A sprue is the main channel that carries molten plastic from the injection molding machine's nozzle into the mold, feeding the runners and gates that fill the cavity. In a cold-runner mold the sprue solidifies with every shot and is removed from the part; a hot-runner mold does away with it.
In short: the sprue is the entry point for plastic into the mold — not part of the product, but part of every shot.
What Does Sprue Mean in Injection Molding?
The sprue runs from the outside of the mold, where the machine nozzle seats, to the parting line, where it meets the runner system. It is formed by the sprue bushing, a hardened steel insert with a tapered bore: wide at the parting line and narrow at the nozzle. The taper lets the solidified sprue pull free of the bushing when the mold opens.
In a cold-runner mold the plastic in the sprue cools along with the plastic in the part, so each cycle produces the parts plus a solid sprue-and-runner tree. A sprue puller on the core side grips the base of the sprue and drags it out with the moving half; the tree is then cut from the parts and is usually reground.
Hot-runner molds keep the melt channel heated all the way to the gate, so there is no solidified sprue or runner to remove — one reason hot runners are chosen for high-volume, material-efficient production.
Some large single-cavity parts are sprue-gated, meaning the sprue feeds the cavity directly with no runner. This leaves a noticeable vestige in the centre of the part that must be trimmed.
The same term is used in die casting and vacuum casting, where the sprue is the channel the metal or resin is poured or injected through. (It is unrelated to the medical condition of the same name.)
Sprue vs Runner vs Gate
The three form one path from nozzle to cavity, and they are often confused.
| Sprue | Runner | Gate | |
|---|---|---|---|
| What it does | Brings melt from the nozzle into the mold | Distributes melt from the sprue to each cavity | Meters melt into the cavity and seals when it freezes |
| Where it sits | Through the fixed mold half, ending at the parting line | Cut into the parting surface | The small opening between runner and cavity |
| Relative size | Largest cross-section | Smaller than the sprue | Smallest — often a few millimetres or less |
| Left on the part? | Only if sprue-gated; otherwise trimmed away | Trimmed away (cold runner) or absent (hot runner) | Leaves a small vestige where it was cut |
How Is a Sprue Designed?
- Keep it as short as the mold base allows: a long sprue takes longer to cool and adds scrap on every shot.
- Match the orifice to the machine nozzle — the sprue bushing bore at the nozzle end is slightly larger than the nozzle orifice so the sprue does not hang up.
- Size the sprue larger than the runner it feeds, so it is not the flow restriction.
- Add a cold-slug well opposite the sprue to catch the cooled plug that forms at the nozzle tip between shots, so it does not travel into the part.
- Decide early whether the program justifies a hot-runner system, which removes the sprue and runner scrap entirely but raises tooling cost.
Feed-system design, including the sprue, runner layout and gate positions, is set during mold design for every plastic injection molding project, and it is one of the things reviewed with the customer before tooling is machined.
Frequently Asked Questions
What is a sprue in injection molding?
It is the channel that carries molten plastic from the machine nozzle into the mold. It connects to the runners, which connect to the gates that fill each cavity. In cold-runner molds it comes out as a solid tapered stub attached to the runner.
Is the sprue part of the finished product?
No. The sprue is scrap. It is separated from the molded parts after ejection and, for most thermoplastics, reground and reused. Only a sprue-gated part carries a trace of it, as a trimmed vestige.
What is the difference between a sprue and a runner?
The sprue is the single entry channel from the nozzle to the parting line. The runners are the channels that branch from the sprue along the parting surface to carry plastic to each gate and cavity.
Related Services at NICE Rapid
- Plastic Injection Molding — single- and multi-cavity tooling where the sprue, runner and gate layout is designed in-house.
- Vacuum Casting — polyurethane parts cast through a sprue in a silicone mold, for small series before tooling investment.
- Request a Quote — send a STEP file and quantities to get feed-system and gating recommendations with the quote.
Sources
- MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — "A sprue is a channel through which to transfer molten plastics injected from the injector nozzle into the mold"; runner and gate roles — ocw.mit.edu
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