Analysis of low-pressure casting technology for aluminum casting.
1 introduction to casting
The crankcase on a certain type of engine is an important part of the engine. The casting shape is 1414mm x 980mm x 480mm, the average wall thickness is 27mm, the blank weight is about 350kg, the geometric shape is complex, the hot section is much and dispersed. The material is ZL702A alloy. The mechanical performance requirements are as follows: the amount of the product is greater than 250MPa, and the value of the product is greater than 2.5%, and the hardness is 95 ~ 125HB. Pressure experiment requirements are as follows: water chamber on hydraulic pressure test, the water temperature of 40 ~ 60 ℃, pressure is 294 ~ 343 kpa, time for 5 min, can not have leakage. Casting is difficult.
2. Molding materials and tooling.
The mold is made of dry sand core. Low pressure casting requires high strength of cast type. PZPSET self-hardening resin sand is used in sand, which is high in strength, low in air volume, good in break and easy to clean. Because the low pressure casting liquid is solidified under certain pressure, the surface of the core is affected by heat and burn, which can produce the defect of sand. Therefore, the surface of the core is coated with 0.3-0.5mm thick alcohol or water-based coating. Due to the high temperature of the coating, it can prevent the surface slime phenomenon during low pressure casting.
The tooling is all metal. Manufacturing metal tooling, all tooling cavity module is first to use three dimensional CAD modelling, and then together, according to the drawing to check the size of each part, and then through the numerical control programming, to ensure the mold manufacturing success at a time, and adopted some new technology in mould design, such as plate cold core box, magnetic field generated by electromagnet electric power field theory, cold iron fixed electromagnet method, solves the cold core box core-making cold iron can't fixed the problem.
After using this technique, the efficiency of the core is greatly improved. Before using the cold core box, use the manual core, because of cold iron is placed, a baffle plate core is first divided into two cores, core making together, after a worker 4 h can 8 cores, can combine four plate core, with cold core box, one of the workers can 1 h 6 diaphragm core.
When designing the tooling, to ensure the dimensional accuracy, reduce the labor intensity of the workers, improve the production efficiency, and give priority to the use of machine core. Under the above type, medium, type, type core size is 1800 mm * 1100 mm * (200 ~ 400) mm, system of artificial cores, flip it very difficult to draw, and core boxes filled with resin sand, no heating equipment, curing speed, low efficiency, and the resin sand molding line core, have automatic turnover stripping mechanism and curing heating device. Water cavity core and transmission case core at the same time, due to the structure of the core itself restrictions, handmade core sand filling is difficult, and poor compactness, choose the MSL - H30 cold core machine after machine core, tight smooth surface, size stability, and high production efficiency, 1 h core 6 pieces.
3 casting process
The cast structure is divided into upper, middle and lower three layers. The lower type is the resin sand type with sand box structure, and the matching size accuracy of each core depends mainly on the precision of the mold. At the same time, a set of inspection guide is used to check the size of each part after adjusting the core.
The ZL702A is smelted in a resistance furnace. Alloy smelting, pour is preheated to 700 ℃ above casting crucible, with blowing argon rotating jet gas removal, prevent castings in most a thick pinhole.
The parameters of low pressure pouring process mainly include filling pressure, filling rate, shell time, pressure pressure, time, pressure keeping time and pouring temperature, etc. In low pressure casting, the filling pressure is generally 35 ~ 50kPa; Because of the sand type, and the casting wall is thicker, the slower filling speed can be selected, and the general control is 0.03 ~ 0.04m.
S - 1; The shell time is generally controlled at 15 ~ 20s; The pressurization pressure is generally controlled at 30 ~ 35kPa and the pressurization time is generally controlled at 5 ~ 7s. The time of holding pressure is related to the structure of the casting, the type of casting and the casting temperature of the alloy, etc., usually determined by the test, the large aluminum castings are generally controlled at 15 ~ 30min; At the low pressure casting, because the liquid metal filling conditions improved, and the good heat preservation, and directly from the seal pot into the mold, casting, pouring temperature general proportion force 10 ~ 20 ℃, low control at about 700 ℃.
Through constant research and practice, the quality of the castings is stable at more than 75% by selecting reasonable technological parameters.
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