Zhuojin Pipe Fitting Co., LtdInjection Mold Design and Process for HPVC Tee Connectors
Literature Overview
This paper by Wu Maochang, published in the Journal of Dalian Polytechnic University in 2009, addresses the injection mold design for a plastic tee connector made from heat-resistant PVC (HPVC). The study was supported by the National Natural Science Foundation of China (Grant No. 50775073). The author analyzes the product structure and identifies the key process challenges in injection molding, ultimately selecting a single-cavity mold configuration with a hydraulic-driven thread ejection mechanism.
Core Technical Analysis
The primary challenge in injection molding a tee connector with internal threads lies in the demolding of the threaded feature. Conventional ejector pins cannot release internal threads without damaging the molded part. The author proposes a hydraulic-driven thread ejection mechanism that rotates the mold core during the ejection cycle, allowing the internal threads to be disengaged cleanly from the part.
Key Design Decisions
| Design Element | Specification | Rationale |
|---|---|---|
| Mold configuration | One cavity per mold | Simplifies alignment and reduces mold complexity for a small production run |
| Material | HPVC (heat-resistant PVC) | Provides chemical resistance and dimensional stability for plumbing applications |
| Ejection mechanism | Hydraulic-driven thread rotation | Solves internal thread demolding without part damage |
| Temperature control | Dedicated temperature control system | Maintains melt flowability and prevents premature solidification in the runner |
Process Parameter Considerations
HPVC injection molding requires careful control of melt temperature (typically 180–220 °C), mold temperature (50–80 °C), injection pressure (60–90 MPa), and holding pressure (30–50 MPa). The author notes that excessive melt temperature can cause decomposition of PVC, leading to discoloration and reduced mechanical properties. The single-cavity configuration was chosen partly to minimize the number of temperature control circuits and reduce the risk of uneven cooling.
Engineering Practice Insights
From a practical standpoint, this paper highlights a common challenge in plastic fitting production: the integration of functional features (threads, internal passages) with mold manufacturability. The hydraulic thread ejection mechanism adds cost and complexity to the mold but is essential for high-quality thread formation. Engineers should note that the single-cavity approach may not be economical for high-volume production; multi-cavity designs with synchronized thread ejection would be necessary for mass production scenarios. The temperature control system design is critical, as HPVC has a narrow processing window, and any deviation can result in surface defects or dimensional inaccuracy.
Study Reflections
This paper serves as a useful reference for understanding how mold design must accommodate functional geometry. The hydraulic thread ejection solution is elegant but should be evaluated against alternative approaches such as side-core actions or collapsible cores. For engineers working on plastic pipe fittings, the key takeaway is that the mold design phase must begin with a thorough analysis of the part's demolding constraints, as these often dictate the overall mold architecture and cost.