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Sprue Bushing and Nozzle Guide — The Injection Mold's Feed System
The sprue bushing and injection nozzle form the critical interface between the injection molding machine's barrel and the mold cavity. Despite being small components, they are responsible for more process issues — drool, stringing, pressure drop, and contamination — than most mold designers acknowledge.
This guide covers sprue bushing types, nozzle selection, sealing, wear management, and what buyers should check during mold commissioning.
Sprue Bushing Overview
The sprue bushing is the steel component mounted in the mold's A-plate that receives molten plastic from the machine nozzle and directs it into the runner system. The sprue bushing's internal taper (typically 2-5° included angle) allows the solidified sprue to be pulled from the machine nozzle when the mold opens.
Sprue Bushing Types
| Type | Description | Best For |
|---|
| Standard (No Back Flow) | Simple tapered bore; no valve mechanism | General purpose; commodity plastics |
| Shut-off / Valve Sprue | Spring-loaded pin seals the bore after injection | Thin-flow materials (PP, PA); prevents drool |
| Extended Sprue | Longer body reaches into the runner/part for center gating | Three-plate molds; center-gated parts |
| Heated Sprue | Cartridge heaters and thermocouple embedded in the bushing | High-temperature materials; long cooling cycles |
| Modified Sprue (Dual Taper) | Two different taper angles along the bore — steeper at the nozzle side, shallower at the cavity side | Materials prone to sticking (nylon, acetal) |
Buyer's Tip: Many Chinese mold factories use standard open-bore sprue bushings for everything, even for nylon and PP where the low-viscosity melt tends to drool between cycles. The result: a cold slug injects into the cavity at the start of every cycle, causing surface defects, splay, or even short shots. For thin-flow materials, insist on a shut-off sprue bushing — it costs $80-150 extra but eliminates drool-related defects that cause 2-5% scrap. Also check that the sprue bushing material is beryllium copper for applications requiring high heat transfer near the gate, not standard tool steel. BeCu bushings cost more but reduce gate freeze variability.
Nozzle Selection
The machine nozzle connects the heating barrel to the sprue bushing. Nozzle selection affects three things: pressure loss, material degradation, and gate sealing behavior.
Nozzle Types
- Open nozzle: Simple bore, low pressure drop. Risk: drool and stringing with low-viscosity materials.
- Shut-off nozzle (mechanical): Spring-loaded or hydraulic pin blocks the orifice between cycles. Prevents drool. Higher pressure drop.
- Shut-off nozzle (needle valve): Pneumatic actuation of a needle into the orifice. Most reliable shut-off but highest pressure loss and cost.
- Insulated nozzle: A small frozen plug of material at the tip prevents drool; the plug remelts each cycle. Used for materials that degrade easily (PVC).
The Nozzle-Sprue Interface
The spherical radius at the nozzle tip must match the spherical radius at the sprue bushing seat. The industry standard is R10mm (small machines) or R15mm-R20mm (large machines). If the radii don't match, the nozzle only contacts the bushing at a small point, causing:
- Plastic leaks at the interface (nozzle drool externally visible)
- High localized stress that cracks the sprue bushing
- Pressure loss as material forces into the gap
- Temperature gradient at the leak point affecting material flow
The nozzle tip orifice diameter should be approximately 0.5-1.0mm smaller than the sprue bushing bore to prevent a step-edge that traps material.
Sprue Puller Design
The sprue must pull cleanly from the sprue bushing when the mold opens. Three common puller methods:
- Z-puller (undercut): A small Z-shaped undercut at the base of the sprue forces it to stay on the A-plate side. Most common, works for all materials.
- Spruce puller pin: A small pin extends into a counterbore at the sprue base, creating a mechanical lock. Less common, used when Z-puller cannot fit.
- Reverse taper sprue bushing: The taper angle is reversed (narrower towards the runner), relying on shrinkage to hold the sprue on the A-side. Only works with high-shrinkage materials (PP, PA, POM).
Wear and Maintenance
Sprue bushings wear at the nozzle contact surface over time. The high-pressure contact (800-2,000 bar) and thermal cycling cause the spherical seat to deform. Signs of wear: visible flash at the nozzle-sprue interface, material weeping, and inconsistent sprue pull. A worn sprue bushing should be replaced immediately — it costs $30-80 for the part. Delaying replacement causes nozzle tip damage ($150-400) and can lead to a cracked A-plate in extreme cases.
Buyer's Checklist
- What type of sprue bushing is specified? For thin-flow materials, verify a shut-off type is used.
- What is the sprue bushing material? Beryllium copper for high-thermal-demand applications.
- Does the mold drawing show the sprue puller method? If not, ask.
- Is the nozzle-sphere radius specification included in the mold RFQ? Both R10 and R20 should be listed to ensure compatibility with your molder's machine.
- What is the sprue bushing bore diameter? Match to the nozzle orifice within 0.5-1.0mm.
- Is the sprue bushing replaceable without removing the A-plate from the machine? A bolted bushing is serviceable; a press-fit bushing requires mold disassembly.
What This Means for Your Project: The sprue bushing and nozzle are small, low-cost parts that cause a disproportionate share of process problems. If you see splay, drool marks, or inconsistent part weights during first-article inspection, the sprue bushing is one of the first things to check. For high-cavitation molds running commodity materials, the cost of upgrading to shut-off bushings and matching nozzles is trivial ($150-300 per mold) compared to the scrap reduction. When specifying a new mold, include the sprue bushing type and shut-off requirement in your RFQ — don't assume the molder will default to the best option. For existing molds, carry spare sprue bushings on your maintenance shelf; they're the most frequently replaced wear component on any injection mold.
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