The Importance and Design of Cast Iron Riser Neck

1. The design points of cast iron riser neck are as follows:

Size determination diameter: The diameter of the riser neck is generally 0.3-0.8 times the diameter of the hot spot circle of the casting. The diameter of the hot spot circle of the casting is large, with a value biased towards 0.3; The diameter of the hot spot circle is small, with a value biased towards 0.8. Length: usually between 20-50mm. For small cast iron parts, the length of the riser neck can be taken as the lower limit; Large cast iron parts are subject to an upper limit. Common shapes for shape design include cylindrical, trapezoidal, etc. The cylindrical riser neck is easy to process and suitable for most situations; The trapezoidal riser neck is beneficial for compensating shrinkage and is widely used in castings with high requirements for compensating shrinkage.

The position selection of the riser neck should be set at the hot junction of the casting, so that the metal liquid in the riser can flow preferentially to the hot junction, achieve sequential solidification, and effectively supplement shrinkage. Try to avoid setting it in the stress concentration area of the casting to prevent stress caused by solidification shrinkage of the riser neck, which can exacerbate the deformation and cracking tendency of the casting. The quantity is determined based on the size of the casting, the complexity of the structure, and the distribution of hot spots. Small and simple castings may only require one riser neck, while large and complex castings may require multiple riser necks to ensure sufficient shrinkage at each hot joint. The connection between the riser and the casting should have a smooth transition, avoiding right or sharp corners to reduce the resistance to the flow of molten metal. The connection between the riser neck and the casting should be firm to prevent breakage due to the impact of molten metal during the casting process. At the same time, the shape and size of the connection should be designed reasonably to avoid the formation of excessive heat affected zones on the casting, which may cause defects in the casting.

2.  Design Case Analysis of Cast Iron Riser Neck

Most alloys exhibit consistent and predictable behavior during the cooling process from liquid to solid at temperature. There are two different stages of contraction. Firstly, when the alloy casting temperature cools to the liquidus line, this is commonly referred to as liquid shrinkage or superheated shrinkage. Secondly, when an alloy cools from liquid to solid, it is commonly referred to as solidification shrinkage. On the other hand, graphite cast iron parts (including gray cast iron, ductile iron, and malleable cast iron) are accompanied by an unusual phenomenon during cooling and solidification, where the metal begins to expand. This expansion is usually attributed to the precipitation of lower density graphite phases, overcoming and exceeding the shrinkage associated with coolant and austenite solidification. So far, the most important aspect of designing risers and gating systems for cast iron is the requirement to maintain positive liquid pressure throughout the entire solidification process. Initially, atmospheric pressure must be allowed to act on the liquid in the riser, and in order for this to occur, the riser must be (compressed). Once expansion begins, a carefully designed riser system controls the expansion pressure and ensures automatic shrinkage of the casting during the remaining solidification process. This is in contrast to steel, aluminum, copper, etc., as they do not involve expansion, which requires the addition of molten metal to the casting during solidification.

3. Control pressure

The riser neck may be the most critical component in riser system design, as it typically determines the magnitude of residual pressure on the liquid. The contact surface of the riser neck must be large enough to transfer the molten metal from the riser to the casting over a long period of time. If necessary, excessive pressure in the mold cavity should be released, but it should be appropriate to maintain positive pressure of the liquid at the end of solidification and to facilitate the removal of the riser from the casting. The riser neck can be regarded as a "safety valve" on pressure vessels, and its design should ensure that the pressure inside the casting is maintained at a manageable level. The molding material, or more specifically, the sand mold that can withstand expansion pressure without expanding, usually determines the degree of controllability. If the mold material is weak, such as when using clay sand molds, a riser neck should be designed to release some expansion pressure to avoid mold expansion. This is achieved by designing the riser neck to solidify at a relatively late stage, allowing some pressure to be released to the riser through the riser neck. By using stronger and harder model bonding materials (such as resin systems), the riser neck can be designed to be smaller, allowing it to solidify earlier during the expansion phase and maintain higher residual liquid pressure. However, a too small riser neck can lead to excessive residual pressure within the casting, resulting in porosity related to mold expansion. An excessively large riser neck usually leads to a loss of positive pressure on the liquid before solidification is complete, resulting in shrinkage and gas discharge from the metal liquid related to solidification. The size of the riser neck in design rules is usually based on the geometric modulus (Mc) of the casting. The typical value of cast iron produced in clay sand is between 0.6 (Mc) and 0.9 (Mc). The accurate value depends on the hardness of the sand mold material, the chemical composition and inoculation degree of iron, and the cooling rate of the casting. If the riser is moved closer to the casting, the heating effect on the sand between the casting and the riser neck will reduce the geometric modulus of contact while maintaining the equivalent thermal modulus. If the neck is short enough to be equal to or less than the smaller contact cross-sectional size, the geometric modulus can be safely reduced by 0.6 times, i.e. the modulus of the longer neck (Mn (short)=0.6mn (long)). This indicates a reduction of approximately 65% in contact area.

conclusion

The successful shrinkage of graphite cast iron involves maintaining and controlling the positive pressure of liquid iron throughout the solidification process. Correctly designing the riser and pouring system, and controlling the metallurgical and pouring time well, are crucial for the production of graphite cast iron parts without shrinkage.