Printing substrates and inks: paper, board, films and ink chemistry

In short

A print job is only as good as the surface it lands on and the fluid that lands on it. This topic covers the three substrate families — paper, paperboard/corrugated, and plastic films and foils — their structure, measurable properties and test methods, and the chemistry of printing inks: pigment, vehicle, solvent and additives, and how each ink dries or cures. It is examined in every Printing Technology syllabus (AICTE/CBCS "Printing Materials" or "Substrates and Inks" paper), and it is the single most practical area in interviews at Manipal Technologies, Thomson Press, Parksons/ITC PSPD, Huhtamaki, Uflex, Jindal Poly, TCPL, Essel Propack, Siegwerk, DIC, Flint and Hubergroup. On the job it decides paper indenting, reel costing, pouch yield, ink mileage and whether a laminate passes a food-contact or shelf-life test.

Explained

Paper and board: how they are made, and the grades that result

• Fibre sources: softwood/hardwood chemical (kraft) pulp — long, strong, bleached or unbleached; mechanical pulp (groundwood/TMP) — cheap, bulky, high lignin, yellows; recycled/deinked pulp — short fibres, lower strength; in India also agro-residues (bagasse, wheat straw) in many mills.
• Wet end: fibres + fillers (ground/precipitated calcium carbonate, china clay — add brightness, opacity, smoothness, cut cost but reduce strength) + internal size (AKD or rosin — controls water penetration) + retention aids, dyes, optical brightening agents (OBAs).
• Machine: headbox → wire (forming, drainage) → press section → dryers → size press (surface starch, raises pick resistance) → calender (gloss and smoothness by pressure+heat) → reel. Coated grades get a blade/film/curtain coating of pigment (clay, CaCO₃) + latex binder, then supercalendering.
• Grain (machine) direction: fibres align along the machine; the sheet is stiffer and stronger along the grain, and expands/curls across it with moisture. Fold parallel to grain; feed long-grain for register stability.
• Paper grades: newsprint (~42–52 gsm, mechanical pulp), maplitho/uncoated woodfree (writing/printing, 54–120 gsm), coated art/chromo (gloss, matt, silk, 70–300 gsm), bond/bank, MG poster, kraft (sack, liner), tissue, speciality (NCR, thermal, metallised paper).
• Boards: FBB/SBB (folding boxboard, bleached, multi-ply — pharma, cosmetics), SBS (solid bleached sulphate), SUS/kraftback (unbleached back — high strength), WLC/duplex and triplex (white-lined chipboard from recycled stock — the Indian mono-carton workhorse), greyboard (rigid boxes).
• Corrugated board: liner + fluted medium + liner. Flute profiles: A (~4.7 mm, cushioning), C (~3.6 mm, general purpose), B (~2.4 mm, crush resistance and print), E (~1.5 mm) and F/N micro-flutes (retail printed boxes). Constructions: single-face, single-wall, double-wall, triple-wall. Adhesive is starch; flute orientation must be vertical in the box wall to carry stacking load.

Paper and board properties, and how they are tested

• Grammage (GSM) — mass per unit area, g/m² (ISO 536). The basis of all paper costing in India. Caliper/thickness in µm or mm (ISO 534). Bulk = caliper/grammage (cm³/g); density is its reciprocal.
• Moisture content 4.5–7% typical; too dry → static and brittleness, too wet → wavy edges, poor ink set. Paper must be conditioned to press-room RH (about 50–55% RH) before printing.
• Strength: tensile and TEA, bursting strength (Mullen, kgf/cm²), tear (Elmendorf, mN), folding endurance (double folds), stiffness (Taber, mN·m) for cartons, ring crush/ECT (edgewise crush, kN/m) for corrugated, SCT short-span compression.
• Surface: smoothness/roughness (Bendtsen or Sheffield ml/min, PPS µm), gloss (75°), porosity/air permeance (Bendtsen or Gurley), surface strength — IGT pick test and wax pick (picking and linting cause hickeys and piling).
• Absorbency: Cobb value = water absorbed in g/m² in 60 s (Cobb₆₀) — critical for offset dampening, gluing and corrugation; low Cobb (20–30) for sized liners.
• Optical: ISO brightness (diffuse blue reflectance at 457 nm, ISO 2470), whiteness (CIE), opacity (ISO 2471 — printing opacity matters for show-through in duplex printing), shade.
• Chemical: pH of surface/extract (acidic paper gums offset blankets and ages badly), ash content (filler), chlorine-free status.
• Test standards: ISO/TAPPI methods; in India the IS 1060 series, "Methods of sampling and test for paper and allied products" (several parts) is the reference, with IS 4661 glossary of paper terms and corrugated-box specifications under IS 2771 (corrugated fibreboard boxes). Where an old IS number has been withdrawn, specify by subject plus the ISO method number in the purchase order.

Plastic films, foils and laminates

• Films by polymer:
  • LDPE/LLDPE — cheap, tough, excellent heat-seal and moisture barrier, poor gas barrier; the sealant layer of most pouches.
  • HDPE — stiffer, better moisture barrier, paper-like (MDO-HDPE).
  • PP: CPP/cast (seal layer, clarity), BOPP (biaxially oriented — stiff, clear, good moisture barrier, printed reverse or surface; the commonest Indian snack-pack face film), pearlised and matt BOPP.
  • BOPET/polyester — high strength and heat resistance, good printability, moderate barrier; metallised PET (MET-PET) gives a near-total light, moisture and oxygen barrier at low cost.
  • PA/Nylon — toughness and puncture resistance, good oxygen barrier but hygroscopic; used for vacuum and thermoform packs.
  • EVOH and PVDC — outstanding oxygen barrier; EVOH loses barrier when wet, so it is buried between PE/PP layers.
  • Aluminium foil (7–40 µm) — absolute barrier above ~20 µm; pinholes and flex-cracking below.
  • Compostable/bio — PLA, PBAT, starch blends; mechanically weaker, heat-sensitive.
• Barrier metrics: WVTR (g/m²·day at 38 °C/90% RH) and OTR (cc/m²·day at 23 °C, 0% RH). Both are inversely proportional to thickness, so quote permeability = WVTR × thickness to compare materials.
• Film properties examined: thickness and gauge variation, yield (m²/kg), tensile and elongation, COF (slip, for machinability), haze and gloss, shrink, seal initiation temperature, hot tack, dart impact.
• Surface energy and treatment: polyolefins are non-polar (~29–31 dyne/cm) and reject ink and adhesive. Corona (or flame) treatment oxidises the surface to 38–44 dyne/cm; measured with dyne pens/test inks. Treatment decays with time and heat — print within the supplier's validity window.
• Laminate construction: adhesive lamination (solvent-based, solventless or water-based polyurethane adhesive, 1.5–4 g/m²; needs a curing period before slitting), extrusion lamination (molten PE as both adhesive and layer), extrusion coating, thermal/wax lamination. Printing is usually reverse-printed on the inner face of the outer film so ink is protected and gloss is high.
• Typical structures: BOPP/BOPP for biscuits; PET/MET-PET/PE for snacks; PET/Al foil/PE for retort and pharma; paper/PE for sugar and sachets; PE/PE mono-material for recyclability under EPR.

Ink chemistry: what is in the can

• Pigment (6–25%) — the colorant; insoluble particles dispersed in the vehicle. Organic: diarylide yellow (PY12/13), lithol rubine (PR57:1), phthalocyanine blue (PB15:3) and green; inorganic: carbon black (PBk7), titanium dioxide white, iron oxides; plus metallics (Al, bronze) and fluorescents. Pigments are chosen for shade, strength, lightfastness (Blue Wool scale), transparency (needed for process trapping), and chemical/alkali/soap resistance. Dyes (soluble) are used mainly in inkjet and some flexo for brightness, at the cost of lightfastness.
• Vehicle/binder (resin system) — carries and anchors the pigment: rosin-modified phenolics, alkyds and drying oils (litho), nitrocellulose, polyurethane, polyamide, acrylic (liquid inks), oligomers+monomers (UV/EB).
• Solvent/diluent — controls viscosity and drying: mineral/vegetable oils in paste inks; ethyl acetate, ethanol, isopropanol, n-propyl acetate in gravure/flexo; water in water-based flexo (water + co-solvent + amine); UV inks are nominally solvent-free (the monomer is the diluent).
• Additives (1–10%) — driers (cobalt/manganese soaps, for oxidation), waxes (slip, rub resistance, anti-set-off), plasticisers, anti-oxidants (to stop skinning), defoamers, surfactants/wetting agents, extenders and anti-static agents.
• Dispersion matters: ink is made by mixing then milling (bead mill, three-roll mill) to break agglomerates; poor dispersion shows as low colour strength, grittiness, filter clogging and gravure doctor-blade lines.
• Process-ink colorimetry is standardised by ISO 2846-1 (pigment colour and transparency of the four process inks); the printed targets belong to ISO 12647 and are handled in colour management.

Ink by process: drying, curing and rheology

• Drying mechanisms:
  1. Absorption/penetration — vehicle soaks into porous stock (newsprint, coldset web offset).
  2. Oxidative polymerisation — drying oils crosslink with atmospheric O₂, catalysed by driers; slow (hours), used in sheetfed offset; needs anti-set-off spray powder.
  3. Evaporation — solvent or water leaves in a hot-air dryer; gravure and flexo, speeds up to 600 m/min.
  4. Thermal/heatset — oil evaporates in a dryer oven then chill rolls set the film (heatset web offset, magazines).
  5. Radiation cure — UV: photoinitiators (e.g. acylphosphine oxides) absorb UV, generate free radicals, polymerise acrylate monomers/oligomers instantly, zero solvent; EB: electron beam cures without photoinitiators. Also UV LED (395 nm) and cationic UV (epoxide, continues curing in the dark).
  6. Precipitation/cold-set of water-based resins as pH drops (amine evaporates).
• Rheology:
  • Paste inks (offset, letterpress, screen) are high viscosity (roughly 10–100 Pa·s), pseudoplastic and thixotropic, and have a yield value — they must stand in the duct and still flow on the roller train. Measured on a falling-rod (Laray) viscometer or cone-and-plate.
  • Tack (resistance to film splitting) is measured on an Inkometer in arbitrary tack units. For wet-on-wet multicolour printing use a decreasing tack sequence (first-down ink highest) or the later ink will not trap. Excess tack causes picking and blanket piling; too little causes poor trap and misting/flying.
  • Liquid inks (flexo, gravure, inkjet) are 50–500 mPa·s, near-Newtonian, run at a flow-cup reading — typically 16–22 s in a DIN 4/Zahn 2 cup for gravure, 18–25 s for flexo; viscosity is topped up with solvent continuously, ideally by an automatic viscosity controller.
  • Inkjet inks are 2–20 mPa·s with tight surface tension (28–35 mN/m) for stable drop formation; dye- or pigment-based, aqueous, solvent, UV or latex.
• Ink–process map: offset = paste, oxidative/heatset, needs water tolerance and no emulsification; flexo = liquid, evaporative, soft plate, high solvent/water tolerance; gravure = liquid, evaporative, cell-engraved cylinder, needs low viscosity and fast solvent release; screen = high-viscosity paste or UV, thick film (8–25 µm) for opacity; digital = toner (fusing) or inkjet.

Substrate–ink interaction: printability and runnability

• Printability = how well the surface accepts and holds ink (smoothness, absorbency, surface energy, gloss, ink holdout). Runnability = how well it passes through the press (strength, moisture, dimensional stability, cleanliness, flatness, curl).
• Wetting rule: ink/adhesive wets a surface only if the substrate's surface energy exceeds the liquid's surface tension by roughly 7–10 dyne/cm — hence the 38 dyne/cm minimum on films.
• Offset specifics: paper pH should be near neutral (acidic coatings retard drying); adequate pick resistance; low Cobb to limit fibre swelling; ink–water balance — over-emulsified ink loses density, tints and dries slowly. Absorbent uncoated stock gives higher dot gain and lower gloss than coated.
• Ink holdout: a coated sheet keeps the pigment on the surface, giving high density with little ink; an uncoated sheet absorbs the vehicle, so solids look weak and need more ink.
• Film specifics: check treat level, use primers/anchor coats for difficult inks, match solvent blend to drying capacity (retained solvent gives odour and failed migration tests), and allow adhesive cure before slitting and seal testing. Adhesion checked by tape/cross-hatch pull, bond strength by T-peel on a tensile tester.
• Common defects and their substrate/ink cause: set-off and blocking (slow drying, too much ink), chalking (vehicle absorbed, pigment left loose), picking (weak surface or high tack), mottle (uneven absorbency), ghosting, scumming, hickeys (dust, dried ink skin), banding/doctor streaks (poor dispersion), poor rub resistance (no wax, under-cure), curl (moisture or one-side wetting), static (dry paper or film).

Standards, safety and Indian regulation

• Food contact: the Food Safety and Standards (Packaging) Regulations, 2018 require packaging materials to conform to the relevant Indian Standards — for example polyethylene and polypropylene for food contact (IS 10146 and IS 10910 respectively) — and restrict recycled plastic and printed surfaces in direct food contact. Printing ink must not be in contact with food, and printed material must comply with the standard on printing inks for food packages; low-migration UV inks, toluene- and ketone-free gravure inks and controls on mineral oil (MOSH/MOAH) are now normal for Indian food work.
• Heavy metals: the industry norm (EU toy/packaging practice followed by Indian exporters) is a sum of lead, cadmium, mercury and hexavalent chromium below 100 ppm; the EuPIA exclusion list governs raw materials.
• Labelling: declarations on retail packs follow the Legal Metrology (Packaged Commodities) Rules, 2011 as amended (name, net quantity, MRP, consumer-care details, date); medicines follow the Drugs Rules, food the FSS labelling regulations. Carton/laminate artwork must leave space and contrast for these and for the barcode/QR.
• Plastic waste: the Plastic Waste Management Rules, 2016 as amended ban identified single-use plastic items (from 1 July 2022), set a minimum 120-micron thickness for plastic carry bags, and impose EPR with registration on the CPCB portal and recycled-content/recyclability obligations — the reason brands are converting multi-material laminates to mono-material PE or PP.
• Workplace: solvent storage and handling fall under the Petroleum Act/Rules (flash-point classes) and fire norms; occupational safety, ventilation, LEL monitoring and PPE in ink and lamination areas are governed by the Occupational Safety, Health and Working Conditions Code, 2020, in force from 21 November 2025 (it replaced the Factories Act). Solvent recovery and VOC limits come under the Air (Prevention and Control of Pollution) Act via State Pollution Control Board consent conditions. GST on printing inks is 18% and plastic packaging films and articles are at 18%, while paper and paperboard cartons and corrugated boxes attract 5%, under the 5/18/40% structure effective 22 September 2025.
• Quality systems: ISO 9001:2026 is the current quality-management edition; BRCGS/FSSC 22000 packaging certifications are customer-driven (voluntary, but contractually required by most FMCG and pharma buyers).

Key points

• Grammage - mass per unit area in g/m²; the commercial basis of paper (ISO 536), independent of thickness.
• Caliper and bulk - thickness in µm; bulk = caliper/grammage (cm³/g); high bulk means a thick, stiff but light board.
• Grain direction - fibre alignment; stiffer along grain, dimensional change and curl across it; fold along grain.
• Cobb value - water absorbed in g/m² in 60 s; low for sized liners and offset stock, high for corrugating medium.
• Bursting strength and burst factor - Mullen strength in kgf/cm²; burst factor = burst × 1000/GSM, normalises for weight.
• ECT - edgewise crush of corrugated board (kN/m); the single best predictor of box stacking strength.
• ISO brightness vs whiteness vs opacity - blue reflectance at 457 nm; perceived neutral white; resistance to show-through.
• Coated vs uncoated - coating gives smoothness, gloss and ink holdout, so less ink and lower dot gain.
• Film yield - m²/kg = 10⁶/(thickness in µm × density in kg/m³); the basis of flexible-packaging costing.
• WVTR/OTR - moisture and oxygen transmission; inversely proportional to thickness; metallised PET, foil and EVOH are the barrier layers.
• Corona treatment - raises film surface energy to 38–44 dyne/cm so ink and adhesive wet it; decays with time.
• Ink = pigment + vehicle + solvent + additives - pigment gives colour, vehicle gives adhesion and gloss, solvent gives flow, additives give rub, slip and drying.
• Six drying routes - absorption, oxidation, evaporation, heatset, UV/EB cure, precipitation.
• Paste vs liquid ink - 10–100 Pa·s thixotropic with a yield value (offset) against 50–500 mPa·s near-Newtonian (flexo/gravure).
• Tack sequence - print in decreasing tack order for wet-on-wet trapping; excess tack picks the sheet.
• Ink mileage - area covered per kg; follows from wet film thickness and ink density, not from guesswork.
• Printability vs runnability - ink acceptance vs trouble-free passage through the press; both are specified when indenting paper.
• Food-contact rule - ink never touches food; reverse-print or use low-migration, toluene-free inks and comply with the FSS Packaging Regulations, 2018.

Formulas and examples

Example 1 — paper indenting and cost. A brochure job needs 20,000 printed sheets of 100 gsm coated art paper, sheet size 64 × 90 cm; allow 5% spoilage. Price ₹95/kg.

• Sheet area = 0.64 × 0.90 = 0.576 m²; mass/sheet = 0.576 × 100 = 57.6 g.
• Sheets to buy = 20,000 × 1.05 = 21,000.
• Mass = 21,000 × 57.6 g = 1,209,600 g = 1,209.6 kg.
• Cost = 1,209.6 × 95 = ₹1,14,912. Ream (500 sheets) weight = 500 × 57.6 = 28.8 kg.

Example 2 — reel length and board bulk. (a) A newsprint reel of net mass 450 kg, 68 gsm, width 865 mm.

• L = 450 / (0.068 × 0.865) = 450 / 0.05882 = 7,650 m.

(b) A folding boxboard is 300 gsm with caliper 0.400 mm.

• b = 0.0400 cm / 0.0300 g/cm² = 1.33 cm³/g; density = 0.75 g/cm³ — a bulky, stiff carton board.

Example 3 — pouch yield and film cost. Structure: BOPET 12 µm (ρ 1,400) + adhesive 2 g/m² + metallised… use MET-PET 12 µm already counted; here BOPET 12 / adhesive 2 gsm / LDPE 50 µm (ρ 922). Pouch 150 × 220 mm, three-side seal, 10% trim and process waste. Laminate price ₹230/kg.

• BOPET grammage = 12 × 1400/1000 = 16.8 g/m²; LDPE = 50 × 922/1000 = 46.1 g/m²; + adhesive 2 → total 64.9 g/m² → yield = 10³/64.9 ≈ 15.4 m²/kg.
• Film per pouch = 2 × 0.150 × 0.220 = 0.0660 m²; + 10% = 0.0726 m² → mass = 0.0726 × 64.9 = 4.71 g.
• Cost = 0.00471 × 230 = ₹1.08 per pouch; 1 tonne of laminate ≈ 15,400 m² ≈ 212,000 pouches.

Example 4 — ink mileage for a gravure run. Wet film 2.5 µm, ink density 1,050 kg/m³, print area 50,000 m² at 60% average ink coverage. Ink ₹280/kg.

• Mileage = 10⁶/(2.5 × 1050) = 10⁶/2,625 = 381 m²/kg of solid-inked area.
• Inked area = 50,000 × 0.60 = 30,000 m² → ink = 30,000/381 = 78.8 kg → ₹22,060.
• Sheetfed offset check: at ~1.5 g/m² of solid, 1 kg covers 1/0.0015 = 667 m² of solid area.

Example 5 — corrugated box strength. Board: bursting strength 2.8 kgf/cm², combined grammage 440 g/m², ECT 6.5 kN/m, caliper 4.2 mm. Box 450 × 350 mm, product 12 kg, stacked 5 high.

• Burst factor = 2.8 × 1000/440 = 6.4.
• Z = 2(450 + 350) = 1,600 mm. BCT ≈ 8.59 × 6.5^0.746 × 4.2^0.254 × 1600^0.492 = 8.59 × 4.04 × 1.44 × 37.7 ≈ 1,884 N ≈ 192 kgf.
• Load on the bottom box = 4 × 12 = 48 kgf. Safety factor = 192/48 = 4.0 — acceptable for short, dry storage; for monsoon warehousing at a factor of 6 you need ≈ 288 kgf, so move to a higher-ECT board or double wall.

Example 6 — barrier scaling. A 25 µm BOPP has WVTR 6.0 g/m²·day at 38 °C/90% RH. Permeability = 6.0 × 0.025 = 0.15 g·mm/m²·day. A 20 µm film of the same polymer → WVTR = 0.15/0.020 = 7.5 g/m²·day. To reach 0.5 g/m²·day you would need 0.30 mm of BOPP — impractical, which is why a metallised or foil layer is used instead.

Where it is used

• Publication and commercial printing: paper indenting against GSM, bulk, brightness, opacity and pick resistance — Manipal Technologies, Thomson Press, Repro India, Replika, newspaper groups buying newsprint reels by length and mass.
• Mono-carton and label converting: board selection (FBB for pharma, duplex for FMCG), stiffness and scoring behaviour, UV ink and coating choices — Parksons Packaging, TCPL, ITC PSPD, Avery Dennison, Huhtamaki labels.
• Flexible packaging: film and laminate design against WVTR/OTR and shelf-life, corona treat level, solventless adhesive lamination, toluene-free gravure inks — Uflex, Jindal Poly Films, Cosmo First, SB Packagings, Paharpur 3P.
• Corrugated packaging and e-commerce: liner/medium GSM, burst factor, ECT and McKee-based box design for Flipkart/Amazon stacking and drop specifications.
• Ink manufacture and technical service: dispersion, rheology control, migration and heavy-metal compliance, customer trials — Siegwerk, DIC India, Hubergroup, Flint, Sakata INX.
• Job roles: substrate/QA executive running a paper testing lab, converting process engineer, ink technologist, packaging development engineer in an FMCG or pharma company writing the specification the converter must meet.

Interview questions

• Q: What is GSM, and why is paper bought by GSM rather than thickness? A: GSM is grammage, the mass per square metre; it fixes the amount of material and hence the price and the postal/transport weight, while thickness varies with bulk and calendering for the same GSM.
• Q: How do you find the length of paper on a reel without unwinding it? A: L = net mass / (grammage × width), using consistent SI units — a 450 kg, 68 gsm, 865 mm reel holds about 7,650 m.
• Q: Why does grain direction matter in carton making? A: The board is stiffer along the grain and changes dimension across it, so creases must run parallel to the grain for a clean fold, and sheet feeding direction is chosen to control register and curl.
• Q: What does a Cobb test tell you and where does it matter? A: The water absorbed in g/m² in 60 s; it predicts offset dampening behaviour, gluing and how a corrugated liner will behave in the humid section and in storage.
• Q: Why must a polyolefin film be corona treated before printing? A: Untreated PE/PP has about 29–31 dyne/cm surface energy, below the ink's surface tension, so ink beads up; corona oxidation raises it to 38–44 dyne/cm, which allows wetting and adhesion.
• Q: Name the four components of a printing ink and what each does. A: Pigment gives colour and opacity, the vehicle/resin binds the pigment to the substrate and gives gloss and rub resistance, the solvent controls viscosity and drying, and additives tune slip, wax rub, drying and foam.
• Q: Why is tack sequence important in multicolour wet-on-wet printing? A: Each successive ink must have lower tack than the one beneath, or it will fail to trap and will instead pick the previous layer off the sheet.
• Q: How does a UV ink dry, and what is its main advantage and limitation? A: Photoinitiators absorb UV and crosslink acrylate monomers instantly, giving zero VOC and immediate finishing; limitations are higher cost, lamp energy, possible migration and odour if under-cured, and poor adhesion to some untreated films.
• Q: Two films, same polymer, one 20 µm and one 40 µm — how do their WVTRs compare? A: Transmission is inversely proportional to thickness, so the 40 µm film has about half the WVTR of the 20 µm film; permeability (WVTR × thickness) is the same.
• Q: What is ECT and how does it relate to box compression strength? A: Edgewise crush test is the force per unit width a corrugated board sustains on edge, in kN/m; the McKee equation estimates box compression from ECT, caliper and perimeter, and ECT is the strongest single term in it.
• Q: Why does a solid area look weak on uncoated paper but rich on coated paper at the same ink film? A: Uncoated stock absorbs the vehicle and some pigment into the fibre network (ink holdout is poor), scattering light; a coating keeps the pigment on a smooth surface, giving higher density and gloss.
• Q: What precautions apply to inks for food packaging? A: Keep ink out of direct food contact (reverse print or an inner barrier), use low-migration/toluene- and ketone-free formulations, control retained solvent and MOSH/MOAH, keep heavy metals under 100 ppm, and comply with the FSS (Packaging) Regulations, 2018 and the relevant IS for the food-contact polymer.
• Q: A laminated pouch delaminates in peel test — where do you look first? A: Treat level on the film (dyne check), adhesive coating weight and mix ratio, insufficient curing time, retained solvent, and ink over-coverage or an incompatible ink binder at the bond line.

Common mistakes

• Confusing grammage with thickness, or quoting "GSM" for a film where the trade works in microns and m²/kg — always convert through density.
• Forgetting spoilage, gripper margin and trim when computing sheets or film required, so the indent falls short.
• Mixing units in reel-length and yield formulas (mm with m, g/m² with kg/m², cm³ with m³) — the single commonest numerical error in exams.
• Treating WVTR or OTR as a property of the material alone; they are thickness- and condition-dependent, so always state the gauge and the 38 °C/90% RH or 23 °C/0% RH basis.
• Assuming corona treatment is permanent, or printing on film outside the treat validity window — then blaming the ink for poor adhesion.
• Printing an increasing tack sequence, or expecting an offset ink to work on a non-absorbent film without a UV or treated system.
• Quoting a withdrawn IS number from an old textbook; specify by subject and current edition, or by the ISO/TAPPI test method.
• Saying "ink must be food grade" without the real rule: the ink should not contact the food at all, and the binder/pigment must be migration-compliant.

Related topics

• Colour theory · Colour management and print quality control: densitometry, spectrophotometry and ICC profiles · Offset, flexography and gravure printing processes · Prepress, plate making and imposition · Packaging design, converting and finishing · Print finishing: coating, lamination and adhesives.