The production of crankshaft blank by wet mold sand process is still the main traditional, while the representative advanced process is high-pressure molding. Its tooling and mold are single, which has high dimensional repetition accuracy of castings and is easy to be produced in large quantities. However, because the external mold is wet mold sand, its castings are prone to air holes, pinholes and shrinkage defects, and the dimensional accuracy and surface finish of castings are low, In addition, the production line needs to build a huge sand treatment system. In order to improve the casting quality, it often needs to add auxiliary cold iron. Therefore, it mainly produces medium and low-grade engine crankshafts of multi cylinder machines.
In order to overcome the casting defects brought by the wet mold sand process to the crankshaft production, the iron mold sand coating process was developed in China in the early 1980s. So far, many iron mold sand coating crankshaft production lines have been built one after another. Although the iron mold sand coating production line overcomes the disadvantage of poor chilling effect of wet mold sand production line on castings, the production line still has the following problems:
(1) a large number of iron molds (20-40 pairs) are required, and the one-time investment is large (RMB 4000-6000 per pair of iron molds for single cylinder crankshaft);
(2) the iron mold must be preheated before production to increase the preparation work and time before production;
(3) the iron mold generates alternating thermal stress in the production process, and the sand is dropped at the same time during the sand dropping, resulting in deformation of the iron mold, frequent mold repair and reducing the service life of the mold;
(4) the dimensional accuracy and surface finish of the casting are reduced due to the deformation of the iron mold during use;
(5) during the production process, the iron mold is required to have a relatively stable temperature (220-260 ℃), and the ambient temperature and pouring cooling time directly affect the use effect of the production line, so its production beat is difficult to control.
Due to its inherent shortcomings, the iron mold sanding line mainly produces single cylinder crankshaft. In order to overcome the shortcomings of the wet mold sand and iron mold sand coating process and produce high-grade automobile engine crankshafts, an automobile manufacturer took the lead in introducing a shell production line to produce car four cylinder engine crankshafts in China. The metallographic structure, mechanical properties, dimensional accuracy and surface finish of the crankshafts produced by this process have been significantly improved. However, due to large one-time investment and high production cost, there are still only two lines of automobile factories in China, This makes it difficult for the shell crankshaft to be popularized and applied in China.
The above three processes are comprehensively compared (see table):
|Tidal mold sand
|Iron mold coated sand
|Dimensional accuracy ISO
|Surface finish Ra
|One time investment
|In terms of import
|Tooling and mould cost
Through the analysis of crankshaft casting production process and production cost, it is of great significance to study and develop the shell mold casting process of nodular iron crankshaft for single cylinder machine by using domestic equipment, so as to ensure the casting quality of crankshaft and reduce the investment of process equipment and production cost.