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Lesson 2.5ShapeChapter 2 · Molding plastics and rubber

Rotational molding

Powder coats a hot, turning mold

Rotational molding (rotomolding) puts plastic powder inside a hollow metal mold, then heats the mold while turning it slowly on two axes. The melting powder coats the inside walls evenly, like chocolate coating the inside of an egg mold. Water tanks, kayaks and playground slides are made this way, often in one seamless piece.

Rotational molding: a real photo
Photo: Rishi saran, CC BY-SA 3.0, via Wikimedia Commons
Powder in a hot, turning mold coats the walls and becomes a hollow partRotational molding, cross-section of one mold through a cycleLoadlidplastic powderhollow metal moldHeat and turnovenpowder melts onto the wallswhile turning on two axesCoolair or water spray cools itwhile it keeps turningUnloadlifted outhollow one-piece parteven walls, no seams1234plastic (powder, then part)oven heatcooling air or watermold and arm
Rotational molding · one mold through 4 stagesCircled numbers match the steps below. Each panel shows the same mold at a later moment in the cycle.

How it works

  1. Load. A measured amount of plastic powder (or liquid plastic) is poured into one half of a thin metal mold, and the mold is closed. The weight of powder sets the wall thickness.

  2. Heat and turn. The mold moves into an oven, typically about 250–400 °C, and turns slowly on two axes at once. The powder tumbles over the hot walls, melts and sticks to them layer by layer.

  3. Cool. The mold leaves the oven and keeps turning while fans or a water mist cool it. The plastic hardens into an even shell.

  4. Unload. The mold opens and the hollow part is lifted out. Machines often carry several molds on arms, so one is loading while another heats and a third cools.

There is no pressure involved, so molds can be thin sheet metal or cast aluminum, and the part has very little built-in stress.

Key facts

Materials
Mostly polyethylene powder; also polypropylene, nylon and PVC (as a liquid paste)
Typical parts
Water and fuel tanks, kayaks, playground slides, traffic barriers, road cones, large bins
Part size
From a few liters to tanks of tens of thousands of liters
Precision
Loose: typically about ±1 mm or more on larger parts, because the plastic shrinks freely
Surface finish
Outside copies the mold; the inside is rougher and never touches a tool
Batch size it suits
Tens to thousands of parts a year per mold
Tooling and machine cost
Molds from a few thousand to tens of thousands of dollars; machines from tens of thousands to several hundred thousand dollars
Speed
Slow: typically 20–90 minutes per cycle, depending on size and wall thickness

Strengths and limits

Strengths

  • Very large hollow parts in one seamless piece
  • Cheap, simple molds; low risk for small batches
  • Even walls with thick, strong corners
  • Almost no internal stress; metal inserts and double walls possible

Limits

  • Long cycles, so cost per part is higher at high volume
  • Few plastics work well; mostly polyethylene
  • Precision is loose and fine details are soft
  • Wall thickness cannot change much from place to place

Where it's used

  • Water and chemicalsRainwater tanks, septic tanks, chemical containers
  • LeisureKayaks, canoes, coolers
  • PlaygroundsSlides, tunnels, play panels
  • Road and safetyTraffic barriers, cones, bollards
  • Industry and agricultureLarge bins, hoppers, fuel tanks for machines

How to recognize it

  • A large hollow part with no seam and evenly thick walls, often 3–10 mm.
  • Rounded, slightly thicker corners and a matte, slightly orange-peel surface.
  • A rough inside surface that was never touched by a tool.
  • A small vent tube hole or plug, left to let air in and out of the mold.

Compare with

Blow molding2.3also makes hollow parts, much faster and thinner, but needs costly machines and suits bottles more than big tanks.Thermoforming2.4another low-cost way to make large parts, but only open shells from a sheet.Centrifugal casting1.5a spinning mold too, but molten metal is flung outward to make pipes and rings.

Watch it

Joseph Whitaker DT · A simple classroom-style explanation

RotationalMolders · A camera inside the turning mold shows the powder melting onto the walls

Ecosure · The full process, step by step, in a factory

Skill Footage · Raw factory footage of large water tanks being molded

See real photos

Remember

  • Powder in, heat while turning on two axes, cool, open: a hollow part with no seam.
  • No pressure means cheap molds and huge parts, but slow cycles.
  • Best for big, tough, hollow items made in tens to thousands.

More in Molding plastics and rubber

All 100 processes
2.1ShortInjection moldingMelt forced into a closed steel moldOpen the lesson2.2Overmolding and two-shot moldingA second plastic molded onto the firstOpen the lesson2.3Blow molding (extrusion and injection blow molding)Hot tube or preform inflated in a moldOpen the lesson2.4ThermoformingHeated sheet sucked onto a moldOpen the lesson2.6Compression molding (SMC, BMC, rubber)A charge pressed and cured in a hot moldOpen the lesson2.7CalenderingCompound squeezed into sheet between rollsOpen the lesson

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