Structural Analysis of a Union Nut Injection Mold with Collapsible Core

• Product Name: Union Nut (Circular pipe fitting/fastener with internal threads)
• Number of Cavities: 4 cavities (1-out-of-4), striking an optimal balance between production efficiency and mold size.
• Gate Type: Submarine Gate (Tunnel Gate)
• Core Structure: Collapsible Core Mechanism (consisting of 1 central main core and 6 outer collapsible segments).
• Automation Level: Fully automatic ejection (automatic gate cutting; product and runner drop automatically).

For products with internal threads, traditional molding methods often rely on an auto-unscrewing mechanism (hydraulic motor + gears). However, unscrewing molds have several drawbacks, including bulky mold sizes, longer cooling cycles, and high susceptibility to mechanical wear.

This case study innovatively adopts a collapsible core mechanism. This assembly features a central main pin surrounded by six outer sliding segments. When the central main core is pulled out, the six outer segments lose their internal support and instantly collapse inward toward the center. This reduces their outer diameter, instantly freeing them from the product's internal threads. This design not only drastically shortens the molding cycle but also allows for a much more compact mold structure, easily accommodating a multi-cavity (1-out-of-4) layout.

The ingenuity of this mold lies in controlling the sequential movement of the mold plates via limit mechanisms. The entire mold opening and ejection process is divided into three critical stages:

When the injection is complete and the mold begins to open, the entire moving half first travels backward for a very short distance.

• Purpose: To cleanly shear the submarine gate, separating the cold runner from the Union nut product.
• Engineering Logic (Why must this happen first?): Shearing a submarine gate requires a resisting force, which is provided by the product being held rigidly inside the mold. If the collapsible core were to shrink first, the product would lose its internal metallic support. Consequently, at the moment of shearing, the product could be warped, pulled, or the gate cutting could fail entirely. Therefore, the gate must be sheared while the core still tightly supports the inside of the product.

As the mold continues to open, the internal limit mechanisms come into play.

• Action Process: Guided by the friction and limiting action of resin latch lock (friction pullers) and pull bars, the backing plate and the B-plate are forced to stop moving. The rest of the moving half continues its backward stroke.
• Core Extraction Principle: Accompanying the retreat of the moving half, the central main pin of the collapsible mechanism is pulled backward. Without the support of the main core, the 6 threaded sliding segments rapidly collapse toward the center. At this point, the overall outer diameter of the threaded core becomes smaller than the inner diameter of the product, successfully disengaging from the internal threads.

• Action Process: Once the threaded core has completely collapsed and separated from the product, mechanical pullers drag the backing plate and the B-plate to resume their backward movement.
• Ejection & Drop: When the moving half reaches its final stroke limit, the injection molding machine's ejector system is triggered, pushing the B-plate forward. Since the internal threads of the products are now completely free of any constraints, both the Union nut products and the separated runner system are smoothly ejected from the B-plate, falling automatically under the force of gravity.

Through highly integrated mechanical movements, this Union nut mold showcases the logical beauty of advanced mold design. The combination of "Submarine Gate + Collapsible Core + Sequential Mold Opening" perfectly resolves the industry pain points of slow ejection and the inability to auto-shear gates for internal threaded parts. For injection molding manufacturers requiring high-volume, high-efficiency production of pipe fittings and nuts, this mold structure serves as a highly valuable reference for reducing costs and boosting productivity.