Designing for economical coring--Part I. (Casting Design Notebook).

One of the major advantages of the metalcasting process is its ability to produce components with complex internal cavities (such as undercuts and skewed internal walls). The shapes used to form internal cavities within the casting are called cores, which can be made from different materials (sand, ceramic and metal) depending on the application. The same principles that govern making molds also are valid for making cores.

The cores are made in coreboxes and must be removed from the corebox after curing. That implies that cores cannot have undercuts that would prevent them from being removed from the corebox. In addition, deep pockets in the corebox must have a sufficient draft angle to prevent high friction between the core and corebox during removal, which could cause a core to break. For deep pockets, additional loose pieces are used in the corebox to assist the core removal.

Advantage of Cores

Although cores increase the cost of castings, they provide a number of advantages. The most important is the shaping of internal passages, which can be of almost any arbitrary shape and with non-uniform cross sections. Another advantage is shaping the geometry of the part to optimize wall thickness. Cores can provide casting geometry that can make stress distribution uniform throughout the part, thereby reducing its weight. This capability has not been extensively exploited in the past due to the complexity of stress analysis in highly complex shapes, but with the advancements in finite element analysis (FEA) tools, this design advantage of metalcastings becomes a significant advantage.

Cores also can form zero draft angles on castings and can be used to allow for undercuts on the vertical walls of the casting to reduce or eliminate secondary machining.

Core Location

Cores can he placed within the mold cavity in a number of different ways, each offering certain advantages and also posing certain limitations. For example, the bell shaped casting in Fig. 1 requires a core to reproduce the interior shape. The core can be located in the drag section of the mold and protruding into the cope section (Fig. 11) or hanging into the drag section (Fig. 1r). The first case allows for almost any core size, while the second limits the core size because of the stress in a core induced by its own weight.

Sand cores are fairly weak and can break if not supported properly. Thus, it is important to support cores within the mold cavity in such a way as to prevent their fracture and/or excessive deflection. For heavier cores, the location of the core as in Fig. 11 is more desirable. Alternately, locating the core within the mold as in Fig. 1r allows better venting and fewer potential problems with gas porosity. Also, locating the casting as in Fig. 1r will result in a better definition of the outer surface due to the gravity that pushes metal against the drag section of the mold. This may be especially important when fine detail on the casting needs to be accurately reproduced.

The process used for making the core also has significant influence on the core's strength and its ability to hold under its own weight. For example, shell cores typically are stronger than oil-based cores and can allow more freedom with respect to core location within the mold. In addition some geometries and coremaking processes allow the cores to be hollow, reducing the core's weight and stresses.

When the casting is very long, such as the one in Fig. 2a, supporting the core in the same manner as in Fig. 1 may not be possible due to the limited flask size and gating system issues. In this case, the mold cavity and core are positioned horizontally within the mold. However, this orientation of the core presents additional problems.

The stresses caused by the core's own weight (Fig. 2b) can be higher than the core's tensile strength, thus causing the core to break. Even if the core does not break, the body force on the core will cause it to deflect downward before the metal is poured, while buoyancy forces will cause it to bend upward after the metal has been poured.

To prevent or minimize these problems, designers redesign the casting (Fig. 2c) to allow for additional support. The resulting stresses and deflection (Fig. 2d) can be reduced by an order of magnitude with this simple modification. If necessary, holes on the casting can be plugged after casting and cleaning.

Another method to support the core in the mold cavity to lower stress and minimize deflection is to use chaplets (Fig. 3). The chaplets are metal shapes made of the same material or compatible material to the one used for casting. Since they are placed within the mold cavity, chaplets become an integral part of the casting. Hence, the foundry engineer must make sure tight bonding is achieved between the casting and chaplet.

Care Size Limitation

The stresses and deflection of cores caused by the body and buoyancy farces are of great importance in manufacturing good castings. These two phenomena depend on the core shape, core material, nature of core support within the mold and metal being poured. To determine the stress distribution and deflection of the core before and after the metal has been poured, one has to apply the basic principles of statics, strength of materials and fluid dynamics to the particular core configuration.

Figure 4 shows the recommended relationship between core dimensions, method of core support, casting wall thickness and type of metal poured. The most important factor for maximum core length is the type of core support within the mold cavity. Vertical placement of the core within the cavity allows for longer cores than horizontal orientation (due to the elimination of a buoyancy force acting on the core). However, this orientation of the core within the mold cavity can cause damage to the mold during mold assembly. In the case of horizontally positioned cores, the core supported on both ends can be longer than the one supported only on one end.

In the case of the cantilevered core, the minimum length of the core print should be 33%L as shown in Fig. 4. In addition, the core print section should be of sufficient mass to allow the core to remain in place for mold closure. The maximum allowable core length also increases as the core diameter increases (as strength of the core increases due to the increased cross section). The casting wall thickness also influences the core length (casting wall thickness decreases the maximum recommended core length increases). This is caused by the decreased amount of metal in the mold cavity and thus reduced buoyancy force.

The type of metal also has a certain level of influence on the recommended maximum length of the core. Steel has a higher density than most metals and results in a higher buoyancy force than aluminum and magnesium. As a result, the maximum recommended core length is smaller for steel than for aluminum and magnesium.

[FIGURE 4 OMITTED]