13
01.2021

The Process of Die Casting

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Die casting is a metal fabrication process usually used to produce metal castings with non-ferrous metals such as aluminum, zinc, and copper. Junying is specialized in aluminum die casting and zinc die casting.

 

The traditional die casting process is mainly composed of four steps, or called high-pressure die casting. These four steps include mold preparation, filling, injection, and sanding, which are also the basis of various improved die casting processes. During the preparation process, the lubricant should be sprayed into the mold cavity. The lubricant can not only help to control the temperature of the mold but also help to demould the casting. The mold can then be closed and molten metal is injected into the mold under high pressure, which ranges from about 10 MPa to 175 MPa. When the molten metal is filled, the pressure is maintained until the casting solidifies. Then the pushrod will push out all the castings. Since there may be multiple cavities in a mold, multiple castings may be produced in each casting process. In the process of sand falling, it is necessary to separate the residue, including mold gate, runner, gate and flash. This process is usually accomplished by squeezing the casting with a special trimming die. Other methods of sanding include sawing and grinding. If the gate is fragile, the casting can be broken directly, which can save manpower. The excess molding port can be reused after melting. The usual yield is about 67%.

 

High-pressure injection causes the mold to fill very quickly so that molten metal can fill the entire mold before any part solidifies. In this way, even thin-walled parts that are difficult to fill can avoid surface discontinuities. However, this can also cause air entrapment, as it is difficult for air to escape during the rapid filling of the mold. This problem can be reduced by placing vents on the parting line, but even very sophisticated processes can leave pores in the center of the casting. Most of the die casting can be completed by secondary processing, such as drilling and polishing.

 

The most common defects include stagnant flow (insufficient pouring) and cold scar. These defects may be caused by the insufficient temperature of the mold or molten metal, metal mixed with impurities, too few vents, too much lubricant and other reasons. Other defects include porosity, shrinkage, hot cracking and flow marks. Flow marks are the marks left on the surface of castings by gate defects, sharp corners, or excessive lubricant.

 

Water-based lubricants, known as emulsions, are the most commonly used type of lubricants for health, environmental and safety reasons. Unlike solvent-based lubricants, if the minerals in the water are removed by proper process, it will not leave by-products in the casting. If the water treatment process is not proper, the minerals in the water will cause surface defects and discontinuities. There are mainly four kinds of water-based lubricants: water mixed with oil, oil mixed with water, semi-synthetic and synthetic. The lubricant with water and oil is the best, because when the lubricant is used, the water will cool the surface of the mold through evaporation while depositing oil, which can help demoulding. Generally, the proportion of this kind of lubricant is 30 parts of water mixed with 1 part of the oil. In extreme cases, the ratio can reach 100:1.

 

Oils that can be used as lubricants include heavy oils, animal fats, vegetable fats and synthetic fats. Heavy residual oil has high viscosity at room temperature, but it will become thin film at high temperature in die casting process. The addition of other substances in the lubricant can control the viscosity and thermal properties of the emulsion. These materials include graphite, aluminum and mica. Other chemical additives can avoid dust and oxidation. Emulsifiers can be added to water-based lubricants, so that oil-based lubricants can be added to water, including soap, alcohol and ethylene oxide.

 

For a long time, commonly used solvent-based lubricants include diesel and gasoline. They are conducive to casting out, however, a small explosion occurs during each die casting process, which leads to the accumulation of carbon on the cavity wall. Solvent-based lubricants are more uniform than water-based lubricants.

發表於2021.1.13
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