Design
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Bosses
Did you know?
Typically, the boss OD = 2 x ID with 1/2° minimum draft. Draft on boss ID should be 1/2° minimum. Some draft is required on ID for part ejection but the amount of draft will depend on the requirement of the specific fastener or insert. The wall thickness at the base of the boss should be less than 60% of the nominal wall thickness.
The rule of thumb for boss OD allows for a suitable wall thickness even as the ID increases. Avoiding overly thick wall sections at the base is important for minimizing the chance of appearance or molding problems such as sink and voids (see figure 1).
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Figure 1. Design Improvement on a Typical Boss |
Did you know?
Also, as a rule of thumb, boss height should be less than 3 x OD.
A tall boss with the included draft will generate a material mass at the base. The reduced mass minimizes possibility of sink and voids on the adjoining wall. In addition, the shorter core pin creating the ID will be easier to cool during molding (see figure 1).
Did you know?
The boss should be radiused at the base with a radii of between 25% to 50% of the nominal wall thickness (see figure 1). The end of the cored hole in the boss should have a minimum radius of 0.010”.
Bosses are typically attachment points and can carry significant loads. The radius reduces the stresses built up from molding and application. In addition to the radius, the boss can be strengthened for side load by the use of gussets or ribs tying the boss to a side wall (see figure 2).
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Figure 2. Boss Designs Separated from but Tied to a Wall Section |
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Bosses adjacent to an external wall should be placed inboard a minimum of 0.125” to the edge of the boss OD (see figure 2).
Not tying the boss to the wall reduces the mass of material at the wall thereby reducing filling time and reducing risk of sink at the wall. The boss can be connected to the wall by a rib of suitable thickness.
Did you know?
The minimum distance between two bosses should be kept to twice the nominal wall thickness.
When features are located too close together to each other, thin difficult to cool sections of the mold steel will exist and can create quality and molding difficulties. The same condition may exist if ribs are placed close together as in tying to a boss (see figure 3)
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Figure 3. Typical Attachment Ribs – Note Cored Out Approach in First and Last Designs |
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