Injection Mold Design Guide May 2026

Injection Mold Design Guide

4. Gating & Runner System

Part 6: The Runner and Gate System (The Plumbing)

How you get the plastic from the nozzle to the cavity dictates part quality.

Part 2: Material Selection & Shrinkage (The Golden Rule)

The single biggest mistake novice designers make is designing a mold as if it were a solid block of steel. Plastic shrinks.

Every polymer has a specific shrinkage rate: injection mold design guide

• Amorphous Plastics (ABS, PC, PS): Shrinkage is low (0.4% – 0.7%) and uniform.
• Semi-Crystalline Plastics (Nylon, POM, PP): Shrinkage is high (1.5% – 2.5%) and anisotropic (different in flow direction vs. transverse direction).

Actionable Guide Rule:

1. Request the Material Data Sheet before designing the steel.
2. Apply isotropic scaling to the 3D model (e.g., 1.006 for ABS).
3. Critical: Inform the mold maker if you will switch materials later. A mold cut for PP cannot run ABS without dimensional failure.

Gate Location Strategy

1. Fill thick to thin: Place gates at the thickest cross-section so material flows into thin sections smoothly. Never gate into a thin wall to fill a thick one.
2. Avoid dead zones: Material should flow across the cavity, not bounce back into itself.
3. Weld lines: When two flow fronts meet, they create a weld line (weak point). Place gates so these lines occur at low-stress areas (e.g., a rib, not a load-bearing wall).
4. Critical aesthetics: Place the gate on a non-cosmetic surface (inside a lid, under a label boss). For clear parts (polycarbonate), a fan or diaphragm gate prevents flow lines.

2. Mold Components (The Anatomy of a Tool)

A mold is a precision tool made of several plates and components. Injection Mold Design Guide 4

• Cavity (A-Side): The female side; forms the outside shape of the part. Usually mounted to the stationary platen.
• Core (B-Side): The male side; forms the inside shape of the part. Usually mounted to the moving platen.
• Sprue: The channel through which molten plastic enters the mold from the nozzle.
• Runners: Channels that direct the plastic from the sprue to the cavity gates.
• Gates: The entry point where the plastic flows into the cavity. This is the critical control point for flow.
• Cooling Lines: Channels machined into the plates through which water or oil circulates to cool the part.
• Ejector System: Pins or sleeves that push the part out of the mold after cooling.
• Leader Pins & Bushings: Align the two halves of the mold precisely.

2. Part Design Requirements (Inputs to Mold)

The mold can only be as good as the part design. Verify these first:

| Requirement | Why it matters | |-------------|----------------| | Uniform wall thickness (0.5–4 mm typical) | Prevents sink, warpage, and fill imbalances | | Draft angle – 0.5° to 2° (per side) | Allows part release without drag marks | | Radii at corners – 0.25–0.5 × wall thickness | Reduces stress concentration and improves flow | | Nominal wall progression – gradual changes only | Avoids flow hesitation and freeze-off | Amorphous Plastics (ABS, PC, PS): Shrinkage is low (0

❌ Avoid: Thick-to-thin sudden steps, sharp internal corners, zero-draft vertical walls.