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Why Radii Matter in Rotomoulded Product Design
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A sharp corner may look precise in CAD. Inside a rotating mould, it behaves more like a warning.
Rotomoulding does not force molten plastic into a cavity under high pressure. A measured charge of polymer, usually in powder form, moves through a heated mould as it rotates around two axes. The particles contact the hot surface, begin to adhere and gradually form the wall of the product.
This difference matters wherever two surfaces meet.
In rotomoulding, a radius is not simply a styling decision. It influences how predictably material reaches a corner, how the wall develops and how loads pass through the finished part.
“The objective is not to round everything. It is to give every transition enough geometry to work.”
ILLION
In short
Generous radii generally support more consistent moulding and distribute stress over a broader area. Sharp inside corners are more difficult to fill and tend to form thinner walls. Outside corners behave differently and often become thicker than the surrounding wall.
The right radius depends on nominal wall thickness, material, corner angle, geometry and product function. It should be discussed before the CAD model is released for tooling.
Rounded transitions in a rotomoulded PE product. Radii support more predictable wall formation and reduce local stress concentration.
A corner is not just a line
On screen, a corner can be reduced to two surfaces meeting at an exact angle. During production, it is a thermal and material-distribution zone.
An inside corner may be among the last areas of the mould to reach the temperature needed for effective powder pickup. Powder can also bridge across a tight recess instead of reaching its deepest point. The result may be a locally thinner wall, trapped air, incomplete fusion or an inconsistent corner.
An outside corner tends to heat earlier and begin collecting material sooner. It will often become thicker than the nominal wall. That additional thickness can be useful because corners frequently carry higher stress, but uncontrolled accumulation may also influence cooling and dimensional behaviour.
This is why “uniform wall thickness” in rotomoulding should never be interpreted as identical thickness at every point. Geometry changes the way the wall forms.
Radii improve more than mouldability
A rounded transition spreads stress across a wider area. A sharp transition concentrates it. The difference becomes important near load points, handles, fixing areas and features exposed to repeated impact or flexing.
Adding more polymer to the mould does not automatically correct a poorly designed corner. It increases the overall part weight and cycle demand, while the local geometry may continue to produce the same weak area. A better transition can be more effective than another millimetre added everywhere. This is also why long product life depends on more than wall thickness.
The Association of Rotational Molders design guide identifies two main functions of corner radii: improving corner moulding and distributing stress. For polyethylene parts, it describes a radius equal to around 75% of nominal wall thickness as a useful optimum, while radii below 25% may generate high stress at inside corners.
These values are guidance, not a universal CAD preset. A 90-degree corner, an acute recess and a large structural transition do not present the same conditions. Material grade, powder quality, mould construction, heat transfer and the orientation of the part during moulding also matter.
Not every edge has to look soft
Designing for rotomoulding does not mean turning every product into a featureless rounded object.
A product can still have a precise visual language. Surface transitions, split lines, textures, changes of plane and secondary details can create definition. The important distinction is between an edge that communicates the design and a tight corner that creates unnecessary production risk.
Good design does not hide the manufacturing process. It uses its behaviour deliberately.
The ILLION approach
At ILLION, radii are reviewed together with wall thickness, material, loads, mould construction and demoulding. We look at where a corner sits, how it is likely to form and what the product must withstand in use.
This review belongs early in development. As we explain in From Rough Sketch to Real Mould, CAD geometry is not only a description of appearance. It becomes a manufacturing decision.
A radius drawn before tooling can look like a small detail. In the finished product, it may determine whether the corner is predictable, strong and repeatable.
Key takeaways
n rotomoulding, corners influence material distribution, wall thickness and structural performance. Inside corners tend to become thinner, while outside corners often become thicker. Appropriate radii help the material form more consistently and reduce stress concentration.
The objective is not to round everything. It is to give every transition enough geometry to work.
References and industry guidance
Frequently Asked Questions
They can heat later than surrounding surfaces, while powder may bridge across the recess instead of reaching its deepest point. This can produce a thinner or incompletely formed corner.
Yes. Outside corners usually collect more material and become thicker than the nominal wall. A controlled radius helps make the transition more predictable and supports better stress distribution.
Not reliably. Increasing the material charge makes the entire part heavier, but it does not remove the local thermal and distribution problem created by the geometry.
No. The appropriate radius depends on wall thickness, material, corner angle, geometry and function. Guidance based on nominal wall thickness is a starting point, not a substitute for reviewing the complete part.
