Rotational Moulding: The Process & Advantages

 

Rotational Moulding: The Process & Advantages

Rotational moulding, also known as rotomoulding, is a plastics moulding technology which is ideal for making hollow articles. It is a casting technic but unlike most other plastics processes there is no pressure involved. Moulds for the process are relatively inexpensive as they do not have to withstand pressure and therefore relatively short production runs can be made very economically.

Rotomoulding is used to make a very diverse range of products. The process offers the product designer exceptional freedom as just about any shape can be produced. There is almost no limit to the size of mouldings and there are literally thousands of applications.

The Process

 

The concept of rotational moulding is very simple. A plastic material, normally in powder form is placed into a hollow mould, usually made from cast aluminium or fabricated from sheet steel. The mould is closed and rotated slowly on two axis. The mould is then heated in an oven whilst rotating and the polymer gradually melts and ‘lays-up’ on the inside of the mould. Once the polymer powder has fully melted the mould is moved to a cooling station and cooled usually with air and sometimes a fine mist of water. As the mould cools the part solidifies. When the material has finally cooled sufficiently to release away from the mould surface the process is stopped and the product is taken out of the mould (de-moulded).

Whilst the concept of rotational moulding is extremely simple anyone who has ever been involved in the process will tell you that it is in fact extremely complicated. It is a casting process and there is normally no pressure involved which means that the material cannot be controlled in the same way as high pressure processes such as injection moulding. There are many variables that can affect production and the end product such as ambient temperature and humidity, type of mould, material specification and powder quality.

Key advantages

Rotomoulding has several unique characteristics and advantages compared to other moulding processes.

• Low tooling costs. As this is a casting process there is no pressure. This means moulds are inexpensive and low volume can be economic. If you have a great idea for a new product but don’t know how many you’ll sell or simply want a low volume production run, rotomoulding’s low initial investment makes it particularly attractive.
• It’s easy to make complicated shapes. Rotomoulding readily accommodates production complexities such as stiffening ribs, moulded inserts and different surface textures.
• Uniform wall thickness. Rotomoulding achieves consistent wall thickness with corners tending to be thicker. This increases product strength and integrity. Other processes such as blow moulding stretch the molten material at corners or sharp edges creating potential thin spots and weaknesses.
• Rotational moulding machine costs are low compared to other processes and the investment required is small. There is great production flexibility with the process.
• There is almost no limit to the size of products and several different products can be moulded at the same time.