The main machine of excavator balance weight is an export product, and the requirements of surface quality and geometric size are very strict. In the past, resin sand casting technology was mostly used; However, the resin sand casting process should be equipped with special tooling. In addition, the recycling rate of old sand is low and the efficiency is not high. In 2010, ZHY casting began to produce the balance weight of sw18u excavator by vacuum casting process, which was successful. The balance weight casting of the excavator weighs 445kg, the thinnest wall is only 20mm thick, and the material is HT150. The requirement for this product is that the casting surface of the balance weight of the excavator shall be free of cracks, bulges, slag inclusion, sand flushing and other defects, and the surface roughness shall be 6.3 ~ 12.5 μ m。
1. Production conditions
Production equipment: vacuum molding machine, mobile drying rack, vacuum negative pressure system, professional sand box (upper box size: 2500mm) × 1500mm × 300mm, lower box size: 2500mm × 1500mm × 600mm）； 2T induction furnace, 2T pouring ladle.
2. Analysis of casting structure and technology
See the figure for the casting shape of excavator balance weight. As the balance weight casting of excavator is the balance weight of export excavator, it is required that the balance weight casting of excavator must be free of shrinkage cavity, air hole, slag inclusion, sand flushing and other defects; The surface quality shall be smooth and smooth; The geometric dimension error is + 1.5mm, and the upper and lower surfaces of the excavator balance weight casting must be processed. It is required that the structure of the excavator balance weight casting is dense. Therefore, in the design of gating system, the flow of liquid metal, exhaust, slag discharge and air outlet of sand core must be considered. It is made of two pieces in one box.
3. Process design
In order to ensure the quality of the balance weight casting of the excavator and prevent the occurrence of sand flushing defects, the gating system adopts a semi closed gating system, and the sand core is made of water glass sand. The sectional area of the ingate is 16cm2; If the internal sprue is 4, the cross-sectional area of each internal sprue is f = 16 ÷ 4 = 4.0cm2; The sprue cross-sectional area ratio is: F straight ∶ f transverse ∶ f inner = 1.2 ∶ 1.4 ∶ l.0, and the pouring time is 35 ~ 40s.
3.1 Key points of mold design
(1) Due to the large shape of the balance weight casting of the excavator and the large internal cavity, the mold is designed as two pieces in one box.
(2) The mold is divided according to the maximum section of the excavator balance weight casting, and the pattern shrinkage rate is 1%. The inner sprue of the excavator balance weight casting is set at the sand mold under the foundation of the casting. The machining allowance of the machining surface of the excavator balance weight casting is 4mm, and the machining allowance is reserved on the upper surface of the pouring position.
(3) The mold is designed as a symmetrical structure, with a sprue in the middle, upper and lower box modeling, and the outer surface of the balance weight casting of the excavator is placed in the lower box to ensure that the surface quality of the casting meets the technical requirements.
(4) To ensure the effective strength of the sand mold, the strength and sand consumption of the sand mold shall be considered in the selection of the sand box. The total height of the upper and lower sand boxes is 900mm.
3.2 Molding process
The production of excavator balance weight castings by V method requires not only the surface finish and dimensional accuracy of castings, but also the internal organization and performance of excavator balance weight castings. Therefore, it is necessary to prevent the occurrence of defects such as sand holes, pores, sand sticking and core lifting. In actual production, it is necessary to ensure the integrity of the film during Vacuum molding and prevent sand holes and sand hole defects caused by local damage of the film. In order to prevent core lifting during pouring, support steel pipes for support and air outlet are welded on the box stop of the sand box. After the sand mold is sprayed with paint, the paint on the surface of the sand mold shall be baked and dried in order to prevent the water in the paint and other liquids from generating immersion pores after high temperature. When assembling the sand core, cast mud strips are used to prevent the middle seam.
3.3 Other production process parameters
Pouring temperature ≥ 1320 ℃, pressure maintaining for 2h, heat preservation and slow cooling for 8h, unpacking, vacuum degree of pouring and molding process ≥ 55kpa.
4. Casting production
After strictly implementing the above V method process scheme, the first batch of 20 excavator balance weight castings have been produced. After inspection and processing verification, the excavator balance weight castings fully meet the drawing design requirements, and the surface roughness of excavator balance weight castings is 1 ~ 2 levels higher than that of resin sand casting.