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Subject 2 : Rotational moulding Corners

Corner Radii – Corner Radius for Rotational Moulding

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)

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