What is gravity casting?
Gravity casting and Counter—Gravity Casting(Vacuum casting)
Gravity casting refers to the process of injecting molten metal into a mold under the action of the earth’s gravity, also known as gravity casting.
The generalized gravity casting includes sand casting, metal casting, investment casting, lost foam casting, mud casting, etc. The narrow gravity casting mainly refers to metal casting.
Classification of gravity casting
There are many process methods to make metal materials into required products. Such as casting, forging, extrusion, rolling, drawing, stamping, cutting, powder metallurgy and so on. Among them, casting is the most basic, most commonly used and most extensive process.
The molten metal is poured into a hollow mold made of high-temperature resistant materials. And the product with the desired shape is obtained after condensation. This is casting. The resulting product is a casting.
Classified by casting material
Divided into ferrous metal casting (including cast iron, cast steel) and non-ferrous metal casting (including aluminum alloy, copper alloy, zinc alloy, magnesium alloy, etc.).
Non-ferrous precision casting factory specializes in non-ferrous metal casting, focusing on aluminum alloy and zinc alloy casting.
According to the material of the mold
Divided into sand casting and metal casting. Precision casting factories are very comfortable with these two casting processes, and design and manufacture these two types of casting molds by themselves.
Classified by molten metal pouring process
Divided into gravity casting and pressure casting.
Gravity casting refers to the process of injecting molten metal into a mold under the action of the earth’s gravity, also known as casting. The generalized gravity casting includes sand casting, metal casting, investment casting, lost foam casting, mud casting, etc.; the narrow gravity casting specifically refers to metal casting.
Pressure casting refers to the process of injecting molten metal into the mold under the action of other external forces (excluding gravity). Die casting in a broad sense includes pressure casting and vacuum casting of die casting machine, low pressure casting, centrifugal casting, etc.; pressure casting in a narrow sense refers to metal die casting of die casting machine, referred to as die casting.
The precision casting factory has long been engaged in the gravity casting of sand and metal molds. These casting processes are the most commonly used in non-ferrous metal casting and the lowest relative price.
Sand casting
Sand casting is a traditional casting process that uses sand as the main modeling material to make molds. Gravity casting is generally used for sand molds, and low-pressure casting, centrifugal casting and other processes can also be used when there are special requirements. Sand casting has a wide range of adaptability. Small pieces, large pieces, simple pieces, complex pieces, single pieces, and large quantities can be used.
Sand casting molds were mostly made of wood in the past, but later changed to aluminum alloy molds or resin molds with higher dimensional accuracy and longer service life. Although the price has increased, it is still much cheaper than the mold for metal casting. In the production of small batches and large parts, the price advantage is particularly prominent.
In addition, sand molds have higher refractoriness than metal molds. So materials with higher melting points such as copper alloys and ferrous metals also use this process.
Disadvantages of sand casting
However, sand casting also has some shortcomings: because each sand mold can only be poured once. The mold is damaged after the casting is obtained and must be reshaped. So the production efficiency of sand casting is low.
And because the overall nature of sand is soft and porous. The dimensional accuracy of sand castings is low, and the surface is rough. However, the precision casting factory has accumulated many years of technology accumulation. And has greatly improved the surface condition of sand castings, and the effect after shot blasting is comparable to that of metal castings.
Metal mold casting
It is a modern technology for making hollow casting molds for casting with heat-resistant alloy steel. The metal mold can be either gravity casting or pressure casting. The metal mold can be used repeatedly. Every time the molten metal is poured, a casting is obtained. It has a long life and high production efficiency.
The metal casting not only has good dimensional accuracy, but also has a smooth surface. And in the case of pouring the same molten metal. The strength of the casting is higher than that of the sand mold. And it is less likely to be damaged. Therefore, in the mass production of medium and small non-ferrous metal castings. As long as the melting point of the casting material is not too high, metal casting is generally preferred.
Disadvantage of metal mold casting
Because heat-resistant alloy steel and the processing of making a hollow cavity on it are relatively expensive, the cost of metal molds is expensive, but the overall cost of die-casting molds is much cheaper. For small batch production, the mold cost allocated to each product is obviously too high, which is generally not easy to accept.
And because the metal mold is limited by the size of the mold material and the capacity of the cavity processing equipment and the casting equipment, it is also helpless for particularly large castings. Therefore, metal mold casting is rarely used in the production of small batches and large parts.
In addition, although the metal mold adopts heat-resistant alloy steel, its heat-resistant ability is still limited. Generally, it is mostly used for casting of aluminum alloy, zinc alloy, and magnesium alloy. It has been less used in copper alloy casting, but is used for ferrous metal casting. It’s even less.
Die casting
Die casting is a metal mold pressure performed on a die casting machine, which is the most efficient casting process. Die casting machines are divided into hot chamber die casting machines and cold chamber die casting machines.
The hot chamber die casting machine has a high degree of automation, less material loss, and higher production efficiency than the cold chamber die casting machine. However, due to the heat resistance of the machine parts, it can only be used for the production of low melting point materials such as zinc alloy and magnesium alloy.
Aluminum alloy die castings, which are widely used today, can only be produced on cold chamber die casting machines due to their high melting point. The main feature of die casting is that molten metal fills the cavity under high pressure and high speed, and is formed and solidified under high pressure.
Disadvantages of die casting
Because the molten metal fills the cavity under high pressure and high speed, the air in the cavity is inevitably entrapped inside the casting, forming subcutaneous pores, so aluminum alloy die castings are not suitable for heat treatment, and zinc alloy die castings are not suitable for surface spraying. (But can be painted).
Otherwise, when the pores inside the casting are heated for the above treatment, they will expand when exposed to heat and cause the casting to be deformed or bubbling.
In addition, the machining allowance of die castings should also be smaller. Generally, it is about 0.5mm, which can reduce the weight of castings and reduce the amount of cutting to reduce costs. It can also avoid penetrating the dense layer of the surface and exposing the subcutaneous pores, causing the workpiece to be scrapped.
Counter—Gravity Casting (CGC)
Counter-gravity casting, also called anti-gravity casting or vacuum casting.
It is a casting forming process developed in the 1950s. It is the application of Pascal’s principle in casting production.
As far as its process is concerned, it is a liquid forming method between pressure casting and gravity casting. The driving force of the alloy liquid filling the mold is opposite to the direction of gravity. And the alloy liquid flows in the direction opposite to the gravity.
Compared with gravity casting
During the anti-gravity casting liquid forming process. The flow state of the melt is controllable. And the solidification feeding ability can be enhanced through the action of external force. Therefore, this process can achieve stable liquid filling, dense casting structure, and effective casting defects. Control is the preferred method for producing high-quality castings.
Become a controllable process.
During the filling process of molten metal, the filling of different filling speeds can be realized by controlling the magnitude of the applied force to meet the requirements of different processes.
At the same time, the external force can continue to be increased after the filling is completed. The casting is solidified under a relatively large force, and the feeding ability of the molten metal is improved. Reduce casting defects such as shrinkage, porosity and pinholes.
In recent decades, a variety of CGC methods have emerged one after another. According to the different forms of application of the driving force of the molten metal filling mold, CGC technology can be divided into low pressure casting, differential pressure casting, and pressure regulating casting.
