Radius [rey-dee-uhs] – noun (plural -di-i). 1. a straight line extending from the center of a circle or sphere to the circumference or surface: The radius of a circle is half the diameter. 2. a rounded corner or edge on a machined, injection moulded or cast piece of metal or plastic.

The humble radius is an all-important factor of your product design, arguably as (or even more) important than the straight edge or plane. The radius must be calculated when designing for rounding corners, hole width, inside and outside wall radii, not to mention any curved plane. Every surface that is not dead straight will have been designed using a radius. Here is a run-down of radius use when designing your product to be sent for injection moulding.

  • Corners. Sharp corners are a bad idea in product design. Straight 90 degree angles can lead to a high concentration of stress in one small area (especially on the inside corner of an angled wall). It is very important to smooth out your corners using a consistent radius. The typically accepted formula holds that the outside radius should be 1.5 times the wall thickness and the inside should be 0.5 times the wall thickness. For instance, if your wall thickness is 6mm and you have two walls that meet at a 90 degree angle, your corners will need to be rounded. The outside radius will be 9mm, whilst the inside radius will be 3 mm.
  • Fillet radius. A fillet radius is a radius that smooths the connection between two parts, similar to the function of an inside radius for a corner. This has the same advantage – reducing stress – and may be complemented by a rib. A fillet radius may be used for attaching a boss to a plane or a rib to a wall. A fillet radius should be applied wherever two planes meet to create an angle, whether sharp or obtuse.
  • If your product design concept involves a curved plane of any kind, a radius should be applied to ensure a consistent and aesthetically pleasing design. In the same way that you would use a ruler to draw a straight line, you must use a radius to draw a curved line.

Applying radii to your design has many benefits. A radius allows the plastic to flow much more smoothly than a corner would. If a mould were to be created with sharp corners there is less of a guarantee that the entire mould would be filled, leaving gaps and a messy edge. A radius can handle stress a lot better than a corner or edge. An inside edge is nothing more than the starting place for a crack; it is where all the stress will be concentrated should your part be subjected to any amount of pressure, Finally and simply, rounded corners typically look better that sharp angles.