Rotational Molding

Rotational molding forms large, hollow, seamless plastic parts by heating and rotating a powder-filled mold, ideal for uniform walls and integrated features.

Overview

Rotational molding (rotomolding, rotary molding) creates hollow, seamless plastic parts by heating a powder inside a rotating mold so it coats and fuses to the cavity walls. The process runs at low pressure, uses relatively simple tooling, and excels at large parts with consistent wall thickness, complex outer shapes, and integrated bosses or inserts.

It shines for low to medium production volumes where you need big, durable, hollow parts and want to avoid the high tooling cost of standard injection molding. Expect thicker walls, looser tolerances, and slower cycles compared to high-pressure molding, but excellent stress distribution and low risk of weld lines or knit lines. Rotational molding is not ideal for very small, highly detailed features, sharp corners, or tight dimensional control, but it’s extremely cost-effective for tanks, enclosures, and structural plastic shells where robustness and size matter more than precision.

Common Materials

  • LLDPE
  • HDPE
  • Polypropylene
  • PVC plastisol
  • Nylon 6
  • Nylon 12

Tolerances

±0.03" to ±0.06"

Applications

  • Storage and fuel tanks
  • Kayaks and small boats
  • Insulated coolers and cases
  • Playground and recreational equipment
  • Large ducting and housings
  • Material handling bins and pallets

When to Choose Rotational Molding

Choose rotational molding for large, hollow, seamless plastic parts where strength, durability, and uniform wall thickness matter more than tight tolerances. It fits low to medium volumes, thick walls, and parts that benefit from integrated features and low tooling cost. Ideal when the part is too large or too costly to tool for high-pressure molding.

vs Standard Injection Molding

Pick rotational molding when you need large, hollow parts with thicker walls and want to avoid expensive, complex injection tooling. It’s better for low to medium volumes where a single-piece hollow shell replaces multi-part assemblies, and where looser tolerances and slower cycle times are acceptable.

vs Overmolding

Choose rotational molding instead of overmolding when a single-material, hollow shell can meet your functional needs without bonded soft-touch or secondary materials. Rotomolding suits large, rugged housings and tanks, where part size and wall uniformity matter more than multi-material interfaces or localized grip features.

vs Insert Molding

Use rotational molding over insert molding when you only need a few metal inserts or threaded features embedded in a large hollow plastic body. Rotomolding can capture inserts in low-pressure conditions and avoid the high tooling and clamp-force requirements of insert molding on very large parts.

vs Blow Molding

Prefer rotational molding when you need thick walls, complex outer geometries, and low internal stresses in large hollow parts, even at lower production rates. It’s also better when you want molded-in features like bosses, variable wall sections, or complex corners that are harder to achieve with extrusion-based blow molding.

vs Compression Molding

Select rotational molding instead of compression molding when the part must be hollow, seamless, and relatively lightweight rather than a solid thermoset or thick solid thermoplastic. Rotomolding is more suitable for large, enclosed shapes like tanks or enclosures where internal cavities and uniform hollow sections are critical.

Design Considerations

  • Target uniform wall thickness; avoid large jumps in section thickness to control shrink and improve material distribution
  • Use generous internal and external radii (typically ≥3 mm) to promote even material flow and reduce stress concentrations
  • Avoid very small features, sharp text, and fine details; plan to machine or add critical features post-mold if tight control is needed
  • Include flat pads or bosses where inserts, fasteners, or machined features will be located, and clearly call out post-machining operations in the drawing
  • Specify realistic tolerances based on part size; reserve tight tolerances only for critical interfaces and keep them localized
  • Provide adequate draft and smooth transitions between surfaces to aid material flow, demolding, and improve surface quality on large parts