What Is Rotational Molding?
Rotational molding (rotomolding) produces hollow parts by slowly rotating a heated mold on two axes while powdered plastic melts and coats the interior walls. No pressure is involved — gravity and rotation do all the work. The result is a seamless, stress-free hollow part with uniform wall thickness.
Kayaks, playground equipment, water tanks, coolers (including the premium brands), septic tanks, and large custom enclosures are all rotomolded.
How the Process Works
- Loading. A measured charge of powdered polyethylene (usually) is placed inside the mold.
- Heating and rotation. The mold is moved into an oven and rotated slowly on two perpendicular axes (typically 4–12 RPM). As the mold heats, the powder melts and coats the inner walls evenly.
- Cooling. The mold moves to a cooling station (forced air, water mist, or both) while still rotating. The plastic solidifies against the mold walls.
- Demolding. The mold is opened and the part is removed. Total cycle time: 15–45 minutes depending on size and wall thickness.
Common Materials
- Linear Low-Density Polyethylene (LLDPE) — The default. ~85% of all rotomolded parts. Tough, UV-resistant, inexpensive.
- Cross-linked Polyethylene (XLPE) — Better chemical resistance and impact strength. Used for fuel tanks and chemical containers.
- Polypropylene (PP) — Higher temperature resistance. Autoclavable medical containers.
- Nylon (PA) — Superior strength and wear. Rotomolded nylon is used for fuel tanks and demanding structural parts, but requires careful process control.
- PVC Plastisol — Liquid material (not powder). Used for soft, flexible parts like balls and dolls.
What It Costs
Rotomolding has the lowest tooling cost of any molding process. The molds don’t need to withstand pressure, so they can be made from cast aluminum or even fabricated sheet metal.
| Cost Element | Typical Range |
|---|---|
| Simple cast aluminum mold | $2,000 – $15,000 |
| Complex multi-piece mold | $15,000 – $50,000 |
| CNC machined aluminum mold | $10,000 – $75,000 |
| Part cost (medium-sized) | $20 – $500+ |
Per-part cost is higher than injection or blow molding because cycle times are long. Rotomolding makes economic sense from 1 part up to a few thousand — beyond that, blow molding usually wins on unit cost.
Design Considerations
- Uniform wall thickness. One of rotomolding’s biggest advantages. The rotating mold distributes material evenly. Typical walls: 3–12 mm.
- No internal pressure. Unlike injection molding, there’s no packing pressure, so parts are virtually stress-free. This makes them extremely impact-resistant.
- Inserts and hardware. Metal inserts, bosses, and fittings can be placed in the mold before the cycle and become encapsulated in plastic.
- Multi-layer walls. You can create foam-core sandwich walls by adding foaming agent between solid skin layers. Great for insulated containers and flotation.
- Draft. Needed but less critical than injection molding — the part shrinks away from the mold as it cools. 1° minimum is typical.
- Surface finish. Exterior matches the mold surface (can be polished or textured). Interior is always slightly rough.
- Tolerances. Looser than injection molding. ±3–5% on dimensions is typical. If you need tight tolerances on specific features, plan for secondary machining.
When to Use Rotational Molding
- Large hollow parts (coolers, tanks, kayaks, playground structures)
- Low to medium volumes (1 to a few thousand)
- Seamless, stress-free construction needed
- Double-wall or foam-core construction
- Budget constraints on tooling
- Parts with molded-in hardware or inserts
When to Consider Alternatives
- High volumes: Blow molding is faster at 10,000+ units
- Tight tolerances: Injection molding
- Small parts: The long cycle time makes rotomolding uneconomical for small, simple shapes
- Continuous profiles: Extrusion
Bottom Line
Rotomolding is the most accessible entry point into custom hollow plastic parts. Tooling is inexpensive, part sizes can be enormous, and the stress-free construction produces remarkably tough products. The trade-offs are long cycle times, looser tolerances, and limited material options compared to injection molding. If you need a large, hollow, tough part and you’re not making a million of them, rotomolding deserves a serious look.