When to Use Tension Links vs Parting Locks for Mold Opening Control
Key Takeaway: Tension links and parting locks are not interchangeable �?they solve different problems. Parting locks control which surface opens first (sequence). Tension links control how far each plate travels (stroke). Most three-plate molds need both working together.
Fundamental Difference: Sequence vs Stroke
The confusion between tension links and parting locks comes from the fact that both are installed at parting surfaces of injection molds. But they control entirely different aspects of mold opening:
| Aspect | Parting Locks | Tension Links |
|---|---|---|
| Controls | Opening sequence �?which surface opens first | Opening stroke �?how far each plate travels |
| Mechanism | Holding force locks plates together until overcome | Rigid link with oval hole limits travel distance |
| When it acts | During mold opening �?prevents premature separation | During mold opening �?stops plate at maximum stroke |
| Adjustable parameter | Holding force (by lock type, diameter, sleeve condition) | Stroke distance (by oval hole length, configurable in 5 mm increments) |
| Mounting position | PL face or mold exterior (depends on type) | Mold exterior �?bolted across parting surface |
| Wear item | Yes �?sleeves degrade over 200K-500K cycles | No �?steel link lasts millions of cycles |
When You Need Parting Locks Only
Parting locks alone are sufficient when:
- The machine controls plate travel. If the injection molding machine has programmable multi-stage opening (position-controlled), the machine defines stroke distances. Parting locks only need to ensure the correct surface opens first.
- Exact stroke distance is not critical. In molds where the runner drops by gravity and the required clearance is generous, precise stroke control is less important than correct sequence.
- Space constraints prevent external tension links. Tension links mount on the mold exterior and require clearance for the link body. In tight-tolerance press installations, parting locks (which mount internally) may be the only option.
When You Need Tension Links Only
Tension links without parting locks are used when:
- Opening sequence is inherently correct. Some mold designs (e.g., with spring-loaded plates or gravity-assisted opening per standard mold design) naturally open in the correct order. In these cases, only stroke control is needed.
- The mold has only one parting line but needs stroke limiting. Two-plate molds with extended opening stroke may need tension links as safety limiters to prevent the mold from opening beyond the guide pin engagement length.
- Safety restraint is the primary goal. Tension chains (a type of tension link) are used purely as safety backup during maintenance �?not for production sequence control.
When You Need Both Together
Most production three-plate molds require both parting locks and tension links working as an integrated system:
- Parting locks ensure PL1 opens before PL2 by applying graduated holding forces.
- Tension links limit PL1 stroke to exactly the distance needed for runner clearance �?preventing over-travel that could stress the runner plate or damage guide pins.
- Stopper bolts provide hard stops as backup to tension links.
This combination provides both sequence control (correct order) and stroke control (correct distance) �?the two independent requirements for reliable three-plate mold operation.
Installation Relationship
When both are installed, they work in series during the opening cycle:
- Phase 1: Machine opens. Parting locks hold PL2 closed while PL1 opens. Tension link at PL1 has slack (bolt moving in oval hole).
- Phase 2: PL1 reaches maximum stroke �?tension link bolt reaches the end of oval hole, creating a hard stop. PL1 movement stops.
- Phase 3: Continued machine opening force overcomes PL2 parting locks. PL2 begins to open. PL1 remains at its defined position.
- Phase 4: PL2 reaches maximum stroke �?second tension link (or stopper bolt) stops PL2. Ejection activates.
Quick Selection Matrix
| Mold Configuration | Parting Locks? | Tension Links? | Reason |
|---|---|---|---|
| Two-plate mold, standard | No | No | Single parting line, no sequence control needed |
| Two-plate mold, long stroke | No | Yes | Stroke limiter to protect guide pins |
| Three-plate mold, machine-controlled stroke | Yes | Optional | Sequence control needed, machine handles stroke |
| Three-plate mold, standard | Yes | Yes | Both sequence and stroke control needed |
| Stack mold | Yes | Yes | Multiple parting lines require full control |
Frequently Asked Questions
Do tension links replace parting locks?+
No. They serve different functions: parting locks control opening sequence, tension links control opening stroke. They are complementary, not interchangeable. Most three-plate molds need both.
Can I use tension links without parting locks?+
Yes, but the opening sequence will be uncontrolled and depend on friction. This is acceptable only when the sequence is inherently correct (spring-loaded or gravity-assisted designs) or the mold has only one parting line.
How many tension links does a mold need?+
Minimum two per stage, symmetrically placed. For molds wider than 600 mm, use four in a rectangular pattern. Each link should be the same configuration (same oval hole length) to ensure uniform stroke limiting.