Investment casting, also known as lost-wax casting, has become one of the most critical manufacturing processes for producing complex geometric components with high dimensional accuracy and excellent surface finish. This process is widely adopted in aerospace, energy, chemical engineering, and other high-end equipment manufacturing sectors. Among the various parts produced via investment casting, hollow long rods serve as core load-bearing components in industrial machinery. The internal quality and dimensional precision of these rods directly determine the service performance and reliability of the entire system. However, during mass production of such parts, severe casting eccentricity defects frequently occur, manifested as uneven wall thickness and deviation of the inner hole geometry from the design model. Our early investigations revealed that these eccentricity defects originate from the instability of the internal core during the casting process, leading to a high scrap rate. This not only causes significant economic losses but also severely delays product delivery schedules. Therefore, solving the eccentricity control problem for hollow long rod castings has become an urgent practical necessity.
The typical hollow long rod we studied exhibits a slender tubular structure with a total length of 362.7 mm. The inner hole features a variable cross-section design, with a maximum diameter of 34.9 mm and a minimum diameter of only 19.3 mm. Such a large length-to-diameter ratio poses extremely high demands on the inorganic non-metallic core that forms this elongated cavity. The core must maintain its geometric shape and spatial position throughout the entire process chain: shell building (coating with refractory slurry, stuccoing, drying and shrinkage), and casting (thermal shock from molten metal, mechanical erosion). Any minor deformation or displacement of the core will directly lead to unacceptable wall thickness variation in the final casting. To address this challenge, we designed two distinct core reinforcement methods: a metal skeleton approach and a high-purity quartz glass rod approach. This paper systematically investigates both methods and presents our findings on how to overcome the eccentricity defect in investment casting of hollow long rods.
1. Experimental Design and Process Methods
1.1 Metal Skeleton Reinforcement
Inspired by the core reinforcement concept used in sand casting, our first reinforcement strategy involved inserting a low-carbon steel rod into the central hole of the ceramic core after the shell-building process. The idea was to provide rigid support through the metal skeleton, thereby enhancing the overall bending and impact resistance of the core. The steel rod was embedded into the core cavity after shell building and subsequent grouting, as schematically illustrated in our experimental plan. However, as we will discuss later, this approach encountered fundamental metallurgical incompatibility.
1.2 High-Purity Quartz Glass Rod Reinforcement
High-purity quartz glass requires a SiO₂ content of at least 99.9%, meaning total metallic impurity levels below 1000 ppm. This material retains its shape and strength at extremely high temperatures. Its thermal expansion coefficient is exceptionally low, approximately 0.55 × 10⁻⁶ /K, which means its dimensions remain almost unchanged under severe temperature fluctuations. Consequently, its thermal shock resistance is outstanding. Although high-purity quartz glass possesses excellent properties in thermal, optical, chemical, and electrical aspects, its application in investment casting has rarely been reported. Notably, a high-purity quartz glass rod with a diameter of 10 mm and a length of 2000 mm costs only about 100 RMB. The required rod length for different products can be easily obtained by scoring with a glass cutter, and rods of various diameters can be custom ordered from suppliers. Thus, high-purity quartz glass rods offer the advantages of low cost, convenience, and practicality.
Our second reinforcement method selected high-purity quartz glass rods with SiO₂ content ≥ 99.9% and a fire resistance of up to 1710 °C as the novel reinforcing medium. We prepared four groups of samples for comparative study. The control group consisted of one sample reinforced with a conventional sand core rod. The experimental group comprised three samples reinforced with high-purity quartz glass rods. All samples followed the standard investment casting process: after shell building, the quartz glass rod was accurately and firmly placed into the central position of the core cavity. The exact placement locations were carefully chosen at the most vulnerable sections of the slender core.
2. Results and Analysis
2.1 Root Cause Analysis of Eccentricity Defect
The hollow long rod product, designated as a CF8M stainless steel casting, exhibited an inner cavity with a maximum diameter of 34.9 mm, a minimum diameter of 19.3 mm, and a total length of 362.7 mm. During the casting process, the internal core was prone to crack initiation at the 19.3 mm diameter section. After pouring, the impact of molten steel caused the core to break at this critical cross-section, leading to eccentricity and uneven wall thickness. We performed macroscopic and microscopic analyses of defective castings along with process history tracing to identify the root cause. The shell firing temperature was 1150 °C ± 10 °C, with a firing time of at least 90 minutes. The pouring temperature was 1600 °C ± 10 °C. At the moment of molten metal entry, the high-temperature liquid steel exerts a severe thermal shock and fluid erosion force on the slender core, especially at the 19.3 mm weak section. This combined effect readily induces microcracks at that location. As cavity filling proceeds, the cracks propagate rapidly under the coupled action of thermal stress and mechanical force, ultimately leading to complete core fracture at this critical cross-section. The occurrence rate of this fracture was 100% in the initial production trials. Once fractured, the core loses its precise positioning and drifts under the buoyancy and impact forces of the molten metal, resulting in a shifted inner hole position after solidification. The wall thickness difference reached up to 5 mm, as confirmed by X-ray inspection and macroscopic photographs.
2.2 Results of Metal Skeleton Reinforcement
We produced the casting using the standard investment casting process with the steel rod inserted. After pouring and shakeout, we discovered that the inner cavity of the casting was blocked by a metallic tumor. Analysis revealed that the inserted steel rod melted during the high-temperature shell firing and subsequent thermal shock from the 1600 °C molten steel. The melting point of low-carbon steel is approximately 1500 °C, well below the pouring temperature. Surrounded by the high-temperature liquid metal, the steel rod rapidly melted and fused with the casting body. Instead of providing support, it contaminated the inner cavity. This outcome demonstrated that the metal skeleton approach is metallurgically infeasible for investment casting of hollow long rods under these conditions.
2.3 Results of High-Purity Quartz Glass Rod Reinforcement
In stark contrast, the high-purity quartz glass rod reinforcement performed excellently. We produced four samples: one control (sand core rod) and three experimental (quartz glass rod). After casting, we examined the wall thickness of each sample from multiple circumferential directions (0°, 90°, 180°, 270°). Table 1 summarizes the measured wall thickness values for all four castings.
| Reinforcement Material | Required Wall Thickness (mm) | 0° (mm) | 90° (mm) | 180° (mm) | 270° (mm) |
|---|---|---|---|---|---|
| Sand Core Rod (Control) | 16.0 | 14.5 | 14.8 | 17.4 | 17.2 |
| Quartz Glass Rod #1 | 16.0 | 15.9 | 16.4 | 16.5 | 16.1 |
| Quartz Glass Rod #2 | 16.0 | 16.4 | 16.5 | 16.7 | 16.8 |
| Quartz Glass Rod #3 | 16.0 | 15.2 | 15.8 | 15.4 | 15.6 |
Table 2 summarizes the key evaluation criteria and the final judgment for each sample group.
| Inspection Item | Control Group (Sand Core Rod) | Experimental Group (Quartz Glass Rod) | Drawing Requirement |
|---|---|---|---|
| Wall Thickness Uniformity | Significant deviation, some out of tolerance | Uniform and consistent | ±1.0 mm |
| Eccentricity Amount | Exceeds permissible range | Controlled within permissible range | ±1.0 mm |
| Conclusion | Unqualified | Qualified | – |
The wall thickness uniformity of the three experimental castings reinforced with quartz glass rods met the engineering requirement of ±1.0 mm. In contrast, the control sample showed a maximum wall thickness variation of 2.9 mm (from 14.5 mm to 17.4 mm), which is unacceptable. Internal inspection using a borescope revealed that the inner cavities of the experimental castings were clean and geometrically intact, while the control casting exhibited localized deformation traces. X-ray non-destructive testing (NDT) of all four castings showed no shrinkage cavities, hot tears, or other internal defects. This indicates that neither the sand core rod nor the quartz glass rod introduced new metallurgical problems.
Further analysis confirmed that both sand core rods and high-purity quartz glass rods can provide reinforcement, but the quartz glass rods are far more effective in constraining core displacement and ensuring dimensional accuracy. We then proceeded to produce 84 pieces in a batch production run using the high-purity quartz glass rod reinforcement method. None of these castings were scrapped due to eccentricity issues. This milestone demonstrated that the quartz glass rod core reinforcement process successfully solved the eccentricity problem of hollow long rod parts in investment casting, raising the yield from 0% (previous attempts) to 100%.
3. Application to a New Product: Low-Temperature High-Pressure Valve
Beyond solving the eccentricity defect for the original hollow long rod product, we successfully transferred the quartz glass rod core reinforcement technology to a new product: a low-temperature high-pressure valve body. This valve body features two slender oil passage holes at both ends, structurally similar to the hollow long rod. Initial production trials using conventional methods suffered from the same wall thickness non-uniformity issue. After applying the quartz glass rod reinforcement, we produced four sample castings and then a batch of 20 pieces. X-ray NDT confirmed no shrinkage, hot tears, or other internal defects. The wall thickness was uniform, and the first-pass yield jumped from 0% (three rounds of conventional trials) to 100%. This successful application validates the versatility and effectiveness of the quartz glass rod core reinforcement technique for various slender cavity structures produced by investment casting.
4. Discussion
4.1 Mechanism of Eccentricity Prevention
The fundamental reason for the success of high-purity quartz glass rods lies in their exceptional thermal and mechanical properties. During the investment casting process, the core undergoes three severe thermal cycles: shell drying (typically 150-200 °C), shell firing (1150 °C), and pouring (1600 °C). The low thermal expansion coefficient of quartz glass $$ \alpha_{SiO_2} = 0.55 \times 10^{-6} \, \text{K}^{-1} $$ ensures that the rod does not expand or contract significantly during these temperature changes. Consequently, the rod maintains intimate contact with the core material throughout the process, providing continuous mechanical support. In contrast, conventional sand core rods have much higher thermal expansion coefficients, leading to differential expansion and contraction that can loosen the support or even induce cracks in the core.
Moreover, the high melting point (approximately 1710 °C for quartz glass) means the rod remains solid even when surrounded by molten steel at 1600 °C. This prevents the rod from melting and contaminating the casting, which was the fatal flaw of the metal skeleton approach. The high purity of the quartz glass (SiO₂ ≥ 99.9%) also avoids any chemical reaction with the molten metal, preserving the internal cavity quality.
4.2 Quantitative Analysis of Core Stress
To further understand the mechanical advantage of the quartz glass rod, we can model the core as a slender beam subjected to bending moment from the molten metal flow. The bending stress $$ \sigma $$ at the critical cross-section (diameter 19.3 mm) can be estimated by:
$$ \sigma = \frac{M y}{I} $$
where $$ M $$ is the bending moment, $$ y $$ is the distance from the neutral axis, and $$ I $$ is the second moment of area. The core without reinforcement has a hollow cross-section with inner diameter $$ d_i = 19.3 $$ mm and outer diameter $$ d_o = 34.9 $$ mm. The second moment of area for a hollow circular section is:
$$ I_{\text{core}} = \frac{\pi}{64} (d_o^4 – d_i^4) $$
After inserting a solid quartz glass rod of diameter $$ d_r = 10 $$ mm into the core cavity, the composite section becomes more resistant to bending because the quartz glass rod has a high elastic modulus (about 70 GPa) compared to the porous ceramic core material (typically 10-30 GPa). The effective bending stiffness $$ EI $$ of the composite increases, reducing the deflection and stress under the same load. This provides a quantitative explanation for the observed improvement in dimensional stability.
4.3 Cost-Effectiveness and Practicality
From a practical standpoint, the quartz glass rod reinforcement method is remarkably cost-effective. As mentioned, a 10 mm diameter, 2000 mm long rod costs only 100 RMB. For a single hollow long rod casting requiring a 360 mm long rod, the material cost per piece is about 18 RMB (360/2000 × 100). Considering that the scrap rate was 100% before this method, and each rejected casting represents a loss of tens to hundreds of RMB (including materials, labor, and energy), the return on investment is extremely high. Furthermore, the process is easy to implement: the rod can be cut to length with a simple glass cutter, and placement requires only careful positioning during shell building. No additional equipment or complex fixturing is needed.
5. Conclusions and Future Work
5.1 Conclusions
Through systematic investigation, we have drawn the following main conclusions:
- We revealed the mechanism of eccentricity defects in investment casting of hollow long rods: the slender ceramic core fractures at its weakest cross-section under the coupled thermal and mechanical loads during pouring, leading to core displacement. The fracture occurs due to thermal shock and fluid erosion at the narrowest diameter (19.3 mm).
- We successfully developed an efficient core reinforcement technique using high-purity quartz glass rods (SiO₂ ≥ 99.9%, fire resistance ≥ 1710 °C). This method significantly enhances the high-temperature stability of the core, preventing fracture and displacement.
- Practical validation demonstrated that the quartz glass rod reinforcement ensures uniform wall thickness of castings, completely solving the eccentricity problem. The yield rate improved from 0% to 100% in batch production. The technique proved transferable to other products with slender internal cavities, such as the low-temperature high-pressure valve body, achieving the same outstanding results.
The research results have immediate practical impact: by optimizing the core reinforcement process, we can directly enhance product qualification rates, reduce resource waste, ensure supply chain stability, and strengthen the core competitiveness of the enterprise. Technically, this study addresses the typical “slender deep hole” forming challenge in investment casting, providing a proven process route and technical reserve for solving similar manufacturing bottlenecks. This contributes to the advancement of precision casting technology and the expansion of its application boundaries.
5.2 Future Work
Looking ahead, we plan to deepen the research from the following perspectives:
- Process parameter optimization: Using finite element simulation and orthogonal experiments to quantify the effects of quartz glass rod diameter, insertion depth, and surface treatment on reinforcement effectiveness, aiming for the optimal solution.
- Exploration of new materials: Evaluate the application potential of higher-performance engineering ceramics such as zirconia (ZrO₂) or silicon nitride (Si₃N₄) under extreme conditions where even quartz glass might approach its limits.
- Intelligent process monitoring: Explore embedding miniature sensors within the shell to achieve real-time monitoring of core displacement and temperature during pouring, building a more predictive quality control system for investment casting.
References
[1] Liu, B. (2012). Design and investment casting of nickel-based superalloy composite lattice sandwich panels. Master’s thesis, Harbin Institute of Technology.
[2] Fang, T., Liu, Y., Bai, J., et al. (2023). Research on electric heating process of hollow rod for offshore electric submersible pump wells. China Offshore Oil and Gas, 35(2), 163-167.
[3] Qin, W., Zhao, E., & Zhou, H. (2007). Analysis and countermeasures of eccentricity of precision cast hollow rod-like parts. Special Casting & Nonferrous Alloys, 27(12), 951-952.
[4] Du, X., & Ma, Z. (2015). Defects and improvement of hollow ingot in hot-top casting of aluminum alloy. Special Casting & Nonferrous Alloys, 35(1), 100-102.