What is Permanent Mold Casting?

In the field of modern industrial manufacturing, permanent mold casting is an important metal forming process. It utilizes reusable metal molds to produce a variety of metal parts and is widely applied in industries such as automotive, machinery, and aerospace. This process is favored due to its many unique advantages, such as the ability to produce castings with precise dimensions and smooth surfaces, as well as high production efficiency and relatively low cost. However, it also has some limitations, such as high mold costs and certain restrictions on the selection of metal materials. This article will provide a detailed introduction to the process flow, main types, advantages and disadvantages of permanent mold casting, as well as its applications in actual production, to help readers better understand this practical casting technology.

Overview of Permanent Mold Casting

Permanent mold casting is a casting process that uses reusable metal molds for metal forming. These molds, commonly referred to as "permanent molds," are typically made of metals like gray cast iron, steel, bronze, or graphite. The main feature of this process is that the mold can be used multiple times, thereby increasing production efficiency and reducing costs. Typical applications of permanent mold casting include the production of parts such as studs, wheel rims, gears, gearboxes, pipe fittings, fuel injection system housings, and vehicle engine pistons.

Main Process Flow of Permanent Mold Casting

After understanding the basic concept of permanent mold casting, let's take a look at its main process flow. This process includes several key steps, each of which plays an important role in the quality of the final casting.

1. Mold Preheating

Before casting, the mold needs to be preheated, usually to a temperature between 150°C and 200°C. The purpose of preheating is to make the flow of metal smoother and reduce casting defects. A preheated mold can better withstand the impact of high-temperature molten metal and also helps to control the cooling rate of the casting.

2. Mold Preparation

After preheating, the mold needs to undergo surface treatment, typically by applying a refractory material or a mold release agent to the cavity. The purpose of this step is to prevent the casting from adhering to the mold, thereby extending the mold's service life and ensuring that the casting can be easily removed from the mold. In some complex casting processes, sand cores or metal inserts may be installed to form the internal structure of the casting.

3. Metal Filling

Pouring molten metal into the mold is the most critical step in the casting process. Depending on the casting method, the way the metal is filled also varies. Common filling methods include gravity casting, low-pressure casting, and vacuum casting. In gravity casting, the molten metal flows into the mold by its own weight; in low-pressure casting, low-pressure gas is used to push the molten metal into the cavity; vacuum casting controls the flow of metal through a vacuum environment. Different filling methods have a significant impact on the quality and performance of the casting.

4. Casting Cooling and Ejection

After the metal filling is complete, the casting needs to cool and solidify in the mold. To prevent hot cracking of the casting during cooling, the mold is usually opened immediately after the casting has solidified. During the ejection process, if the casting is difficult to remove from the mold, auxiliary elements such as ejector pins can be used. These elements may leave small circular marks on the surface of the casting, but they do not affect the overall quality of the casting.

5. Post-Processing

After ejection, the casting usually needs to undergo some post-processing operations, such as removing the gate, cleaning the surface, and performing heat treatment. The purpose of these operations is to improve the surface quality and mechanical properties of the casting, so that it can meet the requirements for actual use.

Main Types of Permanent Mold Casting

There are several different types of permanent mold casting, each with its own unique characteristics and application scenarios. These types include gravity casting, slush casting, vacuum casting, and low-pressure casting, all of which play important roles in various industrial applications.

1. Gravity Casting

Gravity casting is the most common method of permanent mold casting. It relies on the weight of the molten metal to flow into the mold, with a relatively slow filling speed. The advantages of gravity casting are simple equipment, easy operation, and low cost, making it suitable for producing castings with simple shapes and thicker walls. However, due to the slow filling speed, gravity casting is prone to defects such as porosity and slag inclusion, resulting in relatively poor mechanical properties of the casting.

2. Slush Casting

Slush casting is a special type of gravity casting, mainly used for producing hollow castings. The process involves pouring molten metal into the mold, allowing it to cool and form a shell, and then pouring out the remaining liquid to leave a hollow casting. The advantage of this method is material savings, enabling the production of lightweight and low-cost products. However, slush casting also has significant drawbacks. Due to the uneven wall thickness of the casting, it is prone to defects such as pinholes, and it requires a high level of skill from the operators.

3. Vacuum Casting

Vacuum casting is an advanced permanent mold casting technique. It creates a vacuum environment inside the mold, allowing the molten metal to fill the mold under negative pressure. The advantage of vacuum casting is that it can reduce the dissolved gases in the molten metal, improving the cleanliness and surface quality of the casting. The mechanical properties of the castings are usually 10% to 15% higher than those of gravity castings, and the surface roughness is excellent. However, the equipment cost for vacuum casting is high, and the mold's airtightness requirements are strict, which limits its practical application to some extent.

4. Low-Pressure Casting

Low-pressure casting is a casting method that uses low-pressure gas to push the molten metal into the cavity. The working principle is to place the molten metal in a sealed container and apply low-pressure gas (generally between 3 PSIg and 15 PSIg) to make the metal flow upward along the insulated pouring tube and finally enter the bottom of the mold. The advantages of low-pressure casting are smooth filling, fewer pores and slag, better mechanical properties of the casting, and high material utilization. In addition, low-pressure casting does not require a riser because the applied pressure can compensate for the shrinkage of the casting. However, the disadvantages of low-pressure casting are longer cycle times and higher equipment costs.

Advantages of Permanent Mold Casting

Permanent mold casting stands out among many casting methods because of its many significant advantages. These advantages not only improve production efficiency but also ensure the quality and performance of the castings, making it widely used in industrial manufacturing.

1. Reusable Mold

One of the greatest advantages of permanent mold casting is that the mold can be used multiple times. Compared with sand casting, permanent mold casting does not require the mold to be remade after each casting, greatly improving production efficiency and reducing production costs. Generally, the service life of a permanent mold can reach 10,000 to 120,000 cycles, depending on factors such as mold material, pouring temperature, mold temperature, and mold structure.

2. Good Surface Roughness and Dimensional Accuracy

Permanent mold casting can produce castings with excellent surface quality, with a surface roughness typically ranging from 2.5μm to 7.5μm RMS. This surface quality can meet the requirements in many applications without the need for additional machining. In addition, the dimensional accuracy of permanent mold casting is also high. Generally speaking, the tolerance for dimensions within 25mm is 0.4mm, with an additional 0.02mm for each additional centimeter. If the dimension spans across the parting line, an additional 0.25mm is added. Castings with such high precision have wide application value in fields such as mechanical manufacturing and automotive parts.

3. Rapid Cooling and Refinement of Grain Structure

Due to the good thermal conductivity of metal molds, the cooling rate of permanent mold casting is relatively fast. This rapid cooling allows the casting to form a finer grain structure, thereby improving the mechanical properties of the casting. Compared with sand casting, castings produced by permanent mold casting have higher strength and better toughness, meeting the requirements for higher performance applications.

4. Directional Solidification and Flexible Design

By changing the mold wall thickness or heating or cooling certain parts of the mold, permanent mold casting can achieve directional solidification. This directional solidification technology can effectively control the solidification sequence of the casting, reduce internal defects, and improve the quality of the casting. In addition, permanent mold casting can also use removable metal inserts to form complex undercut structures, allowing the mold to still operate quickly and meet the production needs of complex-shaped castings.

Disadvantages of Permanent Mold Casting

No process is perfect, and permanent mold casting is no exception. It also has some limitations in practical applications, which may affect its effectiveness and economy in certain specific scenarios.

1. High Mold Cost

Permanent mold casting molds are usually made of metal and have a high manufacturing cost. Especially for complex molds, a significant amount of time and money needs to be invested in the design and manufacturing process. The high mold cost makes permanent mold casting uneconomical for small-batch production and is generally suitable for large-scale production.

2. Restrictions on Molten Metal

Permanent mold casting is mainly suitable for low-melting-point metals such as aluminum, magnesium, and copper alloys. For high-melting-point metals like steel and iron, the service life of the permanent mold casting mold will be significantly reduced. This is because the pouring temperature of high-melting-point metals is higher, causing greater thermal shock and corrosion to the mold, thereby shortening the mold's service life.

3. Limited Mold Life

Although the mold in permanent mold casting can be used multiple times, its service life is still limited. Factors affecting mold life include mold material, pouring temperature, mold temperature, and mold structure. High pouring temperatures can lead to increased thermal fatigue and corrosion of the mold, shortening its service life. In addition, significant differences in the mold's cross-sectional thickness can also affect its service life. Generally, the service life of a permanent mold is between 10,000 and 120,000 cycles, depending on the above factors.

Application Fields of Permanent Mold Casting

Finally, let's take a look at its wide range of applications in actual production. With its unique advantages, permanent mold casting plays an indispensable role in many fields.

1. Automotive Industry

The automotive industry is one of the main application fields of permanent mold casting. Many automotive parts, such as wheel rims, engine pistons, and gearboxes, are produced through permanent mold casting. Permanent mold casting can meet the strict requirements of automotive parts for dimensional accuracy, surface quality, and mechanical properties, and its efficient production method can also meet the large-scale production needs of the automotive industry.

2. Mechanical Manufacturing

In the field of mechanical manufacturing, permanent mold casting is widely used to produce various mechanical parts, such as gears and pipe fittings. These parts usually require high strength and precision, which permanent mold casting can meet well. In addition, the characteristics of rapid cooling and refinement of grain structure in permanent mold casting also make the castings more wear-resistant and corrosion-resistant, thereby extending the service life of mechanical parts.

3. Aerospace Industry

The aerospace industry has extremely high requirements for the quality and performance of parts. Permanent mold casting can produce high-quality, high-performance castings, so it also has certain applications in the aerospace field. For example, some parts of aviation engines and aircraft structural components can be produced through permanent mold casting. These castings not only need to meet the requirements of high strength and high precision but also need to have good fatigue resistance and corrosion resistance.

Conclusion

Permanent mold casting, as an efficient metal forming process, holds an important position in modern industrial manufacturing. With its advantages of reusable molds, good surface roughness and dimensional accuracy, rapid cooling and refinement of grain structure, it has been widely used in fields such as automotive industry, mechanical manufacturing, aerospace industry, and ornament manufacturing. However, permanent mold casting also has disadvantages such as high mold cost, restrictions on molten metal, and limited mold life. With the advancement of science and technology and the development of industrial manufacturing, permanent mold casting will develop in the directions of intelligence, automation, new materials, new processes, environmental protection and sustainable development, and interdisciplinary integration.