Die casting process of die casting parts
Traditional die casting mainly consists of four steps, also known as high-pressure die casting. These four steps include mold preparation, filling, injection, and sand removal, which form the basis of various modified die casting processes. During preparation, lubricant is sprayed into the mold cavity. Besides helping to control the mold temperature, the lubricant also aids in demolding the casting. Then, the mold is closed, and molten metal is injected into the mold under high pressure, ranging from approximately 10 to 175 MPa. Once the molten metal has filled, the pressure is maintained until the casting solidifies. Then, an ejector pushes out all the castings. Since a mold may have multiple cavities, multiple castings may be produced in each casting process. The sand removal process involves separating residues, including the mold gate, runner, sprue, and flash. This process is usually accomplished by squeezing the casting with a special finishing die. Other sand removal methods include sawing and grinding. If the sprue is fragile, the casting can be directly smashed, saving labor. Excess mold gates can be reused after melting.
High-pressure injection results in extremely fast mold filling, allowing molten metal to completely fill the mold before any part solidifies. This avoids surface discontinuities, even in thin-walled sections that are difficult to fill. However, this also leads to air trapping, as air is difficult to escape during rapid mold filling. This problem can be mitigated by placing vents on the parting line, but even with very precise processes, porosity may remain in the center of the casting. Most die castings can achieve structures that cannot be completed by casting through secondary machining, such as drilling and polishing.
After sand removal, defects can be inspected. The most common defects include incomplete filling (not filling the mold completely) and cold spots. These defects can be caused by insufficient mold or molten metal temperature, impurities in the metal, insufficient vents, or excessive lubricant. Other defects include porosity, shrinkage cavities, hot cracks, and flow marks. Flow marks are traces left on the casting surface by gate defects, sharp corners, or excessive lubricant.
Water-based lubricants, also known as emulsions, are the most commonly used type of lubricant due to health, environmental, and safety considerations. Unlike solvent-based lubricants, water-based lubricants do not leave byproducts in castings if minerals are removed using appropriate processes. Improper water treatment can lead to surface defects and discontinuities in castings. There are four main types of water-based lubricants: water-infused oil, oil-infused water, semi-synthetic, and synthetic. Water-infused oil lubricants are the best because, during lubrication, the water evaporates and cools the mold surface, aiding in demolding.
Oils that can be used as lubricants include heavy oils, animal fats, vegetable fats, and synthetic greases. Heavy residual oils have high viscosity at room temperature but become a thin film at the high temperatures of die casting. Adding other substances to lubricants can control emulsion viscosity and thermal properties. These substances include graphite, aluminum, and mica. Other chemical additives can prevent dust and oxidation. Emulsifiers can be added to water-based lubricants, allowing oil-based lubricants to be added to water; these include soap, alcohol, and ethylene oxide.
Long ago, solvent-based lubricants commonly used included diesel and gasoline. They facilitate casting removal; however, small explosions occur during each die-casting process, leading to carbon accumulation on the mold cavity walls. Solvent-based lubricants are more uniform than water-based lubricants.
