Design for Injection Molding

Overview

Design for Manufacturing (DFM) ensures parts can be efficiently and economically produced. For injection molding, key principles are:

• Uniform wall thickness
• Proper draft angles
• Appropriate ribs and bosses
• Radii and fillets
• Avoiding undercuts

Wall Thickness

Uniform Walls

The most important DFM rule: maintain uniform wall thickness throughout the part.

• Uneven cooling causes warpage
• Thick sections cause sink marks
• Thick sections extend cycle time
• Flow imbalances cause weld line issues

Transitioning Thickness

When thickness changes are unavoidable:

• Gradual transitions (3:1 slope minimum)
• Never abrupt steps
• Locate transitions away from stress areas

Draft Angles

Draft is a slight taper on vertical walls that allows the part to release from the mold.

• Part shrinks onto core during cooling
• Without draft, ejection force can damage part
• Vacuum can form between part and mold

Ribs

Ribs add stiffness without increasing wall thickness.

Design Guidelines

• Thicker ribs cause sink marks on opposite surface
• Thinner ribs may not fill properly

Rib Orientation

• Perpendicular to load for bending resistance
• Crosshatch pattern for bidirectional stiffness
• Consider flow direction during fill

Bosses

Bosses provide mounting points for screws, pins, or inserts.

Design Guidelines

Common Mistakes

Radii and Fillets

Sharp corners create:

• Stress concentrations (crack initiation)
• Flow hesitation (incomplete fill)
• Cooling variations (warpage)

Guidelines

Undercuts

Features that prevent straight pull from the mold:

Types of Undercuts

• External undercuts — Protrusions on outside surface
• Internal undercuts — Cavities or holes inside the part

Solutions

• Mechanical cams move cores perpendicular to mold open
• Add cost and complexity
• Require maintenance

• Angled pins that move during ejection
• For internal undercuts
• Limited undercut depth

• Cores that collapse inward during ejection
• For internal threads, deep undercuts
• High complexity and cost

• Part flexes over undercut during ejection
• Only for flexible materials (PP, PE)
• Undercut <5% of diameter

Avoid When Possible

Best practice: Design to eliminate undercuts entirely.

Gate Considerations for Design

Gate location affects:

• Appearance — Gate mark visible
• Strength — High stress at gate
• Fill pattern — Weld line locations

• Leave flat area for gate
• Avoid gating on cosmetic surfaces
• Gate at thickest section
• Consider flow length to all areas

Parting Line

The line where mold halves meet:

• Visible witness line on part
• Flash can occur at parting line
• Affects draft direction

• Place on non-cosmetic surface when possible
• Design for simple, straight parting line
• Complex parting lines add tooling cost

DFM Checklist

Key Takeaways

• Uniform wall thickness is the #1 rule—variations cause defects
• Ribs and bosses: 50-60% of wall thickness to avoid sink marks
• Draft: 0.5-1° minimum, more for textured surfaces
• Radius all corners: inside = 0.5×wall, outside = 1.5×wall
• Avoid undercuts when possible; use slides/lifters when necessary
• Consider gate location and parting line in early design

---