Thermoforming, rotational and compression moulding

In short

These three processes shape polymers without the high melt pressures of injection moulding. Thermoforming heats a thermoplastic *sheet* to its rubbery state and stretches it onto a mould by vacuum, air pressure or a plug; rotational moulding tumbles polymer powder inside a heated, biaxially rotating hollow mould so it sinters onto the wall; compression moulding squeezes a measured charge of *thermosetting* material between heated mould halves where it flows and then cross-links. They dominate Indian manufacture of packaging and refrigerator liners, overhead water tanks and large hollow articles, and electrical/automotive thermoset parts. GATE (XE-polymer, PI) and university polymer-processing papers ask draw ratio and thickness distribution, the four stages of rotomoulding with PIAT, bulk factor, press tonnage and cure time. Employers include Sintex, Vectus, Supreme, Ashirvad, Huhtamaki, Uflex, Bajaj Electricals, Hindustan Composites, Godrej and Voltas (liners), and most plastic-tank and electrical-accessory plants.

Explained

Thermoforming — principle and process window

• A sheet is clamped, heated to the forming temperature (above Tg for amorphous polymers, just below/near Tm for semicrystalline), stretched onto a single-sided mould, cooled under restraint, then trimmed.
• Forming pressure is tiny: vacuum forming can use at most atmospheric pressure, practically 80–90 kPa; pressure forming adds 0.2–0.7 MPa of compressed air for sharper detail and texture.
• Amorphous grades (HIPS, ABS, APET, PVC, PMMA, PC) have a wide rubbery plateau and are easy to form; semicrystalline grades (PP, HDPE) have a narrow window and sag badly — PP needs high-melt-strength grades and sag control.
• Heating: infrared radiant heaters (ceramic, quartz) on both faces; thin sheets heat in seconds, thick sheets are limited by conduction so heating time rises roughly with the square of thickness.
• Mould materials: cast/machined aluminium (production), epoxy or wood (prototype); vent holes 0.5–0.8 mm; male (positive) mould gives accurate inside dimensions, female (negative) mould accurate outside.
• Shrinkage allowance typically 0.3–0.8% for styrenics, more for PP/HDPE; parts carry residual orientation, so they shrink back if reheated (basis of the "boiling water" test).

Thermoforming — variants and thickness control

1. Drape forming: hot sheet is draped over a male mould and vacuum pulled — simple, but the top face stays thick while the sheet thins most at the bottom of the side walls, drawn last.
2. Straight vacuum forming (cavity): sheet drawn into a female cavity — corners of the base become thinnest.
3. Plug-assist forming: a heated plug (syntactic foam, machined ~85–90% of cavity) pre-stretches the sheet and carries material to the base before vacuum — the standard route for deep cups and yoghurt pots.
4. Billow / snap-back and reverse-draw: sheet is pre-blown into a bubble to stretch it uniformly, then snapped back onto the mould.
5. Matched-mould (matched-die) forming: two mating heated halves give best detail and thickness control; higher tooling cost.
6. Free (drape-free) blowing: no mould face touches the surface — optically clear PMMA skylights and aircraft canopies.
7. Twin-sheet forming: two sheets formed simultaneously and welded at the pinch line to give hollow panels — pallets, fuel/water tanks, truck decks.
8. Thin-gauge roll-fed (continuous) lines run in-line with an extruder, often form–fill–seal; heavy-gauge cut-sheet for liners, panels and luggage.
9. Thinning is unavoidable: final thickness ≈ initial thickness ÷ areal draw ratio. Deep draws (H/D > 1) need plug assist, pre-stretch, differential heating or a thicker starting sheet.
10. Trimming: in-mould steel-rule/punch trim for thin gauge; router, CNC or waterjet for heavy gauge. Edge trim is reground and re-extruded (an economic advantage of the process).
11. Advantages: very low tooling cost, large thin-walled parts, short lead time, good for low/medium volumes and for very large areas.
12. Limitations: only one surface is mould-controlled, non-uniform wall, no ribs/bosses/undercuts, high trim scrap (20–40% for cut sheet), sheet cost higher than granules.

Rotational moulding — the four stages

• Stage 1 — Charging: a weighed shot of polymer powder (ground to ~35 mesh, 300–500 µm, dry-flow and bulk density controlled) is loaded into a cold/warm mould; the mould is closed and clamped. Powder fills only 10–25% of the mould volume.
• Stage 2 — Heating/fusion: the mould enters an oven at 250–350 °C and rotates biaxially (major arm 4–12 rpm, minor axis geared to it, typical rotation ratio 4:1). There is no centrifugal throwing — speeds are low; powder *tumbles*, sticks to the hot wall, sinters, then densifies as trapped air bubbles dissolve/escape.
• Stage 3 — Cooling: forced air and/or water mist while rotation continues until the part is rigid; cooling rate sets crystallinity, shrinkage and warpage — too fast causes warpage and internal stress, too slow wastes cycle and can distort.
• Stage 4 — Demoulding: mould opened, part stripped, mould re-charged. Total cycle commonly 15–60 min.
• PIAT (peak internal air temperature) is the single most important control variable — measured by a wireless probe through the vent tube. For PE, ~195–215 °C: too low leaves bubbles/poor impact (under-cure), too high causes oxidative degradation, yellowing and brittleness.
• Every mould needs a vent tube (PTFE/steel packed with glass wool) to equalise pressure; a blocked vent causes blown or collapsed parts.
• Machine types: carousel (turret, 3–4 arms — most common for tanks), shuttle, clamshell, rock-and-roll (long parts like kayaks and boats), and oil-jacketed/direct-heated moulds.
• Materials: ~85–90% polyethylene (LLDPE/HDPE, MFI ~2–6 g/10 min, rotomoulding grades with antioxidant and UV stabiliser), plus PP, PA 6/11/12, PVC plastisol (liquid — toys, bellows), PC, crosslinked PE (XLPE) for chemical tanks.
• Advantages: stress-free seamless hollow parts, uniform wall thickness including corners (material builds up at corners rather than thinning), very low tooling cost (sheet-steel or cast-aluminium moulds), easy multi-layer and foam-core walls, metal inserts and graphics mouldable in, part size from 1 L to 50,000 L.
• Limitations: long cycle and high labour, narrow material choice (needs powder + thermal stability), loose tolerances (±2–5%), no threads/fine detail, high energy per kg, no truly flat large faces (they warp).

Compression moulding — thermosets

• A measured charge (powder, preform/pellet, dough or SMC blank) is placed in a heated open cavity; the press closes, the material flows to fill, and heat completes cross-linking (cure); the part is ejected hot and needs no cooling in the mould.
• Typical conditions: mould 140–200 °C; pressure 15–50 MPa on the projected area for phenolics and amino plastics, 3–10 MPa for SMC/BMC; cure 20–60 s per mm of thickness.
• Steps: (1) preheat the charge (RF/dielectric or IR) to cut cure time and improve flow; (2) load the charge centrally, covering 50–70% of the cavity area for SMC; (3) close fast to contact, then slow; (4) breathe/degas — crack the mould briefly to let out steam and volatiles (water and formaldehyde from condensation polymers); (5) hold under full pressure for cure; (6) eject, deflash, and post-cure/anneal if required.
• Mould types: flash (open) type — simplest, excess escapes as flash, density least controlled; positive type — plunger fully enters, all charge trapped, exact weighing essential, best density, poor venting and wear; semi-positive — combines both, gives density control with some flash escape; landed positive — stops give constant thickness.
• Bulk factor = bulk volume of charge ÷ volume of the moulded part (typically 1.8–3.0 for powders, ~2 for preforms): it decides the loading-space depth of the cavity.
• Transfer moulding (an allied thermoset process): the charge is heated in a separate pot and forced through a sprue and runners into a closed cavity — gives flash-free parts, fragile inserts (semiconductor encapsulation, coil formers) and intricate shapes, but wastes the cull and runner.
• Materials: phenol-formaldehyde (PF), urea- and melamine-formaldehyde (UF, MF — crockery, switchgear), diallyl phthalate, epoxy moulding compounds, unsaturated polyester SMC (sheet moulding compound) and BMC/DMC (bulk/dough), GMT (glass-mat thermoplastic, compression moulded hot), and rubber compounds.
• Advantages: low and uniform flow means little fibre breakage and little orientation — best process for long-fibre composites; no sprue/runner scrap; large flat parts possible; cheap, robust moulds; low shear so filled compounds are easy.
• Limitations: labour-intensive charge handling, long cure cycles, flash removal, poor for deep thin ribs and side cores/undercuts, thermoset scrap is not recyclable as new moulding material (used only as filler), weight tolerance sensitive.

Comparing the three (and with injection moulding)

• Pressure: thermoforming ≤0.7 MPa, rotomoulding ≈0 (atmospheric), compression 3–50 MPa, injection 50–150 MPa — so tooling cost and press cost scale in the same order.
• Starting form: sheet / powder / charge (powder, dough or blank).
• Wall: thermoforming — thins with draw; rotomoulding — uniform, thick corners; compression — set by mould gap.
• Economic zone: thermoforming and compression suit medium volumes with low tooling spend; rotomoulding suits large hollow parts at low-to-medium volume; injection wins at high volume.

Indian standards, regulation and plant practice

• Rotomoulded water tanks: IS 12701, *Rotational moulded polyethylene water storage tanks — Specification* (wall-thickness, weight, UV and carbon-black requirements); BIS also certifies polyethylene materials for potable-water contact. Tank markings carry the BIS licence number where the manufacturer is licensed.
• Food-contact thermoformed packaging: FSSAI (Packaging) Regulations, 2018 require compliance with the relevant BIS specification for the polymer (for example the standards for polystyrene, PET and polypropylene food-contact materials) and overall-migration testing to IS 9845.
• Plastic waste: Plastic Waste Management Rules, 2016 as amended, with EPR registration and targets on brand owners/producers (thermoformed packaging is a major EPR category); the Solid Waste Management Rules, 2026 replaced the 2016 Rules from 1 April 2026; specified single-use plastic items, including some thermoformed cutlery and stirrers, are banned.
• Safety: the OSH&WC Code, 2020, in force from 21 November 2025, replaced the Factories Act, 1948 — press guarding (two-hand control, light curtains), interlocks on rotomoulding oven doors and arm guarding, and formaldehyde/styrene exposure control fall under it; electrical installations follow the CEA (Measures relating to Safety and Electric Supply) Regulations, 2023.
• Quality: ISO 9001:2026 is the current edition of the QMS standard; automotive suppliers also work to IATF 16949.
• GST on most plastic articles is in the 5%/18%/40% structure applicable since 22 September 2025 — the rate depends on the specific HSN heading.

Key points

• Thermoforming - stretching a heated thermoplastic sheet onto a one-sided mould with vacuum, air pressure or a plug.
• Forming window - amorphous polymers form over a wide rubbery range above Tg; semicrystalline ones only in a narrow band near Tm.
• Vacuum limit - a vacuum can never give more than about 100 kPa; detail and texture need pressure forming.
• Areal draw ratio - part surface area ÷ sheet area consumed; average thickness = sheet thickness ÷ areal draw ratio.
• Plug assist - pre-stretches and carries material to the base of deep draws, evening out the wall.
• Twin-sheet forming - two sheets welded at the pinch line to make hollow panels and pallets.
• Rotomoulding stages - charge, heat with biaxial rotation, cool, demould; typical rotation ratio 4:1 at 4–12 rpm.
• PIAT - peak internal air temperature (~195–215 °C for PE) is the master cure indicator in rotomoulding.
• Sintering and densification - powder tacks to the hot wall, coalesces, then bubbles dissolve; under-cure leaves voids, over-cure degrades.
• Vent tube - mandatory in rotational moulds to equalise internal pressure.
• Compression moulding - heated charge of thermoset cured under pressure between mould halves; ejected hot.
• Bulk factor - bulk volume of charge ÷ moulded volume; fixes the loading space in the cavity.
• Mould classes - flash, positive, semi-positive and landed positive, in rising order of density control.
• Breathing (degassing) - brief mould opening to vent steam/volatiles from condensation thermosets; skipping it causes blisters and porosity.
• Cure time - roughly proportional to the square of wall thickness; preheating the charge shortens it.
• Press tonnage - F = moulding pressure × projected area; always use projected, not surface, area.
• Low-shear advantage - compression moulding preserves fibre length, so SMC/BMC and GMT parts are strong.
• Scrap - thermoforming trim is regrindable; thermoset flash is not reprocessable into new moulding compound.

Formulas and examples

Example 1 — Draw ratio and wall thickness (thermoforming). A cylindrical container, inside diameter D = 100 mm and depth H = 120 mm with a flat base, is vacuum formed from 2.0 mm HIPS sheet.

• Part surface area = πDH + πD²/4 = π(100)(120) + π(100)²/4 = 37,699 + 7,854 = 45,553 mm².
• Sheet area consumed (the opening) = πD²/4 = 7,854 mm².
• R_a = 45,553 / 7,854 = 5.80; average wall = 2.0 / 5.80 = 0.345 mm.
• H/D = 120/100 = 1.2 — a deep draw, so straight vacuum forming would leave the base corners far below 0.345 mm; a plug assist (plug ≈ 0.85 × cavity) is required, and a 3 mm sheet would be used if 0.5 mm minimum wall is specified.
• Forming force on the clamp area at 85 kPa = 0.085 × 10⁶ × 7,854 × 10⁻⁶ = 668 N only — hence cheap tooling.

Example 2 — Sheet heating time. 2.0 mm HIPS (ρ = 1050 kg/m³, c = 1900 J/kg·K) heated from 20 °C to 130 °C by heaters at 450 °C on both faces, effective ε = 0.9, mean sheet temperature 75 °C (348 K).

• q = 0.9 × 5.67 × 10⁻⁸ × (723⁴ − 348⁴) = 0.9 × 5.67 × 10⁻⁸ × (2.732 × 10¹¹ − 0.147 × 10¹¹) ≈ 13.2 kW/m² per side → 26.4 kW/m² for two sides.
• Heat needed = ρ c t₀ ΔT = 1050 × 1900 × 0.002 × 110 = 438,900 J/m².
• t = 438,900 / 26,400 ≈ 16.6 s (ideal). Real lines allow 25–35 s for conduction lag and surface-temperature limits — and about 4× this for a 4 mm sheet, since t ∝ t₀².

Example 3 — Rotomoulding shot weight for a water tank. Cylindrical LLDPE tank, D = 1.0 m, H = 1.3 m (≈1,021 L), wall 5 mm, moulded density 938 kg/m³, powder bulk density 350 kg/m³.

• Surface area = πDH + 2(πD²/4) = 4.084 + 1.571 = 5.655 m².
• Polymer volume = 5.655 × 0.005 = 0.02827 m³ → shot weight = 0.02827 × 938 ≈ 26.5 kg.
• Bulk volume of powder = 26.5 / 350 = 0.0757 m³ = 75.7 L, i.e. 7.4% of the 1,021 L mould — just below the 10–25% typical range, as expected for a thin wall in a large tank.
• At ₹115/kg for UV-stabilised rotomoulding grade, material cost ≈ ₹3,050 per tank (illustrative price; grade prices move with crude).

Example 4 — Compression moulding: tonnage, charge and bulk factor. A phenolic switch plate, 200 mm × 150 mm projected, 6 mm thick, moulded at 30 MPa; moulded density 1.45 g/cm³, bulk density of the powder 0.58 g/cm³.

• A_projected = 200 × 150 = 30,000 mm² → F = 30 N/mm² × 30,000 = 900,000 N = 900 kN ≈ 91.7 tonne → select a 100 t press (allow ~10% margin).
• V_part = 200 × 150 × 6 = 180,000 mm³ = 180 cm³ → part mass = 180 × 1.45 = 261 g; with 3% flash allowance, charge ≈ 269 g.
• Bulk factor B = 1.45/0.58 = 2.5 → loading space = 2.5 × 180 = 450 cm³, i.e. the cavity must be about 15 mm deep before closing.

Example 5 — Cure time scaling. The 6 mm plate above cures in 90 s. For a 9 mm version at the same temperature: t = 90 × (9/6)² = 90 × 2.25 = 202.5 s ≈ 3.4 min. Preheating the preform by RF to 110 °C typically cuts this by 30–40% (to roughly 2.0–2.4 min) and reduces internal porosity.

Where it is used

• Packaging (thin-gauge thermoforming): PET/PP cups, blister and clamshell packs, FFS curd and ice-cream containers, pharma blisters — Huhtamaki, Uflex, Jindal Poly Films, Cosmo and hundreds of MSME thermoformers in Daman, Vapi, Noida and Hyderabad; roles include thermoforming line operator, process engineer and tool setter.
• Appliance and transport panels (heavy-gauge thermoforming): refrigerator inner door and cabinet liners (HIPS/ABS), bus and rail interior panels, tractor and earth-mover bonnets, luggage shells, acrylic sanitaryware and skylights — Godrej, Voltas, Whirlpool, VIP/Samsonite suppliers.
• Water storage and bulk handling (rotomoulding): overhead and underground PE tanks to IS 12701, septic and biogas tanks, chemical and pickling tanks (XLPE), road barriers, dustbins, pallets, insulated ice boxes, agricultural sprayer tanks, playground equipment — Sintex, Vectus, Supreme, Ashirvad, Plasto, Penta and a large rotomoulding cluster in Gujarat and Maharashtra.
• Electrical and electronic thermosets (compression/transfer moulding): phenolic and melamine switch plates, holders, starter bases, circuit breaker housings, commutators, brake-lining and clutch-facing pads, epoxy-encapsulated semiconductors and coils — Bajaj Electricals, Havells, Anchor, HBL, Hindustan Composites.
• Composite panels (SMC/BMC/GMT compression): electric meter boxes, FRP manhole covers, tractor and automotive front-end modules, PMGSY/railway cable troughs, washing-machine bases, water-tank panels — Permali Wallace, Plastic Omnium, EPP and FRP moulders.
• Job roles: process engineer (set oven time and PIAT, draw ratio and plug design, cure cycle), tool-room/mould maintenance, quality (wall-thickness mapping, drop and hydrostatic tests on tanks), production planning in multi-arm carousel plants.

Interview questions

• Q: Why does a thermoformed part always have a non-uniform wall? A: Only one face contacts the mould, and the sheet thins wherever it stretches; the first area to touch the cold mould freezes and stops stretching, so later-contacting regions (deep corners) end up thinnest.
• Q: What is the maximum pressure available in vacuum forming, and what do you do when it is not enough? A: About 100 kPa (practically 80–90 kPa); for sharper detail, texture and undercut definition you switch to pressure forming at 0.2–0.7 MPa or matched-mould forming.
• Q: Why is polypropylene harder to thermoform than HIPS? A: PP is semicrystalline with a narrow softening window near its melting point and low melt strength, so it sags and tears; high-melt-strength or nucleated grades plus tight temperature control are needed.
• Q: What does a plug assist do? A: It mechanically pre-stretches the hot sheet and carries material into the base of a deep cavity before vacuum is applied, giving a much more uniform wall.
• Q: In rotational moulding, is the polymer thrown onto the wall by centrifugal force? A: No — rotation is slow (4–12 rpm). The powder tumbles and sticks to the hot mould surface, then sinters and densifies.
• Q: What is PIAT and why is it monitored? A: Peak internal air temperature, the highest air temperature reached inside the mould (~195–215 °C for PE). It is the best single indicator of complete fusion: too low leaves bubbles and poor impact strength, too high degrades the polymer.
• Q: Why does a rotational mould need a vent tube? A: Air inside expands on heating and contracts on cooling; without a vent the mould blows open or the part collapses and flashes at the parting line.
• Q: Why are rotomoulded parts said to be stress-free, and why do corners come out thicker? A: There is no pressure or shear during forming, so no orientation or moulded-in stress; powder tends to accumulate in corners during tumbling, so corners thicken rather than thin — the opposite of thermoforming and blow moulding.
• Q: What is bulk factor and where do you use it? A: The ratio of the charge's bulk volume to the moulded part volume (ρ_part/ρ_bulk, typically 1.8–3.0); it sizes the loading space of the compression cavity.
• Q: Why is the mould "breathed" during compression moulding of phenolics? A: Condensation cure releases water vapour and formaldehyde; a brief opening vents them, preventing blisters, porosity and dimensional defects.
• Q: Compression or transfer moulding for an intricate part with delicate inserts? A: Transfer moulding — the cavity is closed before the softened charge enters, so inserts are not disturbed and no flash forms on the part, at the cost of cull and runner waste.
• Q: Why is compression moulding preferred to injection moulding for long-glass-fibre composites? A: Flow lengths and shear rates are low, so fibres are not broken or strongly oriented, which preserves strength and keeps warpage low.
• Q: Which standard governs rotationally moulded PE water tanks in India? A: IS 12701, which specifies material, wall thickness/weight, carbon-black and UV requirements for rotational moulded polyethylene water storage tanks.

Common mistakes

• Using surface area instead of projected area when calculating press tonnage, or forgetting to convert N to tonne (÷9810).
• Taking the thermoforming draw ratio as a *linear* ratio when the thickness reduction follows the areal ratio — and then quoting a wall thickness several times too high.
• Assuming thermoforming thins the part uniformly; the minimum wall at the deepest corner, not the average, governs the design.
• Claiming centrifugal force spreads the polymer in rotomoulding, or confusing it with centrifugal casting; also quoting rpm in hundreds instead of 4–12.
• Treating oven temperature as the cure criterion in rotomoulding instead of PIAT and internal-air profile.
• Forgetting the bulk factor, so the designed cavity cannot hold the loose charge; or weighing the charge carelessly in a positive mould, where excess cannot escape and thickness goes out of tolerance.
• Saying a compression-moulded thermoset part is cooled in the mould like a thermoplastic — it is ejected hot after cure; and assuming the flash can be reground into fresh moulding compound.
• Mixing units in heating and charge calculations (mm³ with cm³, kPa with MPa, g/cm³ with kg/m³).

Related topics

Plastics processing and injection moulding · Extrusion and blow moulding · Mould and die design for plastics · Polymer rheology and melt flow behaviour · Composites and FRP processing (SMC, BMC, GMT) · Polymer testing, quality control and plastics recycling.