This article provides an in – depth exploration of low – pressure casting technology. It covers the historical development, fundamental principles, process parameters, technological characteristics, common defects and countermeasures, as well as the future trends of low – pressure casting. Through detailed analysis and the use of tables for summary, readers can gain a comprehensive understanding of this important casting process.
1. Introduction
Low – pressure casting is a crucial manufacturing process in the field of metal casting, especially for non – ferrous metals. With the increasing demand for high – quality, precision, and lightweight components in various industries such as automotive, aerospace, and electronics, low – pressure casting has attracted more and more attention. This section briefly introduces the significance and application scope of low – pressure casting.
2. Historical Development of Low – pressure Casting
| Time | Development Stage | Key Events |
|---|---|---|
| Over 100 years ago | Invention | Proposed by British scientist E.F.LAKE, but did not achieve large – scale industrial application at first |
| 1945 | Initial industrial application | British Alumasc Company used this process to mass – produce rainwater pipes and beer containers |
| After 1950 | Rapid development | The automotive industry in the United States applied it to produce engine parts, leading to significant progress in technology and equipment |
| Around 1957 | Introduction to China | China officially introduced low – pressure casting equipment and began related research |
This table summarizes the key milestones in the development of low – pressure casting, showing how it has evolved from its initial conception to a widely used industrial process.
3. Fundamental Principles of Low – pressure Casting
3.1 Process Flow
Low – pressure casting involves several steps. First, 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 along the riser tube steadily. The molten metal then enters the mold cavity through the ingate. After filling the cavity, the gas pressure on the surface of the molten metal is maintained until the casting is completely solidified. Finally, the pressure is released, and the un – solidified molten metal in the riser tube flows back into the crucible, and the casting is ejected. A diagram of the low – pressure casting process can be inserted here to illustrate the process more vividly.
3.2 Pressure Range
The pressure used in low – pressure casting is relatively low, typically in the range of 0.02 MPa – 0.06 MPa. This low – pressure environment is sufficient to ensure the smooth filling of the molten metal into the mold while minimizing the risk of defects caused by excessive pressure.
4. Process Parameters of Low – pressure Casting
4.1 Lift – up Pressure
| Parameter | Description | Impact on Casting | Optimal Range |
|---|---|---|---|
| Lift – up Pressure | The pressure required to raise the molten metal surface to the vicinity of the ingate | Reflects the rising speed of the molten metal in the riser tube. A slow rising speed is beneficial for gas discharge and preventing splashing | Determined by factors such as the height of the riser tube and the fluidity of the molten metal |
4.2 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.
4.3 Filling Speed
| Parameter | Description | Impact on Casting | Control Method |
|---|---|---|---|
| Filling Speed | The rising speed of the molten metal surface during filling | Too slow may cause cold shuts and misruns; too fast may lead to gas entrapment and oxidation inclusions | Adjusted by controlling the pressure increase rate and the cross – sectional area of the riser tube |
4.4 Crystallization Pressure
Higher crystallization pressure can improve the feeding effect of the casting, resulting in a denser structure and better mechanical properties. However, it cannot be increased indefinitely. The typical value range is 0.1 MPa – 0.25 MPa.
4.5 Holding Time
| Parameter | Description | Impact of Insufficient Time | Impact of Excessive Time |
|---|---|---|---|
| Holding Time | The time the molten metal is maintained at the crystallization pressure until solidification | The casting may be “empty” due to the 回流 of molten metal, resulting in scrap | Prolonged holding time can lead to excessive residue at the ingate, reducing the process yield and making it difficult to eject the casting |
4.6 Mold Temperature
For non – metal molds, the working temperature is usually room temperature. For metal molds, when casting aluminum alloys, the working temperature is generally controlled at 200 °C – 250 °C, and it can be as high as 300 °C – 350 °C for thin – walled and complex parts.
4.7 Pouring Temperature
Under the premise of ensuring casting formation, a lower pouring temperature is preferred. Low – pressure casting pouring temperatures are generally 10 °C – 20 °C lower than those of gravity casting.
5. Technological Characteristics of Low – pressure Casting
5.1 Advantages
| Advantage | Explanation | Significance |
|---|---|---|
| High Purity of Molten Metal | The bottom – pouring process reduces the chance of slag entering the mold, resulting in high – purity molten metal and fewer inclusion defects | Improves the quality of the casting and its mechanical properties |
| Stable Filling | The bottom – pouring and balanced pressure ensure stable filling of the molten metal, reducing the risk of turbulence and splashing | Minimizes the formation of double – layer oxide films and oxide inclusions |
| Good Casting Surface Quality | The pressure – assisted filling enhances the fluidity of the molten metal, facilitating the formation of castings with smooth surfaces and clear outlines, especially for complex thin – walled parts | Meets the requirements for high – precision and aesthetically pleasing components |
| Dense Structure | The casting solidifies under pressure and can achieve top – down sequential solidification, resulting in a good feeding effect and fewer shrinkage porosity and shrinkage cavity defects | Improves the internal quality and mechanical strength of the casting |
| High Metal Yield | Generally, no riser is needed, and the un – solidified molten metal in the riser tube can be recycled, with a metal yield of over 90% | Saves raw materials and reduces production costs |
A picture showing a high – quality low – pressure casting product can be inserted here to visually demonstrate these advantages.
5.2 Disadvantages
| Disadvantage | Explanation | Impact on Production |
|---|---|---|
| High Equipment Cost | The equipment for low – pressure casting is expensive, resulting in a large initial investment | Limits the entry of small – scale manufacturers and increases the cost of production lines |
| Low Production Efficiency | The production speed is relatively slow, which is not suitable for mass – production requirements in some industries | Reduces the overall output and may increase production time and cost |
| Corrosion of Crucible and Riser Tube | When producing aluminum alloy castings, the crucible and riser tube are easily corroded by the molten metal, and the molten metal may also absorb iron, deteriorating the casting performance | Increases the frequency of equipment replacement and affects the quality stability of the casting |
6. Common Defects in Low – pressure Casting and Countermeasures
6.1 Porosity
| Defect Feature | Cause | Countermeasure |
|---|---|---|
| Round or oval – shaped pores with smooth inner walls and a slightly oxidized color | 1. Excessive filling speed leading to gas entrapment. 2. Gas generated by sand molds and cores invading the molten metal. 3. Poor exhaust | 1. Select an appropriate filling speed. 2. Optimize the exhaust conditions of sand molds and cores and reduce the resin content. 3. Ensure unobstructed exhaust structures |
A microscopic image of porosity defects can be inserted to help readers understand the appearance of this defect.
6.2 Shrinkage Porosity and Shrinkage Cavity
| Defect Feature | Cause | Countermeasure |
|---|---|---|
| Irregularly shaped holes with rough inner walls and dendritic protrusions, usually occurring at the center of hot spots or the last – solidifying parts | Failure to form a top – down temperature gradient during solidification, resulting in poor feeding at hot spots | 1. Enhance the heat dissipation at hot spots (e.g., by placing chills). 2. Lower the pouring temperature. 3. Increase the holding pressure during solidification |
6.3 Cold Shut
| Defect Feature | Cause | Countermeasure |
|---|---|---|
| The presence of cold shut lines where the fronts of two converging molten metal streams fail to fuse properly | 1. Low pouring temperature or strong cooling capacity of the mold. 2. Insufficient filling pressure and poor fluidity of the molten metal | 1. 适当提高浇注温度. 2. Heat the chills if necessary. 3. Increase the filling pressure to improve fluidity |
6.4 Inclusion
| Defect Feature | Cause | Countermeasure |
|---|---|---|
| Irregular – shaped defects with a significant color difference from the casting body | 1. Foreign objects in the mold cavity during mold preparation. 2. Oxide inclusions due to high filling speed and turbulent molten metal | 1. Thoroughly clean the mold cavity during mold preparation. 2. Adjust the filling pressure or add a filter at the riser inlet |
7. Future Trends of Low – pressure Casting
7.1 Numerical Simulation Technology
The use of computer – aided numerical simulation technology to develop accurate models that simulate the filling flow and solidification crystallization of molten metal more realistically. This technology can help predict and prevent casting defects in advance, reducing the number of trial – and – error processes in production.
7.2 In – situ Detection Technology
Developing in – situ detection methods such as using ultrasonic equipment to detect residual stress in castings and thermal analysis technology to predict the mechanical properties of key parts of castings. These methods can avoid destructive testing and provide real – time and accurate information on the casting process.
7.3 Material and Equipment Innovation
Research and development of new crucible and riser tube materials to reduce corrosion and improve the quality of molten metal. At the same time, the development of more efficient and intelligent low – pressure casting equipment can improve production efficiency and casting quality.
8. Conclusion
Low – pressure casting is an important casting process with unique advantages and some limitations. Understanding its principles, process parameters, characteristics, and common defects is crucial for improving the quality of castings. With the continuous development of technology, the future of low – pressure casting looks promising, with the potential to overcome existing problems and expand its application scope in more industries.
