This article focuses on the lost foam shell casting coating and investigates the effects of mixing time and standing time on its viscosity and rheological properties. The research findings provide crucial technical support and theoretical references for the proper utilization of this coating. Through experiments and analyses, it is determined that the mixing time should be no less than 50 minutes and the standing time should not exceed 40 hours to ensure the coating’s performance.
1. Introduction
Lost foam shell casting combines the advantages of foam lost foam casting and investment casting, aligning with modern casting concepts such as “clean casting,” “green casting,” and “environmental protection casting.” The coating used in this process plays a vital role as it forms a complex shell after brushing and firing. This shell must possess sufficient strength to withstand the erosion of high-temperature metal liquid during pouring and good air permeability to ensure gas expulsion. These properties are closely related to the viscosity and rheological properties of the coating. Many previous studies have explored different aspects of casting coatings, but the influence of mixing time and standing time on the lost foam shell casting coating requires further investigation.
2. Experimental Materials and Methods
2.1 Materials
The coating used in this study consists of powder and powder liquid. The powder contains Al2O3 and SiO2 as main refractory fillers with an average particle size diameter of 300 – 800 μm, along with small amounts of iron oxide powder, sodium bentonite, and carboxymethyl cellulose sodium. The powder liquid is a water-based solution with effective components such as latex and SN thickener, having a pH value of 8 – 9 and a density of 1.2 g/cm³.
2.2 Preparation and Measurement
First, 140 kg of powder liquid was poured into a stirrer, and then 250 kg of powder was gradually added while the stirrer was running at a speed of 800 r/min. During the stirring process, 500 mL of coating was taken from the stirrer at regular intervals. The viscosity, rheological curve, and shear thinning ratio were measured using an LVDV – 2T viscometer with a No. 3 rotor at different rotation speeds (6, 12, 30, 60 r/min). After 60 minutes of stirring, 800 mL of coating was placed, and the same properties were measured at regular intervals during the standing process. All measurements were carried out at a natural environmental temperature of 18°C.
3. Results and Analysis
3.1 The Influence of Mixing Time on Coating Properties
3.1.1 Viscosity
As shown in Figure 3, under different rotor speeds, the viscosity of the coating decreased rapidly with the increase of mixing time in the initial stage of stirring. As the mixing time continued to increase, the viscosity value showed a slow downward trend. When the mixing time exceeded 50 minutes, the measured coating viscosity tended to a stable value.
| Rotor Speed (r/min) | Viscosity Change with Mixing Time |
|---|---|
| 6 | Decreased rapidly initially, then slowly, and stabilized after 50 min |
| 12 | Similar to 6r/min |
| 30 | Similar to 6r/min |
| 60 | Similar to 6r/min |
3.1.2 Rheological Curve
As depicted in Figure 4, when the mixing time reached 50 minutes, the coating performance tended to be stable. Continuing to increase the mixing time to 60 minutes, the rheological curve of the coating did not change significantly, which was consistent with the change trend of the coating viscosity with the mixing time.
3.1.3 Shear Thinning Ratio
As illustrated in Figure 5, in the initial stage of stirring, the shear thinning ratio of the coating was less than 4.0. With the increase of mixing time, the shear thinning ratio gradually increased and reached the maximum value when the mixing time was 40 minutes. When the mixing time was increased to more than 50 minutes, the shear thinning ratio slightly decreased, with a value range of 4.2 – 4.3. A higher shear thinning ratio indicates better coating brushing performance.
3.2 The Influence of Standing Time on Coating Properties
3.2.1 Viscosity
From Figure 6, it can be seen that when the standing time was within 7 hours, the apparent viscosity of the coating slightly increased overall. When the standing time exceeded 20 hours, the viscosity increased rapidly and reached the maximum value at about 35 hours. When the standing time exceeded 40 hours, the coating viscosity began to decrease, possibly due to the settlement of the coating and the failure of some components.
| Standing Time (h) | Viscosity Change |
|---|---|
| 0 – 7 | Slightly increased |
| 7 – 20 | Continued to increase slowly |
| 20 – 35 | Increased rapidly and reached maximum |
| 35 – 40 | Began to decrease |
| > 40 | Decreased significantly |
3.2.2 Rheological Curve
As shown in Figure 7, with the increase of standing time, the shear stress that the coating could withstand generally increased. When the standing time exceeded 40 hours, the rheological curve tended to be stable. The change trend of the coating rheological curve was basically consistent with the changes in viscosity and shear thinning ratio, further verifying that the standing time should be controlled within 40 hours.
3.2.3 Shear Thinning Ratio
As depicted in Figure 8, with the increase of standing time, the shear thinning ratio of the coating first increased and then decreased. When the standing time exceeded 40 hours, the shear thinning ratio decreased significantly, but overall, the shear thinning ratio values were all above 4.0. From the perspective of shear thinning effect alone, long-term standing had little impact on the coating brushing performance.
3.3 Analysis of Coating Property Changes
The viscosity of the coating during the preparation process decreased with the increase of mixing time and finally tended to a stable value. In the initial stage of coating placement, the viscosity increased with the increase of standing time, and after a long time of placement, the viscosity began to decrease. The viscosity changes of the coating are directly related to the internal structure changes during the stirring and standing processes. The lost foam shell casting coating viscosity consists of structural viscosity and plastic viscosity. The plastic viscosity is mainly determined by the gravitational force between molecules, collisions between powder particles, and internal friction, and is less affected by shear force. The structural viscosity is affected by certain special structures formed inside the coating, such as the chain structure formed by polymer and the hydrated film network structure formed by bentonite. During the stirring process, the internal network structure is broken under the action of shear force, reducing the structural viscosity. When the mixing time is long enough, the coating viscosity reaches a stable value. During short-term placement, the network structure gradually recovers, increasing the viscosity. After long-term placement, the effective components of the coating weaken and the particles settle, causing the viscosity to decrease.
4. Conclusions
In conclusion, for the lost foam shell casting coating:
(1) To obtain a uniform coating, the mixing time should be no less than 50 minutes.
(2) To ensure the coating’s use performance, the standing time after mixing and stirring should not exceed 40 hours.
These conclusions provide important guidance for the preparation and use of the lost foam shell casting coating, helping to improve the quality of castings and the efficiency of the casting process.
5. Discussion
5.1 Significance of Mixing Time and Standing Time Control
The control of mixing time and standing time is of great significance for the performance of the lost foam shell casting coating. Adequate mixing time ensures the uniform dispersion of powder and powder liquid components, which is crucial for the formation of a homogeneous coating structure. If the mixing time is insufficient, the coating may have inconsistent properties, leading to poor performance during the casting process. For example, insufficient mixing may result in uneven viscosity, which can cause problems such as incomplete coating coverage or poor adhesion to the mold surface.
On the other hand, the standing time also affects the coating properties. If the coating is left standing for too long, the changes in viscosity and other properties can impact its usability. As seen in the experimental results, when the standing time exceeds 40 hours, the coating viscosity may start to decrease due to component settlement and degradation. This can lead to a thinner coating layer than expected during the casting process, reducing its effectiveness in protecting the mold and influencing the quality of the final casting.
5.2 Comparison with Other Casting Coatings
When compared to other casting coatings, the lost foam shell casting coating has its unique characteristics. For instance, in traditional sand casting coatings, the focus may be more on the permeability and strength of the coating to withstand the pressure of the molten metal and the abrasive action of the sand grains. In contrast, the lost foam shell casting coating needs to consider not only these factors but also the interaction with the foam pattern and the subsequent formation of a hollow shell.
The rheological properties of different casting coatings also vary. Some coatings may have a more pronounced shear thinning effect at different mixing or standing times compared to the lost foam shell casting coating studied here. This difference in rheological behavior is related to the composition and intended application of each coating. For example, a coating used in investment casting may require a different balance of viscosity and shear thinning properties to ensure the precise replication of the mold shape.
5.3 Implications for Industrial Applications
In industrial applications, the findings regarding mixing time and standing time can have a direct impact on the production process. Manufacturers can optimize their coating preparation procedures by ensuring that the mixing time is set according to the recommended 50 minutes or more. This will help to produce coatings with consistent properties, reducing variability in the casting quality.
Regarding the standing time, by controlling it within 40 hours, companies can avoid potential issues associated with coating degradation. This is especially important in large-scale production settings where batches of coating may be prepared in advance. By adhering to these guidelines, the efficiency and quality of the lost foam shell casting process can be significantly improved, leading to cost savings and better product quality.
6. Future Research Directions
6.1 Optimization of Coating Composition
Future research could focus on further optimizing the composition of the lost foam shell casting coating. This could involve exploring different combinations of refractory fillers, binders, and additives to enhance the coating’s performance. For example, investigating the use of new types of nanoparticles as fillers to improve the coating’s strength and thermal stability. Additionally, studying the effect of different binder systems on the coating’s adhesion to the foam pattern and its ability to form a uniform shell could be an area of interest.
6.2 Investigation of Coating Behavior under Different Conditions
Another direction for future research could be to study the coating’s behavior under different environmental and processing conditions. This includes examining how the coating responds to changes in temperature, humidity, and pressure during the casting process. Understanding these factors can help in predicting and controlling the coating’s performance more accurately. For example, in a high-humidity environment, the coating may absorb moisture, which could affect its viscosity and drying time. By studying such phenomena, appropriate measures can be taken to ensure the coating’s effectiveness.
6.3 Development of Real-time Monitoring Techniques
The development of real-time monitoring techniques for the coating properties during the mixing and standing processes could be a valuable research area. This would involve using sensors or other monitoring devices to continuously measure parameters such as viscosity, temperature, and composition. Real-time data collection and analysis could provide immediate feedback on the coating’s quality, allowing for timely adjustments to the mixing or standing procedures if necessary. This would further enhance the control and optimization of the coating preparation process.
7. Conclusion
In summary, this study has comprehensively investigated the influence of mixing time and standing time on the properties of the lost foam shell casting coating. The experimental results have provided clear guidelines for the preparation and use of this coating. By controlling the mixing time to be no less than 50 minutes and the standing time to not exceed 40 hours, the coating can exhibit satisfactory performance in terms of viscosity and rheological properties. These findings have important implications for the lost foam shell casting industry, enabling manufacturers to improve the quality and efficiency of their production processes. Looking ahead, future research directions such as coating composition optimization, investigation of coating behavior under different conditions, and development of real-time monitoring techniques offer potential opportunities for further enhancing the performance of the lost foam shell casting coating.