Designing a collapsible core mold for threaded consumer product parts centers around using an innovative mechanical structure to “fold” or “shrink” specific parts of the core during ejection, thereby enabling the undamaged demolding of plastic parts with complex features like internal threads or undercuts.
The table below compares three mainstream technical solutions for achieving the “collapsible/shrinkable” function. You can choose based on your production needs:
| Aspect | Option 1: Lifter/Slider with Thread Insert | Option 2: Rotational Thread-Unscrewing Mechanism | Option 3: Removable Thread Core (Linked with Ejection System) |
|---|---|---|---|
| Core Principle | A small threaded insert is embedded in a lifter or slider. During demolding, the lifter/slider first performs a lateral core-pulling motion to disengage the thread insert from the part, followed by ejection. | The threaded core rotates in place, driven by a motor, gears, or ejector pins, to unscrew itself from the plastic part. | The threaded core is entirely pushed out by an independent ejection mechanism during the ejection phase. It is demolded together with the part and then separated manually or automatically. |
| Method of “Folding/Shrinking” | The lateral movement of the lifter or slider achieves a radial shrinkage of the threaded core section. | The core itself does not shrink; the spiral engagement with the part is released through rotational movement. | The core retracts axially as a whole. Its structure may include movable, segmented inserts to achieve “folding” during the ejection process. |
| Advantages | Relatively simple and reliable structure, lower cost. Suitable for external threads or internal threads located near an edge. | High demolding precision, advantageous for deep or long threads, excellent thread quality, high degree of automation. | Thread cores can be replaced quickly, facilitating maintenance and mold repair. Suitable for multiple specifications and small-batch production. |
| Disadvantages | Limited by space; difficult to implement for complex internal threads or deep-hole threads; may leave noticeable parting line marks. | Complex structure, high design and machining costs, increased mold thickness, longer production cycle. | Adds a secondary operation after ejection (separating core from part), which may affect full-automation efficiency. |
| Typical Application Scenarios | Bottle caps, simple threaded fittings, consumer product housings with external threads. | Cosmetic jars/bottles, pen barrels, precision instrument components—products with high thread quality requirements. | Medical devices, electronic products—scenarios requiring frequent thread specification changes or where threads are prone to wear during trial and production runs. |
Key Design Points and Considerations
When designing a “collapsible core,” beyond selecting a solution, the following points must be systematically considered:
- Motion Interference and Sequence Control: This is the core of the design. Mold motion simulation analysis is essential to ensure all moving components (lifters, sliders, ejector plates, rotational mechanisms) coordinate precisely in both time and space, with absolutely no collisions allowed. The typical demolding sequence is: first perform core-pulling or rotation at the thread, then proceed with the main ejection.
- Precision Manufacturing and Polishing of the Core: Collapsible cores are often composed of multiple segmented inserts. The machining precision (typically within ±0.01mm) and fit clearance of these segments are crucial. Surfaces require mirror polishing to ensure a smooth part surface and facilitate smooth sliding during demolding.
- Material and Cooling: Core components should be made of mold steel with high hardness and high thermal fatigue strength. The complex core structure internally must incorporate uniform and efficient cooling channels to prevent localized overheating that could cause uneven part shrinkage or deformation.
- Consideration of Plastic Material: The material’s shrinkage rate, rigidity, and elasticity directly affect demolding. For relatively hard or brittle materials like PC/PMMA, sufficient draft angles are necessary, and forced ejection should be avoided to prevent thread damage or part cracking.
How to Proceed with Your Design
It is recommended that you follow these steps:
- Define Part and Production Requirements: This is the foundation for selecting a solution.
- Part Drawing Analysis: Is the thread internal or external? What are the length and precision requirements for the thread? What is the overall structure of the part?
- Production Requirements: What is the expected production volume (high-volume/low-volume)? What are the automation requirements? What are the budget and lifespan expectations for the mold?
- Conduct Preliminary Conceptual Design: Based on the initial analysis, select 1-2 technical solutions from the table above and create a rough layout design using 3D software.
- Perform Mold Motion Simulation Analysis: This is the most critical step for technical validation. Specialized mold flow analysis (e.g., Moldflow) and mold motion simulation software must be used to simulate the motion paths, timing, and speeds of all moving components, identifying and resolving interference issues in advance.
- Detailed Design and Machining: After successful simulation validation, proceed with detailed engineering drawing design, focusing on tolerance fits, heat treatment requirements, and cooling system planning.