1. Introduction
Low – pressure casting is a crucial manufacturing process in the foundry industry, especially for the production of high – quality non – ferrous metal castings. With the increasing demand for lightweight, precision, and complex components in various industries such as automotive, aerospace, and electronics, low – pressure casting has gained significant attention. This article aims to provide a detailed overview of low – pressure casting, covering its basic principles, process parameters, process characteristics, common defects, and future development trends.
2. Basic Principles of Low – pressure Casting
2.1 Definition and Pressure Range
Low – pressure casting is a casting process in which molten metal is made to fill the mold cavity under a certain pressure. The pressure used in this process is relatively low, typically ranging from 0.02 MPa to 0.06 MPa, which is why it is named “low – pressure casting”.
2.2 Process Flow
The basic process flow of low – pressure casting is as follows (see Figure 1):
- Sealing and Gas Introduction: The molten metal is placed in a sealed crucible (or sealed tank). Dry compressed air or inert gas is then introduced into the crucible.
- Ascent of Molten Metal: Under the pressure of the gas, the molten metal in the crucible rises steadily along the riser tube.
- Filling the Mold Cavity: The molten metal enters the mold cavity through the inner gate.
- Maintaining Pressure: The gas pressure on the surface of the molten metal in the crucible is maintained until the casting is completely solidified.
- Pressure Release and Return of Molten Metal: After the casting is solidified, the gas pressure is released. The non – solidified molten metal in the riser tube flows back into the crucible.
- Ejection of the Casting: Finally, the casting is ejected from the mold.
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. A slow rising speed of the molten metal in the riser tube is preferred. This is beneficial for the discharge of gas in the mold cavity and can prevent the splashing of 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, especially for complex – shaped molds.
3.3 Filling Speed
The filling speed refers to the rising speed of the molten metal surface during the filling process. To avoid defects such as cold shuts and misruns, the filling speed should be higher than the minimum value. However, it should not be too fast to prevent the entrainment of gas by the turbulent molten metal, which may lead to oxide inclusions.
| Parameter | Significance | Control Principle |
|---|---|---|
| Lift – up Pressure | Affects the rising speed of molten metal in the riser tube | Keep it slow to facilitate gas discharge and prevent splashing |
| Filling Pressure | Ensures complete filling of the mold cavity | Adjust according to mold shape and size |
| Filling Speed | Avoids cold shuts and misruns while preventing gas entrainment | Higher than the minimum but not too fast |
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, which 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 pressure to improve the casting quality cannot be applied in all cases. The typical value range of the crystallization pressure is 0.1 MPa – 0.25 MPa.
3.5 Holding Time
The holding time is the period during which the pressure of the molten metal is increased to the crystallization pressure and maintained until the casting is completely solidified. If the holding time is insufficient, the casting may not be fully solidified when the pressure is released, causing the molten metal in the mold cavity to flow back into the crucible, resulting in a “hollow” casting. On the other hand, if the holding time is too long, it will lead to a long residue at the gate, reducing the process yield and making it difficult to eject the casting. Therefore, an appropriate holding time must be selected in production.
| Parameter | Influence on Casting Quality | Consequences of Improper Control |
|---|---|---|
| Crystallization Pressure | Improves feeding effect, densifies microstructure, and enhances mechanical properties | Cannot be increased indefinitely; may cause other problems if too high |
| Holding Time | Ensures complete solidification of the casting | Insufficient time leads to “hollow” casting; too long time reduces process yield and makes ejection difficult |
3.6 Mold Temperature
Low – pressure casting can use various types of 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, it can even be as high as 300 °C – 350 °C.
3.7 Pouring Temperature
Practice has proven that, under the premise of ensuring casting formation, a lower pouring temperature is better. The pouring temperature in low – pressure casting is generally 10 °C – 20 °C lower than that in gravity casting.
| Mold Type | Working Temperature Range |
|---|---|
| Non – metal Mold | Room temperature |
| Metal Mold (for Aluminum Alloy Casting) | 200 °C – 250 °C (200 – 250 °C for general cases, 300 – 350 °C for thin – walled and complex parts) |
| Pouring Temperature | 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 by using the molten metal at the bottom of the crucible through the riser tube, it can effectively prevent slag from entering the mold cavity. As a result, the purity of the molten metal is high, and there are fewer inclusion defects in the casting.
- Smooth Filling: The low – pressure casting process adopts a bottom – pouring method with balanced pressure, so the filling of the molten metal 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 slag inclusions in the casting.
- Good Surface Finish and Formability: 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 good surface finish and clear outlines, especially for the formation of complex thin – walled castings.
- Dense Microstructure: The casting solidifies under a certain pressure and can achieve top – down sequential solidification. This results in a better feeding effect and a denser microstructure of the casting, with fewer shrinkage porosity and shrinkage cavity defects.
- High Metal Yield: Low – pressure casting generally does not require a riser, and the non – solidified molten metal in the riser tube can flow back into the crucible for reuse. Therefore, the metal yield is high, generally reaching over 90%.
| Advantage | 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 Surface Finish and Formability | Pressure – assisted filling improves fluidity for better surface and complex part formation |
| Dense Microstructure | Sequential solidification under pressure reduces shrinkage defects |
| High Metal Yield | No need for riser and reusable molten metal in the riser tube increase the yield |
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. Moreover, the production efficiency is relatively low, so it is generally used for casting non – ferrous alloys.
- Erosion and Performance Deterioration: When producing aluminum alloy castings, the crucible and riser tube are in long – term contact with the molten metal, which is prone to erosion and scrapping. At the same time, it may also cause the iron content in the molten metal to increase, deteriorating the performance of the casting.
| Disadvantage | Explanation |
|---|---|
| High Equipment Cost and Low Productivity | High – cost equipment and low production efficiency limit its application |
| Erosion and Performance Deterioration | Contact with molten metal causes erosion and affects casting performance |
5. Common Defects in Low – pressure Casting and Countermeasures
5.1 Porosity
- Morphology: Porosity in low – pressure castings usually appears as round or oval – shaped holes with smooth inner walls, often with a slight oxidized color.
- Causes:
- Fast Filling Speed: If the filling speed of the molten metal is too fast, the liquid flow will be turbulent, entraining gas that cannot be discharged in time, resulting in porosity.
- Gas Emission from Molds and Cores: After the molten metal fills the mold, due to the long – term heating of the sand mold and sand core, gas is emitted and penetrates into the non – fully solidified molten metal, causing porosity.
- Poor Ventilation: If there are gas – trapping areas in the mold cavity due to poor ventilation, porosity will occur.
- Countermeasures:
- Optimize Filling Speed: Select an appropriate filling speed to ensure the smooth filling of the molten metal without causing cold shuts and misruns, and avoid gas entrainment.
- Improve Ventilation Conditions: Optimize the ventilation conditions of the sand mold and sand core to prevent gas from entering the molten metal. Reduce the resin content of the sand mold and sand core under the premise of ensuring their strength to reduce gas emission.
- Prevent Ventilation Blockage: Avoid blockages in ventilation holes, vents, and other ventilation structures.
| Defect | Morphology | Causes | Countermeasures |
|---|---|---|---|
| Porosity | Round or oval holes with smooth inner walls and a slight oxidized color | Fast filling speed, gas emission from molds and cores, poor ventilation | Optimize filling speed, improve ventilation conditions, prevent ventilation blockage |
5.2 Shrinkage Porosity and Shrinkage Cavity
- Morphology: Shrinkage porosity and shrinkage cavity usually appear as irregular holes with rough inner walls and many dendritic protrusions. They mostly occur in the center of hot spots or the last – solidifying parts.
- Causes: After the molten metal fills the mold, during the solidification process, if a top – down temperature gradient cannot be formed, sequential solidification cannot be achieved. As a result, the hot – spot parts that solidify last cannot be properly fed, leading to shrinkage porosity and shrinkage cavity.
- Countermeasures:
- Enhance Heat Dissipation: Strengthen the heat – dissipation capacity of hot – spot parts, such as by arranging chills.
- Lower Pouring Temperature: Reduce the pouring temperature as much as possible without causing cold shuts.
- Increase Holding Pressure: Increase the holding pressure during solidification to improve the fluidity of the molten metal and enhance the feeding ability to the shrinkage – prone parts.
| Defect | Morphology | Causes | Countermeasures |
|---|---|---|---|
| Shrinkage Porosity and Shrinkage Cavity | Irregular holes with rough inner walls and dendritic protrusions, occurring in hot – spot centers or last – solidifying parts | Failure to form a temperature gradient for sequential solidification | Enhance heat dissipation, lower pouring temperature, increase holding pressure |
5.3 Cold Shut
- Morphology: A cold shut is characterized by the presence of a cold – shut line, where the fronts of two streams of molten metal fail to fuse well.
- Causes:
- Low Pouring Temperature or High Cooling Capacity: A low pouring temperature or a strong cooling capacity of the mold can easily cause cold shuts at the confluence of the molten metal.
- Insufficient Filling Pressure: Insufficient filling pressure leads to poor fluidity of the molten metal, also resulting in cold shuts.
- Countermeasures:
- Raise Pouring Temperature: 适当提高金属液的浇注温度.
- Heat Chills: For cold – shut lines caused by the rapid cooling of chills, the chills can be moderately heated.
- Increase Filling Pressure: 适当提高金属液的充型压力，提高金属液的流动性.
| Defect | Morphology | Causes | Countermeasures |
|---|---|---|---|
| Cold Shut | Presence of a cold – shut line where molten metal fronts do not fuse well | Low pouring temperature or high cooling capacity, insufficient filling pressure | Raise pouring temperature, heat chills, increase filling pressure |
5.4 Inclusions
- Morphology: Inclusions are irregular in shape, and the color at the defect location is significantly different from that of the casting body.
- Causes:
- Non – metallic Inclusions: Non – metallic inclusions are mostly formed due to foreign objects that fall into the mold cavity during the mold – preparation process and are not cleaned up, and then enter the molten metal during the filling process.
- Oxide Inclusions: Oxide inclusions are often caused by a fast filling speed of the molten metal, resulting in turbulent splashing of the liquid surface and the formation of double – layer oxide films.
- Countermeasures:
- Clean the Mold Cavity: Thoroughly clean the mold cavity during the mold – preparation process.
- Adjust Filling Pressure and Install Filters: Adjust the filling pressure to avoid turbulent splashing of the molten metal due to too fast a filling speed. Or install a filter at the riser inlet to filter and stabilize the molten metal.
| Defect | Morphology | Causes | Countermeasures |
|---|---|---|---|
| Inclusions | Irregular shape with a color different from the casting body | Non – metallic inclusions from uncleaned foreign objects, oxide inclusions from fast – filling – induced turbulence | Clean the mold cavity, adjust filling pressure, install filters |
6. Future Development Trends of Low – pressure Casting
6.1 Numerical Simulation Technology
The use of computer – aided numerical simulation technology is becoming increasingly important in low – pressure casting. By developing accurate numerical models that simulate the filling flow and solidification crystallization sequence of molten metal, which are consistent with the actual processes, foundries can predict potential defects in advance. This allows for the optimization of process parameters before actual production, reducing the number of trial – and – error processes, saving time and costs.
6.2 In – situ Detection Technology
Improving the in – situ detection capabilities of casting performance is another significant trend. For example, using ultrasonic equipment to detect the residual stress in castings or thermal analysis technology to predict the mechanical properties of key parts of castings. These techniques can not only avoid destructive testing of castings but also provide real – time and accurate information about the casting process, enabling precise control of the casting process and casting performance.
6.3 Process Integration and Automation
In the future, low – pressure casting is expected to be more integrated with other processes, such as heat treatment and surface finishing. This integration can improve the overall quality and performance of castings. At the same time, the automation level of the low – pressure casting process will be further enhanced, reducing human – error factors and improving production efficiency and product consistency.
7. Conclusion
Low – pressure casting is a valuable casting process with unique advantages in producing high – quality non – ferrous metal castings. Although it has some disadvantages, with the continuous development of technology, such as the application of numerical simulation and in – situ detection technology, these problems can be gradually overcome. The future development of low – pressure casting will focus on improving process accuracy, product quality, and production efficiency, making it play an even more important role in various industries.