Sprue Bushing Internal Bore Taper: Why It Matters and How to Specify
Why Taper Is Necessary
During injection, molten resin fills the sprue bushing bore and solidifies into a conical slug during cooling. As the resin solidifies, it shrinks radially, gripping the bore wall. The taper creates a gradually widening channel from the nozzle end to the runner end — when the mold opens and the sprue puller pulls the sprue from the nozzle end, the expanding taper allows the solidified sprue to slide out smoothly.
Taper Angle Selection
| Taper Angle (per side) | Included Angle | Best For | Limitation |
|---|---|---|---|
| 1° | 2° | Short sprues (< 40 mm), low-shrinkage resins (ABS, PC) | May stick with long sprues or high-shrinkage resins |
| 1.5° | 3° | General purpose, medium-length sprues | — |
| 2° | 4° | Industry default, most resins and lengths | Slightly larger runner volume |
| 3° | 6° | Long sprues (> 80 mm), POM, PA, PP (high shrinkage) | Largest runner volume, more waste |
Bore Finish Requirements
The bore internal surface finish directly affects sprue ejection reliability and resin flow quality:
- Ra ≤ 0.4 μm (polished) — Required for reliable ejection and clean resin flow. Rough surfaces increase ejection force and can cause the sprue to stick.
- Polish direction — Must be axial (along the bore length), not circumferential. Circumferential polish marks create microscopic undercuts that resist ejection.
- No tool marks — Gun drill marks, reamer marks, or EDM texture must be removed by polishing before use.
According to ScienceDirect's sprue bushing reference, bore surface roughness above Ra 0.8 increases sprue ejection force by 50–100%, which can cause sprue sticking, Z-pull failure, and machine cycle interruptions.
Bore Diameter Specification
The sprue bushing bore has two diameters:
- Port diameter (P) — The small end, at the nozzle contact face. Must be 0.5–1.0 mm larger than the machine nozzle orifice to prevent material hang-up at the entry.
- Runner-side diameter — The large end, calculated from: P + 2 × L × tan(taper angle), where L is the bore length.
For a 70 mm bore length with a 3.0 mm port and 2° taper per side:
- Runner-side diameter = 3.0 + 2 × 70 × tan(2°) = 3.0 + 2 × 70 × 0.0349 = 3.0 + 4.9 = 7.9 mm
According to JIS B 5111, both the port diameter and the taper angle must be specified on the sprue bushing drawing.
Frequently Asked Questions
What is the standard bore taper angle for sprue bushings?+
Why does the sprue bushing bore need a taper?+
Does bore taper affect injection pressure?+
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