Bosses in Product Design

Bosses are small cylindrical elements in a design. They are used for accepting screws or other fastening components (threaded inserts usually). Essentially they are locating, mounting and assembly devices built in to the design of the part and if not for accepting screws, will attach to the mating part of another component of the design. To ensure the highest quality during the injection moulding of your part, there are design guidelines to follow when designing bosses.

• Boss design elements:
  • Thickness ratio. Similarly to ribs, the wall thickness of bosses should be no more than 60 percent of the main wall thickness of the part.
  • Base radius ratio. The radius of the base should be at least 25 percent of the main wall thickness. Thus, the diameter of the base of the boss should be at least 50 percent.
  • Bosses should always be connected to adjacent walls using ribs or gussets. Often bosses are designed with thick wall parts or are connected to walls, resulting in a thick section of the part which can warp during manufacture.
  • The base of bosses, as with any attachment between two surfaces, should always be designed with a fillet ratio included. This will ensure that stresses experienced by the boss - such as through the insertion of a screw - will not result in fracture.

• Boss failures and how to avoid them:
  • High hoop stresses. These are particularly annoying as they may only become apparent after the part is designed and the entire run has been injection moulded. High hoop stresses are caused when the insert damages the internal radius of the boss.
  • Knit lines. These lines appear when the flow of molten material is party cooled when it reaches the top of the boss. These lines are visible and can easily crack when pressure is applied. Knit lines can be controlled by moving the parting line of the mould. If this is not possible the addition of a gusset or rib is advised.

Bosses require careful thought when being designed. Due to their nature they are both one of the most fragile parts of the design and one of the elements in most need of strengthening.