What Is the Difference Between Stopper Bolts and Puller Bolts in Three-Plate Molds
Two Bolts, Opposite Functions
Despite similar appearance, stopper bolts and puller bolts serve fundamentally opposite mechanical functions in three-plate injection molds:
| Property | Stopper Bolt | Puller Bolt |
|---|---|---|
| Function | Limits plate travel — creates a hard stop | Creates separation force — actively pulls plates apart |
| Force direction | Compressive (bolt head pushes against plate) | Tensile (bolt pulls plate away from adjacent plate) |
| When it acts | At maximum stroke — bolt head contacts plate | At beginning of opening — bolt engages and pulls |
| Thread type | Male thread into one plate, head contacts the other | Female thread (puller bolt) or male thread (short tip variant) |
| Critical dimension | Bolt length = desired opening distance | Thread engagement depth = pull force capacity |
| Mold damage if too short | Plate over-travels, guide pins disengage | Insufficient pull force, plates stick together |
| Mold damage if too long | Plate doesn't open far enough for runner/part clearance | Excessive pull force, potential bolt failure |
How Stopper Bolts Work
A stopper bolt is threaded into one plate with its head extending into the space between plates. As the mold opens, the plate carrying the bolt moves away from the adjacent plate. When the bolt head contacts the stationary plate, further movement is mechanically blocked:
- Material: Alloy steel, heat-treated for impact resistance at the contact surface.
- Length determines opening distance: The exposed bolt length (from head contact surface to the plate it's threaded into) exactly equals the maximum plate separation.
- Typically paired with springs: Collar springs mounted on the bolt provide initial separation force while the bolt head approaches its stop position.
How Puller Bolts Work
A puller bolt threads through one plate and mechanically engages the adjacent plate. As the mold opens, the puller bolt pulls one plate away from the other:
- Female-thread variant: The bolt has a female-threaded bore. A separate stud threaded into the adjacent plate screws into the puller bolt, creating a positive connection. Adjustable by turning the stud depth.
- Male-thread, short tip variant: The bolt tip threads directly into the adjacent plate. Simpler installation but less adjustable.
- Pull force capacity: Determined by thread engagement depth and bolt material. Typical capacity: 200-500 N per bolt.
Puller bolts are critical when resin adhesion or vacuum effects cause plates to stick despite parting lock release. Without puller bolts, the only separation force comes from the machine — which may not be sufficient or may apply unevenly.
Puller Bolt Collars
Puller bolt collars serve as spacers and guide elements. Installed around the puller bolt shank, they center the bolt in the plate bore and prevent lateral play. Collars also act as thrust surfaces when puller bolts provide separation force — distributing the pull force over a larger plate area to prevent localized deformation.
How They Work Together in a Three-Plate Mold
In a typical three-plate mold opening sequence:
- Phase 1: Mold begins opening. Parting locks hold PL2 while PL1 separates. Puller bolts at PL1 provide initial separation force to break adhesion.
- Phase 2: PL1 continues opening. When stopper bolts at PL1 contact the runner plate, PL1 stops at the designed distance.
- Phase 3: Continued opening force transfers to PL2 (parting locks release). Puller bolts at PL2 provide separation force.
- Phase 4: PL2 reaches stopper bolt limit. Mold is fully open. Ejection activates.