Die casting process

Update:21-01-2019
Summary:

The traditional die casting process consists of four st […]

The traditional die casting process consists of four steps, or high pressure die casting. These four steps include mold preparation, filling, injection, and falling sand, which are also the basis for various improved die casting processes. During the preparation process, it is necessary to spray a lubricant into the cavity. In addition to helping to control the temperature of the mold, the lubricant can also help the mold to be demolded. The mold can then be closed and the molten metal injected into the mold with high pressure, which is in the range of about 10 to 175 MPa. When the molten metal is filled, the pressure is maintained until the casting solidifies. Then the push rod will push out all the castings. Since there may be multiple cavities in one mold, multiple castings may be produced during each casting process. The process of falling sand requires separation of debris, including mold openings, runners, gates, and flash. This process is usually done by extruding the casting through a special dressing die. Other sand falling methods include sawing and sanding. If the gate is relatively fragile, you can directly drop the casting, which saves manpower. Excess mold openings can be reused after melting. The usual yield is about 67%.
High pressure injection results in a very fast filling of the mold so that the molten metal can fill the entire mold before any part solidifies. In this way, surface discontinuities can be avoided even in thin-walled sections that are difficult to fill. However, this can also cause air to stay because the air is hard to escape when filling the mold quickly. This problem can be reduced by placing a vent on the parting line, but even a very precise process will leave a hole in the center of the casting. Most die castings can be used to perform structures that cannot be completed by casting, such as drilling and polishing.
Defects can be checked after the falling sand is completed. The most common defects include stagnation (pouring) and cold heading. These defects may be caused by insufficient temperature of the mold or molten metal, impurities in the metal, too little vent, and too much lubricant. Other defects include pores, shrinkage cavities, hot cracks, and flow marks. Flow marks are traces left on the surface of the casting by gate defects, sharp corners or excessive lubricant.
Water-based lubricants, known as emulsions, are the most commonly used type of lubricant for health, environmental, and safety reasons. Unlike solvent-based lubricants, if the minerals in the water are removed using a suitable process, it does not leave by-products in the casting. If the process of treating water is not appropriate, minerals in the water can cause surface defects and discontinuities in the casting. There are four main water based lubricants: water blending, oil blending, semi-synthesis and synthesis. Water-oiled lubricants are preferred because water can be used to release the mold by evaporation while cooling the surface of the mold while depositing the oil. Usually, the ratio of such a lubricant is 30 parts of water mixed with 1 part of oil. In extreme cases, this ratio can reach 100:1.
Oils that can be used in lubricants include heavy oils, animal fats, vegetable fats, and synthetic oils. The heavy residual oil has a higher viscosity at room temperature, and it becomes a film at a high temperature in the die casting process. Adding other substances to the lubricant controls the viscosity and thermal properties of the emulsion. These materials include graphite, aluminum and mica. Other chemical additives can avoid dust and oxidation. An emulsifier can be added to the water-based lubricant so that the oil-based lubricant can be added to the water, including soap, alcohol, and ethylene oxide.
Solvent-based lubricants commonly used include diesel and gasoline for a long time. They facilitate the casting out, but small explosions occur during each die casting process, which results in the accumulation of carbon on the walls of the cavity. Solvent-based lubricants are more uniform than water-based lubricants.

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