The process in one paragraph
A measured charge of thermoplastic powder goes into a hollow mold. The mold closes, turns slowly on two perpendicular axes inside an oven at 260–320 °C, and the powder tumbles, melts and coats the inner surface. After the charge is fully fused the mold moves to a cooling station, still rotating, until the part is rigid enough to hold its shape. Cycle times run 20–45 minutes and the tooling is cheap relative to injection, which is why rotomolding owns large hollow parts: tanks, kayaks, coolers, road barriers, planters, bins and playground shells.
Powder specification is the whole ball game
Rotomolding does not use pellets. The resin is ground to a powder, and the particle size distribution controls how evenly it coats the mold.
| Parameter | Typical value | Why it matters |
|---|---|---|
| Mesh size | 35 mesh (about 500 µm) | Coarser powder melts unevenly and gives a rough inner surface |
| Fines content | Under 5% below 200 mesh | Fines melt early, form bubbles and smoke in the oven |
| Melt index | 2–5 g/10 min (190 °C / 2.16 kg) | Low MI gives impact strength, high MI gives better flow into details |
| Density | 0.935–0.940 g/cm³ | Standard mLLDPE rotomolding grade |
| Antioxidant package | Required | The powder sits at oven temperature for 20+ minutes |
| Dry blend colour | Pigment pre-compounded | Dry-blended pigment causes streaking and weak points |
Most production parts use medium-density mLLDPE because it balances stiffness with falling-dart impact at low temperature. Cross-linked grades (XLPE) go into fuel tanks and chemical tanks where solvent resistance matters; nylon, PC and PPS powders serve high-temperature or transparent applications and cost several times more per kilogram.
Wall thickness: what tolerance is realistic
| Part size | Typical wall | Achievable tolerance |
|---|---|---|
| Under 500 mm | 2.5–4 mm | ±0.5 mm |
| 500–1,500 mm | 3–8 mm | ±20% of nominal |
| Above 1,500 mm | 6–12 mm | ±25% of nominal |
Rotomolded walls cannot be held to injection tolerances. Gravity and rotation set the distribution, so corners and the ends of long sections run thicker while flat panels run thinner. A drawing that calls for ±0.2 mm on a 1,200 mm tank is not manufacturable, and a supplier who accepts it will fail at first article.
The practical answer is to specify a minimum wall at the thinnest point — usually the design load case — and let the rest run to the process capability.
Mold selection
| Mold material | Cost | Life | Best for |
|---|---|---|---|
| Cast aluminum | Moderate | 5,000–20,000 cycles | Most production parts; best heat transfer |
| Sheet steel (fabricated) | Low | 2,000–10,000 cycles | Prototypes, very large parts, low volume |
| Stainless steel | High | 50,000+ cycles | Corrosive resins, long programs |
| Electroformed nickel | Highest | High | Fine texture, high gloss, complex detail |
Cast aluminum wins on heat transfer, which shortens the oven stage, and it holds texture well. Sheet steel is welded from plate, so it is cheap and fast to build but has weld seams that telegraph into the part surface and hot spots that cause local over-cure.
Mold design details that decide part quality:
- Venting. A vent tube at the highest point prevents pressure build-up and draws air back in during cooling; without it, parts blow out or suck in.
- Draft. 1–3° on vertical walls; less and the part fights the mold on release.
- Parting line. Place it where it can be trimmed or hidden, and expect visible flash that needs finishing.
- Mold mass balance. Unequal mass heats and cools unevenly, which warps the part before it leaves the mold.
Cycle control: PIV rather than time
Experienced molders control the oven stage by peak internal air temperature (PIV) rather than by clock. For LLDPE, PIV typically peaks around 190–210 °C. Stopping the cycle early leaves unmelted powder and a laminated wall; running long over-cures the inner surface, which reduces impact strength and yellows the part.
Cooling rate is the other half. Fast cooling with forced air or water mist shortens the cycle but locks in warpage; slow cooling in still air holds dimensions. Warp-sensitive parts get staged cooling: forced air first, then still air.
Design rules
| Feature | Rule |
|---|---|
| Outside radius | At least 1.5 × wall thickness, ideally 3 × |
| Inside radius | At least 2 × wall thickness; sharp internal corners trap powder |
| Draft angle | 1–3°, more on textured surfaces |
| Flat panels | Crown them 1–2% or add ribs; unsupported flats warp |
| Ribs | Rounded, 1.5–2 × wall thick, tapered |
| Kiss-offs | Use to tie opposite walls and stiffen tanks |
| Undercuts | Avoid; they need a split mold or a removable core |
| Wall transitions | Taper over at least 3 × the thickness change |
| Shrinkage allowance | 1.5–3% for LLDPE, applied to the mold |
Molded-in inserts are possible but need care: a cold metal insert in a hot mold acts as a heat sink and starves the surrounding plastic. Heated inserts, or inserts with a coarse knurl and a generous boss, work; through-bolted designs with a sealing washer are more reliable and cheaper.
Quality control tests
| Test | Method | Purpose |
|---|---|---|
| Wall thickness | Ultrasonic gauge, 8+ points per part | Confirms distribution, not just nominal |
| Falling dart impact | ASTM D1709 style, at −20 °C and 23 °C | Cold impact is the failure mode that kills tanks |
| Density | ASTM D792 | Confirms grade, catches wrong resin |
| Gel count / dispersion | Visual or microscope section | Detects unmelted particles and contamination |
| Warp | Fixture gauge or CMM | Flatness and roundness against drawing |
| Colour | Spectrophotometer against master | Batch consistency |
| Leak / hydrostatic | Fill and pressure hold | For tanks and fluid vessels |
When rotomolding is the wrong process
| Process | Economic volume | Notes |
|---|---|---|
| Rotomolding | 100–5,000 pcs/yr | Cheap tool, high labour, large hollow parts |
| Blow molding | 10,000+ pcs/yr | Better for bottles and tight-neck containers |
| Thermoforming | 500–50,000 pcs/yr | Trays, shells, thin-wall covers; needs trim tooling |
| Injection molding | 10,000+ pcs/yr | Tight tolerances, small parts, high cavitation |
If the part is under 300 mm and needs ±0.1 mm, rotomolding is the wrong answer regardless of volume. If the part is a 2,000 litre tank with a molded-in sump and no seam, rotomolding is usually the only economical route.
RFQ checklist
- 3D model plus a drawing that states the minimum wall and the load case
- Resin grade, colour, and whether UV stabilisation is required for outdoor service
- Service temperature range, including cold impact requirements
- Insert list with load direction and sealing requirement
- Surface finish: texture number, gloss level, and whether the inner surface is cosmetic
- Annual volume, expected program life, and whether tooling is owned or amortised
- Test reports for impact, wall thickness distribution and density
- Packaging and nesting requirement for shipping
- Whether secondary operations (CNC trimming, assembly, foam filling) are in scope
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
Standards referenced
- ASTM D1709 — dart drop impact resistance
- ASTM D792 — density and specific gravity
- ASTM D1998 — polyethylene storage tanks
- ISO 6601 — friction and wear, for sliding interfaces
- ARM (Association of Rotational Molders) design guidelines
