A sprue is the main channel that carries molten plastic from the injection molding machine’s nozzle into the mold. Think of it as the entry point: plastic flows through the sprue first, then branches into smaller channels called runners, which deliver material to the individual part cavities. A useful analogy is a water main feeding smaller pipes to individual houses.
How the Sprue Fits Into the Flow Path
Every injection-molded part starts as a stream of molten plastic pushed under high pressure from a heated barrel. That stream needs a clear path into the mold, and the sprue provides it. It’s a large-diameter, typically vertical channel that connects the machine nozzle on one end to the runner system on the other. The runners then branch off to gates, which are the final narrow openings where plastic enters the actual part cavity.
The sprue is almost always conical, wider at the bottom than at the top. This taper serves two purposes: it lets molten plastic flow smoothly under pressure, and it allows the solidified sprue to be pulled out of the mold cleanly when the mold opens. Standard taper angles fall between 2° and 4°. For plastics that don’t flow easily, the taper can increase to 6° or even 10° to reduce resistance.
The Sprue Bushing
The sprue channel isn’t machined directly into the mold. Instead, it sits inside a replaceable steel component called a sprue bushing. The bushing press-fits into the mold’s stationary half and creates a precise seat where the machine nozzle makes contact. This contact point matters: if the nozzle doesn’t align perfectly with the bushing, plastic leaks between them, creating flash and wasting material.
Two dimensions on the nozzle end of the bushing are critical. The first is the opening diameter, which must be slightly larger than the nozzle’s orifice so plastic transfers cleanly without a lip that could catch solidified material. The second is the spherical radius at the contact surface, typically 0.50 or 0.75 inches, which must match or exceed the nozzle’s radius to form a tight seal. Flat-type bushings with no spherical radius are also available for certain machine setups. Because the bushing is a wear item, it can be swapped out during maintenance without reworking the entire mold.
The Cold Slug Well
Between shots, a small plug of plastic solidifies inside the nozzle tip where it touches the cold mold steel. If that cold slug gets pushed into the runner system on the next shot, it can block a gate or end up embedded in a finished part. To prevent this, mold designers place a cold slug well at the base of the sprue. It’s a short dead-end pocket directly opposite the sprue opening that catches the slug before it reaches the runners. It’s a simple feature, but skipping it leads to inconsistent filling and cosmetic defects.
Why Sprues Stick in the Mold
One of the most common production headaches is a sprue that refuses to eject when the mold opens. Several things cause this. A surface that’s polished too aggressively can create a vacuum between the plastic and the steel, effectively suctioning the sprue in place. Conversely, a surface that’s too rough won’t release cleanly either. Circular polishing is especially problematic because it leaves tiny scratches that act like microscopic undercuts, giving the solidified plastic something to grip.
The fix during mold maintenance is straightforward: remove any scratches or undercuts in the sprue bushing bore and polish in the direction of draw (along the length of the channel, not around it). Insufficient taper, too-short cooling times, and misalignment between the nozzle and bushing can also contribute. When a sprue sticks repeatedly, production stops every time someone has to manually pull it out, so getting this right has a direct impact on cycle efficiency.
Cold Runner vs. Hot Runner Sprues
Everything described above applies to cold runner molds, where the sprue and runners solidify along with the part each cycle. That solidified sprue is waste. It gets ejected, separated from the finished parts, and either discarded or ground up for reuse. In high-volume production, this waste adds up quickly.
Hot runner systems take a different approach. Heated elements keep the plastic in the sprue and runner channels molten at all times, so when the mold opens, only the finished parts eject. There’s no solidified sprue to trim, no runner waste to regrind, and no extra material to re-melt on the next cycle. This translates to faster cycle times, lower material costs, and less scrap. The trade-off is that hot runner molds are more expensive to build and maintain, with heaters, temperature controllers, and more complex tooling. For short production runs, a cold runner with a conventional sprue is often more cost-effective. For high-volume or material-sensitive applications, hot runners pay for themselves through reduced waste and faster throughput.
Sprue Design Considerations
Getting the sprue dimensions right matters more than most people expect. If the sprue diameter is too small, the plastic experiences excessive pressure drop and may not fill the cavity completely, leading to short shots. If it’s too large, the sprue takes longer to cool and solidify, which extends cycle time and can leave a visible mark (called a sprue vestige) on the part surface.
The sprue’s length also plays a role. Longer sprues mean more material waste per cycle and longer cooling times. Mold designers generally keep the sprue as short as possible while still reaching the runner system. For thick-walled parts, a larger sprue may be necessary to maintain packing pressure during the cooling phase, preventing sink marks on the finished part. For thin-walled parts, a smaller, faster-cooling sprue keeps cycle times tight. In multi-cavity molds, the sprue feeds a balanced runner layout so each cavity fills at the same rate, producing consistent parts across every shot.