This paper comprehensively explores low – pressure casting, a crucial process in the manufacturing of non – ferrous metal components. It delves into the technology’s principle, process parameters, and characteristics, analyzes common casting defects and their prevention methods, and looks at its development trends. By providing a detailed understanding of low – pressure casting, this paper aims to assist manufacturers in improving casting quality and efficiency.
1. Introduction
Low – pressure casting has a history of over 100 years. It was first proposed by the British scientist E.F.LAKE, but it did not achieve large – scale industrial application initially. In 1945, the British company Alumasc used this technology to mass – produce rainwater pipes and beer containers. After 1950, with the rapid development of the automotive industry, the United States applied low – pressure casting to manufacture automotive engine parts, promoting significant progress in this technology and related equipment. China introduced low – pressure casting equipment and began related research around 1957.
Low – pressure casting is a casting process in which molten metal is filled into a mold cavity under a certain pressure. The pressure used is relatively low, typically in the range of 0.02 MPa – 0.06 MPa. This process is widely used in the production of non – ferrous metal castings, especially for large – scale, complex, and thin – walled parts.
2. Technical Principle of Low – pressure Casting
The basic process of low – pressure casting is as follows: In a sealed crucible or tank, dry compressed air or inert gas is introduced. This gas pressure forces the molten metal at the bottom of the crucible to rise steadily along the riser tube. The molten metal then enters the mold cavity through the inner gate. The gas pressure on the surface of the molten metal in the crucible is maintained until the casting is completely solidified. After that, the gas pressure is released, and the unfrozen molten metal in the riser tube flows back into the crucible. Finally, the casting is ejected. A schematic diagram of low – pressure casting is shown in Figure 1.
| Component | Function |
|---|---|
| Crucible | Holds the molten metal |
| Riser tube | Guides the molten metal to rise to the mold cavity |
| Inner gate | Controls the flow of molten metal into the mold cavity |
| Mold | Shapes the casting |
| Gas inlet | Supplies compressed air or inert gas |
3. Process Parameters of Low – pressure Casting
3.1 Lift – up Pressure
The lift – up pressure is the pressure required to raise the molten metal surface to the vicinity of the gate. It reflects the rising speed of the molten metal in the riser tube. The molten metal should rise as slowly as possible in the riser tube. This is beneficial for the discharge of gas in the mold cavity and can prevent splashing of the molten metal when it enters the gate.
3.2 Filling Pressure
The filling pressure is the gas pressure required for the molten metal to rise to the top of the mold during the filling process. It ensures that the molten metal can completely fill the mold cavity.
3.3 Filling Speed
The filling speed is the rising speed of the molten metal surface during the filling process. To prevent cold shuts and misruns, the filling speed should be higher than the minimum value. However, it should not be too fast to avoid turbulent flow of the molten metal, which may cause gas entrapment and oxidation inclusions.
3.4 Crystallization Pressure
After the molten metal fills the mold cavity, additional pressure is applied to make the casting solidify under a certain pressure. This pressure is called the crystallization pressure. A higher crystallization pressure can improve the feeding effect of the casting, resulting in a denser microstructure and better mechanical properties. However, increasing the crystallization pressure to improve casting quality cannot be applied in all cases. The usual range of crystallization pressure is 0.1 MPa – 0.25 MPa.
3.5 Holding Time
After the pressure of the molten metal reaches the crystallization pressure, it is maintained for a certain period until the casting is completely solidified. This period is called the holding time. If the holding time is insufficient, the molten metal in the mold cavity may flow back to the crucible before solidification, causing the casting to be defective. If the holding time is too long, it will not only reduce the process yield but also make it difficult to remove the casting from the mold due to the “freezing” of the molten metal in the gate and riser tube.
3.6 Mold Temperature
Low – pressure casting can use various molds. For non – metal molds, the working temperature is usually room temperature. For metal molds, there are specific requirements. For example, when casting aluminum alloys by low – pressure casting, the working temperature of the metal mold is generally controlled at 200 °C – 250 °C. When casting thin – walled and complex parts, the temperature can even reach 300 °C – 350 °C.
3.7 Pouring Temperature
Practice has proven that under the premise of ensuring casting formation, the lower the pouring temperature, the better. The pouring temperature of low – pressure casting is generally 10 °C – 20 °C lower than that of gravity casting.
| Process Parameter | Influence | Recommended Range |
|---|---|---|
| Lift – up Pressure | Affects the rising speed of molten metal in the riser tube | Determined by the height of the riser tube and the requirements for molten metal flow |
| Filling Pressure | Ensures complete filling of the mold cavity | Adjusted according to the complexity of the mold and the fluidity of the molten metal |
| Filling Speed | Affects the formation of casting defects | Higher than the minimum value, but avoid being too fast |
| Crystallization Pressure | Improves the feeding effect and casting microstructure | 0.1 MPa – 0.25 MPa |
| Holding Time | Determines the solidification quality of the casting | Adjusted according to the thickness and material of the casting |
| Mold Temperature | Affects the filling and solidification of the molten metal | Room temperature for non – metal molds; 200 °C – 250 °C (200 – 350 °C for thin – walled complex parts) for metal molds when casting aluminum alloys |
| Pouring Temperature | Influences the fluidity and defect formation of the molten metal | 10 °C – 20 °C lower than gravity casting |
4. Process Characteristics of Low – pressure Casting
4.1 Advantages of Low – pressure Casting
- High Purity of Molten Metal: Since slag generally floats on the surface of the molten metal, and low – pressure casting fills the mold with the molten metal from the bottom of the crucible through the riser tube, it can effectively prevent slag from entering the mold cavity. As a result, the molten metal has a high purity, and the casting has fewer inclusion defects.
- Smooth Filling: The low – pressure casting process uses a bottom – pouring method with balanced pressure, so the molten metal filling is relatively smooth. This can effectively reduce or avoid turbulence and splashing of the molten metal during the filling process, reducing the possibility of double – layer oxide films and oxide inclusions in the casting.
- Good Casting Surface Quality: The molten metal fills the mold under pressure, which can improve the fluidity of the molten metal to a certain extent. This is conducive to forming castings with a smooth surface and clear contours, especially for the formation of complex thin – walled castings.
- Dense Casting Structure: The casting solidifies under a certain pressure and can achieve top – down sequential solidification. This results in a good feeding effect and a relatively dense casting structure, with fewer shrinkage porosity and shrinkage cavity defects.
- High Metal Yield: Low – pressure casting generally does not require a riser, and the unfrozen molten metal in the riser tube can flow back to the crucible for reuse. Therefore, the metal yield is relatively high, usually reaching more than 90%.
4.2 Disadvantages of Low – pressure Casting
- High Equipment Cost and Low Productivity: The equipment cost of low – pressure casting is high, and the initial investment is large. The production efficiency is relatively low, so it is generally used for casting non – ferrous alloys.
- Corrosion Problem: When producing aluminum alloy castings, the crucible and riser tube are in long – term contact with the molten metal, making them vulnerable to erosion and scrapping. At the same time, it can also cause the iron content in the molten metal to increase, deteriorating the performance of the casting.
| Advantages | Explanation |
|---|---|
| High Purity of Molten Metal | Slag is prevented from entering the mold cavity, reducing inclusion defects |
| Smooth Filling | Bottom – pouring with balanced pressure reduces turbulence and splashing |
| Good Casting Surface Quality | Pressure – assisted filling improves fluidity for better surface finish |
| Dense Casting Structure | Sequential solidification under pressure leads to fewer shrinkage defects |
| High Metal Yield | No need for a riser and recyclable molten metal increase the yield |
| Disadvantages | Explanation |
| High Equipment Cost and Low Productivity | High – cost equipment and low production efficiency limit its application |
| Corrosion Problem | Crucible and riser tube are easily corroded, affecting casting performance |
5. Common Defects in Low – pressure Casting and Countermeasures
5.1 Porosity
- Morphology: Porosity is usually round or oval in shape, with a smooth inner wall and often a slight oxidation color.
- Causes:
- The filling speed of the molten metal is too fast, causing the liquid flow to be turbulent and entrap gas, which cannot be discharged in time.
- After the molten metal filling is completed, the sand mold and sand core release gas due to long – term heating, and the gas invades the unfrozen molten metal.
- The exhaust is not smooth, and there are gas – trapped areas in the mold cavity.
- Countermeasures:
- Select an appropriate filling speed to ensure the smooth filling of the molten metal and avoid gas entrapment while ensuring no cold shuts and misruns.
- Optimize the exhaust conditions of the sand mold and sand core to prevent the gas generated from entering the molten metal. Also, reduce the resin content of the sand mold and sand core under the premise of ensuring their strength to reduce gas generation.
- Ensure that the exhaust holes, exhaust plugs, and other exhaust structures are not blocked.
5.2 Shrinkage Porosity and Shrinkage Cavity
- Morphology: Shrinkage porosity and shrinkage cavity are characterized by irregular holes with rough inner walls and many dendritic protrusions. They usually occur in the center of hot spots or the last – solidified parts.
- Causes: During the solidification process after the molten metal filling is completed, a top – down temperature gradient cannot be formed, so sequential solidification cannot be achieved. As a result, the hot – spot parts that solidify last cannot be effectively fed, leading to shrinkage defects.
- Countermeasures:
- Strengthen the heat dissipation capacity of the hot – spot parts, such as by arranging chills.
- Lower the pouring temperature as much as possible without causing cold shuts.
- Increase the holding pressure during solidification to improve the fluidity of the molten metal and enhance the feeding ability to the shrinkage parts.
5.3 Cold Shut
- Morphology: Cold shut is characterized by the presence of cold – shut lines, where the fronts of two converging molten metal flows fail to fuse properly.
- Causes:
- The pouring temperature is too low or the cooling capacity of the mold is too strong, which is likely to cause cold – shut defects at the meeting point of the molten metal.
- Insufficient filling pressure and poor fluidity of the molten metal can also lead to cold – shut defects.
- Countermeasures:。
- For cold – shut lines caused by the excessive cooling rate of the cold iron bar, the cold iron bar can be appropriately heated.。
5.4 Inclusions
- Morphology: Inclusions are irregular in shape, and the color at the defect site is significantly different from that of the casting body.
- Causes:
- Non – metallic inclusions are mostly formed due to foreign objects falling into the mold cavity during the mold preparation process and not being cleaned up, which then enter the molten metal during the filling process.
- Oxide inclusions are often caused by the rapid filling speed of the molten metal, resulting in turbulent liquid surface splashing and the formation of double – layer oxide films.
- Countermeasures:
- Clean the mold cavity thoroughly during the mold preparation process.
- Adjust the filling pressure to avoid excessive filling speed and turbulent splashing of the molten metal. Or add a filter at the riser inlet to filter and stabilize the molten metal.
| Defect | Morphology | Causes | Countermeasures |
|---|---|---|---|
| Porosity | Round or oval with smooth inner wall and slight oxidation color | Fast filling speed, gas from sand mold/core, poor exhaust | Adjust filling speed, optimize exhaust, reduce resin content |
| Shrinkage Porosity and Shrinkage Cavity | Irregular holes with rough inner walls and dendritic protrusions in hot – spot or last – solidified parts | Lack of sequential solidification | Strengthen heat dissipation, lower pouring temperature, increase holding pressure |
| Cold Shut | Presence of cold – shut lines where molten metal fails to fuse properly | Low pouring temperature, strong mold cooling, insufficient filling pressure | Increase pouring temperature, heat cold iron bar if needed, increase filling pressure |
| Inclusions | Irregular shape with color difference from casting body | Foreign objects in mold cavity, rapid filling causing oxide films | Clean mold cavity, adjust filling pressure, add filter |
6. Development Trends of Low – pressure Casting
6.1 Numerical Simulation Technology
The use of computer – aided numerical simulation technology is becoming increasingly important. By developing accurate numerical simulation models that can simulate the filling flow and solidification crystallization sequence of molten metal to match the actual situation, manufacturers can predict potential defects in advance and optimize the casting process. This helps to improve casting quality and reduce the number of trial – and – error experiments.
6.2 In – situ Detection Technology
Enhancing the in – situ detection ability of casting performance, such as using ultrasonic equipment to detect the residual stress of castings or thermal analysis technology to predict the mechanical properties of key parts of castings, can avoid destructive testing of castings. It can also provide real – time and accurate information about the casting process, enabling precise control of the casting process and casting performance.
6.3 Process Optimization and Integration
Continuous optimization of the low – pressure casting process, combined with other advanced manufacturing technologies, such as additive manufacturing and heat treatment, can further improve the quality and performance of castings. This integration can also expand the application scope of low – pressure casting in high – end manufacturing fields.
7. Conclusion
Low – pressure casting is an important casting process for non – ferrous metals, with advantages such as good casting quality and high process yield. However, like any casting process, it also faces challenges in the form of various defects. By understanding the technical principle, process parameters, and common defects of low – pressure casting, and taking appropriate countermeasures, manufacturers can improve the quality of castings. In addition, with the continuous development of technology, the application of numerical simulation, in – situ detection, and process integration will further promote the development of low – pressure casting technology, making it play an even more important role in the manufacturing industry.
