(1) carbon equivalent shall be increased as much as possible without causing graphite floating. After the test, the carbon equivalent of 4.4-4.5% is better. In order to ensure the spheroidizing grade, nodular cast iron should have appropriate amount of residual magnesium, residual rare earth residue (medium and small crankshafts: Mg residue: 0.04-0.06%; re residue: 0.025-0.045%; large crankshafts: Mg residue: 0.04-0.07%; re residue: 0.015-0.025%) and the ratio of Mg residue% to re residue% (mg residue: re residue% ≥ 1).
(2) on the premise of ensuring the stiffness and strength of the shell, try to improve the compactness of the shell filled iron shot. Practice has proved that the coated sand shell mold has almost no strength under the high temperature of molten iron. Therefore, the high and uniform compactness of filled iron shot is the key to prevent casting defects such as mold expansion, mold shell cracking and molten iron leakage.
(3) strengthen liquid feeding and reduce liquid shrinkage. Because the expansion of graphitization is limited, the solidification shrinkage is a fixed value, and the stiffness of shell mold is difficult to ensure that the shape of shell mold casting crankshaft does not swell. Therefore, it is particularly important to minimize liquid shrinkage to prevent shrinkage porosity and shrinkage casting defects.
(4) use riser or gating system for liquid feeding: the ratio of the weight or volume of shell casting crankshaft to the riser or gating system for liquid feeding is (2-3): 1.
(5) improve the compactness of the filled iron shot and increase the heat capacity of the iron shot (the temperature of the filled iron shot is ≤ 80 ℃). In order to prevent shrinkage and looseness at the hot spot where heat dissipation is difficult, chill iron shall be used appropriately; The reasonable thickness of mold shell is selected at the crankshaft of shell mold casting, so as to strengthen the cooling capacity of mold shell and mold.
(6) when determining the process of shell casting crankshaft, try to reduce the machining allowance of the fan plate, reduce the thickness of the fan plate, increase the opening size of the two fan plates, improve the heat dissipation and increase the cooling effect.
(7) vibrating compaction of filled iron shot: the filled iron shot must be vibrated to improve the compaction rate of iron shot. However, the vibrating platform with low amplitude and high frequency should be selected in the real-time vibration, and the vibrating time should not be too long. It should avoid cracking the formwork and causing molten iron leakage. After the test, the vibration time should be 5-8s.
(8) increasing the pouring speed is an effective measure to prevent the subcutaneous air hole of shell casting crankshaft.
(9) the exhaust passage shall be fully considered in the mold shell to ensure the smooth discharge of a large amount of gas during pouring.
(10) the uniformity and convenience of the mold shell shall be solved in the mold shell manufacturing to reduce the shell scrap rate.
(11) in order to strengthen the chilling effect of the filled iron shot on the mold and have certain strength and rigidity to the mold shell, the particle size of the iron shot must be controlled, and the diameter is recommended Φ 5.5、 Φ 4、 Φ 3, the particle size mixing ratio is 40%, 30% and 30%.
(12) when the mold shell is bonded, multi-point clamping on the mold shell or follow-up fixture shall be adopted.
⒀ in order to reduce the workload of machining, according to the process test, the author believes that the machining allowance on the inner side of the fan plate can be reduced to 0.5-0.7mm.
Through the production test and research, the production of crankshaft by shell mold process has the remarkable advantages of improving the quality of shell mold cast crankshaft, high repetition precision, saving investment, low production cost and high productivity. It is especially suitable for the production of crankshaft below four cylinder engine. Under the reasonable process design, the qualified nodular iron castings such as crankshaft and camshaft can be produced in large quantities by using domestic equipment and raw and auxiliary materials, which lays an experimental foundation for the popularization and application of shell iron shot process in the production of nodular iron castings such as crankshaft and camshaft.