Die casting is a metal casting process characterized by the use of a mold cavity to apply high pressure to molten metal. Molds are usually machined from stronger alloys, a process somewhat similar to injection molding. Most die castings are iron-free, such as zinc, copper, aluminum, magnesium, lead, tin, and lead-tin alloys and their alloys. Depending on the type of die casting, a cold chamber dies casting machine or a hot chamber die casting machine is required.
Casting equipment and molds are expensive, so the die-casting process is generally only used for mass-produced products. Manufacturing die-cast parts is relatively easy, generally requiring only four major steps, with a very low incremental cost per item. Die casting is particularly suitable for the manufacture of a large number of small and medium-sized castings, so die casting is the most widely used of various casting processes. Compared with other casting techniques, die casting has a flatter surface and higher dimensional consistency.
Die casting process
The traditional die casting process mainly consists of four steps, or high pressure dies 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 to control the temperature of the mold, the lubricant can also help the casting release. The mold can then be closed and the molten metal injected into the mold at high pressure, which ranges from 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 since there may be multiple cavities in a mold, there may be multiple castings per casting. The process of falling sand requires the separation of residues, including mold openings, runners, gates, and flash. This process is usually done by squeezing the casting through a special trimming die. Other methods of falling sand include sawing and sanding. If the gate is fragile, you can directly beat the casting, which saves manpower. Excess mold openings can be reused after melting. The usual yield is about 67%.
High-pressure injection results in very fast filling of the mold so that the entire mold can be filled with molten metal 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 be reduced by placing vents on the parting line, 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.
Defects can be inspected after the fallout is complete. The most common defects include stagnation (under pouring) 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, and 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 will 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. There are four main types of water-based lubricants: water-oil, oil-water, semi-synthetic, and synthetic. Oil-in-water lubricants are the best because when using lubricants 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, this ratio can reach 100:1.
Oils that 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 other substances to the lubricant can control the viscosity and thermal properties of the emulsion. These substances include graphite, aluminum, and mica. Other chemical additives can avoid 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.
metal for die casting
The minimum cross-sectional area and the minimum draft angle corresponding to each material are listed in the following table, and the thickest section should be less than 13 mm.
| Metal | Minimum cross-sectional area | Minimum draft angle |
| Aluminum alloy | 0.89 mm (0.035 in) | 1:100 (0.6°) |
| Brass and Bronze | 1.27 mm (0.050 in) | 1:80 (0.7°) |
| magnesium alloy | 1.27 mm (0.050 in) | 1:100 (0.6°) |
| Zinc alloy | 0.63 mm (0.025 in) | 1:200 (0.3°) |
Metals used for die casting mainly include zinc, copper, aluminum, magnesium, lead, tin, and lead-tin alloys, although die-cast iron is rare but feasible. More special die-casting metals include ZAMAK, aluminum-zinc alloys, and American Aluminum Association standards: AA380, AA384, AA386, AA390, and AZ91D magnesium. The characteristics of die casting of various metals are as follows:
Zinc: The easiest metal to die-cast, economical to make small parts, easy to coat, high compressive strength, plasticity, and long-lasting life.
Aluminum: Lightweight, high dimensional stability when making complex and thin-walled castings, strong corrosion resistance, good mechanical properties, high thermal and electrical conductivity, and high strength at high temperatures.
Magnesium: Easy to machine, high strength-to-weight ratio, and the lightest of commonly used die-cast metals.
Copper: high hardness, strong corrosion-resistance, the best mechanical properties of commonly used die-casting metals, wear resistance, and strength close to steel.
Lead and tin: high density, very high dimensional accuracy, can be used as special corrosion protection parts. For public health reasons, this alloy cannot be used in food processing and storage equipment. Alloys of lead, tin, and antimony (sometimes with a little copper) can be used to make hand lettering in letterpress printing and bronzing.
The upper mass limits for die casting with aluminum, copper, magnesium, and zinc are 70 lbs (32 kg), 10 lbs (4.5 kg), 44 lbs (20 kg), and 75 lbs (34 kg), respectively.
Features and advantages and disadvantages
Die casting, referred to as die casting, is a casting method that pours molten alloy liquid into a pressure chamber, fills the cavity of a steel mold at a high speed, and solidifies the alloy liquid under pressure to form a casting. The main features of die casting that distinguish it from other casting methods are high pressure and high speed.
① The molten metal fills the cavity under pressure and crystallizes and solidifies under higher pressure. The common pressure is 15-100MPa.
② The molten metal fills the cavity at a high speed, usually at 10-50 m/s, and some can exceed 80 m/s (the linear speed of the cavity introduced through the inner gate—the speed of the inner gate), so the molten metal is The filling time is very short, and the cavity can be filled in about 0.01-0.2 seconds (depending on the size of the casting).
Die casting is a precision casting method. The dimensional tolerances of die castings cast by die casting are very small and the surface accuracy is very high. In most cases, die castings can be assembled and applied without turning. Parts can also be cast directly. Small parts such as general camera parts, typewriter parts, electronic computing devices, and ornaments, as well as complex parts of vehicles such as automobiles, locomotives, and airplanes, are mostly manufactured by die casting.
Advantages：
1. Good product quality
Castings have high dimensional accuracy, generally equivalent to 6~7 grades, even up to 4 grades; good surface finish, generally equivalent to 5~8 grades; high strength and hardness, the strength is generally 25~30% higher than that of sand casting, but the extension The rate is reduced by about 70%; the size is stable, and the interchangeability is good; it can die-cast thin-walled and complex castings. For example, the current minimum wall thickness of zinc alloy die castings can reach 0.3mm; aluminum alloy castings can reach 0.5mm; the minimum casting hole diameter is 0.7mm; the minimum pitch is 0.75mm.
2. High production efficiency
The machine has high productivity. For example, the domestic JIII3 horizontal cold air die-casting machine can die-cast 600-700 times on average in eight hours, and the small hot-chamber die-casting machine can die-cast 3,000-7,000 times on average every eight hours; Die-casting bell alloys have a lifespan of hundreds of thousands of times or even millions of times; it is easy to realize mechanization and automation.
3. Excellent economic effect
Due to the precise dimensions of the die-casting parts, the surface is smooth and clean. Generally, it is used directly without mechanical processing, or the processing volume is very small, so it not only improves the metal utilization rate but also reduces a lot of processing equipment and man-hours; the price of castings is easy; combination die-casting can be used to make other metal or non-metal materials. Saves both assembly man-hours and metal.
Disadvantages：
For example：
1). During die casting, due to the high speed of liquid metal filling the cavity and the unstable flow state, the general die casting method is used, and the casting is prone to pores and cannot be heat treated;
2). Die-casting is more difficult for complex concave castings;
3). High melting point alloys (such as copper, ferrous metals), the die-casting mold life is low;
4). It is not suitable for small batch production. The main reason is that the manufacturing cost of the die casting mold is high, the production efficiency of the die casting machine is high, and the small-batch production is uneconomical.
Die casting is one of the most advanced metal forming methods. It is an effective way to achieve fewer chips and no chips. It is widely used and developed rapidly. The size and weight of the casting depend on the power of the die casting machine. Due to the increasing power of the die-casting machine, the size of the casting can be from a few millimeters to 1-2m; the weight can be from a few grams to tens of kilograms. Abroad can die-cast aluminum castings with a diameter of 2m and a weight of 50kg.
Die casting parts are no longer limited to the automobile industry and instrument industry, and gradually expanded to other industrial sectors, such as agricultural machinery, machine tool industry, electronics industry, defense industry, computers, medical equipment, clocks, cameras, and daily hardware. Industry, specifically: auto parts, furniture accessories, bathroom accessories (bathroom), lighting parts, toys, shaver, tie clips, electrical and electronic parts, belt buckles, watch cases, metal buckles, locks, zippers, etc. In terms of die-casting technology, new technologies such as vacuum die-casting, oxygen-added die-casting, precision and fast-density die-casting and the application of soluble cores have emerged.
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