Bosses are used for the purpose of registration of mating parts or for attaching fasteners such as screwsor accepting threaded inserts (molded-in, press-fitted, ultrasonically or thermally inserted).
The wall thicknesses should be less than 60 % of nominal wall to minimize sinking. However, if the boss is not in a visible area, then the wall thickness can be increased to allow for increased stresses imposed by self-tapping screws.
The base radius should be a minimum of 0.25 x thickness
The boss can be strengthened by gussets at the base, and by attaching it to nearby walls with connecting ribs.
The maximum insertion (or withdrawal) force Fmax and the maximum hoop stress, occurring at the inner diameter of the boss, smax is given by
Failures of a boss are usually attributable to:
High hoop stresses caused because of too much interference of the internal diameter with the insert (or screw).
Knit lines -these are cold lines of flow meeting at the boss from opposite sides, causing weak bonds. These can split easily when stress is applied.
Knit lines should be relocated away from the boss, if possible. If not possible, then a supporting gusset should be added near the knit line.
BOSSES
Bosses are used to facilitate the registration of mating parts, for attaching fasteners such as screws, or for accepting threaded inserts.
Wall thicknesses for bosses should be less than 60 percent of the nominal wall to minimize sinking. However, if the boss is not in a visible area, then the wall thickness can be increased to allow for increased stresses imposed by self-tapping screws.
The base radius should be a minimum of 0.25 X thickness. Bosses can be strengthened by incorporating gussets at the base or by using connecting ribs attaching to nearby walls.
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Bosses
Bosses are used for mounting purposes or to serve as
reinforcement around holes. Good and poor design is
shown in Figure 3.09.
As a rule, the outside diameter of a boss should be 2 to
21⁄2 times the hole diameter to ensure adequate
strength. The same principles used in designing ribs
pertain to designing bosses, that is, heavy sections
should be avoided to prevent the formation of voids or
sink marks and cycle time penalty. Boss design
recommendations are shown in Figure 3.10.
Walsh's Plastic consulting. 