How to properly design ribs and bosses in plastic parts?

Ribs and bosses are essential features in plastic part design. They add strength, support, and help with assembly — but if not designed correctly, they can lead to cosmetic defects, weak joints, and mold complications.

This article explains:

• What ribs and bosses are used for

• Recommended design guidelines

• Common design mistakes to avoid

• Practical tips for strong, defect-free ribs and bosses

• How GoodTech MFG helps optimize structural features

  
  

What Are Ribs and Bosses?

• Ribs are thin, vertical support structures that add rigidity and reduce material usage without increasing wall thickness.

• Bosses are cylindrical features often used for screws, fasteners, or alignment in assemblies.

Design Guidelines for Ribs

Rib Thickness

• Should be 40–60% of the adjacent wall thickness

• Too thick → causes sink marks and uneven cooling

Rib Height

• Typically 2.5 to 3 times the wall thickness

• Tall ribs may require multiple short ribs or gussets instead

Draft Angle

• Apply 0.5° to 1° draft to each side for easy mold release

Rib Spacing

• Space ribs at least 2 to 3 times their thickness apart to avoid cooling hotspots

Design Guidelines for Bosses

Boss Wall Thickness

• Should be 60% of the outer wall thickness max

• Avoid solid bosses — core them out to prevent sink marks

Boss Base Support

• Add connecting ribs between boss and base to distribute load

• Use fillets at the base to reduce stress concentration

Draft & Hole Design

• Add 0.5°–1° draft on outer walls and through holes

• Leave 0.5 mm clearance in through-holes for screw guidance

Common Mistakes in Rib and Boss Design

• Too thick ribs → sink marks on the visible surface

• Unsupported bosses →  weak screw retention or cracking

• Sharp corners → stress concentration and flow hesitation

•  Zero draft → difficult or damaged ejection

Pro Tips for Design Optimization

• Use ribs instead of increasing wall thickness for strength

• Avoid overlapping thick bosses with other solid features — core them out

• Run mold flow analysis to detect sink-prone areas

• Use fillets and radii to guide plastic flow and reduce stress

• Add venting near ribs and bosses to help avoid short shots and burns

Visual Example

• A proper thickness of ribs (Good)

  
  

• Ribs are too thick (Not Good)

Goodtech MFG — Your Engineering Partner

At Goodtech MFG Group, we help you balance structural strength and cosmetic quality. Our engineering team reviews your 3D models and provides DFM reports, rib/boss optimization, and mold flow simulation support.

Have a design you'd like us to check? Contact us for a free DFM consultation.