Corner radii are angles at the internal and external corners of a part. They add strength by distributing stresses. For designers, this Q&A from Gregstrom provides answers to some frequently asked questions about corner radii in rotomolded products.
What is a corner radius used for?
In rotational molding, a corner radius serves two purposes. First, it distributes stress from the corner of a part over a broader area. Second, it improves quality by ensuring the adequate build-up of plastic material in the corner of a rotomolded part.
Why are sharp inside corners a problem?
In a metal mold, sharp inside corners tend to be some of the last parts of the tool to reach the required molding temperature. Corners like this can cause plastic to migrate away from the mold, leading to an unwanted reduction in wall thickness.
Is wall thickness the only concern with sharp inside corners?
No. Sharp inside corners can also increase porosity and lead to excessive shrinkage.
Are sharp outside corners also problematic?
Yes. Typically, sharp outside corners are the closest part of the mold to the source of the oven’s heat. That means that they reach molding temperature first. As the mold turns, the plastic tends to accumulate in the outside corners. This can create walls that are too thick.
What are some best practices with corner radii?
According to the Association of Rotational Molders (ARM), a corner radius that’s less than 25% of the nominal wall thickness places significant stress on a part. However, a corner radius up to 75% of the nominal wall thickness greatly strengthens the corner.
Do all rotomolded plastics need the same corner radii?
No. Because different types of plastics have different molding characteristics, designers need to specify a corner radius that’s appropriate for the material. For example, the minimum recommended angle at the corner of a polyethylene part is 30%.
Who can answer my corner radius questions?
Gregstrom Corporation of Woburn, Massachusetts (USA) is an experienced rotational molder with value-added services that include design assistance. To learn more about us and how we can help strengthen your next plastics project, contact us.