Screen printing · the versatile specialist process
Screen printing (serigraphy) forces ink through a fine mesh screen onto the substrate below. A stencil on the screen blocks ink in non-printing areas; a squeegee draws the ink across the screen, forcing it through the open mesh onto the substrate. Each colour requires a separate screen. Screen printing is the correct process when thick, opaque, and durable ink films are needed on substrates where other processes cannot print, textiles, glass, metal, rigid plastic, ceramics, and circuit boards.
In Indian commercial print, screen printing is primarily used for: point-of-sale displays and signage requiring opaque coverage on coloured or dark substrates; T-shirt and textile printing; glass and bottle decoration; industrial product marking; and specialty packaging with very thick decorative ink effects (glitter, raised inks, UV-cured specialty coatings). It is not used for high-volume commercial printing or standard packaging, offset, flexo, and digital are faster and more cost-effective at those applications.
How screen printing works · mesh, stencil, and squeegee
Mesh count and ink deposit
The mesh count (threads per centimetre or per inch) determines the ink deposit thickness and the fineness of detail achievable. Lower mesh count = larger mesh openings = thicker ink deposit but coarser detail. Higher mesh count = finer mesh = thinner ink deposit but finer detail.
| Mesh count | Ink deposit | Best for |
|---|---|---|
| 32–50 T/cm (80–125 T/inch) | Very thick, 40–80 micron dry film | Heavy glitter inks, thermochromic inks, puff inks, speciality thick coatings, industrial applications |
| 60–77 T/cm (150–195 T/inch) | Thick, 20–40 micron dry film | Opaque white on dark substrates, solid spot colours, textile printing |
| 90–120 T/cm (230–305 T/inch) | Medium, 10–20 micron dry film | Process colour halftone work, detailed graphics, fine line printing |
| 140–165 T/cm (355–420 T/inch) | Thin, 5–10 micron dry film | Very fine detail, fine halftone, high-resolution graphics on smooth substrates |
Screen printing characteristics
- Ink opacity: screen printing deposits significantly more ink than any other process, 10–80 microns dry film compared to 1–3 microns for offset. This makes it the only process that can print a truly opaque white on a black substrate.
- Substrate versatility: screen printing can print on almost any flat or cylindrical substrate, paper, board, fabric, glass, metal, plastic, ceramic, and wood. No other single process covers this range.
- Print speed: slow compared to offset or flexo, manual printing at 100–500 sheets per hour, automatic flat-bed screens at 1,000–3,000 per hour. Not suitable for high-volume applications.
- Setup cost: moderate, one screen per colour, typically ₹500–₹2,000 per screen depending on size and mesh. Economic for short to medium run lengths where the thick ink deposit or substrate versatility of screen printing is required.
Screen printing ink types and key applications in India
| Ink type | Curing | Applications |
|---|---|---|
| Plastisol (PVC-based) | Heat cure 150–165°C | Textile printing, the dominant ink for Indian T-shirt and garment decoration. Opaque, flexible, washfast. Not food-safe. |
| Water-based textile | Heat cure 120–150°C or air dry with accelerator | Eco-friendly textile printing for export garments and organic brands. Lower opacity than plastisol. Growing use in Indian export garment sector. |
| UV screen ink | UV cure (instant) | Paper, board, plastic signage, POS displays. High gloss, excellent chemical resistance. Standard for Indian commercial screen printing on paper and plastic. |
| Solvent screen ink | Air dry / forced air | Metal, glass, some plastics. Chemical-resistant finish. Used for industrial marking and promotional products. |
| Specialty inks | Various | Glitter, metallic, fluorescent, thermochromic (colour-change with temperature), glow-in-dark, puff (raised 3D effect). All used in Indian promotional and novelty printing. |
Food-safe inks · the complete compliance reference for Indian packaging
Food-safe inks are ink formulations that comply with defined limits on the transfer (migration) of ink components into food. The need for food-safe inks arises because conventional printing inks contain components, pigments, binders, solvents, photoinitiators, that may migrate through the packaging material into the food product. If the concentration of migrating substances in the food exceeds regulatory limits, the packaging is non-compliant and the product may require recall.
"Food-safe" is not a single standard, it is a collection of regulatory requirements that vary by country and application. In Indian packaging, the relevant framework is primarily the Food Safety and Standards (Packaging) Regulations under FSSAI, supplemented by international standards (European Council of Europe Resolution AP(2005)2 for printing inks, Swiss Ordinance for food contact materials, and EU Regulation 10/2011 for plastic packaging) which Indian brand owners increasingly use as reference standards even when not legally required.
Ink migration · how it happens and how it is measured
Ink migration occurs through two mechanisms: set-off migration (direct transfer from the printed surface to an adjacent food contact surface during storage of stacked printed materials), and through-migration (diffusion of ink components through the packaging material itself into the food). Set-off is the more common concern for folding cartons; through-migration is the primary concern for flexible films.
Specific migration limits
European regulations (widely used as reference in India) specify overall migration limits of 10 mg/dm² of packaging surface and specific migration limits (SML) for individual substances. For food packaging inks, the practical implication is that all ink components must either be on the positive list of approved substances with confirmed migration below the SML, or the packager must demonstrate through migration testing that the total migration is below 10 mg/dm².
Photoinitiators · the highest-risk ink component
UV-cured inks contain photoinitiators that can migrate through packaging materials. Several common photoinitiators (ITX, isopropylthioxanthone, benzophenone) have been found in food products packaged in UV-printed cartons and are now restricted or banned in food packaging in the EU. For Indian food packaging using UV offset or UV flexo inks, the ink supplier must confirm in writing that all photoinitiators in the ink formulation comply with migration limits for the specific packaging application. This is a mandatory supplier documentation requirement, not optional.
Standard commercial offset inks are not food-safe. For any folding carton or label that is in direct or near-direct contact with food, low-migration (LM) ink formulations must be specified. LM inks use pigments, binders, and photoinitiators (for UV LM inks) that have been selected and tested for low migration potential. Specifying LM inks adds typically 15–30% to ink cost but is non-negotiable for food contact applications. The correct specification procedure is: (1) confirm with the ink supplier that the specific ink set meets the migration requirements for the packaging application; (2) obtain a Declaration of Compliance from the ink supplier confirming compliance with the applicable regulation; (3) archive the Declaration of Compliance with the batch production records.
Food-safe ink compliance in India · practical guidance
| Packaging type | Ink requirement | Documentation needed |
|---|---|---|
| Folding carton, direct food contact inner surface | Food-contact grade ink on inner surface. LM ink on outer surface to prevent set-off migration. | Ink supplier Declaration of Compliance for both inner and outer surface inks. |
| Folding carton, not direct food contact (product in separate barrier packaging inside carton) | LM ink strongly recommended for outer surface. Standard ink on inner surface if no food contact. | LM Declaration of Compliance for outer surface if EU-compliant specification required. |
| Flexible film, reverse-printed (ink between film layers) | LM gravure or flexo ink. Ink sandwiched between laminate layers reduces migration risk but does not eliminate it. | Ink supplier Declaration of Compliance. Migration test data for the specific laminate structure. |
| Direct food contact labels (applied to food surface) | Food-contact grade adhesive and face stock. Ink must comply with food contact regulations, typically ink-free or food-grade digital print required. | Full food contact Declaration of Compliance for all components including adhesive, face stock, and ink. |
| Pharma carton (drug packaging) | LM ink is standard. Pharmaceutical packaging board requirements (Schedule M) require virgin board, ink must also comply with Schedule M requirements. | Ink supplier GMP Declaration. Certificate of Analysis confirming absence of restricted substances. |
Offset press chemistry · the complete consumables reference
Press chemistry encompasses all the chemical consumables used in offset printing beyond ink and paper: fountain solution, roller wash (blanket and roller cleaning solvents), plate chemistry, anti-setoff powder, and specialty additives. Together these chemicals maintain the press in correct operating condition and directly affect print quality, ink drying, and press longevity. Selecting and managing press chemistry correctly is a significant operational and quality management responsibility that receives insufficient attention in many Indian press rooms.
Roller wash and blanket wash · selection, safety, and environmental compliance
Roller wash (also called blanket wash or press wash) is the solvent used to clean ink from the rubber blanket, impression cylinder, ink rollers, and ink ducts between jobs or during maintenance. The solvent must dissolve the ink binder effectively, evaporate quickly enough not to contaminate the next job, and be compatible with the rubber rollers and blankets, some solvents swell or degrade rubber over time.
Wash types
| Wash type | Solvency | Safety | Best for |
|---|---|---|---|
| Hydrocarbon (petroleum-based) wash | Excellent for oil-based inks | Flash point typically 55–70°C. Fire risk if stored incorrectly. | Standard conventional offset inks. The most common press wash in Indian press rooms. Cost-effective. |
| Vegetable oil-based (soy, sunflower) wash | Good, slower than hydrocarbon but very effective | Very high flash point (>150°C). Very low VOC. Safe in press room. | Eco-conscious press rooms, FSC-certified printers, food packaging applications. Growing use in Indian commercial print. |
| UV press wash | Formulated specifically for UV inks, standard hydrocarbon wash is less effective on UV ink binder systems | Variable, check specific product SDS | Any press running UV inks. UV inks require UV-specific wash for complete removal. |
| Alcohol-based wash | Good for water-based inks (flexo, gravure) | Low flash point if IPA-based. Use only with adequate ventilation. | Flexo and gravure press cleaning for water-based and alcohol-based inks. |
Environmental compliance for press wash in India
Press wash solvents are classified as hazardous waste under India's Hazardous and Other Wastes (Management and Transboundary Movement) Rules 2016. Used press wash (solvent contaminated with ink) cannot be disposed of via municipal drains or waste disposal. Press rooms must: store used wash in labelled closed containers; engage an authorised hazardous waste disposal contractor registered with the State Pollution Control Board for collection and treatment; and maintain records of disposal as required by CPCB regulations. Non-compliance is subject to significant penalties under the Environment Protection Act.
Plate chemistry · processing, gumming, and plate care
CTP plate processing chemistry
Modern Computer-to-Plate (CTP) thermal plates used in Indian press rooms are processed using either chemical development or are processless (requiring no chemistry). Chemically processed plates use an alkaline developer to remove the unexposed coating from non-image areas, leaving the photopolymer image coating in image areas. The chemistry must be maintained at the correct concentration, temperature (typically 22–25°C), and replenishment rate to produce consistent plate quality.
| Parameter | Target | Effect of deviation |
|---|---|---|
| Developer concentration | Per manufacturer specification, typically measured by conductivity or titration | Under-concentration: incomplete image development, soft dot edges, low print run length. Over-concentration: aggressive development attacking image areas, reduced plate life. |
| Developer temperature | Typically 22–25°C | Temperature above 28°C: over-development risk. Below 18°C: under-development. Indian summer conditions require temperature control in the plate room. |
| Developer replenishment | Per manufacturer specification based on plate area processed per day | Under-replenishment: developer exhaustion, inconsistent plate quality. Over-replenishment: increased chemistry cost with no quality benefit. |
Plate gumming
After processing, offset plates are coated with a thin layer of gum (typically gum arabic or synthetic equivalent) to protect the non-image areas from ink adhesion during storage and press standstills. Plates that are stored ungummed, or that stand for extended periods without gumming, may develop scumming from the unprotected non-image surface oxidising and becoming ink-receptive. Always gum plates immediately after processing and before storage. Regum a plate that has been standing for more than 2 hours on the press without printing.
Anti-setoff spray powder
Anti-setoff spray powder is applied to the printed sheet surface in the delivery unit to prevent freshly printed sheets from sticking together (blocking) in the delivery pile. The powder creates a microscopic air gap between sheets, preventing ink-to-ink contact while the ink dries. Standard specifications: particle size 15–20 micron for coated paper, 20–30 micron for uncoated. Application rate: typically 3–8 grams per 1,000 sheets depending on ink coverage. Too little powder: setoff and blocking. Too much: visible powder on the printed surface and contamination of the press environment.