Plastics processing and injection moulding

In short

Plastics processing converts polymer granules or powder into finished articles by heating them into a soft or molten state, forcing them to take the shape of a die or mould, and cooling or curing them to hold that shape. Injection moulding is the dominant process by value and the most examined: a reciprocating screw melts the polymer, injects it into a closed mould under high pressure, holds it while it cools, and ejects the part - often in under a minute, in millions of cycles. It carries marks in Manufacturing Technology and Polymer Engineering papers and appears in GATE-ME production questions on shot volume, clamping force and shrinkage. Indian employers who test it include Maruti Suzuki, Tata Motors, Bajaj, Whirlpool, Havells, Supreme Industries, Nilkamal, Cipla and Sundaram Auto Components, plus the tool rooms of CIPET, IGTR and Indo-German Tool Rooms.

Explained

Polymers: what the material does to the process

• Thermoplastics soften on heating and harden on cooling, reversibly - they can be reground and reprocessed. Examples: PP, PE (LDPE/HDPE), PS, ABS, PVC, PA (nylon), PC, POM, PMMA, PET.
• Thermosets cross-link irreversibly on curing (heat plus pressure, or a catalyst); they cannot be remelted. Examples: phenol-formaldehyde (bakelite), urea- and melamine-formaldehyde, epoxy, unsaturated polyester, silicone rubber.
• Elastomers are lightly cross-linked polymers with very large reversible strain - natural rubber, SBR, EPDM, nitrile, butyl.
• Amorphous polymers (PS, ABS, PC, PMMA) have randomly coiled chains, soften gradually above the glass transition temperature Tg, are usually transparent and shrink little (0.3-0.8%).
• Semi-crystalline polymers (PE, PP, PA, POM, PET) have ordered regions, melt sharply at Tm, are opaque and tough, and shrink much more (1.0-2.5%) because crystallising chains pack closer.
• Thermoplastic elastomers (TPE/TPV/TPU) behave like rubber in use but are not cross-linked, so they are injection-mouldable - used for over-moulded grips and seals.
• Additives: plasticisers (flexible PVC), fillers (talc, CaCO₃, glass fibre - raise stiffness, cut shrinkage), stabilisers (UV, heat), flame retardants, colour masterbatch, lubricants, blowing agents, impact modifiers.
• Melt flow index (MFI), grams extruded in 10 minutes through a standard die at a set temperature and load (ISO 1133 / ASTM D1238), is the shop-floor measure of flowability - high MFI means thin, easy-filling melt for thin-wall parts; low MFI means high molecular weight and better strength.
• Polymer melts are non-Newtonian and pseudoplastic (shear-thinning): viscosity falls as shear rate rises, which is why thin gates fill despite their small area. They are also viscoelastic - stored elastic strain causes die swell in extrusion and frozen-in stress in mouldings.
• Hygroscopic polymers (PA, PC, PET, ABS, PMMA) must be dried before moulding (typically 2-4 h at 80-120 °C, grade-specific; PET needs 150-170 °C for 4-6 h); moisture causes splay marks and, in PC/PET, hydrolytic chain scission that permanently lowers strength.

Injection moulding machine and the cycle

• Three units: the injection (plasticising) unit - hopper, barrel with heater bands, reciprocating screw, non-return valve, nozzle; the clamping unit - fixed and moving platens, tie bars, toggle or hydraulic clamp; the control unit - temperature, pressure, position and time control, now largely servo-electric or hybrid.
• The reciprocating screw both rotates (to melt and meter) and slides axially (to act as a plunger) - the key advance over the older plunger machine, which heated poorly and gave inconsistent shots.
• Screw zones: feed (deep flights, conveys solid granules), compression/transition (flights shallow, melting by conduction plus shear), metering (shallow, homogenises and pressurises). Compression ratio of a screw = feed channel depth ÷ metering channel depth, typically 2:1 to 3:1. L/D is usually 18:1 to 24:1.
• The cycle, in order: 1) mould closes and clamps; 2) injection (filling) - screw moves forward, melt fills the cavity at high speed under velocity control; 3) packing/holding - switchover to pressure control, more melt is pushed in to compensate shrinkage until the gate freezes; 4) cooling with the screw rotating to plasticise the next shot (recovery) - this runs in parallel and is usually the longest phase, 50-80% of cycle time; 5) mould opens; 6) ejection by pins/sleeves/stripper plate; 7) mould closes again.
• Switchover (V-P transfer) from velocity to pressure control at roughly 95-98% cavity fill is the single most important setting for consistency: too late gives flash, too early gives short shots.
• Cushion: a small melt pad (3-10 mm of screw stroke) left in front of the screw at the end of holding, so the screw can still transmit packing pressure. Zero cushion means no holding and sunken parts.
• Back pressure (resistance to screw retraction during recovery, typically 5-20 bar hydraulic) improves melt homogeneity and colour dispersion, but too much degrades the polymer and lengthens recovery.
• Machines are rated by clamping force in tonnes (common Indian range 50 t to 1,300 t, e.g. Electronica, Windsor, Ferromatik Milacron India, Toshiba, Haitian) and by shot capacity in grams of polystyrene.

The mould and its elements

• Two halves: the fixed (cavity) half on the injection side carrying the sprue bush and locating ring, and the moving (core) half carrying the ejection system. The joint is the parting line.
• Feed system: sprue (tapered, from nozzle inward, with a sprue puller) → runner (distributes melt; full-round is thermally most efficient, trapezoidal is easier to machine in one plate) → gate (restricted entry that shears and heats the melt, freezes early and allows easy de-gating).
• Gate types: edge/side, submarine (tunnel - self-shearing on ejection), pin-point (3-plate moulds), direct/sprue gate (large parts, leaves a mark), fan, film/flash (flat warp-free parts), diaphragm (concentric round parts), hot-tip.
• Runner systems: cold runner (solidifies with the part, must be regrind - simple, cheap) versus hot runner (manifold and heated nozzles keep melt molten; no runner scrap, faster cycle, better for high volumes, but costlier and harder to colour-change).
• Cooling system: drilled channels, baffles, bubblers and beryllium-copper inserts in deep cores; channel diameter typically 8-12 mm, placed about 1-2 × diameter from the cavity surface and 3-5 × diameter apart. Cooling controls both cycle time and warpage - uneven cooling warps the part.
• Ejection: ejector pins, blade ejectors, sleeve ejectors, stripper plate (for thin-wall containers), air valves. Draft angle of 0.5-3° per side (more on textured surfaces, roughly 1° per 0.025 mm texture depth) is essential for release.
• Venting: shallow slots 0.02-0.05 mm deep at the parting line let trapped air escape; without them the compressed air burns the melt (diesel effect).
• Side cores and slides form undercuts, driven by angle pins, cam slides or hydraulic cylinders; unscrewing cores form internal threads; collapsible cores form internal undercuts.
• Mould materials: P20 pre-hardened steel for medium runs, H13 and hardened tool steels for high volume and abrasive glass-filled grades, stainless 420 for PVC (corrosive HCl), aluminium for prototypes. Mould bases are bought as standards (Hasco, DME, LKM); Indian mould cost for a simple single-cavity tool runs from roughly ₹2-5 lakh, complex multi-cavity hot-runner tools ₹20 lakh and above - indicative, varies with size and source.
• Multi-cavity layouts should be naturally balanced (geometrically symmetric runner paths, e.g. H-pattern for 2ⁿ cavities), otherwise cavity-to-cavity weight varies; artificially balanced runners resize gates instead.

Process variables, defects and design rules

• Key settings: melt temperature (200-300 °C typical, polymer-specific), mould temperature (20-80 °C for commodity, 80-140 °C for PC - stress relief - and for PA/PET, where it sets crystallinity), injection speed, holding pressure and time, back pressure, screw speed, clamp force.
• Injection pressure at the nozzle is commonly 700-1,800 bar; cavity pressure is lower, typically 300-700 bar, and is what clamping force must resist.
• Shrinkage: the mould is cut oversize by a shrinkage allowance. Typical linear values: PS 0.4-0.6%, ABS 0.4-0.7%, PC 0.5-0.7%, PP 1.2-2.0%, HDPE 1.5-3.0%, PA6 0.8-1.5%, POM 1.8-2.2%, glass-filled grades much lower and anisotropic (less along flow than across it, which causes warp).
• Wall thickness should be uniform - typically 1-4 mm (PP 1.5-3, ABS 1.5-3.5). Ribs should be 50-60% of wall thickness (to avoid sink), with 0.5-1° draft; bosses supported by ribs or gussets, not thick solid lumps; generous fillets (inside radius ≥ 0.5 × wall) to cut stress concentration.
• Common defects and causes:
  • Short shot - insufficient material, low pressure/temperature, thin section, poor venting.
  • Flash - clamp force too low, worn parting line, excessive pressure or melt temperature.
  • Sink mark - thick section or rib shrinking inwards; fix by thinning the section, more holding pressure/time, larger gate.
  • Void - internal vacuum bubble in thick sections, same root cause as sink.
  • Weld (knit) line - two melt fronts meeting (around a hole or core); it is a strength-weakening cosmetic line - raise melt and mould temperature, move the gate, add an overflow well.
  • Warpage - differential shrinkage or cooling, or fibre orientation; fix cooling balance and gate position.
  • Burn mark (diesel) - trapped air ignition; add vents, reduce injection speed.
  • Splay/silver streaks - moisture or volatiles; dry the material.
  • Jetting - a snake of melt from a small gate into a large cavity; use a larger or impinging gate, lower initial speed.
  • Flow lines, black specks (degradation/contamination), dimensional variation, sticking in the mould (insufficient draft or over-packing).
• Mould flow simulation (Autodesk Moldflow, Moldex3D, SolidWorks Plastics) predicts fill pattern, weld-line position, air traps, clamp force and warpage before cutting steel - a standard step in Indian tier-1 automotive tooling.

Other plastics processing routes

• Extrusion: continuous melting and forcing through a die to make pipe, profile, sheet, film, wire coating, filament. Die swell makes the extrudate larger than the die. Output is set by drag flow minus pressure flow; downstream sizing, cooling bath, haul-off and cutter complete the line. The base process for PVC pipes (IS 4985), HDPE pipes (IS 4984) and PP-R pipes.
• Blow moulding: a hollow parison or preform is inflated against a mould. Extrusion blow moulding (continuous parison; jerrycans, lube bottles, fuel tanks); injection blow moulding (precision necks, pharma bottles); injection stretch blow moulding (ISBM) - PET preform reheated, stretched axially and blown, giving biaxial orientation - all PET water and soft-drink bottles.
• Compression moulding: a measured charge of thermoset (or SMC/DMC) is placed in a heated open mould and squeezed to shape while it cures - electrical switchgear, melamine crockery, brake pads. Low tooling cost, no gates, minimal flow orientation, but longer cycles.
• Transfer moulding: thermoset charge melted in a pot and transferred through a sprue into a closed cavity - better for thin sections and inserts (semiconductor encapsulation).
• Thermoforming: a heated thermoplastic sheet is drawn over a mould by vacuum, pressure or a plug - disposable cups, trays, refrigerator liners, blister packs. Cheap tooling, thin walls, trim scrap.
• Rotational (roto) moulding: powder in a closed biaxially rotating heated mould fuses on the wall - seamless large hollow items: water tanks (Sintex, Supreme), road barriers, kayaks. Very low mould cost and no residual stress, but long cycles.
• Calendering (PVC sheet and leathercloth), casting (acrylic sheet, PU), reaction injection moulding (RIM) (two reactive liquids mixed and injected - PU bumpers), FRP hand lay-up, pultrusion and filament winding for composites, and welding/joining of plastics (ultrasonic, hot-plate, spin, vibration, solvent, snap-fits).
• Injection moulding variants: gas-assisted (hollow thick sections), multi-shot/two-colour, insert and over-moulding, in-mould decoration/labelling, micro-moulding, structural foam, metal injection moulding (MIM - modified machines running metal powder/binder feedstock, then debound and sintered).

Indian standards, regulation and recycling

• Resin identification codes 1-7 (PET, HDPE, PVC, LDPE, PP, PS, Others) identify the resin for sorting and recycling; marking follows IS 14534.
• Plastic Waste Management Rules 2016, amended several times since: the Plastic Waste Management (Amendment) Rules 2021 banned identified single-use plastic items from 1 July 2022 and raised carry-bag thickness limits; the EPR Guidelines 2022 place Extended Producer Responsibility obligations (collection and recycled-content targets) on producers, importers and brand owners, administered through the CPCB portal. Targets and dates have been revised more than once - check the current CPCB notification before quoting figures.
• Product standards: IS 4985 (uPVC pipes), IS 4984 (HDPE pipes), IS 12701 (rotationally moulded water storage tanks), IS 9845 / IS 10146-10910 series for food-contact plastics, with FSSAI packaging regulations governing food use.
• IS 14534 gives the guidelines for recycling of plastics; IS 17088 covers compostable plastics. BIS certification is now required for several polymer resins under quality control orders.
• CIPET (Central Institute of Petrochemicals Engineering and Technology, Chennai and many centres) is the national training, testing and tooling body; IGTR/IGTRs at Aurangabad, Indore and Ahmedabad train mould designers.
• Recycling: mechanical (sort → shred → wash → pelletise, with regrind usually limited to 10-30% blended with virgin), chemical (glycolysis/pyrolysis), and energy recovery. Thermosets cannot be remelted - they are ground as filler.
• Safety and environment are governed by the Occupational Safety, Health and Working Conditions Code, 2020 (OSH Code, in force 21 November 2025, which repealed the Factories Act, 1948) (machine guarding, interlocks on moving platens) and emissions under the Environment (Protection) Act 1986.

Key points

• Thermoplastic vs thermoset - thermoplastics remelt and are recyclable; thermosets cross-link permanently and cannot be remoulded.
• Reciprocating screw - rotates to melt and meter, then slides forward as a plunger to inject; replaced the plunger machine.
• Screw zones - feed, compression, metering; melting is mostly by shear, not just barrel heaters.
• Cycle - close, inject, pack/hold, cool (longest, 50-80%), open, eject; cooling time scales with the square of wall thickness.
• V-P switchover - change from velocity to pressure control at 95-98% fill; the main lever on part-weight consistency.
• Cushion - residual melt pad that lets holding pressure reach the cavity; never let it fall to zero.
• Gate - the restricted entry; it shears and heats the melt and freezes first, ending the packing phase.
• Hot runner vs cold runner - hot runner gives zero runner scrap and shorter cycles at higher tool cost.
• Clamping force must exceed projected area × average cavity pressure, with a safety margin of about 10-20%.
• Shrinkage - semi-crystalline polymers (PP, PE, POM) shrink far more than amorphous ones (PS, ABS, PC); the mould is cut oversize to compensate.
• Draft angle - 0.5-3° per side, more for textured surfaces; without it the part sticks or scuffs.
• Uniform wall thickness - ribs 50-60% of wall thickness; thick sections cause sinks and voids.
• Weld line - where two flow fronts meet; a weak, visible line, repositioned by moving the gate.
• Venting - 0.02-0.05 mm slots prevent burn marks from adiabatic compression of trapped air.
• Drying - PA, PC, PET, ABS and PMMA are hygroscopic; undried PC and PET suffer permanent hydrolytic degradation.
• Process-to-product map - injection for discrete solid parts, extrusion for continuous sections, blow for hollow bottles, roto for large seamless tanks, thermoforming for thin sheet parts, compression for thermosets.
• Melt is shear-thinning - viscosity drops at high shear, which is why thin gates fill at all.
• EPR under PWM Rules 2016 (amended) - producers, importers and brand owners carry collection and recycled-content obligations via CPCB.

Formulas and examples

Example 1 - Clamping force and machine selection. A rectangular ABS cover 250 mm × 180 mm is moulded in a 2-cavity mould. The runner's projected area is 1,500 mm². Average cavity pressure = 35 MPa.

• Projected area of one part = 250 × 180 = 45,000 mm²; two cavities = 90,000 mm².
• Total projected area = 90,000 + 1,500 = 91,500 mm².
• F = 35 N/mm² × 91,500 mm² = 3,202,500 N = 3,202.5 kN.
• In tonnes: 3,202,500 / 9,810 ≈ 326 t. With a 15% margin ≈ 375 t → select a 400 t machine (the next standard size).

Example 2 - Shrinkage allowance. A PP part must measure 120.00 mm after cooling. Linear shrinkage for the grade is 1.6%.

• Simple rule: L_mould = 120 × 1.016 = 121.92 mm.
• Exact rule: L_mould = 120/(1 − 0.016) = 120/0.984 = 121.95 mm.
• The two differ by 0.03 mm - negligible here, but the exact form matters on long dimensions. If the same part were moulded in ABS at 0.5%, the cavity would be 120.6 mm - a mould is therefore material-specific and cannot be switched from PP to ABS without dimensional loss.

Example 3 - Cooling time and cycle time. A PP box of 2.5 mm wall thickness: melt temperature 230 °C, mould wall 40 °C, ejection temperature 90 °C, α = 7 × 10⁻⁸ m²/s.

• h = 2.5 mm = 0.0025 m; h² = 6.25 × 10⁻⁶ m².
• π²α = 9.8696 × 7 × 10⁻⁸ = 6.909 × 10⁻⁷.
• h²/(π²α) = 6.25 × 10⁻⁶ / 6.909 × 10⁻⁷ = 9.046 s.
• (T_melt − T_mould)/(T_eject − T_mould) = (230 − 40)/(90 − 40) = 190/50 = 3.8.
• (4/π) × 3.8 = 1.2732 × 3.8 = 4.838; ln(4.838) = 1.5765.
• t_c = 9.046 × 1.5765 ≈ 14.3 s.
• If closing, injection, holding, opening and ejection together take 8 s, cycle ≈ 22.3 s. Doubling the wall to 5 mm quadruples cooling to ≈ 57 s and pushes the cycle to ≈ 65 s - which is why wall thickness, not machine speed, governs productivity.

Example 4 - Shot weight, plasticising rate and cost per part. A 4-cavity mould makes PP caps of 6 cm³ each; the cold runner and sprue add 9 cm³ per shot. Solid PP density = 0.905 g/cm³. Cycle time 15 s.

• Shot volume = 4 × 6 + 9 = 33 cm³; shot weight = 33 × 0.905 = 29.87 g.
• Shots per hour = 3,600/15 = 240; parts per hour = 240 × 4 = 960.
• Plasticising rate required = 29.87 g × 240 = 7,169 g/h ≈ 7.17 kg/h - well within a 100 t machine.
• Runner share = 9/33 = 27% of the shot is scrap each cycle; a hot runner would eliminate it. At PP ≈ ₹110/kg and 240 shots/h, runner material = 9 × 0.905 × 240 = 1,955 g/h ≈ 1.955 kg/h ≈ ₹215/h of regrind handling - the usual business case for a hot runner. (Resin prices move with crude; treat ₹110/kg as indicative.)
• Material cost per part = 6 cm³ × 0.905 g/cm³ = 5.43 g = 0.00543 kg × ₹110 ≈ ₹0.60. If machine plus labour plus overhead is ₹900/h, conversion cost per part = 900/960 ≈ ₹0.94, so cost before tooling ≈ ₹1.54 per cap.

Example 5 - Ejection force check (approximate). A PP cup shrinks onto a cylindrical core of 50 mm diameter and 60 mm engagement length. Contact pressure from shrinkage ≈ 2 MPa, coefficient of friction between PP and polished steel ≈ 0.3.

• Contact area = π × 50 × 60 = 9,425 mm².
• Normal force = 2 N/mm² × 9,425 = 18,850 N.
• Ejection force = 0.3 × 18,850 ≈ 5,655 N ≈ 5.66 kN (plus a vacuum term if no air valve is fitted) - which is why deep cores need air poppet valves and adequate draft.

Where it is used

• Automotive: bumpers, dashboards, air-intake manifolds (glass-filled PA), light housings (PC), connectors and clips - tier-1 moulders supplying Maruti Suzuki, Tata Motors, Hyundai and Mahindra at Gurgaon, Chennai, Pune and Sanand.
• Packaging and FMCG: caps and closures, thin-wall containers (Nilkamal, Supreme Industries, Mold-Tek), PET bottles by ISBM, extruded multilayer film.
• Pharmaceutical and medical: syringes, IV components, inhaler bodies and closures, moulded in clean rooms to ISO 13485 and FSSAI/CDSCO requirements (Cipla, Sun Pharma suppliers, HLL Lifecare).
• Electrical and appliances: switches and modular plates (Havells, Anchor, Legrand), fan parts, washing-machine tubs, fridge liners by thermoforming.
• Infrastructure and agriculture: uPVC and HDPE pipes by extrusion (Finolex, Astral, Prince), roto-moulded water tanks (Sintex), drip-irrigation components (Jain Irrigation).
• Job roles: mould designer (UG NX, CATIA, Moldflow), tool-room engineer, process/setting engineer, production and quality engineer, applications engineer with machine suppliers - trained at CIPET, IGTR and polytechnic tool-room courses.

Interview questions

• Q: Why is a reciprocating screw better than a plunger for injection? A: The screw melts by shear as well as conduction, giving a uniform, homogeneous melt and accurate metering, while still acting as a plunger for injection.
• Q: How do you calculate the clamping force needed for a job? A: Multiply the total projected area on the parting plane (parts plus runner) by the average cavity pressure, then add 10-20% margin and pick the next standard machine size.
• Q: What is the cushion and why must it never reach zero? A: A small melt pad left in front of the screw at the end of holding; with zero cushion the screw bottoms out and holding pressure cannot reach the cavity, so parts shrink and sink.
• Q: Why does PP shrink more than ABS? A: PP is semi-crystalline - its chains crystallise on cooling and pack closer, giving 1.2-2.0% shrinkage, while amorphous ABS freezes in a random coil at 0.4-0.7%.
• Q: What causes a weld line and is it only cosmetic? A: Two melt fronts meeting after flowing around a hole or core; it is also a mechanical weakness because the chains do not fully interdiffuse across the interface, especially with glass fibre or pigment present.
• Q: Why is cooling time proportional to the square of wall thickness? A: Heat is removed by conduction through the wall; the solution to the transient conduction equation makes time scale with the square of the conduction path length.
• Q: When would you choose a hot runner over a cold runner? A: High-volume production where the runner scrap, regrind handling and extra cooling time cost more than the higher tool price and the risk of leakage and colour-change difficulty.
• Q: A part shows sink marks on a flat surface opposite a rib - what do you change first? A: Reduce the rib thickness to 50-60% of the wall; only then adjust holding pressure and time, since a geometric cause cannot be fully cured by process settings.
• Q: Why do PC and PET have to be dried, while PP does not? A: PC and PET are hygroscopic and absorb moisture into the polymer; at melt temperature the water hydrolyses the chains, permanently lowering molecular weight and strength. PP absorbs only surface moisture.
• Q: What is die swell and where does it matter? A: Elastic recovery of the melt after leaving a die makes the extrudate larger than the die opening; extrusion and blow-moulding dies are sized smaller to compensate.
• Q: Why can't a thermoset be injection moulded on a standard thermoplastic machine without changes? A: The material cures with heat, so the barrel must be kept cool and the mould hot - the opposite of thermoplastic moulding - otherwise the material cross-links inside the barrel.
• Q: What governs the cycle time of a moulding, and how do you reduce it? A: Cooling time dominates; reduce wall thickness, lower mould temperature within limits, improve cooling-channel layout, use conformal cooling or Be-Cu inserts in hot spots.

Common mistakes

• Applying clamping force to the total surface area or the part volume instead of the projected area on the parting plane.
• Forgetting the runner and sprue in shot volume, then under-sizing the machine's shot capacity.
• Using one shrinkage value for all polymers, or ignoring that glass-filled grades shrink anisotropically and warp.
• Mixing units in the cooling-time formula - putting h in mm with α in m²/s gives an answer a million times too large.
• Treating a sink mark or warpage as a machine-setting problem when the real cause is thick ribs or unbalanced cooling.
• Saying thermosets can be recycled by remelting - they can only be ground as filler or chemically broken down.
• Confusing injection pressure at the nozzle with cavity pressure, and using the nozzle figure in the clamp calculation.
• Omitting draft angle and vents in a mould-design exam answer - both cost marks and, in practice, scrap parts.

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