Runner & Gate Design

Feed System Components

The feed system delivers molten plastic from the machine nozzle to the part cavity:

Nozzle → Sprue → Runner → Gate → Cavity

Each component affects fill balance, cycle time, and material waste.

Sprue

The sprue is the channel connecting the nozzle to the runner system:

• Taper: 1-3° included angle (for easy pull-out)
• Diameter: Match nozzle orifice at entry, larger at exit
• Length: As short as possible
• Sprue bushing: Hardened steel insert for wear resistance

A feature in the mold (undercut or Z-pin) that holds the sprue on the ejector side during mold opening.

Runner System Types

Cold Runner

Traditional approach where runners solidify each cycle:

• Simple mold design
• Low initial cost
• Easy color/material changes

• Material waste (regrind required)
• Longer cycle time (runner must cool)
• Potential for degraded regrind

• Diameter: 4-12 mm typical
• Length: As short as possible
• Balance: Equal path length to all cavities

Hot Runner

Heated manifold keeps plastic molten:

• Zero runner waste
• Faster cycle time
• Better fill balance
• Direct gating possible

• High initial cost ($20,000-$100,000+)
• Complex maintenance
• Color changes difficult
• Thermal control critical

• Manifold: Heated block distributing melt
• Drops: Channels from manifold to gates
• Nozzles: Heated tips at each gate

Gate Types

Edge Gate (Tab Gate)

Entry from the side at parting line:

• Simple to machine
• Easy to trim
• Leaves visible gate mark

• Flat parts
• Prototyping
• Low to medium volume

Fan Gate

Wide, thin gate for uniform flow:

• Reduces jetting
• Good for long, flat parts
• More gate trim required

• Thin-wall panels
• Parts needing uniform fill
• Fiber-filled materials (maintains orientation)

Tunnel Gate (Submarine Gate)

Angled tunnel below parting line:

• Auto-degating (gate shears on ejection)
• Gate mark on non-cosmetic side
• Limited to flexible materials

• High-volume production
• Auto-degating requirement
• Cosmetic top surfaces

• Entry angle: 30-45°
• Diameter: 0.8-2.5 mm
• Land length: 0.5-1 mm

Pin Gate (Point Gate)

Small circular gate, typically with hot runner:

• Minimal gate vestige
• Auto-degating
• Requires 3-plate mold (cold runner) or hot runner

• Cosmetic parts
• Multi-cavity molds
• Small to medium parts

Valve Gate (Hot Runner)

Hot runner with mechanical shut-off:

• Best gate quality (no vestige)
• Sequential filling possible
• Highest cost

• Premium cosmetic parts
• Large parts (sequential filling)
• Automotive Class A surfaces

Cashew Gate

Curved tunnel gate for underside gating:

• Gates on non-cosmetic surface
• Complex to machine
• Limited material flexibility

• Parts with no side access
• Cosmetic requirements

Gate Location Guidelines

General Rules

• Gate at thickest section — Allows pressure to reach thin areas
• Avoid gating on cosmetic surfaces — Gate mark is visible
• Consider flow length — Maximum L/t ratio depends on material
• Balance flow to all areas — Prevents weld lines, air traps
• Gate away from stress areas — Gate region has high residual stress

Flow Length Limits (L/t Ratio)

Where L = flow length (mm), t = wall thickness (mm)

Gate Sizing

General Formula:

Gate thickness = (50-80%) × wall thickness
Gate width = 1.5-2× gate thickness (for edge gates)

For Tunnel Gates:

Gate diameter = √(4 × part volume / (π × fill time × injection speed))

Typically 0.8-2.5 mm

Multi-Cavity Balancing

For molds with multiple cavities:

• Runner length equal to all cavities
• "H" pattern or radial layout
• Preferred for critical parts

• Vary runner diameters to equalize fill
• Uses Moldflow/simulation
• More complex but flexible layout

Key Takeaways

• Cold runners waste material but are simple; hot runners eliminate waste but cost more
• Gate type selection depends on cosmetics, volume, and material
• Gate at the thickest section to allow packing
• Tunnel gates auto-degate; valve gates provide best cosmetics
• Balance runner lengths or diameters for multi-cavity molds
• Gate size: 50-80% of wall thickness

---