Common Design Mistakes to Avoid in Rotational Molding

Rotational molding (or rotomolding) is a highly adaptable process, ideal for creating hollow parts like containers, tanks, and complex structures. It provides manufacturers with the flexibility to produce large parts in various shapes without the need for significant material waste or complex tooling. Despite these advantages, the success of a rotational molding project depends largely on the design phase. Missteps at this stage can lead to poor product quality, structural weaknesses, or inefficiencies that raise costs and slow down production.

In this blog, we’ll explore common design mistakes often made in rotational molding and discuss how designers can avoid these pitfalls to ensure a seamless production process and high-quality output.

1. Failure to Account for Wall Thickness Distribution

One of the unique challenges in rotational molding is controlling the wall thickness. Unlike other molding processes where pressure fills the mold, rotational molding relies on gravity and rotation to distribute material. This can result in uneven walls if the design isn't well thought out.

Avoiding the issue:
To maintain consistent thickness, ensure your design avoids sharp transitions and deep recesses. Areas with too many variations in geometry, such as tight corners, can cause resin pooling or thinning. Instead, aim for smooth, flowing shapes that allow the material to spread evenly during rotation.

2. Overly Complex Mold Features

Designing molds with intricate details can cause problems during the molding process, especially if these details lead to resin not flowing as intended. Underestimating how material moves inside the mold can result in weak points or areas that don’t fill properly.

Avoiding the issue:
Simplify geometries where possible and avoid unnecessary complexity. Features like undercuts, sharp internal angles, and thin projections should either be eliminated or refined to allow smoother material distribution. If such details are essential, you may need to consider secondary processes to add them post-molding.

3. Insufficient Draft Angles

The absence of adequate draft angles is another common mistake that can make it difficult to remove the final part from the mold. Without the right taper, the part might stick to the mold surface, increasing the likelihood of damage or delays.

Avoiding the issue:
Include draft angles of at least 3-5 degrees on all vertical surfaces. This will help with part release and reduce the chances of damaging the molded component during demolding. The greater the wall height, the more critical it is to implement these angles.

4. Neglecting Shrinkage Tolerance

Plastic materials typically shrink as they cool, and rotational molding is no exception. Designers who fail to account for shrinkage in their plans might end up with parts that are smaller than intended or that don't fit together properly.

Avoiding the issue:
When planning your design, consult with material suppliers or experts to get accurate shrinkage rates for the material being used. Each type of plastic, be it polyethylene or polycarbonate, has a different shrink rate, and designs need to accommodate these values to avoid issues in fit and performance.

5. Overlooking Proper Venting in the Mold

Proper ventilation is critical in rotational molding. Without vents, air can get trapped in the mold, leading to incomplete filling, voids, or surface imperfections like bubbles or warps.

Avoiding the issue:
Make sure your mold design includes well-placed vents that allow trapped air to escape as the resin flows in. These vents should be small enough to prevent resin leakage but large enough to ensure smooth airflow, particularly in complex or large molds.

6. Designing with Inconsistent Surface Textures

Surface texture inconsistency can lead to challenges in both appearance and functionality. Overly rough areas might not release from the mold well, while overly smooth areas might not allow for proper material adherence during production.

Avoiding the issue:
Design the surface finish with production in mind. For areas that require ease of release, smoother finishes are ideal. However, if specific texture is needed for functional or aesthetic reasons, ensure it won’t complicate the part removal process.

7. Undervaluing the Cooling Phase in the Design

Designers often focus heavily on the heating process, but cooling plays an equally crucial role in rotational molding. Uneven or inefficient cooling can lead to warping or distortion in the final product, especially if the design doesn’t account for proper cooling paths or surface exposure.

Avoiding the issue:
Incorporate features that promote uniform cooling throughout the mold. Proper airflow, cooling channels, and balanced surface areas can help control the cooling rate, preventing defects like warping or uneven shrinkage.

8. Poor Placement of Reinforcing Features

Adding reinforcements, such as ribs or bosses, can strengthen a part without adding excessive weight. However, placing these features incorrectly can lead to stress concentrations or cooling challenges, resulting in cracking or warping.

Avoiding the issue:
Design reinforcing features with a focus on balanced weight distribution and ease of resin flow. Keep ribs and bosses away from sharp transitions and edges to prevent undue stress during cooling. As a rule of thumb, reinforcement features should not exceed 60% of the part’s wall thickness.

9. Inadequate Consideration for Post-Molding Processes

Sometimes, designers fail to think ahead about how the part will be handled after molding, whether it’s trimming, assembly, or applying finishes. If the design complicates these processes, it can increase costs and slow down production.

Avoiding the issue:
When designing, always think about post-molding needs. Include trimming allowances where necessary and ensure that the design can easily accommodate any secondary operations like cutting, drilling, or surface finishing. Plan the design to simplify any post-molding steps and reduce unnecessary complexity.

Conclusion

Designing for rotational molding requires a thorough understanding of the process’s unique characteristics and limitations. By avoiding common mistakes like ignoring wall thickness uniformity, neglecting shrinkage allowances, or using overly complex geometries, designers can greatly improve both the efficiency of the process and the quality of the final product.

At Benfan, we specialize in rotational molding solutions that deliver quality and precision. If you’re looking to improve your rotomolding projects or need expert advice, contact us today to see how we can help you streamline your production and achieve outstanding results.