Abstract: This article comprehensively analyzes the investment casting process, including its principles, procedures, and influencing factors. The flange plate production case is used as an example to discuss the dimension out-of-tolerance issue. Fault tree analysis and fishbone diagram analysis are applied to identify the causes, and experimental verification is carried out to compare the shrinkage rates of different wax materials. Through a detailed exploration of investment casting technology, this article aims to provide valuable insights for improving the quality of castings and solving production problems.
1. Introduction
Investment casting is a crucial manufacturing process that has been widely utilized in various industries such as aerospace, automotive, and machinery. It offers the advantage of producing complex-shaped components with high precision and good surface finish, often with minimal or no machining required. The process involves creating a wax pattern, coating it with a refractory material to form a shell, melting out the wax, and then pouring in the molten metal. However, ensuring the dimensional accuracy of the castings is a challenging task as numerous factors can influence the final outcome.
1.1 The Importance of Investment Casting
Investment casting allows for the production of intricate parts that would be difficult or impossible to manufacture using traditional methods. It enables the creation of components with fine details, internal cavities, and complex geometries. This is particularly valuable in industries where high-performance and lightweight components are required. For example, in the aerospace industry, investment-cast turbine blades and engine components can withstand high temperatures and stresses while maintaining their structural integrity.
1.2 Overview of the Investment Casting Process
The investment casting process typically consists of the following steps:
| Step | Description |
|---|---|
| Pattern Making | A wax pattern is created, usually by injection molding or 3D printing. The pattern must have the exact dimensions and shape of the desired casting. |
| Shell Building | The wax pattern is dipped or sprayed with a refractory slurry, which is then dried and hardened to form a shell. Multiple layers may be applied to achieve the required thickness and strength. |
| Wax Removal | The wax is melted out of the shell, usually by using a steam autoclave or a hot oven. This leaves a hollow shell with the exact shape of the casting. |
| Metal Pouring | The molten metal is poured into the shell. The type of metal used depends on the requirements of the final component. |
| Cooling and Solidification | The metal cools and solidifies within the shell, taking the shape of the cavity. |
| Shell Removal and Finishing | The shell is removed, and the casting is cleaned, machined (if necessary), and inspected for quality. |
2. Dimensional Accuracy in Investment Casting
Achieving dimensional accuracy is crucial in investment casting as it directly affects the functionality and assembly of the final component. There are several factors that can influence the dimensional accuracy of the castings.
2.1 Factors Affecting Dimensional Accuracy
| Factor | Description | Impact on Dimensional Accuracy |
|---|---|---|
| Wax Pattern | The quality and accuracy of the wax pattern are essential. Any imperfections or inaccuracies in the pattern will be transferred to the casting. | High |
| Shell Material and Properties | The type of refractory material used for the shell, its thickness, and its porosity can affect the dimensional stability of the casting during the pouring and solidification processes. | Medium |
| Pouring Temperature and Rate | The temperature and rate at which the molten metal is poured into the shell can cause shrinkage or distortion of the casting. | High |
| Cooling Conditions | The rate of cooling and the temperature gradient during cooling can lead to internal stresses and dimensional changes in the casting. | High |
2.2 Importance of Controlling Dimensional Accuracy
Controlling dimensional accuracy is vital for several reasons. Firstly, it ensures that the casting can be assembled with other components without any interference or gaps. Secondly, it affects the mechanical properties of the casting. Incorrect dimensions can lead to stress concentrations and reduced strength. Thirdly, it is essential for meeting the design requirements and specifications of the final product.
3. Flange Plate Production Case Study
In this section, we will discuss a case study of flange plate production using the investment casting process and the analysis of the dimension out-of-tolerance issue.
3.1 Production Overview
The flange plates were produced using ZL101 alloy. The production process involved using a set of molds to press the wax patterns. Initially, a medium-temperature wax was used, but due to certain issues, it was replaced with a low-temperature wax. However, during the production, it was found that some of the flange plate castings had dimensions that did not meet the requirements of the drawing.
| Dimension | Drawing Requirement | Measured Value |
|---|---|---|
| A (diameter) | Φ(70±0.55)mm | Φ70.8±71mm |
| B (cumulative diameter) | Φ(96±0.55)mm | Φ97.8±98mm |
3.2 Fault Tree Analysis
Fault tree analysis was conducted to identify the possible causes of the dimension out-of-tolerance issue. The analysis started from the top event (flange plate casting dimension out-of-tolerance) and traced back the causes through a hierarchical structure.
| Event | Description | Cause Determination |
|---|---|---|
| Wax Pattern Dimension Unqualified | The wax pattern size did not meet the requirements. | Possible cause as the wax material was changed from medium-temperature to low-temperature. |
| Shell Material Change | Any change in the shell material could affect the casting dimension. | No change in shell material compared to previous batches, so this is not a cause. |
| Shell Cracking | Cracking of the shell during the process could lead to dimension errors. | No shell cracking occurred during production, so this is not a cause. |
| Metal Pouring Temperature Change | A change in the pouring temperature could cause shrinkage or expansion of the casting. | The pouring temperature was within the required range and did not change compared to previous batches, so this is not a cause. |
| Gate Grinding Excessively | Excessive grinding of the gate could affect the dimension of the casting near the gate area. | There was no excessive grinding of the gate as the dimension out-of-tolerance area did not have a gate, so this is not a cause. |
3.3 Fishbone Diagram Analysis
The fishbone diagram was used to further analyze the causes of the dimension out-of-tolerance issue from different aspects such as man, machine, material, method, environment, and measurement.
| Aspect | Sub-factors | Impact on Dimension Out-of-Tolerance |
|---|---|---|
| Man | Operator’s skill and compliance with procedures | If operators do not follow the correct procedures, it can lead to dimension errors. In this case, operators followed the procedures, so this is not a major cause. |
| Machine | Mold quality and condition, pressing equipment performance | The mold was in good condition and the pressing equipment had a valid certificate, so this is not a major cause. |
| Material | Wax material quality and change, shell material quality | The wax material change from medium-temperature to low-temperature was identified as a possible cause. The shell material was of good quality and did not change, so this is not a cause related to shell material. |
| Method | Manufacturing process parameters, inspection procedures | The manufacturing process parameters were in accordance with the requirements, and the inspection procedures were followed, so this is not a major cause. |
| Environment | Temperature and humidity during production | The temperature and humidity were within the acceptable range, so this is not a major cause. |
| Measurement | Accuracy of measuring tools | The measuring tools were calibrated and within the valid period, so this is not a major cause. |
3.4 Experimental Verification
To verify the impact of the wax material change on the casting dimension, an experiment was conducted. A total of 8 flange plates were produced in the C-116 furnace batch. Among them, 5 wax patterns were pressed with medium-temperature wax, and 3 wax patterns (marked) were pressed with low-temperature wax. The other production conditions were kept the same.
| Wax Material | Outer Diameter (Theoretical) | Outer Diameter (Measured) | Average Shrinkage Rate |
|---|---|---|---|
| Medium-Temperature Wax | 154.0mm | 155.74mm | 1.014% |
| Low-Temperature Wax | 154.0mm | 156.5mm | 1.010% |
The results showed that the shrinkage rate of the low-temperature wax was less than that of the medium-temperature wax. When using the same mold, the wax patterns pressed with low-temperature wax had larger dimensions than those pressed with medium-temperature wax. This confirmed that the change in wax material was a significant factor contributing to the dimension out-of-tolerance issue.
4. Conclusion
In conclusion, investment casting is a complex process that requires careful control of various factors to ensure the dimensional accuracy of the castings. In the case of the flange plate production, the dimension out-of-tolerance issue was analyzed using fault tree analysis and fishbone diagram analysis. The experimental verification demonstrated that the change in wax material, specifically the difference in shrinkage rates between low-temperature and medium-temperature waxes, was a key factor affecting the casting dimensions. By understanding these factors and taking appropriate measures, it is possible to improve the quality of investment-cast components and reduce the occurrence of dimension out-of-tolerance problems. This study provides valuable insights for the optimization of investment casting processes and the production of high-quality castings in various industries.