This article delves into the testing method of molten metal filling ability in lost foam casting. By analyzing the influencing factors, conducting sample design and optimization, and designing sample molds, a practical testing method is established. This method can effectively measure the filling ability of molten metal in lost foam casting of cast iron parts, providing valuable guidance for optimizing the lost foam casting process.
1. Introduction
Lost foam casting is a green casting production method that uses polystyrene foam patterns. Under the high – temperature action of molten metal, the foam gasifies, and finally, a casting is formed. Due to the use of dry sand molding without resin binders and the easy collection and treatment of polystyrene foam gasification products, this method has the advantages of environmental friendliness, high casting accuracy, low cost, and low labor intensity. It is an important development direction in the casting industry and has shown remarkable application effects in fields such as agricultural machinery, automobiles, and power machinery.
However, currently, the rejection rate of lost foam casting is generally relatively high compared with traditional cavity casting. Among them, filling – related defects account for about 80% of all defects. The optimization of the lost foam process lacks a complete and accurate scientific method and quantitative metal fluidity data. During the lost foam casting process, there are intense physical and chemical reactions between the molten metal and the polystyrene foam pattern. Its filling process is affected by many factors such as pouring temperature, pattern density, vacuum degree, and coating permeability, which is quite different from traditional cavity casting. At present, there are standard detection methods for the filling ability of molten metal in traditional cavity casting, but for lost foam casting, a testing method that can comprehensively reflect the filling ability of molten metal has not been established. Most of the existing research results are aimed at lost foam casting of aluminum alloys. Establishing a testing method for the filling ability of lost foam casting and then studying the influence of various process factors (pouring temperature, pattern density, coating thickness, vacuum degree, and molten metal pressure head, etc.) on the filling ability can effectively guide process designers to optimize the process design and reduce the rejection rate. This article focuses on studying the testing method of the flow ability of molten metal in lost foam casting of cast iron parts based on the common process parameters of lost foam cast iron parts.
2. Analysis of Influencing Factors in Lost Foam Casting Filling Process and Selection of Process Parameters
2.1 Analysis of Influencing Factors in Lost Foam Casting Filling Process
During the filling process of lost foam casting, there are complex physical and chemical reactions between the molten metal and the pattern. The pattern rapidly decomposes under the thermal action of the molten metal, generating a large amount of gas, thus forming an air gap between the molten metal and the pattern. The air gap is filled with the pyrolysis products of the foam plastic and generates a relatively large pressure. Compared with ordinary cavity casting, this pressure greatly reduces the filling speed and also makes its filling form significantly different from that of cavity casting. The speed of the molten metal front depends on the disappearance speed of the lost foam pattern, and the disappearance speed of the pattern is related to process factors such as vacuum degree, pattern density, pouring temperature, and metal pressure head.
- Negative Pressure Degree: The negative pressure degree in the lost foam casting process not only gives the sand mold a certain strength to prevent mold collapse and expansion but also enables the decomposition products of the white mold to quickly leave the mold cavity. Existing research shows that the filling speed of lost foam casting increases linearly with the increase of the negative pressure degree. The greater the negative pressure degree, the faster the filling speed. However, there is no description of its impact on the filling ability. Currently, the commonly used negative pressure degree for lost foam casting of cast iron parts is generally 0.03 – 0.06MPa, as shown in Table 1.
| Negative Pressure Degree (MPa) | Impact on Filling Speed | Common Range for Cast Iron Parts |
|—|—|—|
| 0.03 – 0.06 | Increases linearly with the increase of negative pressure degree | 0.03 – 0.06 | - Pattern Density: The greater the density of the white mold pattern, the more decomposition products per unit volume of the white mold, and the greater the resistance to the filling of the molten metal. Currently, the commonly used pattern materials for lost foam casting of cast iron parts are EPS and STMMA. The commonly used density range of EPS is 20 – 25g/L, and the commonly used density of STMMA is generally controlled at 22 – 26g/L, as presented in Table 2.
| Pattern Material | Density Range (g/L) |
|—|—|
| EPS | 20 – 25 |
| STMMA | 22 – 26 | - Pouring Temperature: Existing research indicates that the filling speed of lost foam casting shows an increasing trend initially as the pouring temperature increases. However, when the pouring temperature continues to rise, the filling speed decreases instead. When the pouring temperature is relatively low, as the pouring temperature increases, the gasification speed of the foam plastic accelerates, resulting in less filling resistance and a faster filling speed. When the pouring temperature rises to a certain extent, the types of decomposition products of the foam plastic pattern change. There are more small – molecule decomposition products, and fewer liquid and high – molecular – weight gaseous decomposition products. This increases the amount of gas products in the air gap in front of the molten metal, and these gas products cannot be discharged quickly, thus affecting the filling speed. In actual production, a high pouring temperature is often used to compensate for the heat absorbed by the decomposition of the white mold and improve the filling ability of the molten metal. However, for some thin – walled parts with complex structures, simply increasing the pouring temperature cannot effectively improve the filling ability of the molten metal. Currently, the commonly used pouring temperature for lost foam casting of cast iron parts is 1480 – 1520℃, as shown in Table 3.
| Pouring Temperature (℃) | Impact on Filling Speed | Common Range for Cast Iron Parts |
|—|—|—|
| 1480 – 1520 | First increases and then decreases with the increase of pouring temperature | 1480 – 1520 | - Coating Permeability: The main function of the coating in lost foam casting is to isolate, that is, to separate the molten metal from the sand mold to avoid sand adhesion. However, the coating permeability has a significant impact on the filling of the molten metal. If the coating permeability is good, it is conducive to the discharge of the decomposition products of the white mold, reducing the gas pressure at the front end of the molten metal and increasing the filling speed. The coating permeability is closely related to the thickness of the coating layer and the number of coating applications. The greater the thickness of the coating layer and the more coating applications, the worse the coating permeability. Currently, the commonly used coating for lost foam casting of cast iron parts is generally applied 2 – 3 times, and the coating thickness is 1 – 1.2mm, as presented in Table 4.
| Number of Coating Applications | Coating Thickness (mm) | Impact on Coating Permeability |
|—|—|—|
| 2 – 3 | 1 – 1.2 | The more applications and the greater the thickness, the worse the permeability | - Static Pressure Head: Generally, the size of the static pressure head depends on the height of the sprue. Under the condition that other process parameters remain unchanged, the larger the static pressure head, the faster the filling speed.
2.2 Selection of Process Parameters
When testing the filling ability of molten iron under different pouring parameters, the main pouring parameters set are pouring temperature, vacuum degree, pattern density, and coating permeability. Each pouring parameter can be determined according to actual testing needs. Generally, the pouring temperature is 1400 – 1500℃; the vacuum degree is generally set at 0.03 – 0.06MPa; the pattern density is 20 – 26g/L; the coating permeability is difficult to control in actual production. Different numbers of coating applications (2 – 4 times) under the same Baume degree of the same coating are used to replace different permeabilities. The static pressure head is replaced by different sprue heights according to testing needs, as shown in Table 5.
| Pouring Parameter | Range |
|---|---|
| Pouring Temperature (℃) | 1400 – 1500 |
| Vacuum Degree (MPa) | 0.03 – 0.06 |
| Pattern Density (g/L) | 20 – 26 |
| Coating Permeability (represented by number of applications) | 2 – 4 |
| Static Pressure Head (represented by sprue height) | Determined according to testing needs |
3. Sample Design and Optimization
3.1 Original Sample Design Scheme
The original sample design scheme is shown in Figure 1. The sample size is as follows: the maximum circle diameter , the sample width , the sample thickness is a variable, set as 6mm, 8mm, and 10mm respectively, and the sample total length .
[Insert Figure 1: Schematic diagram of the original sample design scheme]
Two tests were carried out with the above samples under different process parameters.
- Test 1: The test parameters are set as follows. The fixed parameters are: the static pressure head is 150mm, the pattern density is 20g/L, and the number of coating applications is 2 times. The variable parameters are: different sample thicknesses of 7mm, 10mm, 12mm; pouring temperatures of 1420℃ and 1480℃; vacuum degrees of 0.03MPa and 0.06MPa. The sample assembly process of Test 1 is shown in Figure 2.
[Insert Figure 2: Sample assembly process of Test 1]
According to the established test plan, prepare the patterns. Each pattern cluster contains 3 types of wall – thickness samples at the same time and is assembled as shown in Figure 2. The assembly instructions are: the cross – section size of the sprue is φ40mm×50mm, the cross – section size of the runner is 35mm×30mm, the cross – section size of the ingate is 10mm×35mm. Fiber rods are used to fix the white mold, and each pattern cluster contains 3 types of samples with thicknesses of 6mm, 8mm, and 10mm. The yellow mold and mold – filling 造型 are shown in Figure 3.
[Insert Figure 3: Yellow mold and mold – filling 造型]
Pouring was carried out at 1420℃ and 1480℃ respectively. After cleaning, the samples are shown in Figure 4.
[Insert Figure 4: Samples after pouring and cleaning]
The results show that when the negative pressure degrees are 0.03MPa and 0.06MPa, and pouring is carried out at 1420℃ and 1480℃, all wall – thickness samples can be completely filled. It can be seen that this sample structure cannot sensitively reflect the influence of changes in pouring temperature and negative pressure degree on the flow length.
- Test 2: On the basis of Test 1, the pouring results were compared under different pouring static pressure heads. The sample assembly process of Test 2 is shown in Figure 5.
[Insert Figure 5: Sample assembly process of Test 2]
The distance between samples on the same pattern cluster is 40mm. Pouring was carried out at 1420℃ and 1480℃ respectively, and other pouring parameters are the same as those in Test 1. The pouring results are shown in Figure 6.
[Insert Figure 6: Pouring results]
The pouring results show that when pouring at 1420℃ and 1480℃, all wall – thickness samples can basically be completely filled, and no obvious unfilled phenomenon occurs.
From the pouring results of Test 1 and Test 2, it can be seen that under different pouring parameters such as high and low pouring temperatures, high and low vacuum degrees, and different pressure heads, the samples can basically be completely filled, but they cannot sensitively reflect the filling ability of molten iron under different pouring parameters. It is necessary to further improve the sample size to obtain the best test method.
3.2 Optimization of the Sample Design Scheme
After analysis and improvement, the optimized sample structure is shown in Figure 7. The optimized sample size is: the maximum circle diameter ; the sample width ; the sample thickness ; the sample total length .
[Insert Figure 7: Optimized sample structure]
The assembly process of the optimization scheme is shown in Figure 8 (the 3D assembly diagram is shown in Figure 5a). The assembly instructions are: the sprue is φ40mm×50mm, the cross – section size of the runner is 35mm×30mm, and the cross – section size of the ingate is 35mm×10mm.
[Insert Figure 8: Assembly of the optimization scheme]
Each group is set with 3 samples, and the sample density is 20g/L (the minimum sample density in normal production). The vertical distance between two samples is 100mm to reduce the thermal influence between the two samples and ensure that the static pressure head difference of each sample during pouring is 100mm. The verification test coating is applied 2 times (applying 2 times ensures that the coating has good permeability), the negative pressure degree is 0.06MPa, and the pouring temperatures are 1480℃ and 1420℃. The mold – filling 造型 is shown in Figure 9. The pouring results are shown in Figure 10.
[Insert Figure 9: Mold – filling 造型]
[Insert Figure 10: Pouring results]
It can be seen from Figure 10 that when pouring at 1420℃ and 1480℃, each group of samples cannot be completely filled, and the filling distances are different. In addition, both pouring at 1480℃ and 1420℃ show a trend that the filling length gradually increases from bottom to top. The overall filling length at 1480℃ is longer than that at 1420℃, and at the same static pressure head position, the filling length at 1480℃ is longer than that at 1420℃. Therefore, the optimized samples are very sensitive to changes in process parameters, and the filling abilities are different under different pouring parameters. The influence of each pouring factor on the filling ability of lost foam can be basically judged according to the filling distance of the samples.
4. Sample Mold Design
The above – mentioned samples were made by manually cutting foam boards, which had low efficiency, poor dimensional accuracy, and were not easy to change the pattern density and determine the sample structure according to the test requirements. In response to this situation, a sample lost – foam mold was designed according to the sample structure. Making white molds with the mold not only has high efficiency and high dimensional accuracy but also can change the sample density according to the test requirements. The design of the sample white mold is shown in Figure 11.
[Insert Figure 11: Design of the sample white mold]
The following points need to be noted in the design of the sample white mold:
- Anti – deformation ribs are formed integrally with the sample white mold to prevent the sample from deforming and breaking during demolding and need to be removed after mold assembly.
- The length scale interval is 50mm, which is convenient for directly reading the filling length after pouring.
- The gate bonding place is used to bond the ingate and has a certain distance from the sample to facilitate the removal of the ingate after pouring.
The mold design is shown in Figure 12.
[Insert Figure 12: Mold design]
5. Conclusion
A testing method for the fluidity of molten metal in lost foam casting of cast iron parts has been designed. A spiral fluidity test strip with a width of 20mm and a thickness of 8mm is selected, which can not only represent the filling situation of common castings in actual production but also sensitively reflect the influence of changes in process parameters on the flow length. Using this method, according to the respective process parameter conditions of enterprises, the influence of various factors on the filling ability can be tested, providing basic data for process design and optimization. This testing method plays a positive role in studying the filling characteristics of lost foam casting and optimizing the lost foam casting production process.
