Mould and die design for plastics

In short

Mould and die design converts a part drawing into the steel tooling that shapes molten polymer: cavity and core geometry (part shape plus shrinkage), a feed system (sprue, runner, gate), cooling channels, venting, ejection and guiding elements. The mould fixes cycle time, part dimensions, weld-line and sink-mark positions and roughly 20–40% of the cost of a moulded part, so it is designed before the machine is selected. It is examined in Polymer Processing and Mould/Tool Design papers (clamping force, number of cavities, shrinkage, cooling time, runner balancing) and in GATE-level numericals on heat and flow. Indian tool rooms and OEM tooling departments — CIPET and the MSME Tool Rooms (Indo-German, Indo-Danish, CTTC), Godrej Tooling, Lucas-TVS, Sundaram Clayton, Nilkamal, Supreme, Wipro Enterprises, Maruti/Tata/Hero vendors — recruit directly on this topic.

Explained

Mould terminology and classification

• Cavity forms the outer surface, core the inner surface; the parting line is where they meet and must be chosen to keep the part free of undercuts and to hide the witness mark.
• Mould base: fixed (stationary) half bolted to the injection platen, moving half with ejector assembly; clamp plates, cavity/core plates, support plate, spacer blocks, ejector plate and retainer.
• Guiding and alignment: guide pillars and bushes for coarse alignment, taper interlocks or dowels for fine alignment (essential for thin walls and shut-offs), register ring to centre the mould on the platen.
• Classification by feed system: *two-plate* (one parting line, gate on the parting line — simplest, cheapest); *three-plate* (separate runner parting line allowing pin-point gating on the part face, automatic runner degating); *hot-runner / runnerless* (heated manifold and nozzles, no regrind, lowest pressure loss, costliest and needs good temperature control); *insulated runner*.
• By cavity layout: single, multi-cavity (balanced series or H/naturally-balanced pattern), family mould (different parts in one mould — only for similar volumes and same material), stack mould (two levels, doubles output on the same clamp).
• By mechanism: split/slider moulds with angle pins or cam for external undercuts, lifters/collapsible cores for internal undercuts, unscrewing moulds for threads, two-shot/overmould and insert moulds.
• Dies (for continuous processes) shape a flowing melt rather than fill a closed volume: flat sheet/T and coat-hanger dies, annular pipe dies (spider, breaker-plate or spiral-mandrel), blown-film spiral dies, profile dies, blow-moulding die heads with parison programming.

Cavity and core design: dimensions, draft and finish

• Shrinkage allowance is applied to every dimension: cavity dimension = part dimension × (1 + S) approximately, or part/(1 − S) exactly for a cavity cut on the part size basis. Typical mould shrinkage: amorphous polymers (PS, ABS, PC, PMMA) 0.3–0.8%; semi-crystalline (PP 1.2–2.0%, HDPE 1.5–3.0%, POM 1.8–2.5%, nylon 0.8–2.0%, glass-filled grades much lower and anisotropic).
• Shrinkage is not isotropic: higher along flow for unfilled crystalline polymers, lower along flow for fibre-filled grades (fibres align and restrain the flow direction) — the cause of warpage in flat parts. Values are grade-specific and are taken from the supplier datasheet (measured by the standard plaque method, ISO 294-4 type tests); "steel-safe" tooling (cut undersize on the part, i.e. extra steel left) allows correction after trials.
• Draft (taper) on all vertical faces: 0.5–1° for polished surfaces, 1–3° typical, 3–5° per side for textured/grained surfaces (about 1° extra per 0.025 mm texture depth). Zero draft means drag marks and high ejection force.
• Uniform wall thickness (typically 1.0–3.5 mm for thermoplastics) avoids sink marks and voids; ribs 50–60% of wall thickness, generous fillets (inner radius ≥ 0.5 × wall) to avoid stress concentration and flow hesitation.
• Flow-length-to-thickness ratio (L/t) limits how far a gate can fill: roughly 100–150 for PP/PE, 80–120 for ABS, 50–100 for PC, lower for filled grades — it decides the number of gates.
• Surface finish, polishing direction (in the draw direction) and texture are specified on the mould drawing; hardness of cavity steel and plating (hard chrome, nickel, PVD) suit the polymer (e.g. corrosion-resistant steel for PVC and flame-retardant grades that release HCl).

Feed system: sprue, runner, gate, cold slug well, vents

• Sprue: tapered 2–4° included, larger diameter away from the nozzle, with a radius seat matching the nozzle; a cold slug well at the sprue end (depth ≈ 1–1.5 × runner diameter) traps the cold plug.
• Runner cross-section: full round is best (lowest surface-to-volume, lowest pressure drop, but needs machining in both halves); trapezoidal/modified trapezoidal when cut in one plate only; half-round and rectangular are inefficient. Diameters typically 4–10 mm, stepped down after each branch (a common rule: d_branch ≈ d_main / N^1/3).
• Runner balancing: naturally balanced (geometrically identical) layouts are preferred so all cavities fill simultaneously; otherwise balance artificially by changing runner diameters or gate sizes (verified by short-shot trials or flow simulation).
• Gate types: sprue/direct gate (large parts, high pressure transmission, big witness); edge/side gate; submarine (tunnel) gate — self-degating; pin-point gate (three-plate); hot-tip/valve gate; diaphragm and ring gate (concentric round parts, avoids weld line); fan and flash gate (flat panels); tab gate (relieves gate stresses in PC/PMMA). Gate land is kept short (0.5–1.5 mm).
• Gate location rules: at the thickest section, feeding from thick to thin, placed so that weld lines and jetting fall in non-critical areas, kept away from appearance and load-bearing features; gate cross-section typically 50–80% of local wall thickness (freezes soon after filling for cushion control).
• Venting: air must escape at the last point of fill and at the end of runners. Vent land depth 0.005–0.02 mm for crystalline/low-viscosity polymers, 0.02–0.04 mm for ABS/PC, 6–12 mm land then relieved to atmosphere. No venting means burn marks, short shots and diesel effect.

Cooling, ejection and mould materials

• Cooling removes 50–80% of the cycle time. Channels: diameter 8–12 mm, centre distance from the moulding surface 1.5–2 × diameter, pitch 3–5 × diameter; series circuits for pressure drop control, parallel for uniformity; baffles, bubblers and heat pipes for cores and deep ribs; beryllium-copper or Ampco inserts for local hot spots; turbulent flow (Reynolds number above about 4,000–10,000) is required — laminar coolant flow destroys the heat-transfer coefficient.
• Mould temperature is set by the polymer (PP/PE 20–50 °C, ABS 50–70 °C, PC 80–110 °C, POM 80–100 °C, PA 60–90 °C); higher mould temperature gives better finish and lower stress but longer cycle.
• Ejection system: ejector pins (cheapest, mark-prone), sleeve/tubular ejectors around a core pin, blade ejectors for ribs, stripper plate or ring for thin-walled cups, air-poppet valves for large covers. Ejection area is spread to keep local stress below the polymer's yield at ejection temperature; return springs, ejector guide bushes, early-return interlocks and a positive-return mechanism prevent collisions.
• Ejection force rises with contact area, shrinkage onto cores, friction and lack of draft; core pins are checked for buckling and ejector plate for deflection.
• Materials: P20 (pre-hardened ~30–34 HRC) for general cavities; H13 and 2344 (hardened 48–52 HRC) for abrasive filled grades; 420/S136 stainless for corrosive polymers and medical optical finish; M300/ESR grades for high polish; mild/EN steel and aluminium alloys (7075, QC-7) for prototype and short-run tools; beryllium-copper for cooling inserts. Indian material specification follows the IS 1570 series for steel designation and the Indian standard for *tool and die steels* (IS 3748, latest revision) — most commercial tool steels are bought to AISI/DIN grades, so cross-refer the mill certificate rather than quoting an IS number you are unsure of.
• Mould strength checks: cavity wall and support-plate deflection under cavity pressure, clamping plate bending, tie-bar/platen size fit, shut-off (pinch) land stresses, and parting-line flash if the projected area exceeds the clamp.

Design for the other processes

• Compression and transfer moulds (thermosets): *flash* type (cheap, cured-flash wastage, low density control), *positive* (full density, exact charge weight needed, hard to vent), *semi-positive* (common compromise), *landed positive*; loading chamber volume from bulk factor of the moulding powder, flash land width 3–6 mm, cull and sprue in transfer moulds, and heating by cartridge/plate heaters instead of cooling.
• Blow moulds: no core — the parison is inflated against the cavity; pinch-off lands squeeze and weld the parison (land width ≈ 0.5–1 × parison thickness), many fine vents or sandblasted/porous surfaces for air escape, generous cooling because cooling is one-sided, and aluminium or beryllium-copper cavities are common.
• Thermoforming tools: single-sided male or female tools with vacuum holes (0.5–1 mm, spaced in corners), draft 2–5°, plug-assist shape and temperature-controlled aluminium.
• Extrusion dies: the land (parallel) section sets the flow pattern; die swell (extrudate larger than the die, typically 10–40%, higher for high molecular weight and short land) and draw-down are compensated by sizing the die, not the product.
• Rotational moulding tools: thin sheet-metal or cast aluminium shells, vented, no pressure — so cost per cavity is low.

Making, proving and costing the tool in Indian practice

• Sequence: part DFM review → mould flow analysis (Autodesk Moldflow, Moldex3D) for fill, weld lines, air traps, warpage → mould layout (2D) and 3D model → material and standard mould-base selection (HASCO/DME/LKM-pattern bases are stocked in India) → CNC roughing, hardening, EDM (sinker and wire) for ribs and sharp corners, jig grinding, polishing → assembly, water and air testing → trial on machine, short shots, dimensional layout, steel-safe correction → PPAP/first-article approval.
• Lead time and cost (indicative, 2026): a single-cavity prototype aluminium tool from an Indian tool room may be ₹1.5–5 lakh; a multi-cavity hardened production tool ₹8–40 lakh; a 32-cavity hot-runner cap mould well above ₹50 lakh. Quoted tooling is capital goods — moulds and dies fall under HSN 8480 and attract GST at the rate notified for that heading (18% under the 5/18/40 slab structure in force since 22 September 2025; confirm against the current notification).
• Compliance: moulding-machine and tool-room safety (guarding, two-hand control, lifting tackle for mould handling, eyebolts rated for mould weight) is governed by the Occupational Safety, Health and Working Conditions Code, 2020, in force from 21 November 2025, which replaced the former Factories Act; moulds for food-contact and toy parts must use approved mould-release and plating so the part meets BIS requirements for the end product.

Key points

• Cavity and core - cavity forms the outside, core the inside; parting line chosen to avoid undercuts.
• Shrinkage allowance - cavity is cut oversize by the grade's mould shrinkage; crystalline polymers shrink several times more than amorphous.
• Anisotropic shrinkage - unfilled crystalline shrinks more along flow; fibre-filled shrinks less along flow — the root of warpage.
• Draft angle - 0.5–3° typical, 3–5° for textured surfaces; zero draft causes drag marks and high ejection force.
• Two-plate / three-plate / hot runner - increasing cost, decreasing runner scrap and pressure loss.
• Naturally balanced runner - identical flow path to every cavity; otherwise balance by runner or gate size.
• Full-round runner - lowest pressure drop per unit volume; pressure drop varies as 1/D⁴, so a small diameter increase helps greatly.
• Gate size and position - 50–80% of wall thickness, at the thickest section, feeding thick to thin, controlling weld-line position.
• Cold slug well and vents - trap the cold plug; vent 0.005–0.04 mm deep at the last-fill point to prevent burns and short shots.
• L/t ratio - flow length ÷ wall thickness limits the reach of one gate (≈100–150 for PP, lower for PC and filled grades).
• Cooling layout - channel diameter 8–12 mm, 1.5–2 D from the surface, 3–5 D pitch, turbulent flow mandatory.
• Cooling time ∝ (wall thickness)² - doubling the wall quadruples the cooling time; the biggest cycle-time lever.
• Clamping force = cavity pressure × total projected area (parts + runner), plus 10–20% margin.
• Ejection - pins, sleeves, blades, stripper plate, air valves; spread the force, check pin buckling, use positive return.
• Mould steels - P20 pre-hardened general purpose, H13 for abrasive grades, 420/S136 for corrosive polymers and high polish, aluminium for prototypes.
• Compression mould types - flash, positive, semi-positive, landed positive; charge weight from bulk factor.
• Die swell - extrudate expands on leaving the die; dies are sized for the product after swell and draw-down.
• Steel-safe design - leave extra steel so dimensions can be corrected after trials; removing steel is easy, adding it is not.

Formulas and examples

Example 1 — Clamping force and machine selection. A 4-cavity ABS cover mould: each part projects 150 cm² on the parting plane, runners project 20 cm² in total, mean cavity pressure 40 MPa.

• A_proj = 4 × 150 + 20 = 620 cm² = 0.0620 m².
• F = 40 × 10⁶ × 0.0620 = 2.48 × 10⁶ N = 2,480 kN.
• Tonnage = 2.48 × 10⁶ / 9,810 ≈ 253 tonne-force; with 15% margin ≈ 291 t → select a 300 t machine (and check mould shut height and tie-bar spacing).

Example 2 — Shrinkage allowance and cavity count. A PP part, nominal length 100.00 mm, grade shrinkage 1.8%; required output 2,00,000 parts/month, cycle 25 s, machine availability 20 h/day × 26 days.

• Cavity length = 100 / (1 − 0.018) = 101.83 mm (the approximate form 100 × 1.018 = 101.80 mm; the 0.03 mm difference matters on tight tolerances).
• Available time = 20 × 26 = 520 h = 1,872,000 s. Shots needed per cavity set = 2,00,000 / n, time = 25 × 2,00,000 / n ≤ 1,872,000 → n ≥ 5,000,000 / 1,872,000 = 2.67.
• Choose 4 cavities (even, symmetric, naturally balanced); utilisation = 25 × 50,000 / 1,872,000 ≈ 67% of available time.

Example 3 — Cooling time and the thickness effect. PP part, wall s = 3 mm, k = 0.22 W/m·K, ρ = 900 kg/m³, c_p = 2,450 J/kg·K, T_M = 230 °C, T_W = 30 °C, T_E = 90 °C.

• α = 0.22 / (900 × 2,450) = 9.98 × 10⁻⁸ ≈ 1.0 × 10⁻⁷ m²/s.
• s²/(π²α) = (0.003)² / (9.8696 × 1.0 × 10⁻⁷) = 9 × 10⁻⁶ / 9.87 × 10⁻⁷ = 9.12 s.
• (4/π) × (230 − 30)/(90 − 30) = 1.273 × 3.333 = 4.244; ln 4.244 = 1.446.
• t_c = 9.12 × 1.446 ≈ 13.2 s. If the wall is increased to 6 mm, t_c = 4 × 13.2 ≈ 52.7 s — four times the cooling, for the same mould.

Example 4 — Runner diameter, pressure drop and gate shear rate. Melt viscosity 200 Pa·s (apparent, at the processing shear rate), total flow Q = 60 cm³/s = 6 × 10⁻⁵ m³/s, main runner length 100 mm.

• D = 6 mm: Δp = 128 × 200 × 6 × 10⁻⁵ × 0.1 / (π × (0.006)⁴) = 0.1536 / 4.07 × 10⁻⁹ ≈ 37.7 MPa.
• D = 8 mm: Δp = 0.1536 / 1.287 × 10⁻⁸ ≈ 11.9 MPa — a 33% larger runner cuts the loss by (8/6)⁴ = 3.16 times.
• Gate check, 4 cavities so Q_gate = 15 cm³/s = 1.5 × 10⁻⁵ m³/s through a 1.2 mm round gate (R = 0.6 mm): γ̇ = 4 × 1.5 × 10⁻⁵ / (π × (6 × 10⁻⁴)³) = 6 × 10⁻⁵ / 6.79 × 10⁻¹⁰ ≈ 8.8 × 10⁴ s⁻¹ — at the upper limit, so enlarge the gate slightly or raise melt temperature.

Example 5 — Pipe die sizing. An HDPE pipe of 50 mm OD and 2.0 mm wall (ID 46 mm) is drawn from a die bush of 60 mm bore over a mandrel of 50 mm OD.

• DDR = (60² − 50²) / (50² − 46²) = (3,600 − 2,500) / (2,500 − 2,116) = 1,100 / 384 = 2.86.
• Draw ratio balance (DRB) = (D_o/D_i) / (d_o/d_i) = (60/50)/(50/46) = 1.200 / 1.087 = 1.10 — above 1 means the melt is thinned more than it is stretched in diameter; die gap is adjusted with the die swell of the grade in mind.

Where it is used

• Automotive tooling: bumpers, instrument panels, lamp housings and connectors for Maruti Suzuki, Tata Motors, Hyundai and Bajaj vendors — large multi-slide moulds designed and cut at Indian tool rooms and at Godrej Tooling, Pune/Chennai/Faridabad clusters.
• Packaging and FMCG: thin-wall containers, caps and closures in 16–96-cavity hot-runner moulds (3–6 s cycles), and PET preform moulds — the segment where cooling-time design decides profitability.
• Housewares, furniture and crates: Nilkamal, Supreme, Cello, Prince — very large single-cavity moulds, clamp forces of 1,000–3,000 t, gas-assist and structural foam variants.
• Electrical and electronics: switchgear, meter bodies, connectors in insert and unscrewing moulds; thermoset compression moulds for DMC/SMC and phenolic parts.
• Pipes, profiles, film and sheet: extrusion dies and calibration tooling for PVC/HDPE pipe (Finolex, Astral, Supreme), spiral-mandrel blown-film dies, coat-hanger sheet dies.
• Job roles: tool designer / mould designer (CAD: NX, Creo, SolidWorks Mold Tools), CAE engineer (Moldflow), tool-room engineer (CNC/EDM), process and tooling engineer for trials and PPAP; CIPET and MSME Tool Room diploma/PG courses are the standard entry route.

Interview questions

• Q: Why is the cavity cut larger than the part? A: The moulding shrinks on cooling, so the cavity carries the grade's mould-shrinkage allowance (part/(1 − S)) for every dimension.
• Q: How do you decide the gate location? A: At the thickest section, feeding thick to thin, within the material's L/t flow limit, and positioned so that weld lines, jetting and the gate witness fall on non-critical areas.
• Q: What is a naturally balanced runner and why prefer it? A: A layout in which every cavity has an identical flow path, so all cavities fill and pack at the same pressure and time, giving uniform weights and dimensions without artificial balancing.
• Q: What decides the clamping force needed? A: Mean cavity pressure times the projected area of all cavities and runners on the parting plane, with 10–20% margin; exceeding it causes flash.
• Q: Why does cooling time go as the square of wall thickness? A: Heat conduction through the wall governs; the solution of the transient conduction equation for a plate makes time proportional to (thickness)²/α.
• Q: Coolant flow must be turbulent — why? A: Laminar flow gives a low convective heat-transfer coefficient, so the channel removes far less heat; turbulence (Re above roughly 4,000–10,000) can multiply the coefficient several times.
• Q: Difference between a two-plate and a three-plate mould? A: A two-plate mould has one parting line and gates on it; a three-plate mould has a separate runner plate, allowing pin-point gating on the part face with automatic runner separation, at higher cost and cycle time.
• Q: When is a hot-runner mould justified? A: High volumes, thin walls or expensive/regrind-sensitive materials, where zero runner scrap, lower pressure and shorter cycles repay the higher tool cost and temperature-control complexity.
• Q: What causes burn marks at the last-fill area? A: Trapped air compressed by the advancing melt (diesel effect) because of inadequate venting — add vents of 0.01–0.03 mm land depth at that point.
• Q: How do you mould an external undercut? A: With a side-core/slider driven by an angle pin, cam or hydraulic cylinder, retracted before ejection; internal undercuts use lifters or collapsible cores.
• Q: What is "steel-safe" design? A: Cutting the mould so that dimensions can be corrected by removing steel after trials (part comes out slightly heavy/large), because adding steel means welding or an insert.
• Q: Why does a fibre-filled part warp in the opposite sense to an unfilled one? A: Fibres align along the flow and restrain shrinkage in that direction, so shrinkage is lower along flow and higher across it — reversing the differential that causes warpage.

Common mistakes

• Applying shrinkage to some dimensions only, or using a single shrinkage value for a fibre-filled grade in both directions.
• Forgetting the runner's projected area (and the safety margin) in the clamping-force calculation.
• Using the half-wall thickness in the cooling-time formula when the formula is written for full thickness — always check which convention the text uses, and state it.
• Omitting draft, or putting draft on the cavity and not the core, then blaming the machine for drag marks and ejection whitening.
• Designing thick ribs and thick bosses equal to the wall thickness — guaranteed sink marks and voids.
• Choosing a tiny runner "to save material": pressure drop varies as 1/D⁴, so the machine runs at its pressure limit and cavities fill unevenly.
• Mixing units in numericals — cm² with m², bar with Pa, mm with m — and quoting clamping force in tonnes without dividing by 9,810.
• Treating the cold slug well, vents and ejector-return safety as optional details added after the layout is frozen.

Related topics

Plastics processing and injection moulding · Extrusion and blow moulding · Thermoforming, rotational and compression moulding · Polymer rheology and melt flow · Heat transfer in polymer processing · Tool materials, EDM and CNC machining