1. Radii: Rounded external angle between two surfaces.
2. Fillet: Rounded filling of the internal angle between two surfaces.
· Stresses tend to concentrate at the junction of two sections like corners, ribs, or external corners.
· Rounding of the corners will reduce the stress concentration hence proper radius should be used for a given wall thickness.
· When external corners are sharp, the section at the corner should be increased by about 30%
· Rounding of the corner with an external radius will tend to maintain a uniform thickness and therefore relieves stress concentration.
· Radii and fillets promote the distribution of internal load and stresses and thus release the stress concentration associated with changes in wall thickness and changes in direction of internal loads.
· Radii and fillets also promote even flow of material during molding, hence lower pressure can be used or the cycle time can be reduced.
· When stress concentrations are minimized, the strength of the part is increased.
· When generous radii are used at the junction of two walls, there is less possibility of cracks to occur.·A minimum radius of 1/64” should be provided, if possible.