2.1 Ductile Iron Castings process
The riser, subsidy and the design of the cold iron of the casting are the decisive factors to ensure the internal structure of the gearbox shaft casting. The main function of the riser is to provide the molten steel necessary for the volume shrinkage during the solidification process of the casting to prevent shrinkage and porosity, followed by outgassing and scum, to ensure the compactness of the casting  .
The basic requirements of the riser design: ①The riser must have a long enough solidification time to ensure that the riser finally solidifies. ②Sufficient molten steel required for liquid casting and solidification shrinkage is provided. ③Ensure that there is a smooth feeding channel between the riser and the casting, that is, meet the feeding distance requirement.
Modulus is the ratio of the volume of the casting to the heat transfer surface area. It is a parameter to measure the solidification time of the casting. The calculation formula can be simplified as follows:
First select the area to calculate the modulus of the casting, and calculate the modulus of the riser according to the modulus of the casting to enlarge the corresponding safety factor, select the appropriate riser, and select the appropriate shape and quantity of the riser according to the structural characteristics and feeding range of the casting ,position.
In addition to the modulus calculation, the entire process virtual manufacturing software  and the advanced MAGMA solidification simulation software  are used for design and verification to determine the most reasonable riser and subsidy size specifications. The gearbox shaft casting adopts a feeding method that combines an open riser and a dark riser. The cold iron of the corresponding specification is installed at the end of the feeding area to block the liquid feeding channel between the open and dark risers in advance to improve the feeding. feeding port efficiency; while accounting feeding area, adjusting the temperature field, facilitate solidification three real now effective feeding, to ensure internal dense, non-destructive testing to meet the requirements.
2.2 Molding scheme
The shaft of the 5MW offshore wind power gearbox is a rotary structure, and the feeding process of the riser, subsidy, and cold iron is symmetrical. The solid sample molding plan and the core forming plan can be used. The advantages of the solid wood model molding scheme are that the cavity is directly formed by the model, the dimensional accuracy is high, there is no sand core butt gap, and the process information is in place. The disadvantage is that the internal cavity is difficult to brush and the surface quality is difficult to control. The advantage of the core forming scheme is that the cost of the model is low, the cavity can be brushed in place, and the surface quality of the inner cavity can be controlled. The disadvantage is that the core positioning is extremely demanding, otherwise the size of the casting cannot be guaranteed. Since the gearbox shaft has high requirements for internal and external coaxiality, and the coaxiality is very sensitive to the axial deviation of the shaft, for every deviation of the axial angle by 1 , the radial dimension can be deviated by 20~30mm . For the non-machined surface of the inner cavity, The axial angle deviation of the sand core must be accurately guaranteed. Considering the pros and cons, a solid wood model molding scheme is adopted to focus on controlling the size of the shaft.