pressure casting is a liquid or semi-solid metal or alloy, or a liquid metal or alloy containing reinforcing phase in a relatively high speed under high pressure is filled into the die casting mold cavity, and metal or alloy under pressure casting solidifies to form the casting method. Commonly used pressure die casting 4 ~ 500MPa, the metal filling rate is 0.5-120m / s. Therefore, high-pressure, high-speed die casting method is a fundamental difference with other casting methods, it is also an important feature. In 1838 the first American Indian newspaper casting method type pressure, pressure casting patent appeared the following year. After the 1860s, pressure casting process has been greatly developed, not only capable of producing tin-lead alloy die casting, zinc alloy die casting, but also the production of aluminum, copper alloy and magnesium alloy die castings. After 30 years of iron and steel were tested pressure casting method in the 20th century.
pressure casting principle is the principle of injection molding of molten metal. Casting is generally set by the speed that the die casting machine, to adjust the pressure and speed switching position, the other select line type casting.
1, the advantages of pressure casting
1) high productivity, easy to implement mechanization and automation, complex shapes can be produced thin-walled castings. Zinc alloy minimum wall thickness of only 0.3mm, aluminum die casting minimum wall thickness of approximately 0.5mm, the smallest aperture cast 0.7mm.
2) casting a high dimensional accuracy, surface roughness small. Die casting dimensional tolerance level of up to CT3 ~ CT6, the surface roughness is generally Ra0.8 ~ 3.2μm.
3) may be embedded in the casting die casting parts, saving valuable material and machining working hours, alternatively member assembly process, assembling process can be omitted, simplifying the manufacturing process.
2, the pressure casting disadvantage
1) when the liquid metal die casting high filling speed, it is difficult to completely eliminate gas cavity, casting cracks and pores and prone to disaster oxide debris defects, die-casting is usually not heat treated.
2) of the casting mold complex structure, long manufacturing cycle, high cost, are not suitable for small batch production casting.
3) high cost die casting machine, a large investment by the clamping force and limit the size of the die casting machine, not suitable for large-scale production of die castings.
4) limited by the type of alloy, zinc, magnesium, copper, and other nonferrous alloys.
in the production of casting, die casting machine, and casting the die-cast alloy are three elements. Combination die-casting process sucked the three elements of the right to work and make use of the process. So that various process parameters to meet the needs of die casting. Injection pressure selection should be based on different casting alloys and structural properties. Selection of the filling speed, generally thick-walled or high internal quality casting, filling should choose a lower speed and a high supercharging pressure; for thin-walled or high surface quality of castings and casting complex, should be selected higher than the calendar, and high speed filling.
means that the pouring temperature minoxidil when the average temperature of the liquid metal into the cavity, since the temperature of the liquid metal chamber pressure is not measured convenient, generally indicated by the temperature in the holding furnace.
pouring temperature is too high, large shrinkage, the casting cracks easily occur, a large crystal grain, can also cause stick-type; pouring temperature is too low, easy to produce cold shuts, misrun surface patterns and other defects. Thus the pouring temperature and the pressure should be, the die casting mold and the filling speed taking the temperature.
type casting temperature
type pressure casting before use is preheated to a certain temperature, and more generally with a gas, burner, or electrical induction heating.
in the continuous production, die casting mold temperature is often elevated, especially in casting refractory alloys, increases quickly. Except that the temperature is too high viscosity to produce liquid metal type, the slow cooling of the casting, coarse crystal grains. Therefore, when the die casting mold temperature is too high, cooling measures should be adopted. Typically cooled with compressed air, water or chemical media.
filling pressure sustaining
filling, holding pressure and open type time
1) filling time
since the beginning of the liquid metal into the cavity play a cavity filled with dead, known as the time required to fill time. Filling the length of time depends on the size of the casting volume and complexity. Large and simple casting, filling time to be relatively long, complex and thin-walled castings filling time to be shorter. Filling the gate time or gate within the cross-sectional area is closely related to the size of width and thickness, it must be determined correctly. When
2) and the holding pressure time-open
liquid metal is filled from the gate into the cavity is completely solidified, the continued duration of action of the injection plunger, called holding pressure time. The length of time depends on holding pressure casting materials and wall thickness.
on type after holding pressure should remove the cast piece. From the end of the injection die casting opening time, it is called open-type time-open time should be controlled accurately. -Open time is too short, since the strength of the alloy is still low, and may cause a deformation of the casting from the top of the die casting mold down; open type, but time is too long, the casting temperature is too low, a large contraction, and the top of core pulling out the casting resistance is also large. General-open time by 1 mm thickness of the casting to be 3 seconds is calculated, and any adjustment by the test.
casting process, the casting and the die casting mold in order to avoid welding, casting ejector to reduce frictional resistance and avoid excessive heating and the die casting mold the use of paint. Requirements of the coating:
1) at high temperatures, has good lubricity;
2) low volatility point, at 100 ~ 150 ℃, volatilize quickly diluent ;
3) no casting and die casting corrosive action type;
4) performance stability in air of volatile diluent should not be over-thickened;
5) does not precipitate harmful gases at a high temperature;
6) does not occur in the casting cavity surface fouling.
castings cleaning work is very heavy, its workload is often 10 to 15 times the casting work. Therefore, with increasing die casting machine productivity, increase in production, casting cleaning work mechanization and automation are very important.
1) and the gate burr removal
and the gate burr removal apparatus used mainly presses, hydraulic presses and friction, members in mass production, according to the casting architecture and design special mold shape to complete the clean-up task in a punch.
2) surface cleaning and polishing
ordinary multi-surface cleaning and vibration polygonal drum cleaning device buried. Small quantities of small pieces of simple, clean polygonal drum can be used, requiring high surface decorations, cloth or leather can be used buff polishing. It can be used for mass production of casting shells type vibration cleaning machine.
castings after cleaning in accordance with requirements, and may also be impregnated with surface treatment to increase the gloss, to prevent corrosion and improve airtightness.
Because of the advantages of casting it to obtain a wide range of applications, mainly used in mass production of non-ferrous castings. In the production of die castings, account for the largest proportion of the aluminum die casting, from 30% to 50%; followed by zinc alloy die casting; copper alloy die castings from 1% to 2%. Most die casting applications in the automotive, tractor manufacturing, followed by electronic equipment and instrument manufacturing industry, again for the agricultural machinery, defense industry, computers, medical equipment and other manufacturing industries. Parts produced by die casting an engine cylinder block, cylinder head, transmission housing, the hood, and the camera housing and the instrument holder, pipe joints, gears and the like.
In recent years, technology has been used in die casting field. The three-stage injection pressure control means, the injection speed and the type of gas. Development of special die casting process (such as vacuum casting, directional entraining casting, die casting, etc.) and the application of computer control, effectively remove porosity and improve the casting density, while the development of new materials and heat treatment mold new technology to extend the life of the pressure type, the ferrous metal die-casting has been some progress.