Wed Jun 07 22:51:20 CST 2023
The traditional die casting process consists of four main steps, or high pressure die casting. These four steps include mold preparation, filling, injection, and sand drop, and they are the basis for various modified versions of the die casting process. During the preparation process, the mold cavity is sprayed with lubricant, which helps to control the temperature of the mold and also helps to release the castings from the mold. The mold is then closed and the molten metal is injected into the mold at high pressure, which ranges from about 10 to 175 MPa. Once the molten metal is filled, the pressure is maintained until the casting has solidified. The pusher then pushes out all the castings, and since there may be more than one cavity in a mold, more than one casting may be produced per casting process. The sand drop process then requires the separation of residues, including mold builders, runners, gates, and flying edges. This process is usually accomplished by extruding the casting through a special dressing die. Other methods of sanding include sawing and grinding. If the sprue is more fragile, the casting can be dropped directly, which saves labor. Excess mold making sprues can be reused after melting. The typical yield is about 67%.
High pressure injection results in filling the mold very quickly 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 trapping, as it is difficult for air to escape when filling the mold quickly. This problem can be reduced by placing air vents on the parting line, but even very precise processes can leave air holes in the center of the casting. Most die casting can be done by secondary processing to complete some structures that cannot be done by casting, such as drilling and polishing.
Once the sand has been dropped, it is time to check for defects, the most common of which include stagnation (under-pouring) and cold scars. These defects can be caused by insufficient mold or molten metal temperature, metal mixed with impurities, too little venting, too much lubricant, etc. Other defects include porosity, shrinkage, thermal cracking, and flow marks. Flow marks are left on the surface of the casting by gate defects, sharp corners, or excessive lubricant.
Water-based lubricants, called 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 the casting if the minerals in the water are removed using a proper process. Minerals in the water can cause surface defects and discontinuities in the casting if the water is not treated properly. 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 when the lubricant is used the water cools the surface of the mold by evaporation while depositing oil, which can aid in release. Usually, the ratio of this type of lubricant is 30 parts of water to 1 part of oil. In extreme cases, this ratio can be as high as 100:1.
Oils that can be used as lubricants include heavy oils, animal fats, vegetable fats, and synthetic fats. Heavy residual oils are more viscous at room temperature, while at high temperatures in the die-casting process, it turns into a thin film. Other substances are added to the lubricant to control the emulsion viscosity as well as the 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 so that oil-based lubricants can be added to water, including soap, alcohol, and ethylene oxide.
Solvent-based lubricants that have long been used include diesel as well as gasoline. They facilitate casting release, however, small explosions occur during each die casting process, which leads to the accumulation of carbon elements on the cavity walls. Solvent-based lubricants are more uniform than water-based lubricants.