Zhuojin Pipe Fitting Co., LtdPrecision Investment Casting Process Improvement for Tee Castings
Literature Overview
This 1995 paper by Li Xianwen from Baoji Petroleum Machinery Factory, published in Special Casting and Nonferrous Alloys, documents a process improvement initiative for investment casting of tee fittings used in export products. The factory employed water-glass binder investment casting with quartz sand refractory material and ammonium chloride as a hardener for shell production. The improvement was driven by the need to meet the quality requirements of export-grade tee castings, which demanded higher dimensional accuracy and surface finish than standard domestic products.
Original Process and Identified Problems
The original process used plaster molds for wax pattern production on a high-volume, low-variety basis, while metal molds were reserved for batch production of precision castings. The identified problems included:
- Poor surface finish on export tee castings due to plaster mold wear and inconsistency.
- Shell strength variability caused by uneven ammonium chloride hardening.
- Geometric distortion at the branch junction due to non-uniform solidification.
- High scrap rate for export orders due to dimensional tolerance exceedance.
| Process Parameter | Original Value | Improved Value | Impact |
|---|---|---|---|
| Mold type for wax patterns | Plaster mold | Metal mold | Improved surface finish and dimensional consistency |
| Shell hardener concentration | Variable | Controlled NH4Cl concentration | More uniform shell strength |
| Shell thickness | 4 to 5 layers | 3 to 4 layers with controlled thickness | Reduced distortion |
| Wax pattern tolerance | ±0.5 mm | ±0.2 mm | Improved casting dimensional accuracy |
Process Improvement Measures
The key improvements implemented were:
- Transition from plaster molds to precision metal molds for wax pattern production, which significantly improved the surface finish and dimensional repeatability of the wax patterns.
- Optimization of the shell building process by controlling the number of dipping and stuccoing cycles to achieve a more uniform shell thickness, particularly at the geometric transitions of the tee junction.
- Adjustment of the ammonium chloride hardener concentration to improve shell strength without introducing excessive gas porosity.
- Introduction of a controlled cooling and shakeout procedure to minimize thermal shock cracking of the shell.
Quality Control and Verification
Post-improvement quality assessment revealed measurable gains:
- Surface roughness improved from Ra 12.5 μm to Ra 6.3 μm on the tee body.
- Dimensional tolerance conformance improved from 70% to 95% for critical dimensions.
- Internal porosity was reduced through improved venting and gating design.
- The scrap rate for export tee castings dropped significantly, making the process economically viable for high-value export orders.
Study Insights and Reflections
This paper is a valuable example of incremental process improvement in investment casting, a process that remains widely used for complex-shaped fittings such as tees, elbows, and reducers where the geometry is too complex for forging or machining alone. The transition from plaster to metal molds is a classic improvement strategy that should be considered whenever surface finish and dimensional accuracy become critical. The emphasis on shell thickness uniformity at geometric transitions is particularly relevant for tee castings, where the branch junction represents a stress concentration and a potential location for casting defects. For modern investment casting operations, these principles remain applicable, albeit with updated materials such as organic binders and advanced hardening systems. The paper serves as a reminder that even in traditional casting processes, systematic process optimization can yield significant quality and cost improvements.