Casting equipment and moulds are expensive, so die-casting technology is generally used to do mass production. It is relatively easy to manufacture die-casting parts. Usually, it only needs four main steps, and the increment of single cost is very low. Die casting is especially suitable for manufacturing a large number of small and medium-sized castings, so die casting is one of the most widely used casting processes. Compared with other casting technologies, the surface of die casting is smoother, with higher dimensional consistency.
Die casting is a metal casting process, which is characterized by the use of die cavity to exert high pressure on molten metal. Moulds are usually made of higher strength alloys, which are somewhat similar to injection moulding. Most die castings are iron-free, such as zinc, copper, aluminium, magnesium, lead, tin and lead-tin alloys and their alloys. According to the different types of die casting, use cold chamber die casting machine or hot chamber die casting machine.
On the basis of traditional die-casting process, several improved die-casting processes have been developed, including non-porous die-casting process to reduce casting defects and eliminate blowhole. Mainly used for processing zinc, it can also reduce waste and increase the yield of direct injection process. There are also new die-casting technologies such as precision die-casting technology invented by General Dynamics and semi-solid die-casting.
Traditional die casting process is mainly composed of four steps, or called high pressure die casting. These four steps include die preparation, filling, injection and sand drop, which are also the basis of various improved plate die casting processes. In the preparation process, lubricant is sprayed into the die cavity, and it not only helps to control the temperature of the die, but also helps to demould the casting. Then the die can be closed and the molten metal can be injected into the die at high pressure, which ranges from 10 to 175 MPa. When the molten metal is filled, the pressure will remain until the casting solidifies. The push rod then rolls out all the castings. Since there may be more than one cavity in a die, multiple castings may occur in each casting process. The process of sand drop requires the separation of residues, including moulding mouth, runner, gate and flying edge. This process is usually accomplished by squeezing the castings with a special dressing die. Other methods include sawing and grinding. The castings can be directly smashed if the gate is fragile, which can save manpower. Redundant moulding openings can be reused after melting. Output is usually about 67%.
High pressure injection results in a very rapid filling of the die, so that molten metal can fill the entire die before any part solidifies. In this way, surface discontinuity can be avoided even if the thin-walled part is difficult to fill. However, this will also lead to air retention, air is difficult to escape when filling the mould quickly. This problem can be reduced by placing exhaust vents on the parting line, but even very sophisticated processes can leave holes in the center of the casting. Drilling and polishing usually need secondary processing.
Defects can be inspected after sand drop. The most common defects include stagnation (unsatisfactory pouring) and cold scars. These defects may be caused by insufficient temperature of mould or molten metal, metal mixed with impurities, very few vents, too many lubricants and so on. Other defects include gas holes, shrinkage holes, thermal cracks and flow marks. Flow marks are traces left on the surface of castings by gate defects, sharp corners or excessive lubricants.
Oils that can be used as lubricants include heavy oils, animal fats, vegetable fats and synthetic fats. Heavy residual oil has a high viscosity at room temperature, but it will become thin film at high temperature in die casting process. The addition of other substances in the lubricant can control the viscosity and thermal properties of the emulsion. These substances include graphite, aluminium 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.
For a long time, commonly-used solvent-based lubricants include diesel and gasoline. They are conducive to casting stripping, but small explosions occur during each die casting process, which results in the accumulation of carbon elements on the cavity wall. Solvent-based lubricants are more uniform than water-based lubricants.
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