
Learn about minimum wall separation in rotational molding, a plastics manufacturing process for strong, hollow parts.
Rotational molding can produce parallel walls, but designers need to leave enough space between them. In general, the recommended distance between parallel walls is five times the thickness of the nominal wall, the basic frame that defines the part’s shape. A minimum wall separation of three or four times the nominal is sometimes possible, but there are risks to this approach.
That’s because the plastic material in rotational molding needs to coat all the surfaces of the mold, or tool. If a part’s parallel walls are too close together, the mold cavity won’t fill completely. As the Association of Rotational Molders (ARM) explains, a “bridge” or “dike” can form and become a solid wall of plastic.
Costs, Quality, and Minimum Wall Separation
Solid sections of molded plastic take longer to cool, and this increases costs and extends cycle times. A non-uniform wall thickness can also result in part warpage or excessive mold shrinkage. By designing your rotationally molded parts with a minimum wall separation of five times the nominal wall, you can get good-quality, cost-effective plastic parts.
Wall separations of three times the nominal wall thickness have been rotationally molded with only occasional bridging. However, parallel walls like this are more difficult to produce and, therefore, more expensive to achieve. Because the bulk factor of powdered plastic is about three times that of the solid molded part, there simply must be enough volume in the cavity to receive the powder.
Ask Gregstrom for Rotomolded Parts
Gregstrom Corporation is an experienced rotomolder that can help you specify the proper minimum separation for parallel walls. In addition to design assistance, we offer tooling services that can capture complex part geometries in various sizes and surface finishes. Our value-added services also include plastic finishing, CNC and robotic routing, and assembly and testing.