In our pursuit of excellence in manufacturing, we have dedicated significant efforts to refining the lost wax casting process for large-scale components, particularly in the realm of rock drilling machinery. Through rigorous experimentation and practical application, we have successfully implemented lost wax casting to produce over twenty types of large precision castings, with weights ranging from several kilograms to over fifty kilograms. This achievement has enabled us to overcome limitations in sand casting and the absence of large-scale forging equipment, ensuring a reliable supply of heavy-duty rock drilling machines for mining operations. The internal quality requirements for these castings are stringent, and their complex shapes present numerous technical challenges. Over years of exploration, we have developed several key insights and methodologies to optimize the lost wax casting process.
One of the most critical aspects we have focused on is the gating system design, which profoundly impacts casting quality. We have found that the top-pouring method, where molten metal is introduced from the top of the mold, is highly effective for large components in lost wax casting. This approach offers several advantages: it enhances feeding capacity, facilitating directional solidification, which is crucial for achieving high strength and wear resistance in rock drilling parts. Additionally, the simplicity of this gating system makes it practical for high-volume production with limited personnel. It also aids in the efficient removal of wax patterns, minimizes distortion caused by localized overheating in gating channels, improves metal yield, and reduces cleaning efforts. Depending on the hot spot distribution in the casting, we employ single, double, or multiple ingates to achieve adequate feeding. The pressure head height is determined based on the casting’s height and wall thickness, with empirical values summarized in the table below.
| Wall Thickness (mm) | Pressure Head Height (mm) |
|---|---|
| < 10 | 200–250 |
| 10–20 | 250–300 |
| 20–30 | 300–350 |
| > 30 | 350–400 |
However, the top-pouring method in lost wax casting is not without drawbacks. It can lead to issues such as mold erosion, gas entrapment, slag inclusion, and defects like gas pores, slag holes, sand inclusions, and scabbing near the ingates. When hot spots exist at the bottom of the casting, feeding becomes challenging. To address these problems, we have implemented several measures. For instance, adding a transitional ring between the casting and the riser helps collect gases and slag, which are removed during cleaning along with the riser. This also enlarges the feeding channel cross-section, improving feeding efficiency. In cases where local hot spots at the bottom cannot be adequately fed, we adjust machining allowances to eliminate or reduce these hot spots. Controlling the pouring rate is another strategy; for example, we initially pour rapidly to fill bottom sections and then slow down to allow slower solidification in central areas, supplementing feeding for hot spots. If the riser is undersized, a brief pause before final filling can help. To minimize scabbing and oxidation, we pour metal along the wall from one ingate corner when using multiple ingates, which aids in gas and slag expulsion. During dewaxing, we ensure the dewaxing medium does not boil, and during mold handling and firing, we avoid positioning risers upward to prevent sand and debris ingress, reducing defects like sand holes.
Another significant innovation in our lost wax casting process is the use of low-temperature mold casting. For larger castings, we fire the ceramic shells and then cool them to below 200°C before pouring, or store them for later use. If the shell strength is insufficient, we reinforce it with wire and preheat it to a low temperature before positioning it on the pouring platform. This approach offers several benefits: it accelerates metal solidification, promoting denser castings; prevents shell overheating, reducing the risk of cracking or distortion; minimizes surface oxidation and decarburization; and improves working conditions for operators. However, low-temperature mold casting has its trade-offs, such as increased shrinkage rates and higher internal stresses, making it less suitable for high-precision or thin-walled components in lost wax casting.
Deformation is a common issue in large-scale lost wax casting, and we have adopted various measures to mitigate it. During casting design, we strive for uniform wall thickness and appropriate transition radii, avoiding large flat surfaces whenever possible. If design constraints allow, we replace large planes with ribbed surfaces to prevent distortion, reduce weight, and enhance aesthetics. For U-shaped sections, we use wax pattern correctors instead of process ribs to simplify mold design and production. We control injection wax temperature, demolding time, and employ forced cooling of wax patterns; sometimes, adding water to the wax mixture reduces shrinkage deformation. For large patterns, we use higher injection temperatures to ensure surface smoothness. Additionally, we pay attention to the orientation of wax patterns during water cooling and storage to prevent gravity-induced deformation. During shell firing, proper positioning is crucial to avoid distortion due to weakened strength. For large plate-like components, we include a hot straightening step to minimize rejection rates.
The total shrinkage rate in large lost wax castings is influenced by factors such as higher wax injection temperatures and thicker walls, which necessitate earlier demolding to prevent pattern cracking from restrained shrinkage. Under our conditions, the total shrinkage rates vary as shown in the table below. These values may require adjustment based on specific casting characteristics and operational parameters in the lost wax casting process.
| Dimension Type | Shrinkage Rate (%) |
|---|---|
| External dimensions | 2.0–2.2 |
| Internal dimensions | 1.8–2.0 |
| Step hole depth | 1.5–1.8 |
| Position dimensions | 1.0–1.5 |
We can model the shrinkage behavior using a simplified formula: $$ S = k \cdot \Delta T \cdot L $$ where \( S \) is the shrinkage, \( k \) is a material-dependent constant, \( \Delta T \) is the temperature change, and \( L \) is the characteristic length. For our lost wax casting applications, empirical adjustments are often necessary to account for variations in wall thickness and geometry.
Controlling pouring temperature is another critical aspect of lost wax casting, yet we lack precise measurement methods. Currently, we rely on observing the skin formation time on the molten metal surface to estimate temperature, as optical pyrometers are prone to errors. For large castings, the skin formation time typically ranges from 20 to 30 seconds, which corresponds to an approximate temperature range. The relationship can be expressed as: $$ T = T_0 – \alpha \cdot t $$ where \( T \) is the temperature, \( T_0 \) is the initial temperature, \( \alpha \) is a cooling rate coefficient, and \( t \) is the skin formation time. This empirical approach, while not ideal, has proven effective in maintaining consistency in our lost wax casting operations.
In summary, our experiences with lost wax casting for large rock drilling machinery have led to significant improvements in quality and efficiency. The top-pouring method, combined with low-temperature mold techniques and careful deformation control, has enabled us to produce robust castings that meet rigorous demands. The shrinkage rates and temperature management strategies we have developed are integral to the success of the lost wax casting process. As we continue to refine these methods, we aim to further enhance the reliability and performance of our cast components, solidifying the role of lost wax casting in advanced manufacturing. Through iterative practice and adaptation, we have demonstrated that lost wax casting is a versatile and powerful technique for complex large-scale applications.
Furthermore, the integration of these methodologies in lost wax casting has allowed us to address common defects such as porosity and inclusions more effectively. For instance, by optimizing the gating design, we can reduce turbulence and improve metal flow, which is crucial for maintaining the integrity of the ceramic shell in lost wax casting. The use of transitional elements not only aids in feeding but also serves as a buffer against thermal shocks during pouring. In terms of material selection, we have found that certain alloy compositions respond better to the lost wax casting process, particularly those with lower thermal expansion coefficients, which minimize distortion risks. We also emphasize the importance of pattern quality in lost wax casting; any imperfections in the wax pattern can propagate through to the final casting, so we implement strict inspection protocols at each stage.
To quantify the benefits of our approaches in lost wax casting, we have conducted comparative studies on defect rates and mechanical properties. The results consistently show that components produced using our refined lost wax casting techniques exhibit higher density and fewer internal flaws compared to conventional methods. This is partly due to the enhanced control over solidification dynamics, which we achieve through tailored cooling rates and riser design. In lost wax casting, the ability to manipulate these parameters is a key advantage, allowing for customization based on component geometry and performance requirements. We continue to explore advanced simulation tools to model the lost wax casting process, aiming to predict and mitigate potential issues before physical production.
In conclusion, the evolution of lost wax casting in our operations has been driven by a commitment to innovation and quality. By addressing challenges such as deformation, shrinkage, and temperature control, we have elevated the capabilities of lost wax casting for demanding applications. The collaborative efforts of our team, combined with empirical insights, have positioned lost wax casting as a cornerstone of our manufacturing strategy. As we look to the future, we plan to incorporate more automated systems and real-time monitoring to further optimize the lost wax casting process, ensuring its continued relevance and effectiveness in producing high-integrity large components.