Flat surfaces seem simple, but they can present design challenges in rotational molding. That’s because rotomolded parts are made inside hollow molds without internal cores or structural support. This can cause bowing or warping, especially on large, flat panels. If you’re designing a plastic tank, housing, or enclosure, avoiding these defects is crucial.
Gregstrom is an experienced plastics manufacturer that can help you achieve rotomolding success. Keep reading to learn more, and contact us to discuss your application.
Why Flat Surfaces are Challenging
When a rotomolded part cools, it shrinks away from the mold walls. On large flat surfaces, this shrinkage is rarely uniform. The center of the part often pulls inward more than the edges, creating a dish-like indentation or internal bow. Because the inside surface is unsupported, the material naturally seeks a low‑stress shape instead of a perfectly flat plane.
Commercial Flatness Tolerances
Polyethylene, a commonly used rotational molding material, has commercial flatness tolerances of ±0.020 to ±0.030 in/in for standard applications, according to the Association for Rotational Molders. For precision applications with optimized tooling and tight process control, ±0.010 in/in is achievable. Yet both the part and the mold must be engineered for flat surfaces.
Five Design Strategies to Improve Flatness
Follow these strategies when designing rotomolded parts.
1. Break Up Large Flat Areas
Avoid large flat areas. Consider adding ribs, contours, surface breaks, or functional features. These design elements provide stiffening and reduce the visual impact of minor warpage.
2. Add a Slight Outward Crown
A controlled outward curvature helps to counteract the natural inward bowing that occurs during cooling. Designers often use this strategy with rotomolded lids, covers, and access panels.
3. Use Reinforcing Ribs
Ribs molded into a part’s exterior surface can improve stiffness and flatness. Design them with smooth transitions and appropriate radii to avoid thin spots or inconsistent heating or cooling.
4. Apply Textures or Graphics
Surface textures, patterns, or molded‑in lettering can visually mask minor deviations from flatness. This won’t change the part’s geometry, but it does improve perceptions of quality.
5. Use Secondary Fixtures During Cooling
Cooling fixtures or jigs can hold a rotomolded part in the desired shape as it solidifies. This is especially useful for large panels or parts with tight dimensional requirements.
Gregstrom is a Value-Added Rotomolder
Flatness in rotational molding is possible, but it requires a well-planned design. By understanding how shrinkage, wall thickness, and unsupported surfaces interact, you can create parts that meet both functional and cosmetic requirements. With the right combination of geometry, tooling, and process control, even large flat surfaces can be molded successfully and consistently.
Gregstrom is a value-added rotomolder that offers design assistance. With our 75+ years of rotational molding experience, we’re ready to help. Contact us for a quote or to discuss your project.
