The design of the part to be rotomolded must be handled by an engineer who understands the requirements and principals of roto-molding.

The ideal design for a rotationally molded part is a hollow shape where several aspects in the part are smoothly blended from one to the next. The angles and shapes of the part can be manipulated to add strength and the design process takes this into account. Ribs or kiss off points may be added to the design of the part to add rigidity and strength. Some designs can also allow for foam to be added to the structure for extra strength. Foam filling parts can greatly increase rigidity and the ability to bare weight. Once a design is successfully completed a mold will need to be made for manufacturing. Most molds for roto-molding are cast or fabricated from steel or aluminum. Molds can have multiple chambers creating double wall parts or designed for multiple part manufacturing. How well a mold functions contributes to the quality of the parts manufactured. Making changes to a mold or occasional mold repairs are a common part of rotational molding.

A custom rotationally molded part usually requires the design to be manipulated in order to maximize the potential of this process. An experienced roto-molder can walk a new customer through the process from design idea, through mold making and finally to manufacturing.