November 2025), SPCB consent, Hazardous Waste Rules, SWM Rules 2026, EPR for plastic packaging, FSSAI for food-contact inks.

Formulas and examples

Example 1 - open area and ink volume of a screen A 90-48 polyester mesh: n = 90 threads/cm, thread diameter d = 48 µm, mesh thickness t = 0.080 mm.

• Thread pitch = 10,000/90 = 111.1 µm → opening w = 111.1 − 48 = 63.1 µm.
• Open area = (63.1/111.1)² × 100 = (0.568)² × 100 = 32.3%.
• Theoretical ink volume = 0.323 × 0.080 × 1,000 = 25.8 cm³/m².
• With a transfer factor of 0.7, wet film ≈ 25.8 × 0.7 = 18.1 µm.
• With a solvent ink of 35% solids by volume, dry film = 18.1 × 0.35 = 6.3 µm; with a 100%-solids UV ink the dry film stays 18.1 µm — which is why UV gives the high-build effect.

Example 2 - halftone limit and moiré A design with a 40 lines/cm halftone is to be screen printed.

• Required mesh ≈ 4 × 40 = 160 threads/cm minimum (180 is safer at 1/5 ratio: 5 × 40 = 200, so a 180-31 mesh is the practical choice).
• On a 90 threads/cm mesh the safe ruling is 90/4 ≈ 22 lines/cm (about 56 lpi) — a 40 l/cm job on 90 mesh will moiré and the dots will fall into the mesh openings unevenly.

Example 3 - screen shop output and cost A semi-automatic flat-bed press prints a 2-colour poster. Cycle time per colour = 6 s; the job is 4,000 posters; labour ₹120/h for two operators, ink ₹0.45/poster/colour, screen making ₹350 per screen.

• Output per colour = 3,600/6 = 600 impressions/h → 4,000/600 = 6.67 h per colour → 13.33 h for two colours.
• Labour = 13.33 × 2 operators × ₹120 = ₹3,200.
• Ink = 4,000 × 2 × 0.45 = ₹3,600; screens = 2 × 350 = ₹700.
• Total (excluding substrate) = 3,200 + 3,600 + 700 = ₹7,500 → ₹1.875 per poster.

Example 4 - inkjet film thickness from drop volume A UV inkjet head lays 12 pl drops at 600 × 600 dpi, 100% coverage.

• Drops per inch² = 600 × 600 = 360,000; per cm² = 360,000/6.4516 = 55,800.
• Volume per cm² = 55,800 × 12 pl = 669,600 pl = 669,600 × 10⁻⁹ cm³ = 6.70 × 10⁻⁴ cm³.
• Wet film = volume/area = 6.70 × 10⁻⁴ cm³ / 1 cm² = 6.70 × 10⁻⁴ cm = 6.7 µm (and since UV ink is ~100% solids, the dry film is also about 6.7 µm — about one-third of the screen deposit in Example 1).

Example 5 - digital versus offset break-even A 4-colour A4 leaflet. Offset: 4 plates at ₹250 = ₹1,000, make-ready and waste ₹2,200 → fixed ₹3,200; variable ₹1.10 per copy (ink + paper). Digital toner: fixed ₹150 (file setup); variable ₹4.20 per copy (click + paper).

• Break-even N = (3,200 − 150)/(4.20 − 1.10) = 3,050/3.10 = 984 copies.
• At 500 copies: offset = 3,200 + 550 = ₹3,750 (₹7.50/copy); digital = 150 + 2,100 = ₹2,250 (₹4.50/copy) → digital cheaper.
• At 5,000 copies: offset = 3,200 + 5,500 = ₹8,700 (₹1.74/copy); digital = 150 + 21,000 = ₹21,150 (₹4.23/copy) → offset cheaper.

Example 6 - UV cure dose A UV-LED lamp gives 1,200 mW/cm² and the substrate passes under a 60 mm wide cure window at 30 m/min.

• Exposure time = 0.060 m / (30/60 m/s) = 0.060/0.5 = 0.12 s.
• Dose = 1,200 × 0.12 = 144 mJ/cm² — adequate for a thin inkjet film but short of the 400-800 mJ/cm² a thick screen deposit of the same chemistry typically needs, so the screen job must run slower or use more lamps.

Where it is used

• Garment and textile printing: Tiruppur, Ludhiana and Surat units run carousel screen machines with plastisol and water-based inks; rotary screen for continuous fabric; dye-sublimation and DTG/DTF digital for short-run and personalised apparel.
• Labels and flexible packaging: digital label presses (inkjet and liquid-toner) for short-run SKUs and versioning; screen used for high-build varnish, metallic and Braille on cartons and pharma packs; CIJ coders print batch, MRP and expiry under legal metrology packaging rules.
• Signage, display and large format: UV flatbed and roll-to-roll inkjet for hoardings, backlit displays, vehicle wraps and rigid board; screen for long-run outdoor boards, road signs and retro-reflective sheeting.
• Industrial and functional printing: screen-printed conductive silver on membrane keypads, solar-cell front contacts, PCB solder masks and legends, automotive glass defogger lines and tempered-glass borders; screen decoration on bottles, plastic containers and appliance panels.
• Publishing and transactional: digital POD for textbooks, thesis and photo books (short-run offset alternative), personalised statements, exam certificates and security-numbered documents; web-to-print shops for visiting cards and brochures.
• Job roles: screen maker/stencil technician, screen press operator, DTP/prepress operator, RIP and colour-management specialist, digital press operator, large-format production manager, ink technologist and print-quality QC engineer.

Interview questions

• Q: Why does screen printing give a much thicker ink film than offset? A: The deposit is set by the open volume of the mesh and stencil, typically 15-50 cm³/m², whereas offset transfers a thin film split from a roller train, so screen lays 6-50 µm against 1-2 µm.
• Q: What is off-contact and why is it needed? A: A small gap (0.5-2 mm) between screen and substrate so the mesh peels away behind the squeegee instead of dragging through wet ink; it prevents smearing and sharpens edges.
• Q: How do you decide the mesh count for a job? A: From the detail and the deposit needed — coarse mesh for heavy opaque or textile inks, fine mesh for halftones and fine lines, with the halftone ruling kept to about a quarter of the mesh threads per cm.
• Q: What causes moiré in screen-printed halftones and how is it avoided? A: Interference between the halftone dot grid and the regular mesh weave; avoided by using a much finer mesh than the ruling and offsetting the screen angle from the mesh direction.
• Q: What is EOM and why does it matter? A: Emulsion-over-mesh thickness, the stencil layer standing above the mesh; too little gives sawtooth edges and ink leakage, too much gives thick ink and poor detail.
• Q: List the six steps of electrophotography. A: Charging the photoconductor, exposure to form the latent image, development with toner, transfer to the substrate, fusing with heat and pressure, and cleaning/erasing the drum.
• Q: Difference between thermal and piezoelectric inkjet? A: Thermal heads boil ink to form an ejecting bubble and suit aqueous inks with disposable heads; piezo heads flex a crystal to eject the drop, handle UV, solvent and aqueous inks, give greyscale drops and last much longer.
• Q: When would you choose digital over offset for a job? A: Below the break-even run length, when the data varies per copy, when turnaround is hours, or when many versions/SKUs are needed; above break-even with stable content offset is cheaper.
• Q: What does the RIP do? A: It interprets the PDF/PostScript file, applies colour management and screening, and rasterises it into the bitmap and drop/pixel data the engine needs.
• Q: Why is pre-treatment needed in inkjet and DTG? A: Low-viscosity ink spreads or beads on non-absorbent and coated surfaces; a primer, corona treatment or garment pre-treat fixes the colorant in place, preventing bleed and giving adhesion and white opacity.
• Q: Can digital printing replace screen printing in garments? A: Not fully — digital DTG/DTF wins on photographic detail and one-offs, but screen still gives far better opacity, hand-feel options and cost per piece on medium and long runs.
• Q: A screen print on polypropylene is peeling off with tape. What would you check? A: Surface energy (corona or flame treat the PP and verify by dyne test), correct PP-grade ink and hardener, full cure temperature/dose, and that no slip additive or dust is contaminating the surface.
• Q: What is single-pass inkjet and why is it costly? A: Fixed page-wide head arrays print as the web passes once, giving very high speed, but need many heads, precise stitching, nozzle redundancy and recirculation — so capital cost is high.

Common mistakes

• Treating mesh count as an ink-thickness number on its own — deposit depends on mesh count and thread diameter, mesh thickness, EOM and squeegee settings.
• Forgetting the solids content when converting wet film to dry film, or assuming a solvent ink leaves the same thickness as a 100%-solids UV ink.
• Mixing units in open-area and ink-volume sums (µm with mm, cm³/m² with µm) and so being out by a factor of 1,000.
• Printing a fine halftone on a coarse mesh and blaming the film output when the cause is mesh/dot moiré.
• Calling every plateless process "digital inkjet" — electrophotography, dye sublimation and inkjet are different in imaging, ink and application.
• Claiming digital is always cheaper or always better quality; without computing the break-even quantity the answer carries no marks.
• Ignoring curing/fusing in the answer — an uncured UV screen print or an under-fused plastisol fails rub, wash and adhesion tests even if the printing was perfect.
• Quoting repealed law in the safety or environment part (the Factories Act and the 2016 SWM Rules no longer apply; the OSH&WC Code 2020 and the SWM Rules, 2026 do).

Related topics

Offset lithography · Flexography and gravure printing · Prepress, imposition and CTP workflow · Colour management, densitometry and ISO 12647 · Printing inks and substrates · Print finishing, converting and packaging technology