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Rib and Boss Design Guide for Injection Molding

Ribs and bosses are the most commonly used structural features — and the most common source of sink marks, weld lines, and short shots.

Rib Design

Thickness: 50-60% of wall (most common error: making ribs too thick)
Height: Max 3x wall thickness
Draft: 0.5-1° per side
Base radius: 25-50% of rib thickness
Spacing: Min 2x wall thickness between ribs

Buyer's Tip: Chinese mold makers may reduce rib thickness without asking if they see filling problems. This is usually OK, but if ribs go below 0.5mm base, they may not fill at all. Specify minimum rib thickness on your drawing.

Boss Design

Wall thickness: 50-60% of wall
Height: Max 2.5x outer diameter
Inner diameter: Match screw (typically 0.2mm under pitch diameter for self-tapping)
Depth: At least 3x screw diameter
Bottom clearance: 0.5-1mm
Support tall bosses with 2-4 gusset ribs

Common Defects

Defect	Cause	Solution
Sink behind rib	Rib too thick	Reduce to ≤50% of wall
Rib not filling	Rib too thin/deep	Increase thickness or add vent
Boss cracking	Wall too thin or sharp base	Increase OD, add base radius
Weld line at boss	Flow divides	Move boss or add overflow
Boss sinking	No core-out	Core out to 60% of wall

What This Means for Your Project: Standardize boss dimensions to simplify tooling. For M3 self-tapping screws in ABS: 2.0mm ID, 4.5mm OD, 8mm height. Share a boss standard with your Chinese mold maker in the RFQ.

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