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Overview
Rotational moulding, or rotomoulding, is a unique and flexible plastics manufacturing method, ideal for producing hollow, durable items in virtually any shape and size. Unlike other moulding techniques, rotomoulding uses low pressure and slow rotation to gradually coat the inside of a heated mould, creating even wall thickness and eliminating weak points. This method is especially popular for making large and complex shapes, such as water tanks, outdoor furniture, and playground equipment, that are difficult to achieve with other moulding processes.
With its economical tooling costs and ability to mould intricate designs, rotational moulding is widely used across industries—from industrial containers and automotive components to custom-designed decorative items. While it’s particularly effective for low to medium production runs, the process is highly adaptable, catering to varied project needs and design challenges.
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The Rotational Moulding Process Explained
Rotational moulding is a straightforward but precise process:
Loading – Powdered plastic resin is loaded into a hollow mould.
Heating and Rotating – The mould is heated in an oven while rotating around two axes, ensuring an even coating of melted resin inside.
Cooling – The mould is transferred to a cooling station, allowing the plastic to harden gradually.
Unloading – Once cooled, the mould is opened, and the finished product is removed.
The entire process is low-pressure and produces strong, hollow parts without seams or weld lines, making it ideal for large, complex shapes.
Design Considerations for Rotational Moulding
When designing parts for rotational moulding, there are a few essential aspects to consider:
Wall Thickness: Rotomoulding naturally produces even wall thickness. Corners tend to be thicker, adding strength to these stress points.
Ribs and Reinforcements: Ribs can be added for extra strength, often using a hollow structure for efficiency.
Mould Design: Moulds are typically made from aluminium or steel, balancing durability and cost.
Shrinkage and Draft Angles: To ease product removal, designs often include draft angles, and the mould must account for cooling-related shrinkage.
Types of Rotational Moulding Machines
Several types of machines can be used in rotational moulding, each suited to specific part shapes and sizes.
Rock and Roll Machine
Ideal for long, narrow parts, this machine uses an inclined axis to rotate the mould and a smaller heating chamber, reducing heating costs for large, narrow products.
Clamshell Machine
A single-arm machine that heats and cools the mould in one chamber, the clamshell takes up less space and is suitable for prototyping and small-scale production.
Vertical or Up & Over Rotational Machine
This compact, energy-efficient machine fits in smaller spaces and allows efficient rotation between heating and cooling chambers.
Shuttle Machine
Shuttle machines have two arms, independently rotating the moulds between heating and cooling stations, useful for efficient, medium-sized production runs.
Swing Arm Machine
Swing-arm machines feature up to four arms rotating independently, which is beneficial for long cooling cycles or high-maintenance needs, enabling flexibility in production.
Carousel Machine
The most common type, carousel machines have up to four arms and six stations, rotating moulds between heating, cooling, and loading areas. Independent-arm models allow for varied cycle times, which is ideal for custom parts.
Plastic Materials Used in Rotational Moulding
Rotational moulding commonly uses polyethylene (PE) due to its durability and ease of processing. Other materials include polyvinyl chloride (PVC), nylon, and polypropylene. Because the process relies on powdered resin, materials need to be ground into a fine powder, making high thermal stability important.
Common Applications of Rotational Moulding
Rotational moulding is widely used in various sectors:
Storage Tanks: Large water and chemical tanks, up to 50,000 litres.
Industrial Containers: Bins, crates, pallets, and cooler boxes.
Outdoor Equipment: Kayaks, canoes, garden furniture, road cones, and playground equipment.
Decorative Items: Design-led items like furniture, planters, and artistic pieces.
Automotive and Marine: Fuel tanks, truck mudguards, and buoyancy products for marine use.
Advantages of Rotational Moulding
Low-Cost Tooling: Since the process doesn’t use pressure, the moulds are relatively inexpensive, making rotomoulding cost-effective for short production runs.
Consistent Wall Thickness: The biaxial rotation ensures even coating throughout, providing uniform strength.
Complex Shapes: Rotomoulding accommodates various features, including inserts, textured surfaces, and intricate designs.
Size Flexibility: Almost any size product can be made, from small items to large tanks.
Efficient Use of Material: Minimal waste is produced, and excess material can often be recycled.
Disadvantages of Rotational Moulding
High Cycle Times: Each product takes longer to mould compared to methods like injection moulding.
Material Limitations: Limited to thermoplastics that can be powdered and withstand high heat.
Labour-Intensive: Largely manual, requiring experienced operators.
Limited Repeatability: Soft metal moulds may need refurbishing after around 3,000 cycles, impacting repeatability.
Conclusion
Rotational moulding is a versatile, low-cost process ideal for creating durable, hollow plastic parts in complex shapes. While cycle times are longer compared to other methods, the ability to mould intricate designs with consistent wall thickness offers distinct advantages. For businesses needing low-volume production and flexibility, rotomoulding is an excellent choice.
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