Bosses

Bosses often serve as mounting or fastening points and therefore, for good design, a compromise may have to be reached to achieve good appearance and adequate strength. Thick sections need to be avoided to minimize aesthetic problems such as sink marks. If the boss is to be used to accommodate self tapping screws or inserts the wall section must be controlled to avoid excessive build up of hoop stresses in the boss.

General recommendations include the following:

Nominal boss wall thickness less than 75% nominal wall thickness, note above 50% there is an increased risk of sink marks. Greater wall sections for increased strength will increase molded-in stresses and result in sink marks. A minimum radius of 25% the nominal wall thickness or 0.4 mm at the base of the boss is recommended to reduce stresses. Increasing the length of the core pin so that it penetrates the nominal wall section can reduce the risk of sink marks. The core pin should be radiused (min 0.25 mm) to reduce material turbulence during filling and to help keep stresses to a minimum. This option does increase the risk of other surface defects on the opposite surface. A minimum draft of 0.5 degrees is required on the outside dimension of the boss to ensure release from the mold on ejection. A minimum draft of 0.25 degrees is required on the internal dimension for ejection and or proper engagement with a fastener. Proper boss design.

Further strength can be achieved with gusset ribs or by attaching the boss to a sidewall. Bosses adjacent to external walls should be positioned a minimum of 3 mm (.12 in)  from the outside of the boss to avoid creating a material mass that could result in sink marks and extended cycle times. Correct positioning of bosses.

A minimum distance of twice the nominal wall thickness should be used for determining the spacing between bosses. If placed too close together thin areas that are hard to cool will be created.  These will in turn affect quality and productivity.