Knowledge

"The risers were designed according to the standard. Why are there still shrinkage cavities in the castings?"

This question is indeed worth thinking about!

Especially in the production of gray cast iron and ductile iron, many enterprises still apply the feeding logic of cast steel to design risers and handle cast iron problems. As a result, it backfires, increasing the difficulty of cleaning and raising costs instead.

How should risers for cast iron parts be designed to be both scientific and economical?

The root of solving this problem lies in whether we truly understand the essential difference in solidification between cast iron and cast steel, and on this basis, establish a set of "process philosophy" that belongs to cast iron itself.

 

First of all, what you need to know is that cast iron is not cast steel, and the self-feeding ability is the key.

The most significant difference between cast iron and cast steel lies in the "graphitization expansion" during the solidification process. Grey cast iron and ductile iron will precipitate graphite when cooling, causing volume expansion, which forms a certain "self-feeding" capacity.

That is to say, the feeding of cast iron parts does not rely solely on risers, but is a coordinated system based on the post-feeding of the gating system and the self-feeding due to the expansion of graphite.

Therefore, the primary principle of designing iron risers is to clarify their supplementary role. That is: first rely on the feeding system for shrinkage compensation, then on the expansion of graphite for self-compensation, and only then use risers to make up for the difference. Blindly increasing the size of risers not only backfires but may also create new hot spots, interfering with the natural solidification sequence of the casting, resulting in a loss rather than a gain.

 

Furthermore, since the shrinkage value is not fixed, experience cannot be applied uniformly.

Many people are accustomed to determining the diameter of risers based on "1.5 times the wall thickness of the casting", which is often incorrect in cast iron.

In fact, the diameter of the riser is influenced by multiple factors such as alloy composition, pouring temperature, casting structure, and mold stiffness. For instance, in structural parts like base plates and guide rails, the shrinkage value in the length direction is often much greater than that in the height direction.

 

This also objectively determines that we cannot apply a fixed "proportion" to the riser size of all cast iron parts as we do with cast steel. True scientific design must be based on a profound understanding of specific working conditions.

 

What's more interesting is that in cast iron: thin and small parts usually require feeding, while thick and large ones don't necessarily have to.

Due to the self-feeding property of cast iron, when formulating feeding strategies, we need to break away from conventional thinking: thin and small castings are the ones that require more attention to feeding. Weak and small parts can make full use of the feeding system for feeding, while thick and small parts with concentrated hot spots need to have risers specially set up.

On the contrary, thick and large castings have a lower dependence on risers. As long as the process is appropriate, small risers or even riser-free processes can be adopted. The core issue lies in how to maximize the self-compression ability of graphite expansion through high-stiffness molds and the reasonable application of chills. In this case, there is no need to pursue the riser solidifying later than the casting, and the modulus of the riser can even be less than the wall thickness of the casting.

Besides, details determine success or failure. Where the riser is placed is often more crucial than how large it is.

One of the key points of the balanced solidification process is that the riser should be close to the hot spot, but must not cover it. The root of the inner gate, the root of the riser and the hot spot of the casting must not overlap. At the same time, the formation of "contact hot spot" should be guarded against. The traditional design experience of "taking the riser diameter as 1.5 times the wall thickness of the casting" is very likely to form a large hot spot area at the connection between the riser and the casting, causing this part to solidify last and thus leading to shrinkage cavities and porosity at the root of the casting.

For this reason, we more recommend the use of side feeding forms such as ear risers and flash risers, which can effectively disperse hot spots and optimize the solidification path. In practice, it is also necessary to avoid the direct use of cylindrical and square top risers as much as possible to reduce thermal interference to the casting body. ​​​​​​​

 

A perfect riser design, with the addition of chill iron, doubles its effectiveness.

 

When designing, thick-walled hot spots should be placed as much as possible at the lower part of the pouring position.

When there is a significant difference in the thickness of castings, setting chill irons in the thick-walled areas is an effective means to control the solidification sequence.

When the large plane is on the upper box, using an overflow riser can improve the surface quality of the casting.

However, it should be noted that the type of chill iron should preferably be common gray cast iron, and its usage frequency should be strictly limited. This is because after repeated use, the oxidation on the surface of the gray iron will lead to a decline in its chilling ability, which not only affects the feeding effect but may also cause defects such as porosity or adhesion of castings.

 

In terms of specific design methods, the shrinkage modulus method and empirical proportion method are commonly used for risers in gray cast iron.

 

When using insulated risers, their thermal modulus is approximately 1.3 to 1.4 times that of ordinary risers (when the diameter and height are equal, the coefficient is taken as 1.4), and their feeding efficiency is roughly within the range of 25% to 45% (for those with excellent insulation performance, the upper limit can be taken). In terms of shape, spherical ones are the best choice due to their minimum heat dissipation surface area, followed by cylindrical ones. Additionally, the use of "hot risers" should be avoided as much as possible to prevent the metal liquid from continuously heating the casting area during the filling process, while the metal liquid in the riser cools down first, causing the feeding channel to be interrupted prematurely, making the riser ineffective and greatly reducing the feeding effect.

 

The design of risers for good cast iron parts is far from simply choosing a size. Only by truly and fully understanding the "self-feeding property" of cast iron can we break away from the "cast steel mindset" and design a reliable and economical riser system - and this is precisely the direction that every foundry person should continuously strive for.

 

Vigor has more than 20 years experience and the professional team in casting, die-forging and the post treatment processes. If you have any question or products need to do, please feel free to contact us at info@castings-forging.com