Rotational molding design is an essential part of rotational molding, a plastics manufacturing process for creating strong, hollow parts with features such as multiple walls, undercuts, and molded-in graphics and hardware. Applications for rotational molding services include featured products like this Heineken Draught Keg Cooler, a complex assembly which contains multiple rotomolded parts.
For part designers and engineers, an overview of rotational molding’s design requirements is the best place to start. Functional requirements, material and processing limitations, and production costs are key considerations, but designers and engineers must also follow some best practices for wall thickness. Reinforcing ribs are an important part of rotational molding design, too.
Reinforcing Ribs and Rotomolded Parts
Reinforcing or stiffening ribs are especially important in rotomolded plastic parts where the walls are thin relative to their size. This is often the case with rotomolded parts because designers want to keep nominal wall thickness to a minimum. There are many advantages to rotational molding, but thicker walls can increase cycle times and material costs. By adding reinforcing ribs to their designs, engineers can get strong, lightweight parts with thin walls, short molding cycles, and lower costs. It’s also possible to mold-in other structural components such as metal rods or channels for added strength.
For designers, it’s important to understand that several shallow ribs are easier to produce than a single deep rib. Because deep, narrow ribs are really closely-spaced parallel walls, these supports must not exceed the design guidelines for parallel walls. Also, unlike with injection molded or compression mold parts, the stiffening ribs for rotomolded parts are hollow instead of solid. This makes these design elements similar to vacuum formed parts, which are both hollow and lightweight.
Design Guidelines for Stiffening Ribs
According to the Association for Rotational Molding (ARM), the depth of a rib should be at least four times the nominal wall thickness. In addition, the width of a rib should be at least five times the nominal wall. Depth controls the amount of stiffness, but increasing depth in relation to width adds complexity (and therefore cost) to the rotational molding process. For best results, ARM’s design guidelines state that rib width should exceed rib depth.
Rounded reinforcing ribs are easier to mold, but rectangular ribs provide more stiffness for the same amount of plastic that’s used. This shape-based strength differential is due to the perpendicular positioning of the rectangular rib’s depth. Finally, the side walls of a reinforcing rib need tapers to improve their mold release. We’ll return to tapers in a discussion of draft angles, the subject of a future blog entry in this series from Gregstrom.
Get Rotational Molding Design Assistance
Do you have questions about reinforcing ribs? Would you like to learn more about rotational molding design for plastic parts and products? For over 70 years, Gregstrom Corporation has manufactured rotomolded parts that are Made in the USA. To learn how we ca help you, contact us.