Die casting process consists of four steps, which can be called high pressure die casting. These four steps, including mold preparation, filling, injection, and shakeout, are also the basis for various modified die casting processes.
Lubricant is sprayed into the mold cavity during the preparation process. In addition to helping control the temperature of the mold, the lubricant can also help the casting release. The mold can be closed and the molten metal injected into the mold at high pressure, which ranges about 10 to 175 MPa.
After the molten metal is filled, the pressure is maintained until the casting solidifies. The push rod then pushes out all the castings, and there may be multiple cavities in a mold, there may be multiple castings per casting. The process of falling sand requires separation of residues, including mold openings, runners, gates, flash.
This process is done by squeezing the casting through a special trim die. Other methods of include sawing and sanding. If the gate is fragile, you can directly beat the casting, which can save manpower. Excess mold openings can be reused after melting. The usual yield is about 67%.
High-pressure injection causes the mold to be filled very quickly, so that 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 also leads to air entrapment, as it is difficult for air to escape when filling the mold quickly.
This problem can placing vents on the parting line to reduce, but even very precise processes can leave porosity in the center of the casting. Most die castings can complete some structures that cannot be done by casting through secondary processing, such as drilling and polishing.
After the fallout is complete, defects can be inspected. The most common defects include stagnation (underfill) and cold scars. These defects can be caused by insufficient mold or molten metal temperature, metal contamination, too few vents, too much lubricant, etc.
Other defects include porosity, shrinkage, thermal cracking, flow marks. Flow marks are marks left on the casting surface by gate defects, sharp corners, or excess 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, it does not leave by-products in castings if the minerals in the water are properly removed by a process. If the water is not treated properly, minerals in the water can cause surface defects and discontinuities in the casting.
Four main types of water-based lubricants: water-oil, oil-water, semi-synthetic, and synthetic. Oil-in-water lubricants are the best, because when lubricants are used, the water cools the surface of the mold by evaporation while depositing the oil, which can aid in mold release. Typically, the ratio of such lubricants is 30 parts water to 1 part oil. In extreme cases, ratio can reach 100:1.
Oils can be used in lubricants include heavy oils, animal fats, vegetable fats, and synthetic fats. Heavy residual oil is more viscous at room temperature, and at the high temperatures of the die casting process, it becomes a thin film. The addition of the lubricant can control thermal properties of the emulsion. Include graphite, aluminum, and mica.
Other chemical additives can avoid dust and oxidation. Emulsifiers can add to water-based lubricants so that oil-based lubricants can be added to water, including soap, alcohol, and ethylene oxide.
Historically, solvent-based lubricants commonly used include diesel and gasoline. They are good for casting ejection, however small explosions occur during each die casting process, which results in a build-up of carbon on the cavity walls. Solvent-based lubricants are more uniform than water-based lubricants.