A sprue is a channel that delivers material, whether molten plastic or liquid metal, from its source into a mold. It’s the first entry point in any casting or molding system, and its design directly affects the quality of the finished part, the amount of wasted material, and whether defects like air pockets or rough surfaces show up in the final product.
How Sprues Work in Injection Molding
In plastic injection molding, the sprue is a vertical channel running through the entire thickness of the mold’s stationary plate. It connects the machine’s nozzle to the internal channel system that fills the mold cavities. The mouth of the sprue seals tightly against the nozzle using a component called a sprue bushing, preventing any molten plastic from leaking during injection.
From the sprue, plastic flows into runners (shallow channels milled into the mold face), which branch out to gates (tiny openings leading into each cavity). Think of the sprue as the trunk of a tree, runners as the branches, and gates as the points where leaves attach. Each component gets progressively smaller, and each serves a distinct purpose: the sprue channels material inward, runners distribute it across multiple cavities, and gates constrict flow to build pressure right before the plastic enters the cavity where the actual part forms.
The sprue does more than simply route plastic. It maintains flow pressure and temperature as material travels from the hot nozzle into the cooler mold. It also needs to release cleanly when the mold opens, which is why sprues are tapered, wider at the bottom and narrower at the top. That draft angle lets the solidified sprue pull free without sticking.
Sprues in Metal Casting and Jewelry
In metal casting, sprues serve the same basic function but with additional responsibilities. During lost-wax casting, the kind used to make jewelry and detailed metal parts, wax models are attached to a central wax “tree.” The trunk of that tree is the main sprue, and smaller branches connecting individual pieces are called feed sprues. The whole assembly gets coated in a plaster-like investment material, then heated until the wax melts and drains out, leaving hollow channels behind.
Those channels serve a triple purpose. First, they hold the wax patterns in place while the mold is being built. Second, they provide a path for wax to drain out during burnout. Third, and most critically, they become the conduits through which liquid metal reaches every cavity in the mold. The sprue system’s design controls how much turbulence and heat loss the metal experiences on its way to filling the part, which directly determines casting quality.
Compensating for Shrinkage
Most silver and gold alloys lose 5 to 6% of their volume as they transition from liquid to solid. That’s a significant amount of space that needs to be filled, or the finished piece ends up with internal voids called shrinkage porosity. The feed sprue handles this by staying liquid longer than the part itself. As the metal in the pattern begins to freeze and contract, still-liquid metal in the feed sprue flows in to fill the gaps.
If the feed sprue solidifies before the pattern does, there’s no reservoir of liquid metal left to compensate. The result is porosity trapped inside the casting, weakening the part and creating visible defects. In larger industrial castings, a separate reservoir called a riser is placed at locations where shrinkage is most likely. The sprue and riser work together: the sprue delivers metal into the mold, while the riser provides extra molten metal to feed the casting as it cools and contracts.
Why Sprue Shape Matters
Sprues are almost always tapered rather than straight-walled, and the reason comes down to fluid dynamics. When liquid metal or plastic falls through a vertical channel, it naturally accelerates due to gravity. In a straight-walled sprue, the stream narrows as it speeds up, pulling away from the channel walls and creating gaps where air can be sucked in. That entrained air causes turbulence throughout the entire system and can end up trapped in the final part as bubbles or porosity.
A tapered sprue, narrower at the top and wider at the bottom, matches the natural narrowing of the fluid stream. This keeps the material in contact with the walls, reducing turbulence and preventing air from being drawn in. Research on gravity casting has shown that rapid changes in flow height inside the sprue create turbulence that is nearly impossible to eliminate with a non-tapered design. Optimizing the taper angle further reduces entrained air, free surface defects, and velocity spikes that would otherwise carry through the gating system.
Impact on Casting Quality
The difference between a well-designed and poorly designed sprue shows up clearly in the finished product. A study on titanium dental castings found that using two sprues instead of one reduced internal porosity from 1.5 square millimeters down to 0.2 square millimeters, a reduction of roughly 87%. The double-sprue design also produced smoother casting surfaces. This happens because multiple sprues allow metal to reach the cavity from different directions, reducing the distance any single flow path needs to travel and ensuring more complete filling before the metal begins to solidify.
Poor sprue design can cause a cascade of problems: rough surfaces from turbulent flow, internal voids from inadequate shrinkage feeding, incomplete filling from premature solidification, and cold shuts where two partially cooled metal fronts meet without properly fusing.
Material Waste and Recycling
Every sprue becomes scrap after the part is finished. In injection molding, the solidified sprue and runner system is trimmed off and, in many operations, ground up and blended back into fresh material. In metal casting, the numbers are more dramatic. Zinc die casting, for example, generates roughly five to ten pounds of scrap (including sprue waste and flash) for every pound of finished casting.
That ratio makes recycling essential. Studies at zinc casting facilities have shown that up to 50% of scrap metal can be fed back into production using a blend of about 30% virgin material to 70% recycled scrap without compromising part quality. In jewelry casting, precious metal sprues are routinely remelted, since the material cost of gold or silver makes waste unacceptable. The sprue is always designed with removal in mind: it attaches at a location where the cut mark can be easily filed, polished, or hidden on the finished piece.
Sprues Across Different Industries
While the core function stays the same, sprue design varies significantly by application. In injection molding, sprues are engineered for fast cycle times and minimal material waste, since even a few seconds of cooling time or a few grams of extra plastic add up over millions of parts. In investment casting for jewelry, the priority shifts to surface finish and eliminating porosity in small, detailed pieces. In heavy industrial sand casting, sprues are designed primarily around controlling turbulence and providing enough molten metal to feed large, thick-walled parts as they cool.
Regardless of the industry, the sprue is never part of the final product. It exists purely to get material where it needs to go, under the right conditions, and then gets removed. Its value lies entirely in what it prevents: air entrapment, incomplete filling, shrinkage voids, and turbulent flow that would otherwise ruin the part it helped create.