In modern industrial applications, centrifugal impellers are critical high-speed rotating components used in compressors and turbines. Their complex blade structures, high precision requirements, and challenging manufacturability have long posed significant difficulties in mechanical engineering. Traditional manufacturing methods often struggle to achieve the necessary dimensional accuracy and surface finish for these parts. This study explores an improved approach to centrifugal impeller production by integrating advanced techniques like 3D printing and silicone molding into the classic lost wax investment casting process. Lost wax investment casting, derived from ancient methods, offers exceptional precision for intricate geometries, making it ideal for impellers. We aim to develop a streamlined workflow that enhances efficiency and quality while reducing defects commonly associated with conventional casting.
The core of our methodology revolves around adapting lost wax investment casting to incorporate digital fabrication tools. We begin by designing the impeller model using CAD software, which is then realized through 3D printing. This printed model serves as a master for creating silicone molds, which are used to produce wax patterns essential for the investment casting process. By refining each step—from pattern making to mold preparation and metal pouring—we seek to optimize the entire manufacturing chain. Our focus is on minimizing issues such as incomplete filling and gas entrapment, which are prevalent in complex castings. Through iterative experiments, we have established a robust protocol that leverages the versatility of lost wax investment casting while addressing its limitations through technological innovations.
To fabricate the wax patterns for lost wax investment casting, we employed fused deposition modeling (FDM) 3D printing with ABS thermoplastic material. The impeller design was modeled in CAD software and exported as an STL file. Printer settings were optimized for an 80% infill density to ensure adequate strength, dimensional stability, and surface quality. After printing, support structures were carefully removed to yield the master pattern. This pattern was then used in silicone molding to create flexible molds for wax replication. The silicone rubber and catalyst were mixed in a 100:3 ratio, poured around the pattern in a constructed frame, and allowed to cure. Demolding revealed a detailed negative impression, which was used to inject molten casting wax at 75±5°C. The resulting wax patterns exhibited high fidelity to the original design, confirming the suitability of this hybrid approach for lost wax investment casting.
The gating and riser system design is crucial in lost wax investment casting to ensure complete mold filling and defect-free castings. Initially, we tested a top-gating system, where metal enters from the upper part of the mold. However, this led to issues like mistruns due to rapid cooling, trapped gas, and incomplete dewaxing. The metal fluidity, governed by the Reynolds number (Re), can be expressed as:
$$Re = \frac{\rho v D}{\mu}$$
where $\rho$ is the density, $v$ is the flow velocity, $D$ is the characteristic diameter, and $\mu$ is the dynamic viscosity. In top-gating, high Re values indicate turbulent flow, which increases gas entrapment. To address this, we switched to a bottom-gating system, which promotes laminar flow and better venting. The pressure difference $\Delta P$ driving the flow in a bottom-gate system can be modeled as:
$$\Delta P = \rho g h – \frac{1}{2} \rho v^2$$
where $g$ is gravity and $h$ is the height difference. This modification significantly reduced defects, as bottom-gating allows gradual mold filling and minimizes turbulence. The comparison of gating systems is summarized in Table 1, highlighting key parameters affecting the lost wax investment casting process.
| Gating Type | Flow Characteristics | Defect Rate (%) | Metal Fluidity Index | Recommended Applications |
|---|---|---|---|---|
| Top-Gating | Turbulent, high velocity | 15-20 | Low | Simple geometries |
| Bottom-Gating | Laminar, controlled flow | 5-10 | High | Complex parts like impellers |
For the mold assembly in lost wax investment casting, we used a mixture of gypsum powder and quartz sand to create the investment mold. The composition was carefully controlled to balance thermal expansion and contraction during baking. The water-to-powder ratio was maintained between 38:100 and 41:100, with water temperature at 22°C, to achieve optimal slurry viscosity and strength. After mixing, the slurry was vacuum-degassed to remove entrapped air, which could cause surface defects. The wax patterns with gating systems were placed in steel flasks, and the slurry was poured to submerge them by 15-20 mm. Additional vacuum degassing ensured a dense, uniform mold structure. The molds were then left to set for over 2 hours before proceeding to dewaxing and sintering.
The dewaxing and sintering process in lost wax investment casting involves a multi-stage heating cycle to remove the wax pattern and strengthen the mold. We followed a precise temperature profile to prevent cracking and ensure complete wax removal. The heat transfer during baking can be described by Fourier’s law of heat conduction:
$$\frac{\partial T}{\partial t} = \alpha \nabla^2 T$$
where $T$ is temperature, $t$ is time, and $\alpha$ is thermal diffusivity. The stages included: initial drying at 80°C for 2 hours, gradual heating to 200°C and 400°C each for 2 hours, followed by 600°C for 2 hours, and a final hold at 730°C for 4 hours. This regimen ensures progressive wax elimination and mold sintering, reducing the risk of residual ash that could cause gas defects during pouring. The mold was then cooled to 280-320°C for metal pouring. The temperature schedule is detailed in Table 2, which outlines the critical parameters for each phase of the lost wax investment casting process.
| Stage | Temperature (°C) | Time (hours) | Purpose | Key Considerations |
|---|---|---|---|---|
| 1 | 80 ± 5 | 2 | Initial drying and wax softening | Prevent thermal shock |
| 2 | 200 | 2 | Wax melting and removal | Control vapor pressure |
| 3 | 400 | 2 | Binder burnout | Avoid mold cracking |
| 4 | 600 | 2 | Mold strengthening | Ensure dimensional stability |
| 5 | 730 | 4 | Final sintering | Achieve mold integrity |
| 6 | 280-320 | Cooling | Preparation for pouring | Optimize metal fluidity |
Metal pouring was conducted using a medium-frequency induction furnace, where aluminum alloy was melted and poured into the preheated molds via gravity. The bottom-gating system facilitated smooth filling, with metal flow monitored through observation ports. Upon solidification, the molds were broken away, and the castings were cleaned of gates, risers, and surface imperfections. The final impellers were inspected for dimensional accuracy and surface quality, meeting the required specifications. The success rate improved markedly with the bottom-gating approach, underscoring the importance of gating design in lost wax investment casting. The overall process efficiency can be evaluated using a quality index $Q$ defined as:
$$Q = \frac{\text{Number of defect-free castings}}{\text{Total castings}} \times 100\%$$
In our trials, $Q$ increased from approximately 75% with top-gating to over 90% with bottom-gating, demonstrating the efficacy of our modifications in the lost wax investment casting workflow.
In conclusion, our exploration confirms that integrating 3D printing and silicone molding into the lost wax investment casting process enables the production of high-quality centrifugal impellers. The use of FDM printing for master patterns and silicone molds for wax replication provides a flexible and precise method for creating complex geometries. Optimizing the gating system to bottom-gating significantly reduces defects by improving metal flow and venting. The detailed dewaxing and sintering cycle ensures mold integrity, while controlled pouring parameters enhance casting yield. This adapted lost wax investment casting approach offers a reproducible and scalable solution for manufacturing intricate components, with potential applications in aerospace and automotive industries. Future work could focus on material variations and automated systems to further refine the lost wax investment casting technique for broader adoption.