Time:2023-12-12 Browse: 47
The traditional die casting process mainly consists of four steps, also known as high-pressure die casting. These four steps—mold preparation, filling, injection, and shakeout—serve as the foundation for various improved die casting processes. During the preparation phase, a lubricant is sprayed into the mold cavity. Besides helping control the mold temperature, the lubricant also facilitates the release of the casting from the mold. The mold is then closed, and molten metal is injected into it under high pressure, with the pressure typically ranging from 10 to 175 megapascals (MPa). Once the molten metal has fully filled the mold, the pressure is maintained until the casting solidifies. Ejector pins then push out all the castings. Since a single mold may contain multiple cavities, multiple castings can be produced in one casting cycle. The shakeout process involves separating excess material, including sprue, runners, gates, and flash. This is usually accomplished by squeezing the casting through a special trimming die. Other shakeout methods include sawing and grinding. If the gate is brittle, the casting can be directly knocked to remove the gate, which saves labor costs. The excess sprue can be reused after remelting, with a typical material yield rate of approximately 67%.
High-pressure injection results in an extremely fast mold filling speed, allowing the molten metal to fill the entire mold before any part solidifies. In this way, even hard-to-fill thin-walled sections can avoid surface discontinuities. However, this also leads to air entrapment, as air struggles to escape during rapid mold filling. This issue can be mitigated by installing vents along the parting line, but even highly precise processes may leave porosity at the center of the casting. Most die castings require secondary processing to achieve structures that cannot be formed directly through casting, such as drilling and polishing.
After shakeout, the castings are inspected for defects. The most common defects include misruns (incomplete filling) and cold shuts. These defects may be caused by insufficient mold or molten metal temperature, impurities in the metal, an inadequate number of vents, or excessive lubricant application. Other defects include blowholes, shrinkage cavities, hot cracks, and flow marks. Flow marks are surface blemishes left on the casting due to gate defects, sharp corners, or excessive lubricant.
Water-based lubricants, known as emulsions, are the most widely used type, chosen for health, environmental, and safety considerations. Unlike solvent-based lubricants, water-based lubricants do not leave by-products in the casting if minerals in the water are properly removed through appropriate treatment processes. Improper water treatment, however, can cause surface defects and discontinuities in the casting due to mineral residues. There are four main types of water-based lubricants: oil-in-water, water-in-oil, semi-synthetic, and synthetic. Oil-in-water lubricants are the most effective because, during application, the water cools the mold surface through evaporation while depositing the oil, which aids in casting release. Typically, the ratio of such lubricants is 30 parts water to 1 part oil, and in extreme cases, this ratio can reach 100:1.
Oils suitable for use in lubricants include heavy oil, animal fats, vegetable fats, and synthetic greases. Heavy residual oil has high viscosity at room temperature but forms a thin film at the high temperatures involved in the die casting process. Other substances can be added to lubricants to control emulsion viscosity and thermal properties, including graphite, aluminum, and mica. Additional chemical additives help prevent dust accumulation and oxidation. Emulsifiers, such as soaps, alcohols, and ethylene oxide, can be added to water-based lubricants to enable the incorporation of oil-based lubricants into water.
For a long time, commonly used solvent-based lubricants included diesel oil and gasoline. While they facilitate casting release, small-scale explosions occur during each die casting cycle, leading to carbon buildup on the mold cavity walls. Compared to water-based lubricants, solvent-based lubricants provide a more uniform coating.
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