The traditional die casting process consists of four main steps, or high pressure die casting. These four steps include mold preparation, filling, injection and sanding, which are also the basis of various modified die casting processes. During the preparation process, a lubricant needs to be sprayed into the mold cavity. In addition to helping control the temperature of the mold, the lubricant can also help the casting to be demolded. Then the mold can be closed and the molten metal is injected into the mold with high pressure. The pressure range is about 10 to 175 MPa. When the molten metal is filled, the pressure will be maintained until the casting solidifies. Then the push rod will push out all the castings. Since there may be multiple cavities in a mold, multiple castings may be produced in each casting process. The sanding process requires separating the residue, including the mold gate, runner, gate and flash. This process is usually completed by squeezing the casting through a special trimming die. Other sanding methods include sawing and grinding. If the gate is fragile, the casting can be directly dropped, which can save labor. The excess mold gate can be reused after melting. The typical yield is about 67%.
High pressure injection results in a very fast filling of the mold, so that the molten metal fills 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 lead to air entrapment, as it is difficult for air to escape when the mold is filled quickly. This problem can be reduced by placing vents on the parting line, but even very precise processes will leave pores in the center of the casting. Most die castings can be completed by secondary operations such as drilling and polishing to complete structures that cannot be completed by casting.
After the sand is dropped, it is time to check for defects. The most common defects include stagnation (under-filling) and cold scars. These defects can be caused by insufficient mold or molten metal temperature, impurities in the metal, too few vents, too much lubricant, etc. Other defects include pores, shrinkage, hot cracks and flow marks. Flow marks are traces left on the surface of the casting due to gate defects, sharp corners or too much lubricant.
Water-based lubricants are called emulsions and are the most commonly used type of lubricant due to health, environmental and safety considerations. Unlike solvent-based lubricants, water does not leave byproducts in the casting if the minerals in the water are removed using the proper process. If the water treatment process is not done properly, minerals in the water can cause surface defects and discontinuities in the casting. There are four main types of water-based lubricants: water-in-oil, oil-in-water, semi-synthetic, and synthetic. Water-in-oil lubricants are the best because the water cools the mold surface by evaporation while depositing the oil during lubrication, which can help with mold release. Typically, this type of lubricant is mixed with 30 parts water to 1 part oil. In extreme cases, the ratio can be as high as 100:1.
Oils that can be used for lubricants include heavy oils, animal fats, vegetable fats, and synthetic greases. Heavy residual oils are viscous at room temperature, but they become thin films at the high temperatures of the die casting process. Other substances are added to the lubricant to control the viscosity and thermal properties of the emulsion. These substances include graphite, aluminum, and mica. Other chemical additives can prevent dust and oxidation. Emulsifiers can be added to water-based lubricants so that oil-based lubricants can be added to water, including soaps, alcohols, and ethylene oxide.
Solvent-based lubricants have long been used, including diesel and gasoline. They facilitate casting ejection, but small explosions occur during each die casting process, which causes carbon to accumulate on the cavity wall. Solvent-based lubricants are more uniform than water-based lubricants.
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