Part Two: Designing Products for Rotational Molding

Overview

The product design process is, by definition, about finding a balance between competing requirements. Good designs represent an optimized compromise between such requirements. In terms of rotationally molded plastic parts, an optimized design should simultaneously satisfy the following basic conditions:

• The part must provide the end-use functional performance needed.
• The part design must take into account the limitations of the material from which it is made, as well as the demands and restrictions of the rotomolding process.
• The product must fall within a specified cost range, in order to be economically feasible.
• In many applications, there will be a desire for the part to have aesthetic appeal.

For most designs, a basic shape will initially be created, which satisfies the functional requirements of the product. Attention will then be given to finalizing the detail, in order to ensure that the overall part design is compatible with the material and process that has been selected. For parts with specific mechanical performance specifications, this may include structural analysis, other computations and even product testing.

Rotational molding has its own set of part design requirements, many of which are unique to the process. In rotomolding, there are no directly applied forces on the plastic melt, as there are for injection and blow molding. The mold rotates through a pool of liquid or powdered plastic and the plastic gradually builds up on the inside surface of the mold cavity. Achieving the desired wall thickness is a question of heat and time and it is difficult to create sudden transitions in wall thickness.

The ideal design for a rotationally molded part is a hollow shape where the various elements in the part design are smoothly blended from one contour to the next. Such gradual shape transitions will result in a finished part that has optimum strength, is relatively easy to mold, and is more economical overall.

The correct treatment of design details such as wall thicknesses, corner radii, draft angles and tolerances can make the difference between success and failure. The design guidelines contained within the sections that follow will assist the creation of a well-designed rotationally molded plastic part, which fits the materials and process being used.

As described previously, the majority of rotomolded parts are made from polyethylene. However, the design recommendations will mostly be pertinent to any plastic material that can be rotomolded. Despite this, it should be recognized that some plastics may have specific design requirements.