Hydraulic Side-Actuated Collapsible Core Mold for Rubber Ring Pipe Fittings

In high-pressure fluid handling, drainage, and industrial piping networks, the integrity of a joint relies strictly on elastomeric seals. The subject of this case study is a high-precision thermoplastic pipe fitting featuring an integrated internal annular groove at one end, designed to retain a thick rubber sealing ring. Achieving a tight, leak-proof fit requires the groove to exhibit perfect concentricity, zero parting-line flash, and sharp geometric definition.From an injection molding perspective, this 360-degree continuous internal groove constitutes a severe internal undercut. Traditional demolding mechanisms are completely inadequate for this geometry:

• Standard Slanted Ejectors (Lifters): Incapable of providing the required circumferential stroke and parallel collapsing movement without violating the internal wall thickness boundaries or scratching the sealing surface.
• Conventional Solid Cores: Result in complete physical entrapment, making mechanical part release impossible without catastrophic destruction of the molded article.

To overcome these physical constraints while adhering to the rigorous automated production standards dictated by European procurement mandates, Spark Mould engineered a proprietary side-actuated collapsible core framework.

The innovation of this tooling architecture lies in transferring the complex mechanical collapsing movement to a completely independent, side-mounted hydraulic cylinder assembly. This layout bypasses the constraints of the injection molding machine's standard opening stroke and localized mechanical ejector bars.

The collapsible core assembly is engineered with a high-precision multi-component geometry comprising one central main core pin and eight peripheral interlocking segments (sliders). The exact demolding process executes via a highly synchronized, two-step kinematic profile controlled entirely by a programmable hydraulic circuit:

1. Primary Main Core Retraction: Upon completion of the cooling cycle and mold opening, the side-mounted hydraulic cylinder is actuated. The cylinder pulls back the central main core pin along the vertical axis. This main core pin features precisely ground, multi-angled tapered faces. As it retracts, it vacates the internal support space.
2. Centripetal Segment Collapse: Driven by a series of T-slots and precision-ground dovetail guide tracks, the eight peripheral segments follow the retreating angles of the main core pin. This forces the segments to execute a strict lateral centripetal movement, collapsing inward toward the centerline. This collective inward movement reduces the effective outer diameter of the core segments by an amount greater than the undercut depth, completely disengaging the mold steel from the molded internal annular groove.

To safeguard against catastrophic tool wear and part deformation, a robust mechanical delay system is integrated into the side-action slide block. It is a strict prerequisite that no axial separation between the part and the overall collapsible core module occurs while the segments are still expanding or partially collapsed. 

The integrated delay mechanism enforces a sequence where the slide carrier remains stationary relative to the part during the first part of the hydraulic stroke. Only when the central core pin has traveled its full design distance—confirming that all eight segments have completed their centripetal inward collapse—does a secondary mechanical latch or hydraulic sequence valve trigger, allowing the entire core module to pull back horizontally out of the pipe fitting.

During the design phase with the German client, alternative configurations (such as integrating the collapsible core into the rear moving platen driven by the ejection system) were evaluated. The side-action layout was chosen based on three quantifiable engineering and economic factors:

The client required a solution capable of adapting to fluctuating market demands for two distinct pipe diameters.Spark Mould’s side-action configuration incorporates a highly versatile Universal Mold Frame Layout. To switch production between the two product variants, the core mold base and hydraulic cylinder housing remain fully clamped to the machine platen. The operator only needs to exchange the modular core segments and the central core pin. This reduces production line changeover times to under 90 minutes and slashes secondary tooling capital expenditures by approximately 45% compared to commissioning two separate complete molds.

Integrating a 2-cavity collapsing mechanism directly into the main rear plate requires massive, thick plates to house long mechanical linkages and slider bars. By shifting the actuation to an external side-action hydraulic block, the core inserts inside the mold base are kept remarkably compact. This optimization removed over 30% of redundant tool steel mass, lowering the total weight of the mold frame and allowing the tool to run effortlessly on a smaller tonnage injection molding machine with optimized platen spacing.

Decreasing the mass of the tool steel surrounding the cavity directly minimizes the overall thermal mass of the mold inserts. This structural optimization allowed Spark Mould engineers to route high-velocity cooling channels in extremely close proximity to the molding core area. The resulting heat transfer rate minimizes the required cooling duration, which represents a 15–20% cycle time optimization compared to deep-set mechanical core collapsing systems.

To ensure prolonged, high-efficiency operation in a fully automated manufacturing facility, the entire tool setup avoids manual handling. The full layout specifications are detailed in the matrix below:

With eight individual segments constantly sliding against a central core pin under extreme cyclic injection pressures (Pinj ≥ 110 MPa), managing steel friction and preventing flash formation is paramount. The core components are manufactured using premium European-grade 1.2344 (H13) hot-work tool steel, subjected to rigorous vacuum hardening to achieve a core hardness of HRC 48–52.

To guarantee zero flash within the annular groove, the interlocking seam lines between the eight segments are ground to an absolute tolerance profile of ±0.003 mm. Furthermore, all sliding mating surfaces receive an advanced Diamond-Like Carbon (DLC) coating. This coating lowers the coefficient of friction to μ ≤ 0.1 and introduces an ultra-hard surface layer (HV ≥ 2,200), entirely mitigating the risk of steel galling, adhesive wear, or seizure during high-velocity production runs.

The successful deployment of this 2-cavity hydraulic side-actuated collapsible core mold demonstrates that complex internal geometries do not inherently require slow, capital-intensive manufacturing processes. By matching modular side-action mechanics with advanced European metallurgy and robust delay sequences, Spark Mould provided its German partner with a high-OEE, highly versatile tooling system that sets a new benchmark for rubber ring pipe fitting production efficiency.