The sprue is the first segment of the melt delivery path in any injection mold — the channel through which molten plastic travels from the machine nozzle into the runner system or directly into the cavity. Despite its apparent simplicity, sprue design mistakes are among the most common causes of molding defects, production delays, and mold damage. A sprue that sticks in the bushing can stop production entirely. A sprue with the wrong taper or orifice size can cause cold slugs, flow marks, stringing, and excessive material waste.
This guide provides a comprehensive framework for sprue design in injection molds. We cover the two primary sprue categories — direct (cold) sprue and hot sprue — and explain when each is appropriate. We then examine sprue bushing types in detail, including OB (outboard) and ASA styles, orifice sizing formulas based on nozzle tip diameter, taper angle standards, and material-specific considerations. Next, we address sprue puller mechanisms: the Z-pin, the reverse taper undercut, and the ring puller, with design rules for reliable sprue retention on the A-side or B-side of the mold. Finally, we present a troubleshooting section covering the most common sprue-related defects encountered in production.
Whether you are specifying a sprue bushing for a simple single-cavity prototype mold or optimizing the sprue interface for a high-cavitation production tool, the principles in this guide will help you achieve clean sprue separation, reliable ejection, and defect-free parts.
1. What Is a Sprue in Injection Molding?
In an injection mold, the sprue is the tapered channel that connects the injection molding machine's nozzle to the mold's runner system (in a multi-cavity mold) or directly to the part cavity (in a single-cavity mold). The sprue is formed inside the sprue bushing, a replaceable steel component press-fit into the A-side (stationary half) of the mold. When the mold opens, the sprue must be cleanly separated from the nozzle and pulled out of the bushing by a sprue puller mechanism on the B-side (moving half), so that the entire shot — sprue, runners, and parts — ejects together.
The sprue serves three primary functions:
- Melt delivery: It conveys molten plastic from the machine barrel into the mold's internal runner system at the required flow rate and pressure.
- Cold material retention: The cold slug well at the base of the sprue captures the leading edge of the melt, which has cooled during the nozzle dwell time, preventing cold material from entering the runners and causing flow defects.
- Connection interface: The sprue bushing's spherical radius must match the machine nozzle's radius to create a seal that prevents drool (leakage of molten plastic between the nozzle and bushing during injection).
The geometry of the sprue — its diameter, taper angle, length, and the transition to the runner — is critical. A poorly designed sprue increases pressure drop, generates excessive shear heat, extends cooling time (because the sprue is often the thickest cross-section in the shot and thus the last to solidify), and creates cycle-limiting bottlenecks.
2. Direct (Cold) Sprue vs Hot Sprue
There are two fundamental approaches to sprue design: the cold sprue (also called a direct sprue) and the hot sprue (used in hot runner systems). Each has distinct advantages, limitations, and application scenarios.
2.1 Cold (Direct) Sprue
A cold sprue is a tapered channel machined into a sprue bushing that is maintained at the same temperature as the rest of the mold (typically 20–80°C, controlled by the mold's cooling system). Molten plastic enters the sprue, begins to cool immediately upon contact with the steel walls, and solidifies as part of the shot. The solidified sprue is ejected along with the parts and must be separated (usually by a gate cutter or manually) and either reground and reused or discarded as waste.
Advantages of cold sprue:
- Low cost: A standard sprue bushing costs $20–$100, versus thousands for a hot runner drop.
- Simplicity: No heaters, thermocouples, or controllers required. Maintenance is minimal.
- Universal material compatibility: Works with all thermoplastics including engineering grades, glass-filled compounds, and LSR.
- Ideal for short runs and prototypes where hot runner investment is not justified.
- No risk of material degradation from extended residence time in a heated manifold.
Disadvantages of cold sprue:
- Material waste: The solidified sprue becomes scrap (or regrind, which reduces mechanical properties by 10–30% per generation).
- Cycle time impact: The sprue is typically the thickest section and may be the last to fully solidify, extending the overall cooling time.
- Post-molding operations: The sprue must be separated from the part (for direct-gated parts) or from the runner system, adding labor cost.
- Gate vestige: Direct-gated parts leave a visible sprue mark that may require trimming or finishing.
2.2 Hot Sprue (Hot Runner Drop)
A hot sprue is a heated nozzle that maintains the plastic in a molten state from the machine barrel through the sprue bushing and into the runner or cavity. The hot sprue is the entry point of a hot runner system. Because the material remains molten, there is no solidified sprue to eject — the only solid plastic in the shot is the parts themselves.
Advantages of hot sprue:
- Zero sprue waste: No solidified sprue material to remove, regrind, or discard.
- Faster cycle times: Without the thick sprue section acting as a cooling bottleneck, cycles can be 10–30% faster.
- Cleaner parts: No gate vestige from a large sprue gate; hot valve gates leave minimal marks.
- Lower long-term material cost: Especially significant for expensive engineering plastics like PEEK, PEI, or LCP.
Disadvantages of hot sprue:
- High initial cost: A complete hot runner system with drops, manifold, and controller costs $3,000–$25,000+ depending on cavities and complexity.
- Complexity: Heaters, thermocouples, and controllers add maintenance points. A single failed heater can stop production.
- Material degradation risk: Extended residence time in heated channels can degrade heat-sensitive materials (PVC, POM, some flame-retardant grades).
- Color change difficulty: Purging hot runner systems for color changes can take significant material and time.
2.3 When to Choose Cold Sprue vs Hot Sprue
| Factor | Cold Sprue | Hot Sprue |
|---|---|---|
| Production volume | < 50,000 parts | > 50,000 parts |
| Material cost | Low ($2–10/kg) | High ($30–200/kg) |
| Cycle time sensitivity | Low priority | Critical |
| Tooling budget | Limited | Adequate |
| Material type | Any material | Heat-stable materials preferred |
| Color changes | Frequent/easy | Infrequent/difficult |
3. Sprue Bushing Types and Selection
The sprue bushing is the component that forms the sprue channel. It is a replaceable wear item — designed to be swapped out when the orifice becomes worn, damaged, or when a different nozzle size is needed. Selecting the correct sprue bushing involves choosing the right style, orifice diameter, spherical radius, and taper angle.
3.1 OB (Outboard) Sprue Bushing
The OB-style sprue bushing is the most common type used in North American mold shops. It features a flange at the top (A-side face) that locates the bushing axially. The flange sits flush with the parting line or sub-face of the A-plate. The bushing extends through the A-plate and terminates at the runner interface. The OB style is manufactured to industry-standard dimensions (specified by organizations such as NADCA and MISUMI), making replacements readily available.
OB bushing specifications:
- Standard orifice diameters: 2.5 mm, 3.0 mm, 3.5 mm, 4.0 mm, 5.0 mm, 6.0 mm
- Taper angle: 2–5° per side (included angle 4–10°)
- Spherical radius options: 10 mm (0.5 inch R), 12 mm, 15 mm, 18 mm, 25 mm
- Flange diameter: typically 25–40 mm
- Material: S50C (standard), SKD61 (heat-treated for wear resistance), or tungsten carbide tipped for glass-filled materials
3.2 ASA Sprue Bushing
The ASA-style sprue bushing is more common in European and Asian mold standards (HASCO, MEUSBURGER, FUTABA). Instead of a top-mounted flange, the ASA bushing is retained by a circular groove that accepts a retaining ring, or it is held in place by a threaded locking ring from the back of the A-plate. This design allows the bushing to be replaced without disassembling the entire A-plate stack.
ASA bushing specifications:
- Standard orifice diameters: 3.0 mm, 3.5 mm, 4.0 mm, 4.5 mm, 5.0 mm, 6.0 mm
- Taper angle: typically 3–5° per side
- Spherical radius: 10 mm, 12 mm, 15.5 mm (R3/4"), 18 mm
- Retention: Groove + retaining ring (HASCO K50 standard) or threaded ring
- Material: 1.1730 (DIN standard), 1.2343 (heat-treated), or 1.2379 (for abrasive materials)
3.3 Orifice Diameter Selection
The sprue orifice (the smallest diameter at the inlet of the sprue bushing, where the machine nozzle contacts the bushing) must be sized correctly relative to the machine nozzle orifice. The fundamental rule is:
Sprue orifice diameter = Nozzle orifice diameter + 0.5 to 1.0 mm
This clearance ensures that the sprue forms a slight taper that is larger at the nozzle end than the runner end, allowing it to be pulled cleanly from the bushing during mold opening. If the orifice is too small relative to the nozzle, the sprue will be pinched and may stick in the nozzle rather than the bushing. If the orifice is too large, excessive material waste and extended cooling time result.
Typical orifice diameters by part size:
- Small parts (< 20 g shot weight): 2.5–3.0 mm orifice
- Medium parts (20–200 g shot weight): 3.5–5.0 mm orifice
- Large parts (> 200 g shot weight): 5.0–8.0 mm orifice
3.4 Spherical Radius Matching
The spherical radius at the contact face of the sprue bushing must match (or be slightly larger than) the spherical radius of the machine nozzle tip. This creates a conformal seal that prevents molten plastic from leaking between the nozzle and bushing during injection. Standard nozzle radii are 10 mm (SR10) and 15 mm (SR15), with some machines using 12 mm or 18 mm.
If the bushing radius is smaller than the nozzle radius, contact occurs only at the edge of the orifice, creating a poor seal. If the bushing radius is larger, the nozzle seats properly but with a smaller contact area. Always verify the nozzle radius of the specific injection molding machine before specifying the sprue bushing.
3.5 Taper Angle
The sprue channel must be tapered (wider at the top, narrower at the runner end) to allow the solidified sprue to release from the bushing when the mold opens. Industry standards recommend:
- Minimum taper: 2° per side (4° included) for low-shrinkage materials (PC, PMMA, PSU)
- Standard taper: 3–5° per side (6–10° included) for general-purpose materials (PP, PE, ABS, PS)
- Aggressive taper: 5–7° per side for high-shrinkage materials (PA, POM) and for molds prone to sprue sticking
- Textured bushing: Some moldmakers add a slight polish (Ra 0.4–0.8 μm) in the taper direction to improve release; avoid perpendicular polishing marks that can cause the sprue to hang up
4. Sprue Puller Mechanisms
When the mold opens, the sprue must be pulled out of the A-side bushing and retained on the B-side so that it ejects with the rest of the shot. The mechanism that grips and holds the sprue during mold opening is called the sprue puller. There are three primary designs, each suited to different applications.
4.1 Z-Pin (Z-Shaped Puller Pin)
The Z-pin is the most widely used sprue puller design. It consists of an ejector pin with a Z-shaped (or reverse-Z) undercut machined into its tip, positioned at the base of the sprue in the cold slug well. When the sprue solidifies, the Z-portion of the pin is embedded in the plastic. During mold opening, the Z-pin grips the sprue and pulls it from the bushing. During ejection, the pin pushes the sprue forward while the Z-shape allows it to rotate free.
Z-pin design guidelines:
- Pin diameter: typically 4–8 mm for small to medium molds, 10–12 mm for large molds
- Z-undercut depth: 0.5–1.0 mm (measured from the pin's nominal diameter)
- Z-flat width: 50–70% of the pin diameter
- The Z must be oriented so that the ejector stroke pushes the sprue past the undercut — a common error is installing the pin backwards, which makes ejection impossible without damaging the mold
- Material: SKD61 or H13 heat-treated to HRC 50–55 for wear resistance
- The cold slug well diameter should be 1.0–1.5 mm larger than the sprue exit diameter to provide clearance for the slug
Advantages: Simple, inexpensive, easy to machine and replace. Disadvantages: The Z-shape can weaken the pin; insufficient undercut causes sprue slippage; the orientation-dependent installation can be a maintenance issue.
4.2 Reverse Taper (Undercut) Puller
The reverse taper puller uses a conical undercut machined directly into the B-side core at the base of the sprue, opposite the sprue bushing. As the sprue solidifies, the plastic fills the undercut cavity. During mold opening, the reverse taper holds the sprue firmly on the B-side. During ejection, the ejector plate pushes the entire runner plate forward, and the undercut releases as the sprue is pushed past the taper.
Reverse taper design guidelines:
- Undercut diameter: 1.5–3.0 mm larger than the sprue exit diameter
- Undercut depth: 3–5 mm axial
- Taper angle for release: 2–3° per side (must match or exceed the sprue bushing taper for clean release)
- Surface finish: Polished in the draw direction (Ra 0.4 μm or better) to prevent hang-up
- Ideal for: Soft or low-rigidity materials (PP, PE, TPE) where a Z-pin might not grip reliably
Advantages: Very reliable gripping force; no separate pin to break or misalign. Disadvantages: Requires machining directly into the core or an insert; more expensive to manufacture and modify; cannot be easily adjusted after hardening.
4.3 Ring (Annular Groove) Puller
The ring puller uses a circular undercut groove machined around the inside of the cold slug well. As the sprue solidifies, plastic flows into the annular groove, creating a 360° retention ring. This design provides uniform gripping force and is less likely to cause asymmetrical stress or deflection than a Z-pin.
Ring puller design guidelines:
- Groove width: 2–4 mm
- Groove depth: 0.5–1.0 mm below the cold slug well bore
- Edge breaks: 0.3 mm radius on all edges to prevent stress concentration
- Ideal for: Hard or brittle materials (PC, PMMA, glass-filled nylon) where Z-pin gripping might cause cracking
Advantages: Symmetrical gripping, no orientation issues during installation. Disadvantages: More complex machining; slightly less gripping force than reverse taper for very soft materials.
5. Cold Slug Well Design
The cold slug well is a small cavity directly opposite the sprue bushing, on the B-side of the mold. Its purpose is to catch the cold slug — the small plug of partially solidified plastic that forms at the nozzle tip between injection cycles. If this cold material enters the runner system and reaches the cavity, it causes flow marks, splay, short shots, and structural weaknesses.
Cold slug well design rules:
- Diameter: Equal to or slightly larger than the sprue exit diameter (+ 1–2 mm clearance for the puller mechanism)
- Depth: 1.0–1.5 times the sprue exit diameter
- The well also houses the sprue puller mechanism (Z-pin, undercut, or ring)
- In multi-plate molds with secondary runners, each runner branch should have its own cold slug well at the turn point to catch any cold material that travels through the runner
In hot runner molds, cold slug wells are not required at the sprue because the material remains molten throughout. However, if a cold runner branches off from a hot drop, a cold slug well should still be incorporated at the transition point.
6. Sprue-to-Runner Transition
The junction where the sprue meets the runner is a critical flow transition point. Poor geometry at this junction causes turbulence, pressure loss, and shear heating — all of which contribute to part defects. Key design rules:
- Sprue exit diameter > runner diameter: The sprue should always be larger in cross-section than the runner to maintain consistent flow front velocity. A common rule is sprue exit diameter = runner diameter + 1.0–1.5 mm.
- Radius the transition: Never use a sharp 90° intersection between the sprue and runner. A radius of 1–3 mm at the junction base reduces turbulence and pressure drop.
- Cold slug well at the junction: Extend the runner 1.0–1.5 runner-diameters beyond the sprue centerline to create a natural cold slug well in the runner itself.
- Avoid T-junctions with unequal runners: If the sprue feeds a runner that branches in two directions, ensure the runner cross-sections are equal to maintain balanced flow.
7. Common Sprue Defects and Troubleshooting
7.1 Sprue Stuck in A-Side (Bushing)
Symptom: The sprue remains in the sprue bushing after mold opening, preventing the next cycle from starting.
Causes and solutions:
- Insufficient taper angle: Increase the bushing taper to at least 3° per side. For sticky materials like PA or POM, use 5° per side.
- Undersized sprue puller: Increase the Z-pin undercut depth to 1.0 mm, or switch from Z-pin to reverse taper puller for better grip.
- Over-polished bushing: Mirror-polishing the bushing perpendicular to the draw direction creates micro-grooves that lock the sprue. Always polish in the draw direction.
- Sprue orifice too small: If the orifice is smaller than the nozzle, the sprue is pinched. Increase the orifice to nozzle diameter + 0.5–1.0 mm.
- Nozzle freeze-off: The nozzle temperature is too low, causing the sprue tip to solidify before the main body. Increase nozzle temperature by 5–10°C.
7.2 Sprue Stuck on B-Side (Cannot Eject)
Symptom: The sprue is pulled from the bushing correctly but cannot be ejected from the B-side puller.
Causes and solutions:
- Z-pin installed backwards: Remove and reinstall the pin with the Z-flats oriented toward the ejection direction.
- Excessive undercut: Reduce the Z-undercut to 0.5 mm or reduce the reverse taper depth. The undercut should be just enough to grip during opening — no more.
- Insufficient ejection stroke: Increase the ejection stroke so the sprue clears the undercut entirely before the ejector plate retracts.
- Rough surface finish on puller: Polish the puller undercut in the ejection direction to Ra 0.4 μm.
7.3 Cold Slug in Runner or Part
Symptom: Visible chunks of unmelted or partially solidified plastic in the runner or on the part surface, causing flow marks or structural defects.
Causes and solutions:
- Missing or undersized cold slug well: Ensure the cold slug well depth is at least 1.0× the sprue exit diameter and houses the puller mechanism properly.
- Excessive cushion remaining in the barrel: Reduce the cushion to 3–5 mm so residual material doesn't cool excessively between cycles.
- Nozzle temperature too low: Increase nozzle temperature to match the material's melt temperature range.
7.4 Stringing (Filaments Between Nozzle and Sprue)
Symptom: Thin threads of plastic stretch between the nozzle tip and the sprue bushing during mold opening or after injection, creating waste and potential contamination of the parting line.
Causes and solutions:
- High nozzle temperature: Lower the nozzle temperature by 5–15°C to increase melt viscosity at the nozzle tip, reducing thread formation.
- Nozzle dwell time too long: Reduce the open mold time or add a suck-back (decompression) move of 3–5 mm after injection to break the melt thread.
- Worn nozzle tip: Replace the nozzle tip if the orifice is worn or has burrs that prevent clean separation.
- Material-related: Some materials (PE, PP, LDPE) are inherently stringy. Use a valve-gate nozzle or a reverse-taper nozzle tip designed to break the thread cleanly.
7.5 Drool (Material Leaking from Nozzle)
Symptom: Molten plastic leaks from the nozzle between injection cycles, accumulating on the sprue bushing face or parting line.
Causes and solutions:
- Nozzle temperature too high: Reduce by 5–10°C or add a nozzle heat zone with independent control.
- Residual pressure after injection: Add decompression (suck-back) of 3–5 mm after the hold pressure phase.
- Worn nozzle/sprue bushing interface: Inspect the spherical contact surfaces for wear. Replace if the radius is no longer smooth.
- Use a shut-off nozzle: For materials prone to drool (PP, PE, PA), a spring-loaded or pneumatic shut-off nozzle prevents material flow when not injecting.
8. Sprue Design Calculations
8.1 Sprue Pressure Drop
The pressure drop through a tapered sprue can be estimated using the following simplified formula for a Newtonian fluid (actual values will be higher for shear-thinning polymer melts):
ΔP = (2 × τ × L) / (r₁ + r₂) / 2
Where:
ΔP = pressure drop (MPa)
τ = wall shear stress (MPa, typically 0.1–1.0 for common thermoplastics)
L = sprue length (mm)
r₁ = inlet (top) radius (mm)
r₂ = exit (bottom) radius (mm)
For accurate pressure prediction, use mold flow analysis software (Moldflow, Moldex3D) that accounts for the non-Newtonian rheology of polymer melts.
8.2 Sprue Solidification Time
The time for the sprue to freeze can be estimated using the standard cooling time formula:
t = (s² / (π² × α)) × ln((4/π) × (Tmelt − Tmold) / (Teject − Tmold))
Where:
t = cooling time (s)
s = half-thickness of the sprue at its widest point (mm)
α = thermal diffusivity of the plastic (mm²/s, typically 0.08–0.15)
Tmelt = melt temperature (°C)
Tmold = mold temperature (°C)
Teject = ejection temperature (°C, typically 20–40°C below heat deflection temperature)
Because the sprue is tapered, the maximum wall thickness (at the top) determines the cooling time. This is why reducing the sprue orifice to the minimum practical size is critical for cycle time optimization — a 4 mm orifice sprue may take twice as long to freeze as a 2.5 mm orifice sprue.
9. Best Practices Summary
- Size the orifice correctly: Nozzle orifice + 0.5–1.0 mm. This is the single most important dimension in sprue design.
- Match the spherical radius: Verify the machine nozzle radius before ordering the sprue bushing.
- Use adequate taper: 3–5° per side for most materials; up to 7° for sticky/high-shrinkage materials.
- Design the puller for the material: Z-pin for general use, reverse taper for soft materials, ring groove for brittle materials.
- Always include a cold slug well: At the base of every cold sprue and at every runner junction.
- Polish in the draw direction: Ra 0.4–0.8 μm, longitudinally — never cross-hatch or perpendicular polish.
- Minimize sprue length: Position the sprue bushing as close to the cavity as the mold structure allows to reduce material waste and cooling time.
- Consider thermal isolation: For fast-cycling molds, use a thermally insulating sprue bushing (titanium or ceramic insert) to reduce heat transfer from the melt to the mold body.
- Standardize bushing sizes: Use standard (MISUMI, HASCO, DME) bushings across all molds to simplify inventory and replacement.
- Simulate before cutting steel: Use mold flow analysis to verify sprue sizing, pressure drop, and fill balance — especially for multi-cavity molds and parts with long flow paths.
10. Industry Standards and References
- SPI/ANSI standards for sprue bushing dimensions (North American molds)
- DIN 16765 / HASCO K50 for European-standard sprue bushings
- JIS B 5111 for Japanese-standard mold bases and bushings
- ISO 16915 for hot runner system interface standards
- SPE (Society of Plastics Engineers) technical papers on sprue design and runner balancing
Conclusion
Sprue design may seem like a small detail in the overall mold build, but it is the gateway between the injection molding machine and your mold. A correctly designed sprue bushing and puller system ensures clean parting, reliable ejection, minimal waste, and fast cycle times — all of which translate directly to production efficiency and part quality. By following the sizing rules, material-specific guidelines, and troubleshooting framework presented in this guide, you can avoid the most common sprue-related problems and build molds that run smoothly from the first shot.
At Huanze Technology, our mold design team follows these principles on every project, from single-cavity prototype tools to 32-cavity production molds. If you have questions about sprue design for your specific application — or need help optimizing an existing mold — contact our engineering team for a consultation.
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