Part Details Review

When a boss is used for attaching two components of an assembly with the help of a screw, make sure that the top of the boss contacts the mating part. In the case of a metal insert, the top of the insert should be touching the mating part. Failure to do so will induce bending stress in the mating part and tensile stress in the boss as the screw is installed in it (see figure 1) The stress generated on the thread can damage it or cause the metal insert to pull out. Also, the mating component may experience bending stress.

Bosses are also attached to other bosses or to the side walls with ribs. Sometimes they are reinforced with gussets attached to the base walls. Attachments increase the torsional and bending stiffness of the boss. In rare cases, if an assembly is subjected to separation force, such attachments may provide additional support to the boss against breakage in tension. Generally, this does not happen as the boss might first fail under hoop stress or the threads might strip.

When using attachments, pay attention to proper layout of the bosses, including the spacing between them. Bosses which are too close to each other hamper efficient cooling. Avoid bosses that merge into sidewalls as they create thick sections and form sink marks in the sidewalls. Instead, position the bosses away from the sidewall, and if needed, use connecting ribs or gussets for support. Figure 2 shows various possibilities for accomplishing this.

An additional advantage of using attachments is that they aid in filling the bosses and in pushing the air out during molding.

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