Design rules for rotational moulding
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Daan Buitenhuis
Communication
Rotational moulding is a process in which plastic powder is placed into a hollow mould, also known as a die. This mould is then heated and rotated along two axes, causing the powder to melt and evenly coat the inside of the mould to create a hollow plastic shape. A mould has no internal core; the product is formed by the inner wall of the mould. The production process involves four main stages: filling, heating, cooling, and demoulding. After this phase, the mould is cleaned, and the process starts again.
Rotational moulding offers unique advantages and challenges that must be well understood to design and produce successfully. In this article, we dive into the key design rules and considerations for rotational moulding.
Table of contents
Material selection
There are a limited number of suitable materials for rotational moulding. The most commonly used materials are polyethylene (PE) and polypropylene (PP). Additionally, materials such as polyvinyl chloride (PVC), nylon, and polycarbonate (PC) can also be used, though less frequently. Polyethylene is by far the most popular due to its good processability, chemical resistance, and low cost. The choice of material depends on the required stiffness and strength, the ambient temperature, required chemical resistance, colour and tightness requirements, and UV resistance.
Number of products
The rotational moulding process is suitable for both small and large production runs. It can be applied for series starting from 50 units but is also used for large series of up to 75,000 units per year. If the quantities increase further, another technique is often chosen. The output of a mould running 24 hours a day, five days a week, is about 3,000 units per year with a maximum of 80 products per week. If the annual demand exceeds this number, multiple moulds will need to be made. By using multiple moulds, we can significantly increase production capacity per week or year to meet demand.
Product size and complexity
Rotational moulding is often associated with larger products, but this technique is also well-suited for smaller products. It is possible to use multiple moulds on one arm, allowing optimal machine occupancy. Another advantage of rotational moulding is its design freedom; the process offers virtually unlimited possibilities in design. It can produce hollow shapes and complex contours that are difficult or impossible to make with other methods. The shapes that can be produced with rotational moulding are nearly infinite. Where other production methods require complex assemblies of components for a complex final product, rotational moulding can replace this assembly with a single uniform product. This method of production is stronger and saves costs.
Deformation
A challenge in rotational moulding is deformation. Unlike other moulding processes such as injection moulding, no external pressure is applied to push the material into the mould in rotational moulding. This causes the material to shrink and deform uncontrollably. Plastic material naturally tends to shrink when cooling; for polyethylene, this is about 3%, and for polypropylene, 2%. This is taken into account during the design phase by making the moulds larger than the actual product needs to be.
In addition to shrinkage, warpage is also a common problem. Warpage is the deformation of large surfaces, often occurring during the cooling process. During this cooling phase, it is very important to cool in a uniform and calm manner. This prevents stresses in the material, ultimately leading to warped surfaces. Warpage can also be prevented by creating a wall thickness as uniform as possible. This helps to promote even shrinkage and reduce deformation. Avoiding large, flat surfaces in the design can also help prevent warpage. Reinforcement ribs can be added to increase the stiffness of thin-walled parts without significantly adding weight. These ribs can be either hollow or solid, depending on the product requirements. “Kiss-off” ribs, where two walls nearly but not completely touch, can also be used to improve structural integrity.
Wall thickness and uniformity
One of the advantages of rotational moulding is the ability to adjust the wall thickness of the final product by simply adding more or less material to the mould. The wall thickness can vary from 0.5 mm to 50 mm, with uniformity of ±10% possible. It is important to maintain an even wall thickness to minimize stresses and deformations.
Corner radius and design freedom
A key design aspect in rotational moulding is the use of rounded corners. Rounded corners improve material flow during the moulding process and reduce the risk of stresses and fractures in the final product. The recommended inside radii range from 3.2 mm to 12.7 mm, depending on the material used. Large radii also help maintain uniform wall thickness and improve the structural integrity of the product.
Inserts
Integrating inserts, such as metal bushings and threads, is possible in rotational moulding. These inserts can be placed in the mould during the moulding process to be seamlessly integrated into the final product.
It is also possible to rotate internal or external threads. However, it is not possible to add fine threads using this method. Rotating threads is cost-effective but not durable.
Textures
The finish of rotational moulded products can vary from high gloss to ribbed, depending on the mould and materials used. A smooth, polished finish can be more expensive and time-consuming but offers advantages such as easy graphic application and improved aesthetics. It is even possible to add different surface textures to a single product.
In summary, rotational moulding offers designers enormous freedom and flexibility in creating complex, hollow plastic products. By considering the specific design rules and material considerations, high-quality, durable, and cost-effective products can be produced. Understanding the nuances of the rotational moulding process is essential for optimizing product quality and minimizing production errors. By choosing the right materials, paying attention to wall thickness and corner radii, and using the right reinforcement methods, designers can maximize the benefits of rotational moulding and create innovative solutions that meet the needs of diverse industries. At Pentas, we like to work with our engineering team and the customer to go through these steps and come up with the best product. We believe that close cooperation and a thorough understanding of the process and customer requirements lead to the best end result.