Bolt-Fixing vs. Shoulder Sprue Bushings: Which Mounting Type Is Right?
Key Takeaway: Bolt-fixing is the better choice when frequent bushing replacement is expected (corrosive resins, abrasive fillers). Shoulder type is more compact and self-aligning — ideal for molds with tight space constraints or where the bushing is rarely changed.
Side-by-Side Comparison
| Feature | Bolt-Fixing | Shoulder |
|---|---|---|
| Mounting method | Bolts through flange into plate | Shoulder seats in counterbore, clamped by assembly |
| Alignment method | Bore fit + guide bushing | Shoulder-to-counterbore register (self-centering) |
| Removal access | Back of mold (unbolt) | Requires plate separation |
| Replacement time | 15–30 minutes | 30–60 minutes |
| Space required | Larger (bolt flange needs clearance) | Smaller (compact shoulder design) |
| Axial retention | Positive (bolts hold bushing) | Friction + assembly clamping |
| Standard compliance | JIS, MISUMI catalog standards | JIS, MISUMI catalog standards |
When to Choose Bolt-Fixing
- Frequent replacement expected — Processing corrosive resins (PVC, FR compounds) or abrasive filled resins that wear the bushing bore
- Maintenance accessibility is a priority — Bolt-fixing allows bushing replacement without separating the mold plates
- Hot runner backup — Some molds have a sprue bushing as a backup entry point alongside the hot runner; bolt-fixing makes temporary installation easier
When to Choose Shoulder
- Compact mold design — The shoulder profile requires less plate surface area than a bolt-fixing flange
- Self-centering is important — The shoulder registers directly in the counterbore, providing inherent concentricity without a separate guide bushing
- Long production runs with rare maintenance — Unfilled commodity resins where bushing life exceeds 500,000 shots
According to JIS B 5111 mold component standards, both mounting types are standardized with interchangeable bore diameters and SR specifications. The choice is primarily driven by maintenance requirements and available plate space.
Material Options for Both Types
| Material | Hardness | Best For | Available In |
|---|---|---|---|
| SKD61 (H13) | 48–52 HRC | General purpose, standard resins | Both types |
| SKS3 (O1) | 56–60 HRC | Higher wear resistance, filled resins | Both types |
| SUS420J2 (420 SS) | 50–54 HRC | Corrosive resins (PVC, FR-PA, POM) | Both types |
| Pre-hardened steel | 30–35 HRC | Prototype/short-run molds | Both types |
According to ScienceDirect's sprue bushing engineering reference, sprue bushing material selection should match the resin's processing requirements — corrosion resistance for acidic off-gas resins and wear resistance for abrasive filled compounds.
Frequently Asked Questions
What is the difference between bolt-fixing and shoulder sprue bushings?+
Bolt-fixing bushings are secured with bolts through a flange, allowing easy back-of-mold removal. Shoulder bushings have an integral shoulder that seats in a plate counterbore, held by assembly clamping force. Bolt-fixing is easier to replace; shoulder is more compact and self-aligning.
Which type provides better nozzle alignment?+
Shoulder type provides inherently better alignment through direct shoulder-to-counterbore registration. Bolt-fixing relies on bore fit, which can allow slight misalignment from bolt hole clearance. Adding a guide bushing to bolt-fixing installations achieves comparable alignment.
Which type is easier to replace during mold maintenance?+
Bolt-fixing is 2–3× faster to replace. Remove bolts and pull out the bushing from the back of the mold — no plate separation needed. Shoulder type requires removing the locating ring and often separating the plate stack to access the bushing.
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