Gravity casting refers to the process of injecting molten aluminum into the mold by gravity under the action of earth gravity. Generalized gravity casting is divided into: sand casting, metal mold (steel mold) casting, lost wax casting, etc.

The narrow sense of gravity casting specifically refers to metal mold casting. Metal mold casting is divided into vertical manual casting, inclined casting is now the most widely used metal mold (steel mold) casting. The die is made of heat-resistant alloy steel, and the strength, size and appearance of the cast aluminum are higher than those of other casting processes. The liquid aluminum in gravity casting is usually poured into the gate by hand. Depending on the self weight of the liquid metal, it fills the mold cavity, exhausts, cools, and opens the mold to get the sample. The process flow is generally as follows: molten aluminum smelting, pouring material filling, exhausting, cooling, opening the mold, clearing production, heat treatment, and processing.

The characteristics of aluminum alloy gravity casting parts are as follows:

1. The surface finish of the product is not high, and it is easy to produce pits after shot blasting.

2. There are few pores in aluminum casting, so it can be heat treated.

3. The product has low density, low strength, but high elongation.

4. The cost of the mould is low and the service life of the mould is long.

5. The low production efficiency increases the production cost.

6. The process is simple and not suitable for thin-walled parts.

7. Special aluminum alloy with low fluidity can be used.

When choosing which process to produce the product. The selection is mainly based on the wall thickness of the workpiece. When the wall thickness of the product is greater than 8 mm, die casting will cause a lot of pores in the wall, so the products with thicker wall thickness can choose the gravity casting process.

Introduction to pressure casting:
Pressure casting is divided into high pressure casting and low pressure casting.

High pressure casting is commonly known as die casting. Which liquid aluminum is poured into the pressure chamber, and the mold cavity is filled with high speed by its pressure, and the liquid aluminum is solidified under pressure to form an aluminum casting.

High pressure die casting: a casting method in which liquid or semi-solid metal or alloy, or liquid metal or alloy containing reinforcement phase. It is filled into the cavity of the die casting mold at a high speed under high pressure, and the metal or alloy is solidified under pressure to form a casting. The commonly used pressure for die casting is 4-500mpa, and the metal filling speed is 0.5-120m/s. Therefore, the filling time of liquid metal is very short, and the cavity can be filled in about 0.01-0.2 seconds (depending on the size of the casting). Therefore, high pressure and high speed are the fundamental differences and important characteristics between die casting and other casting methods.

The characteristics of aluminum alloy die castings are as follows

1. The surface finish of the product is good, generally up to ra6.3 or even Ra1.6.

2. Do not heat treat.

3. The product has high air tightness, high strength and surface hardness of casting, but low elongation, too thick wall is easy to produce pores.

4. The cost of the mould is high and the service life is short.

5. High production efficiency.

6. It can produce thin-walled parts with small machining allowance.

7. Special aluminum alloys cannot be used.

Low pressure casting: in the sealed crucible (or sealed pot), dry compressed air is introduced. Under the action of gas pressure, the liquid metal rises along the riser into the sprue, and smoothly enters the mold cavity through the sprue. Also the gas pressure on the liquid level in the crucible is maintained. Until the casting is completely solidified. Then, the gas pressure on the liquid surface is released to make the liquid metal in the riser flow back to the crucible. Re-open the mold and take out the casting. The results show that this casting method has the advantages of good feeding, compact structure, easy to produce large thin-walled and complex castings without riser, and the metal yield can reach 95%. No pollution, easy to realize automation. But the equipment cost is higher and the production efficiency is lower. Generally used for casting nonferrous alloys.

The characteristics of aluminum alloy low pressure parts are as follows:

1. The surface difference of product is stronger than that of die casting

2. There are few pores in aluminum casting, so it can be heat treated.

3. The strength and surface hardness of the casting are higher than those of die casting, but the elongation is lower than that of gravity casting

4. The cost of the mould is low and the service life is low.

5. Production efficiency is lower than gravity, far lower than die casting.

6. It is used to produce high quality castings with complex process.

7. Special aluminum alloy with low fluidity can be used.

In short, low pressure is a compromise between high pressure and gravity.