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Overview: A 'boss' is a circular protrusion, which has a variety of functions in the assembly of plastic parts. Use bosses to support and align two parts or, more commonly, attach two parts using pegs, self-tapping screws, inserts, or threads.
A 'core', as in a mold half, is used to form a depression or hole; this is usually done with a pin.
Bosses
You can easily create a boss feature in CATIA by sketching a circle, specifying a diameter, and using a Pad feature for the height. Since all walls of a part should be tapered, by adding the draft using a Drafted Filleted Pad feature, you avoid having to add a taper feature later.
Be careful when adding a boss to the shell of a part. If you did do this, a problem is immediately visible. A thick chunk of plastic on the surface of the shell is prone to leave a sink mark.
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Even if there is no functional purpose, always core a protrusion, such as a boss, by adding another feature to hollow the protrusion.
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For common purposes in plastic part design, hollowing the boss from the top, rather than the bottom, is acceptable.
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Boss Design Considerations
Like any plastic part side walls, both the inside and outside walls of the boss should include a draft.
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The only exception is with self-tapping screws, which require a straight inside wall.
The corners of the boss should also blend, both for flow and strength. Limit the corner radii on the outside of the boss to 25% of the thickness. The inside bottom corner of the hollow boss can be a miniscule .25mm (.010in).
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By examining the intersection of the boss' wall and the part's shell, it is clear that designing the boss with the nominal thickness breaks the uniform wall thickness rule.
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Reduce the boss' wall thickness to 50% - 75% of the nominal to minimize the likeliness of sink marks. However, making the wall thinner may weaken the boss too much. This compromise is one you must weigh on a case-by-case basis.
An alternate method of strengthening a boss by thickening the wall is to add gussets or stiffeners around the perimeter of the boss.
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When locating a boss, be aware of its proximity to the wall of the part or other features. Though it may be tempting to combine features, remember that it creates thick areas that are susceptible to sink.
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It is better if you can position the boss away from the feature. In this case, you can add a connecting rib for strength.
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