What Are the Key Principles of Plastic Part Design for Injection Molding?

Successful plastic injection molding design follows fundamental principles that ensure parts can be molded efficiently and with consistent quality.

1. Wall Thickness:

• Maintain uniform wall thickness throughout the part
• Typical range: 1.5mm - 4mm depending on material
• Avoid thick sections that cause sink marks and long cycle times
• Gradual transitions when thickness changes are necessary
• Thinner walls = faster cooling = shorter cycles = lower cost

2. Draft Angles:

• Apply draft to all surfaces parallel to mold opening
• Minimum 0.5° per side; 1-2° recommended
• Increase draft for textured surfaces (add 1° per 0.025mm texture depth)
• Draft enables clean part ejection without damage

3. Rib Design:

• Rib thickness = 50-60% of adjoining wall
• Rib height ≤ 3x rib thickness
• Include draft on rib sides
• Add radii at rib base (0.25-0.5x wall thickness)
• Proper ribs add strength without sink marks

4. Boss Design:

• Boss OD = 2-2.5x screw diameter
• Boss wall thickness = 60% of nominal wall
• Connect bosses to walls with ribs, not thick sections
• Include draft and radii

5. Corners and Radii:

• Avoid sharp internal corners (stress concentrators)
• Internal radius = 0.5x wall thickness minimum
• External radius = internal radius + wall thickness
• Radii improve flow and reduce stress

6. Undercuts:

• Avoid undercuts when possible
• If required, design for side actions or lifters
• Consider snap-fit designs that flex during ejection
• Undercuts increase mold cost and complexity