Part Details Review
| Height |
The ideal height of a boss allows adequate length of the thread engagement in the hole. When a boss is also used for the purpose of setting exact assembly height of two components with tall side walls, a taller boss may be needed.
However, bosses greater than five times their outside diameter can result in thin wall sections at the top and thick sections at the base. Draft on the outside walls and the hole, both of which are critical to the smooth ejection of the boss, unfortunately work together in reducing wall thickness at the top of the boss while increasing it at the base. The base wall thickness also increases as the fillet radius is increased.
Thin sections at the top may cause filling problems and thick bases increase the potential for sink marks. Thick sections are also associated with higher molded-in stresses. Additionally, as the top section becomes thinner, its ability to tolerate hoop stress is reduced, which may result in cracked bosses upon screw installation.
Cores in tall bosses can become too long to be efficiently cooled, further accentuating the sink marks at the base. Long and thin cores are also prone to core-shift at the unsupported end, causing uneven wall thickness. Consider coring a tall boss from two sides as shown in figure 1.
If the design permits, a better alternative is to split the boss height between the two components as shown in Figure 2. The split is made such that one component has a blind boss and the other a through boss. Splitting the boss also reduces the negative effects (thin tops and thick bases) of the draft, which is now distributed to both sides.
