In last article, we talked about basic knowledge of structural design of plastic parts in controller - wall thickness. Today we will talk about design of fillet transition and stiffeners.
A fillet is a concave transition surface between two intersecting surfaces. Proper consideration should be given to intersecting surfaces so that there are no sharp corner transitions. Elimination of sharp corners reduces stress concentrations at these points and produces parts with greater structural integrity. Fillet provides a streamlined flow path for molten polymer filling mold and allows for easier ejection of part from mold. Rounded corners make production of molds more economical because they are easier to machine. Outer diameter (Ro) should be equal to sum of inner diameter (Ri) and wall thickness. A recommended minimum radius (Rm) of 0.05mm is usually allowed, even where sharp edges are required, and this radius ensures mold is more economical and lasts longer. A larger radius should be specified whenever possible.
Ro=Ri+t
For smooth transitions, use an inner radius of 50% to 75% of thickness. For example, resulting radius with an inner radius of 60% of wall thickness is:
Ri=0.6t
Ro=0.6t+t
Figure below illustrates effect of fillet radius on stress concentration. Suppose a force P is exerted on cantilever portion shown. When radius R increases with all other dimensions held constant, R/T increases proportionally and stress concentration factor decreases as shown in curve. By increasing fillet radius to thickness ratio sixfold (from 0.1 to 0.6), stress concentration factor was reduced by 50% (from 3.0 to 1.5). This illustrates how easily stress concentration factor can be reduced by using a larger fillet radius. Optimally designed fillet was obtained with an R/T of 0.6±0.1, and further increase in radius only reduced stress concentration by a marginal amount. This is generally true for most shapes; however, other proportions may have to be used on specific parts due to other functional requirements.

 

Similarly, rotational stresses caused by use of threaded fasteners can exert forces at attachment points of threaded bosses. Adding a fillet radius has same effect of reducing stress concentration factor as above.
In figure above, two intersecting surfaces of radius R will have highly concentrated stresses. As long as the fillet of inside corners is less than 25% of nominal wall thickness, stress will build up quickly.
Next, let’s talk about related design of reinforcement ribs.
Ribs can add strength, appearance requirements and/or reduce uneven areas. Ribs should be oriented along bending axis. In discussions of wall thickness, uniformity of thickness is a topic. Same goes for adding ribs.
Add a minimum thickness of ribs to meet structural needs of design. Avoid adding ribs with a thickness close to basic thickness of part, otherwise dents and shrinkage will occur (violating 50%-80% requirement we mentioned earlier).
In areas where shrinkage is not a cosmetic or functional issue, a single larger rib is preferable to several shorter ribs, as taller ribs will increase their resistance to bending.

 

With any type of stiffener, section at the bottom (root) is always thicker due to addition of fillets. To minimize root thickness that can cause shrinkage, bottom fillet should be 40 to 60 percent of rib thickness. In addition, rib root thickness shall not exceed 25% of basic wall thickness. Height of ribs must not exceed 2.5 times basic wall thickness.

 

Image above illustrates concept of basic rib thickness. Diameter of rib's basic thickness is determined by measuring inscribed circle between tangents to fillet radii on both sides of rib and bottom of basic wall thickness. Therefore, for same basic rib thickness, a larger fillet radius at rib root will result in a larger basic rib diameter D. Therefore, fillet at the root of rib cannot be too large, otherwise it will easily cause shrinkage.