Professional Plastic Pipe Fitting Mould Manufacturer With 20 Years Of Experience - Spark Mould
Overcoming Full-Perimeter Internal Undercuts in Plastic Brush Body Injection Mold Design
By implementing a highly synchronized collapsible core and lifter mechanism, this mold delivers reliable, fully automated production without compromising part integrity.
Project Specifications & Engineering Challenge
Primary Challenge: The installation recess features a continuous internal undercut along its entire inner perimeter. Standard force ejection (stripper plates) would deform the rigid polymer, while conventional external sliders are restricted by the outer geometry.
Core Parameters:
- Product: Plastic Brush Body (with a rectangular recess for bristle head installation).
- Cavitation: 2-Cavity (1-out-of-2 layout).
- Gating System: Conventional cold runner system with direct side gating.
Tooling Architecture: The Collapsible Core Solution (9 components)
To facilitate clean ejection within an extremely confined internal space, the core section forming the rectangular recess was segmented into nine independent, interlocking components. This "divide-and-conquer" approach transforms a static steel core into a dynamic kinematic assembly:
- Central Main Core (1 Piece): Positioned at the absolute geometric center of the recess, acting as the structural anchor and space occupier during injection.
- Internal Linear Sliders (4 Pieces): Positioned to form the four straight sidewaalls of the rectangular recess.
- Corner Lifters (4 Pieces): Positioned precisely at the four corners of the recess to manage the intersecting undercut zones.
Kinematics Sequence: Step-by-Step Mold Movement
The success of this complex tool relies on a strictly timed sequence of mechanical actions to prevent component collision and tool wear. The mold cycle operates through four distinct phases:
Phase 1: Delayed Parting Line Opening & Main Core Pull
Upon completion of the cooling cycle, the injection molding machine initiates the opening stroke. However, external latch locks or mechanical pull rods ensure that the A and B plates remain strictly closed during the initial movement. Concurrently, a hydraulic cylinder or secondary mechanical linkage pulls back the Central Main Core. The extraction of this central component creates an immediate cruciform void space at the heart of the core assembly.
Phase 2: Internal Sliders Inward Collapse
With the central void space cleared, the 4 Internal Linear Sliders are driven along precise guiding tracks, collapsing inward toward the center. This motion retracts the steel from the straight sidewalls of the brush body, completely disengaging the primary undercut sections.
Phase 3: Main Tool Parting (A/B Plate Separation)
Once the inward collapse of the linear sliders is mechanically verified, the latch locks disengage. The machine's primary stroke now forcibly opens the A and B plates. The molded part stays with the moving side (B-plate) as it separates from the stationary side (A-plate).
Phase 4: Corner Lifter Ejection and Final Releasev
In the final phase, the machine’s ejector rods push the tool’s ejector retainer plate forward. This drives the 4 Corner Lifters upward. As they lift, they travel along angular guide pins, moving diagonally inward toward the center of the part. This diagonal movement releases the remaining corner undercuts. At the end of the stroke, the brush body is entirely free from all steel elements and drops cleanly via gravity or robotic takeoff.
Gating System Optimization
The tool utilizes a 2-cavity balanced layout fed by a traditional cold runner system utilizing optimized side gates.
Design Benefit: Side gating ensures balanced melt front advancement into both cavities, minimizing peak injection pressures and reducing weld lines around the internal recess. Furthermore, the gate location is optimized to shear automatically or clean easily during post-processing, maintaining a pristine aesthetic finish on the visible surfaces of the brush body while ensuring stable, long-term production cycles.
Advanced Undercut Solution Strategies
For industrial buyers and sourcing engineers evaluating tool designs for complex enclosures, selecting the correct undercut release mechanism is critical for total cost of ownership (TCO) and cycle time efficiency.
| Mechanism Type | Best Applied To | Pros | Cons |
Segmented Collapsible Core (This Case) | Rectangular/Irregular complex internal undercuts in rigid plastics (ABS, PC, POM) | High structural rigidity, excellent tool life, precise parting lines. | High initial machining complexity, requires tight tolerances. |
Standard Proprietary Collapsible Cores | Circular internal threads or continuous round undercuts. | "Off-the-shelf availability, compact footprint. | Restricted to cylindrical geometries; expensive replacement parts. |
Force Ejection (Bumping) | Shallow undercuts in flexible materials (PP, PE, TPE). | Simplified mold structure, lower tooling costs. | Risks stress whitening, deformation, and high scrap rates. |
Conclusion
By opting for a custom-engineered 9-piece mechanical collapse sequence over force-bumping, this design guarantees zero stress whitening or geometric distortion on the brush body. The investment in precise CNC tool-room machining pays dividends in manufacturing longevity, making it the ideal architecture for high-volume, automated production lines.