Abstract: This paper introduces the importance and processes of investment casting technology. To improve the quality of investment casting, it is necessary to control the factors affecting the dimensional accuracy and surface quality of wax patterns. Combining production examples, fault tree analysis (FTA), and fishbone diagram analysis, the reasons for dimensional deviation are identified. The influence of medium-temperature wax and low-temperature wax on quality is analyzed, and the difference in shrinkage rates between them is obtained. This effectively solves quality issues in production, enhances casting quality, and meets precision casting quality control requirements.
1. Overview of Investment Casting
Investment casting, a near-net-shape technology with minimal or no machining, is efficient, cost-effective, and capable of forming complex components. It has been widely applied in aerospace, automotive, and other fields. However, investment casting involves multiple processes and factors that can lead to deformation. Deformation of precision castings not only increases the workload of subsequent cleaning and finishing processes but may also result in product scrap if not properly controlled.
2. Analysis of Dimension Deviation for Flange Plate Casting
2.1 Production Overview
In this production instance, medium-temperature wax was initially used for mold pressing due to its advantages such as smooth mold surface. However, due to its high viscosity, defects such as depressions and flow marks often appeared on the surface of thick mold sections, particularly on non-machined spherical and cylindrical surfaces, which were difficult to smooth out. Excessive grinding often led to dimensional deviations, causing casting scrap. Additionally, during the cooling process of medium-temperature wax mold pressing, warping and deformation of the mold baseplate could occur, resulting in scrap.
To address these issues, the medium-temperature wax (mainly rosin-wax-based) was replaced with low-temperature wax (composed of 50% fully refined paraffin wax and 50% stearic acid), using the same set of molds for pressing. The material used for manufacturing the flange plate castings was ZL101. Upon measurement, two dimensions of the flange plate castings did not meet the drawing requirements.
Table 1: Dimensions Deviation of Flange Plate Casting
| Deviation Dimension | Drawing Dimension | Actual Measurement |
|---|---|---|
| 1 | ϕ(70±0.55)mm | ϕ70.8~71mm |
| 2 | Cumulative dimension based on outer diameter ϕ(81±0.55)mm and two spherical-cylindrical dimensions 2×R(7.5±0.37)mm | ϕ97.8~98mm |
2.2 Fault Tree Analysis
FTA was conducted to analyze the key factors affecting the dimensional accuracy of the castings during the production process. The fault tree identified that the main cause of dimensional deviation was unqualified mold dimensions.
2.3 Fishbone Diagram Analysis
A fishbone diagram was used to comprehensively analyze the causes of mold dimension deviation from aspects such as personnel, machines, materials, methods, environment, and measurement.
Table 2: End Factor Analysis
| Sequential Number | End Factor | Confirmation Method | Confirmation Status | Critical Factor Judgment |
|---|---|---|---|---|
| 1 | Non-compliance with process requirements | Investigation and analysis | Operators followed requirements; process compliance met | No |
| … | … | … | … | … |
| 5 | Change in mold material | Investigation and analysis | Previous batches used medium-temperature wax; faulty batch used low-temperature wax | Yes |
| … | … | … | … | … |
| 11 | Unreasonable instruction preparation | Investigation and analysis | Only ≥ϕ97.4mm was required for the theoretical dimension ϕ96mm; no specific range given | Yes |
2.4 Experimental Verification
To address the dimensional deviation issue, an experimental scheme was formulated to identify and eliminate the causes. First, the reasons for mold material change were investigated. Then, eight flange plates were produced in furnace C-116, with five using medium-temperature wax and three (marked) using low-temperature wax for mold pressing. Other production conditions were consistent to verify the difference in shrinkage rates between medium-temperature and low-temperature waxes.
Table 3: Dimensions of Medium-Temperature Wax Mold
| Theoretical Dimension/mm | Mold Dimension/mm (Average of 5 Pieces) |
|---|---|
| Outer Diameter 154.0 | 155.74 |
| … | … |
Table 4: Dimensions of Low-Temperature Wax Mold
| Theoretical Dimension/mm | Mold Dimension/mm (Average of 3 Pieces) |
|---|---|
| Outer Diameter 154.0 | 156.5 |
| … | … |
Table 5: Comparison of Shrinkage Rates Between Low-Temperature and Medium-Temperature Waxes
| Theoretical Dimension/mm | Mold Dimension/mm (Medium-Temp.) | Shrinkage Rate (%) (Medium-Temp.) | Mold Dimension/mm (Low-Temp.) | Shrinkage Rate (%) (Low-Temp.) |
|---|---|---|---|---|
| Outer Diameter 154.0 | 155.74 | 1.014 | 156.5 | 1.010 |
| … | … | … | … | … |
From Table 5, it is evident that using the same set of molds, the shrinkage rate of low-temperature wax is smaller than that of medium-temperature wax, resulting in larger mold dimensions for low-temperature wax.
3. Conclusion
This paper analyzes the dimensionally deviated flange plate casting using FTA and fishbone diagram analysis. Experimental verification shows that the shrinkage rate of low-temperature wax is smaller than that of medium-temperature wax. When using the same set of molds, low-temperature wax results in larger mold dimensions compared to medium-temperature wax, ultimately leading to dimensional deviation and product scrap.