Boss cracking is mostly a wall-ratio failure. The working numbers: boss wall thickness is 0.5-0.6 times the main wall, boss outer diameter is screw diameter plus twice the boss wall, the root fillet is at least 0.25mm, and draft is 0.5-1 degree. Of those four, the wall ratio is the one that ruins a first batch: exceed it and you hear the crack while driving the first screw.
The root is where three surfaces meet - boss wall, housing wall, and any rib. Material piles up there, the inside and outside cool at different rates, and tensile stress appears at the root first. The thicker the ratio, the more material is stacked and the higher the stress. On a handheld device we had an M2 screw in a 1.4mm boss wall on a 1.8mm main wall, a ratio near 0.78. The first batch cracked while screws went in. We brought the ratio to 0.55 and the cracks stopped. The fix was a tool insert and two days. A ratio is one digit on a drawing and a scrapped batch on the line.
Self-tapping screws form their thread by displacing plastic, so the pilot hole has to be right. The working formula is pilot diameter equals screw diameter minus 0.8 times pitch. For M2.5 with a 0.45mm pitch that gives about 2.1mm, and we specify 2.0-2.1mm. Too small and the boss splits; too large and the screw strips.
Boss height is bounded by two conditions: no more than three times the outer diameter, and at least 1.5 times the screw diameter of thread engagement. When they conflict, keep the engagement and add ribs for stability. A tall, thin boss also lets the core shift during injection, which shows up as a coaxiality problem. For M2.5, an outer diameter of 4.3mm puts the height ceiling near 13mm; most of our projects stay at 8-10mm.
Draft is not optional. Both the outside of the boss and the inside, formed by the core, need it: 0.5-1 degree outside, and the same inside. Without draft the boss wall is dragged on ejection and loses strength. A textured boss needs more: add about one degree per 0.025mm of texture depth, a line that is easy to miss because bosses are often marked a flat 0.5 degrees as if they were never seen.
Cracking and sink are two faces of one decision, and fixing one brings out the other. Thickening the boss wall raises strength, but thick sections cool slowly and sink on the visible surface - and a sink mark on an appearance face is more obvious than a crack and cannot be repaired. The real solution is usually not in the boss but in the housing: thin the local section and add ribs so the load spreads into the shell, leaving the boss to locate and clamp. On a robot housing we added three short radial ribs on the back of each boss and dropped the boss wall from 1.0mm to 0.8mm; sink and cracking both went away.
Rib placement matters: on the outside of the boss, not the inside, never straight into an appearance surface, three ribs evenly spaced, length no more than a third of the boss height, thickness about 0.6 times the boss wall. Longer or thicker ribs turn the boss back into a thick block and the sink returns. The same logic explains why a wall thickness of 1.2mm in a 2mm housing is already past the waterline: it is not the number that breaks the part, it is what the number does to cooling.
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