Common Design Mistakes to Avoid in Rotational Molding

Rotational molding (or rotomolding) is an extremely adaptable process that is well suited for making hollow parts such as containers, tanks, and complex structures. It offers manufacturers the flexibility to produce large parts in a variety of shapes without the need for large amounts of wasted material or the use of complex tooling. Despite these advantages, the success of a rotational molding project depends heavily on the design phase. Mistakes during this phase can result in poor product quality, weak structures, or inefficiencies that increase costs and slow production.

In this post, 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. Not considering wall thickness distribution
One of the unique challenges of rotational molding is controlling wall thickness. Unlike other molding processes that fill the mold with pressure, rotational molding relies on gravity and rotation to distribute the material. This can result in uneven wall thickness if the design is not thoughtful.

Avoid Problems:
To maintain consistent thickness, make sure your design avoids sharp transitions and deep recesses. Areas with too much geometry variation, such as narrow corners, can cause resin to accumulate or thin out. Instead, try to go for smooth, flowing shapes that distribute the material evenly during the spinning process.

2. Overly complex mold features
Designing a mold with complex details can cause problems during the molding process, especially if those details prevent the resin from flowing as expected. Underestimating how the material will move within the mold can lead to weak spots or areas that won’t fill properly.

Problems to avoid:
Simplify the geometry as much as possible and avoid unnecessary complexity. Features such as undercuts, sharp internal corners, and thin protrusions should be eliminated or improved to achieve smoother material distribution. If these details are essential, you may want to consider adding secondary processes after molding.

3. Insufficient draft angles
Lack of adequate draft angles is another common mistake that can make it difficult to remove the final part from the mold. Without the correct taper, the part may stick to the mold surface, increasing the possibility of damage or delays.

Problems to avoid:
Maintain at least 3-5 degrees of draft angle on all vertical surfaces. This will help the part demold and reduce the possibility of damaging the molded part during the demolding process. The greater the wall height, the more important it is to achieve these angles.

4. Ignoring shrinkage tolerances
Plastic materials typically shrink as they cool, and rotational molding is no exception. If designers don’t account for shrinkage when designing, they may end up making parts that are smaller than expected or that won’t fit properly.

Avoid Problems:
When planning your design, consult your material supplier or an expert about the exact shrinkage values ​​for the materials you are using. Each plastic, whether it’s polyethylene or polycarbonate, shrinks differently, and the design needs to accommodate these values ​​to avoid fit and performance issues.


5. Ignoring proper venting of the mold
Proper venting is critical for rotational molding. Without vents, air can become trapped in the mold, causing incomplete fill, voids, or surface defects such as bubbles or warpage.

Avoid Problems:
Make sure your mold design includes well-placed vents to vent trapped air as the resin flows in. These vents should be small enough to prevent resin leaks, but large enough to ensure smooth airflow, especially in complex or large molds.

6. Designing with inconsistent surface textures
Inconsistent surface textures can create challenges for both appearance and functionality. Areas that are too rough may not release well from the mold, while areas that are too smooth may not achieve proper material adhesion during production.

Avoid Problems:
Design surface finishes with production in mind. For areas that need to be easily demolded, smoother surface finishes are ideal. However, if a specific texture is required for functional or aesthetic reasons, make sure it does not complicate the part removal process.

7. Underestimating the cooling stage in the design
Designers often focus a lot on the heating process, but cooling plays an equally important role in rotational molding. Uneven or inefficient cooling can cause warping or deformation of the final product, especially when the design does not consider proper cooling paths or surface exposure.

Avoid Problems:
Incorporate features that promote uniform cooling throughout the mold. Proper airflow, cooling channels, and balanced surface area help control cooling rates and prevent defects such as warping or uneven shrinkage.

8. Improper placement of reinforcements
Adding reinforcements such as ribs or bosses can increase part strength without adding too much weight. However, if these features are not placed properly, they can cause stress concentrations or cooling difficulties, which can lead to cracking or warping.

Avoid Problems:
When designing reinforcement features, focus on balancing weight distribution and ease of resin flow. Keep ribs and bosses away from sharp transitions and edges to prevent excessive stress during cooling. As a rule of thumb, reinforcement features should be no more than 60% of the part wall thickness.

9. Insufficient consideration of post-molding processes
Sometimes, designers fail to consider ahead of time how the part will be handled after molding, whether it is trimming, assembly or finishing. If the design complicates these processes, it increases costs and slows production.

Avoid Problems:
When designing, always consider post-molding needs. Include trimming allowances when necessary, and make sure the design can easily accommodate any secondary operations such as 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 unique characteristics and limitations of the process. By avoiding common mistakes such as ignoring wall thickness uniformity, ignoring shrinkage allowances or using overly complex geometries, designers can greatly improve process efficiency and the quality of the final product.