The Sprue Bushing Of Plastic Injection Mold

The sprue bushing is a vital component in plastic injection molds. It acts as the main channel through which molten plastic flows from the injection molding machine’s nozzle into the runner system of the mold.

Definition

A sprue bushing is a hardened steel, cylindrical insert located at the center of the mold's stationary side (usually the A-plate). It connects directly to the machine nozzle and guides the plastic into the mold.

⚙️ Functions of the Sprue Bushing

1. ✅ Guides molten plastic from the nozzle into the mold.

2. ✅ Seals tightly with the machine nozzle to prevent leakage.

3. ✅ Withstands high pressure and temperature during injection.

4. ✅ Transfers material efficiently into the runner system or cavity (for direct gating).

Types of Sprue Bushings

1. Tapered (Standard)

• Most commonly used.

• Matches the standard nozzle taper (usually 1/2" or 3/4" diameter taper).

2. Insulated Sprue Bushing

• Reduces heat loss in the sprue.

• Helps prevent premature solidification.

3. Heated (Hot Sprue) Bushing

• Used in hot runner systems.

• Keeps plastic molten throughout.

4. Straight-Through Bushing

• Used when the sprue leads directly into the cavity (no runner).

️ Design Considerations

• Taper Angle: Typically 1°–2° per side for easy part ejection.

• Material: Usually hardened tool steel (e.g., H13, SKD61) for wear and heat resistance.

• Length: Must match mold base thickness for proper fit.

• Gate Type: In direct gating, the sprue may also act as the gate.

Tips for Sprue Bushing Use

• Always align the sprue bushing precisely with the machine nozzle.

• Use sprue pullers or cold slug wells to aid in clean separation.

• Regularly check for wear or cracks, especially in high-pressure applications.

• Apply thermal insulation or select heated bushings for thick or slow-filling parts.