What Is a Sprue (Injection Molding)?

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 that is 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.

Sprue vs Runner vs Gate

The three form one path from nozzle to cavity:

• Sprue — brings melt from the nozzle into the mold; sits through the fixed mold half, ending at the parting line; has the largest cross-section; only left on the part if sprue-gated.
• Runner — distributes melt from the sprue to each cavity; cut into the parting surface; smaller than the sprue; trimmed away in cold runners or absent in hot runners.
• Gate — meters melt into the cavity and seals when it freezes; the small opening between runner and cavity; smallest, often a few millimetres or less; 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.