Sprue Bushing Parameters Explained: SR, Sprue Port, Flange & Taper

Parameter Reference Table

SR (Nozzle Touch Radius) — The Most Critical Parameter

SR is the spherical radius of the concave surface where the machine nozzle contacts the sprue bushing. This is the most important parameter because an incorrect SR causes resin leakage at every shot.

• Rule: Sprue bushing SR must be larger than nozzle SR by 0.5–1.0 mm.
• Why: If SR is too small, the nozzle tip protrudes past the bushing surface, creating a gap. If SR is too large, the contact area shrinks to a small ring, increasing contact pressure and wear.
• Common values: SR 11, SR 15, SR 20 — corresponding to standard injection machine nozzle sizes.

P (Sprue Port Diameter) — Orifice Sizing

P is the internal diameter at the nozzle end of the sprue bushing — the smallest point of the sprue channel.

• Rule: P must be 0.5–1.0 mm larger than the nozzle orifice diameter.
• Too small: Resin hangs up at the transition, causing drool and cold slug problems.
• Too large: Higher material waste (thicker sprue) and longer cooling time.
• Common values: P 2.5, P 3.5, P 4.5 mm.

A (Taper Angle) — Sprue Release Control

The internal taper angle controls how easily the solidified sprue pulls out of the bushing during mold opening.

• Narrow taper (1–1.5°): Less material waste, but higher risk of sprue sticking. Use for low-viscosity resins (PP, PE).
• Medium taper (1.5–2.5°): Standard for most applications (ABS, PS, PC).
• Wide taper (2.5–3°): Easy release, but more material waste. Use for high-viscosity, glass-filled, or sticky resins.

According to MoldMaking Technology, insufficient taper angle is one of the top causes of sprue sticking in production molds.

D (Shaft Diameter) and Flange Thickness

D is the outside diameter of the bushing body. It must match the bore in the mold's fixed plate with a precision fit (typically H7/g6). The flange thickness determines how deep the bushing sits in the locating ring counterbore.