Designing thin sections. (Casting Design Notebook).

Metalcasting is unique in that it allows designers to place material where it is necessary and to do it in a non-uniform way, conforming with a nonlinear stress distribution field within the structure. When the part has minimal stress levels, the resulting mass distribution will call for a thin wall casting.

Thin wall castings often are advantageous because of their light weight structure, which allows for increased payload and reduced energy consumption in transportation applications. However, thin wall castings also can pose manufacturability problems associated with mold filling.

Rapid cooling of thin wall sections of the casting reduce the fluidity of the molten metal, which could cause the molten metal to prematurely freeze before it can completely fill the mold cavity, resulting in an incomplete fill or cold shuts. The mold filling capability in thin wall casting depends on a number of different process and design related variables, including the type of metal, pouring temperature, casting shape and metalcasting process.

Guidelines for different metals and processes are given in Table 1. The values given in Table 1 are reflective of compatibility with economical production and not necessarily with physical limitations of the process and material combination. These values are only suggestions for initial design and it is highly recommended that designers allow for modifications (increasing or reducing the values) based upon the results of the casting process.

Determining Wall Thickness

The practical minimum wall thickness for a particular casting configuration can be determined based on experiments using inexpensive and accurately made loose patterns. Often, a foundry engineer will recommend a value to start at based on prior experience. The wall thickness then should be adjusted depending upon if the defect rate is above or below the acceptable range.

For example, the 2.25-in, center-line radius aluminum elbow in Fig. 1 is cast via plaster molding. It has an 80% rejection rate for 0.04-in, wall thickness, 35% for 0.06-in, and 10% for 0.08-in, wall thickness. Since the cost of casting reflects both the complexity of the process and the number of quality castings, it is to the advantage of the designer to take this into consideration when specifying thin wall castings. The overall cost and performance advantage of a part with lower weight must overcome the potentially higher manufacturing cost.

Improving Castability

As indicated in Table 1, mold filling problems are rarely an issue when the wall thickness exceeds 0.375 in. Alternately, when the wall thickness is reduced below 0.125 in. adequate mold filling is virtually impossible except for short distances. The problem is premature freezing of the molten metal. In such instances, a designer can allow for certain geometric features on the casting that can ensure fluidity of the molten metal for the entire length of the mold cavity.

An example of such a feature is shown in Fig. 2, where strategically placing ribs on a casting serves two purposes. They break the surface area of the casting into smaller sections by providing channels for the ready delivery of the molten metal, and they increase the strength and structural rigidity of the part.

Another feature that can be used to improve the castabiity of thin walled castings is the selection of a different alloy. As indicated in Table 1, different metals can produce different wall thickness. For example, aluminum can achieve thinner walls more successfully than steel.

In a number of applications, this drastic alteration of metal can not be made due to the significant difference in mechanical properties of these metals. However, in a situation when the casting is not structural or load bearing, a designer can take this fact into consideration to achieve thinner walls.

Even in a similar group of materials, small deviations in alloy composition can have a significant impact on material fluidity. For example, 300 stainless steels have better fluidity than 300 stainless steels with the addition of molybdenum. Hence, whenever possible, the designer should allow a change in the material if that could lead to the desired casting configuration at a reduced cost.

Table 1

Typical Minimum Section Thickness for Different Materials and Processes.

Casting Method            Minimum Section Thickness (in.)

                          Aluminum             Magnesium

Sand                0.125 [+ or -] 0.031  0.156 [+ or -] 0.031
Permanent mold      0.093 [+ or -] 0.015  0.125 [+ or -] 0.015
lnvestment casting  0.062 [+ or -] 0.010  0.062 [+ or -] 0.010
Diecasting          0.062 [+ or -] 0.010  0.093 [+ or -] 0.010
Plaster mold        0.080 [+ or -] 0.015           --

Casting Method         Minimum Section
                       Thickness (in.)

                            Steel

Sand                0.187 [+ or -] 0.031
Permanent mold               --
lnvestment casting  0.093 [+ or -] 0.010
Diecasting                   --
Plaster mold                 --