Building Better Bosses

Princeton’s WordNet defines “emboss” as “raise in relief,” and that’s exactly what a boss is: a feature raised above a surface. In plastic parts, bosses are typically used to assist in assembly, as a receptacle for a screw or threaded insert or as the locator for a mating pin on another part.

Because of its function, a boss must have sufficient strength to do its job. This dictates a minimum size for the feature. At the same time, because a boss rises from a surface, it thickens the surface at that point raising the risk of sink or development of voids as the part cools. The challenge: bosses should be big enough to do their job but not big enough to cause avoidable sink in the surface from where they rise.

A typical boss is an open-topped cylinder, essentially a round rib (see figure 1a). Standard guidelines suggest that its wall thickness be between 40 and 60 percent of the thickness of the wall from which the boss rises. If your design requires more strength than this guideline would provide, you should consider ways to strengthen the boss without thickening its walls. The most common of these is to surround the boss with gussets to support and strengthen its walls (see figure 1b).