In the realm of modern foundry processes, sand casting remains a cornerstone due to its versatility and cost-effectiveness. Among its advanced variants, vacuum evaporation-pattern casting (VEPC) has emerged as a promising technique, combining the benefits of lost foam casting and vacuum-assisted molding. This process relies on a dry, unbonded sand mold that gains strength solely through the application of vacuum, eliminating the need for traditional binders. However, the success of sand casting in this context hinges critically on the mold’s ability to withstand the dynamic pressures exerted by molten metal during pouring. Insufficient mold strength can lead to defects such as sand erosion, collapse, or dimensional inaccuracies due to cavity wall movement. In our extensive research, we have investigated the key process parameters that govern sand mold strength and wall displacement in VEPC, aiming to provide a framework for optimizing casting quality. Through experimental analysis and theoretical modeling, we have derived quantitative relationships that can guide practitioners in mitigating common issues in sand casting, thereby enhancing the precision and surface finish of cast components.
The fundamental principle behind VEPC is the use of a vacuum to consolidate dry sand particles around a foam pattern. When the pattern is vaporized by molten metal, the sand mold must retain its shape without any chemical bonding. The strength of such a mold is derived from interparticle friction and contact pressures induced by the pressure differential between the atmosphere and the evacuated mold cavity. According to granular mechanics, the compressive strength of a granular assembly is directly proportional to the average macroscopic contact pressure between particles. Similarly, the shear strength $\eta$ is related to the contact pressure $\sigma$ by $\eta = \sigma \tan \theta$, where $\theta$ is the internal friction angle. Therefore, by measuring the internal pressure within the sand mold, we can infer its strength characteristics. This approach forms the basis of our experimental methodology, which we designed to simulate real-world sand casting conditions.
To assess the internal pressure of dry sand under vacuum, we developed a specialized testing apparatus. The setup consisted of a cylindrical sand container with dimensions of 410 mm in diameter and 430 mm in height, sealed with a 0.2 mm thick plastic film. A pressure sensor with a capacity of 15 kg was embedded at various locations and orientations within the sand bed to measure pressure in both vertical and horizontal directions. The sensor was connected to an X-Y recorder via a DC power supply, allowing real-time monitoring of pressure changes as vacuum levels were adjusted using a valve on the vacuum cover. The vacuum degree was controlled precisely, ranging from low to high sub-atmospheric pressures, to mimic different operational conditions in sand casting. Additionally, we constructed a force-displacement testing device to evaluate the resistance of the sand mold to external loads and the corresponding wall movement. This device integrated a sand strength tester with auxiliary fixtures, including a rigid frame, a loading ram, and a dial indicator. By applying force through a handwheel and measuring displacement via a push rod connected to a pressure gauge, we obtained curves relating external force to wall displacement. These experiments were conducted using refined quartz sand with a grain size of 40–70 mesh, which is typical in industrial sand casting applications.
Our investigations revealed that two primary factors significantly influence the internal pressure of the sand mold: the vacuum degree and the distance from the mold surface, often referred to as the sand backing depth or “吃砂量” in traditional terms. We systematically varied these parameters and recorded the resulting pressures at different heights within the mold. The data, summarized in Table 1, demonstrate a clear trend: internal pressure increases with higher vacuum levels and greater depths from the mold opening. This behavior can be explained by the cumulative effect of vacuum-induced suction forces. Since the vacuum source is typically at the bottom of the mold, the force acting on any horizontal plane is the sum of the suction forces on all sand particles above that plane. Thus, deeper locations experience higher pressures due to the greater weight of overlying sand. In sand casting, this implies that regions closer to the mold opening are more susceptible to failure, as they have lower inherent strength.
| Distance from Mold Opening (m) | Internal Pressure at 20 kPa Vacuum (MPa) | Internal Pressure at 30 kPa Vacuum (MPa) | Internal Pressure at 40 kPa Vacuum (MPa) |
|---|---|---|---|
| 0.05 | 0.018 | 0.028 | 0.038 |
| 0.10 | 0.025 | 0.036 | 0.046 |
| 0.15 | 0.032 | 0.043 | 0.053 |
| 0.20 | 0.039 | 0.050 | 0.061 |
| 0.25 | 0.046 | 0.057 | 0.068 |
| 0.30 | 0.053 | 0.064 | 0.076 |
| 0.35 | 0.060 | 0.071 | 0.084 |
To prevent sand erosion defects in sand casting, the internal mold pressure must exceed the dynamic pressure exerted by the molten metal during pouring. The impulsive force of the liquid metal can be estimated using the formula derived from fluid dynamics: $$ P = 2 \rho g H $$ where $\rho$ is the density of the metal (e.g., 7600 kg/m³ for cast iron), $g$ is the acceleration due to gravity (9.8 m/s²), and $H$ is the total fall height of the metal stream, including the distance from the pouring basin to the mold opening. Assuming a typical pouring height of 0.2 m above the mold, the effective fall height at a depth $h$ from the opening is $H = 0.2 + h$. Substituting these values, we obtain: $$ P = 2 \times 7600 \times 9.8 \times (0.2 + h) = 0.152 \times (0.2 + h) \, \text{MPa} $$ This equation allows us to calculate the critical pressure required to avoid sand wash at any given depth. By comparing this with the internal pressure data from our experiments, we can determine the minimum vacuum degree needed for defect-free sand casting. For instance, at a depth of 0.05 m, the dynamic pressure is approximately 0.038 MPa, which necessitates a vacuum of at least 40 kPa according to Table 1. Interestingly, although deeper regions experience higher metal impact forces, the increase in internal pressure with depth is more pronounced, meaning that the vacuum requirement is less stringent for areas farther from the opening. Therefore, in sand casting practice, ensuring adequate vacuum at the minimal sand backing depth (i.e., the closest point between the pattern and mold wall) is sufficient to protect the entire mold from erosion.
Beyond vertical pressure, we also examined the lateral pressure within the sand mold, which is crucial for filling horizontal cavities or undercuts in complex sand casting geometries. Our measurements showed that the side pressure is substantially lower than the vertical pressure, often by a factor of two or more, as illustrated in Figure 4. This disparity arises because the vertical pressure is the primary force driven by the atmospheric pressure differential, while lateral pressure is a secondary component dependent on particle interlocking and friction. In sand casting, low side pressure can hinder proper compaction of sand around horizontal features, leading to incomplete molds or weak spots. To address this, we explored the effect of vibration on enhancing lateral pressure. Using a small eccentric vibrator, we applied mechanical oscillation to the sand bed under a constant vacuum of 35 kPa and monitored the change in side pressure over time. The results, plotted in Figure 5, indicate a significant improvement, with pressure increasing by up to 50% after prolonged vibration. This enhancement is attributed to the reorientation and densification of sand grains, which improves force transmission in horizontal directions. Thus, incorporating vibration into the mold preparation stage can greatly benefit sand casting processes involving intricate designs.
The relationship between external loads and cavity wall displacement is another critical aspect of sand casting quality. Excessive wall movement during metal solidification can cause dimensional deviations, resulting in out-of-tolerance castings. Using our force-displacement apparatus, we simulated the pressure exerted by molten metal on the mold wall and recorded the corresponding displacement under varying vacuum degrees and sand backing depths. The data for two representative depths—50 mm and 200 mm—are presented in Figures 6 and 7, respectively. These curves demonstrate that higher vacuum levels and greater sand backing depths reduce wall displacement for a given external force. For example, at a vacuum of 50 kPa and a sand backing of 200 mm, the displacement under a 0.1 MPa load is less than 0.5 mm, whereas at 20 kPa and 50 mm, it exceeds 2 mm. This trend underscores the importance of optimizing these parameters in sand casting to achieve high dimensional accuracy. Moreover, the curves exhibit a peak resistance point beyond which the sand mold undergoes catastrophic failure, characterized by a sudden drop in load-bearing capacity accompanied by large displacements. This behavior highlights the need to operate within the elastic regime of the sand assembly to prevent mold collapse.
To encapsulate our findings, we have derived empirical equations that relate key process variables in sand casting. The internal pressure $P_{\text{int}}$ as a function of vacuum degree $V$ (in kPa) and depth $h$ (in meters) can be approximated by: $$ P_{\text{int}} = k_1 V + k_2 h $$ where $k_1$ and $k_2$ are constants determined experimentally. For our quartz sand, $k_1 \approx 0.001 \, \text{MPa/kPa}$ and $k_2 \approx 0.12 \, \text{MPa/m}$. Similarly, the maximum allowable external pressure $P_{\text{ext}}$ before wall movement becomes excessive can be expressed as: $$ P_{\text{ext}} = \alpha V + \beta h $$ with $\alpha$ and $\beta$ being coefficients that depend on sand properties and mold geometry. These formulas provide a quantitative basis for setting process parameters in industrial sand casting operations.
The application of these insights extends beyond laboratory settings to real-world sand casting production. For instance, in the manufacture of engine blocks or valve bodies via sand casting, designers can use our data to determine optimal sand backing depths and vacuum settings for critical sections. By ensuring that the mold strength surpasses the anticipated metal dynamic pressures, defects like sand inclusions or surface roughness can be minimized. Additionally, the use of vibration-assisted compaction can improve mold uniformity, especially for cores and complex cavities. It is worth noting that while vacuum-based sand casting offers environmental advantages by eliminating binder emissions, its success relies heavily on precise control of process variables. Our research underscores the interplay between vacuum, sand backing, and vibration in achieving robust molds. Future work could explore the effects of sand grain shape, moisture content, and alternative materials on these relationships, further refining the sand casting process.
In conclusion, our comprehensive study on vacuum evaporation-pattern casting has elucidated the factors governing sand mold strength and cavity wall movement. We have shown that increasing vacuum degree and sand backing depth enhances internal pressure, thereby reducing the risk of sand erosion and dimensional inaccuracies in sand casting. The quantitative relationships we established offer practical guidelines for selecting process parameters to meet specific casting requirements. Moreover, we demonstrated that lateral pressure, often a bottleneck in sand casting for horizontal features, can be significantly boosted through mechanical vibration. These findings contribute to the advancement of sand casting technologies, enabling the production of high-integrity cast components with improved surface finish and dimensional precision. As sand casting continues to evolve, incorporating such insights will be pivotal in harnessing its full potential for complex and high-performance applications.