The traditional die casting process is mainly composed of four steps, or high pressure die casting. These four steps include mold preparation, filling, injection and sand falling, which are also the basis of various modified die casting processes. During the preparation process, lubricant needs to be sprayed into the mold cavity. In addition to helping to control the temperature of the mold, the lubricant can also help to demold the casting. Then the mold can be closed and molten metal is injected into the mold with high pressure. This pressure range is approximately 10 to 175 MPa. When the molten metal is filled, the pressure will remain 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 during each casting process. The process of falling sand requires the separation of residues, including the mold opening, runner, gate and flash. This process is usually done by extruding the casting with a special trimming die. Other sand falling methods include sawing and sanding. If the gate is relatively fragile, you can directly beat the casting, which can save manpower. Excess molding ports can be reused after melting. The usual yield is about 67%.
High-pressure injection causes the mold to fill very quickly, so that the molten metal fills the entire mold before any part solidifies. In this way, even thin-walled parts that are difficult to fill can avoid surface discontinuities. However, this also leads to air entrapment, because 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 sophisticated processes will leave holes in the center of the casting. Most die-casting can complete some structures that cannot be completed by casting, such as drilling and polishing.
After the sand fall is complete, you can check for defects. The most common defects include stagnant flow (unsatisfactory pouring) and cold scars. These defects may be caused by insufficient temperature of the mold or molten metal, impurities in the metal, too few vents, and too much lubricant. Other defects include pores, shrinkage, thermal cracking, and flow marks. Flow marks are traces left on the casting surface due to gate defects, sharp corners or excessive lubricant.
Water-based lubricants are called emulsions and are the most commonly used types of lubricants, due to health, environmental and safety considerations. Unlike solvent-based lubricants, if the minerals in the water are removed by a suitable process, it will not leave by-products in the casting. If the water treatment process is improper, the minerals in the water can cause defects and discontinuities in the casting surface. There are four main types of water-based lubricants: water blended oil, oil blended water, semi-synthetic and synthetic. Water-oiled lubricants are the best, because when using lubricants, water will cool the surface of the mold by evaporation while depositing oil, which can help demolding. Generally, the ratio of this type of 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 for lubricants include heavy oils, animal fats, vegetable fats, and synthetic fats. Heavy residual oil has a higher viscosity at room temperature, but it will become a thin film at high temperatures in the die-casting process. Adding other substances to the lubricant can control the 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 so that oil-based lubricants can be added to water, including soap, alcohol, and ethylene oxide.
For a long time, commonly used solvent-based lubricants include diesel and gasoline. They are helpful for castings to come out, however, a small explosion occurs during each die-casting process, which causes carbon to accumulate on the cavity walls. Compared to water-based lubricants, solvent-based lubricants are more uniform.