How Stopper Bolts and Puller Bolts Control Plate Separation Sequence
Four Phases of Plate Separation
In a standard three-plate injection mold, the opening cycle proceeds in four distinct phases. Stopper bolts and puller bolts play specific roles in each:
Phase 1: Clamp Release and Initial Separation
The injection molding machine releases clamp pressure. At this moment, all plates are still in contact. Collar springs pre-loaded on stopper bolts begin pushing plates apart. If puller bolts are installed, their spring elements also contribute initial separation force.
- Stopper bolt role: Collar springs provide 50-200 N of initial push force per bolt.
- Puller bolt role: Internal springs (if present) pull the runner plate away from the cavity plate.
Phase 2: PL1 Opening (Runner Separation)
The machine's opening force overcomes the PL1 parting lock holding force. The runner plate separates from the cavity plate, breaking the gate connections. Puller bolts at PL1 provide sustained separation force throughout this phase.
- Stopper bolt role: Not yet engaged — bolt head is approaching its contact surface as plates separate.
- Puller bolt role: Active pulling force ensures clean, even separation despite resin adhesion.
Phase 3: PL1 Stop
The runner plate reaches its maximum opening distance. Stopper bolt heads contact the cavity plate, creating a hard mechanical stop. The runner plate halts. Further machine opening force transfers to PL2.
- Stopper bolt role: Active — bolt head creates a positive stop. Impact absorption depends on bolt material and contact surface treatment.
- Puller bolt role: No longer contributing — plates are at maximum separation.
Phase 4: PL2 Opening (Part Exposure)
With PL1 stopped, continued machine force overcomes PL2 parting locks. The cavity plate separates from the core plate, exposing the molded parts. Stopper bolts at PL2 eventually create the second stop. Ejection activates.
Force Analysis at Each Phase
| Phase | Machine Force | Stopper Bolt Force | Puller Bolt Force | Parting Lock Force |
|---|---|---|---|---|
| 1 (Clamp release) | Releasing | Spring push: 50-200 N/bolt | Spring pull: 50-150 N/bolt | Holding: max |
| 2 (PL1 opening) | Active opening | Not engaged | Active pull: 200-500 N/bolt | PL1 releasing, PL2 holding |
| 3 (PL1 stop) | Active (transfers to PL2) | Contact stop | Slack | PL2 releasing |
| 4 (PL2 opening) | Active | PL2 bolts not yet engaged | PL2 puller bolts active | Released |
Sizing Guidelines
- Stopper bolt length: Equal to desired opening distance at that parting line. Measure from bolt head contact surface to the mounting plate surface. Tolerance: ±0.1 mm.
- Stopper bolt thread engagement: Minimum 1.5× bolt diameter into the mounting plate for secure anchoring under impact loading.
- Puller bolt thread engagement: Minimum 2× bolt diameter for tensile load applications. Shorter engagement risks thread stripping under repeated pull cycles.
- Collar spring selection: Spring force should be 10-20% of the expected adhesion force at the parting surface. Too strong: premature opening. Too weak: plates stick.
Common Issues and Fixes
| Issue | Cause | Fix |
|---|---|---|
| Bolt head mushrooms after 100K cycles | Impact stress exceeds bolt material hardness | Upgrade to heat-treated alloy steel bolt, or add a hardened washer as contact surface |
| Plate doesn't stop at correct distance | Bolt length is wrong or bolt has loosened from the mounting plate | Re-measure and replace bolt; add thread-locking compound to mounting threads |
| Plates stick despite puller bolts | Puller bolt force insufficient for adhesion level | Add more puller bolts, or add urethane springs for additional separation force |