1. Introduction
Sand casting is one of the most prevalently used casting methods in the foundry industry. The process involves creating a mold from sand, pouring molten metal into it, and allowing the metal to solidify to form the desired casting. A crucial step in this process is the application of coatings on the surface of the sand mold or core.
Coatings play a vital role in sand casting. They act as a barrier between the molten metal and the sand, preventing direct contact. This helps to reduce surface defects in castings, such as sand inclusions, porosity, and rough surfaces, thereby improving the overall quality of the casting products. Among the various types of casting coatings, alcohol – based coatings are widely used due to their unique properties.
Alcohol – based coatings have certain advantages over other types. For example, compared with water – based coatings, they have a faster drying speed, which can improve production efficiency. However, they also present some challenges, such as excessive penetration into the sand mold, which may affect the coating thickness and ultimately the quality of the casting.
Coating thickness is a key factor that significantly influences the performance of casting coatings. An inappropriate coating thickness can lead to a series of problems. If the coating is too thin, it may not effectively protect the casting from defects. On the other hand, if it is too thick, it may cause issues like coating cracking, peeling, and increased production costs. Therefore, understanding the factors that affect coating thickness is of great importance for ensuring the quality of castings.
This article focuses on the application of alcohol – based coatings in sand casting, with a particular emphasis on analyzing the impact of factors such as the Baumé degree of the coating and the number of brushing times on the coating thickness. Through comprehensive research and analysis, we aim to provide valuable insights and practical guidance for the foundry industry to optimize the use of alcohol – based coatings and improve casting quality.
2. Classification of Casting Coatings
Casting coatings can be classified in multiple ways based on different criteria.
2.1 Classification by Casting Alloy Composition
| Alloy Type | Coating Name | Application Scenarios |
|---|---|---|
| Steel | Cast Steel Coating | For casting steel products, it can withstand high – temperature molten steel and prevent reactions between steel and sand, ensuring the surface quality of steel castings. |
| Iron | Cast Iron Coating | Suitable for casting iron – based products. It helps to improve the surface finish and mechanical properties of iron castings. |
| Aluminum | Cast Aluminum Coating | Used in aluminum casting processes. Aluminum has a relatively low melting point, and the coating needs to have good adhesion and heat – resistance to ensure the quality of aluminum castings. |
| Copper | Cast Copper Coating | Applied in copper casting. Copper alloys have specific requirements for coating properties to prevent oxidation and ensure the integrity of the casting surface. |
2.2 Classification by Coating Carrier Liquid
| Carrier Liquid Type | Coating Name | Characteristics |
|---|---|---|
| Water | Water – based Coating | Environmentally friendly, but it may have issues such as slow drying speed and insufficient penetration into the sand mold. |
| Alcohol and Other Organic Solvents | Alcohol – based Coating | Fast – drying, but it may have problems like high penetration into the sand mold, and some organic solvents may be volatile and harmful to the environment and human health. |
2.3 Classification by Casting Process
| Casting Process | Coating Name | Special Requirements |
|---|---|---|
| Sand Casting | Sand Casting Coating | Needs to be compatible with the sand mold material, have good adhesion, and be able to withstand the high – temperature and pressure during casting. |
| Metal – mold Casting | Metal – mold Casting Coating | Must have excellent heat – resistance and wear – resistance to adapt to the repeated use of metal molds. |
| Lost – foam Casting | Lost – foam Casting Coating | Should be able to cover the foam pattern well and withstand the decomposition process of the foam during casting. |
2.4 Classification by Application Method
| Application Method | Coating Name | Application Features |
|---|---|---|
| Flow Coating | Flow – coating Coating | Suitable for large – scale production, can evenly coat the surface of the mold, but requires specific equipment and process control. |
| Brushing | Brushing Coating | Flexible, suitable for small – batch production and complex – shaped molds, but the coating thickness may be less uniform compared to other methods. |
| Dipping | Dipping Coating | Can quickly coat the entire surface of the mold, but it may be difficult to control the coating thickness accurately. |
| Spraying | Spraying Coating | Can achieve a relatively uniform coating thickness, high – efficiency, but it requires spray equipment and proper ventilation to handle the over – spray. |
3. Experimental Method for Alcohol – based Coatings in Sand Casting
3.1 Preparation of Experimental Materials
In this experiment, we selected a typical alcohol – based coating used in sand casting for steel products. The coating was obtained from a well – known coating manufacturer in the foundry industry. The sand mold used was a standard – type silica – based sand mold commonly used for casting steel parts. The sand mold had a uniform particle size distribution to ensure consistent experimental conditions.
3.2 Experimental Equipment
| Equipment Name | Function |
|---|---|
| Stirring Bucket | Used to continuously mix the alcohol – based coating evenly before brushing to ensure a consistent composition throughout the experiment. |
| Baumé Hydrometer | To measure the Baumé degree of the coating solution, which is an important parameter representing the concentration of the coating. |
| Heating Device | For quickly drying the alcohol – based coating after each brushing. Since alcohol – based coatings are volatile, heating can accelerate the drying process and enable accurate measurement of the dry – coating thickness. |
| Coating Thickness Gauge | Specifically designed to measure the thickness of the coating on the sand mold surface. It can provide accurate measurement results for both wet and dry coatings, but in this experiment, we mainly used the dry – coating measurement method. |
3.3 Experimental Procedure
- Coating Preparation: First, pour the alcohol – based coating into the stirring bucket and start the continuous stirring process. This is to ensure that the coating components are uniformly mixed, as any unevenness in the coating composition may affect the experimental results.
- Measurement of Baumé Degree: Use the Baumé hydrometer to measure the Baumé degree of the coating. In this experiment, we selected four different Baumé degrees: 75°Bé, 78°Bé, 80°Bé, and 82°Bé. The Baumé degree reflects the concentration of the coating solution, and different concentrations may have different effects on the coating thickness.
- Brushing Process: Take an appropriate amount of the coating with the measured Baumé degree for brushing. The brushing is carried out on the surface of the sand mold, including the area in contact with the molten metal and the surface of the sand core. For each sand mold, start with the first brushing. After the first brushing, use the heating device to quickly dry the coating by ignition. This is because alcohol – based coatings are volatile, and rapid drying can fix the coating in place for accurate thickness measurement. After drying, let the sand mold cool down, and then clean the surface gently to remove any loose particles or unevenly distributed coating. Then, perform the second brushing, and repeat the drying, cooling, and cleaning steps for subsequent brushings. The number of brushings in this experiment ranges from 1 to 5 times.
- Coating Thickness Measurement: After each brushing and drying process, use the coating thickness gauge to measure the dry – coating thickness. Since alcohol – based coatings have volatile components, the dry – coating thickness is a more accurate indicator of the final coating thickness that affects the casting quality. Record the measurement results for each brushing and different Baumé degrees.
3.4 Selection of Experimental Samples
The experimental samples were selected as typical steel – casting sand molds. These sand molds were representative of the actual production in the foundry industry. The size and shape of the sand molds were designed to be similar to common steel – casting products, such as medium – sized mechanical parts. The surface area of the sand molds was consistent to ensure that the brushing process and coating thickness measurement were comparable.
4. Experimental Results and Analysis
4.1 Influence of the Number of Brushing Times on Coating Thickness
When the Baumé degree of the coating is 75°Bé, a series of comparative experiments were carried out by brushing the same sand – mold surface 1 – 5 times. The results of the coating – thickness changes are shown in Table 1.
| Number of Brushing Times | Coating Thickness (mm) |
|---|---|
| 1 | 0.17 |
| 2 | 0.24 |
| 3 | 0.51 |
| 4 | 0.79 |
| 5 | 0.91 |
As can be seen from the table, with each additional brushing at the same concentration, the coating thickness increases by approximately 0.1 – 0.25 mm. When the coating is brushed for the first time, it has a high penetration rate into the pores of the sand mold. This penetration ability of the coating into the sand mold is called the coating penetration rate. A high coating penetration rate leads to a decrease in the flow – hanging performance on the surface of the sand mold, resulting in a relatively lower initial coating thickness. After drying and cooling, when the second brushing is carried out, the coating penetration rate gradually decreases, while the surface adhesion ability increases, causing the coating thickness to gradually increase.
In the manual – brushing operation, there are issues such as local coating accumulation and poor coating adhesion. Therefore, it is necessary to conduct surface inspection and treatment after the brushing is completed to ensure the quality of the coating.
Based on the experimental data, it can be concluded that for a coating with a Baumé degree of 75°Bé, increasing the number of brushings can effectively increase the coating thickness. However, it should be noted that excessive brushing may also lead to problems such as coating cracking and uneven thickness distribution.
4.2 Influence of Different Baumé Degrees on Coating Thickness
From the previous experiments, it was found that after 3 and 5 brushings, the coating thickness can meet the requirements of actual steel – casting products. Therefore, further experiments were carried out to study the influence of different Baumé degrees on the coating thickness under 3 – and 5 – brushing conditions.
The experimental data were obtained by taking the average of 16 measurement results for each group of experiments with different Baumé degrees and brushing times. This is to reduce the influence of local abnormalities in manual brushing and measurement errors.
For the 3 – brushing results, as shown in Figure 1, the increase in the Baumé degree has a relatively small impact on the coating thickness, and the coating thickness remains at around 0.63 mm. After 3 brushings, the surface coating thickness becomes relatively fixed. At this time, the penetration performance of the coating into the sand mold decreases, and the adhesion strength increases.
For the 5 – brushing results, the coating thickness increases with the increase in the Baumé degree of the coating, with an increase amplitude of up to 0.3 mm. As the Baumé degree increases, the solid – content rate of the surface coating increases, which reduces the fluidity of the coating and enhances the viscous force on the coating surface, thereby increasing the coating thickness.
By sorting out the 120 groups of experimental data obtained under different brushing times and Baumé degrees, it is found that after 3 brushings, the coating thickness is mainly concentrated in the range of 0.55 – 0.65 mm, and after 5 brushings, the coating thickness is mainly concentrated in the range of 1.0 – 1.1 mm, which can meet the requirements of the coating thickness for on – site steel – casting products.
5. Discussion on the Optimal Application Parameters of Alcohol – based Coatings
5.1 Determination of the Optimal Number of Brushing Times
Based on the experimental results, for conventional steel – casting products, brushing 3 times can generally meet the coating – thickness requirements. When the coating thickness requirements are within the range of 0.4 – 0.7 mm (the common requirement for general – sized steel – casting products), 3 brushings can ensure that the coating thickness is within this range. For example, when the Baumé degree is in the range of 75°Bé – 85°Bé, after 3 brushings, the coating thickness is around 0.63 mm, which meets the quality requirements for most steel – casting products.
However, for some special – sized or high – quality – requirement products, the number of brushings may need to be adjusted. If the product requires a thicker coating, such as large – scale steel – casting parts with a coating – thickness requirement of about 1 mm, 5 brushings may be more appropriate. But it should be noted that increasing the number of brushings also means an increase in production time and cost, so a balance needs to be struck between product quality and production efficiency.
5.2 Selection of the Appropriate Baumé Degree
The selection of the Baumé degree also depends on the specific requirements of the casting process. When the number of brushings is 3 times, the influence of the Baumé degree on the coating thickness is relatively small. In this case, the Baumé degree can be selected according to other factors, such as the cost of the coating raw materials and the drying speed requirements. If the production process requires a faster – drying coating, a slightly higher Baumé degree can be selected, as coatings with higher Baumé degrees generally have a higher solid – content rate and faster – drying speed.
When the number of brushings is 5 times, the Baumé degree has a significant impact on the coating thickness. If a thicker coating is required, a higher Baumé degree can be selected. But it should be noted that coatings with too high a Baumé degree may have poor fluidity, which may lead to difficulties in the brushing process, such as uneven coating distribution. Therefore, when selecting the Baumé degree, it is necessary to comprehensively consider the coating – thickness requirements, the brushing process, and the overall performance of the coating.
5.3 Considerations in the Actual Production Process
In actual production, in addition to the number of brushings and the Baumé degree, there are other factors that need to be considered. For example, the surface roughness of the sand mold can affect the adhesion and penetration of the coating. A rougher sand – mold surface may require a slightly thicker coating to ensure good coverage.
The environmental conditions during the coating application process also play a role. High – humidity environments may slow down the drying speed of alcohol – based coatings, which may affect the coating – thickness measurement and the overall production cycle. Therefore, appropriate measures should be taken to control the environmental humidity, such as using dehumidifiers in the production workshop.
Moreover, the quality of the brushing operation also has a significant impact on the coating quality. Workers should be trained to ensure that the brushing process is uniform and that local coating accumulation or insufficient coating areas are avoided.
6. Challenges and Future Research Directions in the Application of Alcohol – based Coatings
6.1 Existing Challenges
- Environmental and Health Issues: Alcohol – based coatings contain organic solvents, which are volatile. These volatile organic compounds (VOCs) can cause air pollution and are harmful to the health of workers in the foundry. Long – term exposure to VOCs may lead to respiratory problems, headaches, and other health issues.
- Coating Penetration Control: As mentioned earlier, alcohol – based coatings have a relatively high penetration rate into the sand mold. Although a certain degree of penetration is beneficial for the adhesion between the coating and the sand mold, excessive penetration can lead to a reduction in the effective coating thickness on the surface, affecting the protection effect of the coating.
- Cost – effectiveness: The production and use of alcohol – based coatings may be relatively costly. The raw materials for alcohol – based coatings, especially some high – performance organic solvents, are expensive. In addition, the need for special equipment for drying and ventilation during the application process also increases the production cost.
6.2 Future Research Directions
- Development of Environmentally Friendly Alcohol – based Coatings: Future research can focus on developing alcohol – based coatings with lower VOC emissions. This can be achieved by using new types of solvents or additives that are more environmentally friendly and less harmful to human health. For example, some bio – based solvents can be explored as alternatives to traditional organic solvents.
- Improvement of Coating Penetration Control Technology: Research can be carried out to develop methods to better control the penetration of alcohol – based coatings into the sand mold. This may involve modifying the coating formulation or using surface – treatment techniques on the sand mold to adjust the interaction between the coating and the sand mold.
- Cost – reduction Research: To improve the cost – effectiveness of alcohol – based coatings, research can be conducted on optimizing the production process of coatings, finding more cost – effective raw materials, and improving the efficiency of the coating – application process. For example, developing new coating – manufacturing processes that can reduce energy consumption and waste generation.
7. Conclusion
In conclusion, the application of alcohol – based coatings in sand casting is a complex process that is affected by multiple factors. The coating thickness is a crucial parameter that directly affects the quality of castings. Through our experimental research, we have analyzed the influence of the Baumé degree of the coating and the number of brushing times on the coating thickness.
We found that increasing the number of brushings can increase the coating thickness, and the influence of the Baumé degree on the coating thickness varies depending on the number of brushings. For conventional steel – casting products, brushing 3 times with a coating of appropriate Baumé degree can generally meet the coating – thickness requirements. However, for special – sized or high – quality – requirement products, the number of brushings and the Baumé degree need to be adjusted accordingly.
