1. Introduction
Investment casting, also known as the lost wax process, has a long history and is widely used in the manufacturing industry. It is renowned for its ability to produce complex-shaped components with high dimensional accuracy and good surface finish. Centrifugal impellers, being crucial components in centrifugal compressors, pose significant manufacturing challenges due to their complex blade structures, high precision requirements, and difficult manufacturing accessibility. This article focuses on the exploration and research of the manufacturing process of centrifugal impellers using investment casting technology, incorporating 3D printing and silicone rubber demoulding techniques.
1.1 Background of Centrifugal Impellers
Centrifugal impellers typically consist of a rotating disk and blades on one side of the disk. They play a vital role in centrifugal compressors, where they are responsible for increasing the kinetic energy of the fluid. The complex geometry of the blades and the need for high precision make traditional manufacturing methods less than ideal for producing these components.
1.2 Significance of Investment Casting in Impeller Manufacturing
Investment casting offers several advantages for manufacturing centrifugal impellers. It allows for the production of intricate shapes with fine details, which is essential for the complex blade geometries of impellers. The high dimensional accuracy and good surface roughness achievable through investment casting ensure that the impellers meet the strict performance requirements of centrifugal compressors.
2. Wax Pattern Manufacturing for Centrifugal Impellers
2.1 3D Printing Design and Manufacturing of the Pattern
- 3D Model Creation: Using UG 3D drawing software, a three-dimensional model of the impeller is created according to the requirements of the part drawing. The generated STL format file is then imported into a 3D printer.
- Printer and Material Selection: A 熔融沉积 (FDM) 3D printer is used in this experiment, with the printing material being a thermoplastic polymer structure material (ABS).
- Printing Settings: To ensure the dimensional accuracy, surface roughness, and strength of the printed pattern, a filling rate of 80% is set. After printing, the excess supports are removed. The process is summarized in Table 1.
| Step | Description |
|---|---|
| 1 | Create 3D model in UG |
| 2 | Import STL file into FDM printer |
| 3 | Set filling rate to 80% and print with ABS material |
| 4 | Remove excess supports |
2.2 Silicone Rubber Demoulding and Wax Liquid Pouring
- Silicone Casting Mould Preparation: Based on the 3D printed ABS impeller pattern, a silicone casting mould for pouring wax liquid is prepared using silicone rubber demoulding technology. The impeller pattern is divided into upper and lower parts for demoulding considering its structure.
- Wax Pouring: The medium-temperature casting paraffin is heated and melted into wax liquid at a temperature of (75±5) °C and poured into the silicone casting mould through the reserved gating system. After cooling and solidifying, the impeller wax pattern is obtained. The detailed process is presented in Table 2.
| Step | Description |
|---|---|
| 1 | Build a surrounding frame according to the pattern size |
| 2 | Place the lower part of the pattern in the frame, fill with oil clay, and apply vaseline as a release agent |
| 3 | Mix silicone liquid and curing agent in a 100:3 ratio, pour into the frame and wait for solidification |
| 4 | Remove the frame, take out the mould, remove the oil clay, flip the mould, and place the wax pouring channel |
| 5 | Repeat the above steps for the upper part of the pattern |
| 6 | Open the silicone mould along the separation surface, take out the pattern |
| 7 | Heat and melt the paraffin, pour into the silicone mould, and obtain the wax pattern after cooling |
3. Design of the Gating System for Centrifugal Impeller Wax Patterns
3.1 Before Improvement
- Gating System Configuration: The gating system before improvement includes an outer gate, a sprue, an ingate, and a riser, with a top pouring method adopted.
- Defects and Causes: The main defects in the metal casting after pouring are insufficient filling. The reasons include low mould temperature, unreasonable gating system design, and incomplete dewaxing. The analysis is summarized in Table 3.
| Defect | Cause |
|---|---|
| Insufficient filling | Low mould temperature, rapid decrease in aluminium liquid temperature and reduced fluidity |
| Unreasonable gating system design, poor mould venting, increased air pressure in the cavity, reduced metal liquid fluidity | |
| Incomplete dewaxing, wax residue in the mould, reaction with high-temperature metal during pouring, increased air pressure in the cavity, reduced metal liquid fluidity |
3.2 After Improvement
- New Gating System Design: After improvement, a bottom pouring method is adopted. The wax pattern and the gating system are combined as shown.
- Advantages: The bottom pouring method is more conducive to gas discharge in the cavity, resulting in more stable metal liquid filling, less impact on the cavity, and the production of high-quality centrifugal impeller castings with complete structures and low defect rates.
4. Manufacturing and Pouring of Centrifugal Impeller Castings
4.1 Powder Mixture Preparation
- Material Selection and Weighing: The material for the centrifugal impeller casting is a powder mixture of gypsum powder and quartz powder. The weights of the gypsum powder and quartz powder are measured using an electronic scale and mixed evenly.
- Water Addition and Mixing: An appropriate amount of water is measured. The water content affects the quality of the casting. After testing, a water-powder ratio of 100: (38 – 41) is selected, with the water temperature around 22 °C. The water is added to the mixed powder container gradually, and the mixture is stirred evenly while vibrating the container to expel gas. The slurry is then left to stand for a while and vacuumed in a vacuum continuous casting machine for 1 – 1.5 minutes to remove residual gas. The process is detailed in Table 4.
| Step | Description |
|---|---|
| 1 | Weigh gypsum powder and quartz powder |
| 2 | Mix powders evenly |
| 3 | Measure appropriate water, select water-powder ratio of 100: (38 – 41), with water temperature around 22 °C |
| 4 | Add water to powder container gradually, stir evenly while vibrating container |
| 5 | Leave slurry to stand, vacuum in vacuum continuous casting machine for 1 – 1.5 minutes |
4.2 Grouting
- Wax Pattern Placement: The impeller wax pattern with the gating system is placed in a special steel crucible.
- Grouting Process: The prepared slurry is poured into the steel crucible until it submerges the wax pattern by 15 – 20 mm. The crucible is then placed in a vacuum continuous casting machine for vacuuming for 1.5 – 2 minutes and left to stand for more than 2 hours.
4.3 Sintering and Dewaxing
- Multi-stage Heating Process: The sintering and dewaxing process involves multiple stages of heating. The electric heating blast drying box is heated to (80±5) °C and the crucible is placed in it for 2 hours. Then, the box is heated to 200 °C, 400 °C, 600 °C, and 730 °C successively, with each stage having a specific holding time. Finally, the crucible is cooled to 280 – 320 °C with the furnace. The temperature-time curve is shown in Figure 1 (similar to the provided graph in the original PDF).
- Purpose of Each Stage: Each stage of heating serves a specific purpose, such as removing moisture, decomposing wax, and preparing the casting for metal pouring.
4.4 Pouring and Casting Finishing
- Pouring Method: The heating device used is a medium-frequency induction vacuum heating furnace. The metal liquid is poured into the casting cavity by its own weight through the outer gate. The pouring is stopped when the metal liquid overflows the riser, which is observed through the observation hole of the furnace.
- Finishing Operations: After the metal liquid cools and solidifies, the steel crucible is opened, the gypsum casting is taken out, and the sand removal process is completed. The obtained casting is then cleaned by removing burrs, flash, gates, and risers, and the surface of the centrifugal impeller casting is cleaned.
5. Conclusion
5.1 Feasibility of 3D Printing and Silicone Rubber Demoulding for Wax Pattern Making
The use of 3D printing and silicone rubber demoulding technology to produce a silicone rubber demoulding mould for a centrifugal impeller printed by FDM 3D printing and then pouring a wax impeller mould for investment casting proves the feasibility of making complex structure wax patterns using these technologies.
5.2 Validation of the Investment Casting Process Flow
The designed investment casting process flow for centrifugal impellers has been verified through experiments, demonstrating its feasibility. The exploration of the manufacturing process of centrifugal impellers based on investment casting technology has achieved the expected results, providing a practical reference for subsequent research.
5.3 Advantage of the Bottom Pouring Gating System
The improvement of the gating system for centrifugal impeller casting and the conclusion that the bottom pouring method is superior to the top pouring method have important implications for the production of high-quality centrifugal impeller castings.
Overall, the integration of 3D printing and silicone rubber demoulding techniques into the investment casting process for centrifugal impellers has shown great potential in improving the manufacturing quality and efficiency of these critical components. Future research could focus on further optimizing the process parameters and exploring the application of other advanced manufacturing technologies to enhance the performance and reliability of centrifugal impellers produced through investment casting.