surface flatness and reinforcement elements

Rotationally moulded products are hollow and it is generally accepted that it is difficult to assure flatness of a plane surface. Where this is critical, it may be possible to counteract anticipated distortion by adding a crown to the face or reinforcing ribs to the product.

Reinforcing or stiffening ribs are added as hollow elements and act in a similar manner to corrugated sheet materials. The greater the height of the rib the greater the stiffness and the sides of the ribs will be designed with a draft angel or be tapered to assist in removal from the mould. It is a commonly used feature in rotational moulding to use two closely spaced walls, which are moulded together to provide additional strength and/or rigidity. This feature is known as kiss-off.

radii and angles

It is vital to understand the nature of viscosity of the rotational moulding material and how any given material flows and forms within the mould. Sharp corners or difficult to access areas may result in porosity. A large radius for example improves the flow of material at corners and assists in the distribution of stress. It is important to design in draft angles at right angle to the parting line to minimise distortion especially where a product tends to toward or around the mould.