Professional Plastic Pipe Fitting Mould Manufacturer With 20 Years Of Experience - Spark Mould
Mold Case Study: High-Precision Dual shot Injection Mold for PP+TPE Water Traps
In the plumbing and fluid management sector, components like water traps require exceptional sealing capabilities, complex thread geometries, and robust structural integrity. At Spark Mould, we recently engineered a highly complex 2-cavity Dual shot (2K) injection mold to manufacture a sophisticated water trap component. This project required the seamless integration of hard plastic (PP) and a soft elastomer (TPE), combined with multi-directional mechanical actions to handle both internal and external threads, as well as side ports. Here is an in-depth look at the tooling engineering and mold structure that made this possible.
Product Specifications & Material Strategy
Product: Water Trap Component
Material Combination: Polypropylene (PP) + Thermoplastic Elastomer (TPE)
Molding Sequence: Rigid PP is injected first to form the structural skeleton, followed by TPE overmolding to create the integrated sealing elements.
Key Geometries:
- A bottom port featuring both internal and external threads.
- A lateral connecting joint (side port).
- Overall circular geometry requiring specialized molding mechanisms.
Advanced Gating System Design
To optimize cycle times, reduce material waste, and ensure a flawless cosmetic finish, the runner system was custom-engineered for both materials:
- PP Injection (1st Shot): We utilized a full hot runner system with pin-point gates (2 drops per cavity). This ensures precise temperature control, minimal shear stress on the PP material, and exceptional dimensional stability for the threaded sections.
- TPE Injection (2nd Shot): The soft TPE is delivered via a hot runner transitioning into cold banana gates (cashew gates). Specifically, one hot runner drop feeds into two cold runner banana gates. The distinct advantage of the banana gate is its ability to automatically shear (auto-degate) during the ejection phase, leaving a clean gate vestige on the TPE seal without the need for secondary manual trimming.
Innovative Mold Structure & Kinematics
Given the complex geometry of the water trap, a standard Dual shot rotary platen was not the most efficient solution. Instead, our engineering team designed a highly integrated mechanism on the moving half of the mold.
Retractable Core (Core-Back) 2K Technology
Instead of rotating the mold half, this tool relies on movable core inserts (core-back mechanism) to achieve the bi-injection process.
- During the first shot, the active core insert remains in its forward position, allowing the PP substrate to be molded.
- Once the PP solidifies, the movable core retracts, creating a precise empty cavity space.
- The second injection unit then shoots the TPE directly into this newly formed space, bonding it chemically and mechanically to the PP substrate.
- Engineering Insight: The core-back technique significantly reduces tooling footprint and cost compared to rotary molds, while ensuring perfect alignment for concentric circular parts.
Split Cavity (Half-Block) Mechanism for External Features
Because the water trap features a circular overall structure and external threads on the bottom port, traditional demolding would cause interference. To solve this, both the cavity and core are integrated on the movable side of the mold using a Split Cavity (Half-Block) structure.
- External Threads: Driven by heavy-duty angle pins, the split cavities smoothly slide open laterally during mold opening, safely releasing the external threads without stripping or scuffing the plastic.
- Side Port Management: The lateral connecting joint is also managed by side sliders (actuated by angle pins). As the mold opens, the sliders pull away from the side port concurrently with the split cavities, ensuring a collision-free release.
Hydraulic Auto-Unscrewing Mechanism for Internal Threads
The most mechanically demanding feature of this water trap is the internal thread located within the bottom port. To automate the demolding of this section without extending cycle times, the mold is equipped with an Automatic Unscrewing Mechanism.
- Action Flow: A hydraulic cylinder drives a precision rack and pinion (gear) system.
- Before the final ejection of the part, the hydraulic cylinder actuates, rotating the threaded core.
- Simultaneous to the rotation, the threaded core retracts, cleanly unthreading itself from the PP component. Only after the core is completely clear does the final ejector system push the completed water trap off the mold.
Why This Matters for High-Volume Production
Designing a single mold that incorporates core-back bi-injection, split-cavity sliders, and hydraulic auto-unscrewing requires exacting tolerances. By integrating these systems, we eliminate the need for secondary assembly (such as adding separate O-rings or manually unscrewing parts). The result is a highly automated, operator-free molding cycle that guarantees consistent part quality, superior IP-rated sealing from the TPE, and a lower total cost of ownership (TCO) for the injection molding manufacturer.
Are you looking to optimize your next complex Dual shot or auto-unscrewing tooling project? Our engineering team specializes in transforming intricate product designs into reliable, high-yield injection molds.