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When you review a plastic injection mold, you may see terms like sprue, runner, gate, and sprue bushing in the mold layout. These features are usually not part of the final plastic product, but they control how molten plastic enters the mold. A small change in this feeding system can affect filling, cooling, material waste, and visible marks on the molded part.
What Is a Sprue in Injection Molding?
A sprue in injection molding is the first main channel that carries molten plastic from the injection machine nozzle into the mold. It usually connects the machine nozzle to the runner system. From there, the melt flows through runners and gates before entering the mold cavity.
In a cold runner mold, the sprue cools together with the runner system. After the mold opens, the sprue and runner are removed with the molded part or separated during ejection. In some simple single-cavity molds, the sprue may feed directly into the part. This is often called a direct sprue gate.
The phrase mold sprue usually refers to the mold channel itself. After molding, people may also call the solidified leftover material a sprue. One is the mold feature, the other is the cooled plastic material formed inside that feature.
Sprue, Runner, and Gate: What Is the Difference?
Sprue, runner, and gate are part of the same feeding system, but they do different jobs.
| Feature | Main Function | What It Affects |
| Sprue | Brings melt from the nozzle into the mold | Pressure loss, cooling, sprue waste |
| Runner | Distributes melt from the sprue to the gates | Filling balance, material use, cycle time |
| Gate | Controls melt entry into the cavity | Gate mark, flow direction, appearance |
The sprue is the first channel. The runner distributes material. The gate is the final opening into the cavity.In a multi-cavity plastic injection mold, this difference becomes more important. If the runner layout is not balanced, one cavity may fill earlier than another. If the gate is too small or poorly located, the part may show flow marks, weld lines, or short shot problems. If the sprue is poorly matched to the nozzle or runner, the whole feeding system can become unstable.
The Role of the Sprue in the Injection Molding Process
The sprue looks simple, but it does more than just “let plastic in.” In a real plastic molding project, it affects several practical points:
- Melt entry: The sprue guides molten plastic from the machine nozzle into the mold. The contact area must seal well, or leakage and unstable injection may occur.
- Flow stability: The melt should enter the runner system smoothly. A restricted sprue can increase pressure loss, while an oversized sprue can add unnecessary material and cooling time.
- Cooling behavior: In a cold runner tool, the sprue solidifies every cycle. If it is thicker than the molded part, it may control the minimum cooling time.
- Material handling: The cooled sprue must be removed, trimmed, reused as regrind, or discarded. This matters more when the material is expensive or appearance-sensitive.
For example, a small ABS or PC-ABS electronic housing may cool quickly at the wall area. But if the cold sprue is thick, the mold may still need extra cooling before ejection. For low-volume work, this may be acceptable. For higher-volume production, the mold team may review a hot sprue or hot runner option.
What Is a Sprue Bushing?
A sprue bushing is the mold component that forms the sprue channel and connects with the injection machine nozzle. It is usually installed in the mold at the nozzle contact area. During injection, the nozzle presses against the sprue bushing, and molten plastic flows through it into the mold.
The sprue bushing matters because it affects alignment, sealing, and melt flow. If the nozzle and sprue bushing do not match well, several problems may happen:
- Plastic leakage at the nozzle contact area
- Cold slug entering the mold
- Unstable filling
- Pressure loss
- Poor sprue release
- Material drooling or stringing in some cases
The radius, opening size, contact surface, and alignment should match the molding machine and mold design. Buyers do not usually need to specify these details, but the plastic mold manufacturer should check machine and mold fit before trial molding.
Common Types of Mold Sprue
Different mold designs use different sprue solutions. The most common choices are cold sprue, hot sprue, and hot runner systems.
Cold Sprue
A cold sprue is the basic and common solution in many molds. The sprue channel is not heated, so the plastic inside it cools and solidifies with each shot. After molding, the cooled sprue is removed from the mold.
Cold sprue molds are usually simpler and less expensive to build. They are often suitable for lower-volume projects, simple parts, or projects where runner waste is acceptable.
The main drawback is waste. Every cycle produces a sprue, and often a runner as well. This material may be trimmed, recycled as regrind, or discarded depending on the material and product requirements.
Hot Sprue
A hot sprue uses heating around the sprue area, often through a heated sprue bushing. The goal is to keep plastic molten in the sprue area and reduce the cold sprue formed during each cycle.
This can reduce material waste and may help with cycle time in some projects. The trade-off is higher mold cost and more temperature control requirements. A hot sprue is not always necessary, but it can be useful when the sprue waste is large or the material is expensive.
Hot Runner System
A hot runner system goes further than a hot sprue. It keeps the runner system heated so molten plastic can be delivered closer to each cavity without solidifying in the runner.
Hot runner molds can reduce runner waste and support high-volume production. They are often used for multi-cavity molds or parts made from costly materials. But they also need better temperature control, maintenance, and tooling investment.
In simple terms, cold sprue is easier and lower cost. Hot sprue reduces the cold slug in the sprue area. Hot runner reduces runner waste more completely, but it requires a more complex mold.
Basic Sprue Design Points
Sprue design is normally handled by the mold designer, but the basic points are easy to understand.
Sprue Shape and Taper
A sprue is usually tapered. The taper helps the cooled sprue release from the mold. If the sprue is too straight, it may stick or break during ejection. If the taper is too aggressive, it may increase material use or affect flow. The exact design depends on material behavior, mold structure, and machine setup.
Sprue Diameter
Sprue diameter affects how easily melt enters the mold. If the diameter is too small, the mold may need higher injection pressure, and the melt may experience more shear. If it is too large, the sprue becomes heavier and may take longer to cool.
For large parts, thick-wall parts, or materials with poor flow, the sprue may need to be larger. For small parts with easy-flow materials, a smaller sprue may be enough. The design should not be copied blindly from another mold.
Sprue Length
A shorter sprue is generally preferred when the mold structure allows it. A long sprue may increase pressure loss and material waste. In some three-plate molds or deep mold structures, sprue length becomes more important and may affect cycle time.
Nozzle Match
The machine nozzle and sprue bushing should seal correctly. Poor contact can cause leakage or unstable injection. This is one reason mold trials should not only check the part. They should also check sprue release, nozzle contact, and overall feeding stability.
Sprue Marks and Other Common Issues
Sprue design can also affect appearance and molding stability. Not every mark near the feed area is caused by the sprue, but it should be checked during trial molding.
Sprue Marks
Sprue marks are visible marks left where the sprue or direct sprue gate connects to the molded part. They are more common when the sprue feeds directly into the part. The mark may look like a round vestige, raised area, trimmed spot, or stress mark.
For hidden surfaces, sprue marks may be acceptable. For cosmetic housings, covers, or customer-facing surfaces, the gate and sprue location should be reviewed early. Moving the feed point after mold steel is cut can be difficult and expensive.
Short Shot or Poor Filling
A poorly sized sprue can restrict material flow. If the sprue, runner, and gate system has too much pressure loss, the cavity may not fill completely. This is more likely in thin-wall parts, long-flow parts, or materials with lower flowability.
Sprue Sticking
Sprue sticking happens when the cooled sprue does not release properly from the mold. Possible causes include poor taper, rough surface, unsuitable sprue puller design, cooling imbalance, or material behavior. In automatic production, this can interrupt the cycle and create scrap.
Flow Marks or Jetting
If the melt path from sprue to runner to gate is not smooth, the material may enter the cavity in an unstable way. This can contribute to flow marks, jetting, air traps, or visible surface defects. The sprue is only one part of the problem, so the runner and gate should be reviewed together.
Sprue Waste and Regrind
In cold runner molds, the sprue and runner become solid plastic after every shot. This material is separated from the finished part. It may be recycled as regrind in some projects.
Regrind sounds simple, but it must be controlled. Too much regrind can affect color stability, surface appearance, mechanical strength, and moisture control. Some materials are more sensitive than others. Transparent parts, flame-retardant materials, high-gloss housings, and precision components often need stricter regrind limits.
For low-volume projects, cold sprue waste may not be a major problem. For high-volume plastic injection molding, material waste can become part of the real production cost. This is why plastic injection molding companies often review cold runner, hot sprue, or hot runner options based on volume, material cost, and quality requirements.
Cold Sprue, Hot Sprue, or Hot Runner: Which One Fits?
There is no universal answer. The right choice depends on part design, material, volume, tooling budget, and production requirements.
| Project Situation | Common Direction |
| Low-volume or simple part | Cold sprue |
| Medium volume with concern about sprue waste | Hot sprue may be reviewed |
| High-volume or multi-cavity production | Hot runner may be reviewed |
| Expensive engineering plastic | Hot sprue or hot runner may help reduce waste |
| Strict appearance requirement | Gate and feed location need early review |
A cold sprue is not outdated. It is still practical for many molds. A hot runner is not automatically better. It brings cost and maintenance requirements. The best choice is the one that fits the part and the production plan.
FAQs
What is the difference between a sprue and a runner?
A sprue brings molten plastic from the nozzle into the mold. A runner distributes that plastic from the sprue to one or more gates. The gate is the final opening into the mold cavity.
Is the sprue part of the final molded part?
Usually no. In cold runner molds, the sprue cools with the runner system and is removed from the finished part. In direct sprue gating, the feed area may leave a visible sprue mark on the part.
Why is a sprue tapered?
A sprue is tapered to help the cooled plastic release from the mold. Without enough taper, the sprue may stick, break, or cause unstable ejection.
What causes sprue marks?
Sprue marks usually come from the area where the sprue or direct gate connects to the part. The mark can be affected by gate size, feed location, trimming method, cooling, material, and part surface requirements.
Conclusion
A sprue in injection molding is the first channel that guides molten plastic from the machine nozzle into the mold. It works with the runner and gate to complete the feeding system. Although the sprue is not usually part of the final product, it can affect filling stability, cooling, material waste, sprue marks, and production consistency.
A good mold sprue design should match the part, material, machine, runner layout, and production target. For some projects, a simple cold sprue is enough. For others, a hot sprue or hot runner may be reviewed to reduce waste or improve production efficiency.
For projects with plastic injection molding needs, drawings, material requirements, surface expectations, and estimated quantities can be sent to HingTung injection molding factory in China for an initial project review.