Design Fundamentals for Thread-Unscrewing Core-Pull Mechanisms in Injection Molds
Threaded-unscrewing mechanisms are employed in injection molds to form and demold internal threads on plastic parts. Unlike straight-pull actions, these systems require a rotary motion to unscrew the core from the molded thread, making them one of the more complex mold actions. Success hinges on the precise integration of mechanical drives, structural components, and control systems.
1.Mechanical Drive Systems: The Heart of the Mechanism
The choice between hydraulic motors, servo motors, or gear trains is fundamental. For high-torque applications, hydraulic motors are preferred due to their robust power and ability to withstand high inertial loads. Their compact size is an advantage in space-constrained mold designs. Conversely, electric servo motors offer superior control, precision positioning (within ±0.1 degrees), and cleaner operation, making them ideal for fine-pitch threads and environments where oil contamination is a concern.
The drive system must be coupled with a reliable gear reduction unit. Planetary gearheads are commonly specified for their high torque-to-size ratio and low backlash (typically ≤3 arc-min), which is critical for preventing thread damage during the unscrewing sequence.
2. Kinematic Design and Precision Engineering
The kinematic chain—from the motor to the threaded core—must be designed for minimal play and high torsional stiffness. A typical setup involves a primary drive gear engaging with a larger gear on the threaded core or its rack. The gear design is paramount; a module of 1-2mm with a pressure angle of 20° provides a good balance of strength and smooth operation. For longer threads, incorporating a helical gear (with a helix angle of 8-15°) can increase tooth contact and reduce noise and wear.
A critical design element is the anti-rotation mechanism. While the core rotates to unscrew, it must be prevented from rotating during mold closing and plastic injection. This is typically achieved using a key and keyway system. The clearance between the key and keyway must be tightly controlled (e.g., 0.02-0.03mm) to allow smooth linear travel while eliminating rotational slack.
3. Component Manufacturing and Material Selection
The threaded core itself is a high-wear item. It is typically machined from a high-grade tool steel like SKD61 (AISI H13), which is then vacuum heat-treated to a hardness of HRC 48-52 to resist abrasion from glass-filled plastics. The thread profile is often ground or EDM'd to achieve the required surface finish (e.g., Ra ≤ 0.4 µm) and dimensional accuracy.
To further enhance service life, advanced surface treatments such as Physical Vapor Deposition (PVD) or TD (Thermal Diffusion) processing can be applied. These coatings, like chromium nitride or vanadium carbide, create an extremely hard, low-friction surface that can extend the core's life by a factor of five or more in high-volume production.
4. Integration with the Molding Cycle and Control
The unscrewing sequence must be perfectly synchronized with the molding machine's cycle. This requires electrical interlocks between the mold and the machine controller. A standard sequence is:
Mold opens.
The unscrewing mechanism is activated, rotating the core until the thread is fully disengaged.
The core retracts linearly (or the part is ejected off the core).
The mechanism reverses to its starting position.
The mold closes, but only after a sensor confirms the core is fully reset.
Integration of proximity sensors to verify the start and end positions of the rotation and linear travel is essential for preventing catastrophic mold damage.
Conclusion
Designing a robust thread-unscrewing mechanism is a multidisciplinary challenge. It requires a deep understanding of mechanical drives, precision machining, material science, and electronic controls. By meticulously addressing each of these areas—selecting the appropriate drive, engineering a stiff and precise kinematic chain, specifying wear-resistant materials, and implementing foolproof safety controls—mold designers can create systems that produce high-quality threaded parts with exceptional reliability over hundreds of thousands of cycles.
