Definition: A curved surface which is tangent (abutting) to the adjoining planes thus eliminating a sharp internal angle. While two planes may make a 90 degree angle an arc is formed which joins the two lines at either side of the interior angle.
One of the basic rules
This has been described in most all manufactures data that includes design of part and is one of the things that are most often disregarded, until that is there is a part breakage or component failure.
The general rule is that:
All inside corners shall be radiused at minimum .2 T where T is the wall thickness of the part.
An ideal start point for most all materials is 0.5T.
*** note that anything is better than nothing at all.
Now this consideration may change for material types, but any sharp corner on an inside of the part acts as stress concentration point and the part shall most often break there. As the ratio of radius to wall thickness decreases the stress concentration of sharp corner decreases but never goes away.
Below are data points for a type 66 nylon that has been conditioned at 50% RH per a notch IZOD impact test.
|
Radius |
Force to break (Ft.-Lb./in.) |
|
0.001" |
0.5 |
|
0.004" |
1.0 |
|
0.010" |
2.0 |
|
0.020" |
4.0 |
|
0.040" |
12.0 |
|
|
|
As can be seen an increase in radius of the notch has increased the amount of force it takes to break the sample.
The above illustrates
A- No radius not recommended
B- Inside is radiused recommended
C- Ideal as it not only radiuses the inside corner but also maintains the uniform wall thickness.
It is critical to note that as the radius in the corner is increased that it does not get too big and thus create other issues due to non-uniform wall thickness and or stress caused by sinking due to thicker wall thickness. The example uses a radius the same as the wall thickness.
The radius (green) to the adjoining wall should always be smooth and not have a step as this step can lead to a notch or inside sharp corner and thus while the actual corner is radiused the construction of the radius defeats the radius due to mismatch creating its own break point.
Further the author has seen cases of flash on a part creating a sharp corner and thus a notch which caused the parts to break in certain materials.
25032013.01
SL SILVEY
www.silveysplasticconsulting.com
