Flat Walls and Sharp Corners in Rotational Molding

Corner Radiuses | corner radii | flat walls
Corner Radiuses | corner radii | flat walls

Flat walls can be difficult to rotomold. That’s because the tools are hollow and do not have internal cores. To avoid flat walls, consider adding reinforcing ribs, kiss-offs, or crowns to your part design. Each of these features has its advantages.

  • Reinforcing ribs add strength and help keep nominal wall thickness to a minimum for shorter cycle times and lower material costs.
  • Kiss-offs increase the stiffness of rotomolded parts and transfer loads from one side of the part to the other.
  • Crowns also add stiffness and can prevent “oil canning”, a cosmetic issue that causes parts to seem to bow outwards.

Tolerance is also an issue with flat parts. Generally, the commercially accepted tolerance for flat rotomolded parts is ±.20 inch-per-inch. Depending on your design, however, you may need to increase this amount to ±.50 inch-per-inch or more.

Sharp Corners

Sharp corners are also a challenge for part designers. They affect the flow of resin, create stress in the finished product, and tend to collect more heat for thicker wall sections and thinner inside corners. For best results, avoid sharp corners by applying corner radii to your part design.

In a metal mold, sharp inside corners tend to be some of the last parts of the tool to reach the required molding temperature. When corners like this cause plastic to migrate away from the tool, the result is an unwanted reduction in wall thickness.

Sharp outside corners are also a problem. Typically, they’re the closest part of the tool to the source of the oven’s heat. Consequently, they reach molding temperature first. Because the plastic tends to accumulate in the outside corners, this can create walls that are too thick.

Gregstrom is a Rotomolder That Offers Design Assistance

Gregstrom is a third-generation, family-owned, Made in USA rotomolder with over 75 years of plastics manufacturing experience. When you partner with us, you can get the design assistance you need for challenges like flat walls and sharp corners. Our value-added services also include tooling services, parts finishing, and quality assurance (Q&A).

Ready to discuss your next plastics project? Contact Gregstrom and let’s get started.