Rotational Molding

Rotational molding forms hollow plastic parts by rotating heated molds around two axes, producing seamless, low-stress walls with low tooling cost and long cycle times.

Overview

Rotational molding (rotomolding) makes hollow plastic parts by charging a mold with powder, heating while rotating on two axes, then cooling so material coats the cavity uniformly. It excels at large, enclosed shapes with seamless construction, consistent wall thickness, and minimal residual stress—often improving impact performance versus high-shear processes.

Choose rotomolding for medium volumes where tooling budget matters and the part is too large or too hollow for efficient solid molding. Typical sweet spots include tanks, housings, ducts, and buoyancy components. Tradeoffs: cycles are minutes (not seconds), dimensional control is looser than injection molding, and fine details, thin walls, sharp edges, and tight flatness are hard. Post-machining of holes, inserts, and sealing features is common, so plan secondary ops and tolerances accordingly.

Common Materials

  • PE (LLDPE)
  • HDPE
  • PP
  • PVC
  • Nylon 12
  • TPU

Tolerances

±0.030" to ±0.060"

Applications

  • Chemical storage tanks
  • Fuel tanks
  • Large equipment housings/covers
  • Kayaks and canoes
  • Material handling bins and hoppers
  • Ducts and air handling components

When to Choose Rotational Molding

Pick rotational molding for hollow parts with large overall size, integrated features, and moderate production volumes where tooling cost needs to stay low. It fits parts that can tolerate looser dimensional control and longer cycle times, with secondary machining for interfaces and precision holes.

vs Standard Injection Molding

Choose rotational molding when the part is large and hollow and you want a seamless one-piece body without welding. It’s a better fit when injection tooling cost is hard to justify and cycle time is less critical than part size and tooling budget.

vs Overmolding

Choose rotational molding when you need a single-material hollow shell or tank-like geometry and can handle secondary assembly for grips, seals, or soft features. Overmolding is typically for bonded multi-material features with tighter registration than rotomolding can hold.

vs Insert Molding

Choose rotational molding when metal inserts can be installed as molded-in features using insert pockets, spin-welding, or post-installed hardware, and the main value is a large hollow body. Insert molding is the better path when insert location and interface tolerances must be tightly controlled inside an injection-molded part.

vs Compression Molding

Choose rotational molding for enclosed hollow parts with uniform walls and no parting-line flash around a perimeter flange. Compression molding is generally stronger for thick solid sections and sharper feature definition, while rotomolding favors hollow geometry and lower stress.

vs Blow Molding

Choose rotational molding when you need thicker walls, more uniform corners, and complex external features without pinch-off seams. Blow molding is typically faster at high volumes for bottle-like shapes but has constraints on geometry and seam locations.

Design Considerations

  • Target uniform wall thickness and avoid thick-to-thin transitions that drive warpage and long cooling times
  • Use generous radii and avoid sharp corners; they thin out and can weaken impact performance
  • Plan for secondary machining on holes, sealing surfaces, and precision interfaces; call out where tight tolerances actually matter
  • Add draft where possible and avoid deep undercuts; complex slides and trapped features add cost and risk
  • Design ribs, bosses, and mounting features as external pads or thickened areas sized for rotomolding, not injection-style thin ribs
  • Specify insert strategy early (molded-in, post-installed, welded) and provide access for installation and torque tools