Abstract: This paper improves the investment casting process of centrifugal impellers by incorporating 3D printing and silicone rubber demolding technologies. Through extensive practice, a reasonable process flow has been studied and formulated, resulting in the production of qualified centrifugal impeller products. The results demonstrate that centrifugal impeller parts with qualified surface quality and dimensional accuracy can be manufactured through the improved investment casting process integrating 3D printing and silicone rubber demolding.
1. Introduction
Centrifugal impellers, as high-speed rotating components, generally consist of a hub and blades on one side of the hub. They are crucial parts of centrifugal compressors. Due to their complex blade structures, high precision requirements, and poor manufacturing accessibility, they have always been manufacturing challenges in the machinery manufacturing industry. Therefore, relevant research has been conducted on the manufacturing methods and processes of centrifugal impellers by domestic technological personnel. This paper aims to explore and study the manufacturing process and method of centrifugal impellers using the investment casting process, combined with 3D printing technology and silicone rubber demolding technology, to provide technical support for the manufacturing of centrifugal impellers.
Table 1: Overview of Previous Research on Centrifugal Impeller Manufacturing
| Researcher | Method | Technology Used | Outcome |
|---|---|---|---|
| Ma et al. | Milling and Turning Simulation | UG Software | Studied machining processes |
| Chen et al. | 3D Printing | Metal 3D Printing | Produced qualified impeller |
| Xu et al. | Rapid Precision Casting | 3D Technology & Gypsum Casting | Manufactured impeller meeting size accuracy |
2. Manufacturing Process of Centrifugal Impeller Wax Pattern
2.1 3D Printing Design and Manufacture of the Pattern
Based on the requirements of the part drawing, the 3D model of the impeller is drawn using UG software. The STL format file generated by the 3D software is imported into the 3D printer. Fused Deposition Modeling (FDM) 3D printing is adopted, using thermoplastic polymer structural material (ABS) as the printing material. To ensure the dimensional accuracy, surface roughness, and strength of the printed pattern, the print settings are set to 80% infill mode. After printing, excess supports are removed.
Table 2: 3D Printing Parameters
| Parameter | Value |
|---|---|
| Printing Material | ABS |
| Printing Method | FDM |
| Infill Percentage | 80% |
| Post-processing | Support removal |
2.2 Silicone Rubber Demolding and Wax Pouring
Based on the ABS impeller pattern made by 3D printing, silicone rubber demolding technology is used to create a silicone mold for pouring wax. To facilitate the removal of the impeller pattern from the solidified silicone mold and considering its structural characteristics, the impeller is divided into upper and lower parts for silicone molding. The silicone mold is made by pouring silicone liquid mixed with a curing agent into a frame and allowing it to solidify. After solidification, the mold is opened, and the impeller pattern is removed. The wax is then poured into the silicone mold, and after cooling and solidification, the wax pattern of the centrifugal impeller is obtained.
Table 3: Silicone Mold Making Process
| Step | Description |
|---|---|
| Preparation | Assemble frame according to pattern size |
| Pattern Placement | Place lower half of pattern down in the frame |
| Oil Clay Filling | Fill around and below the pattern with oil clay |
| Coating with Vaseline | Apply vaseline as a release agent |
| Mixing Silicone | Mix silicone liquid with curing agent (100:3) |
| Pouring Silicone | Pour mixed silicone into the frame and let it set |
| Demolding | Remove the mold, clean off oil clay, and separate |
3. Design of the Gating and Risering System for Centrifugal Impeller Wax Pattern
3.1 Before Improvement
Before improvement, the gating and risering system adopted a top-pouring method. Defects such as misruns were observed in the cast impeller, mainly due to low mold temperature, unreasonable gating and risering system design, and incomplete dewaxing.
Table 4: Defects and Causes in Top-Pouring Method
| Defect | Causes |
|---|---|
| Misruns | Low mold temperature, fast cooling of aluminum liquid, reduced fluidity |
| Poor排气 | Unreasonable gating and risering system, poor mold ventilation |
| Gas pores | Incomplete dewaxing, reaction between high-temperature metal and residual wax |
3.2 After Improvement
After improvement, a bottom-pouring method is adopted, which facilitates better gas venting within the mold cavity and results in a smoother metal filling process, minimizing the impact on the mold cavity and maximizing the production of high-quality centrifugal impeller castings with complete structures and low defect rates.
4. Manufacturing and Pouring of Centrifugal Impeller Mold
4.1 Mixing Powder Materials
Gypsum powder and quartz powder are weighed using an electronic scale and mixed uniformly. The amount of water directly affects the quality of the mold. After selecting a water-powder ratio of 100:38-41 and water temperature of around 22°C, the water is gradually added to the container with mixed powder and stirred uniformly while vibrating the container to remove gas from the slurry. The mixed slurry is then left to stand for a while before being placed in a vacuum integrated casting machine for degassing for 1-1.5 minutes.
Table 5: Mixing Parameters
| Material | Weight Ratio | Water Content | Water Temperature | Degassing Time |
|---|---|---|---|---|
| Gypsum Powder | – | 100:38-41 | 22°C | 1-1.5 min |
| Quartz Powder | – |
4.2 Slurry Pouring
The impeller wax pattern with the gating and risering system is placed in a special steel cup, and the prepared slurry is poured into the steel cup until it covers the wax pattern by 15-20 mm. The mold cup is then placed in a vacuum integrated casting machine for degassing for 1.5-2 minutes and left to stand for more than 2 hours.
4.3 Baking and Dewaxing
The baking and dewaxing process includes six stages with specific temperatures and durations to ensure proper mold preparation for casting.
Table 6: Baking and Dewaxing Process
| Stage | Temperature (°C) | Duration (h) |
|---|---|---|
| 1 | 80±5 | 2 |
| 2 | 200 | 2 |
| 3 | 400 | 2 |
| 4 | 600 | 2 |
| 5 | 730 | 4 |
| 6 | Cooling to 280-320 | – |
4.4 Pouring and Casting Cleanup
The heating equipment adopts a medium-frequency induction vacuum heating furnace. The pouring process relies on the weight of the molten metal to pour the metal liquid into the mold cavity through the outer gate. After the metal liquid cools and solidifies, the steel cup is opened, the gypsum mold is removed, and the sand is shaken out. The cast impeller is then cleaned by removing burrs, fins, gates, and risers, and the surface is prepared.
5. Conclusion
- Feasibility of Complex Structure Wax Pattern: Using FDM 3D printing for centrifugal impellers and silicone rubber demolding technology, wax impeller molds for investment casting have been successfully poured, proving the feasibility of producing complex structure wax patterns based on 3D printing and silicone rubber demolding technology.
- Validation of Investment Casting Process: The investment casting process flow for centrifugal impellers has been designed and validated through experimentation. The exploration of the manufacturing process of centrifugal impellers based on investment casting technology has achieved the expected results, providing a practical basis for subsequent research.
- Improvement of Gating and Risering System: The gating and risering system for centrifugal impeller casting has been improved, with the conclusion that the bottom-pouring method is superior to the top-pouring method.