As a general guide, internal and external corners of Rotational Mouldings should have a minimum radius of 2mm. This ensures the powders will fuse evenly around or into the corners. With an even wall thickness, internal stresses are reduced and weak points eliminated.
Products are often designed with larger radii. This can be to dissipate structural forces over a greater area. They are also used for purely aesthetic reasons; unlike fabrications, there are virtually no limits to the designer’s imagination.
Having a minimum radius of 2mm is not a hard and fast rule. The following factors can be considered:
Internal corners are the most critical; without a radius, material may not build up to the desired wall thickness. Acute angles will need a larger radius, obtuse will need less.
External corners can have much less of a radius depending on the material and overall design, especially if the corner angle is less than 90 degrees. If the corner angle is more than 90 degrees it will often need a radius greater than 2mm. Sharp edges are possible on surface details.
Radii on the corners of rotationally moulded parts fulfil two functions:
A. They distribute the corner stress of the part of a broader area which adds strength to the part.
B. They help the moulding of these corners by the process – too tight a radius can give an incomplete corner.
A plastic part, on loading, will be highly stressed when the radius R on the inside corner is less than 25% of the nominal wall thickness. The stress is reduced as the radii are increased up to 75% of the wall thickness. Increasing the radius has much less dramatic effect on the stress reduction above this.
As well as the effect on stresses, sharp corners are problem areas in moulding.
Sharp inside corners tend to:
A. Be the last portions of the mould to react to moulding temperatures.
B. The plastic has a tendency to flow quickly over these corners.
These two factors result in a general reduction in wall thickness in the moulded part where there are sharp inside corners.
Sharp outside corners cannot always be filled out completely. If the corner is too sharp the first layers of plastic picked up by the mould tend to bridge across the corner leaving air bubbles and incompletely formed radius
Therefore, in rotational moulding it is recommended that all corners have generous radii. The table below gives the recommended radius for Polyethylene and Nylon.
POLYETHYLENE corner RADIUS for Rotational Moulding
Ideal – 12mm (inside), 6mm (outside)
Minimum – 4mm (inside), 3mm (outside)
NYLON corner RADIUS for Rotational Moulding
Ideal – 20mm (inside), 12mm (outside)
Minimum – 5mm (inside), 5mm (outside)