Rotational moulding (or Rotomolding) is a process that is used to manufacture plastic products. It involves the heating and melting of a thermoplastic powder in a mould, the liquid polymer then coats the mould surface to produce the desired shape. This process is typically used to mould hollow parts, especially those with complex and varied shapes not easily obtainable by other hollow-art processes. It is a virtually shear-free and pressure-free process. The wall thickness uniformity and part weight can be easily maintained and there is very little waste of material due to scrap. The moulds used are normally less expensive than the moulds used in injection and blow moulding.

The Process

The rotational moulding process starts with a good quality mould that is placed in a moulding machine that has a loading, heating, and cooling area. Several moulds may be placed on the machine at the same time. Pre-measured plastic resin is loaded into each mould, and then the moulds are moved into the oven where they are slowly rotated on both the vertical and horizontal axis.
The melting resin sticks to the hot mould and coats every surface evenly. The mould continues to rotate during the cooling cycle so the parts retain an even wall thickness. Once the parts are cooled, they are released from the mould. The rotational speed, heating and cooling times are all controlled throughout the process.

The Process

1. Loading - This is where the polymer powder is loaded into the mould.

2. Heating - The powder is heated in a mould that is rotating on two axes. The melted powder coated the inside of the mould to provide the required shape.

3. Cooling - The part is cooled in a cooling station under the desired temperature is achieved.

4. Unloading - The finished part is removed from the mould.

Design Advantages

Rotational moulding offers design advantages over other moulding processes.

With proper design, parts that are assembled from several pieces can be moulded as one part, eliminating expensive fabrication costs.

The process also has a number of inherent design strengths, such as consistent wall thickness and strong outside corners.

The mouldings produced are virtually stress free. If additional strength is required, reinforcing ribs can be designed into the part.

Rotational moulding delivers the product the designer envisions. Designers can select the best material for their application, including food-grade materials.

Additives to help make the part weather resistant, flame retardant, or static free can be specified. Inserts, threads, handles, minor undercuts, flat surfaces that eliminate draft angles or fine surface detail can all be part of the design. Designers also have the option of multi-wall moulding that can be either hollow or foam filled.

Cost Advantages

When cost is a factor, rotational moulding has the advantage over other types of processes as well. In comparison to injection and blow moulding, rotational moulding can easily produce large and small parts in a cost effective manner.

Tooling is less expensive because there's no internal core to manufacture. Since there is no internal core, minor changes can be easily made to an existing mould.

And because parts are formed with heat and rotation, rather than pressure, moulds don't need to be engineered to withstand the high pressure of injection moulding.

Production costs for product conversions are reduced because lightweight plastics replace heavier, often more costly materials.

Examples of Rotationally Moulded Products:

• - Pipe Fittings - Oil Tanks
  - Chemical Containers- Intermediate Bulk Containers
  - Materials Handling Products - Bottle Banks
  - Instrument Housings- Agricultural Feeders
  - Agricultural Drinkers- Highway Barriers
  - Boats and Kayaks - Tool Carts
  - Planter Pots - Playground equipment
• - Water Tanks