The sprue bushing is the entry point of the injection mold: it receives molten resin from the machine nozzle and directs it into the sprue and runner system. Because it seats directly against the nozzle, its head style, radius, and length have to match both the machine and the mold plate. Get those wrong and you get flashing at the parting line, poor seating, or a bushing that interferes with the mold base.
A, B, and U series
Sprue bushings are commonly grouped into series that define the head style and the sprue geometry. The A series is the standard straight head with a radius to match the machine nozzle; the B series is the extended-length variant used when the sprue must reach deeper into the mold; and the U series is the universal/full-round head style used where a longer, larger-diameter sprue is required for high-flow or high-cavity tools. Each series is stocked in a range of overall lengths, and the part number identifies the series, head style, and length together — for example, A129512 for the A series, B129512 for the B series, and U157512 for the U series.
Sizing the bushing
Three measurements drive the selection: the head style and diameter (which must seat against the nozzle and clear the locating ring), the overall length (which must reach from the top of the sprue plate to the runner plane), and the sprue diameter (which controls flow and pressure drop into the runner). The head's spherical radius should match the machine nozzle tip radius for a positive seal without excessive force.
Common replacement mistakes
- Ordering by 'the old part' without measuring length — a bushing a few thousandths too short won't seat, one too long bottoms out.
- Mixing head styles: an A-series head may not clear a locating ring recess designed for a U-series head.
- Ignoring the nozzle radius match, which causes flashing at the sprue bushing seat.
- Forgetting that the sprue bushing OD must clear the locating ring bore with running clearance.
The fix is to measure before ordering: head diameter and height, overall length, sprue entrance diameter, and the nozzle radius. With those four numbers we can match a stock bushing or identify the exact catalog number.
Catalog item plug: Tooling Components stocks DMS sprue bushings in A, B, and U series, sized by length and head style, plus the matching locating rings and nozzle adapters.