How to Match Sprue Bushings with Locating Rings and Guide Bushings
The Three-Component Assembly
The sprue entry assembly consists of three components that must be dimensionally compatible:
- Locating ring — Centers the mold on the machine platen. Its OD fits the machine's platen locating hole; its ID accepts the sprue bushing.
- Sprue bushing — Creates the resin channel from the nozzle into the mold. Its flange seats against the locating ring; its nozzle touch surface (SR) contacts the machine nozzle.
- Guide bushing (optional) — A hardened sleeve in the plate bore that aligns the sprue bushing and protects the plate from wear.
Critical Dimensional Relationships
| Relationship | Rule | Why It Matters |
|---|---|---|
| Locating ring OD ↔ Machine platen hole | Ring OD = Platen hole diameter (h7 fit) | Centers the mold on the machine |
| Locating ring ID ↔ Sprue bushing flange OD | Ring ID > Bushing flange OD + 0.5 mm clearance | Allows bushing assembly through ring |
| Sprue bushing SR ↔ Machine nozzle SR | Bushing SR = Nozzle SR + 0.5–1.0 mm | Ensures proper nozzle seating |
| Sprue port (P) ↔ Nozzle orifice | Port > Nozzle orifice + 0.5 mm | Prevents resin hang-up at entry |
| Bushing body OD ↔ Plate bore | Bushing OD matches plate bore (h7/H7) | Ensures axial alignment |
Common Machine Platen Sizes
| Machine Tonnage Range | Common Platen Hole ø | Locating Ring OD |
|---|---|---|
| 50–100 ton | ø100, ø120 mm | ø100, ø120 mm |
| 100–250 ton | ø120, ø150 mm | ø120, ø150 mm |
| 250–500 ton | ø150, ø200 mm | ø150, ø200 mm |
| 500–1000 ton | ø200, ø250 mm | ø200, ø250 mm |
Always verify the actual platen hole diameter on your specific machine. Platen hole sizes vary between manufacturers (Arburg, Engel, Sumitomo, JSW, Haitian, etc.) even within the same tonnage range. According to Euromap standards for injection molding machines, platen dimensions are standardized within machine families but can differ between brands.
SR Matching
The SR (Spherical Radius) is the curved surface on the sprue bushing where the machine nozzle contacts. The nozzle tip has a convex sphere; the bushing has a concave sphere. For proper seating:
- Bushing SR must be larger than the nozzle SR so the nozzle tip sits inside the bushing's concave surface
- Common SR values: 10R, 12R, 15R, 20R, 25R (mm)
- If SR values are equal, the nozzle contacts only at the edge of the sphere, causing poor sealing and accelerated wear
According to JIS B 5111 mold component standards, the recommended SR relationship is: Sprue bushing SR = Nozzle SR + 0.5 to 1.0 mm.
Guide Bushing Selection
Guide bushings are optional wear sleeves that protect the mold plate bore:
- Always recommended for bolt-fixing sprue bushings — These bushings float freely in the bore and rely on the guide bushing for alignment
- Optional for shoulder sprue bushings — The shoulder provides inherent alignment, but a guide bushing still extends plate bore life
- Material: SKD61 at 50–54 HRC, OD press-fit into plate bore (H7/h6)
- ID: H7 clearance fit with sprue bushing body
Frequently Asked Questions
Why does the locating ring inner diameter need to match the sprue bushing flange?+
What happens if the sprue bushing SR does not match the machine nozzle?+
Do I need a guide bushing with my sprue bushing?+
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